Br. J . Psychol. (1975), 66, 1, p p . 39-52
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Printed in Great Britain
HANDEDNESS AND CONTROLLED MOVEMENT BY KENNETH FLOWERS Brain and Perception Laboratory, Department of Anatomy, University of Bristol Subjects classified according to degree of hand preference were tested with both hands on two tasks of controlled movement. Strongly lateralized subjects (both left- and right-handers) showed greater skill with their better hand than ambilaterals on a visually controlled aiming test (faster speed and equal errors), but there were no marked differences between groups with the other hand. Between-hand differences were also greater in lateralized subjects. On a ballistic rhythmical tapping test, no marked differences in speed were found, but ambilaterals made slightly fewer errors with the better hand. It is argued (i) that for ballistic movements the hands are equipotential, and skill is a direct function of practice, (ii) that the essential dexterity difference between the preferred and non-preferred hands is in the sensory or feedback control of movements rather than in motor function pev se, and (iii) ambilaterals, especially those with very mixed preferences, have virtually two non-preferred hands in continuously controlled movement tasks, and may therefore rely on ballistic movements more than do pronounced sinistrals and dextrals.
Although some measure of dexterity has often been used in surveys of handedness (e.g. Koeh et al., 1933; Durost, 1934; Benton et al., 1962), there has been little consideration of the nature of differences between the hands in terms of skill (except for Provins, 1967a, b ) . Such varied tasks as picking up pins, typing, cutting with scissors, placing pegs and punching holes with pegs have been used to gauge dexterity, although they almost certainly involve different aspects of controlled movement, and it is not clear whether the handedness difference is the same in all of them. Nor is it clear whether differences in performance of the two hands result from some built-in feature of the nervous system (implying a difference in capacity or organization of the two cerebral hemispheres for sensorimotor skills) and, if so, what the feature is. I n other words, why should it be so much easier to carry out manipulative tasks with one hand rather than the other! I n this paper an attempt is made to specify two distinct kinds of control of movement, and to measure the performance on them of the preferred hand (PH) and non-preferred hand (NPH) of subjects with different hand preference patterns, to see if handedness can be defined in terms of the characteristics of skilled movement. I n considering skilled movement, such as violin-playing, two kinds of dexterity, or control, may be distinguished (Oldfield, 1969). The left hand is used to make very fast finger movements in the stopping, often in predictable sequences which are practised until they can be carried out as a complete ‘block’ of predetermined responses without conscious control. This represents what may be called a ‘ballistic ’ mode of performance, where each movement is triggered off automatically and, after practice, is carried out without any form of feedback monitoring once initiated. Indeed, the whole idea of scales and finger exercises in instrument-playing is to make the finger movements automatic in this way, and thus allow them to be executed accurately at high speeds, when it is impossible to monitor each single response, because of the inbuilt time delay involved in using sensory or feedback information to modify it (Vince, 1948; Welford, 1959; Fitts, 1964). This kind of movement
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requires control in the sense of sequential organization or fineness of movement, but not in the sense of current control of the movements as they are being made. I n contrast to this the right hand, used in bowing, requires a constant monitoring of the response as it is being made, since it is very difficult to control tone and volume over even short periods of time without it. This kind of continuous feedback monitoring represents a ‘ corrective ’ mode of control whereby the output of the motor system is continuously adjusted to correspond to or maintain a set criterion of some sort, and is an essential feature of making precise or graded responses, such as in aiming movements, or controlling the tone on a piano, or the accelerator on a car. The movements in this case may be relatively simple in themselves, but are limited in accuracy by the speed with which the serial corrections can be made (Craik, 1948; Poulton, 1969). If there is a difference between the hands, therefore, it could be in either of these kinds of control, and from the example of the violin, and intuitively, the working hypothesis was formulated that the handedness difference occurs in the corrective mode of control rather than in the ballistic one. These experiments, then, were intended to see if the difference in movement control between the PH and NPH of lateralized subjects showed up on an aimed-movement test (Fitts tapping) where responses are made precisely and under current (visual) sensory control, and/or on a similar test which requires control of the sequence of movement without any need for positional accuracy (rhythmical tapping). The first test was intended as a measure of monitored movement, where the central mechanisms involved in the sensorimotor loop are concerned, and the second one intended to measure the ability to generate ballistic sequences where these processes are less important. As the tests provide quantitative measures of the degree of control exercised by the hand in the two kinds of movement, it was possible to compare the handedness of different subjects in terms of their performance with PH and NPH, comparing between as well as within subjects, and asking questions such as (i) How do ambilateral and ambidextrous subjects compare with lateralized subjects in the control shown with the PH and NPH - are they as skilful with each hand or do they in fact lose some control on both sides? (ii) How well do hand preference measures correlate with the skill levels of the two hands?
CLASSIFICATIONOF SUBJECTS ACCORDING TO HAND PREFERENCE It is difficult to classify subjects as left- or right-handed, if they have any inconsistency of preference, without making some assumptions about the criterion to be employed in the classification. For the purposes of these experiments, therefore, the category ‘left-handed’ or ‘right-handed ’ was reserved for strongly lateralized subjects who professed a definite and consistent preference for one hand; all other subjects were classified as ‘ambilateral ’, and comparisons made between these three groups, with the ambilateral group subsequently subdivided on a purely numerical criterion into those mainly left-handed (‘left ambilaterals ’) or mainly right-handed (‘right ambilaterals’) and those more or less evenly divided in preferences (‘mixed-handers ’). This is of course itself an a priori classification of subjects, but the effect of any misclassification will be to nullify differences between the groups. This approach is consistent with the analysis by Annett (1964, 1967), who showed that where a rigorous criterion of consistency is employed for sinistrals and dextrals
Handedness and controlled movement
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on complete samples, the proportions of left-, right- and ambilateral-handers appear in nearly all cases to be constant according to a binomial distribution, with a ratio of 4, 64 and 32 per cent, respectively. Subjects were thus given a questionnaire before testing, which asked them to state which hand they would use by natural preference for eight unimanual actions (writing, drawing, throwing, racquet games, and using a toothbrush, comb, spoon and scissors) and in which position their hands would be, i.e. in a ‘ RH manner ’ or ‘LH manner’, in ten bimanual ones (sharpening a pencil with a penknife, striking a match on a box of matches, hammering a nail, peeling potatoes, dealing out playing cards, using a broom, using a spade, opening a jar, threading a needle and holding a cricket bat or golf club). They were also asked to check on their preferences by demonstrating with the appropriate tools. Subjects who gave left- or right-hand answers to all the 18 questions were classified as ‘left-handers’ (n = 8) or ‘righthanders’ (n = lo), and those who showed one or more anomalous preferences were regarded as ‘ambilaterals’. This latter group was subdivided for the purpose of skilled-movement comparison into three groups ;those showing one to five anomalous answers, termed ‘left ambilaterals’ (n = 16) and ‘right ambilaterals’ (n = 11) respectively, and those showing more than six (one-third) anomalous answers were called ‘mixed-handers’ (n = 11). The balancing of conditions in the experimental design was carried out within these five groups separately. Subjects were all volunteers obtained by an appeal in the university newsletter, and were drawn from the academic, technical and secretarial staff or their relatives. The groups were matched for age as follows. Left-handers: range 21-41, median 28, mean 29.6 years; righthanders: range 19-46, median 24, mean 27.1 years ; ambilaterals: range 20-61, median 26, mean 29.4 years. Five subjects who admitted to being forced to use the right hand for writing, eating or playing games when young were excluded from this analysis. EXPERIMENT I Fitts’ tapping test
I n a major paper on the measurement of the motor system’s capacity to control movement, Fitts (1954) suggested that the degree of precision required in any movement is a function of the number of alternative movements which could have been made. That is, it can be specified as the amount of information required to select the specified tolerance range from the total range of possible movements. Thus to hit a wide target, any one of a number of similar movements would suffice, and it is not necessary to specify which ; with a small target, however, there are fewer movements which result in a correct response, and these have to be specified more exactly. With increasing amplitude, too, the number of possible alternative movements which have to be excluded increases. Fitts suggested that width of target and amplitude of movement together specified the ‘index of difficulty ’ (Id) of a movement, according to the formula: 2 x amplitude of movement Id = log, bits. Width of the target On this formula, to increase the amplitude of movement or to decrease the width of the target is to increase the difficulty (precision required) of a movement. This gives
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a value in terms of information theory (bits) for any combination of target width and amplitude. Fitts then argued that the speed with which movements are made is limited by the information-processing capacity of the motor system. This means that, given that the subject is performing within the limits of accuracy required (i.e. making no errors and hitting the target every time), the maximum speed at which he can make controlled aiming movements will be determined jointly by his capacity and the information required for each one. By counting the number of movements made by subjects in a given condition, the information rate of the hand-arm can be calculated according to the formula : Id Inf. = -bits per second, t where t = the time taken to make one movement (Fitts found this to be about 10-12 bitslsec. for the right hand of his dextral subjects over a fairly wide range of conditions). Conversely, the relative speed of movement on a task of fixed precision indicates the capacity of the hand used in terms of information rate. I n this test the ‘preferred hand’ of ambilateral subjects was determined empirically as the hand with the best performance on the test, and this meant that for some subjects it was in fact the opposite hand to the one they regarded as their PH.
Method Following Fitts, the subject was required to hit two targets alternately with a stylus, and the number of hits scored in trials of 10 sec. each was noted. The targets were perspex strips 32 ern long, painted white with black edges and mounted on wooden boards on a Climpex frame in front of the subject, who moved the arm and hand across the body with lateral (left-right) movements. Target size was varied by using strips of different widths ( 1 , 2 or 4 em) and amplitude was varied by moving the boards along the Climpex frame to give distances between the centres of the targets of 4 cm (Id values 1, 2 and 3), 8 cm (Id values 2, 3 and 4), 18 cm (Id values 3, 4 and 5) and 32 cm (Id values 4, 5 and G ) . On each side of the target were two metal plates used to indicate errors. The perspex fitted over these plates so that the targets were raised slightly above them. Four counters, connected through the stylus and the metal plates, registered the number of errors made on each target in each trial, and a fifth counter registered the total number of movements made. Subjects were allowed t o use all the area of the targets, and to stand in any comfortable orientation in front of the apparatus. Two trials were made with each hand in each of the 12 conditions, with a rest after every block of 12 trials and whenever asked for by the subject. Each trial of 10 see. was timed with a stopwatch by the experimenter, who switched the counters on and off a t the beginning and end of each trial. Half the subjects in each handedness group were tested first with the left hand and then with the right in each condition, and the other half in the reverse hand order. The amplitude conditions were rotated systematically for successive subjects in each group. For each amplitude the target widths were presented always in the order 4-2-1 cm. Subjects were allowed as much practice as desired, with two dummy trials with each hand at the beginning of testing, and as much practice as required a t any subsequent time. Errors were indicated t o the subject by the sound of the error counters and the different sound of the stylus hitting metal rather than perspex, and all subjects were urged to tap as fast as they could without making too many errors. Any trial where the errors numbered more than 10 per cent or so of the total movements made, or where the subject broke down and stopped completely, were repeated, with the instruction to try and concentrate on being accurate. As some subjects appeared t o find this adjustment difficult, and as both errors and speed were being measured on this test, subsequent trials were accepted as the representative performance of the subject. The average of hits on the two trials was divided into the 10 sec. of the test period t o give the value o f t , which was
Handedness and controlled movement
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Fig. 1. Preferred-hand movement times of the three main groups in the two experiments. 0,Left-handers; 0,right-handers; a, combined ambilaterals; x , Fitts’ subjects (1954). plotted against the I d values (Figs. 1-3). For each hand of each subject there was one measure at Id values 1 and 6, two measures for Id values 2 and 5, and three for Id values 3 and 4. These data were pooled for all subjects in each group to give the estimate of left-hand and right-hand control in each condition. Analyses of variance and t tests were used to compare between hands and between groups.
Results The mean time per movement for PH and NPH of the three main groups is shown in Fig. 1 (which also plots the data of Fitts’ subjects in equivalent conditions) and Fig. 3. The two lateralized groups have a faster rate of movement with the PH at all Ids, the difference becoming marked and reliable a t Ids 4, 5 and 6 (F = 5.30, P < 0.005; F = 7-50, P < 0.001 and F = 5.04, P < 0.01, respectively). I n all three conditions both right-handers and left-handers are faster than the combined ambilaterals at the level of P < 0.05 or better. With the NPH there are no differences in any condition (largest difference for I d = 6, F = 2.07, n.s.). When the combined ambilateral group is split up into right ambilaterals, left ambilaterals and mixedhanders, a regular trend appears for the PH, with the slowest scores associated with the more mixed group, and those of the other groups falling between them and the scores of the lateralized subjects (Fig. 2). The difference between the five subgroups are again significant at Ids 4,5 and 6 (F = 3.61, P < 0.01 ;P = 5.55, P < 0.001 and F = 3-59, P < 0.02, respectively). This represents a difference in terms of information processing of l-l&bitslsec. in the mixed-handers. It thus appears that subjects with inconsistency of hand preference have slightly less control in an aiming task with their PH, without any compensating increase in skill with their NPH. One point to note here is that the PH of the ambilateral group is the hand with the best
KENNETH FLOWERS
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Handedness and controlled movement
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overall performance, and the comparison is therefore clean and unaffected by possible misclassifications of hand preference. It also means that the measured differences are if anything underestimated, since taking any other criterion of what is the PH for the ambilateral groups results in PH scores being lower for these subjects and hence exaggerating the differences. (It does not, however, improve their NPH scores to any marked degree.) This discrepancy, between the hand preferred for the majority of tasks and the best hand in the test situation, occurred in fact for three subjects of the right ambilateral group and two mixed-handers. Between-hand scores for the lateralized groups were significant at all I d levels except the lowest, indicating a substantial superiority at aiming for the PH of these groups of around 1 6 2 4 bits/sec. The ambilateral groups also showed a reliable between-hand score a t Id = 1-5, but this was not surprising since the best-hand scores were selected in these subjects for the PH performance. Even so the difference between hands was somewhat less marked (about 1-18 bits/sec.) and became nonsignificant at all Id levels when the subject’s own preferred hand was taken as PH. Thus the ‘best hand’ of the ambilateral subjects was not well correlated with the preferred hand as the subjects themselves understood the term, and there was considerable overlap between the PH and NPH scores on different conditions even with the ‘best hand’ - ‘non-best hand’ measure, implying that inconsistent hand preference was associated with inconsistent as well as less marked skill differences. Comparison with the lateralized groups shows that this between-hands similarity in the ambilateral groups is due to the PH being worse than dextral or sinistral PHs rather than to a higher level of skill being present in the NPH. Since the measurement of capacity in the hands in terms of speed of movement assumes that all subjects were operating within a set accuracy limit, error rates should have been equal in all groups. This was in fact the case for both hands, differences between groups being small and insignificant in all conditions (largest difference F = 2.24, n.s.). The ambilaterals had a slightly lower mean error score all through, but as they were making fewer movements overall, this means that the percentage of errors was about the same as that of the lateralized subjects. I n all groups, too, differences between the hands in the number of errors made were small and insignificant. Thus the lower speed found for the NPHs and for the PH of ambilaterals is not correlated with a lower error rate. The distribution of errors within the groups followed that reported in the Fitt’s study, with an even split between left-side and right-side errors, a small but consistent tendency in all subjects to undershoot rather than overshoot, and a similar tendency to make more errors with the narrowest targets, indicating that subjects did not adjust their speed to fit the tolerance widths exactly. Since these trends were the same for all groups, they do not alter the main effects found between them. The differences in group scores suggest that there is an association between PH level of aiming skill and degree of hand preference, but does not say how direct this is. A Spearman rank correlation coe€€icientwas calculated for this association at the three I d conditions where reliable inter-group differences were found, and produced low but significant correlations in all cases. Thus at I d 4, rs = 0.270 (P < 0.05) ; at I d 5, rs = 0.302 (P < 0.02); and at Id 6, rs = 0.372 (P < 0.01). When ambilaterals were considered alone, the correlations dropped to rs = 0.246 (P < 0.05) for Id = 6
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KENNETH PLOWERS
and did not reach significance for I d = 4 and Id = 5 . These correlations cannot be considered very striking, and suggest that degree of hand preference is not a very precise index of subjects’ manual skill capacity once they have been classed as lateralized or ambilateral. It would not therefore be surprising to find anomalies fairly frequently between handedness as assessed by hand preference measures and handedness as measured on a dexterity test (Benton et al., 1962).
Discussi0.n As can be seen in Fig. 1, the shape of the curves of the different groups in this experiment resembles that of Fitts’ original subjects, although generally speeds (and thus information rates) appear to be higher. The measure of skill is fairly constant across the various conditions (within 14 bits) except for the two lowest I d levels, where performance may be limited by an extrinsic factor such as arm inertia, as suggested by Fitts himself. The apparent superiority of the subjects here may well be attributed to the great error rate tolerated (10 per cent compared to the 1 per cent average reported by Fitts) and to the fact that the instructions emphasized speed, whereas the original study stressed accuracy of performance. The average error rate with either hand was almost the same for the three main groups and was in fact higher for the two subgroups with the lowest measured information capacity (right ambilaterals and mixed-handers). The relatively low speeds of these subjects therefore cannot be attributed to a more cautious or precise manner of performance, i.e. they did not trade off speed for greater accuracy, and the relative speeds do seem to indicate differences in the information-processing capacity of the groups involved. Further, this capacity is correlated not with dextrality or sinistrality as such, but rather with consistency of hand preference. There is no disadvantage in being lefthanded per se, in fact the LH group showed the highest scores for the PH and almost for the NPH as well. Nor was there any indication that left ambilaterals are less skilful than their equivalent dextral opposites, in fact rather the reverse. But there was a definite inverse association between ambilaterality and degree of control in the PH. The question remains as to how stable are these handedness effects, or whether after practice the observed differences might not disappear (as e.g. with the typewriting task of Provins & Glencross, 1968). It is interesting to note here that Fitts, in his more thorough and prolonged experiment, found that: ‘Practice led to relatively small improvement in performance.’ for both speed and error rates. It may thus be assumed that practice does not change the scores to any great extent, at any rate while the subject is performing in a direct aiming mode. With prolonged practice the subject may well be able to perform ballistically, setting an amplitude and velocity value and then moving the arm without aiming each individual hit. But this would no longer be a corrective mode of response-and the handedness effect may then indeed not apply. The second experiment was designed to compare the levels of ‘dexterity’ in such a situation where aiming was not required.
Ha.ndedness and controlled movement
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EXPERIMENT I1 Rhythmical tapping I n the second experiment a tapping task was employed which required control of movements in a different sense from that of the reciprocal tapping task. Here subjects were asked to tap out rhythms repetitively as fast as they could, without aiming the movements to hit any particular point. Performance was ballistic in the sense that relatively little control of the position or force of each tap was necessary, and also in that it was usually impossible to correct individual errors in the required rhythm within a unit or ‘block’ of the rhythmical sequence. Subjects universally reported that they organized each sequence in a grouping of some sort and ‘fired off ’ each group as a whole. The task thus involved dexterity in the sense of making coordinated sequences of movements as fast as possible, but not the use of feedback information to guide the movements as they were made.
Method The apparatus of the first experiment was used. The two widest strips of perspex were placed next t o each other in the middle of the Climpex frame, and the subject asked t o tap on this area (8 x 32 em) in six different tapping combinations. The black edges of the strips, side by side, formed a vertical line down the centre of the perspex area; some subjects found it easier to tap each side of this line for each alternate group of taps. The subject held the stylus as in the previous test, and the overall number of taps made in periods of 10 see. counted. I n this test errors were considered t o be taps made out of sequence. The six test conditions were: ( a ) Spot tapping - tapping as fast as possible on the spot; ( b ) 2-2 - tapping in pairs with a pause between each pair; (c) 3-3 -tapping in triplets with a pause between each set; ( d ) 4-1 groups of four taps alternating with single taps, with a pause between each set; ( e ) 3-2 tapping in pairs and triplets alternately, with a pause between each group; (f)1-2-3 - single, double and triple taps with a pause between each set. Two trials were made with each hand on each condition. Half the subjects of each experimental group were tested with the PH first and then with the NPH on each condition, and half in the reverse order. All subjects went through the conditions in the order ( a ) t o (f).Practice was allowed with either hand for as long as the subject wished. Trials where errors were made on more than two groups of taps, or where the subject broke down completely, were repeated.
Results The mean time per movement for the PH and NPH of the three experimental groups on tjhe six conditions of this experiment are shown in Figs. 1 and 3. The PH of subjects in the ambilateral groups is taken as that which produced the best performance in Expt. I. There are no significant differences between the groups with either hand on any condition (largest difference P = 1.03 for NPH on condition (f)), and the ambilateral group as a whole perform slightly better than the dextrals and sinistrals on half the conditions. There is thus no effect of lateralization on dexterity as measured on this test. Between-hand comparisons show the opposite effect to that found in the aiming test, with the lateralized subjects producing no marked differences, while the ambilaterals are reliably better with the ‘better’ hand. When this latter group is subdivided, the effect disappears, however, for two subgroups, and only remains in the left ambilaterals. Errors recorded on this test, given in Table 1, show a similar pattern. Here all three ambilateral groups make slightly fewer errors with the PH than do the lateralized ones, a difference which reaches
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significance on three conditions, and when total errors are considered. But only the left ambilateral subgroup shows a reliable between-hand difference, which may be due to chance, or to the higher number of subjects in this group. The differences in speed and accuracy in this group are in any case due to a rather lower level of performance with the NPH than to a markedly higher level with the PH. I n general, then, there is a hint that the ambilaterals may be finding it easier to generate the simple rhythmical sequences asked for here, but this is not a very marked effect.
Discussion On this test the differences between PH and NPH, and between groups, found in the first experiment, seem virtually to have disappeared, implying that the two hands are equipotential for this kind of controlled movement. The rate of movement in this test, moreover, is not determined by some maximum or minimum limit effect, since the rates for conditions ( b ) to (f), where grouping is required, all fall between the fastest and slowest measured rates on the Fitts’ test (see Fig. 1). The results thus cannot be explained solely in terms of some extrinsic factor such as the maximum rate of muscular contraction, or the speed with which upward and downward movements can be alternated. The rate is also different for the different conditions, reflecting the universal report of subjects that some groupings are easier to generate than others. But even the most complex rhythm demanded here, condition (f), produced a rate of movement in all subjects faster than that found on I d conditions 4, 5 and 6 in the Fitts task, where handedness differences became significant. Complexity of sequential organization therefore does not seem to be a crucial factor limiting the speed of controlled movements, nor the basis of handedness differences in skill, at least where the subject can generate patterns of movement in predetermined units or groupings, and this emphasizesthe point made earlier that the difference is not in the motor system as such.
GENERALDISCUSSION Ballistic and monitored movements. A basic assumption of this study has been that the Fitts tapping task (Expt. I) requires each movement to be made under current sensory control (i.e. with every one aimed individually), while the rhythmical tapping task (Expt. 11) is done ballistically, i.e. with movements carried out in groups or blocks ‘fired off’ in such a way as to be unmodifiable individually. This can easily be checked by considering mean movement duration times for the various conditions, since aiming movements which take more than half a second probably involve some correction procedure, and all quick movements (taking less than 300msec. or so) must be ballistic in the sense defined above (Woodworth, 1899; Welford, 1968; Poulton, 1969). The data in Figs. 1-3 justify the assumption for Expt. 11,since the mean movement duration on all conditions is less than 300 msec., and hence few of the movements could have been substantially modified during execution. As regards Expt. I, however, it seems as if the same argument must apply to the three easiest conditions, Ids 1-3, as the mean duration of movements there is also below the criterion. This means that subjects were in fact performing ballistically in these conditions. This may be explained by the fact that the test presents a fixed, stationary target, and subjects do not have to observe or calculate its present or 4
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future position, so they may well be able to maintain a reasonable degree of accuracy in low I d conditions by setting an amplitude-of-movement value and trying to repeat it ballistically, trusting that the variance of arm movements will not produce too many hits outside the acceptable range. (Fitts himself recognized this possibility.) With low I d values, then, movements will be limited, not so much by the information factor of monitored movement, as by the factors which limit ballistic movements. When more than 3-4 bits per movement are involved, however, subjects have to resort to some corrective procedure, if only to check that they are reasonably accurate, and the information-processing factor will become predominant as a limiting factor in the speed of movement. It would be in accordance with the hypothesis of handedness suggested here, therefore, for there to be relatively small differences between groups with the PH on conditions I d 1-3, and that these should become noticeable and significant for Id values above 4 - which is the finding of Expt. I. Handedness difleerences in lateralized subjects. The data reported here suggest that in consistent left- and right-handers there is a reliable difference in performance with PH as against NPH on a visually controlled aiming task, but not where movements are ‘fired off’ ballistically without the need for monitoring. This implies that the crucial difference in movement control concerns the sensorimotor feedback loop, where some central processing operation is performed to transmit sensory and feedback information to the motor system. The NPH seems to have a lower rate of information transmission on such tasks, so that sequential adjustments of ongoing motor activity each take slightly longer than with the PH. The basic handicap of the NPH may thus be said to consist of greater intermittency in the sensory control of movement. It will therefore be at a disadvantage on any task where this kind of control is required. It will be at the greatest disadvantage where rapid serial corrections are required, or where the information per movement is high, and will in any case ‘feel’ less capable in such situations even where it can cope quite adequately. The handedness difference, it should be noted, is not in absolute ‘dexterity’, but rather in the time taken to go through the sensorimotor loop and the sequence of operations necessary to maintain continuous control of movements using current sensory and/or feedback information. This agrees with the finding of Woodworth (1899) in his classic monograph that left-hand movements become ballistic (are not amenable to correction during their course) if they take 750msec. or less, while right-hand ones only become ballistic at 400 msec. or faster. Thus the NPH may be capable of fine adjustment and graded response, but it takes longer and requires more care to achieve it. This may explain the negative findings of the other studies which report that the NPH is as capable as the PH of any simple response, even graded ones, where the time taken for the response is not measured (Provins, 1956), and is also consistent with other accounts specifying that ‘delicate ’ adjustments and aiming tasks give the most reliable between-hand differences(Koeh et al., 1933; Durost, 1934). If the NPH is less capable on monitored movements, it appears to be equipotential for ballistic movements, with the implication that where the subject can perform in this mode he may circumvent the limitations of intermittency, and performance will then be a function of the accuracy of motor commands, which may be largely determined by practice. This factor may well apply in typing, games and music, where response sequences are built up to become ballistic (i.e. automatic) and
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are then fired off as a unit without further conscious control. It would thus be reasonable to find that typists, for instance, were typing faster with the left hand after practice (Provins & Glencross, 1968) if the left hand is used more frequently on the keyboard, since the typical typing response is not continuously graded, and little feedback control is required. Conversely, in reaction time experiments one would expect reliable differences between the hands, as shown for example by Kerr et al. (1963), since here each response is preceded by a decision based on an immediately preceding sensory input signal (the light which indicates which butt’on is to be pressed) and hence the control of the response must be exercised through the sensorimotor loop. This effect should, again, be constant and unmodifiable with practice, because although the response itself is probably ballistic, it nevertheless involves central processing mechanisms for its initiation. This may relate to Oldfield’s (1969) contention that the P H may be more directly concerned with expressions of intention and volitional control, simply because the NPH responds that much slower to decision signals and more intermittently in serial tasks which involve sensorimotor and decision processes. Handedness differences in ambilateral subjects. Ambilaterals were selected out a t the beginning of this study by their hand preference, as if they were a separate category, and their performance on the aiming test was consistent with this classification. Thus all the ambilateral groups appeared to have a somewhat lower level of skill with either hand in the corrective control situation, whatever their pattern of preference. The degree of inconsistency was a general index of the level of skill attained, but this was not sufficiently direct to allow one to predict exactly from hand preference scores how well individual subjects would perform. Nor sometimes was the subject’s most-preferred hand the ‘best’ hand objectively measured, or the subject’s most-preferred hand what he himself claimed to be his PH.This was most true for the right ambilateral subgroup, who would in many cases be classed as righthanders on a bimodal categorization. I n other words, the concept of handedness needs to be redefined in terms of best-hand and worst-hand skill, measured as how well the hands do, rather than in terms of what or how many tasks are carried out with one hand. Ambilaterality, however, seems to be a very different thing from ambidexterity, although all those in this sample who claimed to be ambidextrous fell into one or other of the ambilateral groups (two right ambilaterals, two left ambilaterals and three mixed-handers). Certainly those with very mixed hand usage were not equally good with each hand, but rather equally bad; in other words, these subjects have the equivalent of two ‘non-preferred hands ’ in the sense that neither has developed the faster sensorimotor monitoring achieved by the PH of consistent handers. If the distinction suggested here between the two kinds of controlled movement is valid, then it is possible that subjects may become equally good with either hand for ballistic movement by practice, and that those with mixed-hand preference may have an advantage for this. If so, they may rely on ballistic movements more than do pure sinistrals and dextrals. On the present findings, however, it seems unlikely that anyone with mixed usage will be able to develop a high level of performance at corrective movement with either hand, let alone with both. Since, too, consistent handers appear to have a relatively low level of skill a t the aiming task with the 4-2
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NPH, one is led to predict that true ambidexterity is impossible in corrective movements of this sort. On the contrary, a high level of skill in either hand is associated with strong laterality, and it seems that it is only ‘ambilevous’ or bilaterally clumsy subjects who cannot decide which hand to use by preference - possibly because neither hand feels very comfortable in controlling movements. Assuming that the distinction between ballistic and monitored movements is correct, the results presented here have some bearing on the question of specialization of the hemispheres for motor function. It would appear that before this can be clearly assessed the kind of control involved in any particular task needs to be specified. For monitored movement control, ambilaterals seem to have not two specialized hemispheres, but rather a lack of specialization, or a lack of development in the sense that neither side has a very high level of ability. Whether they have an advantage or disadvantage for developing ballistic control is not yet clear. On the present formulation (that the hands are equipotential for this kind of movement) they would not have any particular advantage. But it does seem that anyone with some inconsistency of hand usage has a different cerebral organization for sensorimotor control function compared to lateralized subjects, and this implies that handedness has some neurological basis. REFERENCES ANNETT,M. (1964). A model of the inheritance of handedness and cerebral dominance. Nature, Lond. 204, 59-60. ANNETT,M. (1967). The binomial distribution of right, mixed and left handedness. &. J . exp. Psychol. 19, 327-333. BENTON,A. L., MEYERS,R. & POLDER, G. J. (1962). Some aspects of handedness. Psych&. Neurol. (Basel) 144, 321-337. CRAIK, K. J. W. (1948). Theory of the human operator in control systems. Br. J . Psychol. 38, 56-61; 142-148. DUROST, W. N. (1934). The development of a battery of objective group tests of manual laterality. Genet. Psychol. Monogr. 16, 225-335. FITTS,P. M. (1954). The information capacity of the human motor system in controlling- the amplitude of movement. J . exp. Psychol. 47, 381-391. FITTS, P. M. (1964). Perceptual-motor skill learning. I n A. W. Melton (ed.), Categories of Human Learning. New York : Academic Press. KERR,M., MINUAY,R. & ELITHORN, A. (1963). Cerebral dominance in reaction-time responses. Br. J . Psychol. 54, 325-336. KOEH, H. L. et al. (1933). A study of the nature, measurement and determination of hand preference. Genet. Psychol. Momgr. 13, 117-221. OLDFIELD,R. C. (1969). Handedness in musicians. Br. J . P8yChOl. 60, 91-99. POULTON, E. C. (1969). Tracking. In E. A. Bilodeau (ed.), Principles of Skill Acquisition. New York : Academic Press. PROVINS, K. A. (1956). ‘Handedness’ and skill. &. J. exp. Psychol. 8, 79-95. PROVINS, K. A. (1967a). Handedness and motor skill. Med. J . Austr. ii, 468-470. PROVINS, K. A. (1967h). Motor skills, handedness and behaviour. Aust. J . Psychol. 19, 137-150. PROVINS, K. A. & GLENCROSS, D. J. (1968). Handwriting, typewriting and handedness. &. J . exp. Psychol. 20, 282-289. VINCE,M. (1948). Intermittency of control movements and the psychological refractory period. Br. J . Psychol. 38, 149-157. WELFORD, A. T. (1959). Evidence of a single-channel decision mechanism limiting performance in a serial reaction task. &. J . m p . Psychol. 11, 193-210. WELFOBD, A. T. (1968). Pundanaentals of Skill. London: Methuen. WOODWORTH, R. S. (1899). The accuracy of voluntary movement. Psychol. Monogr. 3, no. 3.
(Manuscript received 19 October 1973 ; revised manuscript received 7 February 1974)
39
Printed in Great Britain
HANDEDNESS AND CONTROLLED MOVEMENT BY KENNETH FLOWERS Brain and Perception Laboratory, Department of Anatomy, University of Bristol Subjects classified according to degree of hand preference were tested with both hands on two tasks of controlled movement. Strongly lateralized subjects (both left- and right-handers) showed greater skill with their better hand than ambilaterals on a visually controlled aiming test (faster speed and equal errors), but there were no marked differences between groups with the other hand. Between-hand differences were also greater in lateralized subjects. On a ballistic rhythmical tapping test, no marked differences in speed were found, but ambilaterals made slightly fewer errors with the better hand. It is argued (i) that for ballistic movements the hands are equipotential, and skill is a direct function of practice, (ii) that the essential dexterity difference between the preferred and non-preferred hands is in the sensory or feedback control of movements rather than in motor function pev se, and (iii) ambilaterals, especially those with very mixed preferences, have virtually two non-preferred hands in continuously controlled movement tasks, and may therefore rely on ballistic movements more than do pronounced sinistrals and dextrals.
Although some measure of dexterity has often been used in surveys of handedness (e.g. Koeh et al., 1933; Durost, 1934; Benton et al., 1962), there has been little consideration of the nature of differences between the hands in terms of skill (except for Provins, 1967a, b ) . Such varied tasks as picking up pins, typing, cutting with scissors, placing pegs and punching holes with pegs have been used to gauge dexterity, although they almost certainly involve different aspects of controlled movement, and it is not clear whether the handedness difference is the same in all of them. Nor is it clear whether differences in performance of the two hands result from some built-in feature of the nervous system (implying a difference in capacity or organization of the two cerebral hemispheres for sensorimotor skills) and, if so, what the feature is. I n other words, why should it be so much easier to carry out manipulative tasks with one hand rather than the other! I n this paper an attempt is made to specify two distinct kinds of control of movement, and to measure the performance on them of the preferred hand (PH) and non-preferred hand (NPH) of subjects with different hand preference patterns, to see if handedness can be defined in terms of the characteristics of skilled movement. I n considering skilled movement, such as violin-playing, two kinds of dexterity, or control, may be distinguished (Oldfield, 1969). The left hand is used to make very fast finger movements in the stopping, often in predictable sequences which are practised until they can be carried out as a complete ‘block’ of predetermined responses without conscious control. This represents what may be called a ‘ballistic ’ mode of performance, where each movement is triggered off automatically and, after practice, is carried out without any form of feedback monitoring once initiated. Indeed, the whole idea of scales and finger exercises in instrument-playing is to make the finger movements automatic in this way, and thus allow them to be executed accurately at high speeds, when it is impossible to monitor each single response, because of the inbuilt time delay involved in using sensory or feedback information to modify it (Vince, 1948; Welford, 1959; Fitts, 1964). This kind of movement
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requires control in the sense of sequential organization or fineness of movement, but not in the sense of current control of the movements as they are being made. I n contrast to this the right hand, used in bowing, requires a constant monitoring of the response as it is being made, since it is very difficult to control tone and volume over even short periods of time without it. This kind of continuous feedback monitoring represents a ‘ corrective ’ mode of control whereby the output of the motor system is continuously adjusted to correspond to or maintain a set criterion of some sort, and is an essential feature of making precise or graded responses, such as in aiming movements, or controlling the tone on a piano, or the accelerator on a car. The movements in this case may be relatively simple in themselves, but are limited in accuracy by the speed with which the serial corrections can be made (Craik, 1948; Poulton, 1969). If there is a difference between the hands, therefore, it could be in either of these kinds of control, and from the example of the violin, and intuitively, the working hypothesis was formulated that the handedness difference occurs in the corrective mode of control rather than in the ballistic one. These experiments, then, were intended to see if the difference in movement control between the PH and NPH of lateralized subjects showed up on an aimed-movement test (Fitts tapping) where responses are made precisely and under current (visual) sensory control, and/or on a similar test which requires control of the sequence of movement without any need for positional accuracy (rhythmical tapping). The first test was intended as a measure of monitored movement, where the central mechanisms involved in the sensorimotor loop are concerned, and the second one intended to measure the ability to generate ballistic sequences where these processes are less important. As the tests provide quantitative measures of the degree of control exercised by the hand in the two kinds of movement, it was possible to compare the handedness of different subjects in terms of their performance with PH and NPH, comparing between as well as within subjects, and asking questions such as (i) How do ambilateral and ambidextrous subjects compare with lateralized subjects in the control shown with the PH and NPH - are they as skilful with each hand or do they in fact lose some control on both sides? (ii) How well do hand preference measures correlate with the skill levels of the two hands?
CLASSIFICATIONOF SUBJECTS ACCORDING TO HAND PREFERENCE It is difficult to classify subjects as left- or right-handed, if they have any inconsistency of preference, without making some assumptions about the criterion to be employed in the classification. For the purposes of these experiments, therefore, the category ‘left-handed’ or ‘right-handed ’ was reserved for strongly lateralized subjects who professed a definite and consistent preference for one hand; all other subjects were classified as ‘ambilateral ’, and comparisons made between these three groups, with the ambilateral group subsequently subdivided on a purely numerical criterion into those mainly left-handed (‘left ambilaterals ’) or mainly right-handed (‘right ambilaterals’) and those more or less evenly divided in preferences (‘mixed-handers ’). This is of course itself an a priori classification of subjects, but the effect of any misclassification will be to nullify differences between the groups. This approach is consistent with the analysis by Annett (1964, 1967), who showed that where a rigorous criterion of consistency is employed for sinistrals and dextrals
Handedness and controlled movement
41
on complete samples, the proportions of left-, right- and ambilateral-handers appear in nearly all cases to be constant according to a binomial distribution, with a ratio of 4, 64 and 32 per cent, respectively. Subjects were thus given a questionnaire before testing, which asked them to state which hand they would use by natural preference for eight unimanual actions (writing, drawing, throwing, racquet games, and using a toothbrush, comb, spoon and scissors) and in which position their hands would be, i.e. in a ‘ RH manner ’ or ‘LH manner’, in ten bimanual ones (sharpening a pencil with a penknife, striking a match on a box of matches, hammering a nail, peeling potatoes, dealing out playing cards, using a broom, using a spade, opening a jar, threading a needle and holding a cricket bat or golf club). They were also asked to check on their preferences by demonstrating with the appropriate tools. Subjects who gave left- or right-hand answers to all the 18 questions were classified as ‘left-handers’ (n = 8) or ‘righthanders’ (n = lo), and those who showed one or more anomalous preferences were regarded as ‘ambilaterals’. This latter group was subdivided for the purpose of skilled-movement comparison into three groups ;those showing one to five anomalous answers, termed ‘left ambilaterals’ (n = 16) and ‘right ambilaterals’ (n = 11) respectively, and those showing more than six (one-third) anomalous answers were called ‘mixed-handers’ (n = 11). The balancing of conditions in the experimental design was carried out within these five groups separately. Subjects were all volunteers obtained by an appeal in the university newsletter, and were drawn from the academic, technical and secretarial staff or their relatives. The groups were matched for age as follows. Left-handers: range 21-41, median 28, mean 29.6 years; righthanders: range 19-46, median 24, mean 27.1 years ; ambilaterals: range 20-61, median 26, mean 29.4 years. Five subjects who admitted to being forced to use the right hand for writing, eating or playing games when young were excluded from this analysis. EXPERIMENT I Fitts’ tapping test
I n a major paper on the measurement of the motor system’s capacity to control movement, Fitts (1954) suggested that the degree of precision required in any movement is a function of the number of alternative movements which could have been made. That is, it can be specified as the amount of information required to select the specified tolerance range from the total range of possible movements. Thus to hit a wide target, any one of a number of similar movements would suffice, and it is not necessary to specify which ; with a small target, however, there are fewer movements which result in a correct response, and these have to be specified more exactly. With increasing amplitude, too, the number of possible alternative movements which have to be excluded increases. Fitts suggested that width of target and amplitude of movement together specified the ‘index of difficulty ’ (Id) of a movement, according to the formula: 2 x amplitude of movement Id = log, bits. Width of the target On this formula, to increase the amplitude of movement or to decrease the width of the target is to increase the difficulty (precision required) of a movement. This gives
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KENNETH FLOWERS
a value in terms of information theory (bits) for any combination of target width and amplitude. Fitts then argued that the speed with which movements are made is limited by the information-processing capacity of the motor system. This means that, given that the subject is performing within the limits of accuracy required (i.e. making no errors and hitting the target every time), the maximum speed at which he can make controlled aiming movements will be determined jointly by his capacity and the information required for each one. By counting the number of movements made by subjects in a given condition, the information rate of the hand-arm can be calculated according to the formula : Id Inf. = -bits per second, t where t = the time taken to make one movement (Fitts found this to be about 10-12 bitslsec. for the right hand of his dextral subjects over a fairly wide range of conditions). Conversely, the relative speed of movement on a task of fixed precision indicates the capacity of the hand used in terms of information rate. I n this test the ‘preferred hand’ of ambilateral subjects was determined empirically as the hand with the best performance on the test, and this meant that for some subjects it was in fact the opposite hand to the one they regarded as their PH.
Method Following Fitts, the subject was required to hit two targets alternately with a stylus, and the number of hits scored in trials of 10 sec. each was noted. The targets were perspex strips 32 ern long, painted white with black edges and mounted on wooden boards on a Climpex frame in front of the subject, who moved the arm and hand across the body with lateral (left-right) movements. Target size was varied by using strips of different widths ( 1 , 2 or 4 em) and amplitude was varied by moving the boards along the Climpex frame to give distances between the centres of the targets of 4 cm (Id values 1, 2 and 3), 8 cm (Id values 2, 3 and 4), 18 cm (Id values 3, 4 and 5) and 32 cm (Id values 4, 5 and G ) . On each side of the target were two metal plates used to indicate errors. The perspex fitted over these plates so that the targets were raised slightly above them. Four counters, connected through the stylus and the metal plates, registered the number of errors made on each target in each trial, and a fifth counter registered the total number of movements made. Subjects were allowed t o use all the area of the targets, and to stand in any comfortable orientation in front of the apparatus. Two trials were made with each hand in each of the 12 conditions, with a rest after every block of 12 trials and whenever asked for by the subject. Each trial of 10 see. was timed with a stopwatch by the experimenter, who switched the counters on and off a t the beginning and end of each trial. Half the subjects in each handedness group were tested first with the left hand and then with the right in each condition, and the other half in the reverse hand order. The amplitude conditions were rotated systematically for successive subjects in each group. For each amplitude the target widths were presented always in the order 4-2-1 cm. Subjects were allowed as much practice as desired, with two dummy trials with each hand at the beginning of testing, and as much practice as required a t any subsequent time. Errors were indicated t o the subject by the sound of the error counters and the different sound of the stylus hitting metal rather than perspex, and all subjects were urged to tap as fast as they could without making too many errors. Any trial where the errors numbered more than 10 per cent or so of the total movements made, or where the subject broke down and stopped completely, were repeated, with the instruction to try and concentrate on being accurate. As some subjects appeared t o find this adjustment difficult, and as both errors and speed were being measured on this test, subsequent trials were accepted as the representative performance of the subject. The average of hits on the two trials was divided into the 10 sec. of the test period t o give the value o f t , which was
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Fig. 1. Preferred-hand movement times of the three main groups in the two experiments. 0,Left-handers; 0,right-handers; a, combined ambilaterals; x , Fitts’ subjects (1954). plotted against the I d values (Figs. 1-3). For each hand of each subject there was one measure at Id values 1 and 6, two measures for Id values 2 and 5, and three for Id values 3 and 4. These data were pooled for all subjects in each group to give the estimate of left-hand and right-hand control in each condition. Analyses of variance and t tests were used to compare between hands and between groups.
Results The mean time per movement for PH and NPH of the three main groups is shown in Fig. 1 (which also plots the data of Fitts’ subjects in equivalent conditions) and Fig. 3. The two lateralized groups have a faster rate of movement with the PH at all Ids, the difference becoming marked and reliable a t Ids 4, 5 and 6 (F = 5.30, P < 0.005; F = 7-50, P < 0.001 and F = 5.04, P < 0.01, respectively). I n all three conditions both right-handers and left-handers are faster than the combined ambilaterals at the level of P < 0.05 or better. With the NPH there are no differences in any condition (largest difference for I d = 6, F = 2.07, n.s.). When the combined ambilateral group is split up into right ambilaterals, left ambilaterals and mixedhanders, a regular trend appears for the PH, with the slowest scores associated with the more mixed group, and those of the other groups falling between them and the scores of the lateralized subjects (Fig. 2). The difference between the five subgroups are again significant at Ids 4,5 and 6 (F = 3.61, P < 0.01 ;P = 5.55, P < 0.001 and F = 3-59, P < 0.02, respectively). This represents a difference in terms of information processing of l-l&bitslsec. in the mixed-handers. It thus appears that subjects with inconsistency of hand preference have slightly less control in an aiming task with their PH, without any compensating increase in skill with their NPH. One point to note here is that the PH of the ambilateral group is the hand with the best
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overall performance, and the comparison is therefore clean and unaffected by possible misclassifications of hand preference. It also means that the measured differences are if anything underestimated, since taking any other criterion of what is the PH for the ambilateral groups results in PH scores being lower for these subjects and hence exaggerating the differences. (It does not, however, improve their NPH scores to any marked degree.) This discrepancy, between the hand preferred for the majority of tasks and the best hand in the test situation, occurred in fact for three subjects of the right ambilateral group and two mixed-handers. Between-hand scores for the lateralized groups were significant at all I d levels except the lowest, indicating a substantial superiority at aiming for the PH of these groups of around 1 6 2 4 bits/sec. The ambilateral groups also showed a reliable between-hand score a t Id = 1-5, but this was not surprising since the best-hand scores were selected in these subjects for the PH performance. Even so the difference between hands was somewhat less marked (about 1-18 bits/sec.) and became nonsignificant at all Id levels when the subject’s own preferred hand was taken as PH. Thus the ‘best hand’ of the ambilateral subjects was not well correlated with the preferred hand as the subjects themselves understood the term, and there was considerable overlap between the PH and NPH scores on different conditions even with the ‘best hand’ - ‘non-best hand’ measure, implying that inconsistent hand preference was associated with inconsistent as well as less marked skill differences. Comparison with the lateralized groups shows that this between-hands similarity in the ambilateral groups is due to the PH being worse than dextral or sinistral PHs rather than to a higher level of skill being present in the NPH. Since the measurement of capacity in the hands in terms of speed of movement assumes that all subjects were operating within a set accuracy limit, error rates should have been equal in all groups. This was in fact the case for both hands, differences between groups being small and insignificant in all conditions (largest difference F = 2.24, n.s.). The ambilaterals had a slightly lower mean error score all through, but as they were making fewer movements overall, this means that the percentage of errors was about the same as that of the lateralized subjects. I n all groups, too, differences between the hands in the number of errors made were small and insignificant. Thus the lower speed found for the NPHs and for the PH of ambilaterals is not correlated with a lower error rate. The distribution of errors within the groups followed that reported in the Fitt’s study, with an even split between left-side and right-side errors, a small but consistent tendency in all subjects to undershoot rather than overshoot, and a similar tendency to make more errors with the narrowest targets, indicating that subjects did not adjust their speed to fit the tolerance widths exactly. Since these trends were the same for all groups, they do not alter the main effects found between them. The differences in group scores suggest that there is an association between PH level of aiming skill and degree of hand preference, but does not say how direct this is. A Spearman rank correlation coe€€icientwas calculated for this association at the three I d conditions where reliable inter-group differences were found, and produced low but significant correlations in all cases. Thus at I d 4, rs = 0.270 (P < 0.05) ; at I d 5, rs = 0.302 (P < 0.02); and at Id 6, rs = 0.372 (P < 0.01). When ambilaterals were considered alone, the correlations dropped to rs = 0.246 (P < 0.05) for Id = 6
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KENNETH PLOWERS
and did not reach significance for I d = 4 and Id = 5 . These correlations cannot be considered very striking, and suggest that degree of hand preference is not a very precise index of subjects’ manual skill capacity once they have been classed as lateralized or ambilateral. It would not therefore be surprising to find anomalies fairly frequently between handedness as assessed by hand preference measures and handedness as measured on a dexterity test (Benton et al., 1962).
Discussi0.n As can be seen in Fig. 1, the shape of the curves of the different groups in this experiment resembles that of Fitts’ original subjects, although generally speeds (and thus information rates) appear to be higher. The measure of skill is fairly constant across the various conditions (within 14 bits) except for the two lowest I d levels, where performance may be limited by an extrinsic factor such as arm inertia, as suggested by Fitts himself. The apparent superiority of the subjects here may well be attributed to the great error rate tolerated (10 per cent compared to the 1 per cent average reported by Fitts) and to the fact that the instructions emphasized speed, whereas the original study stressed accuracy of performance. The average error rate with either hand was almost the same for the three main groups and was in fact higher for the two subgroups with the lowest measured information capacity (right ambilaterals and mixed-handers). The relatively low speeds of these subjects therefore cannot be attributed to a more cautious or precise manner of performance, i.e. they did not trade off speed for greater accuracy, and the relative speeds do seem to indicate differences in the information-processing capacity of the groups involved. Further, this capacity is correlated not with dextrality or sinistrality as such, but rather with consistency of hand preference. There is no disadvantage in being lefthanded per se, in fact the LH group showed the highest scores for the PH and almost for the NPH as well. Nor was there any indication that left ambilaterals are less skilful than their equivalent dextral opposites, in fact rather the reverse. But there was a definite inverse association between ambilaterality and degree of control in the PH. The question remains as to how stable are these handedness effects, or whether after practice the observed differences might not disappear (as e.g. with the typewriting task of Provins & Glencross, 1968). It is interesting to note here that Fitts, in his more thorough and prolonged experiment, found that: ‘Practice led to relatively small improvement in performance.’ for both speed and error rates. It may thus be assumed that practice does not change the scores to any great extent, at any rate while the subject is performing in a direct aiming mode. With prolonged practice the subject may well be able to perform ballistically, setting an amplitude and velocity value and then moving the arm without aiming each individual hit. But this would no longer be a corrective mode of response-and the handedness effect may then indeed not apply. The second experiment was designed to compare the levels of ‘dexterity’ in such a situation where aiming was not required.
Ha.ndedness and controlled movement
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EXPERIMENT I1 Rhythmical tapping I n the second experiment a tapping task was employed which required control of movements in a different sense from that of the reciprocal tapping task. Here subjects were asked to tap out rhythms repetitively as fast as they could, without aiming the movements to hit any particular point. Performance was ballistic in the sense that relatively little control of the position or force of each tap was necessary, and also in that it was usually impossible to correct individual errors in the required rhythm within a unit or ‘block’ of the rhythmical sequence. Subjects universally reported that they organized each sequence in a grouping of some sort and ‘fired off ’ each group as a whole. The task thus involved dexterity in the sense of making coordinated sequences of movements as fast as possible, but not the use of feedback information to guide the movements as they were made.
Method The apparatus of the first experiment was used. The two widest strips of perspex were placed next t o each other in the middle of the Climpex frame, and the subject asked t o tap on this area (8 x 32 em) in six different tapping combinations. The black edges of the strips, side by side, formed a vertical line down the centre of the perspex area; some subjects found it easier to tap each side of this line for each alternate group of taps. The subject held the stylus as in the previous test, and the overall number of taps made in periods of 10 see. counted. I n this test errors were considered t o be taps made out of sequence. The six test conditions were: ( a ) Spot tapping - tapping as fast as possible on the spot; ( b ) 2-2 - tapping in pairs with a pause between each pair; (c) 3-3 -tapping in triplets with a pause between each set; ( d ) 4-1 groups of four taps alternating with single taps, with a pause between each set; ( e ) 3-2 tapping in pairs and triplets alternately, with a pause between each group; (f)1-2-3 - single, double and triple taps with a pause between each set. Two trials were made with each hand on each condition. Half the subjects of each experimental group were tested with the PH first and then with the NPH on each condition, and half in the reverse order. All subjects went through the conditions in the order ( a ) t o (f).Practice was allowed with either hand for as long as the subject wished. Trials where errors were made on more than two groups of taps, or where the subject broke down completely, were repeated.
Results The mean time per movement for the PH and NPH of the three experimental groups on tjhe six conditions of this experiment are shown in Figs. 1 and 3. The PH of subjects in the ambilateral groups is taken as that which produced the best performance in Expt. I. There are no significant differences between the groups with either hand on any condition (largest difference P = 1.03 for NPH on condition (f)), and the ambilateral group as a whole perform slightly better than the dextrals and sinistrals on half the conditions. There is thus no effect of lateralization on dexterity as measured on this test. Between-hand comparisons show the opposite effect to that found in the aiming test, with the lateralized subjects producing no marked differences, while the ambilaterals are reliably better with the ‘better’ hand. When this latter group is subdivided, the effect disappears, however, for two subgroups, and only remains in the left ambilaterals. Errors recorded on this test, given in Table 1, show a similar pattern. Here all three ambilateral groups make slightly fewer errors with the PH than do the lateralized ones, a difference which reaches
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49
Handedness and controlled movement
significance on three conditions, and when total errors are considered. But only the left ambilateral subgroup shows a reliable between-hand difference, which may be due to chance, or to the higher number of subjects in this group. The differences in speed and accuracy in this group are in any case due to a rather lower level of performance with the NPH than to a markedly higher level with the PH. I n general, then, there is a hint that the ambilaterals may be finding it easier to generate the simple rhythmical sequences asked for here, but this is not a very marked effect.
Discussion On this test the differences between PH and NPH, and between groups, found in the first experiment, seem virtually to have disappeared, implying that the two hands are equipotential for this kind of controlled movement. The rate of movement in this test, moreover, is not determined by some maximum or minimum limit effect, since the rates for conditions ( b ) to (f), where grouping is required, all fall between the fastest and slowest measured rates on the Fitts’ test (see Fig. 1). The results thus cannot be explained solely in terms of some extrinsic factor such as the maximum rate of muscular contraction, or the speed with which upward and downward movements can be alternated. The rate is also different for the different conditions, reflecting the universal report of subjects that some groupings are easier to generate than others. But even the most complex rhythm demanded here, condition (f), produced a rate of movement in all subjects faster than that found on I d conditions 4, 5 and 6 in the Fitts task, where handedness differences became significant. Complexity of sequential organization therefore does not seem to be a crucial factor limiting the speed of controlled movements, nor the basis of handedness differences in skill, at least where the subject can generate patterns of movement in predetermined units or groupings, and this emphasizesthe point made earlier that the difference is not in the motor system as such.
GENERALDISCUSSION Ballistic and monitored movements. A basic assumption of this study has been that the Fitts tapping task (Expt. I) requires each movement to be made under current sensory control (i.e. with every one aimed individually), while the rhythmical tapping task (Expt. 11) is done ballistically, i.e. with movements carried out in groups or blocks ‘fired off’ in such a way as to be unmodifiable individually. This can easily be checked by considering mean movement duration times for the various conditions, since aiming movements which take more than half a second probably involve some correction procedure, and all quick movements (taking less than 300msec. or so) must be ballistic in the sense defined above (Woodworth, 1899; Welford, 1968; Poulton, 1969). The data in Figs. 1-3 justify the assumption for Expt. 11,since the mean movement duration on all conditions is less than 300 msec., and hence few of the movements could have been substantially modified during execution. As regards Expt. I, however, it seems as if the same argument must apply to the three easiest conditions, Ids 1-3, as the mean duration of movements there is also below the criterion. This means that subjects were in fact performing ballistically in these conditions. This may be explained by the fact that the test presents a fixed, stationary target, and subjects do not have to observe or calculate its present or 4
PSY
66
50
KENNETH FLOWERS
future position, so they may well be able to maintain a reasonable degree of accuracy in low I d conditions by setting an amplitude-of-movement value and trying to repeat it ballistically, trusting that the variance of arm movements will not produce too many hits outside the acceptable range. (Fitts himself recognized this possibility.) With low I d values, then, movements will be limited, not so much by the information factor of monitored movement, as by the factors which limit ballistic movements. When more than 3-4 bits per movement are involved, however, subjects have to resort to some corrective procedure, if only to check that they are reasonably accurate, and the information-processing factor will become predominant as a limiting factor in the speed of movement. It would be in accordance with the hypothesis of handedness suggested here, therefore, for there to be relatively small differences between groups with the PH on conditions I d 1-3, and that these should become noticeable and significant for Id values above 4 - which is the finding of Expt. I. Handedness difleerences in lateralized subjects. The data reported here suggest that in consistent left- and right-handers there is a reliable difference in performance with PH as against NPH on a visually controlled aiming task, but not where movements are ‘fired off’ ballistically without the need for monitoring. This implies that the crucial difference in movement control concerns the sensorimotor feedback loop, where some central processing operation is performed to transmit sensory and feedback information to the motor system. The NPH seems to have a lower rate of information transmission on such tasks, so that sequential adjustments of ongoing motor activity each take slightly longer than with the PH. The basic handicap of the NPH may thus be said to consist of greater intermittency in the sensory control of movement. It will therefore be at a disadvantage on any task where this kind of control is required. It will be at the greatest disadvantage where rapid serial corrections are required, or where the information per movement is high, and will in any case ‘feel’ less capable in such situations even where it can cope quite adequately. The handedness difference, it should be noted, is not in absolute ‘dexterity’, but rather in the time taken to go through the sensorimotor loop and the sequence of operations necessary to maintain continuous control of movements using current sensory and/or feedback information. This agrees with the finding of Woodworth (1899) in his classic monograph that left-hand movements become ballistic (are not amenable to correction during their course) if they take 750msec. or less, while right-hand ones only become ballistic at 400 msec. or faster. Thus the NPH may be capable of fine adjustment and graded response, but it takes longer and requires more care to achieve it. This may explain the negative findings of the other studies which report that the NPH is as capable as the PH of any simple response, even graded ones, where the time taken for the response is not measured (Provins, 1956), and is also consistent with other accounts specifying that ‘delicate ’ adjustments and aiming tasks give the most reliable between-hand differences(Koeh et al., 1933; Durost, 1934). If the NPH is less capable on monitored movements, it appears to be equipotential for ballistic movements, with the implication that where the subject can perform in this mode he may circumvent the limitations of intermittency, and performance will then be a function of the accuracy of motor commands, which may be largely determined by practice. This factor may well apply in typing, games and music, where response sequences are built up to become ballistic (i.e. automatic) and
51
Handedness and controlled movement
are then fired off as a unit without further conscious control. It would thus be reasonable to find that typists, for instance, were typing faster with the left hand after practice (Provins & Glencross, 1968) if the left hand is used more frequently on the keyboard, since the typical typing response is not continuously graded, and little feedback control is required. Conversely, in reaction time experiments one would expect reliable differences between the hands, as shown for example by Kerr et al. (1963), since here each response is preceded by a decision based on an immediately preceding sensory input signal (the light which indicates which butt’on is to be pressed) and hence the control of the response must be exercised through the sensorimotor loop. This effect should, again, be constant and unmodifiable with practice, because although the response itself is probably ballistic, it nevertheless involves central processing mechanisms for its initiation. This may relate to Oldfield’s (1969) contention that the P H may be more directly concerned with expressions of intention and volitional control, simply because the NPH responds that much slower to decision signals and more intermittently in serial tasks which involve sensorimotor and decision processes. Handedness differences in ambilateral subjects. Ambilaterals were selected out a t the beginning of this study by their hand preference, as if they were a separate category, and their performance on the aiming test was consistent with this classification. Thus all the ambilateral groups appeared to have a somewhat lower level of skill with either hand in the corrective control situation, whatever their pattern of preference. The degree of inconsistency was a general index of the level of skill attained, but this was not sufficiently direct to allow one to predict exactly from hand preference scores how well individual subjects would perform. Nor sometimes was the subject’s most-preferred hand the ‘best’ hand objectively measured, or the subject’s most-preferred hand what he himself claimed to be his PH.This was most true for the right ambilateral subgroup, who would in many cases be classed as righthanders on a bimodal categorization. I n other words, the concept of handedness needs to be redefined in terms of best-hand and worst-hand skill, measured as how well the hands do, rather than in terms of what or how many tasks are carried out with one hand. Ambilaterality, however, seems to be a very different thing from ambidexterity, although all those in this sample who claimed to be ambidextrous fell into one or other of the ambilateral groups (two right ambilaterals, two left ambilaterals and three mixed-handers). Certainly those with very mixed hand usage were not equally good with each hand, but rather equally bad; in other words, these subjects have the equivalent of two ‘non-preferred hands ’ in the sense that neither has developed the faster sensorimotor monitoring achieved by the PH of consistent handers. If the distinction suggested here between the two kinds of controlled movement is valid, then it is possible that subjects may become equally good with either hand for ballistic movement by practice, and that those with mixed-hand preference may have an advantage for this. If so, they may rely on ballistic movements more than do pure sinistrals and dextrals. On the present findings, however, it seems unlikely that anyone with mixed usage will be able to develop a high level of performance at corrective movement with either hand, let alone with both. Since, too, consistent handers appear to have a relatively low level of skill a t the aiming task with the 4-2
52
KENNETH FLOWERS
NPH, one is led to predict that true ambidexterity is impossible in corrective movements of this sort. On the contrary, a high level of skill in either hand is associated with strong laterality, and it seems that it is only ‘ambilevous’ or bilaterally clumsy subjects who cannot decide which hand to use by preference - possibly because neither hand feels very comfortable in controlling movements. Assuming that the distinction between ballistic and monitored movements is correct, the results presented here have some bearing on the question of specialization of the hemispheres for motor function. It would appear that before this can be clearly assessed the kind of control involved in any particular task needs to be specified. For monitored movement control, ambilaterals seem to have not two specialized hemispheres, but rather a lack of specialization, or a lack of development in the sense that neither side has a very high level of ability. Whether they have an advantage or disadvantage for developing ballistic control is not yet clear. On the present formulation (that the hands are equipotential for this kind of movement) they would not have any particular advantage. But it does seem that anyone with some inconsistency of hand usage has a different cerebral organization for sensorimotor control function compared to lateralized subjects, and this implies that handedness has some neurological basis. REFERENCES ANNETT,M. (1964). A model of the inheritance of handedness and cerebral dominance. Nature, Lond. 204, 59-60. ANNETT,M. (1967). The binomial distribution of right, mixed and left handedness. &. J . exp. Psychol. 19, 327-333. BENTON,A. L., MEYERS,R. & POLDER, G. J. (1962). Some aspects of handedness. Psych&. Neurol. (Basel) 144, 321-337. CRAIK, K. J. W. (1948). Theory of the human operator in control systems. Br. J . Psychol. 38, 56-61; 142-148. DUROST, W. N. (1934). The development of a battery of objective group tests of manual laterality. Genet. Psychol. Monogr. 16, 225-335. FITTS,P. M. (1954). The information capacity of the human motor system in controlling- the amplitude of movement. J . exp. Psychol. 47, 381-391. FITTS, P. M. (1964). Perceptual-motor skill learning. I n A. W. Melton (ed.), Categories of Human Learning. New York : Academic Press. KERR,M., MINUAY,R. & ELITHORN, A. (1963). Cerebral dominance in reaction-time responses. Br. J . Psychol. 54, 325-336. KOEH, H. L. et al. (1933). A study of the nature, measurement and determination of hand preference. Genet. Psychol. Momgr. 13, 117-221. OLDFIELD,R. C. (1969). Handedness in musicians. Br. J . P8yChOl. 60, 91-99. POULTON, E. C. (1969). Tracking. In E. A. Bilodeau (ed.), Principles of Skill Acquisition. New York : Academic Press. PROVINS, K. A. (1956). ‘Handedness’ and skill. &. J. exp. Psychol. 8, 79-95. PROVINS, K. A. (1967a). Handedness and motor skill. Med. J . Austr. ii, 468-470. PROVINS, K. A. (1967h). Motor skills, handedness and behaviour. Aust. J . Psychol. 19, 137-150. PROVINS, K. A. & GLENCROSS, D. J. (1968). Handwriting, typewriting and handedness. &. J . exp. Psychol. 20, 282-289. VINCE,M. (1948). Intermittency of control movements and the psychological refractory period. Br. J . Psychol. 38, 149-157. WELFORD, A. T. (1959). Evidence of a single-channel decision mechanism limiting performance in a serial reaction task. &. J . m p . Psychol. 11, 193-210. WELFOBD, A. T. (1968). Pundanaentals of Skill. London: Methuen. WOODWORTH, R. S. (1899). The accuracy of voluntary movement. Psychol. Monogr. 3, no. 3.
(Manuscript received 19 October 1973 ; revised manuscript received 7 February 1974)
