A’ewopsycholc&. Vol. 29, No. 1, PP. SS91, Printed in Great Britain.
1991
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FACTOR ANALYSIS AND THE CEREBRAL HEMISPHERES: PILOT STUDY AND PARIETAL FUNCTIONS* DAVIDB. BOLES Department of Psychology, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. (Receioed 18 December 1989: accepted 21 August 1990) Abstract-Although research on lateral differences has proliferated, attempts to specify fundamental dichotomies of hemispheric processing have yielded less than satisfactory results. The factor analysis of behavioral asymmetries can provide a different approach by helping to identify common functions underlying disparate tasks. It can also provide information on individual differences and interrelationships among functions. Here, the viability of the approach is demonstrated with a small pilot study (N= 29), and with two larger experiments (N= 70 and N = 60) that stress parietal lobe function. Seven lateral&d functions were identified: auditory lexical, spatial attentive, spatial positional, spatial quantitative, tactile figural, visual emotional and visual lexical. A range of positive, negative and null correlations was found between functions, contrary to lateralization strength, hemisphericity, and independence models of individual differences in lateralization. However, a pattern of intercorrelations was observed among functions believed to be localized to the perisylvian region (Brodmann areas 22 and 39), which is consistent with the neurodevelopment theory of GEXHWIND and GALALIURDA (Cerebral Lateralization Biological Mechanisms, Associations and Pathology, The MIT Press, Cambridge, MA, 1987).
INTRODUCTION THE PASTquarter-century
has witnessed a proliferation of research on lateral differences in perception. The use of such paradigms as visual half-field presentation and dichotic listening has resulted in a literature of sufficient size and scope to be reviewed in a number of recent books [S, 7,25,28,59,92,96]. A few examples of commonly found lateral differences include right visual field (RVF) and right ear advantages in word recognition [14,15,27,61,70,78, 1143; and left visual field (LVF) advantages in face and line slant recognition [2,42,44,83, 1001. In turn, lateral differences are generally interpreted in terms of functional asymmetry between the cerebral hemispheres. Numerous attempts have been made to incorporate lateral differences in perception into theories of brain function. Mostly, such theories have purported to specify a fundamental processing dichotomy that distinguishes between the hemispheres. Among others, verbal-nonverbal, verbal-spatial, .serial-parallel and analytic-holistic dichotomies have been proposed [24,32,37,68,7(j). Unfortunately, the dichotomous approach has proved *Reprint requests and correspondence should be directed to David B. Boles, Department of Psychology, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A., or via BITNET to USERBWWL@RPITSMT?% I would like to thank Linda Jaynes and Michael Hochlerin for running subjects in Experiments 1 and 2, Lew Harvey and the Department of Psychology, University of Colorado at Boulder, for making facilities available for the Pilot study, and Michael Posner for contributing comments on an earlier draft. I would also like to thank Jerre Levy for generously contributing a copy of the chimeric faces test. Portions of this research were presented at the Annual Meeting of the Psychonomic Society, Atlanta, November 1989. 59
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somewhat sterile, since all such theories have been empirically contradicted to some degree [3, 13, 33,94, 1101. A more fruitful approach to theory-building may be to adopt the implicit neuropsychological view that separate processes or functions have separate anatomical loci within the hemispheres [56, 63, 711. These processes need not be linked across hemispheres in dichotomous relationships. If one takes this view, there is a need to identify particular processes or functions from a hodgepodge of tasks whose inter-relationships are poorly understood. For example, it is frequently assumed, usually implicitly, that a common lexical access mechanism underlies lateral differences in visual word recognition following (a) nearthreshold presentation with per cent correct measures; and (b) above-threshold presentation with reaction time measures. This assumption, however, relies less on known correlations between the lateral differences produced by the tasks, than on their face validity as measures of the same construct. Given the need to identify common functions underlying tasks, it seems logical to attempt an application of factor analysis to lateral differences. If, for example, a common lexical mechanism underlies the tasks mentioned, and if individuals vary in the strength to which the lexical mechanism is lateralized, then the visual field (VF) differences produced by the tasks should load on the same factor. It can be expected that in many instances, a factor emerging from a factor analysis of lateral differences will bear a one-to-one correspondence to a processing function underlying the constituent tasks. As described so far, factor analysis has the potential for identifying common functions among tasks and augmenting the development of theories of hemispheric asymmetry. There are at least two other potential benefits of the approach, relating to individual difirences and functional inter-relationships. Each is now addressed in turn. Individual diferences
Besides identifying common functions, a factor analysis approach can also differentiate among several existing models of individual differences in hemispheric asymmetry. The models differ in the inter-correlations they predict between lateralized functions. In discussing the predictions, a convention is followed where RVF or right ear advantages are expressed as positive numbers, while LVF or left ear advantages are expressed as negative numbers. One model of individual differences is that of lateralization strength. The viewpoint states that some individuals are strongly lateralized for all functions, while others are weakly lateralized. If two functions are localized in the same hemisphere, the lateralization strength model predicts a positive correlation between the tasks that draw upon them. For example, individuals who show strong right-side advantages on one task should also show strong right-side advantages on the other; individuals who show weak advantages on one should show weak advantages on the other. The result is a positive correlation between the lateral differences produced by the tasks. On the other hand, the lateralization strength viewpoint predicts a negative correlation between tasks drawing on oppositely-lateralized functions. This is because individuals who show a strong positive right-side advantage on one task should show a strong negative left-side advantage on the other task, while other individuals should show weak positive advantages paired with weak negative advantages. A second view of individual differences that is of historical significance but which is certainly wrong, is what can be called strong hemisphericity. According to this view, within a given individual one hemisphere dominates all lateralized functions. The model is
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inadequate, however, since both left-side and right-side advantages are well established for different tasks, even within the same subjects. The main reason for describing the strong hemisphericity viewpoint is that it makes a prediction with respect to task inter-correlations, which is that when lateral differences are expressed as right-side positive and left-side negative, a positive correlation should exist between all tasks. That is, some individuals should show lateral differences that only favor the right side, while others should show differences favoring only the left side. A more realistic model that makes a similar prediction is the weak hemisphericity viewpoint. Here lateral differences can vary in direction, even within a subject. Nevertheless, a moderately positive correlation still exists. For example, to take two visual tasks, perhaps all subjects might show either (a) a large positive (RVF) advantage for words and a small negative (LVF) advantage for faces, or (b) a small positive (RVF) advantage for words and a large negative (LVF) advantage for faces. Tasks drawing on functions located in the same hemisphere, of course, should also produce a positive correlation. In general, like strong hemisphericity, weak hemisphericity predicts a positive correlation among all tasks, but of lower magnitude. The difference between the lateralization strength and the two hemisphericity viewpoints is that lateralization strength predicts negative correlations between tasks showing opposite lateral differences, while hemisphericity predicts positive correlations. All three viewpoints predict positive correlations between tasks drawing on functions located in the same hemisphere. Logically, the alternative is that the task inter-correlations will all be close to zero, a viewpoint that can be labelled independence. Here, individual differences in lateralization are assumed to exist only with respect to specific functions, and different functions are unrelated. A fifth theory of individual differences is the neurodeuelopmental view of GEMXWINDand GALABURDA [46]. According to this theory, individuals may be strongly or weakly lateralized depending on the particular function, so that to some extent there is independence between functions. However, certain specific relationships among anatomical areas of the cerebral cortex and their underlying functions can lead to either positive or negative correlations between the lateral differences they produce. Since this view stresses functional interrelationships, it is described more fully in the next section. For the present, it is suflicient to state that the theory predicts that some functions will be correlated while others will not be. In a previous report [19], evidence was found against the lateralization strength and independence views. Specifically, two tasks showing opposite lateralization produced asymmetries that were positively correlated. The result was seen as consistent with either the weak hemisphericity or neurodevelopmental views, but with only two tasks having been used, there was no way to differentiate between these models. A factor analysis of a set of tasks can further address the individual differences issue by extracting factors corresponding to functions, and then determining whether and what direction or relationship exists between factors. The pattern of factor inter-correlations can also address the issue of functional inter-relationships, which is discussed next. Functional inter-relationships
According to GESCHWIND and GALABURDA[46], three types of developmental relationships exist between anatomical regions in the brain, which ultimately affect the pattern of adult iateralization. These types can be labelled adjacency, homology and connectedness. Adjacency exists when two anatomical areas subserving different functions
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border on one another. Geshwind and Galaburda proposed that during development, adjacent areas affect one another such that retarded development of one causes increased development of the other. A similar kind of linkage, as termed by the authors, was also proposed to exist for homology. That is, the area of the opposite hemisphere which is homologous to the area of retarded development, will also show increased development. Finally, Geschwind and Galaburda suggested that nonadjacent, nonhomologous areas that show strong neural connections (connectedness) to an area of increased development, will also show increased development. Certain implications of Geschwind and Galaburda’s theory seem clear. First, one might expect that a reciprocal developmental relationship between adjacent or homologous areas will affect lateral differences in tasks drawing on those areas. As an example, consider the functions served by two left hemisphere areas shown in Fig. 1: the angular gyrus area
20
T Fig. 1. Brodmann areas and lobes of the left hemisphere. The lobes are frontal (F), occipital (0), parietal (P) and temporal (T).
(Brodmann area 39), and the bordering Wernicke’s area (Brodmann area 22). There is good evidence that area 39 is involved in visual word recognition [43,45,56,71], while area 22 is involved in speech comprehension [6, 55, 56, 71, 771. According to Geschwind and Galaburda, an individual with retarded development of left hemisphere area 39 should show increased development of the homologous right hemisphere area. This should reduce the left hemisphere contribution to visual word recognition in comparison with the right hemisphere, resulting in a reduced RVF advantage in visual lexical tasks. At the same time, due to adjacency this individual should show increased development of left hemisphere area 22. This increased development should result in an increased right ear advantage in auditory lexical tasks. An individual of the opposite developmental pattern, with increased development of left hemisphere area 39 and retarded development of left hemisphere area 22, should show large RVF and small right ear advantages. The result is that over individuals, a negative correlation should be observed between the lateral differences emerging for visual and auditory lexical tasks. A similar argument can be advanced where functions are linked by homology. In these instances, however, the correlation between lateral differences produced by the functions should be positive. Again, retarded development of an area in one hemisphere should lead to increased development of the corresponding area in the other hemisphere. Tasks subserved by the retarded area would then show reduced asymmetry since the superiority of the area is reduced relative to its counterpart in the other hemisphere. Conversely, tasks subserved by the counterpart should show increased asymmetry since its superiority has been increased.
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This pairing of small advantages deriving from one side (e.g. a small positive difference) and large advantages deriving from the other (e.g. a large negative difference), should produce a positive correlation. In cases of linkage by connectedness, there is also a general expectation of positive correlations since one might expect connected functions to reside largely in the same hemisphere. However, if functions are linked between hemispheres, a negative correlation should exist between tasks drawing on those functions. A final implication of the neurodevelopmental theory of Geschwind and Galaburda is that functions that are not adjacent, homologous or connected should show independent courses of development and should produce null correlations between their constituent tasks. The theory therefore predicts that positive, negative and null correlations are all possible between lateral differences. Factor analysis should be sensitive to these possible interrelationships in that positive or negative correlations should result either in a correlation between separate factors on which the tasks load, or in the tasks loading on the same factor.* In conclusion, it is argued that the application of factor analysis to lateral differences has the potential for extracting a wide range of information. Not the least among the possible advances is the ability of factor analysis to identify common functions underlying various tasks. This should allow progression beyond face validity in linking tasks to constructs. Factor analysis can also distinguish among several models of individual differences in lateralization by showing whether task inter-correlations resulting from similarly- or oppositely-lateralized functions are positive, negative, null, or of mixed valence. And finally, factor analysis can help identify functional inter-relationships. Overview of experiments
In the present paper, three experiments are reported. The first is a small pilot study which was intended to demonstrate the feasibility of the factor analysis approach, and which itself produced theoretically significant findings. The pilot study employed several visual tasks used previously, as well as one free-vision task reported to produce asymmetry, and one auditory (dichotic) task. The choice of tasks was directed in part by their speed of performance (each could be completed in less than 16 min), and in part because common functions were anticipated to be present among the tasks, as well as some independent functions. The second and third studies, called Experiments 1 and 2, were larger and used a more motivated selection of tasks. A review of task asymmetries was completed during the pilot study suggesting that the majority of tasks that were used in the study drew on lateralized processes localized in the parietal lobes [20]. With that beginning, the decision was made to continue the factor analysis of parietal lobe tasks. Each successive experiment took representative tasks from its predecessor and combined them with new tasks. PILOT STUDY Overview
Six tasks were selected to collectively represent processes, some degree of opposite lateralization,
some commonality in underlying and some amount of complete
*Assuming that an oblique rotation is used, which allows factors to be correlated instead of forcing them to be orthogonal.
64
independence between tasks. major remaining constraint sessions, including informed tasks, their approximate run
DAVID B. Born
Since the research depended on unpaid volunteer subjects, the was the desire to run all tasks in no more than two 50 min consent and debriefing procedures. Brief descriptions of the times, and relationship to previous literature are as follows:
(1) Typing (13 min). The subjects saw near-threshold words and typed them into a computer console in an untimed per cent correct paradigm. Near-threshold word recognition produces consistent RVF advantages [14,15,83, 1111. (2) Lexical decision (16 min). The subject saw above-threshold words or nonwords and vocally responded “yes” or “no” to their lexical status, with a voice recognition system timing the responses and scoring their accuracy. Lexical decision typically produces a RVF advantage for words and a reduced or null advantage for nonwords [31,51,73,98]. Most lexical decision experiments have used manual responses. However, this specific version of the task, with voice recognition, was used by BOLES[18] with results similar to those described. (3) Word numbers (12 mitt). The subject saw the word names of numbers (e.g. “ONE”) and responded oddeven on a labelled keyboard, with RT and accuracy the dependent measures. BOLES[16,19] found a RVF advantage in the task. (4) Dichotic digits (14 min). The subject heard dichotic digit pairs and repeated them in a per cent correct task. The stimulus tape was created by KIMIJRA[65], who found a right ear advantage. Similar experiments have likewise described a right ear superiority [27, 61, 701. In addition, an ear-order-of-report measure was taken and included in the analyses. (5) Bargraphs (15 min). The subject saw bargraphs representing whole numbers of 1-8, and responded odd-even on a labelled keyboard in a RT task. BOLFS [16,19] found a robust LVF advantage in the task. (6) Faces (6 min). The subject saw pairs of chimeric faces in free vision, each member of which was composed of a happy right half and a neutral left half, or vice versa. The subject chose the member of the pair that appeared happier. LEVY and colleagues [74] reported that subjects generally find chimeras with a happy face on the viewer’s left to appear happier. This is analogous to a LVF advantage in VF research. The task is believed to tap hemispheric function, since left-handers show reduced asymmetry [62], although there is evidence that directional reading tendencies modify the hemispheric asymmetry [loll. A seventh task, e-cancellation, was also used, requiring subjects to cancel examples of the letter “e” on a printed page. Preliminary work indicated fewer missed es on the left side of the page. However, methodological controls implemented across the pilot study and the two experiments showed this asymmetry to be an artifact of “easier” carrier words having been positioned on the left side. Accordingly, although the task was used in all three studies, it either produced no asymmetry, or an artifactual asymmetry, and so was dropped from the analysis. It will not be further mentioned, although it was used as a task in all three studies, and required about 3 min to perform.
METHOD Subjects
Twenty-nine subjects participated. All were from undergraduate classes at the University of Colorado at Bouider, and received class credit for their participation. Although no restriction was placed on gender, by chance 22 of the subjects were male. Each subject was a self-classified right-hander who also scored as right-handed on the IO-item Edinburgh handedness inventory [81]. It was considered desirable to limit the investigation to right-handers in order to identify lateralized functions and their inter-relationships using the modal population. The feasibility of finding task correlations using right-handers has previously been demonstrated [19]. Apparatus
One of the tasks, faces, used a paper-and-pencil instrument. In addition, the Edinburgh handedness inventory was used, and subjects were given written questions concerning familial handedness. Four tasks were run on an Apple IIe computer with a monochrome Monitor III and the Apple-Psych system of experimental control [4]. Depending on the task, responses were made on the console keyboard (typing), on a twokey external board (word numbers and bargraphs), or with the aid of a head-mounted microphone and an IntroVoice I voice recognition board (lexical decision; [lS]). The remaining task, dichotic digits, was run using an Aiwa CS-Jl cassette recorder and stereo headphones. Stimuli and procedure
The tasks were administered to subjects following orders specified by four complete, different Latin squares, plus an additional order from a fifth square for the odd (29th) subject. Latin squares were used instead of a fixed sequence of tasks in order to balance possible carry over effects that might distort the lateral differences found for each task. For each subject the task set was split into two sessions, divided as evenly as possible by approximate running time. Nearly all subjects participated in sessions either 1 or 2 days apart.
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Since each task involved different stimuli and a somewhat different procedure, each is described separately: (1) Typing. Eighteen three-letter words were used (e.g. COD, DIE, KID). Words were presented vertically. In this and some other tasks, bilateral displays were used because they have been found to produce large, significant and reliable field differences [12,17,21]. A simultaneous arrowhead (““) at fixation cued the proper field for report, with the other field to be. ignored. Words subtended 0.4 by 2.0” horizontally and vertically, and were at 2.7” minimum eccentricity. A trial involved a 750 msec tixation box, followed by a 100 msec blank, a stimulus display for 133 msec, another 100 msec blank, and then a masking display of Xs for 133 msec. The subject next typed the cued word into the console at his or her own pace, having been warned at the outset that not all keys were active. (Only nine letters were actually used.) No practice was given; the task involved 72 trials, consisting of 2 VFs x 18 stimuli x 2 replications, fully crossed and randomized. In this and other relevant tasks, the distractor stimulus in the bilateral display was chosen at random. (2) Lexical decision. The voice recognition board was first trained to recognize the subject’s “yes” and “no” vocalizations. In the actual task, 16 words (e.g. AIR, FOE, ZOO) and 16 nonwords (e.g. ANR, FAE, ZID) were used, all of three letters. Bilateral displays of vertical stimuli were employed, with an arrowhead at fixation; the type of stimulus in one VF was independent of the type in the other VF. Stimuli subtended 0.4 by 2.0 horizontally and vertically, and were at 1.6” minimum eccentricity. A trial involved a 750 msec fixation cross, a 100 msec blank, and a 100 msec stimulus display. As quickly and as accurately as possible, the subject said “yes” for a word, or “no” for a nonword, and feedback on speed and accuracy was displayed. After 64 practice trials, 128 experimental trials were given, consisting of 2 VFs x 32 stimuli x 2 replications, crossed and randomized. (3) Word numbers. The word numbers “ONE”-“EIGHT” were used. Bilateral displays of vertical stimuli were employed, with an arrowhead at fixation. Stimuli subtended 0.4” horizontally and between 2.0” and 3.3” vertically, at 1.6” minimum eccentricity. The trial involved a 750 msec fixation cross, followed by a 100 msec blank and then a 100 msec stimulus display. The subject pressed one of two keys to indicate an “odd” or “even” number; these keys were placed one away from the other in the midline, and one hand was on each key. The hand mapping was balanced over subjects. Feedback was given. After 24 practice trials, 144 experimental trials were given, consisting of 2 VFs x 8 stimuli x 9 replications, crossed and randomized. (4) Bargraphs. Bargraphs representing whole numbers from 16 were used. A bargraph was plotted as a vertical rectangle against horizontal reference lines at the 0,4 and 8 levels (for an illustration see [16]). Bilateral displays with a central arrowhead were employed. Bargraphs subtended 2.1 by 5.7” horizontally and vertically, at 4.9 minimum eccentricity. A trial involved a 750 msec fixation cross, a 100 msec blank, and a 50 msec stimulus display. The subject pressed one of two keys as in the word numbers task to respond, and feedback was shown. After 24 practice trials, 144 experimental trials were given, consisting of 2 VFs x 6 stimuli x 12 replications, crossed and randomized. (5) Dichotic digits. A standard, commerically-available tape was used (DK consultants, 152 Albert Street, Unit 12, London, Ontario, Canada), consisting of 24 trials, most ofwhich were of three dichotic pairs each. The numbers “0NE’“‘TEN” were employed. After each trial the subject named as many of the numbers as possible, in any order. After the 24 trials, the headphones were reversed and the tape replayed for an additional 24 trials. (6) Faces. The faces test was that employed by LEVYet al. [74]. On each page ofa 36page booklet, a pair of chimeric faces was shown. One face was the mirror image of the other, and was smiling on one side while neutral on the other. The subject selected the member of each pair that appeared happier, in a self-paced, forced-choice paradigm. Following the last task of the second session, subjects were given a questionnaire that included the Edinburgh handedness inventory, and questions concerning familial handedness and the subject’s gender. Scoring
As BRYDEN’S[28] review indicated, a wide range of laterality measures has been proposed and advocated. Since consensus has yet to be reached regarding the proper selection of such measures, it simply seemed best to adopt, on an a priori basis, a convention for scoring the results. Here the convention adopted was to calculate (a) VF or ear difference scores for measures reflecting independent events in the VFs or ears, and (b) laterality coefficients (LCs) for measures reflecting lateral presentations that were dependent on one another. For example, since VF presentations were independent of one another, VF difference scores were calculated, but since the faces task and handedness questionnaire involved a forced choice of one side or the other, LCs were calculated. The main defense for this convention is in terms of precedent: studies of VF differences usually use difference scores, whereas handedness research often uses LCs or the equivalent, and LEVY et al. [74] employed an LC for the faces task. Although no formal checks have been made, it is doubtful that the choice of measure greatly affects the results since Iaterality measures are generally highly correlated with one another.* *BIRKE~ [lo] found correlations ranging from +0.70 to +0.97 among four commonly used measures. BRYDEN [28], an advocate of a lambda measure based on the log odds ratio, nevertheless presented as an illustration (p. 36) data showing very high inter-correlations among lambda, a laterality index, and a simple right-left difference (r= +0.991-+0.997). HELLIGEet al. [60] reported inter-correlations among six laterality indices that ranged from +0.56 to +0.99 across three tasks, with a median correlation of +0.95.
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The scoring of the word numbers and bargraphs tasks was handled similarly. Median correct RTs and error percentages were calculated for each VF, and subtracted to produce a VF difference score for each dependent measure. The lexical decision task, also involving RT, was handled in the same way except that words and nonwords were scored separately. For the typing and dichotic listening tasks, the percentage of words correct was calculated for each field or ear and subtracted. In addition, response order was scored for the dichotic task. The rationale for including this measure is that the order of report may itself reflect hemispheric asymmetry: for example, by the selection for first report of the clearest or earliest percept. For each of the two trial runs, the first 12 of the 24 trials involved six different digits, three to each ear, making the scoring of the ear of first report unambigous. Combining the runs for these trials, an LC was calculated as follows: (number of right ear 1st reports-number of left ear 1st reports)/24. The remaining trials involved either two digits to one ear and four to the other, or three to each ear with some repetition (e.g. “seven” to the left ear in the second pair, and to the right ear in the third pair). It seemed likely that an imbalance in the number ofdigits presented to each ear would affect the order of report, and that repetition ofdigits would make the scoring of order of report ambiguous, so these trials were not scored for purposes of this measure. An LC was also calculated for the faces task, as follows: (number of right-face choices-number of left-face choices)/36. Side is here defined from the viewer’s perspective, not that of the poser. In all cases, the difference scores and LCs were calculated so that a positive number signified a right-side or lefthemisphere advantage; a negative number signified a left-side or right-hemisphere advantage. Table 1 summarizes the laterality measures.
Table 1. Lateral differences for each measure in the seven tasks of the pilot study, expressed as a per cent error difference+ RT difference or LC. Numbered measures were selected for the factor analysis Task 1. Typing 2. Lexical decision 3. Lexical decision Lexical decision Lexical decision 4. Word numbers Word numbers 5. Bargraphs Bargraphs 6. Dichotic dieits 7. Dichotic digits 8. Faces
Measure Errors Word RT Word errors Nonword RT Nonword errors RT Errors RT Errors Errors Report order (LC) LC
Lateral difference
t
P
+6.5% + 34 msec +6.3% -11 msec -3.0% + 34 msec +0.2% -25 msec -1.3% + 1.2% +0.22 -0.38
4.62 2.08 3.00 0.96 1.18 4.11 0.17 4.51 0.93 1.79 2.92 4.22
1991
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002~3932j91 S3.00+0.00 1991 Pcrgmon Prem plc
FACTOR ANALYSIS AND THE CEREBRAL HEMISPHERES: PILOT STUDY AND PARIETAL FUNCTIONS* DAVIDB. BOLES Department of Psychology, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. (Receioed 18 December 1989: accepted 21 August 1990) Abstract-Although research on lateral differences has proliferated, attempts to specify fundamental dichotomies of hemispheric processing have yielded less than satisfactory results. The factor analysis of behavioral asymmetries can provide a different approach by helping to identify common functions underlying disparate tasks. It can also provide information on individual differences and interrelationships among functions. Here, the viability of the approach is demonstrated with a small pilot study (N= 29), and with two larger experiments (N= 70 and N = 60) that stress parietal lobe function. Seven lateral&d functions were identified: auditory lexical, spatial attentive, spatial positional, spatial quantitative, tactile figural, visual emotional and visual lexical. A range of positive, negative and null correlations was found between functions, contrary to lateralization strength, hemisphericity, and independence models of individual differences in lateralization. However, a pattern of intercorrelations was observed among functions believed to be localized to the perisylvian region (Brodmann areas 22 and 39), which is consistent with the neurodevelopment theory of GEXHWIND and GALALIURDA (Cerebral Lateralization Biological Mechanisms, Associations and Pathology, The MIT Press, Cambridge, MA, 1987).
INTRODUCTION THE PASTquarter-century
has witnessed a proliferation of research on lateral differences in perception. The use of such paradigms as visual half-field presentation and dichotic listening has resulted in a literature of sufficient size and scope to be reviewed in a number of recent books [S, 7,25,28,59,92,96]. A few examples of commonly found lateral differences include right visual field (RVF) and right ear advantages in word recognition [14,15,27,61,70,78, 1143; and left visual field (LVF) advantages in face and line slant recognition [2,42,44,83, 1001. In turn, lateral differences are generally interpreted in terms of functional asymmetry between the cerebral hemispheres. Numerous attempts have been made to incorporate lateral differences in perception into theories of brain function. Mostly, such theories have purported to specify a fundamental processing dichotomy that distinguishes between the hemispheres. Among others, verbal-nonverbal, verbal-spatial, .serial-parallel and analytic-holistic dichotomies have been proposed [24,32,37,68,7(j). Unfortunately, the dichotomous approach has proved *Reprint requests and correspondence should be directed to David B. Boles, Department of Psychology, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A., or via BITNET to USERBWWL@RPITSMT?% I would like to thank Linda Jaynes and Michael Hochlerin for running subjects in Experiments 1 and 2, Lew Harvey and the Department of Psychology, University of Colorado at Boulder, for making facilities available for the Pilot study, and Michael Posner for contributing comments on an earlier draft. I would also like to thank Jerre Levy for generously contributing a copy of the chimeric faces test. Portions of this research were presented at the Annual Meeting of the Psychonomic Society, Atlanta, November 1989. 59
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somewhat sterile, since all such theories have been empirically contradicted to some degree [3, 13, 33,94, 1101. A more fruitful approach to theory-building may be to adopt the implicit neuropsychological view that separate processes or functions have separate anatomical loci within the hemispheres [56, 63, 711. These processes need not be linked across hemispheres in dichotomous relationships. If one takes this view, there is a need to identify particular processes or functions from a hodgepodge of tasks whose inter-relationships are poorly understood. For example, it is frequently assumed, usually implicitly, that a common lexical access mechanism underlies lateral differences in visual word recognition following (a) nearthreshold presentation with per cent correct measures; and (b) above-threshold presentation with reaction time measures. This assumption, however, relies less on known correlations between the lateral differences produced by the tasks, than on their face validity as measures of the same construct. Given the need to identify common functions underlying tasks, it seems logical to attempt an application of factor analysis to lateral differences. If, for example, a common lexical mechanism underlies the tasks mentioned, and if individuals vary in the strength to which the lexical mechanism is lateralized, then the visual field (VF) differences produced by the tasks should load on the same factor. It can be expected that in many instances, a factor emerging from a factor analysis of lateral differences will bear a one-to-one correspondence to a processing function underlying the constituent tasks. As described so far, factor analysis has the potential for identifying common functions among tasks and augmenting the development of theories of hemispheric asymmetry. There are at least two other potential benefits of the approach, relating to individual difirences and functional inter-relationships. Each is now addressed in turn. Individual diferences
Besides identifying common functions, a factor analysis approach can also differentiate among several existing models of individual differences in hemispheric asymmetry. The models differ in the inter-correlations they predict between lateralized functions. In discussing the predictions, a convention is followed where RVF or right ear advantages are expressed as positive numbers, while LVF or left ear advantages are expressed as negative numbers. One model of individual differences is that of lateralization strength. The viewpoint states that some individuals are strongly lateralized for all functions, while others are weakly lateralized. If two functions are localized in the same hemisphere, the lateralization strength model predicts a positive correlation between the tasks that draw upon them. For example, individuals who show strong right-side advantages on one task should also show strong right-side advantages on the other; individuals who show weak advantages on one should show weak advantages on the other. The result is a positive correlation between the lateral differences produced by the tasks. On the other hand, the lateralization strength viewpoint predicts a negative correlation between tasks drawing on oppositely-lateralized functions. This is because individuals who show a strong positive right-side advantage on one task should show a strong negative left-side advantage on the other task, while other individuals should show weak positive advantages paired with weak negative advantages. A second view of individual differences that is of historical significance but which is certainly wrong, is what can be called strong hemisphericity. According to this view, within a given individual one hemisphere dominates all lateralized functions. The model is
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inadequate, however, since both left-side and right-side advantages are well established for different tasks, even within the same subjects. The main reason for describing the strong hemisphericity viewpoint is that it makes a prediction with respect to task inter-correlations, which is that when lateral differences are expressed as right-side positive and left-side negative, a positive correlation should exist between all tasks. That is, some individuals should show lateral differences that only favor the right side, while others should show differences favoring only the left side. A more realistic model that makes a similar prediction is the weak hemisphericity viewpoint. Here lateral differences can vary in direction, even within a subject. Nevertheless, a moderately positive correlation still exists. For example, to take two visual tasks, perhaps all subjects might show either (a) a large positive (RVF) advantage for words and a small negative (LVF) advantage for faces, or (b) a small positive (RVF) advantage for words and a large negative (LVF) advantage for faces. Tasks drawing on functions located in the same hemisphere, of course, should also produce a positive correlation. In general, like strong hemisphericity, weak hemisphericity predicts a positive correlation among all tasks, but of lower magnitude. The difference between the lateralization strength and the two hemisphericity viewpoints is that lateralization strength predicts negative correlations between tasks showing opposite lateral differences, while hemisphericity predicts positive correlations. All three viewpoints predict positive correlations between tasks drawing on functions located in the same hemisphere. Logically, the alternative is that the task inter-correlations will all be close to zero, a viewpoint that can be labelled independence. Here, individual differences in lateralization are assumed to exist only with respect to specific functions, and different functions are unrelated. A fifth theory of individual differences is the neurodeuelopmental view of GEMXWINDand GALABURDA [46]. According to this theory, individuals may be strongly or weakly lateralized depending on the particular function, so that to some extent there is independence between functions. However, certain specific relationships among anatomical areas of the cerebral cortex and their underlying functions can lead to either positive or negative correlations between the lateral differences they produce. Since this view stresses functional interrelationships, it is described more fully in the next section. For the present, it is suflicient to state that the theory predicts that some functions will be correlated while others will not be. In a previous report [19], evidence was found against the lateralization strength and independence views. Specifically, two tasks showing opposite lateralization produced asymmetries that were positively correlated. The result was seen as consistent with either the weak hemisphericity or neurodevelopmental views, but with only two tasks having been used, there was no way to differentiate between these models. A factor analysis of a set of tasks can further address the individual differences issue by extracting factors corresponding to functions, and then determining whether and what direction or relationship exists between factors. The pattern of factor inter-correlations can also address the issue of functional inter-relationships, which is discussed next. Functional inter-relationships
According to GESCHWIND and GALABURDA[46], three types of developmental relationships exist between anatomical regions in the brain, which ultimately affect the pattern of adult iateralization. These types can be labelled adjacency, homology and connectedness. Adjacency exists when two anatomical areas subserving different functions
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border on one another. Geshwind and Galaburda proposed that during development, adjacent areas affect one another such that retarded development of one causes increased development of the other. A similar kind of linkage, as termed by the authors, was also proposed to exist for homology. That is, the area of the opposite hemisphere which is homologous to the area of retarded development, will also show increased development. Finally, Geschwind and Galaburda suggested that nonadjacent, nonhomologous areas that show strong neural connections (connectedness) to an area of increased development, will also show increased development. Certain implications of Geschwind and Galaburda’s theory seem clear. First, one might expect that a reciprocal developmental relationship between adjacent or homologous areas will affect lateral differences in tasks drawing on those areas. As an example, consider the functions served by two left hemisphere areas shown in Fig. 1: the angular gyrus area
20
T Fig. 1. Brodmann areas and lobes of the left hemisphere. The lobes are frontal (F), occipital (0), parietal (P) and temporal (T).
(Brodmann area 39), and the bordering Wernicke’s area (Brodmann area 22). There is good evidence that area 39 is involved in visual word recognition [43,45,56,71], while area 22 is involved in speech comprehension [6, 55, 56, 71, 771. According to Geschwind and Galaburda, an individual with retarded development of left hemisphere area 39 should show increased development of the homologous right hemisphere area. This should reduce the left hemisphere contribution to visual word recognition in comparison with the right hemisphere, resulting in a reduced RVF advantage in visual lexical tasks. At the same time, due to adjacency this individual should show increased development of left hemisphere area 22. This increased development should result in an increased right ear advantage in auditory lexical tasks. An individual of the opposite developmental pattern, with increased development of left hemisphere area 39 and retarded development of left hemisphere area 22, should show large RVF and small right ear advantages. The result is that over individuals, a negative correlation should be observed between the lateral differences emerging for visual and auditory lexical tasks. A similar argument can be advanced where functions are linked by homology. In these instances, however, the correlation between lateral differences produced by the functions should be positive. Again, retarded development of an area in one hemisphere should lead to increased development of the corresponding area in the other hemisphere. Tasks subserved by the retarded area would then show reduced asymmetry since the superiority of the area is reduced relative to its counterpart in the other hemisphere. Conversely, tasks subserved by the counterpart should show increased asymmetry since its superiority has been increased.
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This pairing of small advantages deriving from one side (e.g. a small positive difference) and large advantages deriving from the other (e.g. a large negative difference), should produce a positive correlation. In cases of linkage by connectedness, there is also a general expectation of positive correlations since one might expect connected functions to reside largely in the same hemisphere. However, if functions are linked between hemispheres, a negative correlation should exist between tasks drawing on those functions. A final implication of the neurodevelopmental theory of Geschwind and Galaburda is that functions that are not adjacent, homologous or connected should show independent courses of development and should produce null correlations between their constituent tasks. The theory therefore predicts that positive, negative and null correlations are all possible between lateral differences. Factor analysis should be sensitive to these possible interrelationships in that positive or negative correlations should result either in a correlation between separate factors on which the tasks load, or in the tasks loading on the same factor.* In conclusion, it is argued that the application of factor analysis to lateral differences has the potential for extracting a wide range of information. Not the least among the possible advances is the ability of factor analysis to identify common functions underlying various tasks. This should allow progression beyond face validity in linking tasks to constructs. Factor analysis can also distinguish among several models of individual differences in lateralization by showing whether task inter-correlations resulting from similarly- or oppositely-lateralized functions are positive, negative, null, or of mixed valence. And finally, factor analysis can help identify functional inter-relationships. Overview of experiments
In the present paper, three experiments are reported. The first is a small pilot study which was intended to demonstrate the feasibility of the factor analysis approach, and which itself produced theoretically significant findings. The pilot study employed several visual tasks used previously, as well as one free-vision task reported to produce asymmetry, and one auditory (dichotic) task. The choice of tasks was directed in part by their speed of performance (each could be completed in less than 16 min), and in part because common functions were anticipated to be present among the tasks, as well as some independent functions. The second and third studies, called Experiments 1 and 2, were larger and used a more motivated selection of tasks. A review of task asymmetries was completed during the pilot study suggesting that the majority of tasks that were used in the study drew on lateralized processes localized in the parietal lobes [20]. With that beginning, the decision was made to continue the factor analysis of parietal lobe tasks. Each successive experiment took representative tasks from its predecessor and combined them with new tasks. PILOT STUDY Overview
Six tasks were selected to collectively represent processes, some degree of opposite lateralization,
some commonality in underlying and some amount of complete
*Assuming that an oblique rotation is used, which allows factors to be correlated instead of forcing them to be orthogonal.
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independence between tasks. major remaining constraint sessions, including informed tasks, their approximate run
DAVID B. Born
Since the research depended on unpaid volunteer subjects, the was the desire to run all tasks in no more than two 50 min consent and debriefing procedures. Brief descriptions of the times, and relationship to previous literature are as follows:
(1) Typing (13 min). The subjects saw near-threshold words and typed them into a computer console in an untimed per cent correct paradigm. Near-threshold word recognition produces consistent RVF advantages [14,15,83, 1111. (2) Lexical decision (16 min). The subject saw above-threshold words or nonwords and vocally responded “yes” or “no” to their lexical status, with a voice recognition system timing the responses and scoring their accuracy. Lexical decision typically produces a RVF advantage for words and a reduced or null advantage for nonwords [31,51,73,98]. Most lexical decision experiments have used manual responses. However, this specific version of the task, with voice recognition, was used by BOLES[18] with results similar to those described. (3) Word numbers (12 mitt). The subject saw the word names of numbers (e.g. “ONE”) and responded oddeven on a labelled keyboard, with RT and accuracy the dependent measures. BOLES[16,19] found a RVF advantage in the task. (4) Dichotic digits (14 min). The subject heard dichotic digit pairs and repeated them in a per cent correct task. The stimulus tape was created by KIMIJRA[65], who found a right ear advantage. Similar experiments have likewise described a right ear superiority [27, 61, 701. In addition, an ear-order-of-report measure was taken and included in the analyses. (5) Bargraphs (15 min). The subject saw bargraphs representing whole numbers of 1-8, and responded odd-even on a labelled keyboard in a RT task. BOLFS [16,19] found a robust LVF advantage in the task. (6) Faces (6 min). The subject saw pairs of chimeric faces in free vision, each member of which was composed of a happy right half and a neutral left half, or vice versa. The subject chose the member of the pair that appeared happier. LEVY and colleagues [74] reported that subjects generally find chimeras with a happy face on the viewer’s left to appear happier. This is analogous to a LVF advantage in VF research. The task is believed to tap hemispheric function, since left-handers show reduced asymmetry [62], although there is evidence that directional reading tendencies modify the hemispheric asymmetry [loll. A seventh task, e-cancellation, was also used, requiring subjects to cancel examples of the letter “e” on a printed page. Preliminary work indicated fewer missed es on the left side of the page. However, methodological controls implemented across the pilot study and the two experiments showed this asymmetry to be an artifact of “easier” carrier words having been positioned on the left side. Accordingly, although the task was used in all three studies, it either produced no asymmetry, or an artifactual asymmetry, and so was dropped from the analysis. It will not be further mentioned, although it was used as a task in all three studies, and required about 3 min to perform.
METHOD Subjects
Twenty-nine subjects participated. All were from undergraduate classes at the University of Colorado at Bouider, and received class credit for their participation. Although no restriction was placed on gender, by chance 22 of the subjects were male. Each subject was a self-classified right-hander who also scored as right-handed on the IO-item Edinburgh handedness inventory [81]. It was considered desirable to limit the investigation to right-handers in order to identify lateralized functions and their inter-relationships using the modal population. The feasibility of finding task correlations using right-handers has previously been demonstrated [19]. Apparatus
One of the tasks, faces, used a paper-and-pencil instrument. In addition, the Edinburgh handedness inventory was used, and subjects were given written questions concerning familial handedness. Four tasks were run on an Apple IIe computer with a monochrome Monitor III and the Apple-Psych system of experimental control [4]. Depending on the task, responses were made on the console keyboard (typing), on a twokey external board (word numbers and bargraphs), or with the aid of a head-mounted microphone and an IntroVoice I voice recognition board (lexical decision; [lS]). The remaining task, dichotic digits, was run using an Aiwa CS-Jl cassette recorder and stereo headphones. Stimuli and procedure
The tasks were administered to subjects following orders specified by four complete, different Latin squares, plus an additional order from a fifth square for the odd (29th) subject. Latin squares were used instead of a fixed sequence of tasks in order to balance possible carry over effects that might distort the lateral differences found for each task. For each subject the task set was split into two sessions, divided as evenly as possible by approximate running time. Nearly all subjects participated in sessions either 1 or 2 days apart.
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Since each task involved different stimuli and a somewhat different procedure, each is described separately: (1) Typing. Eighteen three-letter words were used (e.g. COD, DIE, KID). Words were presented vertically. In this and some other tasks, bilateral displays were used because they have been found to produce large, significant and reliable field differences [12,17,21]. A simultaneous arrowhead (““) at fixation cued the proper field for report, with the other field to be. ignored. Words subtended 0.4 by 2.0” horizontally and vertically, and were at 2.7” minimum eccentricity. A trial involved a 750 msec tixation box, followed by a 100 msec blank, a stimulus display for 133 msec, another 100 msec blank, and then a masking display of Xs for 133 msec. The subject next typed the cued word into the console at his or her own pace, having been warned at the outset that not all keys were active. (Only nine letters were actually used.) No practice was given; the task involved 72 trials, consisting of 2 VFs x 18 stimuli x 2 replications, fully crossed and randomized. In this and other relevant tasks, the distractor stimulus in the bilateral display was chosen at random. (2) Lexical decision. The voice recognition board was first trained to recognize the subject’s “yes” and “no” vocalizations. In the actual task, 16 words (e.g. AIR, FOE, ZOO) and 16 nonwords (e.g. ANR, FAE, ZID) were used, all of three letters. Bilateral displays of vertical stimuli were employed, with an arrowhead at fixation; the type of stimulus in one VF was independent of the type in the other VF. Stimuli subtended 0.4 by 2.0 horizontally and vertically, and were at 1.6” minimum eccentricity. A trial involved a 750 msec fixation cross, a 100 msec blank, and a 100 msec stimulus display. As quickly and as accurately as possible, the subject said “yes” for a word, or “no” for a nonword, and feedback on speed and accuracy was displayed. After 64 practice trials, 128 experimental trials were given, consisting of 2 VFs x 32 stimuli x 2 replications, crossed and randomized. (3) Word numbers. The word numbers “ONE”-“EIGHT” were used. Bilateral displays of vertical stimuli were employed, with an arrowhead at fixation. Stimuli subtended 0.4” horizontally and between 2.0” and 3.3” vertically, at 1.6” minimum eccentricity. The trial involved a 750 msec fixation cross, followed by a 100 msec blank and then a 100 msec stimulus display. The subject pressed one of two keys to indicate an “odd” or “even” number; these keys were placed one away from the other in the midline, and one hand was on each key. The hand mapping was balanced over subjects. Feedback was given. After 24 practice trials, 144 experimental trials were given, consisting of 2 VFs x 8 stimuli x 9 replications, crossed and randomized. (4) Bargraphs. Bargraphs representing whole numbers from 16 were used. A bargraph was plotted as a vertical rectangle against horizontal reference lines at the 0,4 and 8 levels (for an illustration see [16]). Bilateral displays with a central arrowhead were employed. Bargraphs subtended 2.1 by 5.7” horizontally and vertically, at 4.9 minimum eccentricity. A trial involved a 750 msec fixation cross, a 100 msec blank, and a 50 msec stimulus display. The subject pressed one of two keys as in the word numbers task to respond, and feedback was shown. After 24 practice trials, 144 experimental trials were given, consisting of 2 VFs x 6 stimuli x 12 replications, crossed and randomized. (5) Dichotic digits. A standard, commerically-available tape was used (DK consultants, 152 Albert Street, Unit 12, London, Ontario, Canada), consisting of 24 trials, most ofwhich were of three dichotic pairs each. The numbers “0NE’“‘TEN” were employed. After each trial the subject named as many of the numbers as possible, in any order. After the 24 trials, the headphones were reversed and the tape replayed for an additional 24 trials. (6) Faces. The faces test was that employed by LEVYet al. [74]. On each page ofa 36page booklet, a pair of chimeric faces was shown. One face was the mirror image of the other, and was smiling on one side while neutral on the other. The subject selected the member of each pair that appeared happier, in a self-paced, forced-choice paradigm. Following the last task of the second session, subjects were given a questionnaire that included the Edinburgh handedness inventory, and questions concerning familial handedness and the subject’s gender. Scoring
As BRYDEN’S[28] review indicated, a wide range of laterality measures has been proposed and advocated. Since consensus has yet to be reached regarding the proper selection of such measures, it simply seemed best to adopt, on an a priori basis, a convention for scoring the results. Here the convention adopted was to calculate (a) VF or ear difference scores for measures reflecting independent events in the VFs or ears, and (b) laterality coefficients (LCs) for measures reflecting lateral presentations that were dependent on one another. For example, since VF presentations were independent of one another, VF difference scores were calculated, but since the faces task and handedness questionnaire involved a forced choice of one side or the other, LCs were calculated. The main defense for this convention is in terms of precedent: studies of VF differences usually use difference scores, whereas handedness research often uses LCs or the equivalent, and LEVY et al. [74] employed an LC for the faces task. Although no formal checks have been made, it is doubtful that the choice of measure greatly affects the results since Iaterality measures are generally highly correlated with one another.* *BIRKE~ [lo] found correlations ranging from +0.70 to +0.97 among four commonly used measures. BRYDEN [28], an advocate of a lambda measure based on the log odds ratio, nevertheless presented as an illustration (p. 36) data showing very high inter-correlations among lambda, a laterality index, and a simple right-left difference (r= +0.991-+0.997). HELLIGEet al. [60] reported inter-correlations among six laterality indices that ranged from +0.56 to +0.99 across three tasks, with a median correlation of +0.95.
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The scoring of the word numbers and bargraphs tasks was handled similarly. Median correct RTs and error percentages were calculated for each VF, and subtracted to produce a VF difference score for each dependent measure. The lexical decision task, also involving RT, was handled in the same way except that words and nonwords were scored separately. For the typing and dichotic listening tasks, the percentage of words correct was calculated for each field or ear and subtracted. In addition, response order was scored for the dichotic task. The rationale for including this measure is that the order of report may itself reflect hemispheric asymmetry: for example, by the selection for first report of the clearest or earliest percept. For each of the two trial runs, the first 12 of the 24 trials involved six different digits, three to each ear, making the scoring of the ear of first report unambigous. Combining the runs for these trials, an LC was calculated as follows: (number of right ear 1st reports-number of left ear 1st reports)/24. The remaining trials involved either two digits to one ear and four to the other, or three to each ear with some repetition (e.g. “seven” to the left ear in the second pair, and to the right ear in the third pair). It seemed likely that an imbalance in the number ofdigits presented to each ear would affect the order of report, and that repetition ofdigits would make the scoring of order of report ambiguous, so these trials were not scored for purposes of this measure. An LC was also calculated for the faces task, as follows: (number of right-face choices-number of left-face choices)/36. Side is here defined from the viewer’s perspective, not that of the poser. In all cases, the difference scores and LCs were calculated so that a positive number signified a right-side or lefthemisphere advantage; a negative number signified a left-side or right-hemisphere advantage. Table 1 summarizes the laterality measures.
Table 1. Lateral differences for each measure in the seven tasks of the pilot study, expressed as a per cent error difference+ RT difference or LC. Numbered measures were selected for the factor analysis Task 1. Typing 2. Lexical decision 3. Lexical decision Lexical decision Lexical decision 4. Word numbers Word numbers 5. Bargraphs Bargraphs 6. Dichotic dieits 7. Dichotic digits 8. Faces
Measure Errors Word RT Word errors Nonword RT Nonword errors RT Errors RT Errors Errors Report order (LC) LC
Lateral difference
t
P
+6.5% + 34 msec +6.3% -11 msec -3.0% + 34 msec +0.2% -25 msec -1.3% + 1.2% +0.22 -0.38
4.62 2.08 3.00 0.96 1.18 4.11 0.17 4.51 0.93 1.79 2.92 4.22
