Articles Span of Apprehension in Learning Disabled Boys Curtis W. Mclntyre, PhD, Michael E. Murray, PhD Carmody M. Cronin, BS, and Scott L Blackwell, MA
The spans of apprehension of learning disabled and normal boys were compared by means of a forced-choice letter recognition task developed by Estes (1965). This task provides an estimate of the span, which is relatively insensitive to either memory or motivational influences. In experiment 1 the span size was found to be the same for both groups when visual "noise" was absent. In the presence of noise, span size for the learning disabled boys was reduced. It is argued that this reduction in span size represents a true deficit in attention. In experiment 2, the influence of variations in the amount of physical similarity between signal and noise letters on the spans of both groups were compared to determine whether noise letters act as more potent distractors for the learning disabled boys. The results indicate that the spans of both groups were influenced equivalently. No evidence for a distractibility explanation was obtained.
M
uch of the recent literature on learning disabilities in children (Dykman, Ackerman, Clements, & Peters 1971, Keogh 1971, Keogh & Margolis 1976, Ross 1976, Tarver & Hallahan 1974) suggests that one of its central symptoms is a deficit in attention. Unfortunately, as Sroufe (1975) has noted, this suggestion is difficult to
evaluate empirically for two reasons: (1) because vague and simplistic definitions of attentional deficit were used, and (2) because the measurement techniques used often confound attentional deficits with motivational and memory deficits (e.g., sustained vigilance tasks by their very nature are susceptible to motivational influences). In the present study an attempt is made to overcome these difficulties by using an explicit model of one aspect of attention, the span of apprehension, and a measurement technique that is not influenced by nonperceptua! variables. The span of apprehension is a measure of the amount of information processed simultaneously from a brief visual display. It is of basic importance to any theory of attention. In early research on the span, letters were exposed tachistoscopically for brief durations so that eye movements could not occur, and subjects were asked to report the letters they saw. Usually subjects were able to report only four or five letters correctly, even though many more were presented. More recently, however, Sperling (1960) demonstrated that the upper limit on span size previously found was due to memory limitations rather than to perceptual limitations. Moreover, he demonstrated that the span of
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469 apprehension increased with the amount of information in the display when this memory limitation was removed by means of a partial report technique. Following this earlier work, Estes (1965) proposed an explicit quantitative model of the span of apprehension based on an indirect measurement technique derived from signal detection procedures that virtually eliminated memory limitations. For the subject the basic task is simply to make a forced-choice letterrecognition response to tachistoscopic exposures of displays of letters containing one of two signal letters. The subject knows what these signal letters are (e.g., the letter T and the letter F) and that each will be seen in 50% of the exposures. Each signal letter is presented within a display containing irrelevant noise letters. Hence, the subjects must scan through the letters in the display until the signal letter is found. The number of letters scanned defines the span of apprehension, A, and Estes has shown that it can be estimated from the percentage of correct recognitions, Pc, and from the number of letters physically present in the display, D. The formula for estimating the span is A = D (2 Pc - 1). The details of the derivation of this formula are not of central importance here. However, four aspects of this measurement technique are important. First, the subject's task remains the same regardless of the number of letters contained in a display. He merely gives a forced-choice recognition response; for example, it was a T or it was an F. Second, the subject is required to remember only the identity of the signal letter. The identities of the noise letters are irrelevant; hence, the memory demands of this task are minimal. Third, the brief, tachistoscopic presentation of the letter displays precludes eye movements. Any scanning processes that are operative must be central rather than peripheral in nature. (Central is used here to indicate that the scanning process operates on an internal sensory afterimage, while peripheral indicates that the scanning process operates on the 14
external visual array via eye movement shifts from one part of the display to another.) Fourth, each presentation of a display is a discrete event that can be triggered by the subject when he is ready. This subject-paced procedure allows variations in the subject's motivational state to be kept to a minimum. In experiment 1 the spans of learning disabled boys and normal controls were compared under two information conditions. In one condition no noise letters were presented; in the other the signal letter was surrounded by eight noise letters. Explicit a priori predictions were not made since the attentional deficits previously observed with learning disabled children may have reflected either motivational-memory confounds or differences in a peripheral rather than a central scanning process. However, it was expected that any central attentional deficits characteristic of learning disabled children would be revealed by span comparisons.
SUBJECTS Forty boys ranging in age from 6 to 11 years (M = 9.2 years) participated in both experiments 1 and 2. Twenty were diagnosed as learning disabled (with seven having a concommitant diagnosis of hyperactivity) by means of a test battery that examined perceptual-motor (Beery's Developmental Test of Visual-Motor Integration, Bender Gestalt) and intellectual/academic skills (WideRange Achievement Test, Wechsler Intelligence Scale for Children) (see Table I). All the learning disabled boys showed significant deficits in acquiring secondary language/symbol skills (e.g., reading, spelling, and handwriting) despite normal intelligence and adequate educational and cultural opportunities. Eight of the learning disabled boys were taking therapeutic drugs: seven were taking Ritalin while one was taking Tofranil (comparison of results with boys not taking drugs showed no significant differences). All were normal in intelligence, ranging in IQ from 90 to 120, and were from upper middleclass backgrounds. None evidenced brain Journal of Learning Disabilities
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470 TABLE
I. Psychometric
profile
for learning disabled
Source Age Actual grade placement WISC-R (full-scale IQ) WISC-R (verbal) WISC-R (performance) WRAT (word recognition)' WRAT (spelling)* WRAT (arithmetic)* Gilmore Oral Reading Score Gilmore Reading Comprehension Score Berry Test of Visual-Motor Integration
N 20 20 20 20 20 16 16 16 16 16 14
group.
Mean 9.20 2.78 97.53 101.37 95.47 2.03 1.63 2.21 1.69 2.65 6.15
SD 1.98 1.64 7.88 9.37 8.43 1.97 1.40 1.46 1.52 2.20 1.35
SE .44 .37 1.76 2.10 1.89 .49 .35 .37 .38 .55 .36
•1965 Edition.
damage or emotional disturbance. The normal control boys were matched with the learning disabled boys on age, intelligence, and socioeconomic status.
EXPERIMENT 1 Apparatus The stimulus displays consisted of arrays of letters constructed as follows. Each array was made by typing black capital letters (IBM Orator) onto a white index card, which in turn was mounted on a cardboard insert that facilitated rapid insertion and removal from a tachistoscope. To allocate letters in each array, an imaginary matrix of 16 letter spaces was located in the center of each card. Each matrix (four letters wide by four letters high) subtended 3.1° x 2.4° of visual angle. Each array contained one of two target letters (T or F); each letter appeared once on two separate cards in each of 12 possible matrix positions (signal letters were not placed in any of the four corner positions), yielding a total of 24 T arrays and 24 F arrays. One set of 24 arrays — composed of 12 T cards and 12 F cards — contained only the signal letter (matrix size 1). The other set of 24 arrays — 12 T cards and 12 F cards — contained the signal letter
plus eight additional noise letters (matrix size 9). In this latter set, a signal letter was first located at one of the 12 possible positions, and then eight different letters were randomly selected without replacement and randomly allocated to 8 of the 15 remaining positions in the matrix to generate the arrays for matrix size 9. Each letter in the matrix subtended approximately 24' * 20' of visual angle, while letters occurring in adjacent cells were separated vertically by 30' of visual angle and laterally by 20' of visual angle. The stimulus arrays were exposed in a twochannel tachistoscope (Scientific Prototype, Model 800-E). One field, the fixation field, contained a 5' black dot that defined the center of the fixation area. The fixation field luminance was 3.5 fL. The stimulus arrays were exposed for 150 msec, in the other field of the tachistoscope. The exposure field luminance was 20.0 fL.
Procedure Each boy was given a standard set of verbal instructions followed by 24 practice trials. Practice trials were given to habituate each boy to the task and to ensure that he understood the instructions. During the practice trials, two randomly selected blocks of 12 trials were presented. Each block was drawn from a
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471 separate matrix size with the restriction of an equal occurrence of the signal letters (6 Ts,6 Fs). After the practice trials, a two-minute rest period was given before the experimental trial blocks were started. The experimental trials consisted of two sets of two 10-trial blocks, with each block separated by a one-minute rest period. Each set of two 10-trial blocks contained one block drawn from each matrix size. The order of presentation of these blocks was randomized, and each boy received a different randomization of the stimulus arrays. £ announced the matrix size before each of the 10-trial blocks began. The intertrial interval varied with the individual's response rate. (Most averaged approximately one trial every 20 seconds.) Each trial was initiated by the subject when he was ready by pressing a hand-held start button. After a delay of 500 msec, the stimulus field was displayed for 150 msec. The boy then reported which signal letter had appeared by saying either *T" or "F."
Results and Discussion The basic data were the mean probabilities of correct recognition by each of the groups for the two matrix sizes. An analysis of variance revealed significant effects for groups (F = 6.90, df = 1/38, p < .02), for matrix size (F= 163.34, d/ = 1/38, p < .001), and for the interaction of groups by matrix size (F = 4.39, df = 1/38, p < .50). The significant main effect for group indicates that the probability of a correct recognition was higher for the normal controls (M = .898) than for the learning disabled boys (M = .848). The significant main effect for matrix size indicates the probabilities of a correct recognition decreased as matrix size increased, i.e., from M = .988 to M = .759 for matrix sizes 1 and 9, respectively. This pattern was expected; Estes (1965) demonstrated a similar pattern. The significant interaction of group by matrix size indicates that the probability of a correct recognition was similar for both groups at matrix size 1, but while these probabilities dropped for both groups as the matrix size increased, they dropped more rapidly for the learning disabled 16
boys (from .994 to .803 for the normal boys and from .981 to .715 for the learning disabled boys). This interaction is reflected in the estimated spans of apprehension computed from the mean probabilities of correct recognition obtained for each group at each matrix size. Comparison of these spans indicates that the span of the learning disabled boys was equal to that of normal controls when no noise letters were present (the mean spans were .988 and .962 for the normal and learning disabled boys, respectively, at matrix size 1). However, as the number of noise letters increased, the spans of the learning disabled boys fell behind those of the normal controls. The mean spans were 5.45 and 3.87 for the normal and learning disabled groups, respectively, at matrix size 9; therefore, the spans of the learning disabled boys were about 30* less than those of the normal controls. It is not likely that this difference is due to a failure of the learning disabled boys to understand the task; their performance under the matrix size 1 condition was comparable with the normal controls; moreover, the span difference seems to be both substantial and real, as the nature of the task and the design and procedure used preclude alternative explanations based on either motivational or memory factors. Thus, it is reasonable to conclude that the span of apprehension of learning disabled boys is much less than that of normal boys. (In addition to the main comparison of the probabilities of correct recognition obtained for the learning disabled and normal boys, the probabilities obtained for the seven learning disabled hyperactive boys were compared with those obtained for two groups of seven similarly aged boys selected from the normal and learning disabled nonhyperactive subjects. An analysis of variance applied to these probabilities revealed a significant effect for groups (F = 4.07, df = 2/18, p < .05). Subsequent Newman-Keuls multiple comparison tests (p < .05) revealed the mean obtained for the normal boys (M = .907) was significantly greater than the means obtained for the Journal of Learning Disabilities
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472 learning disabled nonhyperactive (M = .823) and the learning disabled hyperactive boys (M = 854); the latter means did not differ significantly from each other.) In addition, the nature of the span task indicates that this reduction is central rather than peripheral in nature. Evidently it reflects some underlying deficiency in the central processing mechanism, that extracts, analyzes, and encodes information from brief visual displays. Furthermore, several possible interpretations of this central processing mechanism are readily apparent if two basic assumptions prevalent in the span literature are accepted (Estes 1965, Sperling 1960): (a) the information in the displays is available after presentation for only a brief period of time in the form of a rapidly decaying afterimage; and (b) the central processing mechanism can extract information from this image only before its decay. The first interpretation is that the pick-up of information from the decaying afterimage may be much slower for learning disabled boys. Second, the decaying afterimage may fade more rapidly for learning disabled boys. Third, the noise letters may act as more potent distractors for the learning disabled boys. This last possibility is of primary interest in experiment 2.
EXPERIMENT 2 Within the context of the span task and its related conceptual models, the distractibility of the noise letters should exert an important effect on the efficiency of the search process. In experiment 1 the noise letters represented a high level of distractibility; i.e., they were randomly selected letters that differed from one another, thereby forcing the search mechanism to devote an approximately equal amount of analytic power to each letter to decide whether it was signal or noise. However, if the noise letters were less distractible, e.g., if they were physically quite dissimilar from the signal letters or if they were all the same (redundant), then the reduced distractibility represented by this dissimilarity
and redundancy might be used by the search process to increase the efficiency of the search. Such an increase in efficiency would manifest itself empirically as an increase in the proportion of correct recognitions. Within the context of the Estes model of the span of apprehension, this increase in correct recognitions for a constant display size would be interpreted as an increase in the number of elements scanned, i.e., an increase in the span of apprehension. Several previous studies have demonstrated increases in the size of the span of apprehension in adult subjects when either signal-noise dissimilarity or noise redundancy was increased while display size remained the same (Estes 1972, 1974, Kinchla 1974, Mclntyre, Fox, & Neale 1970, McLaughlin, Masterson, & Herrmann 1972). In experiment 2 the influence of physical signal-noise similarity on the spans of apprehension of normal and learning disabled boys was examined under conditions of complete noise redundancy. To manipulate physical similarity, four noise letters were selected that varied in their similarity to the target letters, T and F. The similarity dimension was defined in terms of the number of physical features held in common between noise and signal letters (Gibson 1965). The noise letters O and U were selected as being quite dissimilar from the signal letters T and F, while the noise letters / and £ were selected as very similar to these signal letters. Complete noise redundancy was obtained by constructing displays that contained a signal letter surrounded by repetitions of a single noise letter (either all Os, (7s, /s, or £s). Two display sizes were used for each noise condition; i.e., the displays used for each noise condition contained either a signal letter plus eight noise letters (matrix size 9) or a signal letter plus 13 noise letters (matrix size 14). Altogether, a total of ten noise letter conditions were used, five of each matrix size. The eight redundant conditions consisted of displays containing a signal letter surrounded by repetitions of one of four noise letters (O, C/, /, and £). The displays in the other two random-
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17
473 letter baseline noise conditions contained a signal letter surrounded by randomly selected noise letters.
Apparatus The stimulus displays consisted of arrays of letters constructed as in experiment 1. One set of 24 arrays, composed of 12 T cards and 12 F cards, contained the signal letter plus eight additional randomly selected noise letters, which were randomly allocated to eight of the 15 remaining positions in the matrix. Four sets of 24 arrays, each set containing 12 T cards and 12 F cards, were constructed and contained repetitions of a single noise letter (either O, C/, /, or £). These redundant noise arrays were constructed exactly like the random noise arrays except that all the noise letters were replaced by a single noise letter (either O, L\ /, or E). For example, a redundant O-noise array consisted of a signal letter (T or F) and eight noise letters (all Os). To generate the displays for matrix size 14, five parallel sets of 24 arrays were constructed using identical procedures with the exception that 13 noise letters were used in each array instead of eight. Thus, the 10 sets of arrays consisted of 240 cards (12 target positions by two target letters by five noise conditions by two matrix sizes). The apparatus and exposure conditions were the same as used in experiment 1.
Design and Procedure The design and procedure were similar to those used in experiment 1. During the 40 practice trials, 10 randomly selected blocks of four trials each were presented with each block drawn from a separate noise condition and with the restriction of an equal occurrence of the signal letters (2 Ts, 2 Fs ) within each block. After the practice trials, a two-minute rest period was given before the experimental trials began. The experimental trials were given in two sets of ten 10-trial blocks, with each block separated by a one-minute rest period. Each set of ten blocks contained one 10-trial block drawn from each noise condition. The order of presentation of 18
these blocks was randomized, and each boy received a different randomization of the stimulus arrays. The experimenter described the noise condition before each of the 10-trial blocks was started.
Results and Discussion The basic data were the mean probabilities of correct recognition obtained for each group under each noise condition. These data are presented in Table II. A 2 * 2 * 5 analysis of variance (groups by matrix size by noise conditions) applied to these data revealed significant effects for matrix size (F • 9.20, df = 1/38, p < .01) and for noise conditions (F« 49.73, df - 4/152, p < .01). The significant matrix-size effect indicated that the mean probability of correct recognition decreased as matrix size increased, i.e., it was greater for matrix size 9 (M « .868) than for matrix size 14 (M - .841). This relationship was expected, as it is predicted by the Estes model and has been found in previous experiments. Newman-Keuls multiple comparison tests (p < .01) applied to the significant main effect for noise conditions revealed that the mean probability of correct recognition obtained for the random noise condition was significantly lower than those obtained for all the redundant noise conditions. More specifically, the mean probabilities of correct recognition obtained for the redundant O and U conditions (M * .943 and .927, respectively) were significantly greater than that obtained for the redundant / condition (M • .876), which was significantly greater than that obtained for the redundant £ condition (M * .792), which in turn was significantly greater than that obtained for the random noise condition (M» .737). Evidently the similarity of the noise letters to the signal letters influenced the probabilities of correct recognition obtained under the redundant noise conditions. The rank order of the probabilities of correct recognition obtained in the present study (O » U > I > E) is very similar to the rank order predicted by Gibson's analysis of letter similarities (i.e., O > U > I > E). Journal of Learning Disabilities
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474
TABLE
//.
Probabilities of correct
Noise condition Random Redundant Redundant Redundant Redundant Mean
recognition.
Normal
Group Learning disabled
Mean
.773 .965 .942 .890 .801 .874
701 .921 .911 .861 .783 .835
737 .943 .927 .876 .792 855
0 U / E
The interaction matrix of groups by noise conditions presented in Table II was analyzed further by the application of Cicchetti (1972) multiple comparison tests, which allowed the direct pairwise comparison of unconfounded means between groups for each noise condition. These tests revealed that the probability of correct recognitions observed for the normal boys (M = 773) under the random noise condition was significantly greater (p < .01) than that observed for the learning disabled boys (M = .701), but that the probabilities observed for the two groups did not differ significantly under any of the redundant noise conditions. Evidently the similarity of the probabilities obtained for both groups under the redundant noise conditions accounts for the failure to find either a significant main effect for groups or a significant interaction of groups by noise conditions. The estimated spans of apprehension computed for each group under each noise condition are presented in Table III. As can be seen, the average span of the learning disabled boys (7.66) is about 902 as large as the average span of the normal boys (8.52) across all five noise conditions. Moreover, the average spans of apprehension obtained for both groups under the separate noise conditions show substantial increases for the redundant noise conditions. These increases vary with the amount of physical similarity between the signal and noise letters.
Turning next to the relative spans observed for the learning disabled and normal boys under the separate noise conditions, it is clear that the relative size of the learning disabled boys' span decreased under the random noise condition. The span of the learning disabled boys was only 742 as large as that of the normal boys under the random noise condition, whereas it was 932 as large (on the average) as the normal boys' span under the four redundant noise conditions. Considering next the distractibility explanation suggested at the end of experiment 1 for the span decreases observed for learning disabled boys, the results of experiment 2 indicate that the distractibility of the noise letters (at least as represented by signal-noise similarity under maximum redundancy conditions) influenced the efficiency of the central search processor equivalently for both learning disabled and normal boys. Both learning disabled and normal boys were able to make use of signal-noise dissimilarity and noise redundancy to increase their spans of apprehension. Moreover, these results indicate that the recognition of signal letters from noise letters proceeds by analyzing the classes of physical features common to all of the letters rather than by analyzing the combination of physical features that are unique to each letter. Evidently the central search processors of learning disabled and normal boys use signal-noise dissimilarity in a series of tests to
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475
TABLE III. Estimated spans of apprehension. Nolst condition Random Redundant Redundant Redundant Redundant Mean
Os Us is Es
Normal
Group Learning dlaablad
Maan
6.15 10.65 10.02 8.97 6.72 8.52
4.57 9.68 9.45 8.20 6.36 7.66
5.36 10.17 9.74 8.59 6.54 8.09
eliminate very dissimilar letters early in the test hierarchy while allowing more similar letters to pass on to be tested again at later stages. In other words, the central processors of learning disabled and normal boys are capable of making several analyses concurrently, and they do not have to devote an equal amount of analytic power to each letter to decide if it is signal or noise. Information processing systems with similar capacities have been suggested by the results of experiments involving molar visual search (Neisser 1967). Given the lack of support for the distractibility hypothesis suggested at the end of experiment 1, closer consideration must be given to the other two hypotheses. Evidently the underlying deficiency in the central search processor, which results in the decreased spans of apprehension observed for learning disabled boys, is due either to the pick-up of information from the decaying afterimage being much slower for learning disabled boys or to the decaying afterimage fading more rapidly for learning disabled boys than for normals. Fortunately the analytic power of the span task and its related conceptual models allow both possibilities to be examined in future research. ACKNOWLEDGMENT The authors thank the stuff, teachers, and students of Dean Memorial lA'aming Center and Greenhill School, both in Dallas, Texas, for their help and cooperation in providing the subjects for this atudij.
20
REFERENCES Cicchetti, D.V.: Extension of multiple-range tests to interaction tables in the analysis of variance: A rapid approximate solution. Psychological Bulletin, 1972, 77(6), 405-408. Dykmon, R.A., Ackerman, P.T., Clements, S.D., Peters, J.E.: Specific learning disabilities: An attentional deficit syndrome. In H.R. Myklebust (Ed.): Progress in teaming Disabilities, Vol. 2. New York: Grune and Stratton, 1971, pp. 56-76. Estes, W.K.: A technique for assessing variability of perceptual span. Proceedings of the National Academy of Sciences, 1965, 54, 403-407. Estes, W.K.: Interactions of signal and'background variables in visual processing. Perception if Psychophysics, 1972, 12, 278-286. Estes, W.K.: Redundancy of noise elements and signals in visual detection of letters. Perception 6 Psychophysics, 1974, 16(1), 53-60. Gibson, E.J.: Learning to read. Science, 1965, 148, 1066-1072. Keogh, BK.: Hyperactivity and learning disorders: Review and speculation. Exceptional Child, 1971, 38, 101-110. Keogh, B.K., Margolis, J.S.: team to labor and to wait: Attentional problems of children with learning disorders. Journal of teaming Disabilities, 1976, 9, 276-287. Kinchla, H.A.: Detecting target elements in multielement arrays: A confusability model. Perception O Psychophysics, 1974, 15(1), 149-158. Mclntyre, C.W., Fox, R., Neale, J.: Effects of noise similarity and redundancy on the information processed from visual displays. Perception 6 Psychophysics, 1970, 7(6), 328-332. Mcteughlin, J.P., Masterton, F.A., Herrmann, D.J.: Pattern redundancy and detection in very short-term memory. Perception i? Psychophysics, 1972, 12(2B), 205-208. Neissner, V.: Cognitive Psychology. New York: Appleton, Century, Crofts, 1967. Ross, A.O.: Psychological Aspects of teaming Disabilities 6 Reading Disorders. New York: McGraw-Hill, 1976. Sperling, G.: The information available in brief visual presentations. Psychological Monographs, 1960,74 (Whole No. 498). Sroufe, LA.: Drug treatment of children with behavior problems. In F.D. Horowitz (Ed.): Review of Child Development Research, Vol. 4. Chicago: University of Chicago Press, 1975. Tarver, S.G., Hallahan, D.P.: Attention deficits in children with learning disabilities: A review. Journal of Learning Disabilities, 1974, 7, 560-569.
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The spans of apprehension of learning disabled and normal boys were compared by means of a forced-choice letter recognition task developed by Estes (1965). This task provides an estimate of the span, which is relatively insensitive to either memory or motivational influences. In experiment 1 the span size was found to be the same for both groups when visual "noise" was absent. In the presence of noise, span size for the learning disabled boys was reduced. It is argued that this reduction in span size represents a true deficit in attention. In experiment 2, the influence of variations in the amount of physical similarity between signal and noise letters on the spans of both groups were compared to determine whether noise letters act as more potent distractors for the learning disabled boys. The results indicate that the spans of both groups were influenced equivalently. No evidence for a distractibility explanation was obtained.
M
uch of the recent literature on learning disabilities in children (Dykman, Ackerman, Clements, & Peters 1971, Keogh 1971, Keogh & Margolis 1976, Ross 1976, Tarver & Hallahan 1974) suggests that one of its central symptoms is a deficit in attention. Unfortunately, as Sroufe (1975) has noted, this suggestion is difficult to
evaluate empirically for two reasons: (1) because vague and simplistic definitions of attentional deficit were used, and (2) because the measurement techniques used often confound attentional deficits with motivational and memory deficits (e.g., sustained vigilance tasks by their very nature are susceptible to motivational influences). In the present study an attempt is made to overcome these difficulties by using an explicit model of one aspect of attention, the span of apprehension, and a measurement technique that is not influenced by nonperceptua! variables. The span of apprehension is a measure of the amount of information processed simultaneously from a brief visual display. It is of basic importance to any theory of attention. In early research on the span, letters were exposed tachistoscopically for brief durations so that eye movements could not occur, and subjects were asked to report the letters they saw. Usually subjects were able to report only four or five letters correctly, even though many more were presented. More recently, however, Sperling (1960) demonstrated that the upper limit on span size previously found was due to memory limitations rather than to perceptual limitations. Moreover, he demonstrated that the span of
Volume 11, Number 8, October 1978
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13
469 apprehension increased with the amount of information in the display when this memory limitation was removed by means of a partial report technique. Following this earlier work, Estes (1965) proposed an explicit quantitative model of the span of apprehension based on an indirect measurement technique derived from signal detection procedures that virtually eliminated memory limitations. For the subject the basic task is simply to make a forced-choice letterrecognition response to tachistoscopic exposures of displays of letters containing one of two signal letters. The subject knows what these signal letters are (e.g., the letter T and the letter F) and that each will be seen in 50% of the exposures. Each signal letter is presented within a display containing irrelevant noise letters. Hence, the subjects must scan through the letters in the display until the signal letter is found. The number of letters scanned defines the span of apprehension, A, and Estes has shown that it can be estimated from the percentage of correct recognitions, Pc, and from the number of letters physically present in the display, D. The formula for estimating the span is A = D (2 Pc - 1). The details of the derivation of this formula are not of central importance here. However, four aspects of this measurement technique are important. First, the subject's task remains the same regardless of the number of letters contained in a display. He merely gives a forced-choice recognition response; for example, it was a T or it was an F. Second, the subject is required to remember only the identity of the signal letter. The identities of the noise letters are irrelevant; hence, the memory demands of this task are minimal. Third, the brief, tachistoscopic presentation of the letter displays precludes eye movements. Any scanning processes that are operative must be central rather than peripheral in nature. (Central is used here to indicate that the scanning process operates on an internal sensory afterimage, while peripheral indicates that the scanning process operates on the 14
external visual array via eye movement shifts from one part of the display to another.) Fourth, each presentation of a display is a discrete event that can be triggered by the subject when he is ready. This subject-paced procedure allows variations in the subject's motivational state to be kept to a minimum. In experiment 1 the spans of learning disabled boys and normal controls were compared under two information conditions. In one condition no noise letters were presented; in the other the signal letter was surrounded by eight noise letters. Explicit a priori predictions were not made since the attentional deficits previously observed with learning disabled children may have reflected either motivational-memory confounds or differences in a peripheral rather than a central scanning process. However, it was expected that any central attentional deficits characteristic of learning disabled children would be revealed by span comparisons.
SUBJECTS Forty boys ranging in age from 6 to 11 years (M = 9.2 years) participated in both experiments 1 and 2. Twenty were diagnosed as learning disabled (with seven having a concommitant diagnosis of hyperactivity) by means of a test battery that examined perceptual-motor (Beery's Developmental Test of Visual-Motor Integration, Bender Gestalt) and intellectual/academic skills (WideRange Achievement Test, Wechsler Intelligence Scale for Children) (see Table I). All the learning disabled boys showed significant deficits in acquiring secondary language/symbol skills (e.g., reading, spelling, and handwriting) despite normal intelligence and adequate educational and cultural opportunities. Eight of the learning disabled boys were taking therapeutic drugs: seven were taking Ritalin while one was taking Tofranil (comparison of results with boys not taking drugs showed no significant differences). All were normal in intelligence, ranging in IQ from 90 to 120, and were from upper middleclass backgrounds. None evidenced brain Journal of Learning Disabilities
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470 TABLE
I. Psychometric
profile
for learning disabled
Source Age Actual grade placement WISC-R (full-scale IQ) WISC-R (verbal) WISC-R (performance) WRAT (word recognition)' WRAT (spelling)* WRAT (arithmetic)* Gilmore Oral Reading Score Gilmore Reading Comprehension Score Berry Test of Visual-Motor Integration
N 20 20 20 20 20 16 16 16 16 16 14
group.
Mean 9.20 2.78 97.53 101.37 95.47 2.03 1.63 2.21 1.69 2.65 6.15
SD 1.98 1.64 7.88 9.37 8.43 1.97 1.40 1.46 1.52 2.20 1.35
SE .44 .37 1.76 2.10 1.89 .49 .35 .37 .38 .55 .36
•1965 Edition.
damage or emotional disturbance. The normal control boys were matched with the learning disabled boys on age, intelligence, and socioeconomic status.
EXPERIMENT 1 Apparatus The stimulus displays consisted of arrays of letters constructed as follows. Each array was made by typing black capital letters (IBM Orator) onto a white index card, which in turn was mounted on a cardboard insert that facilitated rapid insertion and removal from a tachistoscope. To allocate letters in each array, an imaginary matrix of 16 letter spaces was located in the center of each card. Each matrix (four letters wide by four letters high) subtended 3.1° x 2.4° of visual angle. Each array contained one of two target letters (T or F); each letter appeared once on two separate cards in each of 12 possible matrix positions (signal letters were not placed in any of the four corner positions), yielding a total of 24 T arrays and 24 F arrays. One set of 24 arrays — composed of 12 T cards and 12 F cards — contained only the signal letter (matrix size 1). The other set of 24 arrays — 12 T cards and 12 F cards — contained the signal letter
plus eight additional noise letters (matrix size 9). In this latter set, a signal letter was first located at one of the 12 possible positions, and then eight different letters were randomly selected without replacement and randomly allocated to 8 of the 15 remaining positions in the matrix to generate the arrays for matrix size 9. Each letter in the matrix subtended approximately 24' * 20' of visual angle, while letters occurring in adjacent cells were separated vertically by 30' of visual angle and laterally by 20' of visual angle. The stimulus arrays were exposed in a twochannel tachistoscope (Scientific Prototype, Model 800-E). One field, the fixation field, contained a 5' black dot that defined the center of the fixation area. The fixation field luminance was 3.5 fL. The stimulus arrays were exposed for 150 msec, in the other field of the tachistoscope. The exposure field luminance was 20.0 fL.
Procedure Each boy was given a standard set of verbal instructions followed by 24 practice trials. Practice trials were given to habituate each boy to the task and to ensure that he understood the instructions. During the practice trials, two randomly selected blocks of 12 trials were presented. Each block was drawn from a
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471 separate matrix size with the restriction of an equal occurrence of the signal letters (6 Ts,6 Fs). After the practice trials, a two-minute rest period was given before the experimental trial blocks were started. The experimental trials consisted of two sets of two 10-trial blocks, with each block separated by a one-minute rest period. Each set of two 10-trial blocks contained one block drawn from each matrix size. The order of presentation of these blocks was randomized, and each boy received a different randomization of the stimulus arrays. £ announced the matrix size before each of the 10-trial blocks began. The intertrial interval varied with the individual's response rate. (Most averaged approximately one trial every 20 seconds.) Each trial was initiated by the subject when he was ready by pressing a hand-held start button. After a delay of 500 msec, the stimulus field was displayed for 150 msec. The boy then reported which signal letter had appeared by saying either *T" or "F."
Results and Discussion The basic data were the mean probabilities of correct recognition by each of the groups for the two matrix sizes. An analysis of variance revealed significant effects for groups (F = 6.90, df = 1/38, p < .02), for matrix size (F= 163.34, d/ = 1/38, p < .001), and for the interaction of groups by matrix size (F = 4.39, df = 1/38, p < .50). The significant main effect for group indicates that the probability of a correct recognition was higher for the normal controls (M = .898) than for the learning disabled boys (M = .848). The significant main effect for matrix size indicates the probabilities of a correct recognition decreased as matrix size increased, i.e., from M = .988 to M = .759 for matrix sizes 1 and 9, respectively. This pattern was expected; Estes (1965) demonstrated a similar pattern. The significant interaction of group by matrix size indicates that the probability of a correct recognition was similar for both groups at matrix size 1, but while these probabilities dropped for both groups as the matrix size increased, they dropped more rapidly for the learning disabled 16
boys (from .994 to .803 for the normal boys and from .981 to .715 for the learning disabled boys). This interaction is reflected in the estimated spans of apprehension computed from the mean probabilities of correct recognition obtained for each group at each matrix size. Comparison of these spans indicates that the span of the learning disabled boys was equal to that of normal controls when no noise letters were present (the mean spans were .988 and .962 for the normal and learning disabled boys, respectively, at matrix size 1). However, as the number of noise letters increased, the spans of the learning disabled boys fell behind those of the normal controls. The mean spans were 5.45 and 3.87 for the normal and learning disabled groups, respectively, at matrix size 9; therefore, the spans of the learning disabled boys were about 30* less than those of the normal controls. It is not likely that this difference is due to a failure of the learning disabled boys to understand the task; their performance under the matrix size 1 condition was comparable with the normal controls; moreover, the span difference seems to be both substantial and real, as the nature of the task and the design and procedure used preclude alternative explanations based on either motivational or memory factors. Thus, it is reasonable to conclude that the span of apprehension of learning disabled boys is much less than that of normal boys. (In addition to the main comparison of the probabilities of correct recognition obtained for the learning disabled and normal boys, the probabilities obtained for the seven learning disabled hyperactive boys were compared with those obtained for two groups of seven similarly aged boys selected from the normal and learning disabled nonhyperactive subjects. An analysis of variance applied to these probabilities revealed a significant effect for groups (F = 4.07, df = 2/18, p < .05). Subsequent Newman-Keuls multiple comparison tests (p < .05) revealed the mean obtained for the normal boys (M = .907) was significantly greater than the means obtained for the Journal of Learning Disabilities
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472 learning disabled nonhyperactive (M = .823) and the learning disabled hyperactive boys (M = 854); the latter means did not differ significantly from each other.) In addition, the nature of the span task indicates that this reduction is central rather than peripheral in nature. Evidently it reflects some underlying deficiency in the central processing mechanism, that extracts, analyzes, and encodes information from brief visual displays. Furthermore, several possible interpretations of this central processing mechanism are readily apparent if two basic assumptions prevalent in the span literature are accepted (Estes 1965, Sperling 1960): (a) the information in the displays is available after presentation for only a brief period of time in the form of a rapidly decaying afterimage; and (b) the central processing mechanism can extract information from this image only before its decay. The first interpretation is that the pick-up of information from the decaying afterimage may be much slower for learning disabled boys. Second, the decaying afterimage may fade more rapidly for learning disabled boys. Third, the noise letters may act as more potent distractors for the learning disabled boys. This last possibility is of primary interest in experiment 2.
EXPERIMENT 2 Within the context of the span task and its related conceptual models, the distractibility of the noise letters should exert an important effect on the efficiency of the search process. In experiment 1 the noise letters represented a high level of distractibility; i.e., they were randomly selected letters that differed from one another, thereby forcing the search mechanism to devote an approximately equal amount of analytic power to each letter to decide whether it was signal or noise. However, if the noise letters were less distractible, e.g., if they were physically quite dissimilar from the signal letters or if they were all the same (redundant), then the reduced distractibility represented by this dissimilarity
and redundancy might be used by the search process to increase the efficiency of the search. Such an increase in efficiency would manifest itself empirically as an increase in the proportion of correct recognitions. Within the context of the Estes model of the span of apprehension, this increase in correct recognitions for a constant display size would be interpreted as an increase in the number of elements scanned, i.e., an increase in the span of apprehension. Several previous studies have demonstrated increases in the size of the span of apprehension in adult subjects when either signal-noise dissimilarity or noise redundancy was increased while display size remained the same (Estes 1972, 1974, Kinchla 1974, Mclntyre, Fox, & Neale 1970, McLaughlin, Masterson, & Herrmann 1972). In experiment 2 the influence of physical signal-noise similarity on the spans of apprehension of normal and learning disabled boys was examined under conditions of complete noise redundancy. To manipulate physical similarity, four noise letters were selected that varied in their similarity to the target letters, T and F. The similarity dimension was defined in terms of the number of physical features held in common between noise and signal letters (Gibson 1965). The noise letters O and U were selected as being quite dissimilar from the signal letters T and F, while the noise letters / and £ were selected as very similar to these signal letters. Complete noise redundancy was obtained by constructing displays that contained a signal letter surrounded by repetitions of a single noise letter (either all Os, (7s, /s, or £s). Two display sizes were used for each noise condition; i.e., the displays used for each noise condition contained either a signal letter plus eight noise letters (matrix size 9) or a signal letter plus 13 noise letters (matrix size 14). Altogether, a total of ten noise letter conditions were used, five of each matrix size. The eight redundant conditions consisted of displays containing a signal letter surrounded by repetitions of one of four noise letters (O, C/, /, and £). The displays in the other two random-
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473 letter baseline noise conditions contained a signal letter surrounded by randomly selected noise letters.
Apparatus The stimulus displays consisted of arrays of letters constructed as in experiment 1. One set of 24 arrays, composed of 12 T cards and 12 F cards, contained the signal letter plus eight additional randomly selected noise letters, which were randomly allocated to eight of the 15 remaining positions in the matrix. Four sets of 24 arrays, each set containing 12 T cards and 12 F cards, were constructed and contained repetitions of a single noise letter (either O, C/, /, or £). These redundant noise arrays were constructed exactly like the random noise arrays except that all the noise letters were replaced by a single noise letter (either O, L\ /, or E). For example, a redundant O-noise array consisted of a signal letter (T or F) and eight noise letters (all Os). To generate the displays for matrix size 14, five parallel sets of 24 arrays were constructed using identical procedures with the exception that 13 noise letters were used in each array instead of eight. Thus, the 10 sets of arrays consisted of 240 cards (12 target positions by two target letters by five noise conditions by two matrix sizes). The apparatus and exposure conditions were the same as used in experiment 1.
Design and Procedure The design and procedure were similar to those used in experiment 1. During the 40 practice trials, 10 randomly selected blocks of four trials each were presented with each block drawn from a separate noise condition and with the restriction of an equal occurrence of the signal letters (2 Ts, 2 Fs ) within each block. After the practice trials, a two-minute rest period was given before the experimental trials began. The experimental trials were given in two sets of ten 10-trial blocks, with each block separated by a one-minute rest period. Each set of ten blocks contained one 10-trial block drawn from each noise condition. The order of presentation of 18
these blocks was randomized, and each boy received a different randomization of the stimulus arrays. The experimenter described the noise condition before each of the 10-trial blocks was started.
Results and Discussion The basic data were the mean probabilities of correct recognition obtained for each group under each noise condition. These data are presented in Table II. A 2 * 2 * 5 analysis of variance (groups by matrix size by noise conditions) applied to these data revealed significant effects for matrix size (F • 9.20, df = 1/38, p < .01) and for noise conditions (F« 49.73, df - 4/152, p < .01). The significant matrix-size effect indicated that the mean probability of correct recognition decreased as matrix size increased, i.e., it was greater for matrix size 9 (M « .868) than for matrix size 14 (M - .841). This relationship was expected, as it is predicted by the Estes model and has been found in previous experiments. Newman-Keuls multiple comparison tests (p < .01) applied to the significant main effect for noise conditions revealed that the mean probability of correct recognition obtained for the random noise condition was significantly lower than those obtained for all the redundant noise conditions. More specifically, the mean probabilities of correct recognition obtained for the redundant O and U conditions (M * .943 and .927, respectively) were significantly greater than that obtained for the redundant / condition (M • .876), which was significantly greater than that obtained for the redundant £ condition (M * .792), which in turn was significantly greater than that obtained for the random noise condition (M» .737). Evidently the similarity of the noise letters to the signal letters influenced the probabilities of correct recognition obtained under the redundant noise conditions. The rank order of the probabilities of correct recognition obtained in the present study (O » U > I > E) is very similar to the rank order predicted by Gibson's analysis of letter similarities (i.e., O > U > I > E). Journal of Learning Disabilities
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TABLE
//.
Probabilities of correct
Noise condition Random Redundant Redundant Redundant Redundant Mean
recognition.
Normal
Group Learning disabled
Mean
.773 .965 .942 .890 .801 .874
701 .921 .911 .861 .783 .835
737 .943 .927 .876 .792 855
0 U / E
The interaction matrix of groups by noise conditions presented in Table II was analyzed further by the application of Cicchetti (1972) multiple comparison tests, which allowed the direct pairwise comparison of unconfounded means between groups for each noise condition. These tests revealed that the probability of correct recognitions observed for the normal boys (M = 773) under the random noise condition was significantly greater (p < .01) than that observed for the learning disabled boys (M = .701), but that the probabilities observed for the two groups did not differ significantly under any of the redundant noise conditions. Evidently the similarity of the probabilities obtained for both groups under the redundant noise conditions accounts for the failure to find either a significant main effect for groups or a significant interaction of groups by noise conditions. The estimated spans of apprehension computed for each group under each noise condition are presented in Table III. As can be seen, the average span of the learning disabled boys (7.66) is about 902 as large as the average span of the normal boys (8.52) across all five noise conditions. Moreover, the average spans of apprehension obtained for both groups under the separate noise conditions show substantial increases for the redundant noise conditions. These increases vary with the amount of physical similarity between the signal and noise letters.
Turning next to the relative spans observed for the learning disabled and normal boys under the separate noise conditions, it is clear that the relative size of the learning disabled boys' span decreased under the random noise condition. The span of the learning disabled boys was only 742 as large as that of the normal boys under the random noise condition, whereas it was 932 as large (on the average) as the normal boys' span under the four redundant noise conditions. Considering next the distractibility explanation suggested at the end of experiment 1 for the span decreases observed for learning disabled boys, the results of experiment 2 indicate that the distractibility of the noise letters (at least as represented by signal-noise similarity under maximum redundancy conditions) influenced the efficiency of the central search processor equivalently for both learning disabled and normal boys. Both learning disabled and normal boys were able to make use of signal-noise dissimilarity and noise redundancy to increase their spans of apprehension. Moreover, these results indicate that the recognition of signal letters from noise letters proceeds by analyzing the classes of physical features common to all of the letters rather than by analyzing the combination of physical features that are unique to each letter. Evidently the central search processors of learning disabled and normal boys use signal-noise dissimilarity in a series of tests to
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TABLE III. Estimated spans of apprehension. Nolst condition Random Redundant Redundant Redundant Redundant Mean
Os Us is Es
Normal
Group Learning dlaablad
Maan
6.15 10.65 10.02 8.97 6.72 8.52
4.57 9.68 9.45 8.20 6.36 7.66
5.36 10.17 9.74 8.59 6.54 8.09
eliminate very dissimilar letters early in the test hierarchy while allowing more similar letters to pass on to be tested again at later stages. In other words, the central processors of learning disabled and normal boys are capable of making several analyses concurrently, and they do not have to devote an equal amount of analytic power to each letter to decide if it is signal or noise. Information processing systems with similar capacities have been suggested by the results of experiments involving molar visual search (Neisser 1967). Given the lack of support for the distractibility hypothesis suggested at the end of experiment 1, closer consideration must be given to the other two hypotheses. Evidently the underlying deficiency in the central search processor, which results in the decreased spans of apprehension observed for learning disabled boys, is due either to the pick-up of information from the decaying afterimage being much slower for learning disabled boys or to the decaying afterimage fading more rapidly for learning disabled boys than for normals. Fortunately the analytic power of the span task and its related conceptual models allow both possibilities to be examined in future research. ACKNOWLEDGMENT The authors thank the stuff, teachers, and students of Dean Memorial lA'aming Center and Greenhill School, both in Dallas, Texas, for their help and cooperation in providing the subjects for this atudij.
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REFERENCES Cicchetti, D.V.: Extension of multiple-range tests to interaction tables in the analysis of variance: A rapid approximate solution. Psychological Bulletin, 1972, 77(6), 405-408. Dykmon, R.A., Ackerman, P.T., Clements, S.D., Peters, J.E.: Specific learning disabilities: An attentional deficit syndrome. In H.R. Myklebust (Ed.): Progress in teaming Disabilities, Vol. 2. New York: Grune and Stratton, 1971, pp. 56-76. Estes, W.K.: A technique for assessing variability of perceptual span. Proceedings of the National Academy of Sciences, 1965, 54, 403-407. Estes, W.K.: Interactions of signal and'background variables in visual processing. Perception if Psychophysics, 1972, 12, 278-286. Estes, W.K.: Redundancy of noise elements and signals in visual detection of letters. Perception 6 Psychophysics, 1974, 16(1), 53-60. Gibson, E.J.: Learning to read. Science, 1965, 148, 1066-1072. Keogh, BK.: Hyperactivity and learning disorders: Review and speculation. Exceptional Child, 1971, 38, 101-110. Keogh, B.K., Margolis, J.S.: team to labor and to wait: Attentional problems of children with learning disorders. Journal of teaming Disabilities, 1976, 9, 276-287. Kinchla, H.A.: Detecting target elements in multielement arrays: A confusability model. Perception O Psychophysics, 1974, 15(1), 149-158. Mclntyre, C.W., Fox, R., Neale, J.: Effects of noise similarity and redundancy on the information processed from visual displays. Perception 6 Psychophysics, 1970, 7(6), 328-332. Mcteughlin, J.P., Masterton, F.A., Herrmann, D.J.: Pattern redundancy and detection in very short-term memory. Perception i? Psychophysics, 1972, 12(2B), 205-208. Neissner, V.: Cognitive Psychology. New York: Appleton, Century, Crofts, 1967. Ross, A.O.: Psychological Aspects of teaming Disabilities 6 Reading Disorders. New York: McGraw-Hill, 1976. Sperling, G.: The information available in brief visual presentations. Psychological Monographs, 1960,74 (Whole No. 498). Sroufe, LA.: Drug treatment of children with behavior problems. In F.D. Horowitz (Ed.): Review of Child Development Research, Vol. 4. Chicago: University of Chicago Press, 1975. Tarver, S.G., Hallahan, D.P.: Attention deficits in children with learning disabilities: A review. Journal of Learning Disabilities, 1974, 7, 560-569.
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