Input Capability and Speed of Processing in Mental Retardation

Journal of Abnormal Psychology 1979. Vol. 88, No. 4, 341-345

Input Capability and Speed of Processing in Mental Retardation Dennis P. Saccuzzo, Mary Kerr, Andrea Marcus, and Roger Brown San Diego State University The minimum stimulus duration for criterion accuracy and the minimum interval between presentation of a test and presentation of a masking stimulus for criterion accuracy were determined for mildly and moderately retarded adults and normal controls of the same mental and chronological age. The procedure was replicated three times in three separate sessions. Results indicated that both retarded groups required longer stimulus durations as well as longer masking intervals for criterion accuracy than did both the mental age and the chronological age control groups. Results were interpreted as consistent with deficiencies in both iconic storage and speed of information processing in mental retardation. These deficiencies, furthermore, cannot be accounted for on the basis of low mental age.

Information processing theory, which assumes human behavior can be analyzed in terms of discrete stages or processes, has opened new avenues of research in psychopathology. In viewing a response as only the end product of a series of processes, it becomes possible to specify the correlates of observed performance deficits more accurately than could otherwise be done. In mental retardation research, for example, considerable controversy has centered around the underlying source of performance deficits related to visual information processing (see Spitz, 1973). One major issue concerns whether observed differences between retarded and nonretarded persons on tasks related to visual processing can be explained solely on the basis of low mental age in the retarded persons (e.g., Galbraith & Gliddon, 1972; Spitz & Thor, 1968; Thor, 1970; Welsandt & Meyer, 1974). Another major unresolved issue concerns which stage or stages of processing are most impaired in retarded persons. Libkuman and Friedrich (1972) spoke of limitations in brief perceptual memory or, as Neisser (1967) called it, iconic Mary Kerr is now at the University of California, Berkeley. Roger Brown is now at Stanford University. Requests for reprints should be sent to Dennis P. Saccuzzo, Psychology Clinic, San Diego State University, San Diego, California 92182.

storage. Others (e.g., Harris & Fleer, 1974; Spitz & Thor, 1968), however, have pointed to more central factors, such as speed of information transfer from iconic storage to a more permanent memory system. Backward masking (e.g., Kahneman, 1968) is a tool that has relevance to these and other information processing issues in mental retardation, since the technique can be used to separately analyze a variety of processing stages (e.g., Saccuzzo, Hirt, & Spencer, 1974). Backward masking refers to the phenomenon in which the complete processing of an input stimulus may be limited by the presentation of a subsequently presented stimulus (Spencer, 1969). If, for instance, a briefly presented visual stimulus is rapidly followed by a noninformational pattern stimulus, generally known as the mask, a subject will have a reduced capacity to recognize the initial stimulus whenever the mask has been presented before the informational content of the initial stimulus has been extracted from iconic storage and transferred to more central stages of information processing. On the other hand, the mask will have no effect on a stimulus that has already been transferred from iconic storage to a more central stage of processing. Employing a backward masking paradigm for the first time on a retarded sample, Spitz and Thor (1968) found that educable retarded

Copyright 1979 by the American Psychological Association, Inc. 0021-843X/79/8804-0341$00.75

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adolescents were less resistant to the effects of the masking stimulus than were normals of the same chronological age. No differences were found, however, between the retarded group and normal children of the same mental age. Results were interpreted as indicating slow processing in the retarded group, which was believed to be the result of low mental age. A more recent backward-masking study, however, found that mildly retarded adolescents were less resistant to the mask than were mental age as well as chronological age control groups (Welsandt & Meyer, 1974). Results were interpreted in terms of a deficiency in input capability (i.e., iconic storage), which was hypothesized to be related to factors underlying mental retardation. The present study provides data relevant to the discrepancies between Spitz and Thor (1968) and Welsandt and Meyer (1974). We followed a model of information processing that assumes the information from a briefly exposed stimulus remains available in the nervous system for several hundred milliseconds in a storage system (e.g., iconic storage). This storage system, which comprises the initial input stage of processing, may vary from one group to another in terms of several factors, including capacity, duration, and quality. From this initial input stage, information is transferred to the higher brain centers for additional processing, and groups may also differ in this rate of transfer (i.e., speed of information processing). In the present analysis, possible group differences in input capability were controlled by increasing the stimulus exposure duration until each subject reached criterion accuracy. In this paradigm each subject was provided an exposure duration such that all subjects processed the same amount of information when no mask was used, although the stimulus exposure duration differed from subject to subject. Speed of information processing was then evaluated in a critical interstimulus interval (ISI) experiment (e.g., Saccuzzo & Miller, 1977). The critical ISI is the minimum interval between presentation of a stimulus and a mask at which the mask no longer interferes with processing of the stimulus (e.g., Turvey, 1973). Since all subjects processed the same amount of information without a mask,

differences in the critical ISI would be consistent with differences in how long subjects required to process this amount of information. Differences in the exposure-duration requirements, on the other hand, would be consistent with differences in input capabilities. Method Subjects Five groups of eight subjects were tested. Volunteers from the San Diego Association for the Retarded were assigned to mildly and moderately retarded groups on the basis of Wechsler Adult Intelligence Scale (WAIS) Full-Scale IQs. To determine mental ages of the retarded subjects, each WAIS IQ was located in the Stanford-Binet 1972 revised norm tables. Using 18 as the chronological age, the appropriate mental age for each IQ score was obtained. The mildly retarded group had a mean IQ of 55.25 (SD = 4.4). The mean IQ of the moderate group was 47.25 (SD = 3.37). The IQ ranges varied between 64 and 52 for the milds and between 51 and 41 for the moderates. All retarded subjects were in their 20s, with mean ages of 24.88 and 25.12 for milds and moderates, respectively. Only subjects whose medical histories showed no symptoms or diagnosis of organicity were included in the two retarded groups. Two groups of normal children, whose mean chronological ages were 9.1 and 7.7 years, served as mental age controls for the mild and moderate retardate groups, respectively. The mental ages of the children were determined as they were for the retarded groups, with a prorated IQ based on the Wechsler Intelligence Scale for Children-Revised (WISC-R) Vocabulary subscale serving as the IQ score. These children had been preselected according to their teacher's evaluation that their classroom performance was average. All WISC-R Vocabulary subscale scores fell within the average range for these mental age control subjects. College student volunteers, all in their 20s, with a mean chronological age of 24.12, served as the chronological age control group. All subjects met the criterion of vision correctable to at least 20/30, as measured by the illiterate E test (No. 2867-1241). Subjects were naive about tachistoscopic procedures.

Apparatus and Stimuli Stimuli were presented in an Iconix (Model 6137) four-field tachistoscope. Black paratype (No. 11316) capital Ts and As were presented as the target stimuli; two paratype capital Ws placed side by side served as a pattern mask. The mask completely superimposed itself on the letter when both were simultaneously presented. The target and masking stimuli subtended a visual angle of .32° at a viewing distance of 83.9 cm. The four fields of the tachistoscope contained a fixation cross, the target T, the target A, and the mask. Stimuli were presented according to a prearranged random sequence. Luminance of the fixation field (Field 1) was

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set at 1.59 ftL (5.448 cd/m2). Stimulus and masking fields had a luminance of 15.9 ftL (49.423 cd/m2). The interval between offset of the target stimulus and onset of the masking stimulus was always dark.

Results

Table 1 presents the means and standard deviations for the CSD and critical ISI. For each of these variables, the retarded groups Procedure were four to eight times as variable as the adult The experimental task involved a forced-choice normals. The Fmat test (Winer, 1962) found recognition of the target stimuli. Prior to the experi- this variability statistically significant for both ment, each subject was given practice with the test the CSD, F(S, 40) = 8.81, p < .001, and the stimuli until it was clear he or she understood the task and could identify the letters. Each subject was then critical ISI, F(5, 40) = 6.19, p < .001. To tested in three separate sessions occurring 2 days apart. reduce this heterogeneity of variance, a Each session was a replicate of the previous session. In logarithmic transformation (Kirk, 1968) was the first half of each session, the minimum stimulus ex- performed on the data. Since the F m&K test posure duration for six consecutive correct identifications of the target, the critical stimulus duration (CSD), was results for the transformed scores were stadetermined. The stimulus duration was set initially at tistically nonsignificant for both the CSD and 1 msec and then increased in steps of 1 msec until the the critical ISI (p > .05), transformed scores subject reached the criterion. If a subject was correct were used to evaluate the data. on any given trial, the stimulus duration was left unGroup differences in the CSD were analyzed changed. If, however, the subject was incorrect, the stimulus duration was increased by 1 msec for the in a 5 (Groups) X 3 (Sessions) repeated meafollowing target presentation. A new stimulus was sures analysis. The only significant finding was provided for each trial. the main effect for groups, F(4, 35) = 9.97, In the second half of each session, the critical ISI was p < .001. The Newman-Keuls method of determined. During this procedure the durations of the target and masking stimuli were left constant at the multiple comparisons (Winer, 1962) indicated stimulus duration previously determined in the first normal adults were superior to the 7-year-olds part of the session. The critical ISI was defined as the (P < .05) but not to the 9-year-olds (p > .05), minimum interstimulus interval at which a subject with no significant differences between the two could correctly identify six consecutive presentations groups of normal children. Both groups of of the target stimuli. The interstimulus interval between the offset of the target stimulus and onset of the mask- children were superior (p < .01) to both ing stimulus was set initially at 0 and then increased retarded groups, but the two retarded groups in 2-msec increments. If a subject was correct on any did not differ statistically (p > .05). The given trial, the interstimulus interval was left un- 5 (Groups) X 3 (Sessions) repeated measures changed. If the subject was incorrect, the interstimulus interval was increased by 2 msec for the next target analysis of the critical ISI data was similar to the CSD analysis. Again, the only significant presentation. No feedback or contingent reinforcement was used. finding was the main effect for groups, F(4,35) Subjects were given rest as needed. The retarded sub= 13.68, p < .001. A Newman-Keuls analysis jects and children were given candy at the end of each session. College students received experiment credit for indicated significant differences between 7their participation. and 9-year-olds and between moderately reTable 1 Means and Standard Deviations for Raw and Transformed Scores CSD

ISI Log

Raw score Group Normal adults 9-year-olds 7-year-olds Mildly retarded Moderately retarded

M

SD

6.38 3.15 9.25 3.66 5.09 12.21 24.75 14.62 35.29 28.88

transformation

M

SD

.76 .22 .93 .18 1.05 .20 1.31 .28 1.40 .37

Note. CSD = critical stimulus duration; ISI = interstimulus interval.

Log Raw score

M

SD

16.92 13.95 25.75 21.99 79.33 57.27 82.25 48.21 102.50 86.43

transformation

M

SD

1.11 .33 1.24 .41 1.70 .28 1.85 .24 1.88 .33

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tarded subjects and 7-year-olds (p < .01). The difference between mildly retarded subjects and 7-year-olds was also significant (p < .05), but 9-year-olds and normal adults did not differ statistically (p > .05). Discussion The experiment demonstrated that the two retarded samples required longer stimulus exposure durations as well as longer intervals between the test and masking stimuli for criterion performance than did both chronological and mental age control groups. The results, furthermore, were reliable over the three trials of the experiment. These findings support the results and conclusions of Welsandt and Meyer (1974) and indicate that limitations in the early stages of information processing in retarded persons cannot be explained on the basis of low mental age. Spitz and Thor's (1968) failure to discriminate between their retarded and mental age control samples, on the other hand, may be explainable by their relatively few masking trials, their method of selecting a mental age control group, or by the possibility that their masking paradigm was less discriminating than those of the more recent positive findings. In terms of stages of processing, the data are consistent with limitations in both speed of processing and input capability in mental retardation. The CSD procedure controlled input capability by providing all subjects with the same amount of information prior to employment of the mask. The superiority of the chronological and mental age control groups in the critical ISI procedure therefore revealed that these groups required less time to process the same amount of information as the amount given their retarded counterparts, indicating slower processing in the retarded groups. The significantly longer stimulus duration requirements of the retarded groups, on the other hand, indicated reduced input capability. The CSD data thus support the existence of a deficiency in mental retardation at the level of processing encompassed by Neisser's (1967) concept of iconic storage. The precise nature of this input limitation, however, cannot be stated, since the CSD procedure controlled a variety of factors, in-

cluding icon formation time and capacity, icon quality, and icon duration. Future research should be directed toward determining which of these input factors are not impaired, which are impaired because of low mental age, and which involve some impairment other than low mental age in retarded individuals. In view of the delicate nature of matching for mental age (see Winters, 1977), any study using a mental age control group should be interpreted cautiously. On the other hand, the finding that low mental age could not account for deficits in iconic storage and speed of processing raises an important issue. If low mental age cannot account for these deficiencies in iconic storage and speed of processing, it becomes possible to hypothesize that these deficiencies may be related to or may be among the factors that result in low mental age in certain types of mental retardation. Limitations in the amount and speed of information transfer, for example, would obviously limit learning as well as other cognitive variables and have a cumulating effect in the course of development. Furthermore, that other psychopathological groups may also manifest deficiencies in iconic storage or speed of information processing (e.g., Saccuzzo et al., 1974) in no way clouds the present results, since the conditions under which such deficits develop, as well as their course, may differ entirely for each condition (Saccuzzo, 1977). References Galbraith, G. C., & Gliddon, J. B. Backward visual masking with homogeneous and pattern stimuli: Comparison of retarded and nonretarded subjects. Perceptual and Motor Skills, 1972, 34, 903-908. Harris, G. J., & Fleer, R. E. High speed memory scanning in mental retardates: Evidence for a central processing deficit. Journal of Experimental Child Psychology, 1974,17, 452-459. Kahneman, D. Methods, findings, and theory in studies of visual masking. Psychological Bulletin, 1968, 70, 404-425. Kirk, R. E. Experimental design: Procedures for the behavioral sciences. Monterey, Calif.: Brooks/Cole, 1968. Libkuman, R., & Friedrich, D. Threshold measures of sensory register storage (perceptual memory) on normals and retardates. Psychonomic Science, 1972, 27, 357-358. Neisser, U. Cognitive psychology. New York: AppletonCentury-Crofts, 1967.

MENTAL RETARDATION Saccuzzo, D. P. Bridges between schizophrenia and gerontology: Generalized or specific deficits. Psychological Bulletin, 1977, 84, 595-600. Saccuzzo, D. P., Hirt, M., & Spencer, T. J. Backward masking as a measure of attention in schizophrenia. Journal of Abnormal Psychology, 1974, S3, 512-522. Saccuzzo, D. P., & Miller, S. The critical interstimulus interval in delusional schizophrenics and normals. Journal of Abnormal Psychology, 1977, 86, 261-266. Spencer, T. J. Some effects of different masking stimuli on iconic storage. Journal of Experimental Psychology, 1969, 81, 132-140. Spitz, H. H. The channel capacity of educable mental retardates. In D. K. Routh (Ed.), The experimental psychology of mental retardation. Chicago: Aldine, 1973. Spitz, H. H., & Thor, D. H. Visual backward masking in retardates and normals. Perception &° Psychosics, 1968, 4, 245-246.

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Thor, D. H. Discrimination of succession in visual masking by retarded and nonretarded normal children. Journal of Experimental Psychology, 1970, 83, 380-384. Turvey, M. T. On peripheral and central processes in vision. Psychological Review, 1973, 80, 1-52. Welsandt, R. F., & Meyer, P. A, Visual masking, mental age, and retardation. Journal of Experimental Child Psychology, 1974, 18, 512-519. Winer, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1962. Winters, J. J., Jr. Methodological issues in psychological research with retarded persons. In I. Bialer & M. Sternlicht (Eds.), The psychology of mental retardation: Issues and approaches. New York: Psychological Dimensions, 1977. Received April 10, 1979 •