Memory scanning of shapes, colors, and compounds: A comparison of retarded and nonretarded adults

INTELLIGENCE 4,243-254 (1980)

Memory Scanning of Shapes, Colors, and Compounds: A Comparison of Retarded and Nonretarded Adults* RICHARD A. OWINGS, ALFRED A. BAUMEISTER, RICHARD A. LAINE AND MARK H. LEWIS George PeabodY College of Vanderbilt University

Three experiments were performed to assess memory scanning of shapes, colors, and shape-color compounds by retarded and nonretarded people. Attributes comprising compounds provided either redundant or nonredundant information. Large retarded-nonretarded differences in reaction time were obtained. In contrast to previous reports of slow scanning of digits and nonsense shapes by retarded people, scan rates for shapes and colors did not differ between groups. Retarded subjects were not characterized by a deficient scan rate. Although compound stimuli required twice as many attributes in their representation as did simple stimuli, they were not scanned more slowly, indicating that per item scan rate is not determined by the number of attributes required to define each item. Both groups were able to exploit redundant relevant information to achieve faster processing than in simple conditions. Decision rules for rejecting compound stimuli comprised one, two, or more binary tests. Groups did not differ in speed of performing elementary binary test(s).

The hypothesis that mentally retarded persons are less efficient in accessing short-term memory stems from work demonstrating slower digit scan rates for such subjects (Dugas & Kellas, 1974; Harris & Fleer, 1974). Hunt (1978) has taken these results as evidence for deficient short-term memory function among mentally retarded people. Because of this deficit, memory requirements which are easily within the capacity of normal persons may strain the capacity of retarded persons and, hence, disrupt processing. Others, however, have pointed out that retarded people may not be slow to access memory but, rather, they may be disadvantaged by the requirement that they process unfamiliar material (Silverman, 1974; Stanovich, 1978). Digits as well *Requests for reprints should be sent to A. A. Baumeister at P. O. Box 154, George Peabody College for Teachers of Vanderbilt University, Nashville, Tennessee 37203. The authors thank the staff and residents at Home, Inc. for their participation in this investigation. This research was supported by P.H.S. grants HD-00973, DH-10662-03, and HD-07045.

243

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as other verbal material probably are less familiar to retarded than to nonretarded people. Silverman found no difference in scan rates between normal and retarded subjects when nonsense forms were scanned. Using a somewhat different procedure, Maisto and Jerome (1977) observed a retarded-nonretarded difference for nonsense shapes, exceeding that reported for digits by Harris and Fleer (1974). Such a deficit in rate of scanning may be due to inefficient generation and application of verbal mediators. The availability of such verbal labels has been shown to determine preschoolers' rate of scanning nonsense forms (Baumeister & Maisto, 1977). Cavanagh (1972) has suggested that differences in scan rates between different types of materials (e.g., digits, colors, words, nonsense words) occur because different numbers of attributes are required to define items of each type. It may be that the attribute is the fundamental unit of scanning. Accordingly, scan rate per item should vary with the number of attributes required to define items of each type. Such an argument is circular, however, in that there is no independent index of the number of attributes required to represent each type of material. Given multiple attributes, slow scanning in retarded subjects might result from inefficient stimulus selection, including, perhaps, the tendency to overselect as well as failure to ignore irrelevant attributes. Still in question is whether retarded individuals are deficient in exploiting redundant, though relevant, information to achieve faster reaction times. In this series of three experiments, the Sternberg paradigm (Sternberg, 1969) was used to investigate the scanning of colors, familiar shapes and. color-shape compounds by retarded and nonretarded adults. Scan rates were determined for single attribute stimuli in Experiment I, attributes being either shape or color. In Experiments 2 and 3, scan rates were determined for color-shape compounds. Stimulus attributes in Experiment 2 were redundant in that decisions based upon either attribute could yield a correct response. For example, if a blue triangle constituted the positive stimulus, negative stimuli would be neither blue in color nor triangular in shape. Stimulus attributes (color and shape) were not redundant in Experiment 3. That is, subjects had to consider both attributes in order to reach a correct decision (e.g., correct color, correct shape). Experiment I was designed to examine the relative rates of memory scanning for retarded and nonretarded subjects; Experiment 2 addressed the question of whether slow scanning rates result from inefficient stimulus selection; in Experiment 3, attributes of color and shape were not redundant and thus permitted comparison of relative rates of performing binary decision tests. Finally, comparison of data from Experiment I with those from Experiments 2 and 3 allowed for a determination of the relationship between scan rate and the number attributes per item.

MEMORY SCANNING

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METHOD

Subjects. Four mildly and moderately mentally retarded adults participated in the study. Their ages ranged from 20 to 35 years; their IQs were 44,54,55, and 65 according to eitherthe Wechsler orthe Stanford Binet. Two were male and two were female. The normal subjects were four women. Two were high school graduates, currently employed as secretaries. Two were college graduates, currently attending graduate school. Ages ranged from 20 to 30 years. All subjects were paid $2.00 per session. Materials and apparatus. Shape, color, and compound probes were projected from slides onto a white background with a Kodak carousel projector. The size ofthe projected stimuli was about 8 inches. Stimulus onset was controlled using an electro-mechanical shutter. Reaction time was recorded in milliseconds using a Hunter Klockounter, triggered by means of a voice-operated relay. The response of the subject terminated the stimulus display. Shapes and colors were cut from para-type press-on plastic. Colors were red, green, blue, yellow, orange, brown, purple, and black. Shapes were arrow, circle, diamond, triangle, stop-sign, square, cross, and X. Shapes were presented in grey. Colors were presented as rectangles. In Experiment 1, which determined the scan rates for single attribute stimUli, subjects saw set sizes one, two, three, and four for each color or shape. Within each set size there were four blocks of trials. Each block consisted of64 trials, 32 of which were positive probes requiring a "yes" response and 32 were negative probes. Positive and negative probes were randomly intermixed within each block of trials such that a given response was not required more than four times consecutively. Except for set size one, a particular stimulus never appeared on two successive trials. Presentation to each subject of the 16 blocks of trials for any sample stimulus was determined by means of four 4 )( 4 latin squares. Each set size was represented by one of the four possible blocks of trials within that set size. The 16 blocks for each simple stimulus were completed before going on to the next stimulus or set of stimuli. In Experiments 2 and 3 the requisite number of positive compound stimuli for each block were obtained by sampling from the two attribute sets and then selecting an equal number of negative stimuli. Sampling from each attribute set was without replacement. The presentation of stimulus blocks was determined by means of latin squares, as in Experiment I. Trial blocks for set sizes two, three, and four of Experiment 3 consisted of 56 trials each. Within any sequence of eight trials there were an equal number

OWINGS ET AL.

246

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of positive and negative probes. Each negative probe represented one of four possible types of negative item as represented in Figure l. Negative stimuli were either wrong color, wrong shape; correct color, wrong shape; wrong color, correct shape, or correct color, correct shape but mispaired. In set size one there were 48 trials per block since one category of negative item (correct color, correct shape but mispaired) could not be presented. For each of the eight consecutive sequences of six trials, three were positive probe trials and three were negative, representing the three possible types of negative item. The same subjects were used in each of the three experiments. In order to ensure comparability of the experiments, half of the nonretarded subjects and half of the retarded subjects were presented simple stimuli then compound stimuli, colors before shapes and redundant compounds prior to non­ redundant compounds. For the remaining subjects in each group the order of presentation was reversed.

Procedure Subjects were tested individually in a dimly lighted room, seated 8 feet from a white projection screen. Positive stimuli, drawn on index cards, were presented for study prior to the presentation of a block of trials. Subjects were allowed unlimited time to learn the stimuli (usually about 2 to 3 minutes), and

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then the cards were removed. On each trial, the experimenter asked "Ready?," verified that the subject was oriented toward the screen, and initiated the trial. If subjects did not orient, they were encouraged to do so. The probe appeared and remained on until the subject responded by saying "yes" for positive pro bes and "no" for negative probes. The experimenter recorded the response and reaction time. The next trial was usually initiated after an interval of 2 seconds. There were frequent breaks to ensure adequate attention. Subjects were instructed "Do this as fast as you can but try not to make any errors." In Experiments 1 and 2, if errors exceeded two in either the first or the second half of a block, that half was repeated; that is, error rates as high as 6.25 per 32 trials were tolerated. When a third error occurred, a break was called. In Experiment 3 (56 trials = block) subjects were allowed three errors per block (5.36%) for set sizes two, three, and four, and two errors for set size one (4.17%). Before resuming, subjects were shown the positive set again and were encouraged to be more careful. Sessions ranged in length from one-half to 1 hour, depending on the subject's motivation. Four to eight blocks were presented per session. There were occasions for which a block was terminated before completion because of errors. In such cases, the entire block was repeated at the beginning of the next session. RESULTS The analysis proceeded in four steps: (1) Reaction time was analyzed to discover whether retarded subjects responded more slowly than nonretarded subjects, (2) errors were analyzed to discover whether between-groups reaction time differences were due to the choice of different points on the speed-accuracy curve, (3) set size effects were tested to discover whether crucial assumptions of the Sternberg (1969) model were met, and (4) slopes and intercepts of scan functions were analyzed to discover the extent to which scan and non-scan processes contributed to between-groups reaction time differences.

Experiment 1 Each subject's mean reaction time for each set size in each condition was calculated by averaging over the four blocks. The variance of mean reaction time scores for shapes, colors, and redundant compounds was analyzed using a design that represented three problems (shape, color, compound) by two probe types (positive and negative), by four set sizes (1, 2, 3, 4), and by IQ (retarded vs. nonretarded). A .05 level of significance was adopted throughout this and subsequent analyses.

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In the blocks included in the analysis, errors ranged from I% to 4% and 1% to 3% per set size for retarded and nonretarded subjects, respectively. The number of half blocks repeated due to error rates exceeding criterion was 18 for all four set sizes for retarded subjects and 13 for nonretarded subjects. There was a significant effect of groups F(I, 6) =6.0, MSerror =467,000, retarded subjects being 242 msec slower than nonretarded SUbjects (Figure 2). The shape-color difference was not significant 1(12) = .65. The effect of set size was significant F(3, 18) = 24.18, MSerror = 7090. With the exception of the difference between set sizes three and four, all other comparisons were significant.

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Tests were performed to discover whether the serial exhaustive scanning model is applicable to these data. The between set sizes variance was partitioned into a linear component, which was significant, F(l, 18) = 69.8, and a departure from linearity component, which was not significant, F (2, 18) = 1.4. The set size effect was well described by a straight line, in accordance with the assumption of serial processing. The interaction of set size with probe type was not significant, F (3, 18) = 2.7, MSerror = 310, in accordance with the assumption of exhaustive scanning. No interaction with IQ was significant; the serial exhaustive scan model was applicable to both groups. Analyses were performed to determine whether the retarded-nonretarded difference in reaction time arose from differences in speed of memory scanning. A slope and an intercept were calculated for each condition for each subject. Group averages appear in Table 1. An analysis ofvariancefor slopes yielded no significant between groups difference. The analysis of intercepts mirrored the group differences obtained in the analysis of reaction time. The between groups difference in intercept was 243 msecs, which was significant, F(1, 6) =5.1, MSerror = 140,000, and accounted entirely for the between­ groups difference in reaction time.

Experiment 2 A significant effect of simple vs. compound materials, F(2, 12) = 3.87, MSerror = 7595 was observed. Subjects reacted more quickly to compound than to simple stimuli, t (12) = 2.70. Slopes and intercepts of scan function were analyzed to discover whether the simple-compound difference in reaction time was attributable to the scan process, as opposed to non-scan processes. For slopes, the material by groups interaction was significant, F(2,12) =4.8, MSerror =214. RetardedTABLE 1 Slopes and Intercepts as a Function of Material Retarded

Shapes (Experiment I) Colors (Experiment I) Compounds (Experiment 2)

Nonretarded

Slope

Intercept

Slope

Intercept

42

692

57

412

41

696

44

426

53

614

36

432

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OWINGS ET AL.

nonretarded comparisons revealed that the interaction arose from a between­ groups difference for compounds. Retarded people scanned compounds more slowly than did nonretarded people, and the difference was significant, t(12) = 2.3. Retarded subjects also scanned compounds more quickly: However, for neither group was the simple vs. compound contrast significant. The pattern for intercepts was the reverse of that for slopes. Mentally retarded people encoded more quickly in the compound conditions than in the simple conditions, while the nonretarded encoded more slowly. However, mean contrasts, simple vs. compound, were not significant, as was the intelligence by material interaction, F(2, 12) =3.4, MSerror =3394,p =.07. The error rates per set size ranged from 1% to 5% for retarded subjects and from I % to 4% for nonretarded subjects for blocks included in the analysis. The total number of half blocks repeated across all set sizes was seven for retarded subjects and two for nonretarded subjects.

Experiment 3

As in the previous experiments, the effect of set size on reaction time was significant, F(3, 18) = 17.1, MSerror = 8700 according to an ANOVA that included all four set sizes but excluded the mispaired probe, for which there was no set size one. Small set sizes were reacted to more quickly than large set sizes (Figure 3). Analyses of the linearity of the regression lines were performed separately for the several types of probes and for the two types of subjects. In general, the linear component of the between-set sizes variance was significant and the deviation from linearity component was not. The effect of intelligence on reaction time was significant, F(I, 6) = 6.0, MSerror = 171,000. Nonretarded people were about 150 msec faster than retarded people, and this difference was mirrored both in direction and magnitude in the analysis of the intercepts of the scan functions; however, the between groups effect was not significant, F(l, 6) = 2.1, MSerror = 90,700. Scan rates for retarded people did not differ from those of nonretarded subjects, F(I, 6) = .39, MSerror = 3670, being about 50 msecs per both groups. Slopes and intercepts are reported in Table 2. The type of probe affected reaction time, F(4, 24) = 20.7, MSerror = 1700, according to an ANOVA that included all five probe types, but excluded set size one, for which there was no mispaired probe. The effect was similar, both in direction and magnitude, to that observed for the intercepts of the scan functions, F(4, 24) =2.9, MSerror = 190,000. Correct color, correct shape but mispaired probes were reacted to more slowly than any of the remaining three types of negative item. Wrong color, wrong shape probes were reacted to more quickly than any other types of negative probe. All contrasts were significant. Correct color, wrong shape and wrong color, correct shape probes were immediate and did not differ significantly from each other. Positive probes achieved intermediate reaction times, being insignificantly

251

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Nonretarded

Slope

Intercept

Slope

Intercept

56 38

615 651 676 606 709

48 48 59 37 26

472 448 472 493 631

46

70 47

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OWINGS ET AL.

slower than the fastest negative probe and significantly faster than the slowest negative probe. For positive probes error rates ranged from 1.3% to 4% for all subjects. The range of error rates for negative probes was from .9% to 3.4% Errors were more frequent for the correct shape, correct color mispaired probe than any other probe. The number of blocks repeated due to error rates exceeding criterion was 12 for retarded subjects and I for nonretarded subjects. The interaction between intelligence and type of probe for reaction time was not significant, F(4, 24) =1.3. Retarded people apparently made binary choice decisions as quickly as did nonretarded subjects. The scan process did not differ for the various types of decisions. The interaction between set size and type of probe was not significant, F(8, 48) =.5, MSerror =800, for the ANOVA of five probes and three set sizes, and F(9, 54) =1.1, MSerror =1000, for the ANOVA offourprobes and four set sizes. The analysis of scan slopes revealed no differences as a function of probe type.

DISCUSSION In the present experiments retarded people scanned colors, shapes, and, under some circumstances, compounds, at the same rate as nonretarded individuals. On this basis, the hypothesis of generally deficient scanning by mentally retarded persons must be rejected. However, for redundant compounds as for digits (Dugas & Kellas, 1974; Harris & Fleer, 1974) and nonsense shapes (Maisto & Jerome, 1977, retarded subjects scanned more slowly than did nonretarded subjects. This "scanning deficit" was accompanied by an "encoding advantage" relative to simple stimuli for retarded people. Perhaps the best conclusion is that retarded people sometimes choose a different point on the encoding time-scanning time tradeoff curve than do nonretarded persons. A negative correlation between encoding time and central processing time is frequently reported (Hunt, 1978; Maisto, 1978; Maisto & Jerome, 1977; Silverman, 1978; Sternberg, 1975). Both Hunt and Sternberg attribute the negative correlation to a strategic tradeoff. People may achieve the quickest total reaction time by encoding quickly and scanning slowly or by encoding slowly and scanning quickly. This seems to be an intriguing strategic behavior that merits further intvestigation both in its own right, and as a way of approaching questions of intellectual difference. The present data are also incompatible with the attribute scanning hypothesis (Cavanagh, 1972). The number of attributes required to represent compound stimuli in the present task was roughly twice that required to represent simple stimuli, yet scan rate was unaffected. Differences in intercept were obtained between simple and compound stimuli. The differences

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apparently depended on the nature of the decision required. The effect of using compound stimuli was on the decision stage of processing, not the memory scan stage, and this effect resulted because compounds call for complex decision rules. Neither group was distracted by the addition of relevant redundant information in Experiment 2. The data suggest just the opposite in that redundancy facilitated processing for both groups. There is some question as to how redundancy helps. The data for most of the subjects can be accounted for by noting that minimum reaction times did not differ between simple and compound stimuli. Response time was governed by the dimension subjects were able to process more quickly. There was one subject whose minimum reaction times for compound stimuli were faster than those for simple stimuli, and this was true across set sizes. Different people used redundant relevant information in different ways. It is not clear why subjects reacted to redundant compounds more quickly. Perhaps they chose the dimension with which they were more facile and ignored the other dimension, and faster reactions to compounds was an artifact of averaging. If one subject reacts quickly to shapes and the other reacts quickly to colors, faster average reaction times would be obtained for compounds than for simples, not because compounds are reacted to more quickly, but rather because subjects have the option of attending to their preferred dimension. One nonretarded subject reported that she attended to colors and ignored shapes. Her reaction times for colors and compounds were quite similar and were both markedly faster than her reaction times for shapes. On the other hand, there were three people (two retarded subjects and one nonretarded subject) for whom compounds were scanned significantly more quickly than either type of simple material. Perhaps these people scanned colors when the compounds include less familiar shapes and shapes when compounds include less familiar colors. The data for the one nonretarded subject were clearly incompatible with this hypothesis. At each set size, several of her reaction times to compound stimuli were faster than her fastest reaction time to a simple stimulus. She seemed to be affected by the "bulk of the evidence." In sum, different subjects did different things, implying that individual strategies may affect the scanning of stimuli. The results of Exeriment 3 are compatible with the decision model described by Trabasso, Rollins, and Shaughnessey (1971), who conceived of the judgment process as comprising a series of binary tests. For instance, subjects could only apply tests in the order. (1) Is the color correct? (2) Is the shape correct? Time to reject wrong color, wrong shape probes and wrong color, correct shape probes should be equivalent and faster than time to reject correct color, wrong shape probes. The time difference represents the time necessary to perform a single binary test. The nonsignificant interaction between intelligence and type of negative probe reflected the lack of between­ group difference in the speed of performing binary tests.

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OWINGS ET AL.

Checkosky (1971) found additive, not interactive, effects of decision criterion and set size when compound stimuli were scanned. He concluded that the two variables affect different stages of processing. The data presented here demonstrate that testing is self-terminating. If people performed all the tests and then responded, response times for the four types of negative probe would be equivalent. The present finding that decision criterion and set size do not interact corroborate Checkosky's result (1971). To summarize, the item, not the attribute, was the fundamental unit of scanning in these experiments. Per item scan rate remained constant at about 50 msec per item, independent of the number of attributes comprising each item. Items were scanned once for each type of probe; i.e., probe types and set size did not interact. Probe type affects reaction time in a way that is predictable from a branching decision tree, indicating that testing was self­ terminating. The difference among probe types does not differ between retarded and nonretarded people, indicating that the central judgmental process was as efficient for retarded as for nonretarded people.

REFERENCES Baumeister, A. A., & Maisto, A. A. Memory scanning by children: Meaningfulness and mediation. Journal of Experimental Child Psychology, 1977,24,97-107 Cavanagh, J. P. Relation between the immediate memory span and the memory search rate. Psychological Review, 1971,87, 525-530. Checkosky, S. F. Speeded classification of multi-dimensional stimuli. Journal ofExperimental Psychology, 1971,87, 383-388. Dugas, J. L., & Kellas, G. Encoding and retrieval processes in normal children and retarded adolescents. Journal of Experimental Child Psychology, 1974, 17, 177-185. 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. Hunt, E. Mechanics of verbal ability. Psychological Review, 1978,85, 109-130. Maisto, A. A. Comments on the use of the additive factor method with mentally retarded persons: A reply to Silverman. American Journal of Mental Deficiency, 1978,83,191-193. Maisto, A. A., & Jerome, M. Encoding and high speed memory scanning of retarded and nonretarded adolescents. American Journal of Mental Deficiency. 1977,82, 282-286. Silverman, W. P. High speed scanning of nonalphanumeric symbols in cultural-familially retarded and nonretarded children. American Journal ofMental Deficiency, 1974,79,44-51. Silverman, W. P. Comments on "encoding and high-speed memory scanning ... " by Maisto and Jerome. American Journal of Mental Deficiency, 1978,83, 188-190. Stanovich, K. E. Information processing in the mentally retarded.In N. R. Ellis (Ed.), International review ofresearch in mental retardation. (Vol. 9). New York: Academic Press, 1978. Sternberg, S. The discovery of processing stages: extensions of Donders' method. In W. G. Koster (Ed.), Attention and performance II. Amsterdam: North Holland, 1969. Sternberg, S. Memory scanning: New findings and current controversies. Quarterly Journal of Experimental Psychoogy, 1975,27,421-457. Trabasso, T., Rollins, H., & Shaughnessey, E. Storage and verification stages in processing concepts. Cognitive Psychology, 1971,2. 239-289.