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Nonselective read-out from iconic memory in normal, borderline and retarded adolescents
INTELLIGENCE 4, 363-369 (1980)
Nonselective Read-out from Iconic Memory in Normal, Borderline and Retarded Adolescents TERRY M. LIBKUMAN, ROBERT S. VELLIKY AND DOUGLAS D. FRIEDRICH Central Michigan University
The hypothesis that retarded individuals read-out information from iconic memory at a slower rate, relative to their CA matched peers, was tested.Three IQ groups (normal, borderline and retarded) were selected with 10 subjects in each group. A tachistoscope presented a stimulus array and a post-stimulus cue for 100 msec. each, separated by a variable interstimulus interval (ISI). This stimulus array consisted of 10 letters arranged on two horizontal rows. The post-stimulus cue was a circle which surrounded the position of the correct letter. Nine ISis were used and ranged from -100 (cue before array) to 500 msec. A 3 x 9 repeated-measures analysis of variance revealed non-significant effects for IQ and the IQ x ISI interaction. A significant effect was found for ISis (p < .01) which reflected reduced performance at the intermediate ISis for all intelligence groups. The hypothesis of a read-out deficit in the retarded was not supported. The present findings were discussed in relation to previous research which supported a readout deficit in t h e retarded.
Visual perception has been represented as a temporal sequence of events, which involves stages of storage and the tranformation of information. Information is received during discrete fixation pauses between the eye's rapid movements, called saccads. Visual impressions of continuity from these discrete exposures is facilitated by a brief, but crucial, form of visual persistence which allows processing to occur even after the stimulus has terminated. Neisser (1967) referred to the first visual cognitive process as iconic memory, in which the icon can be maintained for several hundred milliseconds in unaltered form. Furthermore, since the partial report technique developed by S perling (1960) has indicated that more information is available in the icon than can be processed, the subject must be selective. In other words, only a portion of the information available will be selected and processes to short-term memory (STM). Consequently, the quality and quantity of information processing that occurs in the later stages is related to the processing efficiency that occurs in iconic memory. One possible variable governing iconic processing efficiency is intelligence.
363
364
LIBKUMAN, VELLIKY, FRIEDRICH
Recently, several studies have suggested less than optimal iconic processing in the retarded when compared with their normal MA or CA peers (Friedrich, Libkuman, Craig, & Winn, 1977; Libkuman & Friedrich, 1972; Mosley, 1978a; Pennington & Luszcz, 1975; Spitz, 1973; Welsandt & Meyer, 1974). It is not clear, however, as to the nature of the deficit; i.e., is it capacity, decay or read-out? In terms of capacity, the data (Pennington & Luszcz, 1975; Spitz, 1973) reflect a somewhat reduced information store. The data are not as clear, however, with respect to the duration of the icon. Craig (1973) and Pennington and Luszcz (1975) found no evidence for a deficit in the duration of the icon. Spitz (1973), on the other hand, suggested that the retarded person has a more durable icon which has the effect of preventing or distorting information that is presented in rapid sequence. In line with the present investigation, Friedrich et al. (1977) and Libkuman and Friedrich (1972) focused on the possibility of slow read-out in the retarded. In particular, nonselective read-out. In contrast to selective readout, which occurs after the onset of the post-stimulus cue, nonselective readout refers to the transfer of information from the icon to STM prior to the onset of the post-stimulus cue. In support of this notion, both studies found that retarded individuals, relative to CA matched normals, required longer exposure durations of the stimulus array (two, four or six digits in the former study, six digits in the latter study) before correct identification of the designated digit occurred. This was interpreted as indicating that retarded individuals require more time to read-out the information from the icon relative to normals. In the Friedrich et al. study, however, the prediction that performance differences would increase between the IQ groups as the number of digits in the array increased was not supported. In other words, if the retarded person exhibits a read-out deficit, the deficit should become more pronounced with higher information loads. In the present study the notion of a nonselective read-out deficit in the retarded individual was again investigated. The logic underlying the study was based on some of the findings reported by Averbach & Corriell ( 1961). In the first two experiments conducted by these authors a 2 x 8 matrix of letters was exposed for 50 msec. which in turn was immediately followed by a variable interstimulus interval (ISI; the interval between the array and the post-stimulus cue). The post-stimulus cue was either a bar-marker (Experiment 1) or a circle which completely surrounded the position of one of the letters (Experiment 2). When the authors compared the performance curves for the two experiments the functions were similar with the exception of the intermediate ISis (50, 100, 150 msec.) where performance was considerably reduced when the circle was used as the cue. According to Averhach and Coriell, the storage process involves erasure to assure that old information is out of the store before new information is put in. The
ICONIC MEMORY AND INTELLIGENCE
365
backward in time action of the circle implies that the first stimulus was stored but the second stimulus limited the time available for nonselective read-out. The effect of the circle was to remove the previously stored information. The improved performance with increasing delay was attributed to the increased time available for nonselective read-out. The object of the present study was to test the hypothesis of slower nonselective read-out in the retarded by incorporating the methodology that Averbach and Coriell used in Experiment 2. It was expected that intelligence would interact with ISI. In other words, when the time available for read-out is limited (e.g., ISis of 50, 100, and 150 msec.) the performance reduction should be more pronounced for the retarded groups.
METHOD
Subjecls A sample of ten normal (,X CA = 15.8, SD = 1.40; X IQ --- 100.5, S D --- 9.23), ten b o r d e r l i n e (X CA = 15.4, SD = 1.43; X IQ = 81.7, SD--2.54), and ten retarded individuals (X CA = 17.4, SD = 2.12; IQ = 70.2, SD = 4.39) was selected from the Gladwin, Michigan, Public School System. There were five males and five females within each group. These groups were matched for chronological age. Additional criteria included: normal or corrected vision; ability to identify letters presented in the experiment; and no past experience in any perceptual memory experiment. Finally, an attempt was made by the experimenters to arouse motivation in the subjects in order to insure a high level of performance.
Apparatus and Materials A model GB three-channel tachistoscope (Scientific Prototype) presented the stimuli. Measured at the viewing hood, any one channel presented a luminance of approximately 1.95 fL. Two channels presented a luminance of approximately 3.68 fL. (i.e., target and circle presented simultaneously). The stimulus array consisted of ten capital letters, 1.91 centimeters high and 0.794 centimeters wide. They were arranged in two horizontal rows (five letters in each row) on eighteen white cards measuring 12.7 x 17.78 centimeters. The letters were selected at random with no card having a duplication of letters. The fixation point was a black dot (r = 0.238 centimeters)centered on a white card (12.7 x 17.78 centimeters). T h e visual angles a n d visual fields were: visual field (card), horizontal = 8°8 ', vertical = 5°59'20"; target letters, horizontal = 4°5 ', vertical = 1°45'12n; individual letters, horizontal = 22'4 n, vertical = 33'6"; and fixation point = 6'36". One circle marker, i.e., the post-stimulus cue, was
366
LIBKUMAN,VELLIKY,FRIEDRICH
positioned on each of ten white cards (12.7 x 17.78 centimeters), corresponding to the ten positions of letters. The circle completely surrounded, without touching, the position of a given letter. The cards were sprayed with a matte finish in order to reduce glare from the white surface. Procedure
A 2-3 minute period for adjustment to light intensity was necessary, during which the subject's head remained in the viewing hood of the tachistoscope with his/her eyes fixated on the dot in the middle of the white field (which remained on before and after the stimulus sequence was presented). Two practice trials were given with targets not used in the experiment. Two experimenters were required, one to operate the control panel and record responses; the other to change cards. The experimenter at the control panel informed the subject to fix his/her attention on the dot and then reply "ready." The same experimenter then depressed the start button to begin the stimulus sequence. Once the sequence ended, the subject was required to verbalize the letter that he/she thought was designated by the circle. (The subjects were instructed to respond as quickly and accurately as possible.) The subject was then told the correct letter. Three temporal sequences were used. In the first, the stimulus array flashed on for a constant duration of 100 msec., followed by a preselected ISI, after which the circle appeared for a 100 msec. constant duration. The ISI between offset of the array and onset of the circle was either 50, 100, 150, 200, 300,400, or 500 msec. In the second temporal sequence the cue preceded the array by 100 msec. Also, there was a third sequence in which the array arid circle appeared simultaneously. Thus, there were nine ISI's: -100, 0, 50, 100, 150, 200, 300, 400 and 500 msec. In addition, the visual field at no time became darkened. The fixation point was visible before the presentation of the stimulus array (or circle, depending on the sequence), for the duration of the ISI, and following the offset of the circle (or array). The pairing of a circle to a position of the array was random. A total of 18 array cards were used. Each position occurred nine times for each subject with the nine ISI's occurring once for each of ten letter positions. Consequently, there were 90 trials per session per subject. Each subject received the same random order of presentation. The subjects were tested individually in sessions requiring 60 minutes with periodic one-minute rest periods. RESULTS AND DISCUSSION A 3 x 9 (Intelligence x ISI) repeated measures analysis of variance was used to analyze the percent correct response data. The results revealed nonsignificant effects for IQ, F (2,27) = 2.34, p > .05, and the IQ x ISI
367
ICONIC M E M O R Y A N D INTELLIGENCE
interaction, F(16, 216) = 0.55, p > .05. A significant effect was found for the ISis, F (8, 216)= 99.10,p < .01. In Figure 1, percent correct for the three IQ groups was plotted over the 9 ISls. For comparative purposes, the figure incorporates the findings of a previous study that was conducted in our laboratory (Craig, 1973). Briefly, this study was a replication of the Averbach and Coriell (1961) bar-marker experiment with the exception that, in the Craig study, the performance of a normal IQ group ('X IQ = 114) was compared with a borderline group (X IQ = 83). Craig's findings paralleled the findings of the present study, i.e., nonsignificant IQ and IQ x ISI effects, significant ISI effects. A comparison of the two studies, as indicated in Figure 1, essentially reflect the deleterious effect of the circle on performance. Collectively, these two studies replicated the findings of Averbaeh and CorieU's first two experiments. Finally, it can be seen that the performance of the three IQ groups in the present study was
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368
LIBKUMAN, VELLIKY, FRIEDR1CH
comparable. In effect, there is no evidence to suggest that intelligence is related to nonselective read-out. The findings of the present study (as well as the Craig study) do not agree with the results of the previous investigations conducted in our laboratory (Friedrich, et al. 1977; Libkuman & Friedrich, 1972). Although some variation did occur with respect to intelligence and environment (institutionalized vs noninstitutionalized) in the mentioned studies, it does not seem likely that these differences can account for the contradictory findings. For example, significant IQ effects were reported in the Libkuman and Frieddch study where the IQ difference was 49 (X normal IQ = 118; ,X retarded IQ = 69) and in the Friedrich et al. study where the IQ difference was 33 (X normal IQ = I l 1; X retarded IQ = 78), but no significant effects were found in the present study where the difference between the extreme IQ groups was 30. We believe the important differences reside in procedural variations. In comparing the present study (and also the Craig study) with Friedrich et al. and the Libkuman and Friedrich studies some major differences emerged: (a) In the former studies the stimulus array pattern was rectangular while in the latter studies a circular pattern was used. (b) In the former studies the stimulus array duration was a constant 100 msec. In the latter the array duration systematically increased or decreased and depended on the subject's behavior. (c) Finally, in the former studies, a variable ISI followed the offset of the stimulus array while in the latter studies, the post-stimulus cue immediately followed the offset of the stimulus array. Additional research will be required to partial out the important differences. However, as Pennington and Luszcz (1975) have suggested, important differences may not emerge between IQ groups until strategies become important for effective performance. In other words, it may be more reasonable to account for the Friedrich, et al. and the Libkuman and Friedrich data by referring to other factors, such as attention, motivation, or strategies (Hornstein & Mosley, 1979; Mosley, 1978b; Ross & Ward, 1978; Stanovich, 1978) rather than differential decay and/or read-out rates. Finally, Pennington and Luszcz 0975) reported that iconic storage in retarded individuals closely paralleled that of normal subjects, though at a slightly lower level. Over a wide range of stimulus conditions, retarded subject consistently reported about one letter less than normals. The mean number of letters reported correctly per array for retarded subjects was 2.4 in the first experiment and 2.8 in another. Normals under the same conditions average 3.7. In the present study, this one letter difference between the retarded and normals groups was upheld. The mean number correct over all ISis was 3.17 for the retarded group, 3.43 for the borderline group, and 4.04 for the normals. The retarded groups' 3.17 letters performance is close to Spitz's (1973) magical number three.
ICONIC MEMORY AND INTELLIGENCE
369
REFERENCES Averbach, E., & Coriell, A. S. Short-term memory in vision. Bell System Technical Journal, 1961, 60, 309-329. Craig, E. R. Sensory register storage (perceptual memory) in normal and retarded adolescents. Unpublished master's thesis, Central Michigan University, 1973. Friedrich, D., Libkuman, T., Craig, E., & Winn, F. Read-out times from iconic memory in normal and retarded adolescents. Perceptual and Motor Skills, 1977, 44, 467-473. Horustein, H. A., & Mosley, J. L. Iconic-memory processing of unfamiliar stimuli by retarded and nonretarded individuals. American Journal of Mental Deficiency, 1979, 84, 40-48. Libkuman, T., & Friedrich, D. Threshold measures of sensory register storage (perceptual memory) on normals and retardates. Psychonomic Science, 1972, 27, 357-358. Mosley, J. L. The tachistoscopic recognition of letters under whole and partial report procedures as related to intelligence. British Journal of Psychology, 1978, 69, 101-110.(a) Mosley, J. L. Retinal locus and the identification of tachistoscopicaHy presented letters by retarded and nonretarded individuals. American Journal of Mental Deficiency, 1978, 82, 380-385.(b) Neisser, U. Cognitive Psychology. New York: Appleton-Century-Crofts, 1967. Pennington, F. M., & Luszcz, M. A. Some functional properties of iconic storage in retarded and nonretarded subjects. Memory and Cognition, 1975, 3, 295-301. Ross, L. E., & Ward, T. B. The processing of information from short-term visual store: Developmental and intellectual level differences. In N. R. Ellis (Ed.), International Review of Research in Mental Retardation, Vol. 9. New York: Academic Press, 1978. Sperling, G. The information available in brief visual presentations. Psychological Monographs, 1960, 74, (Whole No..498). Spitz, H. H. Consolidating facts into the schematized learning and memory system of educable retardates. In N. R. Ellis (Ed.), International review of research in mental retardation, Vol. 6. New York: Academic Press, 1973. Stanovich, K. E. Information processing in mentally retarded individuals. In N. R. Ellis (Ed.), International review of research in mental retardation Vol. 9. New York: Academic Press, 1978. Welsandt, R. F., & Meyer, P. A. Visual masking, mental age, and retardation. Journal of Experimental Child Psychology, 1974, 18, 512-519.
Nonselective Read-out from Iconic Memory in Normal, Borderline and Retarded Adolescents TERRY M. LIBKUMAN, ROBERT S. VELLIKY AND DOUGLAS D. FRIEDRICH Central Michigan University
The hypothesis that retarded individuals read-out information from iconic memory at a slower rate, relative to their CA matched peers, was tested.Three IQ groups (normal, borderline and retarded) were selected with 10 subjects in each group. A tachistoscope presented a stimulus array and a post-stimulus cue for 100 msec. each, separated by a variable interstimulus interval (ISI). This stimulus array consisted of 10 letters arranged on two horizontal rows. The post-stimulus cue was a circle which surrounded the position of the correct letter. Nine ISis were used and ranged from -100 (cue before array) to 500 msec. A 3 x 9 repeated-measures analysis of variance revealed non-significant effects for IQ and the IQ x ISI interaction. A significant effect was found for ISis (p < .01) which reflected reduced performance at the intermediate ISis for all intelligence groups. The hypothesis of a read-out deficit in the retarded was not supported. The present findings were discussed in relation to previous research which supported a readout deficit in t h e retarded.
Visual perception has been represented as a temporal sequence of events, which involves stages of storage and the tranformation of information. Information is received during discrete fixation pauses between the eye's rapid movements, called saccads. Visual impressions of continuity from these discrete exposures is facilitated by a brief, but crucial, form of visual persistence which allows processing to occur even after the stimulus has terminated. Neisser (1967) referred to the first visual cognitive process as iconic memory, in which the icon can be maintained for several hundred milliseconds in unaltered form. Furthermore, since the partial report technique developed by S perling (1960) has indicated that more information is available in the icon than can be processed, the subject must be selective. In other words, only a portion of the information available will be selected and processes to short-term memory (STM). Consequently, the quality and quantity of information processing that occurs in the later stages is related to the processing efficiency that occurs in iconic memory. One possible variable governing iconic processing efficiency is intelligence.
363
364
LIBKUMAN, VELLIKY, FRIEDRICH
Recently, several studies have suggested less than optimal iconic processing in the retarded when compared with their normal MA or CA peers (Friedrich, Libkuman, Craig, & Winn, 1977; Libkuman & Friedrich, 1972; Mosley, 1978a; Pennington & Luszcz, 1975; Spitz, 1973; Welsandt & Meyer, 1974). It is not clear, however, as to the nature of the deficit; i.e., is it capacity, decay or read-out? In terms of capacity, the data (Pennington & Luszcz, 1975; Spitz, 1973) reflect a somewhat reduced information store. The data are not as clear, however, with respect to the duration of the icon. Craig (1973) and Pennington and Luszcz (1975) found no evidence for a deficit in the duration of the icon. Spitz (1973), on the other hand, suggested that the retarded person has a more durable icon which has the effect of preventing or distorting information that is presented in rapid sequence. In line with the present investigation, Friedrich et al. (1977) and Libkuman and Friedrich (1972) focused on the possibility of slow read-out in the retarded. In particular, nonselective read-out. In contrast to selective readout, which occurs after the onset of the post-stimulus cue, nonselective readout refers to the transfer of information from the icon to STM prior to the onset of the post-stimulus cue. In support of this notion, both studies found that retarded individuals, relative to CA matched normals, required longer exposure durations of the stimulus array (two, four or six digits in the former study, six digits in the latter study) before correct identification of the designated digit occurred. This was interpreted as indicating that retarded individuals require more time to read-out the information from the icon relative to normals. In the Friedrich et al. study, however, the prediction that performance differences would increase between the IQ groups as the number of digits in the array increased was not supported. In other words, if the retarded person exhibits a read-out deficit, the deficit should become more pronounced with higher information loads. In the present study the notion of a nonselective read-out deficit in the retarded individual was again investigated. The logic underlying the study was based on some of the findings reported by Averbach & Corriell ( 1961). In the first two experiments conducted by these authors a 2 x 8 matrix of letters was exposed for 50 msec. which in turn was immediately followed by a variable interstimulus interval (ISI; the interval between the array and the post-stimulus cue). The post-stimulus cue was either a bar-marker (Experiment 1) or a circle which completely surrounded the position of one of the letters (Experiment 2). When the authors compared the performance curves for the two experiments the functions were similar with the exception of the intermediate ISis (50, 100, 150 msec.) where performance was considerably reduced when the circle was used as the cue. According to Averhach and Coriell, the storage process involves erasure to assure that old information is out of the store before new information is put in. The
ICONIC MEMORY AND INTELLIGENCE
365
backward in time action of the circle implies that the first stimulus was stored but the second stimulus limited the time available for nonselective read-out. The effect of the circle was to remove the previously stored information. The improved performance with increasing delay was attributed to the increased time available for nonselective read-out. The object of the present study was to test the hypothesis of slower nonselective read-out in the retarded by incorporating the methodology that Averbach and Coriell used in Experiment 2. It was expected that intelligence would interact with ISI. In other words, when the time available for read-out is limited (e.g., ISis of 50, 100, and 150 msec.) the performance reduction should be more pronounced for the retarded groups.
METHOD
Subjecls A sample of ten normal (,X CA = 15.8, SD = 1.40; X IQ --- 100.5, S D --- 9.23), ten b o r d e r l i n e (X CA = 15.4, SD = 1.43; X IQ = 81.7, SD--2.54), and ten retarded individuals (X CA = 17.4, SD = 2.12; IQ = 70.2, SD = 4.39) was selected from the Gladwin, Michigan, Public School System. There were five males and five females within each group. These groups were matched for chronological age. Additional criteria included: normal or corrected vision; ability to identify letters presented in the experiment; and no past experience in any perceptual memory experiment. Finally, an attempt was made by the experimenters to arouse motivation in the subjects in order to insure a high level of performance.
Apparatus and Materials A model GB three-channel tachistoscope (Scientific Prototype) presented the stimuli. Measured at the viewing hood, any one channel presented a luminance of approximately 1.95 fL. Two channels presented a luminance of approximately 3.68 fL. (i.e., target and circle presented simultaneously). The stimulus array consisted of ten capital letters, 1.91 centimeters high and 0.794 centimeters wide. They were arranged in two horizontal rows (five letters in each row) on eighteen white cards measuring 12.7 x 17.78 centimeters. The letters were selected at random with no card having a duplication of letters. The fixation point was a black dot (r = 0.238 centimeters)centered on a white card (12.7 x 17.78 centimeters). T h e visual angles a n d visual fields were: visual field (card), horizontal = 8°8 ', vertical = 5°59'20"; target letters, horizontal = 4°5 ', vertical = 1°45'12n; individual letters, horizontal = 22'4 n, vertical = 33'6"; and fixation point = 6'36". One circle marker, i.e., the post-stimulus cue, was
366
LIBKUMAN,VELLIKY,FRIEDRICH
positioned on each of ten white cards (12.7 x 17.78 centimeters), corresponding to the ten positions of letters. The circle completely surrounded, without touching, the position of a given letter. The cards were sprayed with a matte finish in order to reduce glare from the white surface. Procedure
A 2-3 minute period for adjustment to light intensity was necessary, during which the subject's head remained in the viewing hood of the tachistoscope with his/her eyes fixated on the dot in the middle of the white field (which remained on before and after the stimulus sequence was presented). Two practice trials were given with targets not used in the experiment. Two experimenters were required, one to operate the control panel and record responses; the other to change cards. The experimenter at the control panel informed the subject to fix his/her attention on the dot and then reply "ready." The same experimenter then depressed the start button to begin the stimulus sequence. Once the sequence ended, the subject was required to verbalize the letter that he/she thought was designated by the circle. (The subjects were instructed to respond as quickly and accurately as possible.) The subject was then told the correct letter. Three temporal sequences were used. In the first, the stimulus array flashed on for a constant duration of 100 msec., followed by a preselected ISI, after which the circle appeared for a 100 msec. constant duration. The ISI between offset of the array and onset of the circle was either 50, 100, 150, 200, 300,400, or 500 msec. In the second temporal sequence the cue preceded the array by 100 msec. Also, there was a third sequence in which the array arid circle appeared simultaneously. Thus, there were nine ISI's: -100, 0, 50, 100, 150, 200, 300, 400 and 500 msec. In addition, the visual field at no time became darkened. The fixation point was visible before the presentation of the stimulus array (or circle, depending on the sequence), for the duration of the ISI, and following the offset of the circle (or array). The pairing of a circle to a position of the array was random. A total of 18 array cards were used. Each position occurred nine times for each subject with the nine ISI's occurring once for each of ten letter positions. Consequently, there were 90 trials per session per subject. Each subject received the same random order of presentation. The subjects were tested individually in sessions requiring 60 minutes with periodic one-minute rest periods. RESULTS AND DISCUSSION A 3 x 9 (Intelligence x ISI) repeated measures analysis of variance was used to analyze the percent correct response data. The results revealed nonsignificant effects for IQ, F (2,27) = 2.34, p > .05, and the IQ x ISI
367
ICONIC M E M O R Y A N D INTELLIGENCE
interaction, F(16, 216) = 0.55, p > .05. A significant effect was found for the ISis, F (8, 216)= 99.10,p < .01. In Figure 1, percent correct for the three IQ groups was plotted over the 9 ISls. For comparative purposes, the figure incorporates the findings of a previous study that was conducted in our laboratory (Craig, 1973). Briefly, this study was a replication of the Averbach and Coriell (1961) bar-marker experiment with the exception that, in the Craig study, the performance of a normal IQ group ('X IQ = 114) was compared with a borderline group (X IQ = 83). Craig's findings paralleled the findings of the present study, i.e., nonsignificant IQ and IQ x ISI effects, significant ISI effects. A comparison of the two studies, as indicated in Figure 1, essentially reflect the deleterious effect of the circle on performance. Collectively, these two studies replicated the findings of Averbaeh and CorieU's first two experiments. Finally, it can be seen that the performance of the three IQ groups in the present study was
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368
LIBKUMAN, VELLIKY, FRIEDR1CH
comparable. In effect, there is no evidence to suggest that intelligence is related to nonselective read-out. The findings of the present study (as well as the Craig study) do not agree with the results of the previous investigations conducted in our laboratory (Friedrich, et al. 1977; Libkuman & Friedrich, 1972). Although some variation did occur with respect to intelligence and environment (institutionalized vs noninstitutionalized) in the mentioned studies, it does not seem likely that these differences can account for the contradictory findings. For example, significant IQ effects were reported in the Libkuman and Frieddch study where the IQ difference was 49 (X normal IQ = 118; ,X retarded IQ = 69) and in the Friedrich et al. study where the IQ difference was 33 (X normal IQ = I l 1; X retarded IQ = 78), but no significant effects were found in the present study where the difference between the extreme IQ groups was 30. We believe the important differences reside in procedural variations. In comparing the present study (and also the Craig study) with Friedrich et al. and the Libkuman and Friedrich studies some major differences emerged: (a) In the former studies the stimulus array pattern was rectangular while in the latter studies a circular pattern was used. (b) In the former studies the stimulus array duration was a constant 100 msec. In the latter the array duration systematically increased or decreased and depended on the subject's behavior. (c) Finally, in the former studies, a variable ISI followed the offset of the stimulus array while in the latter studies, the post-stimulus cue immediately followed the offset of the stimulus array. Additional research will be required to partial out the important differences. However, as Pennington and Luszcz (1975) have suggested, important differences may not emerge between IQ groups until strategies become important for effective performance. In other words, it may be more reasonable to account for the Friedrich, et al. and the Libkuman and Friedrich data by referring to other factors, such as attention, motivation, or strategies (Hornstein & Mosley, 1979; Mosley, 1978b; Ross & Ward, 1978; Stanovich, 1978) rather than differential decay and/or read-out rates. Finally, Pennington and Luszcz 0975) reported that iconic storage in retarded individuals closely paralleled that of normal subjects, though at a slightly lower level. Over a wide range of stimulus conditions, retarded subject consistently reported about one letter less than normals. The mean number of letters reported correctly per array for retarded subjects was 2.4 in the first experiment and 2.8 in another. Normals under the same conditions average 3.7. In the present study, this one letter difference between the retarded and normals groups was upheld. The mean number correct over all ISis was 3.17 for the retarded group, 3.43 for the borderline group, and 4.04 for the normals. The retarded groups' 3.17 letters performance is close to Spitz's (1973) magical number three.
ICONIC MEMORY AND INTELLIGENCE
369
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