Hemispheric Changes in Information Processing During Development

Developmental Psychology 1978, Vol. 14, No. 3, 214-223

Hemispheric Changes in Information Processing During Development CAROL TOMLINSON-KEASEY, RONALD R. KELLY, AND JOHN K. BURTON University of Nebraska—Lincoln Hemispheric processing of visually presented words and pictures was examined in three groups of subjects with mean ages of 8 years 8 months, 12 years 3 months, and 27 years 9 months. Pictorial or symbolic stimuli were presented singly to either the right or left visual hemifield. Subjects had to decide whether the first stimulus in a pair matched the second stimulus. The major results were that (a) age groups differed in the strength of lateral differences, and (b) for all age groups, the right hemisphere was significantly faster in processing unmatched stimuli. The results are interpreted as suggesting that lateral specialization of the left hemisphere is not complete until adolescence and that over the age range tested, the left hemisphere becomes progressively more specialized for the processing of matched data.

Although hemispheric processing of visual and auditory information has been studied extensively in adults, there is a dearth of developmental information that might answer some of the continuing questions in the adult literature. One such question is how does the left hemisphere become so specialized for verbal skills? Infant studies using evoked potential recording techniques (Molfese, Freeman, & Palermo, 1975), photic driving procedures (Crowell, Jones, Kapunai, & Nakagawa, 1973), and dichotic listening tasks (Entus, 1974) have all found evidence of hemispheric differences in the reaction of the brain to different stimuli during the first year of life. Although The authors are grateful to Jim Knowlton for designing and constructing the systems control instrument used in this study, to Eastridge Elementary School, Robin Mickel Junior High, and the Lincoln JAiblic School Administration for their generous cooperation, and to the Research Council at the University of Nebraska—Lincoln for their support made available through National Institutes of Health Biomedical Services Support Grant 5 S05 RR07055-10. Helpful suggestions on an earlier draft of the manuscript were made by Lauren Jay Harris and are gratefully acknowledged. Requests for reprints should be sent to Ronald R. Kelly, Media Development froject for the Deaf, 318 Barkley Memorial Center, University of NebraskaLincoln, Lincoln, Nebraska 68583.

these studies, especially of newborns, give fascinating insight into the nature of the brain, the changes that occur throughout development remain unexplored. This is particularly true for hemispheric changes involving visual information processing. Adult hemispheric specialization for language skills is well documented (Bryden, 1960; Curry, 1967; Heron, 1957; Kimura, 1966, 1967; Nebes, 1974; Sperry, 1974). Much less is known about how this specialization comes about, whether it changes during development, and if it changes, when. Studies of dichotic listening have attempted to chart the auditory changes in hemispheric specialization that occur during development. Krashen (1973) critically reviewed the hemispherectomy literature as well as several dichotic listening studies and concluded that lateralization for speech was completed by age 5. However, more recent dichotic listening studies have resurrected the "lateralization by puberty" hypothesis (Bryden, 1973; Bryden & Allard, 1977) with at least one investigation finding that the right ear advantage continued to increase to age 11 (Satz, Bakker, Teunissen, Goebel, & Van der Vlugt, 1975). Clearly, the developmental course of lateralization in the auditory channel is not settled.

Copyright 1978 by the American Psychological Association, Inc. OOI2-I649/78/I403-0214S00.75

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HEMISPHERIC DEVELOPMENT

Developmental investigations of hemispheric specialization in the visual channel have also yielded inconclusive results (Forgays, 1953; Miller & Turner, 1973; Turner & Miller, 1975). Forgays (1953) did not find specialization for words favoring the left hemisphere until the seventh grade. In their experiments Turner and Miller alternately find minimal age effects and large age effects depending on variations in the task. They conclude that there are a variety of perceptual processes that influence cerebral lateralization for visual materials. Unfortunately, these multiple processes as well as several critical task parameters have not yet been investigated in the visual channel. In light of Krashen's (1973) compelling review of the hemispherectomy literature, one might conclude that speech lateralization is completed at one point, auditory lateralization at another, and visual lateralization at yet a third. If this is the case, then an understanding of the development of visual lateralization becomes even more critical since this channel would seem to have a maximum amount of plasticity. Hence, the present study examined the thesis that hemispheric specialization in the visual channel continues to develop past the elementary years. A second important issue in research on the development of hemispheric specialization is the nature of right hemispheric functioning during development. Adult studies of the right hemisphere have not yielded consistent findings. Several studies have demonstrated a right hemisphere superiority for visual processing of nonsense figures, forms, dots, and pictures of faces (Berlucchi, Heron, Hyman, Rizzolatti, & Umilta, 1971; Ellis & Shepherd, 1975; Geffen, Bradshaw, & Wallace, 1971; Kimura, 1966, 1969), whereas other research shows no differences between hemispheres for processing geometric forms (Bryden, 1960; Heron, 1957; Terrace, 1959). To complicate the issue further, three recent studies have actually shown a left hemisphere superiority for the processing of some pictures (Rizzolatti, Umilta, & Berlucchi, 1971; Turner & Miller, 1975; Kelly & Tomlinson-Keasey, Note 1). Although some of these conflicting results concerned with right hemispheric

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specialization are due to methodological differences between individual studies, a clear picture of right hemispheric functioning does not exist. Zaidel (Note 2) speculates that "left hemispheric specialization for language is accompanied by continuous reorganization (but not disappearance) of linguistic competence in the right hemisphere" (p. 8). One can speculate further that right hemispheric functioning may be critical for processing information about the environment when the sophisticated language skills of the adult are not present. Although several theorists (Bruner, 1964; Piaget, 1962) give considerable credence to the power of imagery or an icon during the prelinguistic phase of development, there are no data to date suggesting that this stage of development is more associated with the right hemisphere than with the left. The examination of right hemispheric processing of both pictures and words in three age groups should illuminate the role of imagery, as well as the changing linguistic competence, of the right hemisphere. Finally, there have been multiple studies of the differential hemispheric processing of matched and unmatched stimuli (Bradshaw, Gates, & Patterson, 1976; Cohen, 1972; Gibson, Dimond, & Gazzaniga, 1972; Klatsky & Atkinson, 1972; Moscovitch, 1972, 1976). Most of these authors agree that there is a right hemispheric superiority for matching when the entire gestalt of the stimulus is involved. A left hemispheric superiority exists when the stimuli must be separated into components for serial processing before a same or different judgment can be made. This two-process pattern of responding (holistic or sequential) seems to hold regardless of whether the stimuli are simple lines or more complex configurations (Geffen, Bradshaw, & Wallace, 1971; Patterson & Bradshaw, 1975; Rizzolatti, Umilta, & Berlucchi, 1971). Thus, both hemispheres appear to be necessary for the various kinds of complex visual processing that are required when an adult must decide whether two stimuli match. Developmentally, how do these two processes mature, and are there different timetables for the achievement of these processes? Is there, at some early

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C. TOMLINSON-KEASEY, R. KELLY, AND J. BURTON

point in development, a single right hemiMethod spheric holistic processor of patterns that blends, later in development, with a left hemispheric sequential processor? In the Subjects present study, matched and unmatched A total of 154 right-handed subjects were used. stimuli were presented in the left or right Handedness was assessed by having subjects indicate visual field to trace the development of the which was their writing hand. This single procedure two hemispheres in processing same or dif- eliminated the obvious left handers but did not eliminate subjects who under more extensive assessment ferent stimuli. might be classified as exhibiting mixed In sum, the present study examined (a) procedures dominance. There were 44 third graders (23 males and the development of left hemispheric 21 females, mean age = 8 years, 8 months), 50 seventh specialization in the visual channel, (b) de- graders (24 males and 26females, mean age = 13 years, velopmental changes in right hemispheric 3 months), and 60 graduate and undergraduate students (30 males and 30 females, mean age = 27 years, 9 processing, and (c) differential processing of months). All of the subjects had normal vision, and matched versus unmatched stimuli. It was none indicated any eye problems that had not been predicted that words would be processed corrected. All subjects were volunteers. more efficiently by the left hemisphere and that pictures would be processed faster in the right hemisphere. Further, it was pre- Procedures dicted that specialization for words would The subjects were tested individually. They sat at a be most noticeable in the two oldest age rear projection booth with their foreheads in a headrest groups, but that pictorial processing would 61.5 cm from the screen. Subjects were instructed to be lateralized in the right for all age groups. focus on a dot on the screen throughout the experiment. to each stimulus pair the experimenter said These predictions reflect the thesis that left Prior "ready." Briefly, the instructions to the subjects were hemispheric specialization in visual process- as follows: ing is due to the acquisition and develop(a) Place your forehead on the headrest; (b) look at ment of symbolic processing skills (see also the dot on the screen when the experimenter says Carmon, Nachson, & Starinsky, 1976). "ready"; (c) without shifting your eyes, try to obThese symbolic processing skills make maserve the first stimulus slide (presentation slide); (d) terial more or less amenable to verbal codthere will be a brief blank period, followed by a second stimulus slide (probe slide); (e) continue to look ing. It is possible, for example, for pictures at the dot during both stimulus slides; and (f) try to to be labeled and coded by the left hemirespond as quickly as possible once you see the secsphere, but they should be processed more ond slide by pushing either the "yes" or "no" button efficiently in the right hemisphere if that to indicate whether it matched the first stimulus slide. hemisphere is particularly specialized for pictorial input. An identical argument can be The experimenter then talked each subject through 10 pairs that were presented at slow speed. made for words that are readily visualized. stimulus The adults and junior high subjects received a total of However, in both cases one would predict 72 trials including eight initial practice pairs presented faster processing when no transformations at 100 msec. Both of the older groups responded to half of the stimuli are necessary (Moscovitch, the stimulus pairs with their right hands, and the other half with their left hands. The trials for each hand were 1976). presented in a block, although the specific order of Because the present study focuses on vi- stimulus pairs was randomly organized for each group. sual processing, age groups that should eviBecause of the shorter attention span of younger dence differential skill in processing visual subjects and because fatigue could greatly affect reacsymbols were selected. The three age tion time, the number of trials for the third graders was to 40. To accomplish this reduction, the hand groups included (a) third-grade students reduced used, which is of minimal importance (Rizzolatti, who should show some ability at processing Umilta, & Berlucchi, 1971; Wallace, 1971), was visual symbols, (b) junior high students who changed. Whereas both the adults and junior high subshould evidence a somewhat higher level of jects responded to half the items with each hand, one symbolic visual processing skills, and (c) group of the third-grade children (n = 22) responded to all the stimulus pairs with their right hands, while a university graduate and undergraduate stu- second group (n = 22) responded to all pairs with their dents who should exhibit a very high level of left hands. With this change in procedure, the experimastery for processing visual symbols. mental session was reduced to 10-15 minutes for the

HEMISPHERIC DEVELOPMENT youngest group as compared to approximately 20 minutes for the two older groups. The presentation stimulus was shown for 100 msec 2° to either side of the fixation point followed by a 2-sec blank period. The probe stimulus was also presented for 100 msec. If a stimulus is presented for a duration of 100 msec or faster while a subject fixates on a point, the stimulus will disappear before the subject can make a new fixation to look directly at it (Woodworth & Schlosberg, 1954, p. 502). Because only one side of each of the retinas and only one cerebral hemisphere are directly stimulated (Kimura, 1973), one is assured that a visual stimulus will initially be presented to a specific hemisphere.

Stimuli The stimulus pairs consisted of either words or pictures presented singly to the visual hemifields. The word stimuli were 18-point Gothic style lower-case letters and were presented only in a horizontal position 2° to either the left or right side of the fixation point. The Gothic style is a relatively easy typeface to identify and hence is likely to show a right field superiority and to elicit fewer errors than a type that is more difficult to discriminate (Bryden & Allard, 1976). The center of the words appeared five to five and a half degrees to either side of fixation depending on the number of letters in the ' word. The picture stimuli when projected were approximately the same width and height as the words—Vi x 5 /i« inch (1.3 x .8 cm) high. Stimuli were projected in a 4 x 6 inch (10.2 x 15.2 cm) field with 64 footcandles illumination. The present study analyzed only those trials on which both stimuli in a pair went to the same hemisphere (left/left or right/right), although it was necessary to intersperse them randomly with an equal number of trials in which the stimulus pairs were presented to both hemispheres (right/left or left/right). This procedure was used to assure that subjects' responses to the left/ left or right/right stimulus pairs would not be influenced by anticipation resulting from too much consistency or repetition. The two modes consisted of word to word and picture to picture pairs. Each content item was used only once in order to eliminate any practice effect. All of the word stimuli were three-, four-, and five-letter high-image words from the list of nouns by Paivio, Yuille, and Madigan (1968). The pictures used also represented words of this list and the majority were taken from the Peabody Picture Vocabulary Test (Dunn, 1959). Those pictures not available in the Peabody test were drawn to correspond to the shaded, black and white outline drawings that typify pictures in the Peabody test. For half of the stimulus pairs the subjects responded to matched items, whereas the other half were not matched. The stimulus pairs that were not matched, however, were alike on some dimensions. The unmatched words always contained the same number of letters (i.e., near and like; ear and owl). The unmatched pictures were always the same size, and the type of drawing was the same (i.e., iron and typewriter; bell and/rog). The stimuli were projected through a Kodak Carousel 750 projector with a tachistoscopic shutter

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mounted on the slide projector lens. A specially constructed sequential tinier advanced the slide projector via a phototransistor and controlled the duration of the presentation and probe slides as well as the blank period between them. The subjects' response buttons were connected directly to the timer. Thus, their reaction time could be accurately measured from the instant that the probe slide was projected to the moment that either response button was pushed.

Results An analysis of the errors made by each group under each of the stimulus conditions was conducted to see if the groups were responding in very different ways to the speed and accuracy requirements of the task. Overall, the adults made errors on 2.18% of the presentations, whereas the junior high subjects and third graders made errors on 6.87% and 6.39% of the items, respectively. A log linear analysis (Bock, 1975) of the frequency of errors in each of the groups and stimulus conditions revealed no significant differences either between the groups or within the stimulus conditions (see Table 1). Hence, it was concluded that the three groups were responding to the speed and accuracy requirements of the task in similar ways. For this reason, the central analyses of the data focused on subjects' reaction times. The reaction times were initially analyzed to see if using the right or left hand made a difference. With the adult and junior high subjects, a 2 (Hands) x 2 (Hemispheric Input) x 2 (Mode) repeated measures design indicated no hand response differences, F( 1, 108) = .19,p > .05. Likewise, comparing the third-grade left-hand response group with the right-hand response group showed no reaction time difference, F(l, 42) = 1.54, p > .05. Hence, all further data analyses were conducted without regard to the hand used for responding. An overall analysis indicated no main effect of the sex of subjects in the third- or seventh-grade groups, F(l, 42) = .33, p > .05, and F(l, 48) = .004, p > .05, respectively. However, in the junior high subjects there was a three-way interaction between the hemispheres, whether the stimuli were matched or not, and the sex of the subject, F(l, 48) = 7.13, p < .01. This interaction

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Table 1: Frequency of Errors for All Groups in All Conditions Matched data Left Age group Third graders (88) Seventh graders (100) Adult (120)

Unmatched data Left

Right

Right

Words

Pictures

Words

Pictures

Words

Pictures

Words

Pictures

8 1 1

7 10 5

8 8

8 5 3

2 10

6 6

2

2

3 12 3

3 5 3

2

Note. The number of responses in each cell is given in parentheses.

males to matched words. Because the adult females were faster than the males in seven of the eight stimulus conditions (see Table 2), it seemed most parsimonious to attribute the differences in adults to a small but general female superiority in reaction time rather than to a specific sex difference in information processing of either hemisphere or mode. For this reason, further analyses were conducted without regard to the sex of the subject. An overall analysis comparing the three age groups on all stimulus conditions required a 3 (Age) x 2 (Hemispheric Input) x 2 (Mode) x 2 (Match-Unmatch) analysis of variance with repeated measures on the last three factors. A significant four-way in-

was due primarily to a mean difference of 255 msec in the reaction time of males to matched versus unmatched stimuli in the left hemisphere. This difference reflects an ease of processing matches that is beginning to be evidenced. By adulthood, this difference between processing matched and unmatched stimuli in the left hemisphere is even larger and holds for both sexes (see Table 2). For the adults there was a nonsignificant trend toward female superiority in visual processing, F(l, 58) = 3.74,p = .058. There was also a significant interaction between the stimulus mode, whether the data were matched or not, and the sex of the subject, F(l, 58) = 7.47, p < .01. This interaction reflects the faster reaction time of the fe-

Table 2: Mean Reaction Times (in msec) and Standard Deviations of Males and Females in All Conditions Matched data

Left Age group

Unmatched data Left

Right

Right

Words

Pictures

Words

Pictures

Words

Pictures

Words

Pictures

1,508 846

1,450 566

1,473 851

1,361

1,685

596

1,397

606

1,391 632

1,420 643

1,334 588

1,409 495

1,597 996

1,579 491

1,505

1,509

723

524

1,537 577

1,561 471

915 406

1,063 415

1,143 543

1,206 394

1,280

1,208

612

542

1,095 441

1,187 483

1,000 308

1,122 333

1,137 328

1,053 304

1,197

1,204

356

345

1,206 488

1,231 563

845 298

1,006 337

1,059 297

1,317 713

1,374

1,183

919

512

1,317 632

1,123 675

769 294

853 340

917 303

952 306

1,108

1,262 415

1,143 376

969 289

3rd graders Males (23)

M SD

530

Females (21)

M SD 7th graders Males (24)

M SD Females (26)

M SD Adults Males (30)

M SD Females (30)

M SD

Note. The number of responses in each group is given in parentheses.

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teraction (Groups x Modes x Hemispheric Input x Match-Unmatch, F(2, 151) = 3.22, p < .05) made it difficult to evaluate any of the main effects. Therefore, it was necessary to conduct subsequent analyses of the performances of the age groups for each of the simple main effects (Kirk, 1968, p. 178). In all of the analyses done after finding a significant interaction, the alpha level was reduced to .01 to compensate for the inflation of alpha. Matched Data

Comparing the three groups of subjects on just the matched stimuli required a 3 (Age) x 2 (Hemisphere Input) x 2 (Mode) analysis of variance with repeated measures on the last two factors. The main effect of age groups was significant, F(2, 151) = 21.08, p < .01. In order to compare the means among age groups, Scheffe's (1953) 5 method, which is appropriate for nonpairwise comparisons and unequal n, was used. These analyses indicated that the adults did not process the matched stimuli faster than the Table 3: Means and Standard Deviations of Group Reaction Times (in msec) in All Condition's

Age group 3rd graders (44) Words M

SD Pictures M SD

7th graders (50) Words M SD Pictures M SD

Matched data

Unmatched data

Hemisphere

Hemisphere

Left

Right

Left

junior high school subjects, F(2,151) = .76, p > .05, but were significantly faster than the third graders, F(2,151) = 3.17,p < .01 (see Table 3). In the overall analysis of the matched data, the hemispheric main effect was also significant, F(l, 151) = 15.97,p < .01, indicating that the left hemisphere processed matched stimuli significantly faster than the right. A separate analysis of the age groups using Scheffe's 5 method indicated that the left hemisphere was not significantly faster than the right in the third graders, F(l, 43) = .81, p > .05, but was significantly faster in both the junior high, F(l, 49) = 3.32,p < .01, and adult groups, F(l, 59) = 4.52, p < .01. The main effect of stimulus modes for the matched data across all groups was also significant, F(l, 151) = 3.90, p < .01, indicating that matched words were processed more efficiently than matched pictures. However, a significant interaction for age groups with modes, F(2, 151) = 3.14, p < .05, required subsequent analyses for each group on the mode dimension. These analyses showed that the superiority for processing matched words over matched pictures did not appear in the third graders, F(l, 43) = .23, p > .05, or junior high subjects, F(l, 49) = 2.75, p > .05, but was very pronounced in the adults, F(l, 59) = 13.07, p < .01.

Right

Matched Versus Unmatched 1,587 667

1,458 605

561

1,448 525

1,485 564

959 358

1,139 440

1,237 493

1,153 465

1,093 372

1,127 355

1,205 446

1,210 522

806 296

987 306

1,240 702

1,229 523

1,429 737

1,530

1,432 529

1,460

911

Adults (60) Words M SD

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Pictures M 929 1,134 1,222 1,046 575 345 464 521 SD Note. The number of subjects in each group is given in parentheses.

Although the overall analysis indicated that the unmatched stimuli took significantly longer to process than the matched stimuli, F(l, 49) = 27.97, p < .01, it was not possible to interpret group performances due to the significant interaction between age groups and the matched-unmatched stimuli, F(2, 141) = 5.43,p < .01. Follow-up analyses to ascertain the performance of each age group with the matched and unmatched stimulus pairs showed that the third graders exhibited no significant differences, F(l, 43) = .54,p > .05. In contrast, however, both the junior high subjects, F(l, 49) = 19.20,p < .01, and the adults, F(l, 59) = 23.63, p < .01, processed the matched stimulus pairs significantly faster.

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Unmatched Data

An overall 3 (Age) x 2 (Hemispheric Input) x 2 (Stimulus Mode) analysis of variance for the unmatched data indicated that the main effect of age group was significant, F(2, 151) = 7.35, p < .01. Interestingly, the differences in reaction time here occurred primarily between third graders and junior high subjects, whereas there were no differences between the adults' and the junior high subjects' abilities to process the unmatched stimuli. The main effect of the hemispheric condition across all three age groups was also significant, F(l, 151) = 5.20, p < .05. The unmatched stimuli presented to the right hemisphere were processed faster than in the left hemisphere. As with the matched data, this hemispheric difference was most pronounced with the adult subjects (see Table 3). The adults were especially efficient in the processing of unmatched pictures in the right hemisphere.

Discussion

The lateralization discussion in hemispheric research continues despite repeated efforts to determine when during development lateral specialization is complete. Because the answer to this question is so critical to decisions about surgery and possible language disruption, researchers have persisted in their attempts to find the answer. The pieces of the puzzle that are now fitting together yield a rich, complex, and unfinished picture. It seems clear that the developmental timetable for lateralization of expressive speech is quite short and probably does not extend much beyond 5 years of age (Krashen, 1973). However, the lateralization that occurs in the comprehension of speech (auditory lateralization) does not appear to be nearly so limited and may even extend to adolescence (Satz et al., 1975). The fact that these two types of specialization differ substantially in terms of age at completion suggests that lateralization of the visual system for printed words may be completed at yet a different period in development. The results of the present study

support this contention. In the third-grade group there was essentially no lateral specialization for words. The left hemisphere processed both words and pictures at the same rate, and this processing did not differ from the right hemisphere. Likewise, in their responses to matched or unmatched stimuli, the third graders did not differ. The third-grade children then were using similar methods to process all visual stimuli, and these methods were not specialized for words or matches. Both the adults and the junior high subjects are evidently processing words that match very differently from the way the third graders are processing matched words, especially in the left hemisphere. In terms of the development of hemispheric specialization then, the present study indicates that lateralization for matched words is not evidenced in the visual channel by the time students reach third grade (1,429 msec vs. 1,530 msec for left and right hemispheres, respectively). However, left hemispheric specialization for matched symbols is significant in both of the other age groups (959 msec vs. 1,139 msec for the junior high subjects and 806 msec vs. 987 msec for the adults). The left hemispheric advantage for matched words is practically identical in the seventh-grade and adult subjects, suggesting that specialization for matched words is complete by the time students reach seventh grade. However, reaction times to both matched words and matched pictures continue to decrease from seventh grade to adulthood, indicating greater efficiency of processing, if not further specialization. It should be cautioned that the changes in lateral specialization that are noted here are not proceeding in isolation. The organism is obviously changing on a variety of dimensions from third grade to college. It may be that changes in other realms, notably cognitive development, interact with neurological organization, producing the increasing specialization that is reported here, The results indicated a surprising amount of symbolic processing in the right hemisphere. The adult subjects even processed matched words in the right hemisphere faster than pictures. This result is in line with the current conceptualizations of the right


hemisphere as retaining significant skills in language comprehension even though minimal expressive capabilities are present (Searleman, 1977). Because the task used required pressing a button rather than giving a verbal response, the results are a measure of this right hemispheric comprehension of language. Of course, the simple word stimuli used facilitated the success of what is characterized as a limited linguistic system (Moscovitch, 1976; Sperry, Gazzaniga, & Bogen, 1969). The substantial right hemispheric language capacity demonstrated underscores the earlier point that the visual channel is probably lateralized later than either the expressive speech function or the auditory channel. A second purpose of the present study was to look for specialization that might be specific to the right hemisphere. In this regard, it was noted that overall the right hemisphere processed unmatched stimuli more effectively than the left. This effect was most pronounced in the adults. A variety of findings in the neurological literature indicate that the cortex is constantly monitoring the environment for unique stimuli or stimuli that do not match (Groves & Thompson, 1970; Luria, 1973; Pribram, 1971; Sokolov, 1960). That the right hemisphere might be functionally specialized to assist in such monitoring fits with descriptions of the right hemisphere that highlight the processing of more global information (Sperry, 1974). The emphasis that Broadbent (1974), Milner (1974), and Moscovitch (1976) place on complementary functioning of the two hemispheres would also be consistent with such an analysis. The right hemisphere in adults would be seen more as a global analyzer of stimuli and processor of unique stimuli (mismatches), whereas the left would be more efficient at translating these stimuli into symbols for the purpose of communicating about them, or operating logically on them. The adult perceptual literature has also shown that holistic patterns are more easily judged as same or different by the right hemisphere, although this difference appears to be quantitative rather than qualitative (Bradshaw, Bradley, & Patterson, 1976; Geffen, Bradshaw, & Wallace, 1971; Moscovitch, 1972, 1976; Patterson &

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Bradshaw, 1975; Robertshaw & Sheldon, 1976). It is possible then to interpret the data from the present study as supporting the existence of two processes—one more holistic, the other more logical—that are particularly suited to the right and left hemispheres, respectively. Because the youngest subjects showed little specialization, their responses might be characterized as global, undifferentiated, and holistic. The adults, however, demonstrated some blend of holistic responding and sequential or analytical responding. The results might also be interpreted simply as indicating progressive left hemispheric specialization because the conceptualization of the right hemisphere as critical for the processing of holistic images prior to left hemispheric specialization for symbols received minimal support. Although the processing of pictures relative to words did change in the three age groups, this appeared to be due to increasing specialization for words. The youngest children in the current study did not show any lateral differences. However, a recent study by Carmon, Nachson, and Starinsky (1976) did find that first graders showed a significant right hemispheric superiority for the perception of single letters. These investigators, therefore, endorsed the idea that right hemispheric processing is initially geared to simple nonverbal configurations and that this processing is followed by verbal encoding in the left hemisphere (Carmon et al., 1976). An experiment with preschool and kindergarten children would provide more relevant data on the importance of pattern processing in the right hemisphere during this age period. A final comparison was between the matched and unmatched data. As mentioned before, the third graders showed no laterality on this dimension. However, the two older age groups were reacting to matched and unmatched stimuli in decidedly different ways. The changes in reaction times across groups suggest that the seventh graders and adults have become very skilled at coding and responding to matches that exist in their environment. That such differential processing of matches should develop is indicated

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by neuropsychological findings. Pribram's (1971) review of the evidence suggests that "at the cortex . . . a patterned change is set up as a function of stimulus repetition" (p. 51). It is not surprising, therefore, that the present results demonstrate a developmental specialization for matched stimuli. It is interesting that specialization for matches followed the same course developmentally as the specialization for symbols. Perhaps the ability to process matches effectively in the environment develops with lateralization for the specific mode under consideration. The relatively late development of hemispheric differentiation in response to visual stimuli that is identified in the present study spawns a host of related questions, all of which require more research. If a child processes stimuli in a manner different from adults and even older junior high children, are there presentation modes that would be more or less efficiently utilized by this processing? Given that the results of the present study support increasing symbolic specialization in the left hemisphere, what are the most effective ways to program the left hemisphere for symbolic visual skills? A concern that springs from the comparatively late lateralization reported here is the progress of reading skills if lateralization for words is slow to develop. Research investigating this relationship might prove to have diagnostic value. Additional studies looking at atypical populations might be able to add to our knowledge about hemispheric development and information processing. For example, do deaf children, who have limited access to auditory cues and symbols, show left hemispheric specialization for visual symbols? Likewise, do blind children show differing kinds of specialization on the cues that are available to them? Neuropsychologists are probing the information processing of the adult brain, and infant researchers are detailing the potential and flexibility of the newborn brain. It is time for developmental psychologists to examine the transformation from the infant brain to the adult brain in all of the sensory modalities and to relate these findings to specific modes of information processing.

REFERENCE NOTES

1. Kelly, R. R., & Tomlinson-Keasey, C.Hemispheric processing of visual and symbolic material: The verbal cue still dominates. Paper presented at the meeting of the American Psychological Association, Washington, D.C., September 1976. 2. Zaidel, E. The case of the elusive right hemisphere. Paper presented at the 13th annual meeting of the Academy of Aphasia, Victoria, British Columbia, October 1975. REFERENCES

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(Received January 10, 1977)