Cerebral lateralization of function and bilingual decision processes: Is thinking lateralized?

BRAIN

AND LANGUAGE

5,

56-71 (1978)

Cerebral Lateralization of Function and Bilingual Decision Processes: Is Thinking Lateralized? CURTIS HARDYCK University

of California,

J. L. TZENG

OVID University

Berkeley

of California,

Riverside

AND WILLIAM University

S-Y. WANG

of California,

Berkeley

Pour experiments utilizing tachistoscopic presentation of verbal and spatial stimuli to visual half-fields are presented. Three experiments failed to find any cerebral lateralization effect of the type predicted from existing models of cerebral lateralization processes. One experiment found marked lateralization effects. Since the experiments differ only in the ratio of trials to experimental stimuli, it is argued that cerebral lateralization experiments are detecting only a memory process occurring after subjects have learned all the stimuli to be presented. When new stimuli are presented on each trial, no cerebral lateralization effects are found, suggesting that active ongoing cognitive processing is independent of lateralization.

The experimental study of lateralization of cortical function has apparently progressed to a stage where the concept of separate processing capacities for the right and left cerebral hemispheres is taken as established fact. It is now common for research reports on lateralization phenomena to begin with some invocation such as: “It is well documented that the left hemisphere is the major hemisphere for mediating linguistic processing and the right hemisphere for mediating nonlinguistic processing.” While belief in the separate but equal cognitive processing capacities of the cerebral hemispheres may be widespread, the evidence is scarcely overpowering. Hemispheric specialization in intact, normal individuals is, as Cohen (1975) commented in her study of cuing effects on hemisphere This work was funded by a Spencer Foundation grant to Curtis Hardyck, Grant No. MH 28003 to Ovid Tzeng, and an NSF grant to William S-Y Wang. We would like to thank the subjects who aided us in this work. ReIjrint requests should be addressed to Curtis Hardyck, Institute of Human Learning, University of California, Berkeley, CA 94720. 0093-934X/78/0051-0056$02.00/0 Copyright 0 1978 by Academic Press, Inc. AU rights of reproduction in any form reserved.

56

CEREBRAL

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differences, “not absolute, not constant, and not simple.” The complex set of results reported in the recent studies by Hellige and Cox (1976) and Moscovitch, Scullion, and Christie (1976) both serve as testimony to the inadequacy of simple models based on right-left division of function. The majority of interpretive statements on hemisphere differences are based on the findings first reported by Sperry (1%8a, 1%8b, 1974) on cerebral commissurotomy patients and by Kimura (1%6, 1973) on normal subjects. A basic premise is that verbal stimuli are processed in the left hemisphere and visuospatial stimuli in the right hemisphere. Stimuli presented exclusively in the right visual field serving the left hemisphere (RVF-LH) are processed more quickly and with fewer errors because they are received directly at a central verbal processor in the left hemisphere. Visuospatial problems such as pattern recognition are processed most effectively in the right hemisphere, allowing an advantage in processing speed and accuracy for stimuli shown to the left visual field projecting to the right hemisphere (LVF-RH). An alternative explanation for these observed differences has been proposed by Kinsbourne (1970, 1973), who argues that lateralization effects are in large part a function of the relative level of activation of the cerebral hemispheres. In the Kinsbourne model, performance, regardless of type of material, will be faster and more accurate for stimuli presented to the hemisphere at a higher level of arousal. In support of this model, Bruce and Kinsbourne (Note 1) reported an experiment in which subjects were asked to remember complex visual forms. When done as a direct recognition task, LVF-RH performance was slightly superior. However, when subjects had to perform the figure recognition task while retaining a list of words in memory, RVF-LH performance was superior. Bruce and Kinsbourne interpret their results as support for the hemispheric activation hypothesis, arguing that the added requirement of retaining verbal material in memory raised the level of activation of the left hemisphere, thus producing a superior left hemisphere performance on a visuospatial task. A similar study was reported by Hellige and Cox (1976) who found similar effects for a light verbal memory load (two to four words) but a decreased effect for a six-word memory load. Their results are not supportive of an activation hypothesis, since the facilitating effects of arousal should be relatively independent of memory load. Another interpretive model has been proposed by Davis and Schmit (1971, 1973) based on their studies of hemisphere differences in reaction times on “same-different” judgments of name identity (Aa) vs physical identity (AA) tasks of the type studied by Posner (1969). Davis and Schmit suggest that (a) the right hemisphere can analyze and compare stimuli only on the basis of visual information, (b) the left hemisphere is capable of analysis both on a verbal and a visual basis, and (c) in judgments of same-different, when the judgment within a

HARDYCK,

TZENG,

AND WANG

hemisphere is “different,” the analysis is transferred to the other hemisphere before a decision is made. Evaluations of the Davis and Schmit model have not been reported by other investigators. In general, research reports and reviews of hemisphere function differences have paid little or no attention to the position taken by Broadbent (1974). Broadbent’s position is basically in opposition to the hemispheric specialization models discussed earlier. He states that, for complex decisions and for input-output stages, man functions as a singlechannel organism. In support of his position, he reviews the evidence on performance of tasks carried out simultaneously, concluding that performance levels drop when two tasks are attempted simultaneously, regardless of the level of practice. This performance drop cannot be attributed to inhibition of motor output, rather “It is only choices or decisions about actions in one task that interfere with choices or decisions in another” (p. 36). Broadbent interprets these findings as evidence against hemispheric specialization. If hemispheric specialization is as pronounced as current models suggest, he would argue, then simultaneous performance of two tasks with known hemisphere differences should be better than indicated by current experimental evidence. It is instructive to examine the experimental evidence in support of cerebral lateralization differences. Research reports on this topic tend to share the following characteristics: (1) a restricted set of stimuli; in Brunswik’s terms, (1949, 1952) sharply delimited object sampling; (2) a great many trials; (3) a small number of subjects, usually all right-handed; and (4) an almost complete reliance on reaction time as the dependent measure of cerebral lateralization. These characteristics are common to many experiments reporting significant RVF-LH superiority for verbal tasks. Davis and Schmit (1973), using a Posner (1969) paradigm, collected reaction times to four letters for 256 trials on 16 subjects. Geffen, Bradshaw, and Wallace (1971) measured reaction times to judgments of same-different to four digits presented in pairs for 240 trials on 10 subjects. Geffen, Bradshaw, and Nettleton (1972), using a Posner paradigm, measured reaction times to four letters in pairs for 192 trials on 12 subjects. Gross (1972) measured reaction times on two sets of eight three-letter words (64 combinations) for 1072 trials on 10 subjects. Moscovitch (1972) examined reaction times to tasks requiring same-different matching for l-to-l and l-to-6 letter sets, using 600 trials and six subjects in each condition. Moscovitch (1976) reported results of three reaction time studies, each requiring judgments of letter pairs selected from a 12-letter set. Each study had 480 trials and 16 subjects. Rizzolati, Umilta, and Berlucchi (1971) measured reaction times for recognition of single letters, using four letters and 640 trials on 12 subjects. Rosen, Curcio, Mackavey, and Hebert (1975) examined accuracy of report for two four-letter sets presented simultane-

CEREBRAL LATERALIZATION

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ously, one to each visual field, with sets drawn from a population of 16 letters shown for 40 trials to 16 subjects. Similar characteristics are common to experiments reporting statistically significant LVF-RH superiority. Cohen (1972) measured reaction times to physical identity judgments of letter pairs, using 10 letters in pairs for 320 trials on 12 subjects. Geffen et al. (1971) measured reaction time for recognition of one out of four human faces to a test face for 80 trials on 15 subjects. Rizzolati et al. (1971) measured reaction times for recognition of single human faces, using four faces for 736 trials on 12 subjects. It is equally instructive to examine the few published lateralization experiments reporting no differences in hemisphere function.’ Dimond, Gibson, and Gazzaniga (1972) recorded accuracy of same-different judgments on 200 four-letter words each shown once with a 3-set delay to 13 subjects and found no differences in performance related to hemisphere, but decreased accuracy when the comparison stimulus appeared in a different visual field from the standard. Geffen et al. (1971) measured reaction times for identification of five faces over 20 trials on 14 subjects and found no hemisphere differences. Given the systematic pattern associated with the presence and absence of hemisphere differences, it seems appropriate to question the experimental context within which hemisphere differences are found. The majority of these:experiments clearly indicate a left hemisphere superiority for verbal processes and a right hemisphere superiority for visuospatial functions. It is also clearly evident that experiments reporting such differences use a limited number of relatively simple stimuli and an experimental procedure that allows a subject to become exhaustively familiar with them, undoubtedly affecting the nature of the response. It is difficult to argue that a subject completing his 600th response to one of a stimulus set of 10 items has no automatic or stereotyped characteristics in his responses. The customary practice of eliminating reaction times exceeding some arbitrary time limit only serves to emphasize the automaticity of these response patterns. By contrast, those experiments using sufficient stimuli relative to trials to allow the possibility of new information appearing on each trial find either miniscule or negative results for hemisphere function differences. Given these observed differences in results as related to experimental procedure, it seems appropriate to examine systematically the conditions under which hemisphere function differences are found. The use of bilingual subjects offers a unique opportunity to examine these processes. ’ The experiments reviewed here are limited to those with procedures comparable to experiments reporting significant laterahzation effects. Experiments reporting no lateralization effects but with procedural limitations that could account for lack of effects (such as tachistoscopic exposure times in excess of 150 msec) are not evaluated.

60

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The use of fluent Chinese-English bilinguals allows us to compare semantic judgments of same-different across two languages having similar frequencies of usage of selected common nouns and no orthographic features in common. Since Chinese characters are constructed on different principles than those used in English orthography, we can be quite sure that semantic judgments are not confounded by visual similarities. EXPERIMENT Apparatus

1

and Stimuli

Ninety-six words selected from a list of common Chinese terms (adopted from the norms developed by Liu and Chuang, 1970) and their English equivalents (most English words used have A or AA Lorge-Thomdike ratings) were prepared on 15.24 x 22.86cm white cards. The Chinese stimuli were drawn with a felt tip pen and spanned 84” of visual angle. The English stimuli were done using Letraset 20-point Helvetica and spanned .84 to 1.30” of visual angle at maximum. The Chinese stimuli were all single characters. The English stimuli ranged from three to six letters. There were 21 three-letter words, 30 four; 29 five; and 16 six-letter words. A comparable scaling for Chinese characters is not possible. However, the range of complexity of Chinese stimuli was judged as comparable to the English in terms of visual elements. All stimuli were placed a minimum of 2.5” of visual angle from the point of central fixation. Letter arrangement for English was horizontal. Thirty-two stimuli were presented in the LVF-RH, 32 in the RVF-LH, and 32 had one member of a pair appearing in each visual field (EVF). Pairs in the RVF-LH and LVF-RH were set 50” of vertical visual angle above and below the central fixation point, upper and lower placement being equal for Chinese and English. In the EVF condition, rightleft placement by language was balanced equally. A blank field with a central + was present except during the presentation of a stimulus. Reaction times were measured by an electronic timer with millisecond accuracy, started as a stimulus was shown and stopped by a voice-activated relay.

Procedure At the beginning of the experiment, all subjects were given six practice trials, using stimuli that were not in the experiment. Subjects were asked to focus on the + in the center of the viewing field when the experimenter said “ready” and to respond quickly “yes” if Chinese and English had the same meaning and “no” if they did not. Stimuli were shown for 150 msec at an illumination level of 9 fL. The intertrial interval was approximately 25-30 set after the subject responded. Two random orders of presentation were used, with the restriction that not more than two successive stimuli appeared in any visual field combination. Stimuli were shown once and no stimulus was repeated.

Subjects Eight subjects, fluent in reading Chinese and English, participated. Two subjects had English as a first language and six Chinese as a first language. All subjects had learned their second language after age 15. The Chinese as first language subjects were graduate students at the University of California, Berkeley, with demonstrated proficiency in reading and speaking English. The two English as first language subjects were graduate students in Oriental languages and were assessed for fluency of reading Chinese by one of the experimenters (W. S-Y. W.). All subjects had normal or corrected to normal vision in both eyes and were right-handed, with no known family history of left-handedness.

CEREBRAL LATERALIZATION

61

Results and Discussion

Reaction times were averaged for each subject within each condition, excluding incorrectly answered items. A preliminary analysis of variance done to assess order and position differences indicated no significant effects, allowing the combining of data into a three visual field (RVF-LH, LVF-RH, EVF) x two type (same-different) repeated measures design. Examination of mean values for subjects by conditions to check on possible first language effects indicated no differences. Analysis of reaction times indicated only one significant main effect for type (F 1,7 = 19.90).2As can be seen by inspection of means and standard deviations in Table 1, judgments of different require more time than judgments of same. Visual field-hemisphere effects were not significant, and no interaction effects reached acceptable significance levels. A separate analysis of errors indicated a significant main effect for visual field-hemisphere (F2,14 = 7.06). The total number of errors in the RVF-LH was 46 and for LVF-RH, 47, with EVF errors totaling 19. This result is in accord with findings reported by Davis and Schmit (1971, 1973) who reported that a task distributed over two hemispheres is performed more accurately than when presentation is to only one hemisphere. However, both reaction time and error results are sharply at variance with what would be predicted from existing models of hemisphere function. If we accept the prediction expected from models stating that the left hemisphere has an advantage in verbal processing, we would expect to find a difference in reaction time and accuracy favoring the RVF-LH. With the use of vocal response as a measure of reaction time, it is surprising that some RVF-LH superiority is not found, especially in view of the evidence indicating location of motor speech mechanisms in the left hemisphere. Our results suggest two possibilities; either vocal reaction time is an inappropriate measure for complex semantic judgments, or alternatively, such processes are not highly lateralized. Examination of the error analysis leads to similar conclusions. Under conditions where one hemisphere is specialized for a given type of processing, it seems reasonable to expect that fewer errors would occur if stimuli of a given type are received in an area specialized for their processing. If RVF-LH specialization for language is assumed, the expectation is that stimuli shown in the RVF-LH will be reported more accurately than similar stimuli shown in the LVF-RH. A corollary assumption of the lateralization model is that stimuli received in cortical areas other than that specialized for their processing are immediately transferred to the appropriate area for analysis. If RVF-LH specialization for language processing is assumed, errors should increase 2 All F values equal or exceed a .05 probability level.

62

HARDYCK,

TZENG,

AND WANG

TABLE

1

MEANS AND STANDARD DEVIATIONS OF REACTION TIMES FOR ALL EXPERIMENTS Experiments 1 Condition English English

jc

2 SD

4

3

??

SD

R

SD

ri:

SD

Same

1185

243

1220

180

767

205

Different

1310

297

1017

150

806

202

Same

1176

261

1264

346

715

178

Different

1320

343

1035

150

740

186

Same

1084

241

1048

189

Different

1367

370

1066

183

Same

1093

269

1087

172

854

236

Different

1141

200

1084

151

7%

249

Same

1188

482

1085

194

834

294

Different

1098

190

1127

191

794

223

Same

1057

249

1202

295

Different

1132

240

1107

195

LVF-RH

RVF-LH

EVF

E;;!:;

LVF-RH

RVF-LH

EVF

;rg.e;;

Same

1413

300

1355

436

Different

1955

673

1606

525

Same

1341

249

1175

250

Different

1976

696

1722

561

Same

1240

355

1229

265

Different

2099

955

1584

512

RVF-LH

LVF-RH

EVF

in frequency from RVF-LH to EVF to LVF-RH, since some loss may be expected when information is transmitted to a new location. Instead, we find that the condition with the lowest error rate is the only condition where some sort of interhemispheric transfer must take place in order to reach a decision. One other aspect of these findings deserves emphasis. Other studies of same-different judgments using verbal materials have entertained the

CEREBRAL LATERALIZATION

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possibility that RVF-LH and LVF-RH judgments of near-equal accuracy have involved two differing processes, semantic judgments in the RVF-LH and pattern matching or physical identity judgments in the LVF-RH. Such a hypothesis is irrelevant to the present study since pattern matching is nonexistent. There is no orthographic correspondence between the Chinese character for “hand” and the English word for “hand,” even though the two symbols have the same semantic referent, are unambiguous nouns, and were so judged by our subjects. The possibility remains that these results are artifactual, either an idiosyncratic finding for these particular subjects, or as a confound between judgment processes and the method of measuring reaction time. Another experiment was done to examine these possibilities. EXPERIMENT

2

The results of Experiment 1 are not in accord with any existing models of hemispheric processing, although the finding of lack of hemisphere differences is consistent with other reports of lack of hemisphere function differences when number of stimuli are relatively equivalent. It is of interest to see if hemisphere differences appear when the possibility of judgments based on physical identity are included. Materials Six words were discarded from the original set of 96 stimuli used in Experiment 1. Thirty words were prepared as English-English (EE) pairs, 30 as Chinese-Chinese (CC) pairs, and 30 as Chinese-English (CE) pairs. In the EE and CC stimulus pairs, 15 were identical

FIG. 1. The Chinese character for “hand.”

64

HARDYCK,

TZENG, AND WANG

and 15 had different words of the same length and approximate frequency. For the CE stimulus pairs, 15 had the same meaning and 15 different. Physical placement, positioning, and visual angle in relation to fixation were as described in Experiment 1.

Procedure The procedure was identical to that in Experiment 1, except for reaction time measurement, which was done by pressing one of two buttons on a horizontal panel in front of the subject, so arranged as to allow comfortable arm placement. Subjects were instructed to press one button if the two items of the pair shown were the same and the other button if they were different. The index finger of each hand was used to respond, and the hand used for a given type of response was balanced across subjects. Stimuli were shown once and no terms were repeated. Approximately 5 min after the close of the tachistoscopic procedure, the subjects were asked to write down as many of the words they had seen as they could remember. No prior indication that this would be requested had been given the subjects. Subjects were allowed as much time as they wished for this task, and the time taken ranged from 3 to 6 min.

Subjects Twenty fluent Chinese-English bihnguals participated. Seventeen subjects had Chinese as a first language, and three hand English as a first language. Five of the 20 subjects had participated in the first experiment. All but one subject were graduate students, the exception being a professor of Oriental languages. Seventeen subjects were right-handed with no family history of left-handedness, two subjects were left-handed, and 1 subject was righthanded with 2 left-handed family members.

Results and Discussion Preliminary analyses were done to assess order and position effects and to compare subjects who participated in Experiment 1 with other subjects. No significant effects appeared for any of these comparisons.3 An analysis of language (EE, CC, CE) x visual field (RVF-LH, LVF-RH, EVF) x judgment type (same-different) indicated significant effects for language (F(2,36) = 8.92), type (F(1,18) = 6.90), and the interaction of language and type (F(2,36) = 3.75). No other comparisons reached acceptable significance levels. Inspection of means (Table 1) indicates that judgments are made more quickly for CC comparisons, with EE next, and CE having the longest reaction times. As in Experiment 1, judgments of same require less time than do judgments of different. The language effects seem to be primarily a function of response times to a first language. Subjects are faster at responding to their first language than to a second, or to a combination of first and second, a finding in accord wth a study of bilingual reaction time previously reported 3 All analyses for Experiment 2 were first done on all subjects (N = 20), repeated with the two left-handed subjects and one right-handed subject with left-handed family members removed (N = 17), and with log transformations of reaction times for both N = 20 and N = 17. The resulting F statistics did not differ noticeably. Values reported here are for N = 20 on untransformed data.

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LATERALIZATION

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by Hardyck (in press [b]). The speed advantage for the CC comparisons was most pronounced for the 17subjects having Chinese as a first language. The CC to EE difference was near zero for the three subjects having English as a first language. A similar process may be operative in the interaction between language and type since the effect for language x type is almost entirely a function of the EE and CE judgments. Mean reaction times for same and different in the CC judgments are almost identical. Since no visual field-hemisphere effects were found in the first analysis, subjects were grouped by responding hand, and the analysis was repeated. The logic of this type of comparison has been specified by Moscovitch (1976): “If reaction times favor the same visual field by a constant amount regardless of responding hand, it indicates that some or all of the task must be mediated by the hemisphere contralateral to that visual field. If, however, reaction times favor the visual field on the same side as the responding hand, or if the size of the reaction time difference changes with responding hand, it suggests either that both hemispheres perform the task particularly well or that one does so more efficiently than the other” (p. 49). Results of this analysis were as unsuccessful in detecting hemisphere differences as was the analysis by first language grouping. Significant effects were present for language (F(2,36) = 15.55), type (F 1,18) = 19.60), and language x type interaction (F(2,36) = 10.93). The results are virtually identical to those found in the earlier analysis. Errors in this experiment were not frequent enough to allow analysis. When the free recall data collected at the close of the experiment are analyzed in a language x visual field x type repeated measures design, a significant left hemisphere effect (F(2,28) = 3.60) is present for Chinese. The mean number of Chinese characters recalled was 5.40 for RVF-LH and 3.67 for LVF-RH. The corresponding values for English are 2.93 for RVF-LH and 2.40 for LVF-RH. Words most often recalled are those originally presented in the RVF-LH, suggesting that lateralization effects may indicate memory location within a hemisphere rather than an active lateralized processing of new information. Such an interpretation requires a reassessment of the concept of cerebral lateralization. In the great majority of research reports on this topic, the assumption is made that hemisphere differences in reaction time reflect the specialization of a particular hemisphere for active cognitive processing. When studies reporting significant lateralization effects are examined for similarities, significant lateralization effects are most pronounced when subjects are exposed to relatively few stimuli for a great many trials. This type of experimental paradigm allows the subject to retain all possible response alternatives in memory and allows his response to become a straightforward matching process. Under such conditions,

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no problem-solving ability is required. The response characteristics seem analogous to responding to a question such as 6 x 4 = ? by retrieving the correct answer from a table of values stored in memory. In the work reported here, using new information on every trial, no such strategy is possible and no lateralization effects are detectable, suggesting that immediate processing of new information is not a highly lateralized process. In this interpretation, lateralization effects as indicated by reaction times are primarily a function of cerebral lateralization of certain kinds of memory. Although immediate processing of new language information may not be lateralized, the location of memory for language may be highly lateralized in the left hemisphere. Correspondingly, visuospatial and pattern memory may be lateralized in the right hemisphere. Such a formulation is experimentally testable. In Experiments 1 and 2, no lateralization effects were found under conditions where new stimuli were presented on every trial. If judgment tasks are simplified to permit judgments of same-different on a semantic and pattern-matching basis versus a pure pattern-matching basis, the prediction derived from current interpretive statements would be that left hemisphere performance should be superior where semantic and pattern information are complementary. For a pattern-matching comparison with no semantic component, right hemisphere performance should be superior. These conditions should hold even if new information is presented on every trial. If evaluation of new information is not a lateralized process, there should be no hemisphere effects, with results being similar to those found for Experiments 1 and 2. If lateralization effects are primarily a function of memory storage, a small subset of the same stimuli shown for a large number of trials should show the decreased reaction times-by-hemisphere effect reported by many investigators. The following two experiments were performed to test such a memory model of cerebral lateralization. Experiment 3 used a relatively large stimulus set and Experiment 4 a small number of stimuli. EXPERIMENT

3

Materials and Procedure The procedure was identical to that of Experiment 2. Stimuli were the 30 EE and CC pairs used in Experiment 2, rerandomized. Each stimulus was shown once, with no repetition.

Subjects Eight graduate students in psychology and education particpated. All subjects were righthanded, with no knowledge of spoken or written Chinese.

67

CEREBRAL LATERALIZATION

Results A Z(language) x 3(visual field) x Z(type) repeated measures analysis indicated no significant effects for any condition or interaction. Judgments were equally accurate for Chinese and for English. Errors were too few to allow analysis.

EXPERIMENT Materials

4

and Procedure

In this experiment, only eight stimulus pairs were used. There were four Chinese characters and four English words. Within each language, every word was paired with every other word for the “different” stimulus pairs and with itself for “same.” Stimuli were shown in the RVF-LH and LVF-RI-I only for 200 trials, order of presentation being randomized for each subject. With these exceptions, procedure was identical to that for Experiment 2.

Subjects Subjects were eight graduate students in psychology, all right-handed, with no knowledge of spoken or written Chinese.

Results

Reaction times were combined into blocks of five trials with incorrect responses discarded. Errors were too few to allow analysis. A language x visual field x type x blocks of trials analysis revealed significant effects for language (F(1,7) = 16.14), visual field (F(1,7) = 8.49), trials (F(4,28) = 3.12) and the interaction of language x trials x visual field (F(4,28) = 4.00). Inspection of mean values indicated a faster reaction time to EE stimuli in the RVF-LH and a similar though smaller effect for CC stimuli in the LVF-RH. Results are illustrated in Fig. 2. Reaction time by hemisphere effects increase systematically over trials, CHINESE

ENGLISH

1000

-low

t CRS

- 600 D= Different

2

3 Trial

FIG.

4

5

I

2

blocks

2. Interaction of language x visual field

3 Trial

x

4

5

blocks

trial blocks in Experiment 4.

68

HARDYCK,

TZENG, AND WANG

producing lateralization differences quite similar to those reported in the studies reviewed earlier in this report. GENERAL

DISCUSSION

In the majority of interpretive models of cortical processing an individual listening to spoken speech or engaged in reading is hypothesized as having language information sent to the left hemisphere for processing, semantic analysis, and understanding. An appropriate language response is also hypothesized as being generated within the left hemisphere, a speech response to listening to speech, or for reading, formulating some train of thought in response to the information gained. A cerebral function model requiring that all information of a given type be sent to a certain area for analysis seems analogous to an inexpensive pocket calculator containing circuitry for certain types of operations but which requires that material to be processed be sent to a specific location. A more sophisticated type of computing process, where operations are dependent on current demands and instructions and capable of conditional changes and reorganizations as needed, seems a more appropriate analog to the process our subjects displayed in our first three experiments. The only evidence for lateralization of language is found in the analysis of the free recall data of Experiment 2, where significantly more words first shown in the RVF-LH were recalled. When new information is present on every trial, no lateralization effects are detectable.4 These results suggest that lateralization effects, as often reported, are not a function of immediate cognitive processing in specialized cortical areas, but do reflect differences in memory storage locations. The experiments reported here and those reviewed are consistent with such a formulation. If a subject has to evaluate new information on each trial, his reaction times do not differ systematically with visual field presentation. Even with the known condition that all judgments are verbal, there is no indication of a RVF-LH superiority. Whatever processes are active during the evaluation of new information seem to be relatively independent of hemisphere location. By contrast, when the set of stimuli are known to the subject to the extent that no new information is received for several trials, a change in processing strategy seems to take place, with the subject responding by a process that is better described as referencing a table of known values than as thinking. It would be of interest to reanalyze data from studies reporting lateralization effects to see if the trends found in our Experiment 4 are present in other studies. For example, the recent work of Moscovitch 4 One cautionary note is perhaps overdue in this discussion: These findings are applicable only to visual half-field experiments. Although the same phenomenon may be present in dichotic listening, we have no evidence relevant to this type of investigation.

69

CEREBRAL LATERALIZATION

et al. (1976), suggesting “precategorical processing” common to both hemispheres, may represent the initial stage in their experiments where all the stimuli to be presented were not yet known by the subjects. As the subjects become familiar with the material presented, the precategorical process may change to a “table reference” strategy, with a consequent drop in reaction times. An activation hypothesis, such as that proposed by Kinsbourne (1970, 1973) also seems inadequate to explain our results. A reasonable expectation for activation would be that a hemisphere specialized for the processing of verbal material would become activated by foreknowledge of having to evaluate verbal material. Our subjects knew exactly what type of task they were to perform and were given practice trials with stimuli identical to the type used in the experiment. Such a procedure should be sufficient to produce activation of an area specialized for language processing. Our results offer no support for the concept of activation. While our results are not supportive of much of current thinking on the topic of cerebral lateralization, they do not, in our opinion, diminish the importance of the study of lateralization processes. The use of such information to determine when a subject shifts from a strategy of evaluating each stimulus as a new event to the strategy of referencing a known set seems to offer a number of interesting possibilities for experimentation. A revised conceptualization of lateralization may be of help in the study of aphasia. Individual differences in lateralization of memory phenomena, such as are shown by many of the left-handed (Hardyck, in press [a]), are also of considerable interest. Such a reorientation may be of value in interpreting hitherto puzzling findings such as that of Bever and Chiarello (1974) who reported musical lateralization effects to differ between musically knowledgeable and musically untrained subjects. A model postulating memory as a lateralized function conceptually separable from active cognitive processing seems to offer more explanatory possiblities than one requiring that particular kinds of processing are limited to specific locations. REFERENCES Bever, T., & Chiarello, R. J. 1974. Cerebral dominance in musicians and nonmusicians. Science, 185, 537-539. Broadbent, D. 1974. Division of function and integration of behavior. In F. 0. Schmitt and F. G. Worden (Eds.), The neurosciences: Third study program. Cambridge, MA: MIT Press. Brunswik, E. 1949. Systematic and representative design of psychological experiments. Berkeley, CA: University of California Press. Brunswik, E. 1952.The conceptual framework of psychology. In International encyclopedia of uni$ed science. Chicago: University of Chicago Press. Cohen, G. 1975. Hemisphere differences in the effects of cuing in visual recognition tasks. Journal

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