Platform Session 5: Suprasegmental Processing and Right Hemisphere Language

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PLATFORM SESSION 5 1. Discourse Encoding Strategies of Right Hemisphere Damaged Patients

Debra Titone,* Arthur Wingfield,* David Caplan,† Gloria Waters,‡ and Kristen Prentice* *Brandeis University, †Massachusetts General Hospital, and ‡McGill University

Introduction. Right hemisphere (RH) functioning is involved in a number of tasks involving higher-order use of lexical-semantic information during discourse comprehension. RH damaged adults, for example, have difficulties comprehending gist or thematic information from texts, recall fewer connections between story elements than age-matched controls, and confabulate relationships between story elements more than age-matched controls (e.g., Hough, 1990; Lojek-Osiejuk, 1996). RH damaged adults also overutilize explicit microstructural information in their recall such as pronoun assignment and underutilize the general context or mood of discourse (Brownell, Caroll, Rehak, & Wingfield, 1992; Rehak, Kaplan, & Gardner, 1992). The present study furthers this work by investigating the specific encoding strategies that RH damaged adults employ during discourse comprehension. To this end, we used a self-paced listening paradigm in which RH damaged adults heard recorded speech interrupted at various linguistic boundaries for later recall. We assumed that the way in which listeners control the flow of speech (i.e., pause between consecutive segments) is a behavioral marker of how processing resources are allocated during comprehension. On the basis of previous studies of discourse comprehension and memory impairments accompanying RH damage, one would make two predictions regarding the pattern of self-paced encoding for RH damaged adults. Pause durations of RH damaged adults should be responsive to low-level text variables (e.g., segment length in syllables, word frequency, syntactic boundary type, and speech rate). Second, their pause durations should be less responsive to high-level text variables (e.g., passage complexity, propositional density, the number of segments accumulated within a sentence, and the number of segments accumulated across a passage). Method. Stimuli: Eight 150-word high and low complexity passages were

This research was supported by NIH grant AG00204 to the first author. We also acknowledge support from NIH grants AG04517 and AG09661. Finally, the authors gratefully acknowledge support from the W. M. Keck Foundation.

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FIG. 24. Mean pause duration in milliseconds as a function of passage complexity, speech rate, and boundary type for right hemisphere damaged adults.

recorded, and speech editing software was used to segment the passages and accelerate the speech rate of segments. Segmentations occurred at within clause boundaries (22.3%), at the end of clause boundaries (41.4%), and at the end of sentence boundaries (32.3%). Procedure: Twelve RH lesion patients between the ages of 44 and 77 heard the passages in a segment by segment fashion, and initiated presentation of the each segment by pressing the spacebar of a computer keyboard. Passage complexity, speech rate, and order of presentation were counterbalanced across subjects and passages. Results. Pause durations shown in Fig. 24 were analyzed using ANOVA as a function of passage type (low and high complexity), boundary type (within clause, end of clause, and end of sentence boundaries) and passage position (first third, second third, and final third). Pause durations for normal rate passages were faster than those for accelerated passages, F(1, 11) 5 7.61, p , .05, and were slower as importance of the syntactic boundary increased, F(2, 22) 5 4.61, p , .05. Pause durations for within clause boundaries were faster as more of the passage was heard, but were unaffected for end of clause and end of sentence boundaries, F(4, 44) 5 2.78, p , .05. There were no significant effects of passage complexity. In addition to the ANOVA, pause durations were analyzed using a hierarchical multiple regression. A set of low- and high-level characteristics of the texts were separately entered as predictor variables. Results indicated that the following low-level text variables significantly predicted pause durations: number of syllables, word frequency, boundary type, and speech rate. In contrast, of the high-level text variables, only the number of preceding segments within a passage significantly predicted pause durations. RH damaged adults’ pause durations became shorter as more of the passage was heard.

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Conclusions. These data showed that encoding strategies of RH damaged adults are sensitive to low-level aspects of discourse. Moreover, their pattern of pause durations shows a sensitivity to low-level text variables that our previous work suggests does not occur for healthy elderly adults (e.g., speech rate; Titone, Prentice, & Wingfield, 1996). In contrast, we found that pause durations of RH damaged adults are unaffected by increases in passage complexity, propositional density, or the number of preceding words within a sentence; all high-level text variables. Finally the regression analysis suggested that pause durations of RH damaged adults decrease as more of the passage is heard. This again, is in contrast to what we have found for healthy elderly adults whose pause durations increase as more of a passage is heard. In the case of normal elderly adults, this reflected an increase in the resources necessary to build a complex representation of discourse material as it accrues over time. The present data from RH damaged adults suggest that they build such a discourse representation during comprehension to a lesser degree. REFERENCES Brownell, H. H., Carroll, J. J., Rehak, A., & Wingfield, A. 1992. The use of pronoun anaphora and speaker mood in the interpretation of conversational utterances by right hemisphere brain damaged subjects. Brain and Language, 43, 121–147. Hough, M. S. 1990. Narrative comprehension in adults with right and left hemisphere braindamage: Theme organization. Brain and Language, 38, 253–277. Lojek-Osiefuk, E. 1996. Knowledge of scripts reflected in discourse of aphasics and rightbrain-damaged patients. Brain and Language, 53, 58–80. Rehak, A., Kaplan, J. A., & Gardner, H. 1992. Sensitivity to conversational deviance in right hemisphere-damaged patients. Brain and Language, 42, 203–217. Titone, D., Prentice, K., & Wingfield, A. 1996. Self-paced listening in discourse comprehension: Speech Rate, passage complexity, and age. Paper presented to the 37th Annual Meeting of the Psychonomic Society, Chicago, Illinois.

2. Discourse Comprehension in Alzheimer’s Disease: The Effects of Pronouns vs Nouns

Amit Almor, Daniel Kempler, Maryellen MacDonald, and Elaine Andersen Program in Neural, Informational and Behavioral Sciences, University of Southern California

Excessive use of pronominal reference in dementia of the Alzheimer’s type (DAT) has been previously attributed to word finding difficulty (Kempler, 1995) and pragmatic deficits (Ripich & Terrel, 1988). However, in light of the widely assumed role of working memory in maintaining reference

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(Daneman & Carpenter, 1980), and the working memory deficits that are widespread in DAT (Baddeley, 1992), the overuse of pronouns may also be attributed to difficulty in maintaining an active representation of a referent over time. The two comprehension experiments presented here were designed to investigate the degree to which AD patients are able to maintain reference over two sentences, and how this ability is affected by the type of reference (nouns vs pronouns). A comprehension paradigm was chosen because it places no production demands on patients and thus clearly differentiates between the word finding difficulty and the reference maintenance difficulty explanations: If patients’ problems with pronouns are due to word finding difficulties, normal comprehension is predicted; if the problems are due to reference maintenance difficulty they will have poor comprehension. Experiment 1. Experiment 1 investigates whether or not DAT patients show a pronoun comprehension deficit relative to healthy aged controls, and if so, whether this deficit is associated with degraded representation of referents. Subjects: Five patients diagnosed with Probable Alzheimer’s disease and ten age-matched healthy controls participated. Procedures and Materials: A cross-modal naming paradigm was used to assess reference maintenance. The task required subjects to listen to a context sentence followed by a sentence fragment, and then read aloud a visually presented single word continuation. Half of the continuations (in caps) were referentially appropriate while half were not. Context sentence: Sentence fragment:

The children loved the silly clown at the party. During the performance, the clown threw candy to Appropriate continuation: THEM. Inappropriate continuation: HIM.

The inappropriate continuations were always anomalous with regard to pronominal number (HIM vs. THEM). There were 20 appropriate/inappropriate pairs, resulting in 40 stimuli. Reaction times to reading appropriate vs. inappropriate continuations were used to indicate whether subjects were able to correctly interpret the pronominal reference as co-referring with a noun in the preceding sentence. Slowed reaction times to the inappropriate (compared to the appropriate) continuations indicate that reference was maintained. Reaction times were normalized with z-scores with respect to subject and session to minimize the effect of reaction time differences between (fast and slow) testing sessions and (fast and slow) subjects. Results: A 2 3 2 ANOVA (Population by Appropriateness) revealed an interaction between population and appropriateness, F(1, 13) 5 4.93, p , .05, such that normal controls were very sensitive to the appropriateness of the pronoun, p , .01 in a Tukey post hoc test, but DAT patients were not, p . .05. This pattern of results shows that DAT patients are impaired in

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processing pronouns in a cross-modal naming task. Given that this experiment did not require word finding, the DAT impairment observed in this experiment must be attributed to a failure to maintain the representation of referents, and not to a word finding difficulty. Experiment 2. If DAT patients have a difficulty maintaining active representation of referents over time, it is plausible that the manner by which repeated reference is made (full nouns vs pronouns) would affect DAT patients’ processing. In particular, we expect that DAT patients would show better performance with more specified references (i.e., full nouns) than with informationally impoverished references (i.e., pronouns), as the former kind of reference would be more effective in reactivating the representation of the antecedent. For normals, however, we expect the type of reference to have little effect on comprehension. Experiment 2 used a cross-modal naming paradigm and had 20 pairs of experimental items with the following structure: a context sentence introduced two human participants, a male and a female. The participant appearing as the object in the leading sentence was modified with a pre-nominal adjective. A second sentence fragment referred to both participants with either the same nouns, or pronouns. The visual target ending the fragment (in caps) was always an adjective modifying the object of the leading sentence. Context sentence: Full noun condition: Pronoun condition:

The housewife watched the clumsy plumber working under the sink. The housewife could not believe that the plumber was so CLUMSY. She could not believe that he was so CLUMSY.

An advantage due to the presence of a noun vs. pronoun would be shown by shorter RTs in that condition. Results: A 2 3 2 ANOVA with factors Population (DAT, Normals) and Reference-Type (Noun vs. Pronoun) revealed an interaction of population and reference type, F(1, 13) 5 5.72, p , .05, such that DAT subjects were faster in the noun condition than in the pronoun condition, p , .05 in a Tukey post hoc test, but NCs showed no difference, p .. 05. This pattern is compatible with our prediction that DAT patients would benefit from a more specified reference, while normals show no sensitivity to the type of reference. Conclusion. We have demonstrated that DAT patients are impaired in their ability to maintain reference across sentences, and that this deficit is linked to a difficulty in processing pronouns. For DAT patients, full nouns activate the representation of the referent significantly better than pronouns. Therefore, it is also likely that in production, the overuse of pronouns reflects degraded representation of the referent and not only word-finding difficulty. The fact that DAT patients overuse pronouns and also show reduced compre-

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hension when processing pronouns, carries significant implications for theories of reference use in general. REFERENCES Baddeley, A. 1992. Working memory. Science, 255(5044), 556–559. Daneman, M., & Carpenter, P. A. 1980. Individual differences in working memory and reading. Journal of Verbal Learning and Verbal Behavior, 19, 450–466. Kempler, D. 1995. Language changes in dementia of the Alzheimer type. In R. Lubliski (Ed.), Dementia and Communication, pp. 98–114. San Diego: Singular. Nicholas, M., Olber, L., Albert, M., & Helm-Estabrooks, N. 1985. Empty speech in Alzheimer’s disease and fluent aphasia. Journal of Speech and Hearing Research, 28, 405–410. Ripich, D. N., & Terrell, B. Y. 1988. Patterns of discourse cohesion and coherence in Alzheimer’s disease. Journal of Speech and Hearing Disorders, 53, 8–19. Ulatowska, H., Allard, L., & Donnell, A. 1988. Discourse performance in subjects with dementia of the Alzheimer type. In Whitaker, H. (Ed.), Neuropsychological studies in nonfocal brain damage, pp. 108–131. New York: Springer-Verlag.

3. Speech in the Disconnected Right Hemisphere

Eran Zaidel University of California, Los Angeles

and Laura Seibert Program in Communication Sciences and Disorders, The University of Texas at Austin

Introduction. It has been known for a long time that complete commissurotomy patient LB of the Los Angeles series can sometimes name pictures and words shown in his left visual hemifield (LVF), i.e., to his nondominant, right hemisphere (RH), although he could never match those stimuli across the vertical meridian (e.g., Butler & Norsell, 1968; Johnson, 1984; Zaidel, 1994). This could reflect, in decreasing order of likelihood, (1) cross-cueing of the ‘‘ignorant’’ left hemisphere (LH) by the informed RH, (2) subcallosal phonological or semantic, but not visual, transfer from the RH to the LH, (3) ipsilateral sensory projection from LVF to the LH via the superior colliculus, or (4) RH speech (Zaidel, 1990). McKeever et al. (1982) showed progressive LVF naming in callosotomy patient POV (VP) from the Toledo series. These authors interpreted their results as evidence for emerging RH speech, but the patient may have remnants of posterior callosal fibers and thus may exhibit only partial disconnection (Gazzaniga et al., 1985). Indeed, Intriligator et al. (1995) described

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a patient with a posterior callosal lesion who could name LVF stimuli but not compare visual stimuli across the two VFs, presumably because different callosal channels are implicated in those two functions. Baynes et al. (1995) observed similar and progressive partial naming of LVF stimuli by callosotomy patient JW from the Hanover series and concluded that he demonstrates RH speech. However, their series of naming and matching experiments are equally consistent with subcortical transfer and LH speech. Here we report three new experiments to determine whether occasional naming of LVF stimuli by LB is due to LH or RH speech. We reasoned that if LVF naming is done by the RH then it should be unaffected by the presence and type of distractors in the RVF, whereas if LVF naming is done by the LH then it should be selectively impaired as a function of the presence and type of distractors in the RVF. In the first experiment we examined LB’s ability to name LVF words (1) without RVF distractors, and (2) with figural RVF distractors. In the second experiments, we examined LB’s ability to name LVF words (1) without RVF distractors, and (2) with lexical RVF distractors consisting of words and nonwords. In the first two experiments, trials with LVF or RVF targets, with and without distractors were intermixed. The third experiment was identical to the second experiment, except that trials with LVF and RVF targets with and without distractors were blocked in order to study the effect of attention on RH speech. Methods. In the first experiment, we presented a list of 192 words, half in the LVF and half in the RVF, half with and half without graphic distractors in the opposite VF. The distractors were nonverbal geometric shapes. The experiment was repeated once. In the second experiment, we presented 12 lists of 96 words each. Half of each list consisted of (underlined) targets in the LVF and half of targets in the RVF. Half of the targets were unilateral and half had distractors in the other VF. The target words varied in length, frequency (high or low), and grapheme-phoneme regularity (regular and irregular). Half of the distractors were words varying in the same dimensions, and half were nonwords matching the targets in length. The experiment was repeated in another session, three months later. In the third experiment, we presented the same stimuli as in the second experiment but VF of target and presence of distractor were blocked. Conditions were presented in the following order: LVF targets without distractors, LVF targets with RVF distractors, RVF targets without distractors, and RVF targets with LVF distractors. For statistical analysis in the first and third experiments, we used the binomial z-test for differences between proportions. The 12 word lists served as the random factor in an ANOVA for the second experiment. The percentage of prompt and correct name responses for LVF-target trials served as the dependent variable in all three experiments.

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FIG. 25. (a) Experiment 1: figural distractors, mixed presentation. (b) Experiment 2: lexical distractors, mixed presentation. (c) Experiment 3: lexical distractors, blocked presentations.

Results and discussion. In the first experiment, the first session showed better naming without (35.4%) than with (16.7%) distractors (z 5 2.093, p 5 0.0364). This pattern is consistent with LH naming. The difference disappeared in the second session (35.4% without distractors, 39.6% with distractors), suggesting RH naming (Fig. 25a). Thus, the first experiment shows a shift from LH to RH naming. In the first session of the second experiment, there was no difference in naming of LVF targets with (33%) and without (32%) distractors. This suggests RH naming of LVF targets (Fig. 25b). There was a frequency effect (F(1, 11) 5 11.793, p 5 .0056) (better reading of frequent than infrequent words) but no regularity effect (F(1, 11) 5 .08, p 5 .7822) (equal reading of regular and irregular words) in this session. This is consistent with the finding that LB’s RH has access to a lexical route but not a nonlexical route in reading (Zaidel, in press). Neither target frequency nor regularity interacted with the presence of distractors, suggesting that the RH named most LVF words regardless of whether they were presented alone. In the second session of the second experiment, prompt naming of LVF words was significantly better without (43%) than with (27%) verbal distractors (F 5 (1, 11) 5 10.089, p 5 .0088), supporting LH naming (Fig. 25b). Thus, the second experiment also shows a shift of control of reading LVF words from one hemisphere to the other; in this case, the shift is from the RH to the LH. There was an overall frequency (F(1, 11) 5 24.971, p 5 .0004) as well as a regularity (F(1, 11) 5 132.0, p 5 .0001) effect in this session. This is consistent with the finding that LB’s LH has access to both a lexical and nonlexical route and that both routes operate in parallel (Zaidel, in press). Neither target frequency nor target regularity interacted with the presence of distractors, suggesting that the LH named most LVF words, regardless of whether they were presented alone. An ANOVA incorporating both sessions of the second experiment showed a significant interaction between session (1, 2) and presence of distractor (F(1, 11) 5 13.2, p 5 .0039). Another ANOVA of the two sessions of the second experiment showed no frequency 3 session interaction (p 5 .9957)

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but a significant regularity 3 session interaction (F(1, 11) 5 10.746, p 5 .0074). In the third experiment, prompt naming of LVF targets with RVF distractors (64%) was significantly better than naming of LVF targets without distractors (56%) (z 5 2.03; p , .05), suggesting that RH naming is actually inhibited by the LH, and that LH inhibition is mitigated by RVF distractors (Fig. 25c). In all three experiments, RVF naming was essentially errorless with or without distractors (Expt. 1: 100%, 100%; Expt. 2: 99%, 98%; Expt. 3: 97%, 98%). Conclusions. LB used a variety of strategies for naming LVF words. Analysis of his nonprompt or incorrect LVF namings showed that often he used a letter-by-letter verbal cross-cueing of the uninformed LH by the knowledgeable RH. On a few occasions he had semantic paralexias, showing subcallosal semantic transfer from the RH to the LH or a greater propensity by the RH for such errors. However, frequently LB named LVF words promptly and this sometimes reflected LH naming, perhaps via ipsilateral visual projection from the LVF to the LH. On other occasions, prompt naming of LVF words reflected RH reading. Since LB’s RH has no grapheme-phoneme translation rules (Zaidel & Peters, 1981), RH reading aloud must use addressed phonology mediated by the lexical route of print to sound. Thus the RH appears to possess the capacity for speech but this capacity is expressed only sporadically. RH naming is sensitive to attention and may sometimes be inhibited by the LH. Genetically specified LH specialization for language (speech) appears to determine hemispheric dominance for performance but allows the development of bilateral competence. REFERENCES Baynes, K., Wessinger, C. M., Fendrich, R., & Gazzaniga, M. S. 1995. The emergence of the capacity to name left visual field stimuli in a callosotomy: Implications for functional plasticity. Neuropsychologia, 33, 1225–1242. Butler, S. R., & Norsell, U. 1968. Vocalization possibly initiated by the minor hemisphere. Nature, 220, 793–794. Gazzaniga, M., Holtzman, J., Deck, M., & Lee, B. 1985. MRI assessment of human callosal surgery with neuropsychological correlates. Neurology, 35, 1763–1766. Intriligator, J., Hanaff, M. A., & Michel, F. 1995. A patient suffering from damage to the posterior portion of the corpus callosum can name items in both visual fields but cannot report whether they are the same or different. Society for Neuroscience Abstracts, 36, 5470. Johnson, L. E. 1984. Vocal responses to left visual field stimuli following forebrain commissurotomy. Neuropsychologia, 22, 153–166. McKeever, W., Sullivan, K., Ferguson, S., & Rayport, M. 1982. Right hemisphere speech development in the anterior commissure-spared commissurotomy patient: A second case. Clinical Neuropsychology, 4(1), 17–22. Zaidel, E. 1990. Language functions in the two hemispheres following cerebral commissurot-

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omy and hemispherectomy. In F. Boller & J. Grafman (Eds.), Handbook of Neuropsychology (Vol. 4, pp. 115–150). Amsterdam: Elsevier. Zaidel, E. 1994. Interhemispheric transfer in the split brain: Long-term status following complete cerebral commissurotomy. In R. H. Davidson & K. Hugdahl (Eds.), Brain Asymmetry (pp. 491–532). Cambridge: MIT Press. Zaidel, E., & Peters, A. 1981. Phonological encoding and ideographic reading by the disconnected right hemisphere: Two case studies. Brain and Language, 14, 205–234. Zaidel, E. In press. Language in the right hemisphere following callosal disconnection. In H. Whitaker and B. Stemmer (Eds.), Handbook of Neurolinguistics. New York: Academic Press.

4. A PET Investigation of Speech Prosody in Tone Languages

Jack Gandour Purdue University

Donald Wong Indiana University School of Medicine

Diana Van Lancker University of Southern California School of Medicine

and Gary Hutchins Indiana University School of Medicine

PET studies have demonstrated activation of right prefrontal cortex in monolingual speakers of English when asked to judge whether pairs of monosyllables differed in pitch, consistent with the importance of righthemisphere mechanisms in pitch perception (Zatorre et al., 1992, 1996). As English does not use pitch phonemically, Zatorre et al.’s two activation conditions obtaining consonant and pitch judgments from native speakers of English, are limited in how much information they can give regarding the functional roles of pitch and segmental phonetic information in linguistic processing, and in hemispheric processing of auditory stimuli. Tone languages, i.e. languages in which pitch patterns are phonologically significant at the syllable level, are ideal for investigating neural substrates of linguistic and nonlinguistic processing mechanisms. The primary aim of this pilot study is to examine the effect of language experience (Thai, English) in how the brain processes pitch in linguistic and nonlinguistic context. It is hypothe-

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sized that pitch will show increased activation in the left hemisphere for Thai speakers only. Method. Ten right-handed, musically untrained subjects, five Thai and five English, participated in the study. A total of four stimulus conditions—baseline, pitch, consonant, tone—were presented twice to each subject for a total of 8 tasks in a single session. Stimuli for the consonant and tone conditions consisted of natural speech Thai words; stimuli for the pitch condition consisted of low-pass filtered versions of the same words. Auditory stimuli were presented in pairs for discrimination judgments. All stimuli were presented binaurally via foam insert earphones at a comfortable listening level. PET imaging was performed using a Siemens 951/31R. PET image processing involved averaging of subtraction images across subjects and statistical testing for identification of brain regions demonstrating task specific significant changes in blood flow. The following five paired-image subtractions were performed on averaged group data: Tone–Pitch (T–P), Tone–Consonant (T– C), Tone–Baseline (T–B), Consonant–Baseline (C–B), and Pitch–Baseline (P–B). Behavioral measures (response time, response accuracy) were also collected during the scanning period. Results. Significant CBF increases were observed in the left frontal operculum (BA 44/45) in both the T–P and T–C subtraction conditions for the Thai group only. In the T–B subtraction, stereotaxic coordinates of the focus in the inferior frontal gyrus (BA 44) for the Thai group were also quite close to those observed in T–P and T–C. In contrast, unilateral CBF increases in the left inferior frontal gyrus were not found in the P–B subtraction for the Thai subjects. Behavioral measures of task performance (response accuracy [d′], response bias [β]; response times) yielded no significant group effects. The three subtractions with the baseline condition (T–B, C–B, P–B) yielded bilateral activation in the superior temporal gyrus (BA 22) for both Thai and English subjects. Also prominent for both Thai and English was significant activation in the left anterior cingulate gyrus (BA 6/32) across subtraction conditions (T–P, T–B, C–B, P–B). Areas of blood flow increase were also observed in the right cerebellum for both Thai (P–B) and English subjects (T–B, P–B). Discussion. The major finding of this pilot study is that significant activation in a portion of Broca’s area was observed only when pitch patterns were presented in a linguistic context to speakers of a tone language. Pitch patterns presented in a nonlinguistic context did not yield similar patterns of activation foci in the left frontal cortex for either the Thai or English subjects. Taken together, these findings emphasize the importance of functional properties of auditory stimuli in mediating the perception of speech prosody, and furthermore, support a model whereby articulatory processes, whether they be segmental or suprasegmental, implicate portions of Broca’s area (cf. Za-

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torre et al., 1992, 1996). This hypothesis is consistent with the motor theory of speech perception (Liberman & Mattingly, 1985), which proposes that phonetic decoding depends on access to information, segmental or suprasegmental, about the articulatory gestures associated with a given speech sound. The differential patterns of brain activation as a function of task and language group cannot be attributed to differences in task performance. No significant group differences were observed on any of our behavioral measures, surprisingly, even the accuracy of tonal discrimination. Yet there are robust, neurophysiological effects of language experience on the processing of these complex auditory stimuli. Significant activation in a portion of Broca’s area (BA 44) in the T–C subtraction for the Thai group may perhaps be neurophysiological evidence of a segmental/suprasegmental distinction in the speech stream. Consistent with problem-solving deficits observed in brain-damaged patients, activation of prefrontal association cortex (BA 10) in the same T–C subtraction is likely related to enhanced processing demands on Thai subjects in making phonetic discrimination judgments in the tone task. Bilateral activation at several foci in the superior temporal gyrus (BA 22) was observed across baseline subtractions for both Thai and English subjects. This suggests that neural processes in the superior temporal gyri are initially responsible for perceptual analysis of suprasegmental as well as segmental properties of complex auditory stimuli. Increased blood flow to the anterior cingulate cortex (BA 6/24/32), which was observed across tasks and groups, is likely to be attributed to the subjects’ receiving of instructions before the task, their preparation, planning, and anticipation of the cognitive task, rather than task-related processing itself (Murtha et al., 1996). Additional significant peaks of activation in the right cerebellum are likely related to the motor demands associated with right-hand key pressing. REFERENCES Liberman, A., & Mattingly, I. 1985. The motor theory of speech perception revised. Cognition, 21, 1–36. Murtha, S., Chertkow, H., Beauregard, H., Dixon, R., & Evans, A. 1996. Anticipation causes increased blood flow to the anterior cingulate cortex. Human Brain Mapping, 4, 103– 112. Zatorre, R., Evans, A., Meyer, E., & Gjedde, A. 1992. Lateralization of phonetic and pitch discrimination in speech processing. Science, 256, 846–849. Zatorre, R., Meyer, E., Gjedde, A., & Evans, A. 1996. PET studies of phonetic processing of speech: Review, replication, and reanalysis. Cerebral Cortex, 6, 21–30. ARTICLE NO. BL971916