Hemispheric specialization for pitch and "tone": Evidence from Thai

Introduction In past dichotic listening studies, linguistic stimuli have shown a right ear advantage, implying left hemisphere dominance for language processing, while other stimuli incorporating pitch distinctions have shown no ear preference or a left ear (right hemisphere) advantage. An experiment was devized to compare ear preferences in tone language speakers for thl,"ee sets of stimuli: pitch differences within language stimuli (tone-words in the tone language, Thai); language stimuli without pitch differences (consonant-vowel words on mid­ tone); and pitch differences alone (hums). Results from 22 native Thai speakers demonstrate that tone-words and consonant-words are better heard at the right ear, while the hums show no ear effect. Preliminary results on English-speaking subjects suggest that the consonant-words give the usual right ear effect, while the tone-words and the hums do not. This study leads to the conclusion that pitch discrimination is lateralized to the left hemisphere when the pitch differences are linguistically processed. Since the discovery by Broca more than a century ago (1861 and Bonin, 1960) that lesions in the left hemisphere produce language deficits while lesions in the right do not, lateraliza­ tion of language processing has been investigated in patients and in normal subjects by a variety of means. It has been repeatedly demonstrated that for most normal, right handed people, the left hemisphere is the dominant language hemisphere. One research technique used is dichotic listening, in which two different stimuli are presented simultaneously to the right and left ears of a subject wearing stereo headphones. Data accumulated over the past decade in this experimental paradigm confirm the belief that language is lateralized to the left cerebral hemisphere; these data include a consistent right ear preference for language stimuli. Investigators have demonstrated a right ear superiority for dichotically presented digits (Broadbent, 1954; Kimura, 1961), nonsense words (Curry, 1967), nouns (Borkowsky, Spreen & Stutz, 1965; Pettit & Noll, 1972), consonant-vowel syllables (Studdert-Kennedy & Shankweiler, 1970; Berlin et al., 1972), backwards speech (Kimura & Folb, 1968), Morse Code signals (Pap9un et al., 1971; 1972) and sentences (Zurif & Sait, 1969). It has also been shown that when simultaneous visual stimuli are presented tachisto­ scopically, the right visual field (left hemisphere) is superior for verbal stimuli, suggesting that it is language, rather than the acoustic stimulus alone, that is lateralized (Faglioni, 7 102 Diana Van Lancker and Victoria A. Fromkin Scotti & Spinnler, 1969). This conclusion is further supported by the fact that evoked potential responses are different over the left and right hemispheres when verbal vs. nonverbal stimuli are presented (Buchsbaum & Fedio, 1970). The hemispheric difference for visual stimuli has been amply demonstrated in split-brain subjects (Gazzaniga & Sperry, 1967). The specialization of the left hemisphere is not just for sounds, nor is it specialized for all sounds, as the right ear superiority is not observed for all acoustic stimuli. Neither ear was preferred in the processing of clicks (Schulhoff & Goodglass, 1970) or steady-state vowels (Shankweiler & Studdert-Kennedy, 1967). Other sounds have produced a left ear (right hemisphere) superiority, especially various kinds of musical stimuli such as baroque melodies (Kimura, 1964) and chords (Gordon, 1970). A right hemisphere superiority for perceiving music has been demonstrated in lobectomized patients. Subjects of listening tasks who previously had their right temporal lobes removed did worse than left lobecto­ mized patients on the Timbre and Tonal Memory subtest of the Seashore Test of Musical Abilities (Milner, 1962), and on recognizing orchestrated melodies (Shankweiler, 1966). Left ear advantage also resulted for environmental sounds (Curry, 1967), sonar signals, (Chaney & Webster, 1966), non-language vocalizations (Carmon, 1972), hummed melodies (King & Kimura, 1972), and the "emotional tone" of sentences (Haggard & Parkinson, 1971). Studies by Day, Cutting & Copeland (1971) have demonstrated that dichotic stimuli processed in terms of their linguistic dimension are better heard at the right ear, although the same stimuli processed according to their non-linguistic dimension (such as pitch) are preferred by the left ear. The verbal-nonverbal dichotomy for acoustic processing in the left versus the right hemispheres has been further confirmed by evoked cortical response studies (Cohn, 1971; Wood, Goff & Day, 1971). The question of hemispheric specialization for pitch has not yet been clarified. The left ear advantage (right hemisphere superiority) for certain musical stimuli is briefly reviewed above. A right hemisphere involvement in pitch-related functions is also evident in the observation that expressive aphasics (sustaining left hemisphere damage) rarely lose control of pitch in normal linguistic intonation, although other aspects of language production are severely impaired. Moreover, aphasics and left hemispherectomies can sing(Smith, 1966; Bogen, 1973; Gordon, 1973). But it is compatible with these facts to say that pitch is bilaterally or subcortically processed. In the Wada test for cerebral dominance, Bogen & Gordon (1971) observed a strong left-brain dominance for language production, but suggest that "tonal abilities" are either a right hemisphere or a bilateral function in the brain. Milner (1962) found no significant change in pitch perception after unilateral lobectomy of either side. Zurif & Mendelsohn (1972), in their dichotic listening tests for sentences, are led to suggest the possibility that "a preliminary and partial analysis. of prosodic contours can be carried out in both hemispheres". Similarly, Curry (1968) found no difference in performances for the dichotic pitch discrimination test in normals and a right hemispherectomized patient. On the basis of the patient's high scores, and previous findings that cats could make pitch discriminations after bilateral ablations of the cortex (Katsuki, 1961, 1962), Curry suggests that pitch is subcortically processed. Itis possible that pitch-processing is set-influenced, and shows laterality effects according to task. This set-influence is exemplified by Spellacy & Blumstein (1970), where vowels in a linguistic context were better recognized by the right ear, while the same stimuli in a non• language context (embedded in music and environmental sounds) were preferred by the left ear. Similarly, when pitch was used linguistically to distinguish voiceless from voiced consonants, a right ear advantage resulted (Haggard & Parkinson, 1971). Hemispheric specialization 103 In all of the experiments involving pitch reported on in the literature, the subjects were speakers of English, a language which utilizes pitch intonational contours, but which does not use pitch distinctively to contrast individual words. That is, in English, the word cat means "cat" whatever the fundamental frequency of the acoustic signal happens to be. In the majority of the world's languages, however, the pitch of individual syllables is as significant as, say, the voicing contrast of the initial consonant in English, which dis­ tinguishes "pit" from "bit". Such languages are known as "tone" languages. The experiment reported on in this paper was conducted to determine whether speakers of a tone language-in this case, Thai-would show a right ear advantage when the dichotic stimuli represented contrasting tones. Secondly, we wished to compare the degree of lateralization for tone stimuli with the degree of lateralization for words, where the contrast depended on consonant substitution. Thirdly, we asked whether the task of pitch discrimination would yield a different result if the same pitches were not in a linguistic context; that is, when the same pitch configurations as are found on the Thai tones were hummed. Furthermore, we wanted to compare speakers of a tone language with those of a non-tone language for this type of task; and to compare these results with speakers of other tone languages, such as Mandarin and Yoruba. Methods and Procedures From a total of 24 Thai speakers, one was eliminated from analysis because he was reportedly left-handed, and another because of abnormally high errors throughout the testing. The results reported on here come from a population of 22 native Thai-speaking subjects, students and residents of Los Angeles. The English-speaking group, not yet complete, is made up of 14 students varying in musical talents, know ledge of tone languages and handedness. We are reporting mainly in the Thai subjects' data at this time. Three sets of stimuli were used: (1) five Thai words differing only in tone (pitch), the "tone-word" stimuli; (2) five Thai words contrasting only in initial consonant, and having Table I 3 sets of stimuli used (1) Tone-words Stimulus Tone Length (ms) English gloss naa mid tone 625 "field" naa low tone 650 ( a nickname) naa falling 575 "face" naa high tone 625 "aunt" " rising 650 "thick" naa (2) Consonant-words 700 (a nickname) daa mid tone naa mid tone 650 "field" saa mid tone 700 "diminish" caa mid tone 650 "tea" 1aa: mid tone 600 "goodbye" (3) Hums 650 mid tone