John J. Sidtis

On the nature of the cortical function underlying right hemisphere auditory perception

Abstract The nature of the function underlying the right hemisphere superiority in processing some types of auditory stimuli was examined by investigating the relationship between the degree of functional asymmetry observed during dichotic testing and the harmonic information conveyed in the test stimuli. Ninety-six right handed subjects received one of four dichotic pitch recognition tests. The tests differed from one another in the number of constituent overtones present in the tonal stimuli. As stimuli increased in complexity from pure tones to square waves, the overall accuracy of pitch discriminations increased and a right hemisphere advantage emerged for both accuracy and latency of response. These results indicate that right hemisphere auditory function is specialized for the analysis of steady state harmonic information rather than for music perception per se .

Selective loss of complex-pitch or speech discrimination after unilateral lesion ☆

Twenty-eight right-handed patients who suffered a single cerebrovascular accident in the distribution of either the left or right middle cerebral artery were tested on their ability to discriminate complex-pitch and speech stimuli presented dichotically. Whereas the left hemisphere lesion group was impaired in dichotic speech but not in dichotic complex-pitch discrimination, the right hemisphere lesion group was impaired in dichotic complex pitch but not in dichotic speech discrimination. Complex-pitch phenomena may provide a useful model for the study of auditory function in the nondominant hemisphere.

The Complex Tone Test: Implications for the assessment of auditory laterality effects.

Abstract The results obtained from 24 right-handed subjects on a dichotic pitch recognition test were analyzed with respect to the magnitude, stability and incidence of right hemisphere advantage indicating that complex pitch discrimination yields perceptual asymmetries comparable to those observed for speech perception, but in the opposite direction. The effects of stimulus competition on the magnitude of laterality effects were also examined and a nearly threefold difference in the magnitude of the laterality measure was demonstrated as a function of acoustic features of the stimuli. The problem of attributing the magnitude of any perceptual asymmetry to a degree of lateralization of cortical function is discussed.

Cortical mechanisms involved in praxis Observations following partial and complete section of the corpus callosum in man

In patients who have undergone complete section of the corpus callosum for intractable epilepsy, lateralized presentation of visual nonverbal stimulation showed that the coordination of motor acts by either hand is controlled exclusively by the contralateral hemisphere. When two patients had serial operations consisting of an initial division of the splenium and posterior 3 cm, followed by complete callosal division, an opportunity arose to test the explicit cortical pathways involved in ipsilateral control. Between operations, these patients could not coordinate movements of the hand ipsilateral to the hemisphere receiving the command. This suggested that for visual nonverbal stimulation, the posterior 3 cm of corpus callosum is necessary for control of the ipsilateral hand; the rostra1 callosum cannot transfer sensorimotor commands. Also, contrary to current views, each hemisphere can carry out sequentially dependent motor activity.

Predicting brain organization from dichotic listening performance: Cortical and subcortical functional asymmetries contribute to perceptual asymmetries

Abstract The patterns of perceptual asymmetry elicited by dichotic speech and complex pitch stimuli were evaluated in a group of 28 normal, right-handed subjects. As in previous studies, between 70 and 75% of the subjects showed a right-ear advantage for speech and left-ear advantage for pitch. However, less than half of the subjects (46%) showed the expected pattern on both tests. It is argued that the assumption of symmetrical, contralateral auditory pathway superiority during dichotic stimulation is only appropriate in roughly half of the dextral population. In the remaining half, significant subcortical asymmetries and/or a lack of contralateral advantage appear to be present. The assessment of complementary cortical functions should provide a way to reduce the confounding of cortical and subcortical contributions to auditory perceptual asymmetries, and thus provide a more accurate behavioral index of brain organization.

Left ear performance in dichotic listening following commissurotomy

Abstract Contribution of the left ear stimulus to dichotic listening performance following commissurotomy was studied in five patients. In two tasks, subjects were asked to identify in writing both members of a pair of competing stimuli, either digits or CV syllables. A third task required subjects to integrate high and low frequency components of a single word presented dichotically. Left ear performance was at chance level for CV syllables but exceeded 80% for four out of five patients on the digit stimuli. All patients showed evidence of being able to utilize left ear information in the dichotic fusion task. Results indicate that apparent supression of left ear material in the dichotic task is a function of spectral-temporal overlap between competing stimuli.

Music, Pitch Perception, and the Mechanisms of Cortical Hearing

The fact that mental functions are asymmetrically represented in the cerebral hemispheres of humans has been central to much of the study of the relationship between brain functions and behavior over the past 25 years. This fact, of course, was recognized much earlier, largely beginning with the observations of Broca and Wernicke that language disturbances were associated with left-hemisphere damage. Subsequently, similar arguments about the lateralization of function in the brain have also been made about such nonlanguage functions as music. Although the behavioral disturbances seen in the neurological syndromes of aphasia or amusia are not typically modality-specific, they have provided important eIues about where 10 beg in the study of how the brain processes linguistic and nonlinguistic auditory patterns. Because of the strong association between language and left-hemisphere function, a useful working assumption has been that the characteristics of the speech perception process are likely to be quite similar, if not identical, to the characteristics of cortical hearing in the left hemisphere. On the other side of the brain, however, the characteristics of cortical hearing are less weIl understood. This chapter, then, focuses on one of the processes underlying the right hemisphere’s purported musical skills. The eIues broadly suggested by the clinical syndromes and narrowed by experimental studies with both normal subjects and patients with focal brain damage are considered here. Based on this evidence, it is argued that one of the areas in which the right hemisphere has a relative advantage over the left is in processing steady-state harmonic information, and it does so in a manner that is important in extracting pitch information from complex periodic sounds. This capacity, then, is a significant contributor to the apparent right-hemisphere advantage for some musical functions, and almost certainly, it plays a role in the right hemisphere’s contribution to the prosody of fluent speech.

Functional connectivity associated with acoustic stability during vowel production: implications for vocal-motor control.

Vowels provide the acoustic foundation of communication through speech and song, but little is known about how the brain orchestrates their production. Positron emission tomography was used to study regional cerebral blood flow (rCBF) during sustained production of the vowel /a/. Acoustic and blood flow data from 13, normal, right-handed, native speakers of American English were analyzed to identify CBF patterns that predicted the stability of the first and second formants of this vowel. Formants are bands of resonance frequencies that provide vowel identity and contribute to voice quality. The results indicated that formant stability was directly associated with blood flow increases and decreases in both left- and right-sided brain regions. Secondary brain regions (those associated with the regions predicting formant stability) were more likely to have an indirect negative relationship with first formant variability, but an indirect positive relationship with second formant variability. These results are not definitive maps of vowel production, but they do suggest that the level of motor control necessary to produce stable vowels is reflected in the complexity of an underlying neural system. These results also extend a systems approach to functional image analysis, previously applied to normal and ataxic speech rate that is solely based on identifying patterns of brain activity associated with specific performance measures. Understanding the complex relationships between multiple brain regions and the acoustic characteristics of vocal stability may provide insight into the pathophysiology of the dysarthrias, vocal disorders, and other speech changes in neurological and psychiatric disorders.

Bilateral Language and Commissurotomy

Although the behavioral study of commissurotomy patients has focused almost exclusively on questions about the functional differences between the disconnected hemispheres, the surgical procedure has also provided an opportunity to study a wide range of interactions between the hemispheres that have significant effects on behavior (e.g., see Holtzman, this volume, Chapter 19). With the development of the staged surgical technique (see Roberts, this volume, Chapter 11) it not only has become possible to study interhemispheric interaction with and without a corpus callosum, but also to study interactions that occur with only the anterior or posterior half of the callosum intact. Behavioral examinations of patients during pre-, inter-, and postoperative periods have provided new insights into sensory and motor interhemispheric interaction (see Volpe, this volume, Chapter 21), as well as a first glimpse of how the two hemispheres can interact on a cognitive level in the absence of direct sensory or perceptual information (Sidtis et al., 1981a). This chapter will focus on cognitive interactions between the hemispheres by describing some of the recent work on semantic information processing carried out with commissurotomy patients who have some bilateral language capacity.