Central auditory processing : III. The “cocktail party” effect and anterior temporal lobectomy

The capacity to selectively attend to only one of multiple, spatially separated, simultaneous sound sources—the “cocktail party” effect—was evaluated in normal subjects and in those with anterior temporal lobectomy using common environmental sounds. A significant deficit in this capacity was observed for those stimuli located on the side of space contralateral to the lobectomy, a finding consistent with the hypothesis that within each anterior temporal lobe is a mechanism that is normally capable of enhancing the perceptual salience of one acoustic stimulus on the opposite side of space, when other sound sources are present on that side. Damage to this mechanism also appears to be associated with a deficit of spatial localization for sounds contralateral to the lesion.     

Central auditory processing : IV. Ear dominance—Spatial and temporal complexity

Ear dominance for dichotically presented tones was measured in 63 right-handed subjects when the frequency difference (Δf) was small compared to the center frequency (f_c) and again when it was large. Although two-thirds of the population exhibited a left-ear dominance in both conditions, a shift toward right-ear dominance occurred when the Δf was increased. An additional study, employing the alternating tone illusion described by Deutsch, revealed the same general effect, i.e., a shift toward right-ear dominance with increasing values of Δf/f_c. The results of these experiments, coupled with a review of previously published data of other dichotic experiments, indicate that as the ratio of Δf/f_c increases, the subjective complexity of the sound image increases, and there is a progressive emergence of a “right-ear advantage” (or ear dominance). A tentative explanation relates these results to the effects of anatomical asymmetries of primary and auditory association cortex and the efferent temporal lobe enhancement mechanism described by R. Efron, P. H. Crandall, B. Koss, P. L. Divenyi, and E. W. Yund (Brain and Language, 1983, 19, 254–263.     

Central auditory processing: I. Ear dominance—A perceptual or an attentional asymmetry?

The phenomenon of ear dominance for pitch described by Efron and Yund has been attributed by them to an asymmetry of sensory origin in the binaural integration of dichotic tone pairs. An explanation of this phenomenon in terms of an attentional bias is rejected on the basis of two experiments where the possibility of such bias was excluded. These and other experiments indicate that a simple explanation of this ear dominance in terms of a hemispheric specialization in the processing of tonal stimuli also must be rejected.     

Central auditory processing : II. Effects of anterior temporal lobectomy

Ear dominance for the pitch of dichotically presented tonal stimuli was measured in nine patients before and after a unilateral anterior temporal lobectomy. Four subjects had a left and five had a right lobectomy. Every patient exhibited a change of ear dominance consistent with the hypothesis that a unilateral lobectomy decreases the perceptual salience of the tone presented to the ear contralateral to the lesion. Depending on the direction and magnitude of the subject's preoperative ear dominance and the side of the lobectomy, the postoperative results either increased or decreased the strength of ear dominance in a predictable fashion. The results support the idea that within each temporal lobe lie physiological mechanisms which can enhance the perceptual salience of the acoustic signal emitted by one sound source when other, concurrent, spatially separated sound sources are present. It is also argued that the same mechanism operates on speech, melodic, and tonal signals.     

The perception of dichotic chords by hemispherectomized subjects

Hemispherectomized subjects display a strong ear dominance in their perception of dichotically presented two-tone chords. The frequency of the tone presented to the ear contralateral to the remaining hemisphere dominated the pitch mixture of the chord. The described effect does not vary with the age at which the hemispherectomy was performed. These results are consistent with the hypothesis, developed to account for findings in normal subjects, that the pitch mixture of a dichotic chord is determined by a subcortical pitch processor. The effects observed in hemispherectomized subjects may result from an interruption of an efferent pathway from the cortex to the subcortical pitch processor or from an asymmetrical degeneration of the processor which may be located in the thalamus (medial geniculate).     

Central auditory processing. VI. Detecting ear dominance by evoked potentials

An electrophysiological method is described for measuring the direction and strength of a subject's ear dominance for pitch using the P3 component of event-related potentials. Results of these experiments reveal that the P3 can be used effectively for these measures in man.