Elsevier

Clinical Neurophysiology

Volume 114, Issue 12, December 2003, Pages 2245-2250
Clinical Neurophysiology

Expedited communication
Neural activity associated with binaural processes for the perceptual segregation of pitch

https://doi.org/10.1016/S1388-2457(03)00247-5Get rights and content

Abstract

Objective: We measured late cortical potentials in a psychophysical procedure for binaural unmasking of a dichotically-embedded pitch.

Methods: Late-latency auditory evoked potentials were measured from 128 recording channels in 13 healthy subjects. Control stimuli consisted of 500 ms segments of broadband acoustic noise presented identically to both ears via earphones, evoking a perception of noise localized in the centre of the head. Dichotic pitch stimuli were created by introducing a dichotic delay to a narrow frequency region of the same noise segments, and resulted in a perception of both the centrally-located noise and a right-lateralized pitch.

Results: Both stimuli evoked late auditory event-related potentials (ERPs) characterized by a P1-N1-P2 complex of waves between 60 and 180 ms after stimulus onset. ERPs associated with the control and dichotic pitch stimuli showed no amplitude differences for the P1 and N1 waves. ERPs to dichotic pitch stimuli became significantly more negative beginning at a latency around 150 ms, an effect that was maximal between 210 and 280 ms. Topographic mapping showed that this late negativity was lateralized to the left hemisphere.

Conclusions: The late negative wave elicited by the dichotic pitch stimulus reflects neural processing that is dependent upon binaural fusion within the auditory system.

Significance: The dichotic pitch paradigm may provide a useful tool for the electrophysiological assessment and study of the temporal processing capabilities of the auditory system. This paradigm may also be useful for the study of binaural mechanisms for the perceptual segregation of concurrent sound sources.

Introduction

Slight differences in the timing of acoustic signals reaching the two ears can be utilized by the auditory system to segregate and localize sound sources. Intracranial and lesion studies in animals have shown that interaural timing differences (ITDs) are first extracted at the level of the superior olivary complex of the brain-stem (Masterton and Imig, 1984), and are utilized at higher levels of the auditory system including auditory cortex (Jenkins and Merzenich, 1984). In the human, it is possible to measure neurophysiological responses to ITDs with evoked potentials. Because binaural fusion of ITDs involves very precise computations of interaural timing relationships, these binaurally-evoked potentials can potentially evaluate very fine-grained temporal processing capabilities in auditory structures of both normal individuals, and those with known or suspected temporal processing deficits underlying problems in hearing or language.

Several psychophysical procedures that index our sensitivity to ITDs have been adapted for use in evoked potential studies. One method employs abrupt interaural time changes in acoustic streams, such as continuous noise (Halliday and Callaway, 1978, McEvoy et al., 1990) or long trains of clicks (Ungan et al., 1989), causing lateralization shifts of the sound image and evoking late cortical responses similar to, but with later latencies than, the P1-N1-P2 responses to onsets of acoustic stimuli. Because the acoustic streams are designed so they contain no detectable monaural changes, these time-shift evoked potentials must result solely from binaural processes in the auditory system, and their prolonged latencies indicate that these restricted acoustic cues require longer processing times. A second psychoacoustic approach uses stimuli that produce a binaural masking level difference (MLD). With this method, if a signal (S) is presented in masking noise (N) with both in phase at the two ears (S0N0), the threshold for detecting the signal is significantly higher than when either the signal or the noise are π radians out of phase (SπN0 or S0Nπ). The late auditory potentials show comparable binaural unmasking effects (Kevanishvili and Lagidze, 1987) and exhibit other properties characteristic of the behavioral MLD (Fowler and Mikami, 1992a, Fowler and Mikami, 1992b). Interestingly, neither the lateralization shift nor the MLD paradigms seem to exert significant effects on potentials prior to the late potentials, including the auditory brain-stem response (ABR) or middle latency responses (MLR) (Kevanishvili and Lagidze, 1987, McEvoy et al., 1990), suggesting that relatively late stages of cortical processing may be crucial for the perceptual elaboration of binaural cues.

The present study extends this body of evoked potential research to another binaural psychoacoustic phenomenon involving the perception of pitches from stimuli that contain no monaural cues to pitch. Typically, dichotic pitch is produced by presenting listeners with two broadband noises with interaurally identical amplitude spectra but with a specific interaural lag over a narrow frequency band. The interaurally-shifted frequency band will be perceptually segregated from the noise, and the resulting pitch will have a tonal quality associated with the center frequency of the dichotically-delayed portion of the spectrum (Cramer and Huggins, 1958). Dichotic pitch is therefore a binaural unmasking phenomenon that is theoretically closely related to the MLD. In this study, we wished to determine if the late auditory evoked potentials are sensitive to the binaural processes involved the perception of dichotic pitch. If so, they may provide a useful neurophysiological tool for study of the temporal processing capabilities of the central auditory system. To our knowledge, there have been no previous reports of the evoked potential consequences of this perceptual phenomenon.

Section snippets

Subjects

Because some listeners may be unable to detect dichotic pitch (Dougherty et al., 1998), 15 volunteers were screened for this ability prior to participation in the main experiment. A dichotic pitch stimulus or a control stimulus was presented to the listener with equal probability in a standard single-interval (yes/no) task. The stimuli were identical to those later employed in the main research project. Listeners were required to state, by pressing one of two keys on a computer keyboard,

Results

Both noise and tone stimuli evoked well-defined auditory ERPs, with maximal amplitudes at fronto-central regions of the scalp, and characterized by a positive-negative-positive complex of waves within the first 300 ms of stimulus onset. Fig. 1 shows grand-averaged waveforms recorded at frontal midline and lateral locations. At the midline electrode, mean P1 latency was 76 ms, mean N1 latency was 108 ms, and mean P2 latency was 172 ms.

Noise and tone ERPs overlay closely during the course of the

Discussion

As the two stimuli used here were discriminable solely by a dichotic delay (in the dichotic pitch stimulus) but were otherwise acoustically identical, we can be confident that the late cortical LN wave reflects neural processing that is dependent upon binaural fusion within the brain. The relatively late latency consequences of binaural processing obtained in the present study are consistent with results from other binaural experiments. Kevanishvili and Lagidze (1987) found that the binaural

Acknowledgements

This work was supported by Royal Society of New Zealand Marsden Grant #UOA813. We thank Alex Morgan for assistance with data collection, and 3 anonymous reviewers whose comments improved this manuscript.

References (16)

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