Experimental evidence for a cochlear source of the precedence effect

Abstract

The precedence effect (PE) refers to the dominance of directional information carried by a direct sound (lead) over the spatial information contained in its multiple reflections (lags) in sound localization. Although the processes underlying the PE have been largely investigated, the extent to which peripheral versus central auditory processes contribute to this perceptual phenomenon has remained unclear. The present study investigated the contribution of peripheral processing to the PE through a comparison of physiological and psychoacoustical data in the same human listeners. The psychoacoustical experiments, comprising a fusion task, an interaural time difference detection task and a lateralization task, demonstrated a time range from 1 to 4.6-5 ms, in which the PE operated (precedence window). Click-evoked otoacoustic emissions (CEOAEs) were recorded in both ears to investigate the lead-lag interactions at the level of the basilar membrane (BM) in the cochlea. The CEOAE-derived peripheral and monaural lag suppression was largest for ICIs of 1-4 ms. Auditory-evoked brainstem responses (ABRs) were used to investigate monaural and binaural lag suppression at the brainstem level. The responses to monaural stimulation reflected the peripheral lag suppression observed in the CEOAE results, while the binaural brainstem responses did not show any substantial contribution of binaural processes to monaural lag suppression. The results demonstrated that the lag suppression occurring at the BM in a time range from 1 to 4 ms, as indicated by the suppression of the lag-CEOAE, was the source of the reduction in the lag-ABRs and a possible peripheral contributor to the PE for click stimuli.

FIG. 1

A Schematic stimulus configurations used in the behavioral experiments: reference and deviant. The reference configuration consists of two diotic click pairs (ITD = 0), delayed by an inter-click interval (ICI). In the deviant configuration, the lead is represented as a diotic click pair (lead-ITD = 0) and the lag as a dichotic click pair (lag-ITD > 0). B Interleaved stimulus presentation used in the CEOAE experiment. Three configurations (SC single click; DC double click; DCI double-click inverted) were repeated 1,800 times within a sequence for each ICI condition and for an ITD of 300 μs. C Stimulus presentation for the ABR experiment. A deviant configuration was repeated 2,000 times, for each ICI condition and for an ITD of 300 μs. The ABRs were recorded by using four electrodes: Fz (ground, positioned at the forehead), Cz (reference, positioned at the vertex), and M1 and M2 (left and right mastoids).

FIG. 2

Psychoacoustical results. A Individual and mean results of the fusion test for binaural (black bars) and monaural stimulation (monaural right, red bars; monaural left, blue bars) by deviants with a lag-ITD of 300 μs. B Mean behavioral thresholds obtained from the lateralization test (circles) and ITD-detection test (squares). The error bars represent the standard error of the mean. C Lateralizations reported the most by the six subjects over three repetitions of the lateralization test (symbols) and mean lateralization threshold (black curve). The different markers represent the six response buttons (left, center, center and center, center and left, center and right, and center and left and right). The size of the symbols indicates at what percentage the lateralization was reported over 18 responses (six subjects, three repetitions): small symbols, below 50 %; medium symbols, between 50 and 70 %; large symbols, above 70 %.

FIG. 3

CEOAE results. A Spectra of the recorded CEOAEs for the single-click condition, i.e., the unsuppressed response (US), and for the derived suppressed response (DS, obtained from (DC-DCI)/2 in Fig. 1B) of the lagging click, for one representative subject KE. The difference between US and DS (the area displayed in gray) represents peripheral lag suppression for ICIs of 2, 4, 8 ms. B Individual (gray curves) and mean (black curves) results of peripheral lag suppression as a function of the ICI for monaural left and right stimulation. The error bars indicate the standard error of the mean.

FIG. 4

ABR results. A ABRs recordings for one representative subject KE, for monaural (right panel, red and blue curves) and binaural (left panel, black curve) stimulation and different ICI conditions. The error bars at a latency of 6 ms indicate the time-averaged SD of the recording. The horizontal dashed lines depict the zero voltage reference, and the bar scale at a latency of 16 ms indicates a voltage of 0.4 μV. B Individual (gray curves) and mean (black, blue, and red curves) results of lag wave V reduction obtained from ABRs recordings for monaural (gray, blue, and red curves) and binaural stimulation (black curve), as a function of ICI. The error bars indicate the standard error of the mean.

FIG. 5

Comparison of mean lag suppression from OAEs (dashed curves), lag wave V reduction from ABRs (solid curves), and behavioral echo thresholds (vertical dashed lines) for monaural and binaural stimulation. The error bars indicate the standard error of the mean.

FIG. 6

Individual comparisons of behavioral lateralization thresholds (solid black curves (in microseconds)) and peripheral lag suppression (dashed blue and red curves (in decibels); blue diamonds, monaural left; red squares, monaural right).