Echo suppression in the human cortex is affected by the spatial and temporal proximity of the primary sound and echo.

Echo suppression in the human auditory cortex was studied with auditory middle latency evoked potentials (AMEP) using virtual reality acoustic stimuli, including distance and elevation cues, presented by earphones. The purpose of the study was to assess the effect of proximity of the source sound and echo on the degree of echo suppression. Sixteen subjects were presented with source-echo pairs in which the preceding source sound was always at the vertex, and the echo varied among ten positions on the coronal plane. Positions varied in elevation, distance and time lag between source and echo. The psychoacoustic location judgment of the fused source-echo pair was closer to the source sound (more echo suppression) the nearer the echo drew to the source in its elevation and time. The equivalent dipole magnitudes of the cortical components of AMEP were significantly reduced (more suppression) with shorter echo lags and when echo elevation was similar to that of the source sound. The distances used in this study did not significantly affect echo suppression. These results indicate that echo suppression in the auditory cortex is more pronounced the closer are the primary sound and echo in locational attributes and timing. As source sound and echo draw apart, echo suppression in the cortex decreases and the perceived localization of the fused source-echo is more biased toward the echo.

Auditory brain stem potentials evoked by clicks in notch-filtered masking noise.

Auditory brain stem evoked potentials (ABEPs) were recorded in response to wide-band clicks in notch-filtered masking noise. Notches (1/2 octave wide, 96 dB/octave slopes) were centered on the audiometric frequencies. Without the notches, masking noise intensity was adjusted to obliterate ABEPs. Intensity series, with both clicks and masking noise attenuated by the same 10 dB steps, were obtained for clicks masked by notched noise, in addition to unmasked clicks. Absolute latencies and interpeak latency differences of ABEPs to clicks in notched noise did not change with notch frequency. Absolute latencies were longer for ABEPs to clicks with notched noise than to unmasked clicks. For all stimuli, latencies were prolonged when stimulus intensity decreased. The findings suggest that ABEPs to clicks in notched noise are initiated by low threshold cochlear activation which may account for the independence of component latency from 'travelling wave' propagation time. These potentials may be representative of only one subset of the constituents of ABEPs to wide-bank unmasked clicks. The clinical application of these potentials must await elucidation of their relevance to the process of hearing.