The contribution of high frequencies to human brain activity underlying horizontal localization of natural spatial sounds

Abstract

Background: In the field of auditory neuroscience, much research has focused on the neural processes underlying human sound localization. A recent magnetoencephalography (MEG) study investigated localization-related brain activity by measuring the N1m event-related response originating in the auditory cortex. It was found that the dynamic range of the right-hemispheric N1m response, defined as the mean difference in response magnitude between contralateral and ipsilateral stimulation, reflects cortical activity related to the discrimination of horizontal sound direction. Interestingly, the results also suggested that the presence of realistic spectral information within horizontally located spatial sounds resulted in a larger right-hemispheric N1m dynamic range. Spectral cues being predominant at high frequencies, the present study further investigated the issue by removing frequencies from the spatial stimuli with low-pass filtering. This resulted in a stepwise elimination of direction-specific spectral information. Interaural time and level differences were kept constant. The original, unfiltered stimuli were broadband noise signals presented from five frontal horizontal directions and binaurally recorded for eight human subjects with miniature microphones placed in each subject's ear canals. Stimuli were presented to the subjects during MEG registration and in a behavioral listening experiment.

Results: The dynamic range of the right-hemispheric N1m amplitude was not significantly affected even when all frequencies above 600 Hz were removed. The dynamic range of the left-hemispheric N1m response was significantly diminished by the removal of frequencies over 7.5 kHz. The subjects' behavioral sound direction discrimination was only affected by the removal of frequencies over 600 Hz.

Conclusion: In accord with previous psychophysical findings, the current results indicate that frontal horizontal sound localization and related right-hemispheric cortical processes are insensitive to the presence of high-frequency spectral information. The previously described changes in localization-related brain activity, reflected in the enlarged N1m dynamic range elicited by natural spatial stimuli, can most likely be attributed to the processing of individualized spatial cues present already at relatively low frequencies. The left-hemispheric effect could be an indication of left-hemispheric processing of high-frequency sound information unrelated to sound localization. Taken together, these results provide converging evidence for a hemispheric asymmetry in sound localization.

Figure 1

Representative N1m responses. Equivalent current dipoles (ECDs) calculated for the grand-averaged N1m response at the average peak latency t. The figure shows ECDs for stimuli with different bandwidths originating from directions -90° and 90°, calculated separately for the left and right hemisphere (contour step 20 fT/cm).

Figure 2

N1m dynamic range. The grand-average dynamic range of the N1m amplitude for stimuli with different bandwidths. For the analysis of the dynamic range within each hemisphere, the N1m amplitudes for ipsilateral stimuli were subtracted from their contralateral counterparts. The left-hemispheric dynamic range was diminished by the reduction in bandwidth, while its right-hemispheric counterpart was not significantly affected. Error bars indicate SEM.

Figure 3

Direction discrimination accuracy. The subjects' grand-average accuracy in discrimination of the sound source direction. Stimuli with four different bandwidths were presented from five frontal horizontal directions. Stimuli with the narrowest bandwidth were poorly localized, particularly if originating from the oblique direction angels (-45°, 45°). Error bars indicate SEM.