Neural cross-correlation and signal decorrelation: insights into coding of auditory space

Development of frequency tuning shaped by spatial cue reliability in the barn owl's auditory midbrain

Sensory systems preferentially strengthen responses to stimuli based on their reliability at conveying accurate information. While previous reports demonstrate that the brain reweighs cues based on dynamic changes in reliability, how the brain may learn and maintain neural responses to sensory statistics expected to be stable over time is unknown. The barn owl's midbrain features a map of auditory space where neurons compute horizontal sound location from the interaural time difference (ITD). Frequency tuning of midbrain map neurons correlates with the most reliable frequencies for the neurons' preferred ITD (Cazettes et al., 2014). Removal of the facial ruff led to a specific decrease in the reliability of high frequencies from frontal space. To directly test whether permanent changes in ITD reliability drive frequency tuning, midbrain map neurons were recorded from adult owls, with the facial ruff removed during development, and juvenile owls, before facial ruff development. In both groups, frontally tuned neurons were tuned to frequencies lower than in normal adult owls, consistent with the change in ITD reliability. In addition, juvenile owls exhibited more heterogeneous frequency tuning, suggesting normal developmental processes refine tuning to match ITD reliability. These results indicate causality of long-term statistics of spatial cues in the development of midbrain frequency tuning properties, implementing probabilistic coding for sound localization.

Cross-modulation interference with lateralization of mixed-modulated waveforms

PURPOSE

This study investigated the ability to use spatial information in mixed-modulated (MM) sounds containing concurrent frequency-modulated (FM) and amplitude-modulated (AM) sounds by exploring patterns of interference when different modulation types originated from different loci as may occur in a multisource acoustic field.

METHOD

Interaural delay thresholds were measured from 5 normal-hearing adults for an AM sound in the presence of interfering FM and vice versa as a function of interferer modulation rate. In addition, the effects of near versus remote interferer rates, and fixed versus randomized interferer interaural delay, were investigated.

RESULTS

AM interfered with lateralization of FM at all modulation rates. However, the FM interfered with AM lateralization only when the FM rate was higher than the AM rate. This rate asymmetry was surprising given the prevalence of low-frequency dominance in lateralization, but was predicted by a cross-correlation model of binaural interaction. Effects were similar for fixed and randomized interferer interaural delays.

CONCLUSIONS

The results suggest that in multisource environments, sources containing different modulation types significantly interfere with localization in complex ways that reveal interactions between modulation type and rate. These findings contribute to the understanding of auditory object formation and localization.

Interaural temporal and coherence cues jointly contribute to successful sound movement perception and activation of parietal cortex

The perception of movement in the auditory modality requires dynamic changes in the input that reaches the two ears (e.g. sequential changes of interaural time differences; dynamic ITDs). However, it is still unclear as to what extent these temporal cues interact with other interaural cues to determine successful movement perception, and which brain regions are involved in sound movement processing. Here, we presented trains of white-noise bursts containing either static or dynamic ITDs, and we varied parametrically the level of binaural coherence (BC) of both types of stimuli. Behaviorally, we found that movement discrimination sensitivity decreased with decreasing levels of BC. fMRI analyses highlighted a network of temporal, frontal and parietal regions where activity decreased with decreasing BC. Critically, in the intra-parietal sulcus and the supra-marginal gyrus brain activity decreased with decreasing BC, but only for dynamic-ITD sounds (BC by ITD interaction). Thus, these regions activated selectively when the sounds contained both dynamic ITDs and high levels of BC; i.e. when subjects perceived sound movement. We conclude that sound movement perception requires both dynamic changes of the auditory input and effective sound-source localization, and that parietal cortex utilizes interaural temporal and coherence cues for the successful perception of sound movement.

Temporal integration in absolute identification of musical pitch

The effect of stimulus duration on absolute identification of musical pitch was measured in a single-interval 12-alternative forced-choice task. Stimuli consisted of pure tones selected randomly on each trial from a set of 60 logarithmically spaced musical note frequencies from 65.4 to 1975.5Hz (C2-B6). Stimulus durations were 5, 10, 25, 50, 100, and 1000ms. Six absolute-pitch musicians identified the pitch of pure tones without feedback, reference sounds, or practice trials. Results showed that a 5ms stimulus is sufficient for producing statistically significant above chance performance. Performance monotonically increased up to the longest duration tested (1000ms). Higher octave stimuli produced better performance, though the rate of improvement declined with increasing octave number. Normalization by the number of waveform cycles showed that 4cycles are sufficient for absolute-pitch identification. Restricting stimuli to a fixed-cycle waveform instead of a fixed-duration still produced monotonic improvements in performance as a function of stimulus octave, demonstrating that better performance at higher frequencies does not exclusively result from a larger number of waveform cycles. Several trends in the data were well predicted by an autocorrelation model of pitch extraction, though the model outperformed observed performance at short durations suggesting an inability to make optimal use of available periodicity information in very brief tones.