Optimal Time Window for the Integration of Spatial Audio-Visual Information in Virtual Environments

Sound duration and location may influence both auditory and visual perception with consequences for the judgement of both auditory-visual event location and integration. This study investigated audio-visual integration in a virtual environment using both short- and long-duration auditory stimuli with visual stimuli temporally offset from the start of the auditory stimulus, to investigate the effects of top-down neural effects on perception. Two tasks were used, an auditory localization task and a detection task (judgement of audio-visual synchrony). Eleven participants took part in the study using a HTC Vive Pro. The short-duration auditory stimuli (35-ms spatialized sound) and long-duration auditory stimuli (600-ms non-spatialized sound followed by 35 ms of spatialized sound) were presented at -60°, -30°, 0°, +30° and +60° degrees azimuth, with the visual stimulus presented synchronously or asynchronously with respect to the start of the auditory stimulus. Results showed that localization errors were larger for the longer-duration stimuli and judgements of audiovisual synchrony tended to be improved for stimuli presented at ±30°. Top-down neural processing can affect spatial localization and audio-visual processing. Auditory localization errors and audio-visual synchrony detection may reveal the effects of underlying neural feedback mechanisms that can be harnessed to optimize audio-visual experiences in virtual environments

Effect of human auditory efferent feedback on cochlear gain and compression

The manunalian auditory system includes a brainstem-mediated efferent pathway from the superior olivary complex by way of the medial olivocochlear system, which reduces the cochlear response to sound (Warr and Guinan, 1979; Liberman et al., 1996). The human medial olivocochlear response has an onset delay of between 25 and 40 ms and rise and decay constants in the region of 280 and 160 ms, respectively (Backus and Guinan, 2006). Physiological studies with nonhuman mammals indicate that onset and decay characteristics of efferent activation are dependent on the temporal and level characteristics of the auditory stimulus (Bacon and Smith, 1991; Guinan and Stankovic, 1996). This study uses a novel psychoacoustical masking technique using a precursor sound to obtain a measure of the efferent effect in humans. This technique avoids confounds currently associated with other psychoacoustical measures. Both temporal and level dependency of the efferent effect was measured, providing a comprehensive measure of the effect of human auditory efferents on cochlear gain and compression. Results indicate that a precursor (>20 dB SPL) induced efferent activation, resulting in a decrease in both maximum gain and maximum compression, with linearization of the compressive function for input sound levels between 50 and 70 dB SPL. Estimated gain decreased as precursor level increased, and increased as the silent interval between the precursor and combined masker-signal stimulus increased, consistent with a decay of the efferent effect Human auditory efferent activation linearizes the cochlear response for mid-level sounds while reducing maximum gain

Masking by inaudible sounds and the linearity of temporal summation.

Many natural sounds, including speech and animal vocalizations, involve rapid sequences that vary in spectrum and amplitude. Each sound within a sequence has the potential to affect the audibility of subsequent sounds in a process known as forward masking. Little is known about the neural mechanisms underlying forward masking, particularly in more realistic situations in which multiple sounds follow each other in rapid succession. A parsimonious hypothesis is that the effects of consecutive sounds combine linearly, so that the total masking effect is a simple sum of the contributions from the individual maskers. The experiment reported here tests a counterintuitive prediction of this linear-summation hypothesis, namely that a sound that itself is inaudible should, under certain circumstances, affect the audibility of subsequent sounds. The results show that, when two forward maskers are combined, the second of the two maskers can continue to produce substantial masking, even when it is completely masked by the first masker. Thus, inaudible sounds can affect the perception of subsequent sounds. A model incorporating instantaneous compression (reflecting the nonlinear response of the basilar membrane in the cochlea), followed by linear summation of the effects of the maskers, provides a good account of the data. Despite the presence of multiple sources of nonlinearity in the auditory system, masking effects by sequential sounds combine in a manner that is well captured by a time-invariant linear system

Estimating peripheral gain and compression using fixed-duration masking curves

Estimates of human basilar membrane gain and compression obtained using temporal masking curve (TMC) and additivity of forward masking (AFM) methods with long-duration maskers or long masker-signal silent intervals may be affected by olivocochlear efferent activation, which reduces basilar membrane gain. The present study introduces a fixed-duration masking curve (FDMC) method, which involves a comparison of off-and on-frequency forward masker levels at threshold as a function of masker and signal duration, with the total masker-signal duration fixed at 25 ms to minimize efferent effects. Gain and compression estimates from the FDMC technique were compared with those from TMC (104-ms maskers) and AFM (10- and 200-ms maskers) methods. Compression estimates over an input-masker range of 40-60 dB sound pressure level were similar for the four methods. Maximum compression occurred at a lower input level for the FDMC compared to the TMC method. Estimates of gain were similar for TMC and FDMC methods. The FDMC method may provide a more reliable estimate of BM gain and compression in the absence of efferent activation and could be a useful method for estimating effects of efferent activity when used with a precursor sound (to trigger efferent activation), presented prior to the combined masker-signal stimulus. (C) 2013 Acoustical Society of America