Neural coding and perception of auditory motion direction based on interaural time differences.While motion is important for parsing a complex auditory scene into perceptual objects, how it is encoded in the auditory system is unclear. Perceptual studies suggest that the ability to identify the direction of motion is limited by the duration of the moving sound, yet we can detect changes in interaural differences at even shorter durations. To understand the source of these distinct temporal limits, we recorded from single units in the inferior collicu-lus (IC) of unanesthetized rabbits in response to noise stimuli containing a brief segment with linearly time-varying interaural time difference ("ITD sweep") temporally embedded in interaurally uncor-related noise. We also tested the ability of human listeners to either detect the ITD sweeps or identify the motion direction. Using a point-process model to separate the contributions of stimulus dependence and spiking history to single-neuron responses, we found that the neurons respond primarily by following the instantaneous ITD rather than exhibiting true direction selectivity. Furthermore, using an optimal classifier to decode the single-neuron responses, we found that neural threshold durations of ITD sweeps for both direction identification and detection overlapped with human threshold durations even though the average response of the neurons could track the instantaneous ITD beyond psychophysical limits. Our results suggest that the IC does not explicitly encode motion direction, but internal neural noise may limit the speed at which we can identify the direction of motion.

Ahrens, A., Marschall, M., & Dau, T. (2020). The effect of spatial energy spread on sound image size and speech intelligibility. The Journal of the Acoustical Society of America, 147 (3), 1368–1378.

van Hoesel, R. J. (2015). Audio-Visual Speech Intelligibility Benefits with Bilateral Cochlear Implants when Talker Location Varies. Journal of the Association for Research in Otolaryngology, 16 (2), 309–315.

Muñoz, R. V., Aspöck, L., & Fels, J. (2019). Spatial release from masking under different reverberant conditions in young and elderly subjects. Journal of Speech, Language, and Hearing Research, 62 (9), 3582–2595.

Lee, G. W., & Kim, H. K. (2018). Personalized HRTF modeling based on deep neural network using anthropometric measurements and images of the ear. Applied Sciences (Switzerland), 8 (11).

Zohourian, M., & Martin, R. (2020). Binaural direct-to-reverberant energy ratio and speaker distance estimation. IEEE/ACM Transactions on Audio, Speech, and Language Processing, 28, 92–104.

Kopco, N., Doreswamy, K. K., Huang, S., Rossi, S., & Ahveninen, J. (2020). Cortical auditory distance representation based on direct-to-reverberant energy ratio. NeuroImage, 208 (September 2019), 116436.

Auditory distance perception and its neuronal mechanisms are poorly understood, mainly because 1) it is difficult to separate distance processing from intensity processing, 2) multiple intensity-independent distance cues are often available, and 3) the cues are combined in a context-dependent way. A recent fMRI study identified human auditory cortical area representing intensity-independent distance for sources presented along the interaural axis (Kopco et al. PNAS, 109, 11019-11024). For these sources, two intensity-independent cues are available, interaural level difference (ILD) and direct-to-reverberant energy ratio (DRR). Thus, the observed activations may have been contributed by not only distance-related, but also direction-encoding neuron populations sensitive to ILD. Here, the paradigm from the previous study was used to examine DRR-based distance representation for sounds originating in front of the listener, where ILD is not available. In a virtual environment, we performed behavioral and fMRI experiments, combined with computational analyses to identify the neural representation of distance based on DRR. The stimuli varied in distance (15-100 cm) while their received intensity was varied randomly and independently of distance. Behavioral performance showed that intensity-independent distance discrimination is accurate for frontal stimuli, even though it is worse than for lateral stimuli. fMRI activations for sounds varying in frontal distance, as compared to varying only in intensity, increased bilaterally in the posterior banks of Heschl's gyri, the planum temporale, and posterior superior temporal gyrus regions. Taken together, these results suggest that posterior human auditory cortex areas contain neuron populations that are sensitive to distance independent of intensity and of binaural cues relevant for directional hearing.

Conroy, C., Mason, C. R., & Kidd, G. (2020). Informational masking of negative masking. The Journal of the Acoustical Society of America, 147 (2), 798–811.

Negative masking (NM) is a ubiquitous finding in near-“threshold” psychophysics in which the detectability of a near-threshold signal improves when added to a copy of itself, i.e., a pedestal or masker. One interpretation of NM suggests that the pedestal acts as an informative cue, thereby reducing uncertainty and improving performance relative to detection in its absence. The purpose of this study was to test this hypothesis. Intensity discrimination thresholds were measured for 100-ms, 1000-Hz near-threshold tones. In the reference condition, thresholds were measured in quiet (no masker other than the pedestal). In comparison conditions, thresholds were measured in the presence of one of two additional maskers: a notched-noise masker or a random-frequency multitone masker. The additional maskers were intended to cause different amounts of uncertainty and, in turn, to differentially influence NM. The results were generally consistent with an uncertainty-based interpretation of NM: NM was found both inquiet and in notched-noise, yet it was eliminated by the multitone masker. A competing interpretation of NM based on nonlinear transduction does not account for all of the results. Profile analysis may have been a factor in performance and this suggests that NM may be attributable to, or influenced by, multiple mechanisms.

Molloy, K., Lavie, N., & Chait, M. (2019). Auditory figure-ground segregation is impaired by high visual load. Journal of Neuroscience, 39 (9), 1699–1708.

Han, C., O’Sullivan, J., Luo, Y., Herrero, J., Mehta, A. D., & Mesgarani, N. (2019). Speaker-independent auditory attention decoding without access to clean speech sources. Science Advances, 5 (5), 1–12.