ON-OFF receptive fields in auditory cortex diverge during development and contribute to directional sweep selectivity.

Neurons in the auditory cortex exhibit distinct frequency tuning to the onset and offset of sounds, but the cause and significance of ON and OFF receptive field (RF) organisation are not understood. Here we demonstrate that distinct ON and OFF frequency tuning is largely absent in immature mouse auditory cortex and is thus a consequence of cortical development. Simulations using a novel implementation of a standard Hebbian plasticity model show that the natural alternation of sound onset and offset is sufficient for the formation of non-overlapping adjacent ON and OFF RFs in cortical neurons. Our model predicts that ON/OFF RF arrangement contributes towards direction selectivity to frequency-modulated tone sweeps, which we confirm by neuronal recordings. These data reveal that a simple and universally accepted learning rule can explain the organisation of ON and OFF RFs and direction selectivity in the developing auditory cortex.

Comodulation Enhances Signal Detection via Priming of Auditory Cortical Circuits.

Acoustic environments are composed of complex overlapping sounds that the auditory system is required to segregate into discrete perceptual objects. The functions of distinct auditory processing stations in this challenging task are poorly understood. Here we show a direct role for mouse auditory cortex in detection and segregation of acoustic information. We measured the sensitivity of auditory cortical neurons to brief tones embedded in masking noise. By altering spectrotemporal characteristics of the masker, we reveal that sensitivity to pure tone stimuli is strongly enhanced in coherently modulated broadband noise, corresponding to the psychoacoustic phenomenon comodulation masking release. Improvements in detection were largest following priming periods of noise alone, indicating that cortical segregation is enhanced over time. Transient opsin-mediated silencing of auditory cortex during the priming period almost completely abolished these improvements, suggesting that cortical processing may play a direct and significant role in detection of quiet sounds in noisy environments. SIGNIFICANCE STATEMENT: Auditory systems are adept at detecting and segregating competing sound sources, but there is little direct evidence of how this process occurs in the mammalian auditory pathway. We demonstrate that coherent broadband noise enhances signal representation in auditory cortex, and that prolonged exposure to noise is necessary to produce this enhancement. Using optogenetic perturbation to selectively silence auditory cortex during early noise processing, we show that cortical processing plays a crucial role in the segregation of competing sounds.

Behavioural estimates of auditory filter widths in ferrets using notched-noise maskers.

Frequency selectivity is a fundamental property of hearing which affects almost all aspects of auditory processing. Here auditory filter widths at 1, 3, 7, and 10 kHz were estimated from behavioural thresholds using the notched-noise method [Patterson, Nimmo-Smith, Weber, and Milroy, J. Acoust. Soc. Am. 72, 1788-1803 (1982)] in ferrets. The mean bandwidth was 21% of the signal frequency, excluding wider bandwidths at 1 kHz (65%). They were comparable although on average broader than equivalent measurements in other mammals (∼11%-20%), and wider than bandwidths measured from the auditory nerve in ferrets (∼18%). In non-human mammals there is considerable variation between individuals, species, and in the correspondence with auditory nerve tuning.