Streaming of repeated noise in primary and secondary fields of auditory cortex

Statistical regularities in natural sounds facilitate the perceptual segregation of auditory sources, or streams. Repetition is one cue that drives stream segregation in humans, but the neural basis of this perceptual phenomenon remains unknown. We trained ferrets to detect a stream of repeating noise samples (foreground) embedded in a stream of random noise samples (background). While they listened passively, we recorded neural activity in primary (A1) and secondary (PEG) fields of auditory cortex. We used context-dependent encoding models to test for evidence of streaming of the repeating stimulus in these brain areas. Separate models tested whether time-varying neural spike rates were better predicted by scaling the response to both streams of the repeating stimulus equally (global response gain), or by scaling the response of one stream relative to another (stream-specific response gain). Consistent with adaptation, we found an overall reduction in global gain when the stimulus was repeated. However, when we measured stream-specific changes in gain, neural responses to the foreground stream were enhanced relative to the background. This enhancement was stronger in PEG than in A1. In A1, the degree of enhancement depended on auditory tuning. It was strongest in units that displayed low sparseness (i.e., broad sensory tuning) and were tuned preferentially to the repeated sample. Thus, while overall auditory responses were reduced by the repeating sound, enhancement of responses to the foreground stream relative to the background provides evidence for stream segregation that emerges in A1 and is refined in PEG.