Neural encoding of auditory temporal context in a songbird basal ganglia nucleus, and its independence of birds' song experience

Some of the most complex auditory neurons known are found in the songbird forebrain, throughout the 'song system', including its basal ganglia nucleus Area X. These cells are selective for the temporal order of the bird's own song (BOS): they typically respond strongly to BOS, but more weakly when the syllable sequence of BOS is played in reverse order (roBOS), indicating that they integrate auditory information over more than single syllables. Here, studying the zebra finch Area X, we found that order selectivity strongly depends on the mean syllable duration of individual songs, decreasing markedly as this duration approaches 150-200 ms. Simply segmenting the same songs differently, creating an increase in average syllable length towards 150 ms, caused a similar decrease in order selectivity. This suggests that song neurons integrate acoustic information over a relatively limited time window, predominantly less than 150 ms. We provided further support for this by showing that a significant fraction of Area X order selectivity was accounted for by the acoustic similarity between each BOS and roBOS, measured using cross-correlation with fixed window sizes, but only when the correlation windows were at least 50 ms and no more than 200 ms long. All the same findings were evident in birds raised without tutor exposure, indicating that tutor learning has little effect on neural mechanisms underlying song temporal selectivity. Our results suggest that song-selective neurons encode much of the temporal context of song using a short, constant time window that is conserved across differences in songs, birds and learning.