Responses to Harmonic and Mistuned Complexes in the Awake Marmoset Inferior Colliculus
Kostlan, Kevin J.
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The auditory system parses complex acoustic scenes. Often, multiple different sounds overlap in both the spectral and temporal representation at the auditory nerve (AN) level. To separate sources in these cases, the AN information is “regrouped” such that each sound becomes a single percept (Licklider, 1954). The system takes advantage of harmonicity in each source (Scheffers and Maria, 1983: p134) in doing so. Sensitivity to periodicity/harmonicity was found in Bendor and Wang (2010) and Feng (2013) in the awake Marmoset cortex. We used the same preparation in the Inferior Colliculus (IC), which is two levels “below” the cortex, but found no preference for harmonics (n=22). Most units responded whenever there was energy in their receptive field and many were inhibited by multi-component complexes such as harmonic stimuli. The rare examples (n=2) that had a strong preference to harmonics over tones did not have the sharp tuning properties as did harmonic units in Feng (2013). Furthermore, units were not much more sharply tuned to harmonics than was predicted by a superposition of pure tone responses combined with side-band inhibition and saturation. Although harmonically selective units could be built from IC units (that were reasonably sharply tuned) by taking the weighted sums of a pseudopopulation (as in May et al., 1998 and Cai et al, 2009) generated from the unit and applying a threshold, the same was also true with a simulated auditory nerve (AN) model from (Zilany et al, 2009). The lack of non-trivial IC responses to harmonics and the success of the AN pseudopopulation method construct to construct harmonically selective units suggests that such selectivity originates de novo in the cortex that the IC does not necessarily play a major role in sound regrouping.