Sparse Codes for Speech Predict Spectrotemporal Receptive Fields in the Inferior Colliculus
Complete and overcomplete sparse coding models trained on spectrograms of speech predict Inferior Colliculus (IC) spectro-temporal receptive field (STRF) shapes with excitatory and suppressive subfields that are localized in frequency but separated in time. (a) Two examples of Gerbil IC neural STRFs  exhibiting ON-type response patterns with excitation following suppression; data courtesy of N.A. Lesica. (b) Representative model dictionary elements from each of three dictionaries that match this pattern of excitation and suppression. The three dictionaries were all trained on spectrogram representations of speech, using a hard sparseness (L0) penalty; the representations were complete (left column; Fig. S2), two-times overcomplete (middle column; Fig. S3), and four-times overcomplete (right column; Fig. 4 and Fig. S4). (c) Two example neuronal STRFs from cat IC  exhibiting OFF-type patterns with excitation preceding suppression; data courtesy of M.A. Escabí. (d) Other model neurons from the same set of three dictionaries as in panel b also exhibit this OFF-type pattern.