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Sparse Codes for Speech Predict Spectrotemporal Receptive Fields in the Inferior Colliculus

Figure 4

A four-times overcomplete, L0-sparse dictionary trained on speech spectrograms.

This dictionary shows a greater diversity of shapes than the undercomplete dictionaries. (a–l) Representative elements a, c, e, g, j, and l resemble those of the half-complete dictionary (see Fig. 3). Other neurons display more complex shapes than those found in less overcomplete dictionaries: (b) a harmonic stack with flanking suppressive subregions; (d) a neuron sensitive to lower frequencies; (f) a short harmonic stack; (h) a localized but complex pattern of excitation with flanking suppression; (i) a localized checkerboard with larger excitatory and suppressive subregions than those in panel l; (k) a checkerboard pattern that extends for many cycles in time. Several of these patterns resemble neural spectro-temporal receptive fields (STRFs) reported in various stages of the auditory pathway that have not been predicted by previous theoretical models (see text and Figs. 68). (m) A graph of usage of the dictionary elements during inference. The different classes of dictionary elements still separate according to usage (see Fig. S4 for the full dictionary) although the notable rises and plateaus as seen in Fig. 3g are less apparent in this larger dictionary.

Figure 4