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Sparse Coding Models Can Exhibit Decreasing Sparseness while Learning Sparse Codes for Natural Images

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V1 developmental data appear to challenge the canonical sparse coding models.

Multi-unit activity in primary visual cortex (V1) of awake young ferrets watching natural movies shows decreasing sparseness over time. The sparseness metrics shown in this figure are defined in the results section of this paper, and the data are courtesy of Pietro Berkes [14], [15]. The plot has a logarithmic horizontal axis. For contrast, one expects that, in sparse coding models, the sparseness should increase over time. This point was emphasized in recent work [14]. In this paper, we show that, in sparse coding models sparseness can actually decrease during the learning process, so the data shown here cannot rule out sparse coding as a theory of sensory coding.

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