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Is the code in ferret V1 a 'sparse code'?

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It has been proposed that the primary visual cortex provides a “sparse code” for the information in natural scenes. It has been difficult to pin down quite what a sparse code would look like! One measure is “life-time sparseness”: how few natural scenes does a given neuron respond to. We investigated how neurons in ferret V1 responded to flashed presentations of digitised photos of natural scenes. Some neurons showed very sparse responses: i.e. they responded to very few photos. The degree of sparseness was correlated with whether the neurons were simple or complex, with the sharpness of their tuning for sinewave gratings and with their general responsivity to gratings. In a sparse code, a neuron would respond weakly to most stimuli but would respond well to a few, giving a leptokurtic response distribution. In V1, response variance increases with increasing response. If a response distribution is replotted against equally-discriminable response steps, the distribution becomes much less kurtotic!

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