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Carl van Vreeswijk
CNRS University Paris Descartes
Wednesday 10 June 2009
16.00
Seminar Room B10 (Basement)
Alexandra House, 17 Queen Square, London, WC1N 3AR
Contrast Invariance and the the Contrast Response Function in a model of V1.
Irregular activity of cortical neurons can be explained by the assumption that the total excitatory and total inhibitory inputs into the neurons in the network are both much larger than the threshold input, but are approximately balanced. In such models, as currently defined, the input-output relation of the network is almost linear. This conflicts with what is observed experimentally, for example for the contrast response function of the different areas in the visual pathways. In this seminar I will discuss a model of V1 that does not rely on the balance hypothesis, but instead assumes an external source of noise the yield irregular activity. The model explains both the contrast invariance of orientation tuning and the shape of the contrast response function of simple and complex cells in V1. We also address the source of heterogeneity in the response of neurons in V1. We consider two alternative sources, diversity in single cell properties and diversity in network connectivity. We show that both could explain the heterogeneity in response observed experimentally, but that they have statistically different distributions of contrast response functions. Thus, if the model is correct, this difference could be used to determine which is the dominant source of heterogeneity in cortex.