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Abstract

Neurons often integrate information from multiple parallel signaling
streams. How a neuron combines these inputs largely determines its
computational role in signal processing. Experimental assessment of
neuronal signal integration, however, is often confounded by
cell-intrinsic nonlinear processes that arise after signal integration
has taken place. To overcome this problem and determine how ganglion
cells in the salamander retina integrate visual contrast over space, we
used automated online analysis of recorded spike trains and closed-loop
control of the visual stimuli to identify different stimulus patterns
that give the same neuronal response. These iso-response stimuli
revealed a threshold-quadratic transformation as a fundamental
nonlinearity within the receptive field center. Moreover, for a subset
of ganglion cells, the method revealed an additional dynamic
nonlinearity that renders these cells particularly sensitive to
spatially homogeneous stimuli. This function is shown to arise from a
local inhibition-mediated dynamic gain control mechanism.