Figure 8.

Schematic of proposed model of adaptation. Excitatory pyramidal cells (open symbols) and inhibitory cells (filled symbols) receive feedforward excitation from thalamus (a) and recurrent excitation from neighboring pyramidal cells (b). The feedforward inhibition (c) sets the threshold of the pyramidal cells and thus controls the degree of contrast adaptation. Increasing stimulus contrast activates thalamic afferents more strongly and generates more feedforward inhibition, and this moves the contrast response function rightward along the contrast axis. The gain, or slope, of the contrast response function is determined by the recurrent inhibition (d), which is proportional to the net excitatory activity in the local network. In practice, both adaptation and gain control could be achieved via the same population of inhibitory neurons. Here we show them separately to emphasize the distinct roles of the two different modes of inhibition [see Douglas et al. (1995) for more details]. The arrangement of the cells in this schematic does not correspond to their location in cortex.