Fig. 10.

Changing the ratio of excitation and inhibition alters forward suppression in a network with synaptic depression. A: schematic of the linear threshold model containing three layers of neurons: “thalamic” neurons (hexagons) in the first layer, excitatory “pyramidal” neurons (triangles) and inhibitory “interneurons” (squares) in the second layer, and a “cortical output” neuron (big triangle) in the third layer. The frequency preference of each neuron is indicated by its color. B, top: the responses of each thalamic neuron to masker tones, as a function of masker frequency (in units of octaves from the probe frequency). Bottom: the responses of each thalamic neuron to the probe alone (PA; circles), and to the probe tone as a function of masker frequency (lines). Thalamic neurons with the probe frequency in their receptive fields (e.g., middle, green) respond to the probe tone, but these responses are not forward suppressed. C: as B, for the pyramidal neurons and interneurons in the second layer, which receive depressing inputs from the thalamic neurons. Interneurons are more broadly tuned than pyramidal neurons and therefore often have higher responses to the PA. D: schematic of a network with weak inhibitory synaptic connections onto the cortical output neuron. E, left: the cortical output neuron receives tuned excitation in response to the masker, and depressed excitation in response to the probe. Middle: the cortical output neuron receives tuned, but weak, inhibition in response to the masker, and depressed and weak inhibition in response to the probe. Right: the net output of the cortical output neuron shows relatively strong forward suppression. F and G: as D and E, with intermediate inhibitory synaptic strength. H and I: as F and G, with strong inhibitory synaptic connections. J: normalized suppression curves of the cortical output neuron with either weak (left), intermediate (middle), or strong (right) inhibitory synaptic strength show strong forward suppression (left), weak forward suppression (middle), or a combination of forward suppression with forward facilitation (right).