Adaptive switches in midbrain circuits

Abstract

In this issue of Neuron, Mysore and Knudsen (2012) describe a simple, anatomically supported circuit that can categorize stimuli into "strongest" and "others," regardless of their absolute strength. Such flexible categorization cannot be achieved by lateral inhibition alone but also requires that the inhibitory neurons reciprocally inhibit each other.

Figure 1. A Circuit with Lateral Inhibition Can Produce Switch-like Responses with Constant Threshold Value

The circuit diagram is shown on the left. Input and output neurons are marked with circles; diamonds mark inhibitory neurons. Right: the responses of output 1 are plotted as a function of “competitor” input 2, for different values of input 1 (corresponding to the stimulus strength within the receptive field). The switch between high and low values of output 1 occurs for the same value of competitor input 2, regardless of input 1. Input and output strength are in arbitrary units.

Figure 2. A Circuit with Reciprocal Inhibition of Feedforward Lateral Inhibition Can Produce Adaptive Switch-like Responses

Notations are as in Figure 1. The main difference is the switch from low to high level in output 1 occurs when “competitor” input 2 exceeds the “main” input 1.