On the Time Required for Recurrent Processing in the Brain

Can one tell which connections are involved in processing information of a given sort in the cortex, by looking at the temporal course of the responses of individual neurons? Thorpe and Imbert32 have argued that the speed with which neurons in the early visual system appear to be able to produce selective responses, e.g. when tested with different orientations of a stimulus, indicates that the processing is of a feedforward nature. They have recorded from orientation selective neurons in the primary visual cortex of the monkey, and orientation tuning curves appear very similar if computed from the firing rate over a 300 ms period, or only over a 50 ms period after response onset. These results have been interpreted as ruling out the involvement of feedback loops, i.e. that a given neuron may alter its response over time, due to the firing of other cells which it itself influences (whether directly or polysynaptically). Examples of some classes of network in which feedback effects may be important include networks with recurrent excitatory connections, such as are found between cortical pyramidal cells, and networks with recurrent lateral inhibition (see Fig. 1). Very similar interpretations to those of Thorpe and Imbert have been drawn by Oram and Perrett,24 from an analysis of the responses of “face” cells in the temporal lobe cortex.