Dynamics of light intensity detection by visual cortical neurons in cats

Abstract

The dynamics of the intensity function of 32 neurons in area 17 of the visual cortex to photic stimuli of optimal size, shape, and orientation flashing in the center of the receptive field was studied by the time slices method, with a step of 10 or 20 msec, in unanesthetized, curarized cats. All neurons tested showed instability of their intensity function reflected in characteristics of successive fragments of the response: It changed both in preferred intensity and in width of the intensity range within which the neuron generated an above-threshold response. In 72% of cases the preferred intensity for the neuron changed successively during the 4–200 msec after the beginning of stimulation by 4–36 dB from greater toward lesser brightnesses, but later it changed more rapidly (in 20–60 msec), rising again apparently in a jump. In four cases the response optimum was shifted up the intensity scale from its initial value by 10–20 dB. Analysis showed that the observed effects are the simple result of the shape of the relationship between temporal characteristics of the response (latent period, time taken to reach the maximum, and time of ending of the burst) to photic stimulus intensity. The possible functional role of these effects for dynamic time coding of information on brightness of photic stimuli by visual cortical neurons is discussed.