Sensitivity to stationary flashing spots of the brisk classes of ganglion cells in the cat retina.

The responses of brisk-sustained (X) and brisk-transient (Y) cat retinal ganglion cells to small stationary spots flashing on and off in their receptive fields were recorded. Response magnitudes were determined as the amplitude of the fundamental frequency component. At 0.5 and 8 Hz the sensitivity of brisk-sustained cells is independent of the size of their receptive field centre, with the mean sensitivity at 0.5 Hz being 16% lower than at 8 Hz. At 8 Hz the sensitivity of brisk-transient cells is inversely proportional to the centre diameter of their receptive fields. The relationship is quite different from that found in brisk-sustained cells. The ratio of the sensitivity at 0.5 Hz to the sensitivity at 8 Hz can be used as a measure of the 'sustainedness' of a ganglion cell. Using this measure brisk-sustained cells show no change in their sustained nature as a function of eccentricity. In contrast, brisk-transient cells appear to become increasingly transient with eccentricity. Within the area centralis the sensitivities of brisk-transient cells at 0.5 and 8 Hz are very similar to the sensitivities of brisk-sustained cells, and hence they have a very obvious sustained component to a light flashed on and off at low frequency. These brisk-transient units meet all other criteria of their class and so there can be no question as to their classification.