Response latency of brisk‐sustained (X) and brisk‐transient (Y) cells in the cat retina

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Abstract
Several methods for evaluating light-evoked response latency and its variability in brisk-sustained (X) and brisk-transient (Y) retinal ganglion cells were tested. The most accurate procedure proved to be that described by Levick (1973), in which the time of occurrence of the 4th impulse after stimulus onset is taken as an estimate of the latency. The shortest response latencies are obtained when the stimuli are the same size as the receptive field center. At medium and high response amplitudes (> 150 impulses/s) the response of brisk-transient (Y) cells to these optimal stimuli is 10-15 ms faster than that of adjacent brisk-sustained (X) cells. The response latency of brisk-sustained (X) cells for stimuli larger than the receptive field center increases, whereas that of brisk-transient (Y) cells remains constant. Brisk-sustained (X) cells respond faster than do brisk-transient (Y) cells to stimuli smaller than the receptive field center. No systematic difference exists between brisk-sustained (X) and brisk-transient (Y) cells in regard to the temporal variability of the response. The SD of the latency for stimuli of optimal size decreases from 2.0-8.0 ms at medium stimulus contrast to 0.6-2.0 ms at high stimulus contrast. The response of OFF-center cells to the disappearance of a light spot is always slower than that of an ON-center cell of the same class to the onset of this stimulus. When OFF-center cells are stimulated with dark spots, their response latency does not differ from that of ON-center cells of the same class. No simple relationship exists between the response latency and the response amplitude. At medium and high discharge rates, most brisk-transient (Y) cells respond faster than an adjacent brisk-sustained (X) cell with equal response. At the same response amplitude, the latencies become shorter as the background illumination is raised. The same discharge rate can be obtained with stimuli of sub-optimal and supra-optimal size, but the latency for the larger stimulus is shorter than that for the smaller one. Latency is an additional parameter characterizing the light-evoked response.