Abstract
Stimulus-response rélations of on-center optic nerve fibers of cat were investigated in reference to the classification of Type I (phasic, ON-I) and Type II (tonic, ON-II) using light spot presented for 25 seconds. The average discharge rate of the transient part of responses (0-500 msec after stimulus onset) varies as a function of spot-to-background luminance ratio for both ON-I and ON-II. The rate of increase of the discharges for ON-II is about twice as much as that of ON-I (80 impulses/sec for each one log unit increase of stimulus intensity for ON-I, 150 impulses/sec for ON-II), and the saturation level for both ON-I and ON-II is about 200 impulses/sec. The average discharge rate of ON-II's tonic part of responses (10-20 sec after stimulus onset) increases as a function of spot-to-background luminance ratio and reaches the peak (60 impulses/see), then, further increase in stimulus intensity causes a decrease of discharge rate. It is found that the inter-spike intervals of this tonic responses become strikingly constant after the peak is reached. The response latencies for ON-I and ON-II vary as a function of spot intensity and they are not largely affected by the background luminance. To the onset of a high contrast stimulus, ON-I responds with a burst discharge followed by a transient pause, while ON-II responds with continuously decreasing rate of discharges.
