Activity in the Optic System Following Stimulation by Brief Flashes of Light. ∗

Abstract

Details of the response of the optic nerve of the rabbit to stimulation by short light flashes, described in a previous paper, are also exhibited in the cat. The most obvious difference is that the cat record is less diagrammatic, more confused by apparently random activity. The cat retina is considerably the more sensitive as measured by the size of nerve responses to weak illumination. Immediate and delayed “on’ responses are recognizable, each involving repetitive firing to the shortest stimuli. A distinct “off’ response is produced by a flash of 50 msec, similar in form to the first “on”. With much shorter flashes, an “off’ response can be recognized superposed upon the second “on’ discharge.

Records from the cortex and superior colliculus following flash stimulation of the retina have been correlated with the nerve response. The successive spikes of the nerve response, occurring at 100 per sec with strong light, show facilitation at the geniculate level, and the first few can be correlated with specific spike-like elevations in the cortical or colliculus record. When the first spike of the nerve record is the highest, the second spike in the cortical record may be higher than the first, presumably due to temporal summation within the 20msec period in which such summation occurs at the geniculate level after electrical shocks. The 3 nerve bursts, 2 “on’ and one “off”, are duplicated at the higher levels after suitable duration of flash.

In the cat, the major response of the colliculus following electrical shocks to the nerve is a slow wave activated by the highest-threshold fibers, in contrast to the cortex which is activated almost exclusively by the lowest-threshold units. The colliculus of the rabbit shows 2 successive elevations, one due to low-threshold and one to high-threshold fibers. In both animals these waves, which are post-synaptic, reverse in sign as a recording electrode passes from the surface of the colliculus downward, at about the level of the stratum opticum. In the rabbit the 2 elevations present reverse at slightly different levels. This enables one so to place an electrode that to electrical stimulation these elevations are of opposite polarity.