The effect of area and intensity on the response of cat retinal ganglion cells to brief light flashes

Abstract

1. The action potentials of single optic tract axons were recorded in cats anaesthetized by pentobarbital. The receptive fields were stimulated with brief light flashes (20 or 25 msec) of variable area (0.18 less than or equal to A less than or equal to 11.8 degrees) and stimulus intensity (0.36 less than or equal to I less than or equal to 360 cd-m-2). 2. Retinal on-center and off-center neurons responded in one of two ways to brief light flashes: oscillatory responses (O-response) to certain stimulus combinations or non-oscillatory responses (N-response) for all stimulus combinations. 3. The O-respnse of on-center neurons was characterized in its PST-histogram by up to five peaks. Neurons exhibiting an O-response at medium and/or strong stimulus intensities responded at low AxI-values with an N-response. Other neurons, however, exhibited for all stimulus combinations an N-response, in which the instantaneous neuronal impulse rate decreased after an initial peak approximately expronentially with time. 4. The O-response of off-center neurons exhibited with medium AxI-values up to four peaks in the PST-histograms. N-type off-center neurons had at low and medium AxI-values a short primary inhibition period which was followed by a fast rising and slowly decaying activity period. 5. Ricco's law was only valid for small AxI-values not more than one 10log unit above threshold. For higher stimulus values (especially in the O-response) an increment of the stimulus intensity was usually more effective than an equal increment of the stimulus area. 6. On-center and off-center neurons located in about the same region of the retina had a strong tendency to discharge alternately with each other. 7. A superposition model is proposed as an explanation of the experimental data. 8. Possible correlations between the neurophysiological findings and the fast oscillatory after-images seen in corresponding psychophysical experiments are discussed.