Cancellation of rod signals by cones, and cone signals by rods in the cat retina.

1. The interaction of rod and cone signals at the level of cat retinal ganglion cells was studied by a method of light exchange. Two spectrally distinct lights were exchanged in such a manner that the rate of photon catch by rods increased in a stepwise manner at the same moment that the cone rate decreased in the same manner, and vice versa. 2. Under any conditions of adaptation, where both rods and cones contributed to the ganglion-cell discharge, it was always possible to adjust the ratio of the magnitudes of the rod and cone stimuli so that no change in ganglion-cell discharge could be detected by listening to the recorded activity via a loudspeaker. We term this condition a silent exchange. 3. On the face of it, the condition of silent exchange arises when rod and cone signals are able to cancel one another, when made opposite in phase by the exchange situation. But was this silence due to a true cancellation of the signals from one photoreceptor type by those of the other type, or was it due to our failure to stimulate the photoreceptors adequately? In order to test whether rod signals can cancel those of cones we bleached both visual pigments and set our exchange apparatus to stimulate the two photoreceptors in the antagonistic manner described above. At first no response could be heard on exchange, for the thresholds of both rods and cones lay above that of our apparatus. But the cones soon recovered and a strong response was heard on exchange. With no change in our stimulating situation, this response diminished with time and silence was again restored. This restoration of silence could not be due to the cones alone, for with time their sensitivity could only further increase. It could only be the increasing sensitivity of the rods that quietened the cone signals. In agreement with this conculsion, the dark-adaptation curve of the rods showed that they became sensitive to our stimulus at the time that the cones began to be silenced. 4. By means of coloured backgrounds we have also shown the converse, namely that rods signals can be cancelled by those of cones.