Isolation of rod and cone contributions to cat ganglion cells by a method of light exchange.

1. The great majority of cat retinal ganglion cells are known to receive signals from rods and from a single (green) cone type. The centre region of the receptive fields of these cells was stimulated by a spot that changed back and forth from orange to white. By adjusting the intensity of the white spot relative to that of the orange a condition could be established at which the photon‐catch rate of the rods remained unchanged during the orange‐white exchange. At this intensity setting, termed the rod isolept, rods are thus unstimulated by the exchange, however intense, and the ganglion‐cell response was found to be due entirely to the green cones. At another intensity setting of the white spot relative to the orange (cone isolept), the photon catch of the green cones remained unchanged during the exchange and ganglion‐cell responses were found to arise entirely from the rods. 2. A neutral wedge in the combined exchange beam (but not in the steady background that covered the whole receptive field) regulated the size of the exchange stimulus and thus the magnitude of the ganglion‐cell discharge heard from a loud speaker to the exchange. Exchange threshold was the wedge setting at which this change in firing rate could only just be heard. 3. At the cone isolept, cones remain unstimulated however intense the exchange stimulus, and the rod increment threshold curve was determined over its full range from absolute threshold up to saturation. Likewise, at the rod isolept, the cone increment threshold curve was determined over the same intensity range as for the rods. Rod saturation was found to occur at the point where the cone increment threshold curve began to rise from its absolute threshold level toward its Weber region. 4. The exchange approach also enabled both rod and cone dark‐adaptation curves following a strong bleaching exposure to be obtained in the same experiment by moving successively between the cone and rod isolepts. At the cone isolept the time course of early rod dark adaptation could thus be determined when the rod threshold to flashing spots lay well above that of the cones.