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Christina Enroth-Cugell^*, B. Gevene Hertz^* and P. Lennie

Departments of Biological Sciences and Electrical Engineering, Northwestern University, Evanston, Illinois, U.S.A.

1. In an attempt to understand the convergence of rod and cone signals in the cat's retina, ganglion cells that received inputs from both rods and cones were stimulated using lights chosen to excite one or other receptor system or both together.

2. If a mesopic background was chosen to allow the ganglion cell to be excited by a blue-green test flash primarily through rods and a deep red flash primarily through cones, one light could not be alternated with the other without eliciting a response from the cell.

3. This appears to be a result of the different temporal properties of the scotopic and photopic systems. On the mesopic background responses to blue-green test flashes were transient. Responses to red test flashes arose with similar latency, but were more sustained.

4. Rod and cone systems responded with similar latencies in the presence of the mesopic background that substantially light-adapted the rod system but left the full sensitivity of the cone system undiminished. When equivalently light-adapted, the cone system was faster.

5. When brief flashes that acted through rods were presented with flashes that acted through cones the ganglion cell's response was the sum of the responses to the two flashes presented separately, as long as the flashes were weak. This linear relation ceased to hold when flashes were strong, but the breakdown appears not to be the result of mutual inhibition between rod and cone signals.

6. When a background light excited both rod and cone systems it appeared to reduce sensitivity independently in each.

7. The scotopic and photopic receptive fields of a given ganglion cell always were of the same type, on- or off-centre, and, within the limits of measurement, the central regions of the receptive fields were concentric and both the same size.

Present address: Laboratory of Experimental Psychology, University of Sussex, Brighton, England.