Ganglion cell performance at absolute threshold in toad retina: effects of dark events in rods.

1. The performance of ganglion cells in detecting flashes of light near the absolute threshold was studied in an isolated eye‐cup preparation of toad retina. Retinal ganglion cells, through which all visual information from the rods must flow to the brain, are in a key position for evaluating the still unproven hypothesis that the absolute light sensitivity is limited by rod noise (Barlow, 1956). 2. The dark‐adapted threshold intensity for these cells, which were selected on the basis of their high sensitivity, averaged 0.029 Rh* flash‐1 (range 0.008‐0.062), where Rh* signifies one photoisomerization per rod. On average, 46 photoisomerizations were needed per receptive field per flash to evoke a threshold response (range 16‐84). 3. In the threshold region, frequency of responses versus mean flash intensity was determined. Threshold performance could be described by theoretical frequency of response curves, allowing intrinsic noise to be estimated in terms of an equivalent rate of photoisomerization‐like (dark) events. In two completely characterized cells the rate of dark events corresponded to 0.03 and 0.06 Rh*DS‐1, where Rh*D signifies one dark event per rod. 4. Threshold elevations produced by dim backgrounds were studied. The results of these experiments are consistent with a dark event rate equivalent to 0.046 Rh*DS‐1, or 0.037 Rh*DS‐1 after correcting for a probable decrease in summation time. 5. The rate of actual dark events (0.028 Rh*DS‐1, 20 degrees C) measured in Bufo rods (Baylor, Lamb & Yau, 1980) is close to the equivalent rates determined here. Thus, for the ganglion cells signalling the dimmest lights, the dark events in rods appear to be the most significant intrinsic retinal noise source limiting detection.