The graded response of bipolar and ganglion cells to test flashes at the receptive field centre, spans only a limited portion of the test intensity domain: more than 90% of the graded response range can be elicited by test flashes differing by less than 100 to 1. 2. In the presence of steady illumination of the receptive field surround, the absolute levels of log test intensities required to elicit 90% of the graded response are increased (reset), but the relation in (1) still applies. 3. Each point in the receptive field surround, when illuminated, contributes to the resetting of the required centre test flash intensities by a weighing that decreases exponentially with distance from the centre. The space constant is 0.25 mm. 4. When the receptive field surround is fully covered with illumination, the centre test flash intensities required to elicit 90% of the response range must be increased by about tenfold for each tenfold increase in surround intensity over a surround intensity domain of about 1000 to 1. 5. The absolute levels of surround and required centre test intensities are inter-related: when the receptive field surround is fully covered, a test flash with intensity equal to that of the surround elicits a half-maximal response. Thus, in the presence of a full field background, the bipolar potential is held near its half-maximum response potential. 6. The graded resetting of the required centre test flash intensities is well correlated with the graded increase in horizontal cell response as the surround intensity and area are varied. It is inferred that units with response and receptive field properties like those of the horizontal cells, when driven by surround illumination, act as interneurones to reset the relationship between required test flash intensity and response in bipolar and ganglion cells.
