When mapped with a small spot of light, the central receptive fields of bipolar cells in the salamander retina are much larger than the extent of bipolar cell dendrites. Furthermore responses of bipolar cells to distant spots of light are considerably delayed relative to proximal spots. Using quantitative modelling, electrical coupling between bipolar cells is examined and rejected as a sufficient explanation of the data. An active process appears to shape signal waveform as signals spread laterally in the bipolar cell layer. Chemical synaptic coupling between bipolar cells is considered and shown to be inconsistent with the data. It is suggested that local, transient negative feedback from amacrine cells is involved in shaping bipolar cell signals, Wilson 1980). It seems probable that the particular spatial and temporal properties of the photoreceptor layer make a useful contribution to the information processing of the retina and some possible benefits have been pointed out (Detwiler et al. 1978;Attwell et al. 1984).
In this paper we look at some of the temporal characteristics of signal spread within the receptive field of bipolar cells. As in the case of the rod network we find that our data are inconsistent with the spread of signal within a passive electrical network. Instead our data suggests that an active process contributes strongly to the processing of visual signals at the level of bipolar cells.