Asymmetric Temporal Properties in the Receptive Field of Transient Amacrine Cells in Carp Retina
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The speed of signal conduction is essential for the temporal properties of neurons and neuronal networks. We observed highly different conduction velocity within the receptive field of fast ON-OFF type transient amacrine cells that tightly coupled each other, forming a retinal network. Photoresponses of the transient amacrine cells in isolated carp retinae were intracellularly recorded using slit photostimuli (110 or 220 µm wide, 7 mm long), which simplified the estimation of the current flow in their network into a one-dimensional problem. The white light slit was scanned along four axes with 45-degree intervals. The averaged conduction velocity (defined by the distance of slit stimuli divided by time to response peak) was about 50 mm/s. This velocity was directionally asymmetric around the observed receptive field centers, with an average maximum ratio of 1:5 in the recorded velocity axis. The fastest speeds were found in the dorsal area of the receptive fields. The asymmetry was similar in the ON-and OFF-part of the responses, and thus independent of the pathway. Despite this, the spatial decay of the graded-voltage photoresponse amplitude in a field was found to be symmetric, separating the temporal and spatial properties of these responses. Thus, it was suggested that the conduction speed can be an independent factor in processing of directional information within the inner retina.