Response to the length of moving visual stimuli of the brisk classes of ganglion cells in the cat retina.

Response histograms were collected for brisk-sustained and brisk-transient ganglion cells in the cat retina as narrow bars were moved backwards and forwards across their receptive fields. When a bar of fixed length was moved across the centre of the receptive field with contrast proportional to velocity, a constant response was obtained as long as the centre of the receptive field was crossed within the summation time. However, if the length of the bar was such that it extended beyond the centre, then there was a small but steady increase in surround antagonism for an increase in velocity. The same response was produced by a brief whole-field flash as by an extended bar moving across the receptive field at high velocity if both stimulus conditions delivered the same energy uniformly across the receptive field. With brisk-sustained cells it was observed, for small bar lengths, that bar length and contrast could be exchanged to give a constant response, even when there was considerable non-linearity in the over-all stimulus-response relationship. Thus conditions that resulted in constant stimulus flux produced a constant response. This property was seen at both high and low velocities for the majority of brisk-sustained units. The stimulus-response relationship had a greater range of linearity at high velocities than at low velocities. From similar experiments with brisk-transient cells it was observed that bar length and contrast could only be exchanged to give a constant response at high velocities. At low velocities there was considerable non-linearity: there appeared to be saturation of the response from local regions and it was necessary to extend the bar outside such a region to obtain an increase in response. At lower velocities, despite the changes seen in length-response curves under different conditions of contrast and velocity, the degree of surround antagonism remained constant for a given cell. Further, both brisk-sustained and brisk-transient cells showed the same degree of surround antagonism.