The dynamics of the receptive fields of retinal horizontal cells were examined by applying a spatio-temporal modulated light signal to the retina. 2. The spatio-temporal receptive fields of both cone- and rod-driven horizontal cells, estimated through cross-correlation between the modulated light signal and the cells' responses, showed their receptive fields (the space-dependent component) to be reduced in size with time. 3. In cone-driven horizontal cells, the reduction in receptive field size was initially small but then rapidly became prominent with time. The time to peak of the time-dependent component of spatio-temporal receptive fields did not depend on the distance from the centre. 4. Application of a small amount of Co2+, an agent blocking the cone-driven horizontal cells' feedback action on cones, or GABA, resulted in a reversal of the time-dependent shrinkage of receptive fields to time-dependent expansion. 5. In rod-driven horizontal cells, the receptive field shrinkage was slow. The time to peak of the time-dependent component decreased with the distance from the centre. 6. Image processing experiments examining the response pattern in the horizontal cell layer (neural image) to a moving square of light showed smudging of the neural image when the time-dependent receptive field expansion was present, while there was essentially no smudging under conditions of receptive field shrinkage.
