Variability of inter-spike intervals of cat's on-center optic tract fibres activated by steady light spot: a comparative study on X- and Y-fibres.

Extracellular recordings were made from single on-center optic tract fibres in cats. Light spots of 10sec duration were focussed on the center of the receptive field under 1cd/m2 adaptation luminance by diffuse light. In the maintained discharges at 1 cd/m2, the shape of inter-spike interval histograms (IH) tended to be more skewed in X-fibres than in Y-fibres. As the luminance of the light spot increased, corresponding with the decrease in irregularity of length of inter-spike intervals, the shape of the IHs from X-fibres became symmetrical without noticeable shift of the modal interval peaks. This change was not clear in Y-fibres. The first order serial correlation coefficients showed small negative values in the case of the maintained discharges. These negative values increased with the decrease of the mean intervals of the discharges induced by the light spots, commonly in X- and Y-fibres. The negative correlation of adjacent inter-spike intervals was associated with a change of the shape of some IHs at lower luminances.