Spatial-frequency characteristics of neurones of area 18 in the cat: dependence on the velocity of the visual stimulus.

The spatial and temporal response properties of neurones of areas 17 and 18 were studied in single units (165) of anaesthetized and paralysed cats. The visual stimuli were drifting or alternating gratings. We confirmed and extended the observation by Tolhurst & Movshon (1975) showing that the spatial-frequency characteristics of neurones of area 17 are largely independent of the temporal parameters of drifting or alternating gratings. The spatial-frequency tuning curves of neurones of area 18 shift along the spatial-frequency axis when the velocity or the temporal frequency of the drifting grating are changed. The effect of an increase either of velocity or temporal frequency is to shift the cell spatial-frequency tuning curve down the spatial-frequency scale, keeping relatively constant the strength and band width of the response. The spatial-frequency tuning curves of neurones of area 18 do not show this temporal-frequency-dependent phenomenon when the stimuli are gratings alternated in phase. In this case neurones of areas 17 and 18 show a similar behaviour. The response properties of neurones of area 18 are compared with recent psychophysical results obtained in similar experimental conditions. The hypothesis is advanced that both areas 17 and 18 are devoted to the processing of spatial information. Area 17 would be responsible for the processing of patterns in stationary or quasi-stationary situations while area 18 would be responsible for that of patterns moving at high velocities.