Spatial filtering by X cells as a function of light level

Purpose. No consensus exists on how the spatial properties of the receptive fields of ganglion cells change with light level. For example, some have found that center sizes increase when going from photopic to scotopic conditions, while others have reported little change. Our objective was to provide a near definitive characterization of X cell spatial properties across light levels. Methods. Discharges were recorded extracellularly from anesthetized adult cats. The properties of the receptive field were investigated across the full operational range of light levels encountered by this animal by measuring frequency responses for sinusoidal gratings and a pattern that isolates the surround mechanism. Results. The spatial structure of the receptive field was found to change only slightly across all light levels. Center size is slightly larger under scotopic than photopic conditions. If surround size changes it is within our measurement error. The size of the center also shows a dependence on temporal frequency under photopic conditions, as we have reported previously, but not under scotopic conditions. Responsivity declines under scotopic but nol photopic conditions. Conclusions. Although there are very distinct pathways through the retina for rod- and conedriven signals, the spatial properties of the receptive field remain relatively constant for both photopic and scotopic vision. Changes in spatial filtering observed psychophysically can be explained only in small part therefore by changes that occur at the single cell level.