Threshold setting by the surround of cat retinal ganglion cells.

Abstract

The slope of curves relating the log increment threshold to log background luminance in cat retinal ganglion cells was affected by the area and duration of the test stimulus, as it is in human psychophysical experiments. Using large area, long duration stimuli the slopes average 0.82 and approach close to 1 (Weber''s Law) in the deepest cases. Small stimuli gave an average of 0.53 for on-center units using brief stimuli, and 0.56 for off-center units, using long stimuli. Slopes under 0.5 (square root law) were not found over an extended range of luminances. On individual units the slope was generally greater for larger and longer test stimulus, but no unit showed the full extent of change from slope of 0.5 to slope of 1. The above differences held for objective measures of quantum/spike ratio, and for thresholds either judged by ear or assessed by calculation. The steeper slope of the curves for large area, long duration test stimuli compared with small, long duration stimuli, was associated with the increased effectiveness of antagonism from the surround at high backgrounds. This change may be less pronounced in off-center units, one of which (probably transient Y-type) showed no difference of slope, and gave parallel area-threshold curves at widely separated background luminances, confirming the importance of differential surround effectiveness in changing the slope of the curves. In on-center units, the increased relative effectiveness of the surround is associated with the part of the raised background light that falls on the receptive field center.