Analysis of visual modulation sensitivity. IV. Validity of the Ferry-Porter law.

The maximum flicker frequency was determined over a 5+6-log-unit range of retinal illuminance for a stimulus configuration designed to isolate the linear response from long-wavelength (R) cones. For a particular retinal location, the data conformed to the Ferry-Porter law and departed significantly from the predictions of the diffusion equation. The slope of the function was an invariant characteristic and was unaffected by stimulus intensity or area, modulation waveform, or modulation amplitude. However, the slope varied substantially with retinal locus, increasing by more than a factor of 2 between the foveola and 35 degrees eccentricity. This increase shows that the time constant of the linear, unadapted visual response decreases with increasing eccentricity. The difference between foveola and periphery remained at high spatial frequencies, implying that it was not attributable to lateral inhibitory effects.