Field additivity of the middle-wavelength cone pathway under various test and field configurations

The effect of photon noise on the detection of white flashes

Thresholds for detecting brief, white, foveal test flashes drop abruptly within 0.2 sec of the offset of a white adapting field. The magnitude of the abrupt drop is proportional to the square root of field intensity (square root of I) correct for bleaching and dark light. Thresholds are then stable out to 1.6 sec for 200 msec tests, or recover only slightly for 20 msec tests. These results exclude some simple deterministic models in which Weber-like gain controls in the luminance pathway are assumed to recover exponentially in the dark, but can be explained parsimoniously if turning off the field abolishes photon-driven noise, improving the S/N ratio while leaving visual responsivity virtually unaltered. This theory was first put forward by Krauskopf and Reeves [(1980) Vision Research, 20, 193-196] for S-cone thresholds; it implies that the Weber law for increment thresholds is not due to a single gain control, but rather expresses the product of two distinct square root of I factors, adjustment of responsivity and photon-driven noise. Removal of the noise, not recovery of gain, permits thresholds to fall in early dark adaptation.

Transient lumanopia: the invisibility of flicker in early dark adaptation

Flicker thresholds for brief white (5-arcmin, 16-18-Hz) foveal flicker bursts can rise for at least 1 s after a steady white adapting field is extinguished. We call this anomalous loss of sensitivity in early dark adaptation lumanopia by analogy with dyschromatopsia, a similar anomaly of the hue pathways. Lumanopia is greatest at 400 Td. Flicker thresholds for 10-12 Hz flicker bursts do not show lumanopia. The temporal-frequency dependence of lumanopia is specific to early dark adaptation, as flicker thresholds vary little from 10 to 18 Hz after steady light or dark adaptation. Lumanopia has implications for models of retinal gain controls.