Properties of the visual channels that underlie adaptation to gradual change of luminance

Following adaptation to a spatially uniform patch of light that is gradually brightening (or dimming), a steady test patch appears to be gradually dimming (or brightening). We measured this ramp aftereffect with a nulling method, as a function of the amplitude and temporal repetition rate of the adapting sawtooth waveform and at various retinal eccentricities and levels of dark adaptation. We conclude that the underlying visual channels respond best to large-amplitude sweeps in luminance of at least 20 dB (1 log unit); but they are fairly insensitive to the temporal rate of this sweep. The channels are present out to an eccentricity of at least 40 degrees but they almost disappear during dark adaptation. The ramp aftereffects were asymmetrical: the subjectively darkening aftereffect produced by a brightening adapting ramp was slightly stronger than vice versa.