Abstract

Studies of luminance increments and decrements have typically used transient pulses of localized stimuli. We employed two new paradigms to examine increment/decrement response asymmetries in grating stimuli presented with a smooth spatio–temporal envelope. For the first paradigm, we studied apparent width of the light and dark bars making up sinusoidal gratings as a function of contrast. In contrast ranges close to threshold, the light bars appeared up the three times wider than the dark bars, with a peak at contrast of ~3%. Similar asymmetries were seen over a range of spatial frequencies. For the second paradigm, the gratings were generated as only luminance increment bars from the grey background (or only luminance decrement bars). We found asymmetries in the response to increments and decrements than varied as a function of spatial frequency of the test grating. At low spatial frequencies, the visual system was more sensitive to decrements. At intermediate spatial frequencies, sensitivity was higher for increments than for decrements by a factor of ~2. Sensitivity to both become the same at high spatial frequencies.

© 1991 Optical Society of America