Abstract

We measured temporal contrast thresholds for positive and negative sawtooths to test for asymmetry in sensitivity to light offset vs light onset. Since mirror-image sawtooth waveforms have identical amplitude spectra and differ only in phase, they also offer a test of the idea that sensitivity to a waveform is governed only by the amplitude of the Fourier fundamental. The stimulus was a 1.8° diam circle, mean retinal illuminance of 500 townsends (Td). Threshold contrast for the sawtooths (positive ramp or fast-off and negative ramp or fast-on) was measured at frequencies from 2 to 26 Hz. A sine-wave contrast sensitivity function was also determined over the same range. Observers were more sensitive to fast-off sawtooths than to fast-on by 0.05–0.16 log unit for stimuli of 2–13 Hz. At higher frequencies, differences were not systematic. This incremental/decremental asymmetry was confirmed with other waveforms. Sensitivity to fast-on sawtooths could be predicted from sine-wave contrast sensitivity with evidence of second harmonic intrusion at very low frequencies. Greater sensitivity to fast-off sawtooths suggests a response to the temporal structure of a waveform vs its spectral content and is consistent with the hypothesis of parallel on and off psychophysical channels.

© 1988 Optical Society of America