Mirror-image sawtooth waveforms (rapid-on and rapid-off) were used to test for differences in sensitivity to incremental and decremental stimuli. Temporal contrast sensitivity functions were measured for rapid-on and rapid-off sawtooths and for sine wave stimuli (for 2-26 Hz, mean retinal illuminance of 500 td, circular target of 1.8 deg, foveal). Rapid-off sawtooths yielded higher sensitivity than rapid-on sawtooths at low and middle frequencies. This advantage for detection of decremental lights was confirmed in an experiment in which contrast sensitivity was determined for the sum of a rapid-on and a rapid-off waveform added over the full range of phase angles. Our data, based on periodic rather than pulsed stimuli, broaden and strengthen the evidence that the visual system is more sensitive to decrements than to increments in light level.