Sawtooth contrast sensitivity: effects of mean illuminance and low temporal frequencies

Temporal contrast sensitivity was measured for mirror-image sawtooth (rapid-on and rapid-off) and sine waveforms for a 1.8 deg foveal target. In one experiment, contrast sensitivity was measured for 2-26 Hz stimuli at target mean illuminance levels of 5-1260 td. At 5 td, contrast sensitivity functions for sawtooth and sine waveforms, expressed in terms of the Fourier fundamental amplitude, are equivalent. At higher light levels, sawtooth sensitivity increasingly exceeds sine sensitivity and rapid-off (decremental) sawtooths show progressively greater sensitivity than rapid-on (incremental) sawtooths. This pattern of results was obtained for two color-normal observers and for a deuteranopic observer. In a second experiment, sawtooth and sine sensitivity was tested at 500 td with an extended low-frequency range, to 0.5 Hz. Rapid-off and rapid-on sensitivities declined only slightly at low temporal frequencies in contrast with sine sensitivity. To interpret our data, we evaluate two single-pathway models (last-stage asymmetric detector and compressive response-intensity non-linearity) and a dual-pathway model in which incremental and decremental waveforms are detected by separate ON and OFF visual mechanisms.