Temporal summation and reaction times for detecting gratings

The effects of the duration of presentation of stimuli on the reaction time for detection of gratings of different spatial frequencies were studied. The product of contrast and duration (the contrast "energy") was kept constant for each duration. These experiments showed that at near-threshold "energies," reaction times did not change with increases in the duration to 15 and 30 msec at low and high spatial frequencies respectively. At high contrast "energies," this critical duration was identical (15 msec) for both low and high spatial frequencies. The effects of the duration of a linearly increasing front for grating presentation on detection reaction times were also studied. Increases in the duration of the front (to 60 msec) significantly lengthened the reaction time at low spatial frequencies at all contrast levels studied. Conversely, at high spatial frequencies, a significant increase in the reaction time was seen only at high contrast levels. These results suggest that reaction time depends on two types of mechanisms (phasic and tonic) at near-threshold contrast and one type (phasic) at high contrast.