Visual spatial attention can increase contrast sensitivity and decrease saccadic or manual reaction times (RTs) in attended compared to unattended locations. As stimulus contrast also influences saccadic RT, we asked whether the effect of attention on saccadic RT can be accounted for by the change in contrast sensitivity. We used a dual task paradigm that recorded saccadic RTs and psychophysical responses to targets of various contrast. On each trial, subjects were presented with a central fixation stimulus, which also carried an endogenous cue (an arrow) to the target's radial direction but not its eccentricity. Across trials, the cue was 75% valid, 12.5 % invalid and 12.5 % neutral (no arrow). Circular gratings m-scaled for eccentricity were flashed for 8 ms in one of 8 radial directions around fixation, at either 3 or 6 deg eccentricity, with one of 9 contrasts. The subjects' tasks were to (1) saccade to the location where the target flashed and (2) respond via button press if the target flashed at the nearer (3 deg) or the farther (6 deg) eccentricity. Saccadic RTs and psychophysical responses to targets at the different contrast levels were analyzed to assess the effects of cueing. Valid cueing produced a factor of 1.2 increase in contrast sensitivity, relative to invalid-cue trials. Valid cueing also reduced saccadic RTs for contrast levels at and above the detection threshold. However, the effect of cueing on saccadic RT is accounted for only partly by the increase in psychophysical contrast sensitivity. A comparison of RT vs. contrast for valid and invalid cueing shows a combined effect: a shift consistent with the observed change in contrast sensitivity, and an overall change in latency of roughly 20 msec. Attention appears to influence saccadic RTs in more than one way.
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