Stimulus location probability regulates spatial attention

Purpose, To test the hypothesis that vbual spatial attention can affect performance on a simple brightness discrimination task. We have quantified attention by measuring performance while manipulating stimulus location probabilities. Methods. Observers (two humans and two monkeys) fixated while a snprathreshold stimulus, on which tl ey were to make a simple brightness discrimination judgement, was briefly pre rented at one of five locations, 12 deg. from fixation. We parametrically manipulated I he distribution of probabilities that governed the trial-by-trial location at which the dis;riminant stimulus would be presented. We determined brightness discrimination thresholds, in a baseline condition, in which the discriminant stimulus occurred 100% of he time at a middle location. Operating at discrimination threshold, we examined four other conditions in blocks; (!) the stimulus appeared 80% of the time at the middle location and 5% at each of four other flanking locations; (2) the stimulus appeared 50% of the time at the middle location and 12 or 13% at the other locations; (3) Itie stimulus appeared with equal probability (20%) at all five locations, and (4) the Uimulus appeared 40% of the time at two flanking locations and 6 or 7% of the tiiru at the three other locations. In Experiment 2 we measured performance using the same distributions of probability but at brightness discriminations above and belt w threshold. Results. In Experiment 1 we found that performance varied as a function ol stimulus location probability. Performance at a given spatial location improved as the probability of stimulus appearance at th it location increased. In Experiment 2 we found that stimulus location probabi ity imposed a smaller effect when the discrimination was performed either abov; or below threshold. Conclusions. We have quantified vhual spatial attention and find that attention affects performance on a simple discrimination task.