Effects of transient covert attention on the psychometric function

Purpose: Directing attention to a particular location is known to improve accuracy and decrease reaction time for stimuli that appear at that location. In this study we assessed how transient covert attention affects the psychometric function by measuring performance on an orientation discrimination task as a function of target contrast while directly manipulating transient attention. Methods: Observers performed a 2-AFC orientation discrimination task on targets (Gabor patches between 2 and 12 cpd) that appeared at 4.5 degrees eccentricity at one of 8 equally spaced locations around a canonical circle. The timing between pre-cue onset and target offset was brief enough to preclude saccadic eye movements. We used the method of constant stimuli and measured performance (percent correct) as a function of stimulus contrast in neutral and peripheral pre-cue conditions. Both pre-cues signaled target onset, but only the peripheral pre-cue indicated target location. Results: Performance increased as a function of stimulus contrast for both neutral and peripheral pre-cue conditions. Overall, performance was higher in the peripheral pre-cue condition, and the largest improvement was in the threshold region (60-90% correct). In the peripheral pre-cue condition, the psychometric function shifted towards lower contrasts and had a shallower slope. In addition, we replicated a horizontal meridian advantage and an impairment for targets that appear directly above fixation (Carrasco, Penpeci-Talgar & Cameron (2001). Spatial Vision, 14.3). Moreover, performance fields showed a similar pattern at all but the highest contrast, where performance was at asymptote. The slope of the psychometric function was steeper for targets on the horizontal than on the vertical meridian. Conclusion: Overall, performance is higher in peripheral than in neutral pre-cue conditions. Attention decreases the slope and shifts the psychometric function to lower contrasts at all target locations.