Covert attention speeds information accrual more along the vertical than the horizontal meridian

Background: 1) Transient covert attention improves discriminability and accelerates the rate of visual information processing in feature and conjunction searches (Carrasco & McElree, 2001). 2) Contrast sensitivity and spatial resolution are better along the horizontal than vertical meridian, and covert attention improves discriminability at all locations to a similar degree (Carrasco et al., 2001; Talgar & Carrasco, In Press). Goal: We investigated whether: a) the rate of information processing differs for different locations at a fixed eccentricity; b) the effect of transient covert attention on the dynamics varies as a function of location. Methods: We used time-course functions derived from the response-signal speed-accuracy trade-off (SAT) procedure for an orientation feature discrimination task. Each trial began with a precue (67 ms), which was either informative (small circle, indicating the target location) or neutral (a circle in the middle of the display). After a 53 ms ISI, Gabor patches, with 0 or 7 distracters, appeared for 40 ms. The target and distracters were presented at 8 equidistant locations from fixation at 4° eccentricity. A tone sounded at 1 of 7 SOAs, ranging from 40 to 2000 ms, to prompt observers to respond. Results: a) SAT asymptotic accuracy (d') was higher and the rate of information processing was faster at the locations on the horizontal than the vertical meridian; b) precueing the target location improved discriminability to a similar extent across the visual field but accelerated information accrual more on the vertical than the horizontal meridian. Conclusion: These results indicate that performance is superior on the horizontal than the vertical meridian because discriminability is higher and temporal dynamics are faster. Interestingly, whereas covert attention affects discriminability to a similar extent across the visual field, it speeds up information accrual more at the least privileged locations, i.e. on the vertical meridian.