Analog versus discrete shifts of attention across the visual field.

Two discrimination experiments were run to investigate analog versus discrete properties of a shift of visual spatial attention. Central cuing was used in Experiment 1, whereas peripheral cuing was used in Experiment 2. Presentation of a probe stimulus between fixation and the target (Distance 1), opposite fixation from the target (Distance 3), or away from an imaginary line running from the target through fixation (Distance 2) permitted a fine-grained analysis of attention at those loci across target-probe delays. D-prime analyses in both experiments suggest that attention is shifted in a discrete manner between locations. Sensitivity to probes was generally greater when the probe was aligned with the target and fixation, with Distance 3 equal to Distance 1, than when it was away (at Distance 2). Analysis of sensitivity to targets across cue-probe delays suggests that attention was directed to the probe upon its appearance.