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Ramakrishna Chakravarthi, Rufin VanRullen; Beam me up, Scotty! Exogenous attention teleports but endogenous attention takes the shuttle. Journal of Vision 2010;10(7):244. doi: 10.1167/10.7.244.
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Analyzing a scene requires shifting attention from object to object. Several studies have attempted to determine the speed of these attentional shifts, coming up with various estimates ranging from two to thirty shifts per second. The discrepancy among these estimates is likely a result of several factors including the type of attention, cue and stimulus processing times, eccentricity, and distance between objects. Here, we adapt a method pioneered by Carlson et al (2006) that directly measures attentional shift times. We present 10 ‘clocks’, with single revolving hands, in a ring around fixation. The observers are asked to report the hand position on one of the clocks at the onset of a transient cue. We use different combinations of exogenous and endogenous cuing to determine shift times for both types of attention. In experiment 1, we first endogenously cue a clock with a central arrow. While the observer attends that clock, on some trials we cue the same clock exogenously to evaluate ‘baseline’ processing time, and on other trials we exogenously cue another clock at a variable distance (1, 2 or 5 clocks away) to determine the shift time for exogenous attention. Similarly in experiment 2, we exogenously cue one clock and ask observers to either report the observed time (baseline), or (in other blocks) endogenously shift their attention to another clock at a variable distance from the cued clock to determine the shift time for endogenous attention. In agreement with previous studies, our results reveal that endogenous attention is much slower than exogenous attention (endogenous: 250–350 ms; exogenous: 100–150 ms). Surprisingly, the dependence of shift time on distance was minimal for exogenous attention, whereas it was several times higher for endogenous attention. This qualitative difference suggests distinct neural mechanisms for the two modes of attention.
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