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The Time Course of the Oblique Effect in Orientation Sensitivity Nestor Matthews, Jennifer Cox & Alana Rojewski Department of Psychology, Denison University, Granville OH 43023 USA Purpose Results It is well established that our ability to see subtle angular differences is better near cardinal (horizontal or vertical) than oblique (diagonal) axes. Here, we investigated the whether this “oblique effect” in orientation sensitivity depends on duration. Experiment 2: The Effect of Mask Type Experiment 1: The Effect of Duration Cardinal Oblique Method Cardinal 1.6 1.4 Oblique Linear (Oblique) 1 Oblique y = 0.0029x + 0.2038 R2 = 0.883 0.8 Cardinal y = 0.0077x + 0.2077 R2 = 0.9792 0.6 0.4 Power (Oblique) 12 Threshold (degrees) 1.2 d' Gabor patches, and judged whether the second was oriented ‘clockwise’ or ‘anti-clockwise’ to the first. Across trials, we randomized both the axis (cardinal versus oblique) and the stimulus duration (8 – 142 mesc). To limit the persistence of orientation-specific neural responses, circular bulls-eye masks preceded and followed each Gabor patch. Exp 2 - We held the stimulus duration constant (108 msec), and systematically varied whether masks preceded or followed. Exp 3 - We repeated Exp 1, but now presented the two stimuli simultaneously on the left and right sides of fixation (see below). Large Angle Linear (Cardinal) Power (Cardinal) 14 Oblique Exp 1 - Participants foveally viewed sequentially presented Cardinal Discussion 10 Oblique y = 16.193x -0.1488 R2 = 0.8957 8 6 4 0.2 0 0 0 20 40 60 80 100 120 Stimulus duration (ms) 140 160 The data from Exp 2 indicated that for sequential orientation discrimination, forward masks are more effective than backward masks, and inner masks are more effective than outer masks. This is true at both cardinal and oblique axes. Cardinal y = 41.717x -0.5468 R2 = 0.9885 2 0 The data from Exp 1 indicated no oblique effect in orientation discrimination at the briefest stimulus duration (8 msec), where performance at each axis was poor yet already significantly better than chance. At all subsequent durations, the oblique effect was evident, and grew in magnitude with increases in duration. Indeed, an ANOVA confirmed a significant axis-by duration interaction, suggesting that the oblique effect in orientation sensitivity is a dynamic phenomenon. 20 40 60 80 100 120 140 160 Stimulus Duration (ms) Experiment 3: The Effect of Simultaneity Data from the two groups (blue & red) overlapped during threshold estimation, showing initial similarity. During the main experiment, however, performance was significantly worse in the masked group. Interestingly, the oblique effect under simultaneous stimulation was much reduced. The data from Exp 3 indicated that across durations the oblique effect in orientation discrimination is nearly eliminated when the stimuli are presented simultaneously. This finding may be difficult to reconcile with purely bottom-up explanations of the oblique effect. The Bottom Line Threshold Estimation Time Time Cardinal Clockwise Or Anti-Clockwise? Clockwise Or Anti-Clockwise? Oblique Main Experiment Precision (d') Proficiency (d'/RT) Reaction Time Physiological studies have suggested that the oblique effect may occur because fewer visual neurons are tuned to oblique axes than to cardinal axes. However, if an over-representation of cardinal neurons alone were sufficient to explain the oblique effect, one might expect the phenomenon to be constant across stimulus durations. The present data suggest, instead, that neural models of the oblique effect in orientation sensitivity will require consideration of orientation-specific temporal recruitment. This poster can be viewed and downloaded at http://denison.edu/~matthewsn/obliqueeffectcns2004.html