The Time Course of the Oblique Effect in Orientation SensitivityNestor Matthews, Jennifer Cox & Alana Rojewski

Department of Psychology, Denison University, Granville OH 43023 USA

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.

Exp 1 - Participants foveally viewed sequentially presented 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).

Discussion

The Bottom LinePhysiological 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.

Results

Proficiency (d'/RT)

Threshold Estimation

Experiment 3: The Effect of Simultaneity

Cardinal Oblique

ClockwiseOr

Anti-Clockwise?

Tim

e

ClockwiseOr

Anti-Clockwise?

Tim

e Precision (d') Reaction TimeCardinal Oblique

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.

This poster can be viewed and downloaded athttp://denison.edu/~matthewsn/obliqueeffectcns2004.html

The data from Exp 1 indicated no oblique effect in orientation discrimination at the briefest stimulus duration (8 msec), whereperformance at each axis was poor yet already significantlybetter than chance. At all subsequent durations, the obliqueeffect was evident, and grew in magnitude with increases induration. Indeed, an ANOVA confirmed a significant axis-byduration interaction, suggesting that the oblique effect inorientation sensitivity is a dynamic phenomenon.

The data from Exp 2 indicated that for sequential orientation discrimination, forward masks are more effective thanbackward masks, and inner masks are more effective thanouter masks. This is true at both cardinal and oblique axes.

The data from Exp 3 indicated that across durations the oblique effect in orientation discrimination is nearly eliminated whenthe stimuli are presented simultaneously. This finding may be difficult to reconcile with purely bottom-up explanations of theoblique effect.

Main Experiment

Experiment 1: The Effect of Duration Experiment 2: The Effect of Mask Type

Method

Purpose

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None Both

Masking Type

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Oblique

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Backward Forward

Maksing Type

Cardinal

Oblique

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Masking Type

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Oblique