Saccades to Explicit and Virtual Features in the Poggendorff Figure Show Perceptual Biases.

Barbara Dillenburger,Michael Morgan

doi:10.1177/2041669517699221

Barbara Dillenburger, Michael Morgan

Open Access

https://doi.org/10.1177/2041669517699221

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Abstract

Human participants made saccadic eye movements to various features in a modified vertical Poggendorff figure, to measure errors in the location of key geometrical features. In one task, subjects (n = 8) made saccades to the vertex of the oblique T-intersection between a diagonal pointer and a vertical line. Results showed both a small tendency to shift the saccade toward the interior of the angle, and a larger bias in the direction of a shorter saccade path to the landing line. In a different kind of task (visual extrapolation), the same subjects fixated the tip of a 45° pointer and made a saccade to the implicit point of intersection between pointer and a distant vertical line. Results showed large errors in the saccade landing positions and the saccade polar angle, in the direction predicted from the perceptual Poggendorff bias. Further experiments manipulated the position of the fixation point relative to the implicit target, such that the Poggendorff bias would be in the opposite direction from a bias toward taking the shortest path to the landing line. The bias was still significant. We conclude that the Poggendorff bias in eye movements is in part due to the mislocation of visible target features but also to biases in planning a saccade to a virtual target across a gap. The latter kind of error comprises both a tendency to take the shortest path to the landing line, and a perceptual error that overestimates the vector component orthogonal to the gap.

Highlights

In a recent article (Morgan & Dillenburger, 2016), we described psychophysical investigations of the famous Poggendorff effect
The absolute magnitude of the P-bias in the two experiments is not significantly different, t(7) 1⁄4 0.98; p > .1. These results confirm that the eye movement bias in extrapolation is not entirely due to a tendency to take a shorter path across the gap
There is a tendency to take the path that would be predicted from the perceptual P-bias, that is, a path that takes the saccade to a target position that is not the point of intersection between the pointer and the landing line

Summary

Introduction

In a recent article (Morgan & Dillenburger, 2016), we described psychophysical investigations of the famous Poggendorff effect. As in the earlier article (Morgan & Dillenburger, 2016), we avoid the loaded term illusion in favor of bias and we refer to the P-bias as any bias in the perception of collinearity in the same direction as that seen in the traditional, upright 4-line Poggendorff figure. Evidence for a location shift was found (Morgan, 1999) using the rather difficult task of matching the perceived orientation of the virtual line between the two intersections to that of a grating. Morgan and Dillenburger (2016) found evidence in a rigorous two-alternative forced choice task for a location shift, using a 2-dot probe stimulus, but the effect was small and not present in all participants Evidence for a location shift was found (Morgan, 1999) using the rather difficult task of matching the perceived orientation of the virtual line between the two intersections to that of a grating. Morgan and Dillenburger (2016) found evidence in a rigorous two-alternative forced choice task for a location shift, using a 2-dot probe stimulus, but the effect was small and not present in all participants

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