Perception, emotion and orienting
- The Elbow Illusion: A Novel Motion-Based Mislocalization.
Dylan R. Nieman, Bhavin R. Sheth & Shinsuke Shimojo
A continuously moving object is mislocalized in the forward direction when another object is flashed briefly in the same location (flash-lag effect: Nijhawan, 1994). We asked whether abrupt transitions, not flashes, lead to the same effect. Two identical balls (0.53 deg. dia., nearly 100% contrast) start moving simultaneously (17.8 deg./s), the top following an upright V drawn left to right (the legs are orthogonal), the bottom an inverted V, drawn right to left. The trajectories never cross and the turn points are 5.7 deg. from the screen center.
Observers judge the horizontal misalignment of the turn points whose physical positions are randomly varied using the method of constant stimuli. a) When the legs of the V are of the same length (3.7 deg.), 2/5 observers perceive the post-turn legs misaligned forward, 3/5 perceive them misaligned behind. The overall group mean misalignment is 0.23 deg. behind. b) When the pre-turn leg length is reduced (0.75 deg.) and the post-turn leg length increased (12.0 deg.), the backward misalignment increases (p < 0.0001; binomial test) to 0.50 deg. c) In a control experiment, we simultaneously flash both complete trajectories. There is no bias in observers alignment judgments, implying that the effect does not have a static analog and must therefore be motion based. d) Degrading the post-turn trajectory display (fading in from black to white) diminishes the backward bias, implying that the Frohlich effect is unlikely to explain the mechanism behind the elbow illusion. Moreover, the direction of the bias–behind the turn-point–indicates a qualititatively different mechanism from other motion-based perceptual effects such as the flash-lag effect and the Frohlich effect.
- Color and Orientation Pop-Outs Differentially Affect Discrimination
Bhavin R. Sheth & Shinsuke Shimojo
Visual properties such as orientation, color and luminance are functionally similar in that each of them individually provide a basis for perceptual pop-out (Treisman & Gelade, 1980). Pop-outs are commonly believed to facilitate information processing in a variety of tasks such as detection (Bravo & Nakayama, 1992) and discrimination. Here, we provide evidence contradicting this intuition. A 3 X 3 matrix of (vertically/ horizontally) oriented grating patches is shown (a matrix of plaid masks follows) either in the periphery (10 deg. eccentricity, 1 X 1 deg. area each, 180-300 ms duration) or in the fovea (0.5 X 0.5 deg. area each, 5 ms duration).
The observer (n=5 throughout) discriminates the central targets color or orientation. If the target is a unique color, discrimination improves, compared to a control in which all items are of the same color. Conversely, if the targets orientation is salient, discrimination does not improve but, in fact, deteriorates compared to a no orientation pop-out control. The effect generalizes to other features, such as luminance, in place of color. There are no attentional differences between conditions. There is not a greater ability of color to pop-out than color.
Phenomenologically, the data show real differences between so-called elementary visual properties (Nothdurft, 1993; Wolfe et al., 1993). Though counterintuitive, a greater degree of suppression by an orthogonally oriented surround as compared to an iso-oriented one, and relatively little suppression of a uniquely colored stimulus by the surround can explain the data, posing new challenges for both physiology and computation (Fitzpatrick, 2000).