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Face perception
Perception, emotion and orienting
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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.
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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).
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