Academic journal article Attention, Perception and Psychophysics

Spatial Mislocalization as a Consequence of Sequential Coding of Stimuli

Academic journal article Attention, Perception and Psychophysics

Spatial Mislocalization as a Consequence of Sequential Coding of Stimuli

Article excerpt

Published online: 17 November 2011

© Psychonomic Society, Inc. 2011

Abstract In three experiments, we tested whether sequentially coding two visual stimuli can create a spatial misperception of a visual moving stimulus. In Experiment 1, we showed that a spatial misperception, the flash-lag effect, is accompanied by a similar temporal misperception of first perceiving the flash and only then a change of the moving stimulus, when in fact the two events were exactly simultaneous. In Experiment 2, we demonstrated that when the spatial misperception of a flash-lag effect is absent, the temporal misperception is also absent. In Experiment 3, we extended these findings and showed that if the stimulus conditions require coding first a flash and subsequently a nearby moving stimulus, a spatial flash-lag effect is found, with the position of the moving stimulus being misperceived as shifted in the direction of its motion, whereas this spatial misperception is reversed so that the moving stimulus is misperceived as shifted in a direction opposite to its motion when the conditions require coding first the moving stimulus and then the flash. Together, the results demonstrate that sequential coding of two stimuli can lead to a spatial misperception whose direction can be predicted from the order of coding the moving object versus the flash. We propose an attentional sequential-coding explanation for the flash-lag effect and discuss its explanatory power with respect to related illusions (e.g., the Fröhlich effect) and other explanations.

Keywords Attention . Visual illusions . Prior entry

At every moment in time, a multitude of visual stimuli impinge on the human retina, but only a few of these stimuli are selected for purposes such as perception, indepth processing, or action control. Attending to different visual locations, stimuli, features, or dimensions boosts the perception and discrimination of fine visual detail (cf. Bashinski & Bacharach, 1980; von Helmholtz 1894) and speeds up processing of visual stimuli at the focus of attention, as well as subsequent saccades to the attended stimuli (cf. Posner, 1980; Rizzolatti, Riggio, Dascola, & Umiltà, 1987). At the same time, attending can prolong the perceived duration of the attended stimuli (cf. Enns, Brehaut, & Shore, 1999; Mattes & Ulrich, 1998).

Apart from these effects, visuospatial attention can also contribute to visual illusions. As everyday observers, we are barely aware of these illusions. We naively presume that the temporal and spatial features that we perceive reflect the physical properties of distal objects. Beginning with the early days of experimental psychology, however, visual illusions have shown that spatial features of distal objects can be misperceived (e.g., Fröhlich, 1929), and related research has suggested that attention could be (partly) responsible for these effects (Müsseler & Aschersleben, 1998). Fröhlich observed that the starting position of an abruptly onsetting moving stimulus was not veridically perceived. Instead, it was perceived at a position shifted farther along its motion trajectory. According to Müsseler and Aschersleben, visual perception of the onset position of the abruptly onsetting moving stimulus depends on allocating attention to its position and is therefore beset with a delay corresponding to the time it takes for attention to focus on the moving stimulus. As a consequence, the moving stimulus will have a perceived onset location that is shifted in the direction of the stimulus movement (cf. Fröhlich, 1929). In line with this assumption, several studies have shown that the Fröhlich effect is reduced when attention is allocated earlier to the motion onset-for instance, when the position of the abruptly onsetting motion stimulus is precued (cf. Ansorge, Carbone, Becker, & Turatto, 2010; Müsseler & Neumann, 1992; Müsseler, Stork, & Kerzel, 2008).

These results reveal that perceptual illusions caused by visuospatial attention may be the flip side of the advantageous effects of visuospatial attention: Because the focusing of visuospatial attention is a necessary precondition for an in-depth representation of a visual stimulus, attention can also delay the perception of a stimulus if it is initially misdirected elsewhere. …

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