The perceived position of a moving object is not the result
of position integration
Mowei Shen, Jifan Zhou, Tao Gao, Junying Liang, Rende Shui*
Department of Psychology and Behaviroal Sciences, Xixi Campus, Zhejiang University, Hangzhou, Zhejiang 310028, PR China
Received 15 May 2007; received in revised form 13 August 2007
The flash-lag effect is a robust visual illusion in which a flash appears to spatially lag a continuously moving stimulus, even though
both stimuli are actually precisely aligned. Some research has been done to test how visual information has been integrated over time.
The position integration model suggests motion integration is a form of interpolation of past positions, and predicts that we cannot per-
ceive the reversal point at its actual position on the trajectory of a moving object which reverses abruptly. In current research, we dem-
onstrate that subjects could perceive the reversal point accurately while the psychometric function measured by a flash does not pass
through the actual turning point. These results do not support the position integration model. We propose that the flash-lag effect is
more likely to be a temporal illusion.
? 2007 Elsevier Ltd. All rights reserved.
Keywords: Flash-lag effect; Motion perception; Postdiction model
The world we live in is always changing. Our visual sys-
tem must deal with information changing time from time.
One important task of visual system is to perceive the posi-
tion of moving objects. Current work on the flash-lag effect
has helped us to understand how the visual system solves
this problem. The flash-lag effect is a robust visual illusion
in which a flash appears to spatially lag a continuously
moving stimulus, even though both stimuli are actually
precisely aligned (Nijhawan, 1994; Whitney, 2002).
It has been argued intensely whether the flash-lag effect
is a temporal illusion or a spatial illusion (Eagleman & Sej-
nowski, 2002). The ‘‘latency difference’’ model takes the
flash-lag effect as a temporal illusion, which asserts that
moving objects are processed more quickly than flashed
objects, so by the time the flashed object reaches the ‘‘per-
ceptual end point’’, the moving object has already moved
to a new position (Whitney & Cavanagh, 2000; Whitney
& Murakami, 1998; Whitney, Murakami, & Cavanagh,
2000). Alternatively, some other researchers suggested that
the flash-lag effect is a spatial illusion. Eagleman and Sej-
nowski proposed a ‘‘postdiction’’ model of the flash-lag
effect, which assumes that the position of moving objects
is estimated by integrating continuous positional signals
within a time window; the flash resets all the integrals, so
only those starting immediately after the flash will produce
a position estimate, and the forward average is necessarily
in advance of the position of the flash (Eagleman & Sej-
nowski, 2000a, 2000b, 2000d, 2002). Krekelberg and Lappe
proposed a similar model, and according to their model,
the perceived position of flashed and moving objects is
based on the temporal integration of moving objects’ posi-
tion after the flash within a larger time window than that in
Eagleman (Krekelberg & Lappe, 2000a, 2000b).
The spatial model has aroused much attention in
recent years. These two spatial models both assume that
the positional perception of moving objects is the result
of integrating positional signals over time. This ‘‘position
integration’’ hypothesis predicts that if a moving object
abruptly reverses direction, the perceived moving object
0042-6989/$ - see front matter ? 2007 Elsevier Ltd. All rights reserved.
E-mail address: email@example.com (R. Shui).
Available online at www.sciencedirect.com
Vision Research 47 (2007) 3088–3095
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M. Shen et al. / Vision Research 47 (2007) 3088–3095