In Experiment 1 and 2, we demonstrated that contrast affected both the footsteps
and fan illusions in the same way regardless if motion signals at the leading or trailing
edges were present (Experiment 1A) or absent (Experiment 2). Furthermore, a motion
capture variation of the fan illusion analogous to that of the footsteps illusion (Howe
et al., 2006) also varied with contrast as predicted (Experiment 1B). These results are
consistent with Anstis’s (2001) and Howe et al’s (2006) explanations of the footsteps
illusion.
However, according to Howe et al.’s (2006) model, straddled and un-straddled
versions of the static motion footsteps illusion should be affected by the change in
contrast of background stripes in the same way. Contrary to this prediction, in the
absence of motion signals from the leading and trailing edges of the moving bar, only
the un-straddled versions of the footsteps and fan illusion had decreased illusion
strength at higher stripe contrasts while the straddled versions remained the same
(Experiment 2). This result indicates that something more is involved in determining
illusion strength other than the contrasts of the edges.
A critical difference between the two versions is the presence of
deletion/accretion cues in the un-straddled versions revealed by contrast-induced
surface segregation of the occluding stripes and occluded bars/fans. The manner
26
objects disappear and subsequently reappear indicate both the presence of an occluder
and the spatiotemporal continuity of the moving objects. Spatiotemporal continuity
has often been regarded as a necessary or even sufficient condition for object
persistence (Scholl, 2007). During occlusion, mid-level representations of objects (the
object files, Treisman, 1992) are maintained by spatiotemporal continuity (Cheries et
al., 2009; also see Flombaum, et al., 2009, for a review) and representation of
spatiotemporal continuity is sufficient for representing identical objects even after
complete feature change during occlusion (the tunnel effect, Michotte, 1991). The
primate brain might have been hard-wired to realistically represent the way objects
move according to physical laws, even when they are occluded (Scholl, 2007).
Human and nonhuman primates prioritize spatiotemporal continuity when
tracking objects (Flombaum et. al., 2009). We propose that (1) the cause of the static
capture illusion was the lack of conditions for tracking and (2) attentive tracking aids
in smoother motion perception. This was supported by Experiment 2 and 3. In
Experiment 2, the presence of depth (surface segregation) and deletion/accretion cues
(in the un-straddled fans) resulted in smoother motion perception. In Experiment 3A,
longer overlapping durations taxing attentional resources further increased the static
capture of equiluminant fans and footsteps displays. In Experiment 3B, interfering
distracters caused the smoother mid-contrast fan in Experiment 2 to “stop.”
27
There may be more factors influencing attentive tracking, such as the importance
of object history to the object updating process (Moore, Mordkoff, & Enns, 2007). In
the straddled versions, the moving fans/bars are never completely in view, but in the
un-straddled versions, they are entirely exposed before disappearing completely.
Therefore, the smoother motion perception in the latter might also be caused by better
attentive tracking with longer viewing histories.
Attentive tracking can explain the difference in illusion strength in the straddled
and un-straddled footsteps illusions, while Howe et al.’s (2006) explanation would
lead to somewhat odd implications. Howe et al. (2006) explain the stronger illusion
for the straddled version with larger contrast-weighted stationary signals from the top
and bottom edges of the moving bar. However, when the moving bar and stationary
stripes were equiluminant, even though there were no motion signals when all leading
and trailing edges overlap with the stripes, the illusion strength was still different for
the two versions (Experiment 2). We explain the stronger illusion of the straddled
version by impairment of attentive tracking, but according to Howe et al. (2006), this
is because the stationary signals of the top and bottom edges of the straddled bar have
made the already stationary display become even more stationary!
Furthermore, motion capture has been shown to be dependent on whether the
stationary and moving objects are represented on the same surface (Cavanagh, 1992;
28
Culham & Cavanagh, 1994; Ramachandran & Anstis, 1986). Therefore, the effect of
contrast manipulation in Experiment 1B could also be caused by the segregation of
surfaces that decreases motion capture. As other motion capture stimuli have been
shown to be modulated by attentive tracking (Cavanagh, 1992; Culham & Cavanagh,
1994), the motion capture variation of the footsteps and fan illusions might also share
common mechanisms related to attentive tracking.
Previous studies in the footsteps illusion have overlooked attention as a factor
influencing perceived motion. Anstis (2001, 2004) mentioned that the footsteps
illusion is stronger in the observer’s peripheral vision. Sunaga, Sato, Arikado, and
Jomoto (2008) demonstrated that in the footsteps illusion, low frequency samplings of
a static contrast-induced mis-alignment illusion contributed largely to the alternating
mis-alignments of the black and white moving bars. As high-spatial-frequency
information is less sensitive in peripheral vision, they concluded that this was the
main cause of the illusion. However, Intriligator and Cavanagh (2001) found that the
resolution of attention scales with larger eccentricity and is coarser in the upper visual
field and along the radial lines from fixation. Therefore, attention and eccentricity
may be confounded in these findings. As this study shows that attentional modulation
can affect and may be the cause of the footsteps and fan illusions, the role of attention
can be a future line of investigation for footsteps and other contrast-dependent motion
29 illusions.
30
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(a)
(b)
Figure 1. The standard footsteps illusion consists of black and white bars that appear to
stop-and-go when moving across black-and-white stripes (a). In the standard fan illusion, a smaller rotating fan also appears to stop-and-go when superimposed with a stationary
35 fan (b).