228 Depth Perception and the Perception of Events
followers of this second proposal used more naturalistic set-
tings, most studies on the perception of heading have used
random-dot displays simulating the optical motion that
would be produced by an observer moving relative to a
ground plane, a three-dimensional cloud of dots, or one or
more fronto-parallel surfaces at different depths.
Overall, empirical investigations on heading show that the
human visual system can indeed recover the heading direc-
tion from velocity fields like those generated by the normal
range of locomotion speeds. The psychophysical studies in
particular have revealed the following about human percep-
tion of heading: It is remarkably robust in noisy flow fields
(van den Berg, 1992); it is capable of making use of sparse
clouds of motion features (Cutting et al., 1992) and of ex-
traretinal information about eye rotation (Royden, Banks, &
Crowell, 1992); and it improves in its performance when
other three-dimensional cues are present in the scene (van
den Berg & Brenner, 1994). Some of the proposed computa-
tional models embody certain of these features, but so far no
model has been capable of mimicking the whole range of ca-
pabilities revealed by human observers.
Vection
Observers sometimes experience an illusory perception of
self-motion while sitting in a stationary train and watching an
adjacent train pulling out of the station. Thistrain illusionis the
best-known example of vection (Fisher & Kornmüller, 1930).
Vection can be induced not only by visual, but also by auditory
(Lackner, 1977), somatosensory (Lackner & DiZio, 1984),
and combined somatosensory and kinesthetic (Bles, 1981) in-
formation. The first studies on visually induced vection can be
dated back to Mach (1875) and were performed using a verti-
cally striped optokinetic drum or an endless belt (Mach, 1922).
Two kinds of vection can be distinguished: circular and linear.
Circular vection typically refers to yaw motion about the verti-
cal axis, whereas linear vection refers to translatory motion
through a vertical or horizontal axis. Vection is calledsatu-
ratedwhen the inducing stimulus appears to be stationary and
only self-motion is perceived (Wertheim, 1994).
Linear vection is typically induced about 1–2 s after the
onset of stimulation (Giannopulu & Lepecq, 1998), circular
vection after about 2–3 s, and saturated vection after about 10 s
(Brandt, Dichgans, & Koenig, 1973). A more compelling
vection is induced by faster speeds of translation or rotation
(Larish & Flach, 1990), by low temporal frequencies (Berthoz,
Lacour, Soechting, & Vidal, 1979), by more or larger elements
(Brandt, Wist, & Dichgans, 1975); this is also the case when
larger retinal areas are stimulated (Brandt et al., 1975) and
when the inducing stimulus belongs to the background relative
to the foreground (Nakamura & Shimojo, 1999).
Visual Control of PosturePostural stability, or stance, is affected by visual, vestibular,
and somatosensory information. Visual and somatosensory
information are more effective in the low-frequency range of
postural sway, whereas vestibular information is more effec-
tive in the high-frequency range (Howard, 1986). A device
known as the moving roomhas been used to demonstrate that
visual information can be used to control posture. In their
original study, Lee and Aronson (1974) required infants to
stand within a room in which the walls were detached from
the floor and could slide back and forth. They reported that
when the walls moved, infants swayed or staggered in spite
of the fact that the floor remained stationary, a finding later
replicated by many other studies (Bertenthal & Bai, 1989; for
adult, see Lee & Lishman, 1975).
Two sources of visual information are available for pos-
tural control: the radial and lamellar motions of front and side
surfaces, respectively, and the motion parallax between ob-
jects at different depths that is generated by the translation of
the observer’s head (Warren, 1995). Evidence has shown that
posture is regulated by compensatory movements that tend to
minimize both of these patterns of optical motion (Lee &
Lishman, 1975; Warren, Kay, & Yilmaz, 1996). Three hy-
potheses have been proposed concerning the locus of retinal
stimulation: (a) theperipheral dominance hypothesis,which
states that the retinal periphery dominates both the perception
of self-motion and the control of stance (Dichgans & Brandt,
1978); (b) theretinal invariance hypothesis,which states that
self-motion and object motion are perceived independently of
the part of the retina being stimulated (Crowell & Banks,
1993); and (c) thefunctional sensitivity hypothesis,which
states that “central vision accurately extracts radial...and
lamellar flow, whereas peripheral vision extracts lamellar flow
but it is less sensitive to radial...flow”(Warren & Kurtz,
1992, p. 451). Empirical findings have contradicted the pe-
ripheral dominance hypothesis (Stoffregen, 1985) and the
functional sensitivity hypothesis (Bardy, Warren, & Kay,
1999). Instead, they support the retinal invariance hypothesis
by emphasizing the importance of the optic-flow structure for
postural control, regardless of the locus of retinal stimulation.Perceiving Approaching ObjectsTime to Contact and Time to PassageCoordinating actions within a dynamic environment often
requires temporal information about events. For example,
when catching a ball, we need to be able to initiate the grasp
before the ball hits our hand. One might suspect that in order
to plan for the ball’s time of arrival, the perceptual system