Biomedical Engineering Reference
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(a)
g(t)
z
z m (t)
z m (t*)
z o (t)
x
W
(b)
()
()
()
z m t
z o t
v min
t
z m
E
E
gt
=
TTC 1 k
(
)
()
E *
E
t
-1
˙
sin
[
]
˙ /2
()
()
z m t
1 tan
obstacle distance (E)
z o t
E =
TTC
=
( /2)
time-to-contact gt
tan
()
E = tan( /2) tan
gap size (E)
z m E
=
m sin 2
approach speed (E)
Fig. 4.4 a Top-down view of observer and a pair of converging obstacles at time t ( black circles )
and time t ( gray circles ). t is the time at which the size of the gap (g) between obstacles is equal
to the observer's body width (W). b Optical specification of minimum walking speed (
ν min )
eyeheights that must be covered in the amount of time remaining until the obstacle
reaches the locomotor path. Because such units are intrinsic rather than extrinsic,
information about
ν min in relation
to maximum locomotor speed, in the same way that eyeheight-scaled information
about aperture size can be calibrated to allow for the perception of aperture size in
relation to body width [ 35 ]. Third, information about
ν min can be calibrated, allowing one to perceive
ν min is available regardless
of whether the observer is stationary or moving. Therefore, unlike the information-
based approach described in the previous section, this approach accounts for the
fact that stationary and moving observers can perceive passability equally well [ 6 ].
Putting these first three points together, detecting and calibrating information about
ν min allows for the direct perception of the passability of a shrinking gap by stationary
and moving observers, taking into account both the width of the observer's body and
his or her locomotor capabilities.
A fourth point is that detecting information about
ν min requires the visual system
to recover the object-motion component of optic flow independent of self-motion.
When a person moves in the presence of other moving objects, the optic flow field is
 
 
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