Biomedical Engineering Reference
In-Depth Information
(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