Biomedical Engineering Reference
In-Depth Information
fact, such metrics need not be defined by dimensions of the body. As anticipated by
Lee [
19
,
34
], dimensions of the actor's movements provide other possible yardsticks.
For example, legged locomotion is normally accompanied by side-to-side head move-
ments (i.e., lateral head sway). As a person approaches an aperture, the size of the
aperture is optically specified in units of lateral head sway amplitude (A) by a higher-
order variable involving the optical expansion rates
( φ)
( θ)
of the
obstacles on the left and right sides (Fig.
4.2
; see also, [
11
] for the full derivation). In
other words, just as there is information about aperture size in units of eyeheight [
35
],
there is also information about aperture size in units of lateral head sway amplitude.
To the degree that lateral head sway amplitude remains roughly constant during
normal locomotion, its relation to body width is relatively stable. Therefore, just
as the perceptual system can calibrate itself to the relation between eyeheight and
body width, it can also calibrate itself to the relation between head sway amplitude
and body width. Head-sway-scaled information constitutes an alternative source of
information for perceiving aperture size in units of body width.
Another dimension of the observer's movement that can serve as a yardstick for
scaling dimensions of the environment is stride length. In his 1980 paper, Lee showed
that the size of an approached object, such as an aperture, is optically specified in
units of stride length (L) by
and drift rates
ατ
α
/
t
s
, where
τ
α
is the ratio of
α
to the first temporal
derivative of
).
Whereas eyeheight-scaled information is static (i.e., available even when the
observer is stationary), head-sway-scaled and stride-length-scaled information are
dynamic in that the observer must be moving. However, both dynamic sources of
information specify aperture width in body-scaled units regardless of whether the
α
and
t
s
is stride duration (see Fig.
4.1
for definition of
α
position of observer when lateral
head position(
x
h
) is equal to
lateral head sway amplitude (A)
A
G
x
h
φ
˙
˙
˙
˙
R
(
t
x
h
=
A
)
L
(
t
x
h
=
A
)
˙
G
A
R
(
t
)
L
(
t
)
˙
1
R
(
t
x
h
=
A
)
+
1
2
˙
˙
R
(
t
)
L
(
t
)
L
(
t
x
h
=
A
)
Fig. 4.2
Optical specification of aperture size by stride-length-scaled information illustrated from
a top-down view. As the observer on the
left
approaches the aperture between two obstacles (
dark
gray circles
)onthe
right
, the size of the gap (G) is optically specified in units of lateral head sway
amplitude (A) by a higher-order variable involving the optical expansion rates
( φ)
and drift rates
( θ)
of the obstacles on the
left
(L) and
right
(R) sides
