Biomedical Engineering Reference
In-Depth Information
problem of how people choose actions that are appropriately gauged to their body
dimensions. In other words, this study provides an example of the kind of solution
that one might seek in attempting to understand the more general problem of how
people take their body dimensions and movement capabilities into account.
Among the most commonly encountered potential impediments to forward loco-
motion is a narrow opening (or aperture) between obstacles, such as a doorway
or the space between a stationary object and a wall. To select safe and efficient
routes through environments, one must be able to perceive whether such apertures
are sufficiently wide to allow safe passage. Further, it must be possible to perceive
passability in advance—for trial-and-error is neither a safe nor efficient option. Of
course, whether or not an aperture is passable depends not only on the size of the
aperture but also on the size of the observer's body. Therefore, the decision about
whether to pass through or circumvent the aperture must be made in a way that takes
into account the size of the aperture in relation to the size of the body.
The availability of
eyeheight-scaled information
[
28
] offers a potential solution to
this problem. As illustrated in Fig.
4.1
, the width of the aperture (G) is optically spec-
ified in units of eyeheight (E) by
[
2tan
(α/
2
)
]
/
tan
γ
, where
α
is the angle subtended
by the inside edges of the obstacle and
is the angle of declination of the base of the
obstacle (see [
35
] for derivation). The fact that aperture width is specified in units
of eyeheight is important because it means that dimensions of the environment are
specified in the same units as dimensions of the body. Further, because body width
(W) is a fixed proportion of standing eyeheight, such information also specifies aper-
ture width in relation to body width, which is sufficient for perceiving the passability
of the aperture. Consistent with the hypothesis that passability is perceived on the
basis of eyeheight-scaled information, subtle increases in the height of the ground
surface beneath the obstacles make apertures appear more passable [
35
].
Aperture width is not the only dimension that is specified by eyeheight-scaled
information. The horizontal and vertical dimensions of any visible surface that is
γ
Proje
cted e
yehei
ght
α
G
γ
G
W
W
E
½
Fig. 4.1
Optical specification of aperture size by eyeheight-scaled information.
α
is the angle
subtended by the inside edges of the obstacle,
is the angle of declination of the base of the
obstacle, G is the size of the gap, E, W, and L are the observer's eyeheight, body width, and stride
length, respectively
γ
