Biomedical Engineering Reference
In-Depth Information
The results of Fajen et al. [
6
] do not support this prediction and demonstrate that the
range of conditions across which people can accurately perceive action-scaled affor-
dances is broader than one would expect based on the information-based account.
This should not come as a surprise, for it is not enough to know whether an action is
within or beyond one's limits only when moving.
4.5.1 An Alternative Account
If people do not rely on information about their current future to perceive action-
scaled affordances, then how are such affordances perceived? Does the fact that
people reliably perceive such affordances even when stationary mean that they must
rely on knowledge of their locomotor capabilities (i.e., an internal model)? In this
section, I will introduce an alternative account that still bypasses the need for knowl-
edge of locomotor capabilities but better accounts for the range of conditions across
which action-scaled affordances are perceived. I will illustrate this approach using
the same shrinking gap task that was described above.
A shrinking gap is passable if the minimum locomotor speed needed to safely pass
through the gap (
ν
min
) is less than the actor's maximum possible locomotor speed.
In terms of spatial variables,
ν
min
is equal to the minimum distance that the observer
must travel to pass between the obstacles divided by the amount of time remaining
until the size of the gap is equal to the width of the observer's body; that is,
)
=
z
m
(
)
/(
t
∗
)
−
t
∗
−
ν
min
(
t
z
o
(
t
t
)
(4.1)
where
z
m
and
z
o
are the positions along the z-axis of the moving obstacle and the
observer, respectively, and
t
∗
is the time at which the size of the gap is equal to the
width of the observer's body (see Fig.
4.4
a). This is equivalent to:
1
k
E
g(
ν
min
(
t
)
[
z
m
(
t
)
−
z
o
(
t
)
]
˙
z
m
E
×
)
=
+
TTC
×
−
(4.2)
t
∗
−
E
/(
t
E
t
)
where
E
is the observer's eyeheight,
z
m
is approach speed of the obstacle,
TTC
(time-
to-contact) is the amount of time remaining until the obstacle reaches the locomotor
axis,
k
is a constant equal to
W
˙
E
, and
g
is the spatial gap between the inside edge of
the obstacle and the z-axis (see Fig.
4.4
a). As shown in Fig.
4.4
b, each component of
Eq.
4.2
(and therefore
/
ν
min
itself) is optically specified. Therefore, one can perceive
the minimum locomotor speed needed to safely pass through the shrinking gap. (See
[
7
] for the full derivation of Eq.
4.2
and its optical specification).
There are four important points to be made about the information for
ν
min
in
Fig.
4.4
.First,
ν
min
is specified in a way that takes into account the physical sizes of
the obstacles and the observer's body. Therefore, by detecting such information, the
passability of the shrinking gap can be perceived in a way that takes these properties
into account. Second,
ν
min
is specified in units of
E
)
, which is the number of
t
∗
−
/(
t
