Biomedical Engineering Reference
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through the gap if current locomotor speed is maintained. Bootsma [
3
] referred to
this as one's
current future
because it concerns one's future assuming that current
conditions (e.g., current locomotor speed) are maintained. For the purposes of this
discussion, the particular optical variable that specifies the sufficiency of one's current
speed is less relevant than the fact that such information could be used to perceive
whether the gap is passable. For example, if the person is moving fast enough to
safely pass through the gap before it closes, then the aforementioned information
specifies that the gap is passable (assuming current speed can be maintained). On
the other hand, if the person is moving as quickly as possible but will not reach the
gap before it closes, then the aforementioned information specifies that the gap is
impassable.
A similar approach was proposed by Oudejans et al. [
25
] to explain how out-
fielders in baseball perceive whether a fly ball is catchable. In that case, the relevant
information specifies whether the fielder's current running speed is sufficient to reach
the landing location in time to catch the ball [
4
,
23
,
24
]. Thus, if an outfielder is run-
ning fast enough to reach the landing location in time to catch the ball and he or she
can maintain that speed, such information can be used to perceive that the ball is
catchable. Likewise, if the outfielder is running as fast as possible but will not reach
the landing location in time, such information can be used to perceive that the ball
is uncatchable.
For the present purposes, the important point is that the availability of information
about the sufficiency of one's current state makes it possible to perceive action-scaled
affordances such as passability and catchability based on visual information alone
without having to rely on knowledge of one's running capabilities. The same would
apply to other tasks for which there is information about one's current future, such
as intercepting a moving target [
10
] and braking to avoid a collision [
18
,
38
].
4.5 Testing the Information-Based Approach
An important corollary of this hypothesis is that affordances are specified only when
the observer is actually moving. When the observer is stationary, the information
specifies only that his or her current speed (which is zero if the observer is station-
ary) is not sufficient. The information does not specify whether the action is still
within the person's capabilities once movement is initiated. This leads to the testable
prediction that movement is necessary to perceive action-scaled affordances; that is,
if action-scaled affordances are perceived on the basis of information about the suf-
ficiency of one's current state, then because such information is available only when
observers are moving, stationary observers should perceive such affordances less
accurately [
25
].
This prediction was recently tested in a set of experiments conducted in an immer-
sive VE [
6
]. The task was to judge whether it was possible to safely pass through a gap
between a pair of converging obstacles before the gap closed. The initial distance to
the point of convergence and the rate of closure varied across trials, yielding a range
