Biomedical Engineering Reference
In-Depth Information
The main finding was that subjects performed the task successfully in all three
conditions, with only a minor degradation in performance in the Wall condition.
That is, subjects successfully timed and appropriately scaled the amplitude of shoul-
der rotation to fit through the aperture in all three conditions. These findings sug-
gest that visual information other than eyeheight-scaled information can be used
to guide locomotion through apertures. In particular, both stride-length-scaled and
head-sway-amplitude-scaled information are sufficient to perceive aperture size in
relation to shoulder width, supporting Lee's [
19
] assertions that dimensions of the
world can be perceived in units related to dimensions of the actor's movements. Thus,
for dimensions of the world that are not specified by eyeheight-scaled information,
the availability of other body-scaled information makes it possible to perceive the
environment in relation to one's body dimensions.
4.4 Perceiving Action-Scaled Affordances
The overwhelming majority of research on affordance perception has focused on the
perception of affordances that are constrained by the observer's body dimensions.
There is an entirely different class of affordances that are defined by one's movement
capabilities—that is,
action-scaled affordances
. Most of the few studies on action-
scaled affordances focus on the accuracy with which such affordances are perceived
(e.g., [
26
]). Much less is known about the means by which such affordances are
perceived and their specification by information.
Earlier in the chapter, I described how the availability of body-scaled information
makes it possible to perceive affordances such as passability without knowledge of
body dimensions. In this section, I will attempt to develop an analogous account of
action-scaled affordances. That is, I will show how action-scaled affordances can be
perceived based on information in optic flow without resorting to internal models of
the body.
4.4.1 The Information-Based Approach
I will begin by considering a first attempt by ecological psychologists to explain how
action-scaled affordances could be perceived using visual information alone without
relying on knowledge of action capabilities. Because this approach relies on visual
information alone, I will refer to it as the
information-based approach
. Consider a
person walking toward a shrinking gap between obstacles such as an elevator with
its doors beginning to close. Whether or not the shrinking gap is passable depends
not only on the person's body dimensions but also his or her locomotor capabilities.
In such situations, there exists visual information that specifies whether the person's
current speed of locomotion is sufficient to safely pass through the gap before it
closes [
36
]. That is, such information informs the person whether he or she will pass
