Affordance Perception and the Visual Control of Locomotion - Human Walking in Virtual Environments - page 79

starting point is to assume some form of knowledge of the dimensions and dynamics
of the body [ 20 ]. For proponents of the computational approach to motor control,
it is axiomatic that the brain builds and exploits internal models of the body and
environment [ 30 ]. Within this framework, there is an abundance of studies aimed at
understanding how the motor system makes use of detailed internal models of the
dimensions of the body [ 15 ], the dynamics of the body [ 29 ], and motor variability
[ 5 , 32 ].
In this chapter, I will consider an alternative approach—one that is inspired by
the ecological approach to perception and action [ 14 ]. Ecological psychologists have
long appreciated the importance of the “fit” between the organism (its dimensions and
movement capabilities) and the environment. Indeed, the idea that the environment
is perceived in relation to the body is one of the conceptual pillars of the ecological
approach. One of the goals of this chapter is to consider how people take their body
dimensions and dynamics into account by appealing to what is provided for free
from the environment in the form of perceptual information.
4.2.2 Affordance Perception and the Control of Locomotion
From an ecological perspective, the ability to take one's body dimensions and move-
ment capabilities into account begins with the perception of affordances—that is,
possibilities for action provided by the environment for an animal with particular
dimensions and capabilities [ 14 , 33 ]. Affordances differ from conventional spatial
properties in that they reflect the fit between the animal and its environment. For
example, the passability of a gap between a tree and a boulder depends not only on
the physical size of the gap but also on the actor's body dimensions. Similarly, the
leapability of a stream depends not only on the width of the stream but also on the
actor's leaping capabilities. Because affordances reflect the fit between the actor and
the environment, the perception of affordances makes it possible to choose actions
that are appropriately gauged to one's body dimensions and dynamics.
There is a vast amount of empirical research assessing the accuracy with which
affordances can be perceived. A review of this literature is beyond the scope of this
chapter (but see [ 9 ] for a recent review). Instead, the focus will be on how affordances
are perceived, the available information that makes it possible to perceive affordances,
and the role of affordance perception in the control of locomotion.
4.2.3 Eyeheight-Scaled Information
Although the main focus of this chapter is on recent developments in the study of
affordance perception, it is worth taking a brief detour to consider a classic study by
Warren and Whang [ 35 ] on the perception of passability. This study is a good place
to start because it offers an instructive example of the ecological approach to the