Biomedical Engineering Reference
In-Depth Information
perpendicular to the line of sight and resting on the ground plane are specified
in intrinsic units by eyeheight-scaled information. Thus, affordances such as the
climbability and sit-on-ability of horizontal surfaces can be directly perceived using
eyeheight-scaled information [
21
,
37
]. Distances along the ground plane are also
optically specified in units of eyeheight [
28
].
Nonetheless, eyeheight-scaled information alone is not sufficient for a general
affordance-based account of visually guided locomotion. There are other important
aspects of locomotion that require people to take their body dimensions and action
capabilities into account but cannot be attributed to the use of eyeheight-scaled infor-
mation alone.
First, eyeheight-scaled information cannot be the only means by which dimen-
sions of the environment are perceived in relation to dimensions of the body. For
such information to reliably specify the size of an object, that object must be resting
on the ground plane. Although objects are not normally suspended in the air, it is not
uncommon for parts of objects to extend outward from their base of support. Thus,
the size of an aperture formed by an overhanging tree branch or a kitchen countertop
that juts out into an opening cannot be perceived on the basis of eyeheight-scaled
information. Moreover, when the environment contains slopes, stairs, or tiers of level
ground surfaces, relying on eyeheight-scaled information can lead to biases in per-
ceived size [
35
,
37
]. Are there sources of information other than eyeheight-scaled
information that specify dimensions of the environment in relation to dimensions of
the body? Second, the affordances that I have discussed in this section are
body-scaled
affordances
in that they reflect relations between dimensions of the environment and
dimensions of the actor's body. There are also affordances that are defined by the
actor's movement capabilities, which are referred to as
action-scaled affordances.
For example, if an aperture is bounded by a pair of moving obstacles that are con-
verging toward each other to form a shrinking gap, then passability depends not only
on body dimensions but also on locomotor capabilities, such as how quickly the
person can move. Can we account for the perception of action-scaled affordances
without invoking internal models of the dynamics of the body? Third, once an appro-
priate action is selected, reaching the goal often involves guiding movement based
on continuously available information, which also requires one to take one's body
dimensions and movement capabilities into account. The role of affordance percep-
tion in selecting actions has been widely investigated but its role in on-line control is
less well understood. In the remainder of this chapter, I will discuss recent attempts
to extend the theory of affordances to address the three aforementioned problems.
4.3 Perceiving Body-Scaled Affordances
Although the role of eyeheight-scaled information is well established, there are other
sources of information that specify dimensions of objects, including those that are
not resting on the ground surface. In other words, eyeheight is not the only yard-
stick by which properties of the world are optically specified in intrinsic units. In
