6
An Action-Specific Approach
to Spatial Perception
Dennis R. Proffitt
People have conjectured about spatial perception for millennia, and
have studied it in earnest for well over a hundred years. As can be
seen in current perception textbooks, spatial perception is typically
viewed as a general-purpose representation of the environment’s layout. The perception of surface layout is generally thought to be unaffected by people’s bodies, what they might be doing, or their internal
physiological states. Textbooks divide spatial perception into topics
defined by distal environmental properties such as the perception of
distance, size, and shape. In this view, spatial perception is specific
to the environmental properties that are perceived.
This chapter provides a different approach. Extending Gibson’s
(1979) theoretical approach, spatial perception is here viewed as a
biological adaptation that supports our species’ ways of life. From
this perspective, spatial perception is action-specific and the subdivision of the field is made relative to the actions that spatial perception supports, such as reaching, grasping, walking, and throwing.
Perception relates spatial layout to one’s abilities to perform intended
actions and also to the inherent costs associated with their performance. In essence, it is proposed that people see the world as “reachers,” “graspers,” “walkers,” and so forth. By this account, perception
relates and is influenced by three factors: the visually specified environment, the body, and purpose.
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The Visual Specification of the Environment
For a moving observer in a natural setting, the environment’s spatial
layout is well specified by optical and ocular-motor variables (Proffitt
& Caudek, 2002; Sedgwick, 1986). Viewed in isolation, a great deal is
known about the human sensitivity to each of the myriad of visual
variables that specify environmental properties. On the other hand,
understanding how these variables are combined when information
is redundant—the problem of cue integration—has proven to be a
tough problem to solve. There are almost no studies that attempt to
model the combination of more than two variables. This is because,
as the number of specifying variables increases, the number of possible combinations of these variables that would need to be investigated becomes prohibitively large (Cutting & Vishton, 1995). The
problem of cue integration has relevance for the current argument
because of the possibility that the combination of visual information
is action-specific.
Most current models of cue integration rely upon some variant of
weighted averaging in which each specifying variable is used to derive
an estimate of the relevant environmental property, each estimate is
then weighted by its prior reliability, and then a weighted average is
taken (cf. Landy, Maloney, Johnston, & Young, 1995). An especially
intriguing alternative has been proposed by Domini, Caudek, and
Tassinari (2006). In their model, information is combined directly
without first deriving the environmental property to which it relates.
In contrast to weighted averaging models, in Domini et al.’s model,
environmental properties are derived only after the information has
been combined.
The intractability of cue integration in natural environments bears
upon a fundamental issue. Currently, it is not known whether cue
integration is influenced by what the perceiver is doing. It is possible
that the processes that weight or combine specifying variables do so
differently, depending upon what the perceiver is trying to do. This is
one of many possible mechanisms by which purpose and action may
influence perception.
Another possible mechanism derives from the fact that specifying variables are sampled differently depending upon the perceiver’s
goals. It has long been know that eye movements are strongly influenced by purpose (Yarbus, 1967). In his review of eye movements
and the control of actions, Land (2006) wrote,
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One of the main conclusions from this review is that eye movement strategies are very task-specific. They principally involve the
acquisition of information needed for the execution of motor actions in
the second or so before they are performed and in the checking of
the execution of each action. (p. 322)
Land reminds us that where people look depends upon what they
are attempting to do, and thus, the sampling of visual information
is action-specific.
Visual attention is also action-specific. Employing a visual search
paradigm, Bekkering and Neggers (2002) presented participants
with physical arrays of blocks that varied in both orientation and
color. A block’s orientation influenced the hand posture that would
be required to grasp it. On each trial, participants attended to a fi xation dot and were instructed to make a saccade to a block having
a specified orientation and color. Following the saccade they were
instructed to either point or grasp the block. It was found that there
were fewer erroneous saccades to blocks having the wrong orientation when participants were intending to grasp the block as opposed
to when they were intending to point to it. The number of saccades
to blocks of the wrong color was unaffected by the intended-action
manipulation. This study showed that intentions to perform an
action such as grasping, which must accommodate to an object’s orientation, can influence the visual processing of object orientation.
The Body
The body has an exterior and an interior. The exterior consists of
the body’s form, which enables a behavioral potential as determined
primarily by the skeleton and skeletal muscles. The exterior body
performs actions in the external environment. The body’s interior
consists of the plethora of organs, glands, and physiological systems
that sustain life. A principal function of the brain is to control the
body so as to achieve desired states in both the external environment
and the body’s internal environment.
Studies in behavioral ecology show that the behavior of organisms
is primarily governed by energetic and reproductive imperatives
(Krebs & Davies, 1993). With respect to energy, organisms have been
shaped by evolution to follow behavioral strategies that optimize
obtaining energy (food), conserving energy, delivering energy to
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their young, and avoiding becoming energy for predators. To meet
these ends, species have evolved behavioral strategies for achieving
desired outcomes in the external physical environment while concurrently maintaining desired states in the internal environment of
the body.
The current account suggests that spatial perception promotes
effective and efficient behavior by directly relating the visually specified environment to the possibilities and costs of intended actions.
To achieve this, perception must be action-specific. For example,
when people intend to walk, they see the world as “walkers.” Their
perception will reveal where walking is possible in relation to their
walking-relative physiological potential as well as the energetic costs
associated with walking. If instead, people view the same scene as
“throwers,” then perceiving the possibilities and cost of locomotion
become irrelevant. People can throw a ball over a gorge that does not
afford walking.
A concrete example from behavioral ecology is here provided to
illustrate a potential advantage of perceiving the environment relative to one’s action potential. An animal’s assessment of the risk of
an approaching predator can be determined by measuring how close
the predator can come before the animal initiates flight (Stankowich & Blumstein, 2005; Ydenberg & Dill, 1986). An iguana with
a cool body temperature will flee from a predator at a greater distance than will one with a warmer body (Rocha & Bergalo, 1990).
Because the reptile is more metabolically efficient at warmer body
temperatures, its maximum escape speed and body temperature are
positively correlated. Thus, the risk of being caught by a predator is a
function of both the distance of the predator and the iguana’s body
temperature. It is, of course, not known what iguanas perceive, but
two possibilities come to mind. It could be that the iguana sees the
distance to the approaching predator as being the same regardless of
body temperature. The iguana might be supposed to have a generalpurpose distance perception system that is unaffected by intended
action or physiological state. In this case, when deciding whether to
flee, the reptile would have to relate the predator’s apparent distance
to its body temperature and the implications of this metabolic state
to its running speed. Another possibility, in line with the current
approach, is that iguanas see predators as being closer when their
bodies are cool as compared to when they are warm. In this case,
the iguana flees whenever it sees the predator’s proximity as falling
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within an invariant flight-specifying distance. The iguana does not
have to relate perception to its potential for action because this has
already been achieved in perception.
The mechanisms by which physiological state might influence
visual processing are many. The brain resides in a chemical milieu
in which many aspects of the body’s interior state are directly manifested in hormones, neurotransmitters, and various dimensions
of blood’s composition. Given that the neural correlates of visual
awareness are associated with very late processing in the temporal
lobe (Koch, 2004), there are also numerous opportunities for neural
influences from both visual and nonvisual areas.
Purpose
The argument that perception is action-specific demands a fundamental role for purpose; perception is specific to the action that is
intended. People in the same situation will see the world differently
depending upon what they are intending to do. People see the world
as “walkers” only if they intend to walk or “throwers” only if they
intend to “throw.” As will be discussed later, a manipulation that
influences the effort required to walk but not to throw, will influence
people’s perception of distance if they intend to walk but not if they
intend to throw (Witt, Proffitt, & Epstein, 2004).
The remainder of this chapter will describe spatial perception
from an action-specific perspective. It will be shown, for example,
that objects within reach appear closer than those that are out of
reach, and that since reachability is extended by holding a tool, apparent distances are influenced, accordingly: Objects that are within
reach when holding a tool, but out of reach when the tool is not held,
appear closer when the tool is held and the “reacher” intends to use
it (Witt, Proffitt, & Epstein, 2005). Other studies show that egocentric extents are expanded when walking is made more effortful due
to the wearing of a heavy backpack (Proffitt, Stefanucci, Banton, &
Epstein, 2003). Such effects cannot be accommodated by approaches
that conceptualize distance perception as a general-purpose representation of the environment. Hand tools influence “reaching distance,” whereas backpacks influence “walking distance.” Perceptions
are here viewed as being action-specific, as opposed to being specific
to distal environmental properties such as distance.
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Reaching
Near space is defined by the extent of a person’s reach or slightly
beyond, and thus, it is an instance of a dimension of spatial layout
that has an action-specific definition. Others have referred to this
region as personal space (Cutting & Vishton, 1995) or peripersonal
space (Lavadas, 2002). Near space can be expanded by providing
people with a hand tool that extends their reach. When this is done,
previously out of reach objects will fall within near space, and as a
consequence, these objects will appear closer than they did before
the tool was held (Witt, Proffitt, & Epstein, 2005). Being reachable
has consequences for an object’s visually perceived distance.
In the Witt et al. studies, participants sat at a table upon which
targets were projected by a digital projector in the ceiling. On each
trial, a target was projected and participants judged its egocentric
distance using a visual-matching task. After making this distance
judgment, participants reached out and touched the target if it was
within reach and pointed to its location if it was not. The experimental manipulation was defined by whether or not the participants held
a conductor’s baton that extended their reach. It was found that targets that were out of reach without the baton, but within reach when
it was held, were perceived to be closer when the baton was held, as if
judgments of proximity incorporated reachability. In another study,
Witt et al. showed that the influence of holding the baton is entirely
dependent upon whether participants intended to reach with it. The
previously described experimental design was repeated except that,
after making the distance judgments, participants never reached out
to touch the targets. In this study, holding the baton had no effect on
the apparent distance to the targets.
Two conclusions can be drawn from the Witt et al. studies. First,
the apparent distance to objects is influenced by whether or not they
can be touched. Extending one’s reach with a tool diminishes the
apparent distance to objects that become touchable only through its
use. Second, reachable space is not rescaled if a tool is held with no
intent to use it. Perception is influenced by the behavioral potential
to perform intended actions.
Interesting parallels to the Witt et al. findings can be found in
the electrophysiology and cognitive neuroscience literatures. Iriki,
Tanaka, and Iwamura (1996) found that the macaque monkey possesses visual neurons in the intraparietal sulcus that fire when a rai-
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sin is in its near space. These cells fire when a visible raisin could be
grasped and eaten but not when it was seen to be out of arm’s reach.
Iriki et al. then trained monkeys to use a rake to acquire raisins that
were beyond their grasp without it. Neurons that had previously
not fired to raisins beyond arm’s reach now fired to raisins within
rake’s reach. This study indicates that macaque monkeys—and most
likely people—possess visual neurons that code for the reachability
of objects and that these cells rescale the spatial range of reachability
when a tool is held and used.
Research with neglect patients has also shown that near space
can become rescaled through tool use. Neglect patients ignore much
of what is present in the left side of their visual field. A common
diagnostic assessment for neglect is to ask patients to bisect a line
presented in the frontal plane before them. People with neglect will
indicate a position on the line that falls far to the right of actual
center, thereby indicating that they have neglected all or most of
the left side of the line. With respect to the symptoms of neglect,
a double dissociation between near and far space has been found.
Some patients show neglect only for lines in near space (Halligan
& Marshall, 1991), whereas others show neglect only for far lines
(Cowey, Small, & Ellis, 1994). Patients who show neglect only in near
space will respond accurately on the bisection task if they use a laser
pointer to indicate the center of a line that is beyond reach. However, if a stick is used that allows them to indicate the line’s center
by touching it, then neglect will again be exhibited (Berti & Frassinetti, 2000; Pegna et al., 2001). These latter findings show that far
space can become remapped into near space through tool use, and
that this remapping has an influence on the perceptual processing of
these patients. Specifically, physical contact, even if indirect, seems
to invoke the mechanisms underlying neglect, whereas distal localization alone does not.
Together, the studies reviewed in this section indicate that reachability has visual consequences. Behavioral studies show that objects
in near space appear closer than those that are not. With tool use,
more distant objects become reachable, and consequently, they are
perceived to be closer. The electrophysiological studies with macaque
monkeys show that visual neurons exist, which code for reachable
objects, and that these cells will rescale reachable space as a result
of learning to use a tool. Finally, studies of patients, who experience
neglect only in near space, indicate that the neural mechanisms
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responsible for their neglect are specific to reachability and not to
absolute distance.
Grasping
Grasping objects requires that people reach to an object’s location
and achieve an appropriate arm and hand posture to grasp and
manipulate the object. If the to-be-grasped object is a hand tool with
a handle, then the orientation of the handle relative to the grasper
can make the tool more or less easy to pick up. Consider a hammer.
If its handle is pointed to the grasper’s right, then the hammer can
be easily grasped with the right hand, but not with the left. It would
be worthwhile, at this point, for the reader to place on a table an
elongated object—a pen will do—and pretending that it is a hammer,
notice how easy it is to pick up with the right hand when the handle
points to the right as opposed to pointing to the left. When doing
this demonstration, be sure to pick up the pretend hammer in a way
that is appropriate for its use; that is, the grasping posture must be
one that affords hammering—the hammer’s head must be above the
hand, not below.
The ease with which a hand tool can be grasped affects its apparent
distance, but surprisingly, only for right-handed people (Linkenauger,
Witt, Stefanucci, & Proffitt, 2006). In these studies, participants sat
at a table. Directly in front of the participants was a small dot on the
edge of the table, which served as the near endpoint when making
distance judgments. On each trial, an experimenter placed a hand
tool at varying distances in front of the participant, with the handle pointing either to the left or to the right. A dot was affi xed to
the center of gravity of each tool. Participants were told to imagine
picking up the tool with their right hand in a manner appropriate
for its use, after which they indicated its apparent distance—the distance between the dot before them on the table and the dot on the
tool—using a visual matching task. Finally, they picked up the tool
and gave it to the experimenter. It was found that, for right-handed
participants, tools appeared nearer when the handle was pointed to
the right as opposed to the left, indicating that the tools appeared
closer when they were easier to grasp and pick up.
Another experiment replicated the above design except that the
right-handed participants were instructed to use their nondominant
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left hand. The results for handle orientation reversed. The tools were
now seen to be nearer when their handles pointed to the left rather
than to the right, a finding which is again consistent with the notion
that apparent grasping-distance is influenced by ease of grasp.
A totally unanticipated finding in the Linkenauger et al. studies was that none of the results with right-handers generalized to
left-handed participants. Left-handers saw the tools as being equally
far away regardless of the pointing direction of the tool’s handle or
which hand was used to pick it up. Left-handers are known to be
more ambidextrous (Gonzalez, Ganel, & Goodale, 2006), and this
may be a reason for why they were unaffected by the orientation of
a tool’s handle. In everyday circumstances, if left-handers see a tool
with its handle pointed away from their dominant hand, then they
would be more likely than a right-hander to pick it up with their
nondominant hand. In addition, left-handers have had a lifetime of
experience coping with such tools as scissors, can openers, and writing desks, which have been designed for right-handed people.
In summary, right-handers see tools as appearing closer when their
handles are oriented in a direction that makes the tool easy to pick
up with the intended dominant or nondominant hand. Left handers
see the world differently; the orientation of the tool’s handle does not
influence their grasping-distance perception, perhaps because they
are more ambidextrous than right-handers.
There exists an extensive literature on the neurophysiology of
grasping (see Castiello, 2005, for a review.) The literature indicates
that the visual guidance of grasping engages the dorsal stream of
visual processing including the anterior intraparietal sulcus and networks of other nearby parietal areas. A human fMRI study by Valyear, Culham, Sharif, Westwood, and Goodale (2006) showed that a
region in the posterior portions of the intraparietal sulcus showed
strong activations associated with changes in the orientation of hand
tools, but not to changes in the tool’s identity. Identity changes of
tools evoked strong activations in the temporal lobe’s fusiform gyrus
but not in parietal regions. These results are consistent with Milner and Goodale’s (1995) proposal that the ventral visual processing
stream is responsible for shape perception and object recognition,
whereas the dorsal stream controls visually guided actions. Grasping
a tool must conform to the orientation of its handle, and this orientation sensitivity was seen in parietal but not temporal activations.
Identifying a tool does not require viewpoint-specific encoding, and
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thus, temporal regions showed sensitivity to changes in an object’s
identity but not to changes in its orientation.
To anticipate the last section of this chapter, “Putting What,
Where, and How Together,” for conscious spatial perception to be
action-specific, aspects of both dorsal and ventral processing must
be combined. Ventral processing is required to identify a hammer
as being a hammer, and dorsal processing is required to take its
orientation into account when picking it up. Interestingly, patients
with ventral stream damage may not be able to identify an object
as being a hammer and, although they can pick it up, they may do
so in a manner that is inappropriate for its use; they may grasp the
handle with the hammer’s head below the hand (Carey, Harvey, &
Milner, 1996). Creem and Proffitt (2001b) showed that people have a
strong tendency to pick up tools by their handles in a manner that is
appropriate for their use, even if the handles are pointing away from
them and appropriate grasping is difficult. However, if participants
are required to do another task that puts a heavy load on semantic processing, which interferes with concurrent object recognition
processing, then they behave like the patients with ventral damage. They pick up tools with the easiest grasp, even if this results
in a posture that is inappropriate for the tools’ use (Creem & Proffitt, 2001b). These studies indicate that grasping a tool appropriately
requires both ventral and dorsal processing. The initial processing of
what, where, and how may be functionally and anatomically distinct
(Creem & Proffitt, 2001a), but the action-specific nature of spatial
perception manifests contributions from all three functions.
Walking
Perceiving the surface layout of the ground is of primary importance for walking. Visual perception provides information about
where walking is possible as well as the difficulty associated with
any chosen path. With respect to the geometry of its spatial layout,
the ground plane has two walker-relative parameters, egocentric
distance and slant. Both of these parameters are influenced by the
energetic costs associated with walking. A recent review provides an
in-depth summary of studies showing energetic influences on perceiving the ground’s layout (Proffitt, 2006). The basic findings are
that hills appear steeper and egocentric distances farther, following
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manipulations of the anticipated metabolic energy costs associated
with walking an extent.
With respect to geographical slant perception, people grossly
overestimate the slant of hills in all circumstances. Five-degree hills
are typically judged to be about 20° and 10° hills appear to be 30°
(Proffitt, Bhalla, Gossweiler, & Midgett, 1995). A large increase in
this overestimation occurs following manipulations of the metabolic
energy required to ascend hills (Bhalla & Proffitt, 1999). In these
studies, the energetic costs associated with walking were experimentally manipulated by having people wear a heavy backpack or become
physically tired by taking an hour-long exhausting run. Other studies selected people based upon their physical fitness, age or health.
Creem-Regehr, Gooch, Sahm, and Thompson (2004) used a harness
to manipulate walking effort in a virtual environment and found
that increased effort was associated with an increase in perceived
geographical slant. Overall, it was found that hills appear steeper
when people are encumbered by a backpack or harness, tired, of low
fitness, elderly, and in declining health.
The overestimation of slant—both normative and experimentally
induced—occurs in conscious awareness. These overestimations
are obvious to anyone looking at a hill with knowledge of its actual
slant. Participants are surprised and incredulous when, following
an experiment they are told that the hill that was judged by them
to be 20° is, in fact, only 5°. The conscious perception of slant was
assessed with both verbal reports and a visual match task. Another
assessment used a visually guided action measure that is dissociated
from conscious awareness. Participants placed their hand flat on a
rotating palmboard and, while looking at the hill but not their hand,
attempted to make the board parallel with the slant of the hills.
These adjustments are quite accurate and unaffected by any of the
energetic manipulations (Bhalla & Proffitt, 1999).
The dissociation between the measures of explicit awareness and
the visually guided action measure may reflect the two streams of
visual processing, which Milner and Goodale (1995) proposed. The
explicit measures may entail ventral processing, whereas the visually
guided action may be controlled by the dorsal stream. At present, no
direct evidence exists for this account of slant perception.
The overestimation of slant in conscious awareness is thought to
promote effective long-term planning of locomotion, whereas accuracy in visually guided actions promotes effective behaviors in the
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immediate proximal environment. It is obvious why actions directed
at the immediate environment should be accurate. People would be
clumsy creatures indeed if, whenever they encountered a 5° hill, they
lifted their foot to accommodate a 20° incline. The functional utility
of overestimation in conscious awareness takes a bit of explaining.
The normative bias to overestimate geographical slant is an instance
of psychophysical response compression that is found in many magnitude estimation tasks—see Proffitt (2006) for an expanded discussion of why response compression leads to overestimation in slant
perception. Another example of response compression occurs in the
human sensitivity to light intensity. When asked to indicate when a
change in brightness has occurred, dark-adapted people in a completely dark environment can detect the presence of a few photons
of light. On the other hand, in a well-illuminated environment, it
takes an increase of orders-of-magnitude more light before people
can notice the change. This is an instance of psychophysical response
compression. Its virtue in the case of luminance detection is that
people have a higher sensitivity to changes in light intensity when
ambient light is low compared to when it is high. Similarly for geographical slant perception, normative overestimation allows people
to be more sensitive to changes in small slants, for example, noticing
the difference between 5° and 6° compared to detecting a difference
between 75° and 76°. Seeing differences in the former has real consequences for planning locomotion, whereas there are no behavioral
consequences that depend upon detecting the difference between the
latter two inclines.
There are two advantages associated with the increase in slant
overestimation that occurs when the effort required to ascend hills
is increased. First, increased overestimation implies an increase in
sensitivity to small geographical slants. This means that as the metabolic costs of ascending hills increases, people become more sensitive to hill slants. Second, when choosing walking speed, people
need not relate their current ability to expend walking energy to the
apparent slant of the hill. Instead, this relationship is immediately
apparent in perception. Recall that the discussion of how an iguana’s
body temperature influences its flight-distance for an approaching
predator. If the iguana sees an invariant flight-distance in which its
body temperature and resulting flight speed influence the apparent distance to the predator, then the iguana does not have to relate
these variables when it is deciding when to flee. Similarly for people
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viewing hills, by seeing their potential to expend energy in the perceived slant of inclines, people can decide how fast to walk based
upon how steep the hill appears. They do not have to relate slant to
their current physiological state because this has already been done
in perception.
Regarding energetic influences on spatial perception, the research
findings for egocentric distance perception are much the same as for
slant. Distances to targets appear greater when people are encumbered
by a backpack or have just gotten off a treadmill—an experience that
causes an adaptation in which the visual/motor system learns that it
takes forward walking effort to go nowhere, and consequently, that it
takes more effort to walk a prescribed distance (Proffitt, Stefanucci et
al., 2003). Across these studies, a variety of dependent measures were
used including verbal reports, visual match tasks, and blind walking, in which participants view a target, don a blindfold, and attempt
to walk to the target’s location without sight.
It has also been found that apparent distances on steep hills are
expanded (Stefanucci, Proffitt, Banton, & Epstein, 2005). Steep hills
require more energy to ascend. The hills that were assessed in these
studies could not be ascended without considerable difficulty. The
finding of increased perceived distance on hills presents a geometrical paradox. Given that the slant of hills is overestimated, when
people look up a hill, the apparent distance to a target should be
underestimated. Given that the angular elevation to the target does
not change, the steeper the apparent hill, the short must be the egocentric extent along the ground to its location. This is what geometry
requires. Such findings of geometrical inconsistencies in perception have been found in earlier studies on perceiving spatial layout
(Epstein, 1977; Epstein, Park, & Casey, 1961; Sedgwick, 1986).
Manipulations that influence the effort required to walk may not
affect the effort required to perform other distance-relative behaviors. For example, walking on a treadmill without experiencing
optic flow causes an adaptation in which more effort is associated
with walking to a target, and consequentially, its apparent distance
increases. However, treadmill walking does not affect the effort
required to throw a beanbag to a target location. In accord with an
action-specific approach to spatial perception, when people view a
target—following a period of treadmill walking—with the intention
of throwing a beanbag to its location, then the treadmill adaptation
has no effect on their distance judgments (Witt, Proffitt, & Epstein,
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2004). Perceiving distances in these cases is specific to what a person
is intending to do next. Walking adaptation influences perception if
a person is a “walker” but not a “thrower.”
That perceived extent has been found to be action-specific is in
accord with prior studies on perceptual-motor adaptation conducted
by Rieser, Pick, Ashmead, and Garing (1995). In their experiments,
Rieser et al. had participants walk on a treadmill that was placed on
a trailer being pulled across a field by a tractor. Through this means,
the rate of optic flow was decoupled from the rate that participants
were walking. Following this adaptation, participants were shown
targets, and after being blindfolded, they attempted to walk to the target locations. Participants whose treadmill-walking rate was greater
than the tractor’s speed walked too far, and conversely, those who
walked at a slower speed than that of the tractor walked too short a
distance. Of particular relevance to the action-specificity argument,
other participants who attempted to throw balls to the location of
targets were unaffected by the treadmill-walking adaptation.
As with slant perception, the advantage of perceiving distances in
terms of walking energy is that long-term motor plans can be based
upon perception as opposed to requiring that perception and physiological state be combined during the planning process. Recall that
the body has both an exterior and an interior. Most of the behaviors performed in the environment with the body’s exterior have as a
goal the maintenance of a desired state in the body’s interior, a good
example being maintaining a desired rate of energy expenditure.
Both aspects of the body are related in walking-specific perception:
In the apparent surface layout of the ground, people see both the
possibilities and the associated energetic costs for walking.
Consider another example from behavioral ecology. A recent
study used GPS to track the movement of elephants in northern
Kenya (Wall, Douglas-Hamilton, and Vollrath, 2006). It was found
that elephants almost never ascended steep hills even when there was
rich vegetation to be had. Wall et al. proposed that a principal reason
for this reluctance to ascend hills was that, because of their body
weight, elephants incur an enormous energetic cost when climbing.
It would cost the elephants more calories to climb the hills than they
would obtain by consuming the vegetation that could be obtained
there. Wall et al. stated, “We conclude that megafauna probably take
a rather different view of their surroundings than more light weight
animals. This is especially true if the heavyweights, like elephants,
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are herbivores for which energy replenishment is so much more time
consuming than it is for carnivores” (pp. R528). We, of course, do
not know what elephants perceive, but from the current perspective
it seems likely that their apparent topography would be highly exaggerated so as to enhance their sensitivity to geographical slant and
to relate the possibilities and associated energetic costs for obtaining
food.
Throwing
When throwing balls to targets, people perceive the distance to the
targets relative to the effort associated with throwing (Witt, Proffitt,
& Epstein, 2004). In these studies, participants viewed sports cones
in a large open field and threw either light or heavy balls to their
locations. After throwing the ball, participants reported the apparent distance to the cone and then threw the ball again. Depending
upon the experiment, either verbal reports or a visual-match task
were used as dependent measures. In both cases, distances were
judged to be greater by those participants throwing heavy balls as
opposed to light ones.
In another experiment, Witt et al. also showed that the influence
of throwing was contingent upon viewing the target cone with the
intention of throwing. The experiment had two groups and both
threw the heavy ball at targets. After throwing the ball, each group
made a distance judgment. The groups differed in what they did next;
one group attempted to throw the ball to the target while blindfolded,
whereas the other group attempted to blind walk to the target’s location. Thus, when viewing the target, one group anticipated throwing
and the other anticipated walking to the target location. The “throwers” viewed the target cones to be farther away than did the “walkers.” Throwing heavy balls makes targets appear farther away, but
only if one is about to throw again. Recall that Witt et al. also showed
the converse of these findings; treadmill-walking adaptation influenced apparent distance only when participants anticipated walking
to a target and not when they anticipated throwing a beanbag to its
location, instead.
A final set of studies was conducted to assess whether these findings were due to changes in perception itself, or to some action-specific postperceptual process (Witt, Proffitt, & Epstein, 2006). Two
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groups of participants were adapted to walking on a treadmill. Both
groups then viewed a target, but with different expectations. One
group anticipated that after donning a blindfold, they would attempt
to walk to the target’s location. The other group expected to throw
a beanbag to the target location while blindfolded. Thus, one group
viewed the target as “walkers,” whereas the other group viewed it as
“throwers.” Both groups put on the blindfolds and those in the walking group attempted to blind walk to the target location, as expected.
After donning their blindfolds, the “throwers” were told that a mistake had been made in the instructions and that, in fact, they were to
attempt to walk to the target location while blindfolded. Those participants in the walking condition were influenced by the treadmill
walking adaptation and walked farther than those participants in
the throwing condition. The “walkers” viewed the target relative to
the energy required to walk to it, and thus, they were influenced by
the treadmill adaptation. The “throwers” had experienced the same
treadmill adaptation; however, because this experience influenced
the effort associated with walking but not throwing, the former
being an action they had not anticipated, they were unaffected by
the adaptation. In other words, it seems to be anticipated effort that
induces re-calibration. In a control experiment, this experiment’s
design was repeated except that the treadmill-walking adaptation
was eliminated and, in this case, the groups did not differ in their
blind walking. Note that, in the initial experiment, both groups
did exactly the same thing; both were adapted to treadmill walking
and both attempted to blind walk to a target. The only difference
between the groups was their behavioral intention when they viewed
the target. The results indicate that each group’s spatial perceptions,
as calibrated by required energy, were specific to these behavioral
intentions.
Falling
Falling is an inherent danger associated with human locomotion. An
adult could be injured by a slip and fall. Body size matters:
A 2 m tall man, when tripping, will have a kinetic energy upon
hitting the ground 20-100 times greater than a small child who
learns to walk. This explains why it is safe for a child to learn to
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walk; whereas adults occasionally break a bone when tripping,
children never do. (Went, 1968, pp. 407)
The cost of injury increases with locomotion speed, and in the
case of falling from a height above the ground, with altitude. As
locomotion speed and attitude increase, fear of falling becomes palpable. All of these factors—speed, altitude, and fear of falling—have
been found to influence spatial perception.
Stefanucci, Proffitt, and Clore (2005) investigated how the risk of
falling on a steep hill at a high speed might affect geographical slant
perception. Participants viewed a steep sidewalk from the top, either
standing on a skateboard or on a box of equivalent height. Given the
steepness and extent of the sidewalk, descending on the skateboard
would be very fast and risky. As in prior studies, explicit awareness of
geographical slant perception was assessed with verbal reports and a
visual matching task. The visually guided palmboard was also used.
In addition, participants provided rating-scale judgments about how
fearful they were of descending the sidewalk. It was found that the
sidewalk appeared steeper—as assessed by explicit awareness measures—for those participants who were standing on the skateboard
and reported feeling frightened compared to those who stood on the
box and reported little or no fear. The visually guided palmboard
adjustments were unaffected by both the skateboard manipulation
and reported levels of fear. Although participants knew that they
would not actually have to ride the skateboard down the hill, fear of
an action seems to elicit the same kind of processing as anticipating
its performance.
Being at the edge of a high drop off, such as a cliff or balcony,
makes people uneasy as the penalty for falling could entail severe
injury or worse. Jackson and Cormack (in press) found that people
overestimate vertical distances and, most importantly, that their
overestimation is much greater when the height is viewed from above
than from below. Jackson and Cormack concluded that the greater
overestimation that is exhibited when heights are viewed from above
is a consequence of an evolved adaptation, which through perceptual
exaggeration, motivates people to avoid falling off heights.
Stefanucci and Proffitt (2006) similarly found that high vertical
extents are overestimated much more from the top than from the
bottom. Participants used a visual matching task to judge vertical
extent. They either stood atop a 26-foot balcony looking down or
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they stood at the bottom looking up. The height of the balcony was
overestimated by about 60% from the top and slightly less than 30%
from the bottom. In addition, participants provide rating scale judgments of their fear of falling. It was found that the assessed anxiety
related to falling was positively correlated with distance estimations.
These studies suggest that an emotion, in this case fear, influences
spatial perception. Other emotional influences on spatial perception
have also been demonstrated (Riener, Stefanucci, Proffitt, & Clore,
2003).
Hitting and Putting
People who play sports often report that the spatial dimensions of
balls, goals, hurdles, swimming pools, and so forth appear to be influenced by how well they are performing. Baseball, which has a rich
journalistic tradition, provides many examples of apparent ball size
being influenced by hitting performance. When describing a massive home run, Mickey Mantle said, “I never really could explain it. I
just saw the ball as big as a grapefruit” (Ultimate New York Yankees,
n.d.). George Scott of the Boston Red Sox said, “When you’re hitting
the ball [well], it comes at you looking like a grapefruit. When you’re
not, it looks like a blackeyed pea” (Baseball Almanac, n.d.). During
a slump, Joe “Ducky” Medwick of the St. Louis Cardinals said he felt
like he was “swinging at aspirins” (ESPNMAG.com, n.d.).
Witt and Proffitt (2005) found that batter’s hitting performance
does, in fact, influence their recollection of the ball’s size. Softball
players were approached after completing a game and were asked
to indicate the size of a softball by selecting one of many differently
sized circles, which were displayed on a poster board. Afterwards,
their batting average for the completed game was obtained. The
recalled size of a softball was found to be positively correlated with
player’s batting average.
Golf is another sport in which reports abound of apparent spatial
distortions. Golfers will claim that when they are putting well, the
hole looks as big as a basket, and when their putting is off, the hole
can look as small as a dime. Witt, Linkenauger, Bakdash, and Proffitt (2006) tested golfers after they had completed an 18-hole round.
Similar to the study with softball players, the golfers were asked to
choose, from among many circles, the one that was the size of a golf
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hole. Following this, other information about their round and golfing ability was obtained. It was found that apparent hole size was
negatively correlated with the golfers’ scores for the 18-hole round.
Because, in golf, low scores are good, this implies that the recollected
hole size was positively related to performance. Of particular interest, it was found that apparent hole size was not related to how good a
player was as assessed by his or her handicap. This implies that good
players do not always see the hole as bigger, but rather that anyone
will see the hole as being bigger on days when he or she is playing
better. Finally, it was found that apparent hole-size was correlated
with putting performance on the last hole but not to overall score on
the last hole, suggesting that these effects are specific to the relevant
task, which was putting.
Both the softball and putting studies obtained size judgments
from memory. Participants had completed play and were not looking at the ball or putting hole. Thus, the results of these studies could
be due to performance influences on perception, memory, or both.
There are, however, other results that suggest that perception could
have been affected. Wesp, Cichello, Gracia, and Davis (2004) conducted a study on dart throwing and perceived target size. They
found that participants who were more successful in hitting the target viewed it to be bigger than did participants who performed less
well. In their study, the target was visibly present when the size judgment was made.
Putting What, Where, and How Together
Current views on the two cortical visual systems make claims, not
only about the anatomical localization of visual functions, but also
about consciousness (Goodale & Milner, 2004; Milner & Goodale,
1995). With respect to function, the ventral stream has been defined
as the “what” system and is responsible for object identification. The
dorsal stream is responsible for processing “where” and “how,” terms
that refer to spatial localization and the visual guidance of actions
(Creem & Proffitt, 2001a). The two cortical pathways have also been
implicated in accounts of the neural correlates of consciousness.
Milner and Goodale (1995) proposed that conscious awareness was
associated with visual processing in the ventral but not the dorsal
stream. They suggested that the term, perception, should be applied
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to conscious visual awareness, whereas the visual guidance of action
should be considered to be a distinct, unconscious visuomotor
process.
The evidence for Milner and Goodale’s proposal is well known
(Goodale & Milner, 2004; Milner & Goodale, 1995). Patients with
brain damage in the temporal lobe may have very limited shape
awareness, and yet, they can accommodate their grasp when picking
up objects as well as fully-sighted persons. Conversely, patients with
parietal damage have unaffected shape perception abilities, but have
difficulty grasping objects effectively. Some have gone so far as to
describe the dorsal stream as a “zombie,” an instance of a system that
is capable of guiding behavior, but without any attendant consciousness or will (Koch, 2004).
It is difficult to imagine, however, how spatial perception could be
action-specific without dorsal processes contributing to conscious
experience. Consider the influences of reachability and graspability on distance perception. Objects appear closer when they can be
touched with a tool compared to when no tool is held and the objects
are out of hand’s reach. Visually guided reaching is a function of the
dorsal stream. The cells in macaque monkeys that responded to reachable raisins are found in the dorsal stream (Iriki et al., 1996). With
respect to grasping, tools appear closer to right-handed people when
the handles are oriented toward the right as opposed to the left hand,
and consequently, are easier to grasp (Linkenauger et al., 2006). The
human brain areas that show sensitivity to the orientation of tools in
fMRI studies are in the dorsal stream (Valyear et al., 2006).
This chapter has argued that spatial perception relates to and is
influenced by the visually specified environment, the body, and purpose. It has been suggested that people see spatial layout in terms of
the actions that they intend to perform and the bodily opportunities and costs of these actions. Anticipated actions are formative in
how spatial relationships are perceived. Recall the study in which
people walked on a treadmill, and thereby, acquired a visual/motor
adaptation in which the effort associated with walking an extent was
increased. Following this adaptation, targets appear farther away
if participants anticipate walking to its location but not if they are
about to throw a beanbag to it (Witt, Proffitt, & Epstein, 2004). The
adaptation acquired through treadmill walking affects “walkers”
but not “throwers.” The world is seen relative to the behavior that is
about to be performed.
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The what, where, and how of the two visual streams are coordinated and combined in perception in accordance with the choice of
purposive behaviors. By choosing to reach, grasp, walk, or throw, a
person sees the world in terms of their body’s abilities to perform
these intended actions and also in relation to the inherent costs associated with their performance.
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