90 Foundations of Visual Perception
is that each must transform incoming visual information for
different purposes.” The principal support for this two-vision
hypothesis has been provided by findings of double disso-
ciations between action and perception—that is, between
assessments of effective action and measures of perceptual
experience—in brain-damaged and intact individuals. These
findings (summarized by Milner & Goodale, 1995, and by
Goodale & Humphrey, 1998) imply that it will be profitable
to adopt dual parallel investigative approaches to the study of
vision, one deploying action-based measures, the other more
traditional “perceptual” measures.
Goodale and Humphrey (1998) and Norman (in press)
proposed that the two-vision model provides a framework for
reconciling the ecological and computational approaches:
“Marrian or ‘reconstructive’ approaches and Gibsonian or
‘purposive animate-behaviorist’ approaches need not be seen
as mutually exclusive, but rather as complementary in their
emphasis on different aspects of visual function” (Goodale &
Humphrey, 1998, p. 181). We suspect that neither Gibson nor
Marr would have endorsed this proposal. (Chapters by Heuer
and by Proffitt and Caudek in this volume also discuss the
distinction between the perceptual system and the action
system.)
Percepts and Neurons
Perceptual processes are realized by a biological vision system
that evolved under circumstances that have favored organisms
(or genetic structures) that sustain contact with the environ-
ment. No one doubts that a description and understanding of
the hardware of the visual system will eventually be part of an
account of perception. Nevertheless, there are important dif-
ferences among theories in their uses of neurophysiology.
One of the tenets of first-generation information-processing
theory (e.g., Johnson-Laird, 1988; Neisser, 1967) is that the
mind is an operating system that runs on the brain and that
the proper business of the psychology of cognition and per-
ception is study of the program and not the computer—the
algorithm and not the hardware. Furthermore, inasmuch as an
algorithm can be implemented by diverse computational archi-
tectures, there is no reason to look to hardware for constraints
on algorithms. Another way of expressing this position is that
the aim of information-processing theory, as a theory of per-
ception, is to identify functional algorithms above the level of
neurophysiology.
The cognitive constructivist shares many of the basic
assumptions of standard information-processing theory and
has adopted the independence stance toward physiology. Of
course, perceptual processes are implemented by biological
hardware. Nevertheless, perceptual theorizing is not closely
constrained by the facts or assumptions of sensory physiol-
ogy. The use of physiology is notably sparse in the principal
documents of cognitive constructivism (e.g., Rock, 1983,
1997). Helmholtz may seem to be an important exception to
this characterization; but, in fact, he was careful to keep
his physiology and psychology separate (e.g., Helmholtz,
1866/2000, Vol. 3).
Physiological talk is also absent in the canonical works of
the ecological theorists, but for different reasons. The ecolo-
gists contend that the questions that have been addressed by
sensory physiologists have been motivated by tacit accep-
tance of a metatheory of perception that is seriously flawed:
the metatheory of the cognitive constructivist. As a conse-
quence, whereas the answers discovered by investigations
of sensory physiologists may be correct, they are not very
useful. For example, the many efforts to identify the proper-
ties of the neuronal structures underlying perception by
recording the responses of single cells to single points of light
seem to reflect the tacit belief that the perceptual system is
designed to detect single points. If the specialization of the
visual system is different, such as detecting spatiotemporal
optical structures, the results of such studies are not likely to
contribute significantly to a theory of perception. In the eco-
logical view what is needed is a new sensory physiology
informed by an ecological stance toward stimulation and the
tasks of perception.
The chief integrative statement of the computational
approach, Marr’s (1982)Vision,is laced with sensory physi-
ology. This is particularly true for the exposition of the
computations of early vision. Nevertheless, in the spirit of
functionalism Marr insists that the chief constraints are
lodged in an analysis of the goals of perceptual computation.
In theorizing about perceptual process (i.e., the study of algo-
rithms) we should be guided by its computational goal, not by
the computational capabilities of the hardware. When an al-
gorithm can satisfy the requirements of the task, we may look
for biological mechanisms that might implement it.
The Gestalt theorists (e.g., Köhler, 1929, 1940) were
forthright in their embrace of physiology. For them, a plausi-
ble theory must postulate processes that are characteristic of
the physical substrate, that is, the brain. Although it is in prin-
ciple possible to implement algorithms in diverse ways, it is
perverse to ignore the fit between the properties of the com-
puter and the properties of the program. This view is in sharp
contrast to the hardware-neutral view of the cognitive con-
structivist: For the Gestalt theorist, the program must be
reconciled with the nature of the machine (Epstein, 1988;
Epstein & Hatfield, 1994). In this respect, Gestalt theory
anticipated current trends in cognitive neuroscience, such as
the connectionist approaches (Epstein, 1988).