Theories and Foundational Questions 89of processes to fulfill its functions. Ramachandran (1990a,
1990b) gives the most explicit expression of this standpoint
in his utilitarian theory.
Eight Foundational Questions
The commonalities and differences among the four theories
under consideration are shaped by their approaches, implicit
or explicit, toward a number of basic questions.
What Is Vision For?
What is the visual system for? The answer to the question can
shape both the goals of experimentation and the procedures
of investigation. For most of the twentieth century one an-
swer has been paramount: The function of the visual system
is to generate or compute representations or descriptions of
the world. Of course, a representation is not to be considered
a picture in the mind. Nevertheless, representations serve a
useful function by mirroring, even if symbolically, the orga-
nization and content of the world to be perceived.
Acceptance of the preeminence of the representational
function is apparent in the Gestalt insistence that the first step
in the scientific analysis of visual perception is application of
the phenomenological method (Kubovy, 1999). This same
endorsement is not as wholehearted in cognitive construc-
tivist approaches (Kubovy & Gepshtein, in press). Neverthe-
less, a review of two of the major documents of cognitive
constructivism, Rock’s (1983) The Logic of Perceptionand
the edited collection Indirect Perception(Rock, 1997), shows
that in every one of the dozens of investigations reported, the
dependent variables were direct or indirect measures of per-
ceptual experience. Marr (1982) was also explicit in allying
himself with the representational view. For Marr, the function
of vision is “discovering from images what is present in
the world.” The task for the vision scientist is to discover the
algorithms that are deployed by the visual system to take the
raw input of sensory stimulation to the ultimate object-
centered representation of the world. Given this conception
of a disembodied visual system and the task for the visual
system, the ideal preparation for the investigation of vision
is the artificial (nonbiological) vision system realized by the
computer.
The ecological realists do not join the broad consensus
concerning the representational function of the visual system.
For Gibson, the primary function of the visual system is to
detect the information in optical structures that specifies the
actions afforded by the environment (e.g., that a surface
affords support, that an object affords grasping). The function
of the visual system is to perceive possible action, that is,
actions that may be successfully executed in particular envi-
ronmental circumstances.
The representationalists also recognize that perception is
frequently in the service of action. Nonetheless, the differ-
ence between the representationalists and the ecological real-
ists is significant. For the representationalists the primary
function of the visual system is description of the world. The
products of the visual system may then be transmitted to the
action system. The perceptual system and the action system
are separate. Gibson, by contrast, dilutes the distinction be-
tween the perceptual system and the action system. The shap-
ing of action does not await perception; action possibilities
are perceived directly.
We might expect that following on the ecological realist
redefinition of the function of the visual system there would
be a redirection of experimental focus to emphasize action
and action measures. However, a redirection along these lines
is not obvious in the ecological realist literature. Although
there are several notable examples of focus on action in the
studies of affordances (e.g., Warren, 1984; Warren & Whang,
1987), overall, in practice it is reformulation of input that has
distinguished the ecological approach. The tasks set for the
subjects and the dependent measures in ecologically moti-
vated studies are usually in the tradition established by the
representationalists.
The last two decades of the twentieth century have wit-
nessed a third answer to the question of function. According
to this new view, which owes much to the work of Milner and
Goodale (1995), the visual system is composed of two major
subsystems supported by different biological structures and
serving different functions. The proposal that there is a func-
tional distinction between the two major projections from
primary visual cortex is found in earlier writing by Schneider
(1969) and Ungerleider and Mishkin (1982). These writers
proposed that there were two visual systems: the “what” sys-
tem designed to process information for object identification
and the “where” system specialized for processing informa-
tion for spatial location. The newer proposal differs from the
older ones in two respects: (a) The functions attributed to the
subsystems are to support object identification (the what
function) and action (the how function), and (b) these func-
tions are implemented not by processing different inputs but
by processing the same input differently in accordance with
the function of the system. As Milner and Goodale (1995,
p. 24) noted, “we propose that the anatomical distinction
between the ventral and dorsal streams corresponds to the
distinction... between perceptual representation and visuo-
motor control.... The reason there are two cortical pathways