Theories and Foundational Questions 91The consensus among contemporary investigators of per-
ception favors a bimodal approach that makes a place for
both the neurophysiological and the algorithmic approaches.
The consensus is that the coevolution of a neurophysiology
that keeps in mind the computational problems of vision and
of a computational theory that keeps in mind the competen-
cies of the biological vision system is most likely to promote
good theory.
Although this bimodal approach might seem to be unex-
ceptionable, important theoretical disagreements persist
concerning its implementation. Consider, as an example,
Barlow’s (1972, 1995) bold proposal called the single-neuron
doctrine:“Active high level neurons directly and simply
cause the elements of our perception” (Barlow, 1972, §6.4,
Fourth Dogma). In a later formulation, “Whenever two stim-
uli can be distinguished reliably, then some analysis of the
neurological messages they cause in some single neuron
would enable them to be distinguished with equal or greater
reliability” (Barlow, 1995, p. 428). The status of the single-
neuron doctrine has been reviewed by Lee (1999) and by
Parker and Newsome (1998). The general experimental para-
digm assesses covariation between neural activity in single
cortical neurons and detection or discrimination at threshold.
The single-neuron doctrine proposes that psychophysical
functions should be comparable to functions describing
neural activity and that decisions made near threshold should
be correlated with trial-to-trial fluctuations of single cortical
neurons (e.g., Britten, Shadlen, Newsome, & Movshon,
1992).
The available data do not allow a clear-cut decision con-
cerning this fundamental prediction. However, whatever the
final outcome may be, disagreements about the significance of
the findings will arise from differences concerning the appro-
priate unit of analysis. Consider first the perceptual side that
was elected for analysis. From the standpoint of the ecologi-
cal realist (e.g., Gibson, 1979), the election of simple
detection and discrimination at threshold is misguided. The
ecological realist holds that the basic function of the visual
system is to detect information in spatiotemporal optical
structure that is specific to the affordances of the environ-
ment. Examining relations between neuronal activity and
psychophysical functions at threshold is at the wrong level of
behavior. As noted before, it is for this reason that the canoni-
cal documents of the ecological approach (Gibson, 1950,
1966, 1979) made no use of psychophysiology.
Similar reservations arise in the Gestalt approach. Since
its inception, Gestalt theory (Hatfield & Epstein, 1985;
Köhler, 1940) has held that only a model of underlying brain
processes can stand as an explanation. In searching for the
brain model, Gestalt theorists were guided by a heuristic: The
brain processes and the perceptual experiences that they sup-
port have common characteristics. Consequently, a careful
and epistemically honest exploration of perceptual experi-
ence should yield important clues to the correct model of the
brain. According to Gestalt theory, phenomenological explo-
ration reveals that global organization is the most salient
property of the perceptual world, and it is a search for the
neurophysiological correlates of global experience that will
bring understanding of perception.
There are analogous differences concerning the choice of
stimulation. If there is to be an examination of the neuro-
physiological correlates of the apprehension of affordances
and global experience, then the stimulus displays must sup-
port such perceptions. Proponents of this prescription suspect
that the promise of the pioneering work of Hubel and Wiesel
(1962) has not been realized because investigators have
opted for the wrong level of stimulation.Concerning InformationThe term informationhas many uses within psychology
(Dretske, 1986). Here the term refers to putative properties of
optical stimulation that could specify the environmental state
of affairs (i.e., environmental properties), structures, or
events that are the distal source of the optical input. To spec-
ify an environmental state is to pick out the actual state of
affairs from the family of candidate states that are compatible
with the given optical stimulation.
Cognitive constructivists have asserted that no properties
of optical stimulation can be found to satisfy the require-
ments of information in this sense because optical stimula-
tion is intractably equivocal. At best optical stimulation
may provide clues—but never unequivocal information—
concerning the state of the world. This assessment was al-
ready entrenched when Berkeley wrote his influential Essay
Towards a New Theory of Vision(Berkeley, 1709/2000), and
the assessment has been preserved over the ensuing three
centuries. The assumption of intractable equivocality is one
of the foundational premises of constructivism; it serves as a
basic motivation of the enterprise. For example, the transac-
tionalists (Ittelson, 1960; Kilpatrick, 1950, chap. 2) lay the
foundation for their neo-Helmholtzian approach by showing
that for any proximal retinal state there is an infinite class of
distal “equivalent configurations” that are compatible with
a given state of the retina. In the same vein, computational
research routinely opens with a mention of the “inverse
projection problem.” If optical stimulation does not carry in-
formation that can specify the environment, we must look
elsewhere for an account of perception.