that the holistic perception of the world arises because the cortex is organized
to function that way. You organize sensory information the way you do be-
cause it is the most economical, simple way to organize the sensory input,
given the structure and physiology of the brain. (Many of the examples of per-
ceptual organization we will discuss in a later section were originated by the
Gestaltists.)
Gibson’s Ecological Optics James Gibson(1966, 1979) proposed a very influential
nativist approach to perception. Instead of trying to understand perception as
a result of an organism’s structure, Gibson suggested that it could be better
understood through an analysis of the immediately surrounding environment
(or its ecology). As one writer put it, Gibson’s approach was, ‘‘Ask not what’s
inside your head, but what your head’s inside of’’ (Mace, 1977). In effect, Gib-
son’stheory of ecological opticswas concerned with the perceived stimuli rather
than with the mechanisms by which you perceive the stimuli. This approach
was a radical departure from all previous theories. Gibson’s ideas emphasized
perceiving asactive explorationof the environment. When an observer ismoving
in the world, the pattern of stimulation on the retina is constantly changing
over time as well as over space. The theory of ecological optics tried to specify
the information about the environment that was available to the eyes of a
moving observer. Theorists in Gibson’s tradition agree that perceptual systems
evolved in organisms who were active—seeking food, water, mates, and shel-
ter—in a complex and changing environment (Gibson, 1979; Pittenger, 1988;
Shaw & Turvey, 1981; Shepard, 1984).
According to Gibson, the answer to the question ‘‘How do you learn about
your world?’’ is simple. You directly pick up information about theinvariant,
or stable, properties of the environment. There is no need to take raw sensa-
tions into account or to look for higher level systems of perceptual inference—
perception is direct. While the retinal size and shape of each environmental
object changes, depending on the object’s distance and on the viewing angle,
these changes are not random. The changes are systematic, and certain proper-
ties of objects remain invariant under all such changes of viewing angles and
viewing distances. Your visual system is tuned to detect such invariances be-
cause humans evolved in the environment in which perception of invariances
was important for survival (Palmer, 1981).
TowardaUnifiedTheoryofPerception These diverse theories can be unified to
set the agenda for successful research on perception. You can recognize that the
different perspectives contribute different insights to the three levels of analysis
a theory of perception must address (Banks & Krajicek, 1991):
.What are the physiological mechanisms involved in perception?This topic
has its history in work with animals, and has more recently been addressed
using neuroimaging techniques (see Part 19). The information impinging
on the sensory receptors is often ambiguous. Stimulus-driven, or bottom-
up processing, works its way up the brain, while expectation-driven, or
top-down processing, complements it..What is the process of perceiving?This question is usually tackled by
researchers who follow in the tradition originated by Helmholtz and thePerception 147