activity. Thus your percept of your hand remains stable over changes in the
size of the image because your interpretation is governed by stable perceptual
activities. Most of the time, sensing and perceiving occur so effortlessly, con-
tinuously, and automatically that you take them for granted. It is our goal
in this chapter to allow you to understand and appreciate the processes that
afford you a suitable account of the world, with such apparent ease. We begin
with an overview of perceptual processes in the visual domain.
Sensing, Organizing, Identifying, and Recognizing
The termperception,in its broad usage, refers to the overall process ofappre-
hendingobjects and events in the external environment—to sense them, under-
stand them, identify and label them, and prepare to react to them. The process
of perception is best understood when we divide it into three stages: sensation,
perceptual organization, and identification/recognition of objects.
Sensationrefers to conversion of physical energy into the neural codes recog-
nized by the brain. Sensation provides a first-pass representation of the basic
facts of the visual field. Your retinal cells are organized to emphasize edges and
contrasts while reacting only weakly to unchanging, constant stimulation. Cells
in your brain’s cortex extract features and spatial frequency information from
this retinal input.
Perceptual organizationrefers to the next stage, in which an internal represen-
tation of an object is formed and a percept of the external stimulus is developed.
The representation provides a working description of the perceiver’s external
environment. Perceptual processes provide estimates of an object’s likely size,
shape, movement, distance, and orientation. Those estimates are based on men-
tal computations that integrate your past knowledge with the present evidence
received from your senses and with the stimulus within its perceptual context.
Perception involvessynthesis(integration and combination) of simple sensory
features, such as colors, edges, and lines, into the percept of an object that can
be recognized later. These mental activities most often occur swiftly and effi-
ciently, without conscious awareness.
To understand the difference between these first two stages more clearly,
consider the case study of Dr. Richard, whose brain damage left his sensation
intact but altered his perceptual processes.
Dr. Richard was a psychologist with considerable training and experience
in introspection. This special skill enabled him to make a unique and val-
uable contribution to psychology. However, tragically, he suffered brain
damage that altered his visual experience of the world. Fortunately, the
damage did not affect the centers of his brain responsible for speech, so he
was able to describe quite clearly his subsequent unusual visual experi-
ences. In general terms, the brain damage seemed to have affected his
ability to put sensory data together properly. For example, Dr. Richard
reported that if he saw a complex object, such as a person, and there were
several other people nearby in his visual field, he sometimes saw the dif-
ferent parts of the person as separate parts, not belonging together in a
single form. He also had difficulty combining the sound and sight of thePerception 135