50 • CHAPTER 3 Perception
Perception Starts at the Receptors: Bottom-Up Processing
The fi rst step in perception is the stimulation of receptors by stimuli from the environ-
ment. Let’s fi rst consider the signals generated in Crystal’s visual receptors. Crystal sees
the umbrella because light refl ected from the umbrella enters her eyes, stimulates recep-
tors, and starts electrical signals traveling toward the visual receiving area of the cortex.
Processing that begins with stimulation of the receptors is called bottom-up processing.
All of our sensory experiences, with the exception of situations in which we might imag-
ine something or “see stars” from getting hit on the head, begins with bottom-up pro-
cessing. We can describe bottom-up processing both physiologically and behaviorally.BOTTOM-UP PROCESSING: PHYSIOLOGICAL
We can describe the physiological approach to bottom-up processing briefl y, because
we have already described, in Chapter 2, the sequence of events that occur after light
refl ected from a tree stimulates the visual receptors in the eye (see page 38). We saw
that stimulation of the receptors triggers a series of events in which electrical signals are
transmitted from the receptors toward the brain. Perceiving the tree or a bird chirping
occurs after electrical signals that start in the receptors reach the brain.
The initial effect of these signals in the cortex has been determined by recording
electrical signals from individual neurons. As we described in Chapter 2, neurons in the
cortex that respond best to simple shapes like lines or bars with specifi c orientations are
called feature detectors because they respond to simple features.
Perceiving a tree, or any other object, depends on activity beyond the visual cortex, but the
feature detectors’ response is the fi rst step in the brain’s response to objects. Thus, when you
look at an object such as a tree, neurons in the visual cortex that respond to specifi c orienta-
tions fi re to features of the tree, such as the trunk and branches, as shown in ● Figure 3.2.(a)(b)● (^) FIGURE 3.2 A tree such as this one can be created from a number of simple features, such
as oriented bars (a few of which are highlighted on the right). When a person looks at the
tree, each feature can activate feature detectors in the cortex that respond best to specifi c
orientations. This occurs at an early stage of cortical processing.
Bruce Goldstein
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