Cognitive Psychology: Connecting Mind, Research and Everyday Experience, 3rd Edition

38 • CHAPTER 2 Cognitive Neuroscience

recognize and perhaps react to the person. But this description, while correct, is too gen-

eral. We want to know how you are able to respond “That’s Bill,” as opposed to identify-

ing the person as Roger or Sally. What is it about the electrical activity in your brain that

goes beyond “That’s a face” to actually representing a specifi c face such as Bill’s? This is

the question of representation, and to begin answering it, we will consider what happens

when you perceive another stimulus—a tree.

REPRESENTING A TREE: FEATURE DETECTORS

Considering how a tree is represented in the nervous system brings us back to one of

the defi nitions of mind presented in Chapter 1, which stated that the mind is a system

that creates representations of the world, so we can act within it to achieve our goals.

Applied to the brain, the major idea behind this statement is that a tree, and everything

else we perceive, is represented in the brain. We can appreciate what this means by con-

sidering what happens as we look at a tree.

We see the tree because light refl ected from the tree enters the eye and an image of the tree

is focused onto the retina, the layer of neurons that lines the back of the eye (● Figure 2.17).

The important word here is image, because it is the image created by light refl ected by the

tree that gets into the eye, not the tree itself. The idea of the tree not getting into the eye may

seem silly because it is so obvious, but the point is an important one: What enters the eye is a

representation of the tree—something that stands for the tree.

One property of this representation is that although it may look like the tree, it is also

different from the tree. It is not only smaller, but may be distorted or blurred because of

the optics of the eye. This difference between the actual tree and its representation becomes

more dramatic about a few thousandths of a seconds later when receptors in the retina

transform the tree’s image into electrical signals, which then travel through the retina, leave

the eye via the optic nerve, and eventually reach the primary visual receiving area of the

brain. Our perception of the tree is therefore based not on direct contact with the tree,

but on the way the tree is represented by action potentials in the brain. Early research on

the nature of this representation led to the proposal that this representation could involve

neurons called feature detectors that respond to features that make up objects.

● FIGURE 2.17 Light refl ected from the tree enters the eye and forms an image of the tree on the

retina. This image is transformed into electrical signals that travel out the back of the eye along the

optic nerve and eventually reach the brain. Our perception of the tree is based on the information

contained in these neural signals.

Electrical

signal to

brain

Retina

Image of

tree on

retina

Light

Optic

nerve

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