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

Categories and the Brain • 261

example, the stimulus on the left is 100 percent cat, the one on the right is 100 percent

dog, and the others are mixtures of the two. Using stimuli like these, plus many oth-

ers that also mixed dog and cat properties, Freedman trained the monkeys to respond

to stimuli that were more than 50 percent “cat” as being in the cat category, and to

respond to the rest as dogs. After several months of training, the monkeys were able to

categorize a 60 percent cat/40 percent dog stimulus as “cat” about 90 percent of the

time. Likewise, monkeys correctly classifi ed 60 percent dog stimuli as “dog” about 90

The Universality of Language

Once the monkeys had learned to categorize the stimuli, they were tested using

the procedure in ● Figure 9.25a. First a sample stimulus (either a cat or a dog) was

presented; then, after a 1-second delay, a test stimulus was presented. The monkey’s

task was to release a lever if it judged the test stimulus to be in the same category as

the sample stimulus. As the monkeys were doing this, Freedman recorded from neurons

in an area of the temporal lobe called the inferotemporal (IT) cortex, which responds to

forms, and from neurons in the prefrontal (PF) cortex, which is involved in memory and

other cognitive processes (see Chapter 5, page 138).

● FIGURE 9.24 Some of the stimuli used in Freedman’s experiment. The far left stimulus is

100 percent cat, the far right is 100 percent dog, and the others are mixtures of the two. The

dashed line is the border between the category “cat” and the category “dog.” (Source: Adapted

from D. J. Freedman et al., “A Comparison of Primate Prefrontal and Inferior Temporal Cortices During Visual

Categorization,” Journal of Neuroscience, 23(12), 5235–5246, Figure 1b. Copyright 2003 Society for Neuroscience.)

100% Cat 80% Cat 60% Cat 60% Dog 80% Dog

Cats Dogs

100% Dog

● FIGURE 9.25 (a) Testing procedure
for determining if monkeys can
categorize cats and dogs. (b) Activity
recorded from a neuron in the
monkey’s IT cortex during the testing
sequence. This neuron responds
more to a 60 percent dog stimulus
than to a 60 percent cat stimulus
during presentation of the sample. (c)
Activity recorded from a neuron in the
monkey’s PF cortex during the testing
sequence. This neuron responds better
to the dog stimulus during the delay
and test periods. (Source: D. J. Freedman
et al., “A Comparison of Primate Prefrontal
and Inferior Temporal Cortices During Visual
Categorization,” Journal of Neuroscience, 23,
5235–5246, Figure 4, 2003. Reprinted by
permission.)

Sample

IT PF

(a)

(b) (c)

Delay

1 sec.

Test

“Is the test stimulus

from the same category

as the sample stimulus?”

Response

20

15

10

5

0

Firing rate

20

15

10

5

Firing rate

–500 0 500 1000 1500 2000

Time from sample onset (ms)

–500 0 500 1000 1500 2000

Time from sample onset (ms)

Fixation Sample Delay Test Fixation Sample Delay Test

60% Cat

60% Dog

Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.