Working Memory and the Brain • 139is then hidden from view, the infant behaves as if the object no longer exists) is that
their frontal and prefrontal cortex does not become adequately developed until about
8 months of age (Goldman-Rakic, 1992).PREFRONTAL NEURONS THAT HOLD INFORMATION
The idea that the PF cortex is important for working memory is also supported by
experiments that have looked at how some neurons in the PF cortex are able to hold
information after the original stimulus is no longer present, by continuing to respond
during a brief delay. Shintaro Funahashi and coworkers (1989) conducted an experi-
ment in which they recorded from neurons in a monkey’s PF cortex while the monkey
carried out a delayed-response task. For the task, the monkey fi rst looked steadily at
a fi xation point, X, while a square was fl ashed at one position on the screen (● Figure
5.25a). In this example, the square was fl ashed in the upper left corner (on the other
trials, the square was fl ashed at different positions on the screen). This causes a small
response in the neuron.
After the square went off, there was a delay of a few seconds. The nerve fi ring
records in Figure 5.25b show that the neuron was fi ring during this delay. This fi ring is
the neural record of the monkey’s working memory for the position of the square. After
the delay, the fi xation X went off. This was a signal for the monkey to move its eyes
to where the square had been fl ashed (Figure 5.25c). The monkey’s ability to do this
provides behavioral evidence that it had, in fact, remembered the location of the square.
The key result of this experiment was that Funahashi found neurons that responded
only when the square was fl ashed in a particular location and that these neurons contin-
ued responding during the delay. For example, some neurons responded only when the
square was fl ashed in the upper right corner and then during the delay; other neurons
responded only when the square was presented at other positions on the screen and
then during the delay. The fi ring of these neurons indicates that an object was presented● FIGURE 5.25 Results of an experiment showing the response of neurons in the
monkey’s PF cortex during an attentional task. Neural responding is indicated by an
asterisk (*). (a) A cue square is fl ashed at a particular position, causing the neuron to
respond. (b) The square goes off , but the neuron continues to respond during the delay.
(c) The fi xation X goes off , and the monkey demonstrates its memory for the location of
the square by moving its eyes to where the square was. (Source: Adapted from S. Funahashi, C. J. Bruce, &
P. S. Goldman-Rakic, “Mnemonic Coding of Visual Space in the Primate Dorsolateral Prefrontal Cortex,” Journal of Neurophysiology
61, 331–349, 1989. Copyright © 1989 by The American Physiological Society. Reproduced by permission.)Square
goes offEye
movement*Electricalactivity**
(a) Cue (b) During delay (c) Monkey moves eyesCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.