224 Chapter 8
Emotion and Memory
Limbic System
Emotions influence memory, in some cases by strengthening,
and in others by hindering, memory formation. The amygdala
is involved in the improvement of memory when the memory
has an emotional content. This is illustrated by the observation
that patients who have damage to both amygdaloid nuclei lose
the usual enhancement of memory by emotion.
Although strong emotions enhance memory encoding within
the amygdala, stress can impair memory consolidation by the hip-
pocampus and the cognitive functions and working memory per-
formed by the prefrontal cortex (discussed next). As a result, stress
can promote the storage of emotionally strong memories but hinder
the retrieval of those memories and working memory. In this regard,
researchers have demonstrated that people with post-traumatic
stress disorder often have hippocampal atrophy. The mechanisms
by which stress affects the brain are not fully understood, but it
is known that during stress there is increased secretion of “stress
hormones” (primarily cortisol from the adrenal cortex; chapter 11,
section 11.4), and that the hippocampus and amygdala are rich in
receptors for these hormones. The hippocampus and amygdala are
thus targets of these hormones, and corticosteroids (including corti-
sol) have been shown to suppress neurogenesis in the hippocampus.Prefrontal Cortex
As previously mentioned, the prefrontal cortex is involved in
higher cognitive functions, including memory, planning, and judg-
ment. It is also required for normal motivation and interpersonal
skills and social behavior. In order to perform such varied tasks,
the prefrontal cortex has numerous connections with other brain
regions, and different regions of the prefrontal cortex are special-
ized along different lines. As revealed by patients with damage to
these areas, the functions of the lateral prefrontal area can be dis-
tinguished from the functions of the orbitofacial prefrontal area.
The orbitofrontal area of the prefrontal cortex ( fig. 8.18 )
seems to confer the ability to consciously experience pleasure and
reward. It receives input from all of the sensory modalities—taste,
smell, vision, sound, touch, and others—and has connections with
many regions of the limbic system. As previously discussed, the
limbic system includes several brain areas that are involved in
emotion and motivation. Connections between the orbitofrontal
cortex, the amygdala, and the cingulate gyrus ( fig. 8.18 ) are nota-
bly important for the emotional reward of goal-directed behavior.
People with damage to the lateral prefrontal area of the pre-
central cortex show a lack of motivation and sexual desire, and
they have deficient cognitive functions. People with damage to the
orbitofrontal area of the prefrontal cortex ( fig. 8.18 ), in contrast,
have their memory and cognitive functions largely spared but
experience severe impulsive behavior, verging on the sociopathic.
One famous example of damage to the orbitofrontal area
of the prefrontal cortex was the first case to be described, in- A 25-year-old railroad foreman named Phineas P. Gage
was tamping blasting powder into a hole in a rock with a metal
rod when the blasting powder exploded. The rod—3 feet
receptors from the postsynaptic membrane, which primarily
accounts for the reduced synaptic transmission in LTD.
In some cases, LTP involves changes in the presynaptic
axon as well. These changes promote an increased Ca^2 1 con-
centration within the axon terminals, leading to greater release
of neurotransmitter by exocytosis of synaptic vesicles. The
enhanced release of neurotransmitter during LTP may be pro-
duced by the release of retrograde messenger molecules—ones
produced by dendrites that travel backward to the presynaptic
axon terminals. There is evidence that nitric oxide (NO) can
act as a retrograde messenger in this way, promoting LTP by
increasing the amount of glutamate released from the presyn-
aptic axon terminal (see fig. 8.16 ).
The postsynaptic neuron also can receive input from other
presynaptic neurons, many of which may release GABA as a
neurotransmitter. Through the release of GABA, these neurons
would inhibit the postsynaptic neuron. The release of GABA
can be reduced, and thus inhibition of the postsynaptic neuron
lessened, by another retrograde messenger produced by the post-
synaptic neuron. The retrograde messenger in this case is an
endocannabinoid, a type of lipid neurotransmitter (chapter 7, sec-
tion 7.6). The release of the endocannabinoids from the postsyn-
aptic neuron is stimulated by depolarization, which is produced
at an excitatory synapse by glutamate binding to its receptors on
the postsynaptic neuron. The endocannabinoids then suppress the
release of GABA at a different, inhibitory synapse. This process,
called depolarization-induced suppression of inhibition, may
also contribute to the synaptic learning of LTP.
Neural Stem Cells in Learning and Memory
As previously described, neurogenesis (the formation of new
neurons from neural stem cells) occurs in the adult mamma-
lian brain in (1) the subventricular zone of the lateral ventri-
cles, and (2) the subgranular zone of the hippocampus, from
which new neurons become functional in the dentate nucleus
of the hippocampus. In humans, it appears that the new neu-
rons formed in the lateral ventricles migrate to the striate cor-
tex. Also, neurogenesis is very active in the hippocampus of
human brains throughout adulthood, with little loss due to age.
A recent study measured a rate of 700 new neurons added to
each human hippocampus per day. This is exciting because the
hippocampus is required for the consolidation of certain kinds
of memory, including episodic and spatial memory.
Neurogenesis in the subventricular zone of the lateral
ventricles is implicated in olfactory memory and learning in
rodents. Neurogenesis in the subgranular zone of the hippo-
campus is stimulated in mice by physical exercise and by an
enriched environment, changes that other studies have demon-
strated to improve memory. Studies in rodents show that adult
neurogenesis in the hippocampus accompanies the ability of
mice to learn a water maze, indicating that it may contribute
to spatial learning, memory, and other cognitive tasks. The
demonstration of a significant rate of neurogenesis in the adult
human hippocampus suggests that it may likewise play a role
in human brain function.