Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 19

Learning, Memory, Language, & Speech 293

implicit memory processes are generally intact. They perform
adequately in terms of conscious memory as long as they con-
centrate on what they are doing. However, if they are distract-
ed for even a very short period, all memory of what they were
doing and what they proposed to do is lost. They are thus ca-
pable of new learning and retain old prelesion memories, but
they cannot form new long-term memories.
The hippocampus is closely associated with the overlying
parahippocampal cortex in the medial frontal lobe (Figure
19–4). Memory processes have now been studied not only
with fMRI but with measurement of evoked potentials (event-
related potentials; ERPs) in epileptic patients with implanted
electrodes. When subjects recall words, activity in their left
frontal lobe and their left parahippocampal cortex increases,
but when they recall pictures or scenes, activity takes place in
their right frontal lobe and the parahippocampal cortex on
both sides.
The connections of the hippocampus to the diencephalon
are also involved in memory. Some people with alcoholism-
related brain damage develop impairment of recent memory,
and the memory loss correlates well with the presence of
pathologic changes in the mamillary bodies, which have
extensive efferent connections to the hippocampus via the
fornix. The mamillary bodies project to the anterior thalamus
via the mamillothalamic tract, and in monkeys, lesions of the
thalamus cause loss of recent memory. From the thalamus,
the fibers concerned with memory project to the prefrontal
cortex and from there to the basal forebrain. From the basal
forebrain, a diffuse cholinergic projection goes to all of the
neocortex, the amygdala, and the hippocampus from the
nucleus basalis of Meynert.
Severe loss of these fibers occurs
in Alzheimer disease.
The amygdala is closely associated with the hippocampus
and is concerned with encoding and recalling emotionally
charged memories. During retrieval of fearful memories, the
theta rhythms of the amygdala and the hippocampus become

synchronized. In normal humans, events associated with

strong emotions are remembered better than events without

an emotional charge, but in patients with bilateral lesions of

the amygdala, this difference is absent.

Confabulation

is an interesting though poorly understood

condition that sometimes occurs in individuals with lesions of

the ventromedial portions of the frontal lobes. These individ-

uals perform poorly on memory tests, but they spontaneously

describe events that never occurred. This has been called

“honest lying.”

NEW BRAIN CELLS?

It is now established that the traditional view that brain cells are

not added after birth is wrong; new neurons form from stem

cells throughout life in two areas: the olfactory bulb and the

hippocampus. This is a process called

neurogenesis.

There is

evidence implicating a role of neurogenesis in the hippocam-

pus with learning and memory. A reduction in the number of

new neurons formed reduces at least one form of hippocampal

memory production. However, a great deal more is still to be

done before the relation of new cells to memory processing can

be considered established.

LONG-TERM MEMORY

While the encoding process for short-term explicit memory

involves the hippocampus, long-term memories are stored in

various parts of the neocortex. Apparently, the various parts of

the memories—visual, olfactory, auditory, etc—are located in

the cortical regions concerned with these functions, and the

pieces are tied together by long-term changes in the strength

of transmission at relevant synaptic junctions so that all the

components are brought to consciousness when the memory

is recalled.

Once long-term memories have been established, they can

be recalled or accessed by a large number of different associa-

tions. For example, the memory of a vivid scene can be evoked

not only by a similar scene but also by a sound or smell associ-

ated with the scene and by words such as “scene,” “vivid,” and

“view.” Thus, each stored memory must have multiple routes

or keys. Furthermore, many memories have an emotional

component or “color,” that is, in simplest terms, memories can

be pleasant or unpleasant.

STRANGENESS & FAMILIARITY

It is interesting that stimulation of some parts of the temporal

lobes in humans causes a change in interpretation of one’s sur-

roundings. For example, when the stimulus is applied, the

subject may feel strange in a familiar place or may feel that

what is happening now has happened before. The occurrence

of a sense of familiarity or a sense of strangeness in appropri-

ate situations probably helps the normal individual adjust to

FIGURE 19–4
Areas concerned with encoding explicit
memories.
The prefrontal cortex and the parahippocampal cortex of
the brain are active during the encoding of memories.
(Modified from
Russ MD: Memories are made of this. Science 1998;281:1151.)

Prefrontal

cortex

Hippocampus

Parahippocampal

cortex