Ganong's Review of Medical Physiology, 23rd Edition

290
SECTION III
Central & Peripheral Neurophysiology

FORMS OF MEMORY

From a physiologic point of view, memory is appropriately di-
vided into explicit and implicit forms (Figure 19–2).
Explicit
or
declarative memory
is associated with consciousness—or
at least awareness—and is dependent on the hippocampus and
other parts of the medial temporal lobes of the brain for its re-
tention. Clinical Box 19–1 describes how tracking a patient
with brain damage has led to an awareness of the role of the
temporal lobe in declarative memory.
Implicit
or
nondeclar-
ative memory
does not involve awareness, and its retention
does not usually involve processing in the hippocampus.
Explicit memory is divided into
episodic memory
for
events and
semantic memory
for facts (eg, words, rules, and
language). Explicit memories initially required for activities
such as riding a bicycle can become implicit once the task is
thoroughly learned.
Implicit memory is subdivided into four types.
Procedural
memory
includes skills and habits, which, once acquired,

become unconscious and automatic.

Priming

is facilitation of

recognition of words or objects by prior exposure to them. An

example is improved recall of a word when presented with the

first few letters of it. In

nonassociative learning,

the organism

learns about a single stimulus. In

associative learning,

the

organism learns about the relation of one stimulus to another.

Explicit memory and many forms of implicit memory involve

(1)

short-term memory,

which lasts seconds to hours, during

which processing in the hippocampus and elsewhere lays down

long-term changes in synaptic strength; and (2)

long-term

memory,

which stores memories for years and sometimes for

life. During short-term memory, the memory traces are sub-

ject to disruption by trauma and various drugs, whereas long-

term memory traces are remarkably resistant to disruption.

Working memory

is a form of short-term memory that keeps

information available, usually for very short periods, while

the individual plans action based on it.

NEURAL BASIS OF MEMORY

The key to memory is alteration in the strength of selected

synaptic connections. In all but the simplest of cases, the alter-

ation involves protein synthesis and activation of genes. This

occurs during the change from short-term working memory

to long-term memory. In animals, acquisition of long-term

learned responses is prevented if, within 5 min after each

training session, the animals are anesthetized, given elec-

troshock, subjected to hypothermia, or given drugs, antibod-

ies, or oligonucleotides that block the synthesis of proteins. If

these interventions are performed 4 h after the training ses-

sions, there is no effect on acquisition.

The human counterpart of this phenomenon is the loss of

memory for the events immediately preceding brain concus-

sion or electroshock therapy

(retrograde amnesia).

This

amnesia encompasses longer periods than it does in experi-

mental animals—sometimes many days—but remote memo-

ries remain intact.

FIGURE 19–1
Drawings of PET scans of the left cerebral
hemisphere showing areas of greatest neuronal activation when
subjects performed various language-based activities.
(From
Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology.
McGraw-Hill, 2008.)

FIGURE 19–2
Forms of long-term memory.
(Modified from Kandel ER, Schwartz JH, Jessell TM [editors]:
Principles of Neural Science,
4th ed
.
McGraw-Hill, 2000.)

Two forms of

long term memory

Implicit

(nondeclarative)

Explicit

(declarative)

Facts

(Semantic)

Medial temporal lobe

Hippocampus

Neocortex Striatum Amygdala Cerebellum

Emotional

responses

Skeletal

musculature

Reflex

pathways

Events

(Episodic)

Priming Procedural

(skills and

habits)

Associative learning:

classical and

operant conditioning

Nonassociative learni

habituation and

sensitization