Ganong's Review of Medical Physiology, 23rd Edition

290 SECTION III Central & Peripheral Neurophysiology
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.)
FORMS OF MEMORY
From a physiologic point of view, memory is appropriately divided
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 retention.
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 nondeclarative
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,
Facts
(Semantic)
Explicit
(declarative)
Events
(Episodic)
Medial temporal lobe
Hippocampus
Two forms of
long term memory
Priming Procedural
(skills and
habits)
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 subject
to disruption by trauma and various drugs, whereas longterm
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 alteration
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 electroshock,
subjected to hypothermia, or given drugs, antibodies,
or oligonucleotides that block the synthesis of proteins. If
these interventions are performed 4 h after the training sessions,
there is no effect on acquisition.
The human counterpart of this phenomenon is the loss of
memory for the events immediately preceding brain concussion
or electroshock therapy (retrograde amnesia). This
amnesia encompasses longer periods than it does in experimental
animals—sometimes many days—but remote memories
remain intact.
Emotional
responses
Implicit
(nondeclarative)
Associative learning:
classical and
operant conditioning
Skeletal
musculature
Neocortex Striatum Amygdala Cerebellum
Nonassociative learni
habituation and
sensitization
Reflex
pathways
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.)