Human Physiology, 14th edition (2016)

The Central Nervous System 221

perform sequential actions), and the inhibition of inappropri-

ate actions (such as answering a stranger’s ringing telephone).

There is evidence that signals are sent from the prefrontal

cortex to the inferior temporal lobes, where visual long-term

memories are stored. Scientists now believe that particular

aspects of a memory—visual, auditory, olfactory, spatial, and

so on—are stored in particular areas that need to cooperate to

elicit the complete memory.

Working memory —the ability to keep information in

your head consciously for a short time—is stored differently

depending on whether it involves keeping several numbers

in your mind until you type them, or whether it involves spa-

tial information, such as backtracking to pick up an item you

skipped while browsing in a new store. However, both types of

working memory require the prefrontal cortex. There are also

certain generalities that can be made about long-term declara-

tive memory and brain location. For example, the ability to

recall names and categories (semantic memory) is localized

to the inferior temporal lobes; different locations seem to be

required for storing episodic memories. Thus, in Alzheimer’s

disease, episodic and semantic memory decline independently

of each other. Some of the brain areas associated with different

systems of memory are summarized in table 8.3.

Alzheimer’s Disease

Alzheimer’s disease is the most common form of dementia,

its incidence increasing with age to greater than one in three

after age 85. People with Alzheimer’s disease have (1) loss of

brain weight and volume, and a decreased cortical thickness

that correlates with cognitive decline, (2) losses of specific

neuronal populations (as in the hippocampus and cerebral cor-

tex), of dendritic spines and synapses, and of neural network

function; (3) an accumulation of extracellular protein depos-

its called amyloid senile plaques; and (4) intracellular protein

deposits forming neurofibrillary tangles.

to specific concepts. For example, single neurons have been
found to increase activity in response to specific actors or clips
from specific TV shows. These and other observations suggest
that memory retrieval related to a specific concept may involve
the activation of a relatively small set (perhaps numbering in
the thousands) of specific neurons.
The amygdala is particularly important in the memory
of fear responses. Studies demonstrate increased neural activ-
ity of the human amygdala during visual processing of fearful
faces, and patients with bilateral damage to the amygdala were
unable to read danger when shown threatening pictures.
The cerebral cortex is thought to store factual informa-
tion, with verbal memories lateralized to the left hemisphere
and visuospatial information to the right hemisphere. The
neurosurgeon Wilder Penfield was the first to electrically
stimulate various brain regions of awake patients, often evok-
ing visual or auditory memories that were extremely vivid.
Electrical stimulation of specific points in the temporal lobe
evoked specific memories so detailed that the patients felt as
if they were reliving the experience. The medial regions of
the temporal lobes, however, cannot be the site where long-
term memory is stored because destruction of these areas in
patients treated for epilepsy did not destroy the memory of
events prior to the surgery. The inferior temporal lobes, on
the other hand, do appear to be sites for the storage of long-
term visual memories.
The left inferior frontal lobe has recently been shown to
participate in performing exact mathematical calculations. Sci-
entists have speculated that this brain region may be involved
because it stores verbally coded facts about numbers. Using
fMRI, researchers have recently demonstrated that complex
problem-solving involves the most anterior portion of the fron-
tal lobes, an area called the prefrontal cortex. Some of the
other functions ascribed to the prefrontal cortex include short-
term memory (as for a phone number that must be kept in mind
to dial but then quickly forgotten), planning (remembering to

Table 8.3 | Categories of Memories and the Major Brain Regions Involved

Memory

Category

Major Brain

Regions Involved

Length of

Memory Storage Examples

Episodic memory (explicit,

declarative)

Medial temporal lobes, thalamus,

fornix, prefrontal cortex

Minutes to years Remembering what you had for breakfast, and

what vacation you took last summer

Semantic memory (explicit,

declarative)

Inferior temporal lobes Minutes to years Knowing facts such as what city is the capital,

your mother’s maiden name, and the different

uses of a hammer and a saw

Procedural memory (explicit

or implicit; nondeclarative)

Basal nuclei, cerebellum,

supplementary motor areas

Minutes to years Knowing how to shift gears in a car and how to

tie your shoelaces

Working memory Words and numbers: prefrontal

cortex, Broca’s area, Wernicke’s

area

Seconds to minutes Words and numbers: keeping a new phone

number in your head until you dial it

Spatial: prefrontal cortex, visual

association areas

Spatial: mentally following a route

Source: Modified from: Budson, Andrew E. and Bruce H. Price.“Memory dysfunction.” New England Journal of Medicine 352 (2005): 692–698.