A large body of evidence indicates that the dorsolateral prefrontal
cortex plays an important role in certain forms of memory work, in
particular those that involve alternating between two memory tasks
and exploring various possibilities before making a choice.
It seems fairly certain that this area of the brain holds information
required for reasoning processes that are in progress. But its
precise role remains the subject of much debate. Does this prefrontal
area basically coordinate the activities of slave sub-systems,
as in Baddeley’s
model of the phonological loop and the visual/spatial sketchpad?
Or does it actually itself serve as a temporary storage area for
certain types of information, as Goldman-Rakic’s research
tends to indicate? Might the level of abstraction of the task be
the deciding factor, or might the size of the workload determine
whether this area comes into play?
As all these unanswered questions suggest, the anatomical substrate
of working memory is far from being understood in detail. Moreover,
the phenomenon of working memory is made all the more complex by
the fact that it takes place over time.
Source: NIMH Laboratory of Brain
and Cognition. Published in Nature, Vol. 386, April
1997, p. 610.
For example, the experimental
results illustrated here show how various areas of the subjects’ brains
alter their activity levels as the subjects are presented
with various visual stimuli. When the subjects are shown
a blurred image, the activity level (represented by the blue
bars in the graph) becomes highest in area 1, the visual
part of the brain. When the subjects are shown an image of
a face, brain activity (black bars) becomes highest in the
associative and frontal regions (4, 5, and 6). Lastly, when
the subjects are retaining an image of a face in their working
memory, brain activity (red bars) is highest in the frontal
regions, while the visual areas are scarcely stimulated at
all.
It has also been observed that
distinct processes appear to be involved in the storage
and recall of items memorized with the phonological loop
and the visual/spatial sketchpad.
One thing is certain: the prefrontal cortex plays a fundamental
role in working
memory. It enables people to keep information available that
they need for their current reasoning processes. For this purpose,
the prefrontal cortex must cooperate with other parts of the cortex
from which it extracts information for brief periods. For this
information to eventually pass
into longer-term memory, the limbic system probably has to
be brought into play.
The hippocampus
receives connections from the cortex’s primary sensory
areas, unimodal associative areas (those that involve only
one sensory modality), and multimodal associative areas,
as well as from the rhinal and entorhinal cortexes. While
these anterograde connections converge at the hippocampus,
other, retrograde pathways emerge from it, returning to
the primary cortexes, where they record in the cortical
synapses the associations facilitated by the hippocampus.
Thus, even in the mechanism of memorization, we find the
feedback loops so often encountered at all levels in the
living world.
For
a piece of information to be recorded in long-term memory,
it must pass through Papez’s circuit. Injuries
to this circuit can result in memory impairments.
For example, a lesion in the mammillary bodies is responsible
for an amnesic syndrome whose most classic example is Korsakoff’s
syndrome. In addition to the confabulation, confusion, and
disorientation that accompany this syndrome, patients suffer
from anterograde amnesia: they cannot store new information
in their long-term memory. The most typical cause of this
syndrome is vitamin B1 deficiency, often seen in chronic
alcoholics.
LONG-TERM MEMORY
Recent research
has provided a complex, highly intricate picture of memory
functions and their loci in the brain. The
hippocampus, the temporal lobes, and the
structures of the limbic system that are connected to
them are essential for the consolidation of long-term memory.
The hippocampus facilitates associations
among various parts of the cortex, for example, between a tune
that you heard at a dinner party and the faces of the other
guests who were at the table. However, all other things being
equal, such associations would naturally fade over time,
so that your mind did not become cluttered with useless
memories. What might cause such associations to be strengthened
and eventually etched into long-term memory very often
depends on “limbic” factors, such as how
interested you were in the occasion, or what emotional
charge it may have had for you, or how gratifying you
found its content.
The various structures
of the limbic system exert their influence on the hippocampus
and the temporal lobe via Papez’s circuit, also known
as the hippocampal/mammillothalamic tract. This circuit
is a sub-set of the numerous connections that the limbic
structures have with one another. The diagram here shows
the route that information travels from the hippocampus
to the mammillary bodies of the hypothalamus, then on to
the anterior thalamic nucleus, the cingulate cortex, and
the entorhinal cortex, before finally returning to the hippocampus.
Once the
temporary associations of cortical neurons generated
by a particular event have made a certain number of such “passes”
through Papez’s circuit, they will have undergone
a physical remodelling that consolidates them. Eventually,
these associations will have been strengthened so much
that they will stabilize and become independent of the
hippocampus. Bilateral lesions of the hippocampus will
prevent new long-term memories from forming, but will not
erase those that were encoded before the injury.
With this gradual disengagement
of the limbic system, the memories will no longer pass
through Papez’s
circuit, but instead will be encoded in specific areas
of the cortex: the same ones where the sensory information
that created the memories was initially received (the occipital
cortex for visual memories, the temporal cortex for auditory
memories, etc.).
