Memory
Memory involves the retention of experiences by alterations in brain
structure and function. Intelligence requires a short-term retention of
experience so that links between current events and features stored in
long-term memory can be compared. This comparison is the basis for recognizing
new events and making quick decisions. The frontal cortex provides a variety of
processing areas that hold short and medium term memories and extract features
of current events to regulate behavior. There are several short term memory
buffers that allow us to carry out routine tasks. For example, we have a buffer
function that can store about 7 word or number items plus or minus 2 for a few
seconds. A seven digit telephone number is relatively easy to remember but a
sixteen digit VISA card number is hard or impossible to recall from this buffer.
The most important characteristic of a short-term store is, clearly,
that it retains information for a limited amount of time only. Most definitions
of short-term memory limit the duration of storage to less than a minute; no
more than about 30 seconds, and in some models as little as 2 seconds. Memory
that exceeds short-term memory duration limits is known as long-term memory. All
memories decay over time. Short-term means that memory decays over seconds to
minutes. If you are trying to remember a phone number, the strategy is to repeat
the number out loud or covertly using your selftalk facility. Memory decay is
reduced by repeating at regular intervals. When memory tasks involve several
components such as digits, words, and pictures there is competition for storage
and recall. New content can replace older content unless the older content is
refreshed.
Tovee suggested [i] that working memory is
temporary information storage that provides continuity between our past
experience and our present situation, and allows us to plan ahead for what we
are likely to encounter in the immediate future. Deficits in working memory lead
to attention deficits and behavioral disorganization. The human frontal
cortex is involved in temporary storage and manipulation of information. The
ability to sequence tasks is disrupted by forgetting what you just did. Working
memory maintains information for a brief period of time, available for all brain
modules to use. If you cannot remember what you just did or wanted to do,
cognitive performance is impaired; you become disorganized and life becomes
difficult. Learning is impaired by the inability to retain an impression of
recent events.
Reading, for example, requires words to be held briefly in recent memory
so that the meaning of whole sentences and then paragraphs can be assembled. If
short-term memory fails to work properly, you cannot understand what you read.
Smith and Jonides suggested: [ii] “Working memory
includes two components: short-term storage (on the order of seconds) and
executive processes that operate on the contents of storage...different frontal
regions are activated for different kinds of information. Storage for verbal
materials activates Broca's area and left-hemisphere supplementary and premotor
areas; storage of spatial information activates the right-hemisphere premotor
cortex; and storage of object information activates other areas of the
prefrontal cortex.” They [iii] also suggested that
there is a: Verbal working memory in the left-hemisphere posterior parietal
cortex, subvocal rehearsal component in left-hemisphere speech areas (selftalk),
including Broca's area as well as the premotor and supplementary motor areas
Spatial working memory mostly in the right-hemisphere spanning posterior
parietal, occipital, and frontal cortices, divides into storage and rehearsal
regions, with right-hemisphere posterior parietal and premotor regions
subserving spatial rehearsal.
In addition they described executive
regulation of the processing of working-memory contents. Inhibition of verbal
working memory is mediated by the left-hemisphere prefrontal region and that it
can be dissociated from verbal storage and rehearsal processes. Ungerleider
[iv] et al suggested that working memory actively maintains
a representation of events briefly. Jonides et al [v]
suggested that parietal regions are part of a network of short-term storage and
retrieval of verbal sound representations. They described two divisions of the
medial temporal lobe, the perirhinal and hippocampal cortices. The perirhinal
cortex judges familiarity. Neurons in the perirhinal cortex respond maximally to
first presentations of visual signals but less to subsequent presentations.
Individual neurons signal different types of information that may comprise a
judgment of prior occurrence. For example, there are recency neurons that know if an event has
occurred recently irrespective of whether it
has been seen many or few times previously.
Familiarity neurons encode the
relative familiarity of an event, making a judgment about how many times the
event has been seen previously without regard to recency. Other
neurons respond best to novel stimuli. Yet other neurons change their response
progressively as initially unfamiliar stimuli are repeated and become more
familiar. Wang [vi] stated:: "Delay-period activity of
prefrontal cortical cells, the neural hallmark of working memory, is generally
assumed to be sustained by reverberating synaptic excitation in the prefrontal
cortical circuit… the biophysical properties of cortical synaptic transmission
are important to the generation and stabilization of a network persistent state…
to achieve a stable persistent state, recurrent excitatory synapses must be
dominated by a slow component. If neuronal firings are asynchronous, the
synaptic decay time constant needs to be comparable to that of the negative
feedback; whereas in the case of partially synchronous dynamics, it needs to be
comparable to a typical interspike interval (or oscillation period). Slow
synaptic current kinetics also leads to the saturation of synaptic drive at high
firing frequencies that contributes to rate control in a persistent state. For
these reasons the slow NMDA receptor-mediated synaptic transmission is likely
required for sustaining persistent network activity at low firing rates. This
result suggests a critical role of the NMDA receptor channels in normal working
memory function of the prefrontal cortex."
The prefrontal cortex is important in working memory and damage to this
part of the brain causes memory deficits. The prefrontal cortex has connections
to most other areas of the brain that process sensory information. Fuster
suggests that the prefrontal cortex "serves the overarching function of the
temporal organization of behavior" by driving networks that maintain currently
important information in an active state. Updating memory is related to
mid-dorsolateral prefrontal activation and right inferior parietal region,
probably related to a visuospatial strategy used to maintain the information in
short-term memory. Information storage within separate sensory systems is
brief; less than a second in visual sensory storage. Information that moves into
a schematic or conceptual form is maintained for many seconds and can be stored
indefinitely in LTM if repeated and recalled in a learning sequence. Visual
memory does not involve storing images the way a digital camera stores images.
Vision is so information-rich it is real-time processing only. Visual memory
consists of feature extractions and a visual equivalent of paraphrasing – a
pseudo-image. When you close you eyes, you cannot see a real picture, but you
have a sense of what is out there. Recall based on quick exposure to images
permits for example the recognition of images already seen in a sequence.
Performance on image recognition tasks decreases with sequence length. Visual
recognition depends on features extracted from the visual information and does
not require that intact images are preserved in STM and compared in an image
processor. Visual working memory involves interaction among the prefrontal
cortex and the higher association areas. This activity is modulated by dopamine.
Other neurotransmitters, such as acetylcholine play a modulatory role in
prefrontal memory function. In monkeys, visual working memory involves posterior
areas in visual cortex and anterior areas in prefrontal cortex. In the visual
cortex, ventral areas are involved in object vision, and dorsal areas are
involved in spatial vision. In the prefrontal cortex,
ventrolateral areas are involved in working memory for objects and dorsolateral
areas for spatial locations. In humans, object vision has a more inferior
location in temporal cortex, and spatial vision a more superior location in the
parietal cortex.
[i] Ibid see ref 93
[ii] Edward E. Smith, John Jonides. Storage and
Executive Processes in the Frontal Lobes Volume 276, Number 5313 Issue of 2 May
1997, pp. 821 - 824 The American Association for the Advancement of
Science
[iii] Edward E. Smith* and John Jonides.
Neuroimaging analyses of human working memory PNAS Vol. 95, Issue 20,
12061-12068, September 29, 1998
[iv] Leslie G.
Ungerleider* Susan M. Courtney and James V. Haxby Vol. 95, Issue 3, 883-890,
February 3, 1998 This paper was presented at a colloquium entitled "Neuroimaging
of Human Brain Function," organized by Michael Posner and Marcus E.Raichle,
held May 29-31, 1997,sponsored by the National Academy of Sciences at the
Arnold and Mabel Beckman Center in Irvine, CA.
[v]
John Jonides, Eric H. Schumacher, Edward E. Smith1, Robert A. Koeppe, Edward Awh,
Patricia A. Reuter-Lorenz, Christy Marshuetz1, and Christopher R. Willis The
Role of Parietal Cortex in Verbal Working Memory . The Journal of Neuroscience,
July 1998,18(13):
[vi] Xiao-Jing Wang.Synaptic Basis
of Cortical Persistent Activity: the Importance of NMDA Receptors to Working
Memory. Jour of Neuroscience, 1999, 19(21):9587-9603
