432 Neuroanatomy: Draw It to Know It
Memory: Capacity & Consolidation
Here, we will address the concept of memory capacity
and we will create a workfl ow for the consolidation of
memory. First, in regards to short-term memory capac-
ity, indicate that short-term memory is able to hold seven
bits of information (plus or minus two) at any given
time. However, these bits of information are routinely
manipulated to ease their memory burden. Show that
the most commonly discussed example of information
manipulation is the process of chunking , wherein we
take a large amount of information and organize it into
fewer chunks or bits. For instance, we can represent the
string of digits in a telephone number as seven separate
digits, which we may struggle to remember and have to
frequently rehearse to avoid their decay, or we can use
the principles of chunking to combine digits into larger
chunks and reduce their memory burden. For example,
even the 11-digit number 18005551212 can easily be
memorized when we chunk it into just three bits of
information: 1-800, 555, 1212.
Chunking can also occur through more abstract means,
as well. As a remarkable example, albeit rare, through syn-
esthesia, certain individuals unconsciously perceive infor-
mation in sensorial rather than semantic fashion, which
can have disastrous eff ects on comprehension but can
allow for an astonishing ability to chunk extremely large
quantities of information and dramatically boost memory
capacity.^12 , 13 For the autistic savant Daniel Tammet, num-
bers appear to him as colors and sounds and carry person-
alities; he unconsciously creates landscape-like projections
of numbers in his mind that allow him to remember stag-
gering amounts of information.^14
Now, let’s shift our focus to the consolidation of
memory. Show that memory consolidation relies upon
encoding (the process in which information is transformed
into a format in which it can be stored and retrieved), stor-
age (the stockpiling of memory into its stored state), and
retrieval (the accessing of stored memories).
In regards to the neuroanatomic circuitry for memory
processing, draw a medial face of a cerebral hemisphere,
and show that memories pass from the sensory associa-
tion cortices to the limbic lobe where they are encoded
(most notably, in the Papez and amygdaloid circuits),
and then pass to the neocortex, where they are stored
and from where they are retrieved. However, this model
predicts noninvolvement of the limbic lobe (more spe-
cifi cally, the hippocampus) in the storage process, but
multiple memory trace theory has shown that the hip-
pocampal–neocortical bond for episodic memory does
not decay but, instead, persists as part of a memory scaf-
fold. Th us, indicate that every time an episodic memory
is retrieved, an additional memory trace is formed, and
the hippocampal–neocortical ensemble of that memory
is strengthened.^15 , 16
Lastly, let’s learn about the clinical corollary of amne-
sia (ie, loss of memory), through the oft en-written-about
patient Henry Gustav Molaison, universally referred to
by the initials H.M., who in 1953 underwent bilateral
medial temporal lobe resection for intractable epilepsy.
Th e primary dysfunction in H.M. was the encoding of
new memories; for example, people who came daily to
visit H.M. were strangers anew every day. On the con-
trary, H.M.’s sensory memory was unaff ected and his
short-term memory was only partially aff ected. Indicate
that anterograde amnesia is the inability to form new
memories aft er the development of amnesia, whereas
retrograde amnesia is the inability to retrieve memories
that occurred prior to the development of the amnesia.
H.M. had complete anterograde amnesia and also partial
retrograde amnesia, which demonstrated a temporal gra-
dient as follows: the degree of his retrograde amnesia
was at its maximum for memories just prior to the opera-
tion and at its minimum for memories 11 years prior to
the operation — note that the true duration of H.M.'s
temporal gradient is debated.^17