256 CHAPTER 6
All that explains is the “where” of memory. Did scientists ever
find out the “what” or the exact physical change that happens in the
brain when memories are stored?Several studies have offered evidence that memory is not simply one physical
change but many: changes in the number of receptor sites, changes in the sensitivity of
the synapse through repeated stimulation (called long-term potentiation), and changes in
the dendrites and specifically in the proteins within the neurons (Alkon, 1989; Kandel &
Schwartz, 1982; Squire & Kandel, 1999). The changes underlying synaptic plasticity and
memory storage have been attributed to six molecular mechanisms (cAMP, PKA, CRE,
CREB-1 and CREB-2, CPEB) that signify information changing from short-term memory
to long-term memory, and apply to both explicit and implicit memory (Kandel, 2012).
In addition to multiple changes occurring, changes in synaptic function have to occur
across collections of neurons as part of a larger circuit (Kandel, 2012). Collectively, the
synaptic alterations, changes in neuronal structure, protein synthesis, and other changes
that take place as a memory is forming are called consolidation (Deger et al., 2012; Fioriti
et al., 2015; Griggs et al., 2013; Hill et al., 2015; Krüttner et al., 2012). Consolidation may
take only a few minutes for some memories, such as learning a new friend’s name, but
may take years for others, such as learning a new language (Dudai, 2004).
In the discussion of the hippocampus (a part of the limbic system) in Chapter Two, it
was identified as the part of the brain that is responsible for the formation of new long-
term declarative memories. One of the clearest pieces of evidence of this function comes
from the study of a man known as H.M. (Milner et al., 1968).
H.M. was 16 when he began to suffer from severe epileptic seizures. Eleven years
later, H.M.’s hippocampi and adjacent medial temporal lobe structures were removed in
an experimental operation that the surgeon hoped would stop his seizures. The last thing
H.M. could remember was being rolled to the operating room, and from then on his abil-
ity to form new declarative memories was profoundly impaired. The hippocampus was
not the source of his problem (his seizures were reduced but not eliminated), but it was
apparently the source of his ability to consolidate and store any new factual information he
encountered, because without either hippocampus, he was completely unable to remem-
ber new events or facts. Consolidation had become impossible. He had a magazine that
he carried around, reading and rereading the stories, because each time he did so the sto-
ries were completely new to him. As with most amnesic patients of this type (although
H.M.’s case was quite severe), his nondeclarative memory was still intact*. It was only
new declarative memory—both semantic and episodic—that was lost. H.M., who can now
be revealed as Henry Gustav Molaison, died in December 2008 at the age of 82. His expe-
rience and his brain will continue to educate students and neuroscientists, as he agreed
many years ago that his brain would be donated for further scientific study upon his death.
It has now been cut into 2,401 slices, each about the width of a human hair, in preparation
for further study. You can read more about H.M.’s contributions to science at Suzanne Cor-
kin’s Web site at http://web.mit.edu/bnl/publications.htm. To learn about the H.M. post-
mortem project being conducted go to http:// thebrainobservatory.org (Carey, 2009).
There is some evidence that memories of the same event may involve different
areas of the hippocampus (Collin et al., 2015). The different areas seem to correspond to
different degrees of memory detail for the event, such as remembering reading a specific
text message from your partner before going to class (a fine detail) or recalling going out
to eat after class (a broader event).
Another area of the brain involved in the formation of long-term memories is
the posterior cingulate, an area of the cortex located near the rear of the corpus callosum.consolidation
the changes that take place in the
structure and functioning of neurons
when a memory is formed. *intact: whole or complete.