Self and Memory 211“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityand its adjacent area entorhinal cortex) from both brain hemispheres.^4
The operation stopped the seizures and HM retained his normal intelli-
gence and perceptual ability, and had a decent vocabulary. But, alas, he
could not remember anything new since surgery! HM had developed
what we now call anterograde amnesia, the inability to make new mem-
ories. For example, he could not recognize persons whom he routinely
saw on a monthly basis, he could not retain information about an object
for more than a few minutes, and he had great difficulty learning to get
around a new house. This incident established the role of the medial
temporal lobe in human memory.^5 The kind of memory lost in HM was
episodic memory, knowledge of events with a time-place reference, such
as “I had dinner last night with Mr. Smith in an Italian restaurant on
Fifth Avenue.” On the other hand, semantic memory, which concerns
general factual statements such as “the Earth is round,” was less affected.
Episodic and semantic memories both belong to the so-called declara-
tive (or explicit) memory, one that requires the full participation of con-
sciousness. The opposite type of memory, known as non-declarative (or
implicit) memory, which consists of training for motor skills and other
types of learning that do not necessitate conscious effort to recall, was
normal in HM. Motor skill learning depends on brain regions outside
of the medial temporal lobe, and involves structures such as the basal
ganglia and cerebellum. What is also significant about HM’s memory is
that events from the remote past were generally (but not totally) pre-
served, such as his childhood experiences. The lesson from HM is that
the hippocampus is important for converting short-term to long-term
memory of the declarative type. Figures 10.1 to 10.3 show the medial
temporal lobe of the human brain and the parts relevant to memory,
while Tables 10.1 and 10.2 compare the various types of memory.
Long-term memories are those that last for months and years after
the experience; they are correlated with permanent physical changes in
the brain, in particular the neurons and their synapses in multiple cor-
tical areas. Short-term memories are those that stay in the brain for less
than an hour before permanent physical changes set in. In HM, it lasted