Download The Neurobiology of Learning and Memory, 2e

15
The Content of Memory:
Memory Systems and the
Hippocampus
The Content of Experience Matters to the Brain:
Example 2: Personal Facts and Skills
The many hours of practice required to play a musical
instrument or a sport leave an enduring impression on your
brain that now supports the skill.
In addition to acquiring a skill, however, you will also remember
much about the practice sessions—where they occurred, who
your instructors were, and how difficult it was initially to
perform.
Nevertheless, these aspects of your memory have absolutely
nothing to do with your ability to perform the skill.
We know this because people who are amnesic in the sense of
having no recollection of the training episodes can still
perform.
The inescapable conclusion is that the memory system that
supports skillful behaviors is outside of the region of the brain
that supports our ability to recollect the training episodes.
The Content of Experience Matters to the Brain:
Multiple Memory Systems
Memory researchers believe
• A complete understanding of memory can only be
achieved by recognizing that the content of
experience is important.
• Memories are segregated into different brain
regions according to their content.
These ideas are the central premises of what is called
the multiple memory system perspective.
The Hippocampus and Episodic Memory:
H.M. Contribution to Memory Research
The multiple memory systems view and the significance of
the hippocampus begins with Brenda Milner’s analysis of
patient H.M.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
Henry Gustav Molaison (H.M.) is the
most famous patient in the history of
memory research. He suffered
profound epilepsy. To alleviate this
problem, a region of the brain called
the medial temporal lobe was
bilaterally removed. This region
included the hippocampus,
amygdala, and surrounding cortex.
As a consequence, H.M. became
severely amnestic.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
Retrograde amnesia
Anterograde amnesia
Surgery
H.M.’s retrograde amnesia (memory for events that
occurred before surgery) extended back to childhood. His
anterograde amnesia (memory for events that occurred
after surgery) was permanent.
H.M. lost all contact with the past. He could not recognize
people he saw every day nor himself.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research: Not All Memory Functions Were Lost
H.M.’s short-term memory remained
intact but he could not convert a
short-term memory into a long-term
declarative memory.
He could acquire new skills such as
mirror tracing.
Even though his performance
improved, H.M. could not remember
participating in the tasks.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
“Right now, I’m wondering. Have I done or said
anything amiss. You see at this moment everything
looks clear to me, but what happened just before?
That’s what worries me. It’s like waking from a
dream; I just don’t remember.”
—H.M., quoted by Brenda Milner.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
He had a conscience. For example, when explaining
why he could not fulfill his dream of being a
neurosurgeon, he cited the fact that he wore
glasses, and that blood might spurt up onto his
glasses, creating an obstacle to his vision and
causing him to miss his target in the patient's brain,
thereby causing the patient harm.
In addition, he had good insight into his memory
disorder. When I asked him, "What do you do to try
to remember?" he replied, "Well, that I don't know
'cause I don't remember (laugh) what I tried." He had
a sense of humor, and often made jokes.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
H.M. was not the first patient to display amnesia for
certain types of information. What was unique was
that the location of his brain damage was known.
This meant that specific regions of the brain may be
critical to memory and, for the first time, researchers
had a testable hypothesis about just what regions
were critical.
That his intellectual capacities were intact also meant
that memory functions could be separated from other
cognitive abilities.
That H.M.’s anterograde and retrograde amnesia was
restricted to certain kinds of content is also part of
the foundation for the multiple memory systems view.
The Hippocampus and Episodic Memory: H.M.’s Contribution to
Memory Research
Today many researchers believe that the removal of
HM’s medial temporal lobes disrupted what is called
the episodic memory system.
This memory system extracts and stores the content
of our personal experiences in a manner that allows
them to be consciously retrieved.
The Hippocampus and Episodic Memory: The Extent of H.M.’s
Brain Lesion
William Scoville
The Hippocampus and Episodic Memory: Animal Models of
H.M.’s Amnesia
Given the extent of H.M.’s brain damage, it was
difficult to know if any particular region was more
critical to his amnesia than another. Thus,
researchers turned to animal models to determine
what regions of the brain were critical to H.M.’s
amnesia.
The research strategy:
• Devise a memory task that could be used with
primates to study episodic memory.
• Surgically remove various regions of the brain that
were removed from H.M. and test animals on this
task.
The Hippocampus and Episodic Memory: The Delayed
Nonmatching-to-Sample Task Developed by Mishkin
The Hippocampus and Episodic Memory: Mishkin’s Experiments
% Correct
C
A
H
AH
10 30 60 120
Delay Interval (sec)
The Hippocampus and Episodic Memory: Damage to the Rhinal
Cortices Impairs Performance on the DNMS Task
Larry Squire suggested that it is the cortices surrounding the
hippocampus that support DNMS performance. Primates in this
experiment had selective damage to the hippocampus and amygdala or
to the underlying perirhinal cortex. Note that only damage to the rhinal
cortex impaired performance.
The DNMS Paradox Resolved
Dual Process Theories of
Recognition Memory: The
DNMS task does not
depend only on the episodic
memory system. It has two
solutions—one that depends
on the hippocampus and
one that does not. According
to this analysis, monkeys
without a hippocampus can
still perform correctly on the
DNMS task because they
still have the neural system
(perirhinal cortex) needed to
make familiarity-based
judgments.
The Hippocampus and Episodic Memory: Patients with Selective
Damage to the Hippocampus
Other patients who have much
more focal damage to the
hippocampus, such as patient
R.B., also show a similar pattern
of spared and impaired
memories. However, R.B’s
impairments were not as severe
as H.M.’s.
Damage to the hippocampus of
patient R.B. was restricted to a
massive loss of neurons in the
CA1 field, outlined in white in
the figure, thereby disrupting the
sequence of information flow
through the hippocampus.