Transcript NLM2e Ch17 Lecturex

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The MTH System: Episodic
Memory, Semantic Memory,
and Ribot’s Law
Divisions of Declarative Memory
Semantic memory supports our memory for facts and our
ability to make generalizations from multiple experiences
so that you can answer questions like is a violin a
musical instrument or an automobile, or what is your
mothers birthday?
To do this requires intentional retrieval and explicit
recollection. However, the content of semantic memory is
said to be context free—not tied to the place where it
was acquired.
Two Views of the Role of the Hippocampus in Episodic and
Semantic Memory
(A) The unitary view of the
medial temporal
hippocampal (MTH)
system. Supporters of this
view believe that the
system is needed to
support both episodic and
semantic memory. (B) The
modular view. Supporters
of this view believe that
the entire system,
including the hippocampal
formation, is required for
episodic memory but that
semantic memory does
not require the
hippocampus.
A Modular MTH System: Growing up without a Hippocampus
Farneh Vargha-Khadem and her
colleagues studied children that
had experienced an anoxicischemic insult that bilaterally
damaged their hippocampus.
They were between the age of 4 and 9 years
when the damage occurred.
These children had very impaired episodic
memory but developed normal language and
social skills.
They were able to read and write and acquire
new factual information.
A Modular MTH System: Recognition Memory, Primates
Monkeys with damage to the hippocampus can
recognize the sample object in the DNMS task. Humans
with damage to the hippocampus can also use a
familiarity based recognition system but do depend on
the hippocampus to recollect the supporting information.
This result suggests a modular organization of the MTH
system.
A Modular MTH System: Recognition Memory, Rodents
Rodents with
damage to the
hippocampus
recognize previously
experienced objects
but cannot
remember the
context in which an
object was
experienced. This
result suggests a
modular organization
of the MTH system.
What Happens When Memories Age?
What happens to episodic memories as they
age? Should they stay or should they go?
• In some cases the memory trace is likely lost.
• However, in some cases the trace might
endure or even strengthen.
• The outcome depends in part on the content.
• Much of what is initially stored is of no
significance and can be lost without any
consequence.
Ribot’s Law
Ribot’s Law: Ribot also proposed that old
memories are more resistant to
disease/disruption than new memories.
The MTH System and Ribot’s Law
Ribot’s Law suggests that as memories age they
become resistant to disruption (see Chapter 1). By
itself this claim is not surprising because (a) the
experience that produced the initial memory is
more likely to be repeated and (b) older memories,
compared to new memories, are more likely to
have been recalled a few times. Both of these
factors would increase the strength of the memory.
However, in the modern era, Ribot’s Law has been
nuanced to give an explicit role for the MTH
system in protecting old memories from disruption.
So, we will examine this position.
The Standard Model of Systems Consolidation
David Marr (center) was the
first neuroscientist to suggest
a role for the hippocampal
system.
However, Squire, Cohen, and
Nadel (1984) are primarily
responsible for the spread of
this idea.
Based primarily on the patient H.M, who was
originally thought to have a temporally limited
retrograde amnesia, they proposed what is
now called the standard model of systems
consolidation.
The Standard Model of Systems Consolidation
A schematic representation of the standard model of systems
consolidation. Initially the memory trace consists of weakly connected
neocortical representations of the features (purple circles) of the
experience held together by their temporary connections with the medial
temporal hippocampal (MTH) system. New memories require the MTH
system for retrieval. As the memory ages, intrinsic processes result in the
consolidation or strengthening of the connections among the neocortical
representations. Because of the strengthened connections the memory
can now be retrieved without the hippocampus.
Cellular and Systems Consolidation
Two types of
processes are
thought to
contribute to the
consolidation of
long-term stability
of memories.
Challenges to the Standard Model
These graphs illustrate patterns of results that would either support
or be evidence against the standard model of systems
consolidation. (A) This pattern would support the model because it
shows that damage to the hippocampus results in temporally
graded retrograde amnesia. (B) This pattern would be evidence
against the standard model because it shows that damage to the
hippocampus produces a flat retrograde amnesia.
Challenges to the Standard Model
After reviewing the post H.M. patient literature, Lynn
Nadel and Morris Moscovitch concluded that both old and
new episodic memory always depend on the
hippocampus. This is because patients with almost
complete damage to the hippocampus could not recall
either new or old episodic memories. Some evidence for
old episodic memories was found in patients with only
partial damage to the hippocampus.
Patient V.C
This figure illustrates V.C.’s flat retrograde amnesia for
recall of famous public events. This memory test was
conducted in 1998. Control subjects were chosen to match
V.C.’s age and educational level.
Patient H.M. Revisited
When Suzanne Corkin reexamined Henry
Molaison, she found that his old episodic
memories were not spare.
Multiple Trace Theory
The assumptions of the multiple trace theory of systems consolidation.
Old memories still depend on the hippocampus but are more resistant to
disruption because they have had more opportunity to be reactivated
than new memories, and each reactivation generates another index in
the hippocampus. Because these copies are distributed, the memory can
survive partial but not complete damage to the hippocampus and will be
more resistant to other insults such as a brain concussion.
Other Evidence: Human Brain Imaging
The figure on the right illustrates the predictions that the
standard model (SM) and multiple trace theory (MTT) of
systems consolidation make about activation in the medial
temporal hippocampal (MTH) system. Multiple trace theory
predicts that retrieval of both new and old memories should
activate the MTH system. The standard model predicts that
the retrieval of only new memories should activate the
system.
Interpreting Imaging Data: A Caveat
The human imaging data supports the multiple trace theory.
However, it does not rule out the standard model.
Suppose, as the standard model assumes, that an old memory is
retrieved directly from the neocortical sites. Once these sites are
activated they will project to the MTH system to cause activation
there. If this happens, then the activity in the MTH system will not
reflect retrieval of the memory through activating the existing index
but instead will reflect the retrieval experience laying down a new
copy of the trace—generating a new index.
Advantages of Animal Studies
Animal studies
• Provide animals with a known behavioral
experience
• Vary the exact time between the experience
and the occurrence of the brain damage
• Vary the extent of the brain damage
It is also hold constant the length of time
between when the brain is damaged and when
the animals are tested.
Contextual Fear Studies Provide No Support for the Systems
Consolidation View
(A) These images illustrate the extent of the damage to the
hippocampus. (Images courtesy of Robert Sutherland.) (B) These
data show that over the 6-month retention interval, control rats
showed evidence of forgetting. Note, however, that there was no
evidence that the 3-month and 6-month-old memories were
protected from damage to the hippocampus.
Optogenetic Control of Remote Memories
Control
Optogenetic inhibition
% Freezing
60
30
40
20
20
10
Light on
Light off
Cued Fear Memory
Light on
Old Contextual Fear Memory
Optogenetic inhibition of CA1 neurons has no effect on retrieval of
a cued fear memory, but blocks the retrieval of an old contextual
fear memory. Implication: under normal conditions, hippocampal
neurons are always involved in the retrieval of old contextual fear
memories.
Distributed Conditioning Sessions Spare New Memories from
Damage to the Hippocampus
% Freezing
Sham
Hippocampus
1 Session
3 sessions
In this contextual fear experiment, rats were
shocked 3 times. In the 1-session group the rats
were placed in the context and given all 3 shocks
in a single session. In the 3-session condition the
shocks were distributed over 3 sessions
separated by 24 hours. Twenty-four hours after
the trainings, the rats were assigned to either a
sham surgery control condition or surgery that
damaged the hippocampus. Several days later
they were tested for contextual fear. Note that
damage to the hippocampus did not impair
contextual fear in rats in the 3 session condition.
Implication
This finding reinforces an important point: the
hippocampus is required to rapidly form episodic
memories. However, other brain regions also can
capture representations of experience when the
experience is often repeated or recalled.
The Complementary Learning System View
This view assumes that different learning systems evolved to serve
different and sometimes incompatible functions. In this context,
appropriate behavioral adaptations require one memory system
that can rapidly acquire information about single episodes and
another system that gradually collects information about repeated
experiences to build representations of stable features of the
environment. The complementary memory systems framework
provides a natural way of understanding how memories can
become independent of the MTH.
The Age of the Memory: Summary
Thus, this animal literature is consistent with the human
literature, indicating that episodic memories always require
the hippocampus. The hippocampus is required to retrieve
new and old episodic memories. Nevertheless, neocortical
regions can support memories for experiences that might
also be captured by the hippocampal system.
Such cortical representations may be the result of repeated
experiences or repeated recall and can be retrieved without
a functioning hippocampus, regardless of their age.
This conclusion suggests that the age of a memory has
limited value in explaining the resistance of a memory trace
to disruption. Other variables such as repetition and
frequency of reactivation or recall of the memory, which are
more likely to be the case for old memories, are probably
the important variables.