Fifty-nine years ago, Henry
Gustav Molaison—better known as H.M.—underwent a bilateral medial temporal lobe
(MTL) resection that included the hippocampus. Ever since, and thanks also to
several other studies in human and non-human animals, the hippocampus has been
tightly associated with memory. You loose your hippocampus, you loose your
capacity to consolidate new memories. End of the story. And if recent
meta-analyses revising the role of the hippocampus and adjacent regions during
episodic memory retrieval are correct (e.g., Nadel and Moscovitch, 1997),
extensive damage in the MTL may also result in complete incapacity to recollect
events from one’s past. You loose your MTL, you loose your capacity to remember
the past. End of the history. So it is not surprising that, for the last half a
century, the hippocampus was as closely tied to memory as Broca’s area was to
language production.
Perhaps for this reason the
recent observation that the MTL (especially the hippocampus) plays a critical
role in future simulation constitutes such a surprising discovery. In 1985,
Endel Tulving observed that patients with amnesia due to hippocampal damage had
trouble coming up with vivid simulations of possible personal future events
(Tulving, 1985). Is not that they were unable to give an answer to a question
like “can you tell me how your next Christmas is going to be?” It is rather
that their answers were formulaic, devoid of detail, kind of semantic: “I guess there will be a tree, and presents, and
maybe family”, and not what one would expect from an individual that can run,
as it were, a detailed episodic simulation of a possible future Christmas in
her mind: “I guess I’ll see aunt Annie with her loud voice, drinking her gin
and tonic, as always, and going about singing Christmas carols, because this
year we have new nieces, you know? And she’s going to be all over them…” No
such mental movie. When asked about specific details of their mental simulation
of future events, individuals with amnesia are unable to articulate detailed
descriptions of what they are imagining, limiting their answers to “that’s all
I see” or “nothing else comes to mind”. The observation that the hippocampus
plays a pivotal role in personal future thinking has been further corroborated
by numerous behavioral and neuroimaging studies.
However, in 2010, Larry Squire
and collaborators tested a handful of individuals with hippocampal damage on
their ability to construct mental simulations of possible future events.
Surprisingly, they found that their capacity to think abut possible personal
futures did not differ from controls. What gives? In response to this report,
Mcguire and Hassabis (2011) observed that the patients used by Squire and
colleagues had some remnant hippocampal tissue that may have been sufficient to
support the construction of future simulations. But this was merely a
conjecture. Empirical evidence was needed.
In a recent paper, Mullally,
Hassabis and Maguire (2012) report such evidence. As it turns out, one of the
patients (P01) studied by Hassabis and collaborators in 2007, despite having
extensive hippocampal damage, was nonetheless able to produce relatively
detailed descriptions of mental future simulations. Compared with the other
patients, though, P01 had some remnant hippocampal tissue. Could it be possible
that this difference really made such a difference? To answer this question,
Mullally et al (2012) asked P01 to visualize possible future scenes while
undergoing fMRI. The results were astonishing: the little bit of preserved
right hippocampal tissue was very much engaged during future scene
construction. In addition, all other regions engaged in episodic future
thinking overlapped with those that were engaged by the control group.
Moreover, the activity of said regions coupled with that of the hippocampus
when successful episodic future simulation was achieved. Again, you loose your
hippocampus, you loose your capacity to vividly think about your personal
future in a detailed manner. But if some hippocampal tissue is spared, maybe
you won’t.
What I really like about Mullally
et al’s (2012) study, besides the very interesting result, is the underlying
structure of the argument with which the data is put forth as evidence for the
hypothesis. Normally, in neuropsychology, it is assumed that the ultimate
source of evidence for a particular brain region being necessary for a certain
cognitive process is double dissociation. Initially introduced by Teuber in
1955, the notion of double dissociation was supposed to help elucidate whenever
two processes were orthogonal to one another, provided the researcher could
show that two experimental manipulations could differentially affect two
independent variables. In cognitive neuroscience, this principle is implemented
by way of showing that damage to brain region A impairs process X but not Y,
whereas damage to brain region B impairs process Y but not X. But double dissociation cannot show
sufficiency. Usually, claims about sufficiency are much harder to come by, and
when you do, they are usually questionable and pretty local. What I find so
impressive about the Mullally et al (2012) study is that they conjectured that
a specific brain region, in this case the very posterior tissue of the right
hippocampus, was sufficient for engaging the brain network required for future
simulation. Thus, unlike demonstrations of double dissociation, in which the
relevant brain area is shown to be necessary but not sufficient for a
particular cognitive process to occur, Mullally and collaborators (2012) managed
to demonstrate that a portion of a certain brain region, which is normally
functionally connected to other neural areas to support future thinking, is
sufficient to engage such a network. A highly recommend paper, which
orchestrates careful neuropsychological assessment and skillful neuroimaging
analysis.
Maguire, E. A., Hassabis, D.
(2011). Role of the hippocampus in imagination and future thinking. Proc
Natl Acad Sci U S A 108(11), E39- doi:10.1073/pnas.1018876108.
Mullally, S. L., Hassabis, D.,
Maguire, E. A. (2012). Scene Construction in Amnesia: An fMRI Study. J
Neurosci 32(16), 5646-5653
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