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My
research addresses a broad range of cognitive neuroscience issues in learning,
memory, language and cognitive
development. I view cognitive functions as emerging from
the parallel, distributed
processing activity of neural populations, with learning occurring
through the adaptation of
connections among participating neurons. Research opportunities
in my lab revolve around
efforts to develop explicit computational models based on these
ideas; to test, refine,
and extend the principles embodied in the models; and then to apply
the models to substantive
research questions through behavioral experiment, computer
simulation, functional brain
imaging, and mathematical analysis.
One line of research in my
lab was inspired by the striking pattern of spared and impaired
memory in patients with
damage to the hippocampal region, suggesting that different parts
of the brain play different,
specialized roles in memory. With a neuroscience colleague and
a student, we suggested
that the hippocampus and neocortex may play complementary
roles in learning and memory.
The neocortex uses a very gradual learning procedure that
allows it to exploit the
structure in ensembles of inputs. The hippocampus is needed to
complement the neocortex,
providing a mechanism for rapid learning of the specific
arbitrary aspects of particular
items. The results of this rapid learning are gradually
integrated into the neocortical
system, accounting for the pattern of retrograde amnesia
seen in many amnesic patients.
We are now considering how the neocortex may learn
and represent semantic knowledge,
addressing children's acquisition of knowledge of living
things and the deterioration
of this knowledge in dementia. Developmental data suggest a
progressive differentiation
of concepts: We distinguish animals from plants before we
distinguish birds from fish,
or canaries from robins. In semantic dementia this trend
reverses, so that the finer
distinctions are lost before more general ones. These findings
coexist with the fact that
in some tasks, there appears to be a priority for accessing and
naming concepts at an intermediate
("basic") level of specificity (e.g., bird is more
accessible than animal or
robin). Current work with a student in the lab addresses all
these phenomena in a single
model, where they reflect the interplay of frequency and
concept similarity in determining
the semantic representations of concepts and the ease
with which distinct names
may be assigned to them.
Another project considers
why the neocortical learning system in adults, clearly capable
of learning in many cases,
nevertheless shows some important failures. We focus on the
failure of Japanese adults
to learn the distinction between /r/ and /l/ as an example. Some
approaches suggest this
failure reflects a simple switching off of learning about speech
sounds as a function of
age or puberty. We suggest that it may reflect, at least in part,
a characteristic of the
neural mechanism that underlies learning, which may be based on
Hebbian synaptic modification:
when one neuron participates in firing another, the
connection between them
is strengthened. This form of learning can tend to strengthen
a network's tendency to
keep doing what it is already doing. Thus, Japanese adults'
tendency to perceive /r/
and /l/ as the same may simply be strengthened each time they
hear either sound. A model
implemented by a student in my lab shows how these ideas,
in conjunction with some
other biologically motivated assumptions, can lead to failure to
learn new phonemic distinctions
in adulthood. The model suggests training methods that
may allow adults to learn
such distinctions. We are beginning to test the efficacy of these
methods, with initially
positive results.
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