SPATIAL MEMORY AND EXPERIENCE-DEPENDENT SYNAPTIC CHANGE

The potential functional significance of a newly discovered form of
synaptic plasticity observed in the mammalian hippocampal formation will
be examined specifically with regard to its possible direct or indirect
role in learning and memory operations. This phenomenon, which we call
short-term exploratory modulation (STEM), results from behaviors that
can roughly be categorized as exploratory in nature, but is independent
of the motor components of this behavior. The primary aim of the
proposal is to determine whether the STEM mechanism is one that could
plausibly be used for high capacity information storage, or whether it
represents a form of global system modulation, perhaps important for the
storage process, but not directly involved in it. The proposed approach
to these questions include: a determination of the extent to which STEM
is related to the relatively-well-studied but artificial form of
synaptic enhancement (LTE/LTP) that can be induced by convergent high-
frequency activity on hippocampal excitatory pathways; an assessment of
the role of known heterosynaptic modulation systems in the hippocampus;
the correlation of STEM with actual learning; and an investigation of
the information content of the STEM phenomenon. Specifically, five
different lines of experimental questions will be addressed: 1) Is STEM found only in the temporal lobe afferents to the fascia
dentata, or do other extrinsic or intrinsic hippocampal synapses show
this form of spontaneous plasticity? 2) Do STEM and LTE occlude, is STEM regulated by NMDA receptor
dependent mechanism, and does STEM require spontaneous activity on the
perforant path fibers on which it is expressed? 3) Do heterosynaptic transmitter systems cause STEM or contribute
permissively in some way? 4) Is STEM related to explicit or incidental spatial learning? Are
STEM and learning ability positively correlated? 5) Does STEM reflect information-specific changes in different synaptic
subsets? It is expected that these studies will further our understanding of the
types of plasticity that occur in the mammalian hippocampus, their
relation to learning and memory operations, and their potential
contribution to normal cognition.