University of Edinburgh. College of Science and Engineering. School of Informatics.
Abstract
Institute for Adaptive and Neural ComputationCA1 is the main source of afferents from the hippocampus, but the function of
CA1 and its perforant path (PP) input remains unclear. In this thesis, Marr’s model
of the hippocampus is used to investigate previously hypothesized functions, and also
to investigate some of Marr’s unexplored theoretical ideas. The last part of the thesis
explains the excitatory responses to PP activity in vivo, despite inhibitory responses in
vitro.
Quantitative support for the idea of CA1 as a relay of information from CA3 to the
neocortex and subiculum is provided by constraining Marr’s model to experimental
data. Using the same approach, the much smaller capacity of the PP input by comparison
implies it is not a one-shot learning network. In turn, it is argued that the
entorhinal-CA1 connections cannot operate as a short-term memory network through
reverberating activity.
The PP input to CA1 has been hypothesized to control the activity of CA1 pyramidal
cells. Marr suggested an algorithm for self-organising the output activity during
pattern storage. Analytic calculations show a greater capacity for self-organised patterns
than random patterns for low connectivities and high loads, confirmed in simulations
over a broader parameter range. This superior performance is maintained in the
absence of complex thresholding mechanisms, normally required to maintain performance
levels in the sparsely connected networks. These results provide computational
motivation for CA3 to establish patterns of CA1 activity without involvement from the
PP input.
The recent report of CA1 place cell activity with CA3 lesioned (Brun et al., 2002.
Science, 296(5576):2243-6) is investigated using an integrate-and-fire neuron model
of the entorhinal-CA1 network. CA1 place field activity is learnt, despite a completely
inhibitory response to the stimulation of entorhinal afferents. In the model, this is
achieved using N-methyl-D-asparate receptors to mediate a significant proportion of
the excitatory response. Place field learning occurs over a broad parameter space. It is
proposed that differences between similar contexts are slowly learnt in the PP and as a
result are amplified in CA1. This would provide improved spatial memory in similar
but different contexts