3,876 research outputs found
Storage of phase-coded patterns via STDP in fully-connected and sparse network: a study of the network capacity
We study the storage and retrieval of phase-coded patterns as stable
dynamical attractors in recurrent neural networks, for both an analog and a
integrate-and-fire spiking model. The synaptic strength is determined by a
learning rule based on spike-time-dependent plasticity, with an asymmetric time
window depending on the relative timing between pre- and post-synaptic
activity. We store multiple patterns and study the network capacity.
For the analog model, we find that the network capacity scales linearly with
the network size, and that both capacity and the oscillation frequency of the
retrieval state depend on the asymmetry of the learning time window. In
addition to fully-connected networks, we study sparse networks, where each
neuron is connected only to a small number z << N of other neurons. Connections
can be short range, between neighboring neurons placed on a regular lattice, or
long range, between randomly chosen pairs of neurons. We find that a small
fraction of long range connections is able to amplify the capacity of the
network. This imply that a small-world-network topology is optimal, as a
compromise between the cost of long range connections and the capacity
increase.
Also in the spiking integrate and fire model the crucial result of storing
and retrieval of multiple phase-coded patterns is observed. The capacity of the
fully-connected spiking network is investigated, together with the relation
between oscillation frequency of retrieval state and window asymmetry
How feedback inhibition shapes spike-timing-dependent plasticity and its implications for recent Schizophrenia models
It has been shown that plasticity is not a fixed property but, in fact, changes depending on the location of the synapse on the neuron and/or changes of biophysical parameters. Here we investigate how plasticity is shaped by feedback inhibition in a cortical microcircuit. We use a differential Hebbian learning rule to model spike-timing dependent plasticity and show analytically that the feedback inhibition shortens the time window for LTD during spike-timing dependent plasticity but not for LTP. We then use a realistic GENESIS model to test two hypothesis about interneuron hypofunction and conclude that a reduction in GAD67 is the most likely candidate as the cause for hypofrontality as observed in Schizophrenia
Robust short-term memory without synaptic learning
Short-term memory in the brain cannot in general be explained the way
long-term memory can -- as a gradual modification of synaptic weights -- since
it takes place too quickly. Theories based on some form of cellular
bistability, however, do not seem able to account for the fact that noisy
neurons can collectively store information in a robust manner. We show how a
sufficiently clustered network of simple model neurons can be instantly induced
into metastable states capable of retaining information for a short time (a few
seconds). The mechanism is robust to different network topologies and kinds of
neural model. This could constitute a viable means available to the brain for
sensory and/or short-term memory with no need of synaptic learning. Relevant
phenomena described by neurobiology and psychology, such as local
synchronization of synaptic inputs and power-law statistics of forgetting
avalanches, emerge naturally from this mechanism, and we suggest possible
experiments to test its viability in more biological settings.Comment: 20 pages, 9 figures. Amended to include section on spiking neurons,
with general rewrit
- …