6 research outputs found
Optimal learning rules for discrete synapses
There is evidence that biological synapses have a limited number of discrete weight states. Memory storage with such synapses behaves quite differently from synapses with unbounded, continuous weights, as old memories are automatically overwritten by new memories. Consequently, there has been substantial discussion about how this affects learning and storage capacity. In this paper, we calculate the storage capacity of discrete, bounded synapses in terms of Shannon information. We use this to optimize the learning rules and investigate how the maximum information capacity depends on the number of synapses, the number of synaptic states, and the coding sparseness. Below a certain critical number of synapses per neuron (comparable to numbers found in biology), we find that storage is similar to unbounded, continuous synapses. Hence, discrete synapses do not necessarily have lower storage capacity
Soft-bound synaptic plasticity increases storage capacity
Accurate models of synaptic plasticity are essential to understand the adaptive properties of the nervous system and for realistic models of learning and memory. Experiments have shown that synaptic plasticity depends not only on pre- and post-synaptic activity patterns, but also on the strength of the connection itself. Namely, weaker synapses are more easily strengthened than already strong ones. This so called soft-bound plasticity automatically constrains the synaptic strengths. It is known that this has important consequences for the dynamics of plasticity and the synaptic weight distribution, but its impact on information storage is unknown. In this modeling study we introduce an information theoretic framework to analyse memory storage in an online learning setting. We show that soft-bound plasticity increases a variety of performance criteria by about 18% over hard-bound plasticity, and likely maximizes the storage capacity of synapses
Binary Associative Memories as a Benchmark for Spiking Neuromorphic Hardware
Stöckel A, Jenzen C, Thies M, Rückert U. Binary Associative Memories as a Benchmark for Spiking Neuromorphic Hardware. Frontiers in Computational Neuroscience. 2017;11: 71.Large-scale neuromorphic hardware platforms, specialized computer systems for energy efficient simulation of spiking neural networks, are being developed around the world, for example as part of the European Human Brain Project (HBP). Due to conceptual differences, a universal performance analysis of these systems in terms of runtime, accuracy and energy efficiency is non-trivial, yet indispensable for further hard- and software development. In this paper we describe a scalable benchmark based on a spiking neural network implementation of the binary neural associative memory. We treat neuromorphic hardware and software simulators as black-boxes and execute exactly the same network description across all devices. Experiments on the HBP platforms under varying configurations of the associative memory show that the presented method allows to test the quality of the neuron model implementation, and to explain significant deviations from the expected reference output
Memory stability and synaptic plasticity
Numerous experiments have demonstrated that the activity of neurons can alter the
strength of excitatory synapses. This synaptic plasticity is bidirectional and synapses
can be strengthened (potentiation) or weakened (depression). Synaptic plasticity offers
a mechanism that links the ongoing activity of the brain with persistent physical
changes to its structure. For this reason it is widely believed that synaptic plasticity
mediates learning and memory.
The hypothesis that synapses store memories by modifying their strengths raises
an important issue. There should be a balance between the necessity that synapses
change frequently, allowing new memories to be stored with high fidelity, and the
necessity that synapses retain previously stored information. This is the plasticity stability
dilemma. In this thesis the plasticity stability dilemma is studied in the context
of the two dominant paradigms of activity dependent synaptic plasticity: Spike timing
dependent plasticity (STDP) and long term potentiation and depression (LTP/D).
Models of biological synapses are analysed and processes that might ameliorate the
plasticity stability dilemma are identified.
Two popular existing models of STDP are compared. Through this comparison it is
demonstrated that the synaptic weight dynamics of STDP has a large impact upon the
retention time of correlation between the weights of a single neuron and a memory. In
networks it is shown that lateral inhibition stabilises the synaptic weights and receptive
fields.
To analyse LTP a novel model of LTP/D is proposed. The model centres on
the distinction between early LTP/D, when synaptic modifications are persistent on
a short timescale, and late LTP/D when synaptic modifications are persistent on a long
timescale. In the context of the hippocampus it is proposed that early LTP/D allows the
rapid and continuous storage of short lasting memory traces over a long lasting trace
established with late LTP/D. It is shown that this might confer a longer memory retention
time than in a system with only one phase of LTP/D. Experimental predictions
about the dynamics of amnesia based upon this model are proposed.
Synaptic tagging is a phenomenon whereby early LTP can be converted into late
LTP, by subsequent induction of late LTP in a separate but nearby input. Synaptic
tagging is incorporated into the LTP/D framework. Using this model it is demonstrated
that synaptic tagging could lead to the conversion of a short lasting memory trace into
a longer lasting trace. It is proposed that this allows the rescue of memory traces that
were initially destined for complete decay. When combined with early and late LTP/D
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synaptic tagging might allow the management of hippocampal memory traces, such
that not all memories must be stored on the longest, most stable late phase timescale.
This lessens the plasticity stability dilemma in the hippocampus, where it has been
hypothesised that memory traces must be frequently and vividly formed, but that not
all traces demand eventual consolidation at the systems level