Causal Inference and Explaining Away in a Spiking Network.
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Here we demonstrate that a family of high-dimensional quadratic optimization problems with non-negativity constraints can be solved exactly and efficiently by a network of spiking neurons.The network naturally imposes the non-negativity of causal contributions that is fundamental to causal inference, and uses simple operations, such as linear synapses with realistic time constants, and neural spike generation and reset non-linearities.This type of network might underlie tasks such as odor identification and classification.
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Affiliation: Department of Technologies of Information and Communication, University Pompeu Fabra, 08018 Barcelona, Spain.
ABSTRACT
While the brain uses spiking neurons for communication, theoretical research on brain computations has mostly focused on non-spiking networks. The nature of spike-based algorithms that achieve complex computations, such as object probabilistic inference, is largely unknown. Here we demonstrate that a family of high-dimensional quadratic optimization problems with non-negativity constraints can be solved exactly and efficiently by a network of spiking neurons. The network naturally imposes the non-negativity of causal contributions that is fundamental to causal inference, and uses simple operations, such as linear synapses with realistic time constants, and neural spike generation and reset non-linearities. The network infers the set of most likely causes from an observation using explaining away, which is dynamically implemented by spike-based, tuned inhibition. The algorithm performs remarkably well even when the network intrinsically generates variable spike trains, the timing of spikes is scrambled by external sources of noise, or the network is mistuned. This type of network might underlie tasks such as odor identification and classification. No MeSH data available. |
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Mentions: We first address the question of whether intrinsically generated spiking variability harms the performance of our networks. To generate spiking variability intrinsically by neuronal dynamics, we created a spiking network where the dimensionality of the stimulus was much lower than the number of neurons, M ≪ N. Because the N × N connectivity matrix J is a low rank matrix with rank M ≪ N, the neuronal dynamics offers a highly overcomplete representation of the input space and becomes a multi-dimensional attractor35. Without any regularization of the dynamics and in absence of noise, the corresponding rate-based network converges to a point on this multi-dimensional manifold attractor, determined by the initial conditions. The spike-based implementation can be interpreted as a noisy version of the rate-based network, such that the spiking network traverses the attracting manifold in a quasi-random walk, despite not having any truly stochastic component in its dynamics. In this scenario, which is specific to the overcomplete representation of inputs, the same stimulus can be faithfully represented by potentially many different activity patterns consisting of different sets of neurons being active and representing different combination of causes13. This representation can evolve over time, and the observed complex dynamics can be interpreted as variability. Our simulations show that, for each neuron, firing is very irregular (Fig. 4a), with a broad distribution of high inter-spike-intervals (ISI) (Fig. 4b). The population averaged coefficient of variation of the inter-spike-intervals (CVISI) was CVISI = 3.20, larger than the one typically observed in sensory cortex3334, but consistent with the larger variability found in prefrontal areas36. The presence of variability was robust to changes in the synaptic kernels used. When instead of using exponential kernels we used delta-function kernels with no delay or with 2 ms delay, the network generated high variability with population averaged CVISI = 3.48 and CVISI = 2.89, respectively. The variability observed in larger networks of up to N = 500 cells (CVISI = 2.98) was also comparable to the variability observed in smaller networks of N = 100 cells (CVISI = 3.48). Despite the sheer irregular activity in the network, the encoding of the stimulus is fairly stable over time (Fig. 4c). A relatively stable decoding error of around 1deg is attained (Fig. 4d, blue line). Therefore, the spiking network is able to represent a complex input pattern in a reliable way over time in spite of intrinsically generated spiking variability. |
View Article: PubMed Central - PubMed
Affiliation: Department of Technologies of Information and Communication, University Pompeu Fabra, 08018 Barcelona, Spain.
No MeSH data available.