Causal Inference and Explaining Away in a Spiking Network.
Bottom Line:
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. Related in: MedlinePlus |
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Mentions: This difference in algorithms is also reflected in a difference in the details of the implementation. First, signal-tracking networks mostly require instantaneous inhibition to operate efficiently. This is because inhibition is in charge of immediately suppressing the firing of other neurons once a particular neuron that represents the stimulus is active, to avoid over-representing the stimulus. Our networks, in contrast, do not require instantaneous inhibition: exponential synaptic kernels, or even delta-kernels with a delay, can be safely added to the dynamics and the network still operates efficiently (see Figs 2 and 6). This is made possible by considering the steady-state solution instead of a greedy, dynamic loss minimizer. Second, signal-tracking networks usually operate with leaky integrate-and-fire cells. Neurons in cortex feature such a leak, such that leaky networks can be considered more realistic than pure non-leaky networks. As we have shown in Fig. 3, the effect of leak in our network approximately implements L1-norm regularization. However, in general, the presence of leak makes deriving the steady-state solution of the system intractable, such that the exact computations underlying this solution remain elusive. Another upside of our non-leaky network is the integration of information without any information loss31, which is crucial for optimal functioning. The importance of using non-leaky networks for optimal computations have also been recently recognized in updated versions of signal-tracking networks43 and in networks for stable representation of memories13. In this work43, the authors have relaxed the instantaneous inhibition requirement of signal-tracking networks by using alpha function synapses. This makes their optimal network parameters depend on the shape of the synaptic kernel, while our optimal solution does not have to obey such dependency. Third and finally, the way L1 regularization is implemented in the network dynamics differs in signal-tracking networks and our networks: while signal-tracking networks implement L1 through an increase of both the spiking threshold and reset voltage, our networks implement L1 through global inhibition. While such a simultaneous increase of threshold and reset voltages can be realized by global inhibition when the network is leaky, this mapping becomes impossible for non-leaky networks. Furthermore, in some network implementations of signal-tracking networks14, the parameters values for L1 and L2 regularization substantially differ from those of our network (Eqs. (19)-(20), ). In summary, focusing on the steady-state solution in non-leaky networks allowed us to solve the quadratic programming problem already considered in23 by spiking networks with significantly less constraints on synaptic kernels and a different implementation of L1 and L2-norm regularization. |
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Affiliation: Department of Technologies of Information and Communication, University Pompeu Fabra, 08018 Barcelona, Spain.
No MeSH data available.