71 research outputs found
Deterministic networks for probabilistic computing
Neural-network models of high-level brain functions such as memory recall and
reasoning often rely on the presence of stochasticity. The majority of these
models assumes that each neuron in the functional network is equipped with its
own private source of randomness, often in the form of uncorrelated external
noise. However, both in vivo and in silico, the number of noise sources is
limited due to space and bandwidth constraints. Hence, neurons in large
networks usually need to share noise sources. Here, we show that the resulting
shared-noise correlations can significantly impair the performance of
stochastic network models. We demonstrate that this problem can be overcome by
using deterministic recurrent neural networks as sources of uncorrelated noise,
exploiting the decorrelating effect of inhibitory feedback. Consequently, even
a single recurrent network of a few hundred neurons can serve as a natural
noise source for large ensembles of functional networks, each comprising
thousands of units. We successfully apply the proposed framework to a diverse
set of binary-unit networks with different dimensionalities and entropies, as
well as to a network reproducing handwritten digits with distinct predefined
frequencies. Finally, we show that the same design transfers to functional
networks of spiking neurons.Comment: 22 pages, 11 figure
Spiking neurons with short-term synaptic plasticity form superior generative networks
Spiking networks that perform probabilistic inference have been proposed both
as models of cortical computation and as candidates for solving problems in
machine learning. However, the evidence for spike-based computation being in
any way superior to non-spiking alternatives remains scarce. We propose that
short-term plasticity can provide spiking networks with distinct computational
advantages compared to their classical counterparts. In this work, we use
networks of leaky integrate-and-fire neurons that are trained to perform both
discriminative and generative tasks in their forward and backward information
processing paths, respectively. During training, the energy landscape
associated with their dynamics becomes highly diverse, with deep attractor
basins separated by high barriers. Classical algorithms solve this problem by
employing various tempering techniques, which are both computationally
demanding and require global state updates. We demonstrate how similar results
can be achieved in spiking networks endowed with local short-term synaptic
plasticity. Additionally, we discuss how these networks can even outperform
tempering-based approaches when the training data is imbalanced. We thereby
show how biologically inspired, local, spike-triggered synaptic dynamics based
simply on a limited pool of synaptic resources can allow spiking networks to
outperform their non-spiking relatives.Comment: corrected typo in abstrac
Stochasticity from function -- why the Bayesian brain may need no noise
An increasing body of evidence suggests that the trial-to-trial variability
of spiking activity in the brain is not mere noise, but rather the reflection
of a sampling-based encoding scheme for probabilistic computing. Since the
precise statistical properties of neural activity are important in this
context, many models assume an ad-hoc source of well-behaved, explicit noise,
either on the input or on the output side of single neuron dynamics, most often
assuming an independent Poisson process in either case. However, these
assumptions are somewhat problematic: neighboring neurons tend to share
receptive fields, rendering both their input and their output correlated; at
the same time, neurons are known to behave largely deterministically, as a
function of their membrane potential and conductance. We suggest that spiking
neural networks may, in fact, have no need for noise to perform sampling-based
Bayesian inference. We study analytically the effect of auto- and
cross-correlations in functionally Bayesian spiking networks and demonstrate
how their effect translates to synaptic interaction strengths, rendering them
controllable through synaptic plasticity. This allows even small ensembles of
interconnected deterministic spiking networks to simultaneously and
co-dependently shape their output activity through learning, enabling them to
perform complex Bayesian computation without any need for noise, which we
demonstrate in silico, both in classical simulation and in neuromorphic
emulation. These results close a gap between the abstract models and the
biology of functionally Bayesian spiking networks, effectively reducing the
architectural constraints imposed on physical neural substrates required to
perform probabilistic computing, be they biological or artificial
Characterization and Compensation of Network-Level Anomalies in Mixed-Signal Neuromorphic Modeling Platforms
Advancing the size and complexity of neural network models leads to an ever
increasing demand for computational resources for their simulation.
Neuromorphic devices offer a number of advantages over conventional computing
architectures, such as high emulation speed or low power consumption, but this
usually comes at the price of reduced configurability and precision. In this
article, we investigate the consequences of several such factors that are
common to neuromorphic devices, more specifically limited hardware resources,
limited parameter configurability and parameter variations. Our final aim is to
provide an array of methods for coping with such inevitable distortion
mechanisms. As a platform for testing our proposed strategies, we use an
executable system specification (ESS) of the BrainScaleS neuromorphic system,
which has been designed as a universal emulation back-end for neuroscientific
modeling. We address the most essential limitations of this device in detail
and study their effects on three prototypical benchmark network models within a
well-defined, systematic workflow. For each network model, we start by defining
quantifiable functionality measures by which we then assess the effects of
typical hardware-specific distortion mechanisms, both in idealized software
simulations and on the ESS. For those effects that cause unacceptable
deviations from the original network dynamics, we suggest generic compensation
mechanisms and demonstrate their effectiveness. Both the suggested workflow and
the investigated compensation mechanisms are largely back-end independent and
do not require additional hardware configurability beyond the one required to
emulate the benchmark networks in the first place. We hereby provide a generic
methodological environment for configurable neuromorphic devices that are
targeted at emulating large-scale, functional neural networks
Demonstrating Analog Inference on the BrainScaleS-2 Mobile System
We present the BrainScaleS-2 mobile system as a compact analog inference
engine based on the BrainScaleS-2 ASIC and demonstrate its capabilities at
classifying a medical electrocardiogram dataset. The analog network core of the
ASIC is utilized to perform the multiply-accumulate operations of a
convolutional deep neural network. At a system power consumption of 5.6W, we
measure a total energy consumption of 192uJ for the ASIC and achieve a
classification time of 276us per electrocardiographic patient sample. Patients
with atrial fibrillation are correctly identified with a detection rate of
(93.70.7)% at (14.01.0)% false positives. The system is directly
applicable to edge inference applications due to its small size, power
envelope, and flexible I/O capabilities. It has enabled the BrainScaleS-2 ASIC
to be operated reliably outside a specialized lab setting. In future
applications, the system allows for a combination of conventional machine
learning layers with online learning in spiking neural networks on a single
neuromorphic platform
Cortical oscillations implement a backbone for sampling-based computation in spiking neural networks
Brains need to deal with an uncertain world. Often, this requires visiting
multiple interpretations of the available information or multiple solutions to
an encountered problem. This gives rise to the so-called mixing problem: since
all of these "valid" states represent powerful attractors, but between
themselves can be very dissimilar, switching between such states can be
difficult. We propose that cortical oscillations can be effectively used to
overcome this challenge. By acting as an effective temperature, background
spiking activity modulates exploration. Rhythmic changes induced by cortical
oscillations can then be interpreted as a form of simulated tempering. We
provide a rigorous mathematical discussion of this link and study some of its
phenomenological implications in computer simulations. This identifies a new
computational role of cortical oscillations and connects them to various
phenomena in the brain, such as sampling-based probabilistic inference, memory
replay, multisensory cue combination and place cell flickering.Comment: 30 pages, 11 figure
Accelerated physical emulation of Bayesian inference in spiking neural networks
The massively parallel nature of biological information processing plays an
important role for its superiority to human-engineered computing devices. In
particular, it may hold the key to overcoming the von Neumann bottleneck that
limits contemporary computer architectures. Physical-model neuromorphic devices
seek to replicate not only this inherent parallelism, but also aspects of its
microscopic dynamics in analog circuits emulating neurons and synapses.
However, these machines require network models that are not only adept at
solving particular tasks, but that can also cope with the inherent
imperfections of analog substrates. We present a spiking network model that
performs Bayesian inference through sampling on the BrainScaleS neuromorphic
platform, where we use it for generative and discriminative computations on
visual data. By illustrating its functionality on this platform, we implicitly
demonstrate its robustness to various substrate-specific distortive effects, as
well as its accelerated capability for computation. These results showcase the
advantages of brain-inspired physical computation and provide important
building blocks for large-scale neuromorphic applications.Comment: This preprint has been published 2019 November 14. Please cite as:
Kungl A. F. et al. (2019) Accelerated Physical Emulation of Bayesian
Inference in Spiking Neural Networks. Front. Neurosci. 13:1201. doi:
10.3389/fnins.2019.0120
Fast and deep: energy-efficient neuromorphic learning with first-spike times
For a biological agent operating under environmental pressure, energy
consumption and reaction times are of critical importance. Similarly,
engineered systems also strive for short time-to-solution and low
energy-to-solution characteristics. At the level of neuronal implementation,
this implies achieving the desired results with as few and as early spikes as
possible. In the time-to-first-spike-coding framework, both of these goals are
inherently emerging features of learning. Here, we describe a rigorous
derivation of learning such first-spike times in networks of leaky
integrate-and-fire neurons, relying solely on input and output spike times, and
show how it can implement error backpropagation in hierarchical spiking
networks. Furthermore, we emulate our framework on the BrainScaleS-2
neuromorphic system and demonstrate its capability of harnessing the chip's
speed and energy characteristics. Finally, we examine how our approach
generalizes to other neuromorphic platforms by studying how its performance is
affected by typical distortive effects induced by neuromorphic substrates.Comment: 20 pages, 8 figure
Inference with Artificial Neural Networks on Analog Neuromorphic Hardware
The neuromorphic BrainScaleS-2 ASIC comprises mixed-signal neurons and
synapse circuits as well as two versatile digital microprocessors. Primarily
designed to emulate spiking neural networks, the system can also operate in a
vector-matrix multiplication and accumulation mode for artificial neural
networks. Analog multiplication is carried out in the synapse circuits, while
the results are accumulated on the neurons' membrane capacitors. Designed as an
analog, in-memory computing device, it promises high energy efficiency.
Fixed-pattern noise and trial-to-trial variations, however, require the
implemented networks to cope with a certain level of perturbations. Further
limitations are imposed by the digital resolution of the input values (5 bit),
matrix weights (6 bit) and resulting neuron activations (8 bit). In this paper,
we discuss BrainScaleS-2 as an analog inference accelerator and present
calibration as well as optimization strategies, highlighting the advantages of
training with hardware in the loop. Among other benchmarks, we classify the
MNIST handwritten digits dataset using a two-dimensional convolution and two
dense layers. We reach 98.0% test accuracy, closely matching the performance of
the same network evaluated in software
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