20,894 research outputs found
Direct Feedback Alignment with Sparse Connections for Local Learning
Recent advances in deep neural networks (DNNs) owe their success to training
algorithms that use backpropagation and gradient-descent. Backpropagation,
while highly effective on von Neumann architectures, becomes inefficient when
scaling to large networks. Commonly referred to as the weight transport
problem, each neuron's dependence on the weights and errors located deeper in
the network require exhaustive data movement which presents a key problem in
enhancing the performance and energy-efficiency of machine-learning hardware.
In this work, we propose a bio-plausible alternative to backpropagation drawing
from advances in feedback alignment algorithms in which the error computation
at a single synapse reduces to the product of three scalar values. Using a
sparse feedback matrix, we show that a neuron needs only a fraction of the
information previously used by the feedback alignment algorithms. Consequently,
memory and compute can be partitioned and distributed whichever way produces
the most efficient forward pass so long as a single error can be delivered to
each neuron. Our results show orders of magnitude improvement in data movement
and improvement in multiply-and-accumulate operations over
backpropagation. Like previous work, we observe that any variant of feedback
alignment suffers significant losses in classification accuracy on deep
convolutional neural networks. By transferring trained convolutional layers and
training the fully connected layers using direct feedback alignment, we
demonstrate that direct feedback alignment can obtain results competitive with
backpropagation. Furthermore, we observe that using an extremely sparse
feedback matrix, rather than a dense one, results in a small accuracy drop
while yielding hardware advantages. All the code and results are available
under https://github.com/bcrafton/ssdfa.Comment: 15 pages, 8 figure
Direct Feedback Alignment With Sparse Connections for Local Learning
Recent advances in deep neural networks (DNNs) owe their success to training algorithms that use backpropagation and gradient-descent. Backpropagation, while highly effective on von Neumann architectures, becomes inefficient when scaling to large networks. Commonly referred to as the weight transport problem, each neuron's dependence on the weights and errors located deeper in the network require exhaustive data movement which presents a key problem in enhancing the performance and energy-efficiency of machine-learning hardware. In this work, we propose a bio-plausible alternative to backpropagation drawing from advances in feedback alignment algorithms in which the error computation at a single synapse reduces to the product of three scalar values. Using a sparse feedback matrix, we show that a neuron needs only a fraction of the information previously used by the feedback alignment algorithms. Consequently, memory and compute can be partitioned and distributed whichever way produces the most efficient forward pass so long as a single error can be delivered to each neuron. We evaluate our algorithm using standard datasets, including ImageNet, to address the concern of scaling to challenging problems. Our results show orders of magnitude improvement in data movement and 2× improvement in multiply-and-accumulate operations over backpropagation. Like previous work, we observe that any variant of feedback alignment suffers significant losses in classification accuracy on deep convolutional neural networks. By transferring trained convolutional layers and training the fully connected layers using direct feedback alignment, we demonstrate that direct feedback alignment can obtain results competitive with backpropagation. Furthermore, we observe that using an extremely sparse feedback matrix, rather than a dense one, results in a small accuracy drop while yielding hardware advantages. All the code and results are available under https://github.com/bcrafton/ssdfa
Biologically plausible deep learning -- but how far can we go with shallow networks?
Training deep neural networks with the error backpropagation algorithm is
considered implausible from a biological perspective. Numerous recent
publications suggest elaborate models for biologically plausible variants of
deep learning, typically defining success as reaching around 98% test accuracy
on the MNIST data set. Here, we investigate how far we can go on digit (MNIST)
and object (CIFAR10) classification with biologically plausible, local learning
rules in a network with one hidden layer and a single readout layer. The hidden
layer weights are either fixed (random or random Gabor filters) or trained with
unsupervised methods (PCA, ICA or Sparse Coding) that can be implemented by
local learning rules. The readout layer is trained with a supervised, local
learning rule. We first implement these models with rate neurons. This
comparison reveals, first, that unsupervised learning does not lead to better
performance than fixed random projections or Gabor filters for large hidden
layers. Second, networks with localized receptive fields perform significantly
better than networks with all-to-all connectivity and can reach backpropagation
performance on MNIST. We then implement two of the networks - fixed, localized,
random & random Gabor filters in the hidden layer - with spiking leaky
integrate-and-fire neurons and spike timing dependent plasticity to train the
readout layer. These spiking models achieve > 98.2% test accuracy on MNIST,
which is close to the performance of rate networks with one hidden layer
trained with backpropagation. The performance of our shallow network models is
comparable to most current biologically plausible models of deep learning.
Furthermore, our results with a shallow spiking network provide an important
reference and suggest the use of datasets other than MNIST for testing the
performance of future models of biologically plausible deep learning.Comment: 14 pages, 4 figure
Deep supervised learning using local errors
Error backpropagation is a highly effective mechanism for learning
high-quality hierarchical features in deep networks. Updating the features or
weights in one layer, however, requires waiting for the propagation of error
signals from higher layers. Learning using delayed and non-local errors makes
it hard to reconcile backpropagation with the learning mechanisms observed in
biological neural networks as it requires the neurons to maintain a memory of
the input long enough until the higher-layer errors arrive. In this paper, we
propose an alternative learning mechanism where errors are generated locally in
each layer using fixed, random auxiliary classifiers. Lower layers could thus
be trained independently of higher layers and training could either proceed
layer by layer, or simultaneously in all layers using local error information.
We address biological plausibility concerns such as weight symmetry
requirements and show that the proposed learning mechanism based on fixed,
broad, and random tuning of each neuron to the classification categories
outperforms the biologically-motivated feedback alignment learning technique on
the MNIST, CIFAR10, and SVHN datasets, approaching the performance of standard
backpropagation. Our approach highlights a potential biological mechanism for
the supervised, or task-dependent, learning of feature hierarchies. In
addition, we show that it is well suited for learning deep networks in custom
hardware where it can drastically reduce memory traffic and data communication
overheads
Forward Signal Propagation Learning
We propose a new learning algorithm for propagating a learning signal and
updating neural network parameters via a forward pass, as an alternative to
backpropagation. In forward signal propagation learning (sigprop), there is
only the forward path for learning and inference, so there are no additional
structural or computational constraints on learning, such as feedback
connectivity, weight transport, or a backward pass, which exist under
backpropagation. Sigprop enables global supervised learning with only a forward
path. This is ideal for parallel training of layers or modules. In biology,
this explains how neurons without feedback connections can still receive a
global learning signal. In hardware, this provides an approach for global
supervised learning without backward connectivity. Sigprop by design has better
compatibility with models of learning in the brain and in hardware than
backpropagation and alternative approaches to relaxing learning constraints. We
also demonstrate that sigprop is more efficient in time and memory than they
are. To further explain the behavior of sigprop, we provide evidence that
sigprop provides useful learning signals in context to backpropagation. To
further support relevance to biological and hardware learning, we use sigprop
to train continuous time neural networks with Hebbian updates and train spiking
neural networks without surrogate functions
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