On-the-Fly Learning in a Perpetual Learning
Machine
Andrew J.R. Simpson
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Centre for Vision, Speech and Signal Processing, University of Surrey,UK
1
Andrew.Simpson@Surrey.ac.uk
Abstract—Despite the promise of brain-inspired machine
learning, deep neural networks (DNN) have frustratingly failed
to bridge the deceptively large gap between learning and memory.
Here, we introduce a Perpetual Learning Machine; a new type of
DNN that is capable of brain-like dynamic ‘on the fly’ learning
because it exists in a self-supervised state of Perpetual Stochastic
Gradient Descent. Thus, we provide the means to unify learning
and memory within a machine learning framework. We also
explore the elegant duality of abstraction and synthesis: the Yin
and Yang of deep learning.
Index terms—Perpetual Learning Machine, Perpetual
Stochastic Gradient Descent, self-supervised learning, parallel
dither, Yin and Yang.
I. I
NTRODUCTION
It is an embarassing fact that while deep neural networks
(DNN) are frequently compared to the brain, and even their
performance found to be similar in specific static tasks, there
remains a critical difference; DNN do not exhibit the fluid and
dynamic learning of the brain but are static once trained. For
example, to add a new class of data to a trained DNN it is
necessary to add the respective new training data to the pre-
existing training data and re-train (probably from scratch) to
account for the new class. By contrast, learning is essentially
additive in the brain – if we want to learn a new thing, we do.
Thus, whilst there is little doubt that the learning of the
brain and machine learning are essentially the same, the
learning of the brain involves the emergent phenomenon of
memory which has failed to emerge from machine learning.
Indeed, recent machine-inspired approaches to ‘memory’ have
involved explicit add-on storage facilities [e.g., 1] which
explicitly discriminate between learning (training – i.e., of
weights) and memory (storage – i.e., of data). Thus, the
problem has been brushed under the carpet.
In this article, we describe a novel form of supervised
learning model, which we call a Perpetual Learning Machine,
which gives rise to the basic properties of memory. Our model
involves two DNNs, one for storage and the other for recall.
The storage DNN learns the classes of some training images.
The recall DNN learns to synthesise the same images from the
same classes. Together, the two networks hold, encoded, the
training set. We then place these pair of DNNs in a self-
supervised and homeostatic state of Perpetual Stochastic
Gradient Descent (PSGD). During each step of PSGD, a
random class is chosen and an image synthesised from the
recall DNN. This randomly synthesised image is then used in
combination with the random class to train both DNNs via
non-batch SGD. I.e., the PSGD is driven by training data that
is synthesised from memory according to random classes. We
next demonstrate that new classes may be learned on the fly
by introducing them, via ‘new experience’ SGD steps, into the
path of PSGD. Over time, new classes are assimilated without
disruption of earlier learning and hence we demonstrate a
machine which both learns and remembers.
Fig. 1. Recall of training images. On the left are plotted MNIST
digits and on the right are plotted the same digits synthesised using
the recall DNN.
II. M
ETHOD
We chose the well-known MNIST hand-written digit
dataset [2]. First, we unpacked the images of 28x28 pixels into
vectors of length 784. Example digits are given in Fig. 1. Pixel
intensities were normalized to zero mean.
Perpetual Memory. In order to test the idea of perpetual
memory, through perpetual learning, we required our model to
learn to identify a collection of images. We took the first 75 of
the MNIST digits and assigned each to an arbitrary class (this
is arbitrary associative learning). This gave 75 unique classes,
each associated with a single, specific digit. The task of the
model was to recognise the images and assign to them the
correct (arbitrary) classes. We split the 75 digits into 50 ‘learn
during training’ examples and 25 ‘learn later on the fly’ test
examples. The first 50 training examples were learned with
typical SGD and discarded. Hence, they were not available for
later use during assimilation of additional classes. The latter
25 examples were held back for insertion during PSGD.