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Title: Critical Learning Periods in Deep Networks

Paper content:
Similar to humans and animals, deep artificial neural networks exhibit critical periods during which a temporary stimulus deficit can impair the development of a skill.
The extent of the impairment depends on the onset and length of the deficit window, as in animal models, and on the size of the neural network.
Deficits that do not affect low-level statistics, such as vertical flipping of the images, have no lasting effect on performance and can be overcome with further training.
To better understand this phenomenon, we use the Fisher Information of the weights to measure the effective connectivity between layers of a network during training.
Counterintuitively, information rises rapidly in the early phases of training, and then decreases, preventing redistribution of information resources in a phenomenon we refer to as a loss of "Information Plasticity".
Our analysis suggests that the first few epochs are critical for the creation of strong connections that are optimal relative to the input data distribution.
Once such strong connections are created, they do not appear to change during additional training.
These findings suggest that the initial learning transient, under-scrutinized compared to asymptotic behavior, plays a key role in determining the outcome of the training process.
Our findings, combined with recent theoretical results in the literature, also suggest that forgetting (decrease of information in the weights) is critical to achieving invariance and disentanglement in representation learning.
Finally, critical periods are not restricted to biological systems, but can emerge naturally in learning systems, whether biological or artificial, due to fundamental constrains arising from learning dynamics and information processing.
Critical periods are time windows of early post-natal development during which sensory deficits can lead to permanent skill impairment BID12 .
Researchers have documented critical periods affecting a range of species and systems, from visual acuity in kittens BID35 BID33 to song learning in birds BID18 .
Uncorrected eye defects (e.g., strabismus, cataracts) during the critical period for visual development lead to amblyopia in one in fifty adults.The cause of critical periods is ascribed to the biochemical modulation of windows of neuronal plasticity BID10 .
In this paper, however, we show that deep neural networks (DNNs), while completely devoid of such regulations, respond to sensory deficits in ways similar to those observed in humans and animal models.
This surprising result suggests that critical periods may arise from information processing, rather than biochemical, phenomena.We propose using the information in the weights, measured by an efficient approximation of the Fisher Information, to study critical period phenomena in DNNs.
We show that, counterintuitively, the information in the weights does not increase monotonically during training.
Instead, a rapid growth in information ("memorization phase") is followed by a reduction of information ("reorganization" or "forgetting" phase), even as classification performance keeps increasing.
This behavior is consistent across different tasks and network architectures.
Critical periods are centered in the memorization phase.Under review as a conference paper at ICLR 2019 Performance is permanently impaired if the deficit is not corrected early enough, regardless of how much additional training is performed.
As in animal models, critical periods coincide with the early learning phase during which test accuracy would rapidly increase in the absence of deficits (dashed).
(B) For comparison, we report acuity for kittens monocularly deprived since birth and tested at the time of eye-opening (solid), and normal development visual acuity in kittens as a function of age (dashed) BID7 BID23 .artificial
neural networks (ANNs) are only loosely inspired by biological systems (Hassabis et al., 2017) .Most studies
to date have focused either on the behavior of networks at convergence (Representation Learning) or on the asymptotic properties of the numerical scheme used to get there (Optimization). The role of
the initial transient, especially its effect in biasing the network towards "good" regions of the complex and high-dimensional optimization problem, is rarely addressed. To study this
initial learning phase of ANNs, we replicate experiments performed in animal models and find that the responses to early deficits are remarkably similar, despite the large underlying differences between the two systems. In particular
, we show that the quality of the solution depends only minimally on the final, relatively well-understood, phase of the training process or on its very first epochs; instead, it depends critically on the period prior to initial convergence.In animals, sensory deficits introduced during critical periods induce changes in the architecture of the corresponding areas BID4 BID34 BID9 . To determine
whether a similar phenomenon exists in ANNs, we compute the Fisher Information of the weights of the network as a proxy to measure its "effective connectivity", that is, the density of connections that are effectively used by the network in order to solve the task. Like others
before us BID28 , we observe two distinct phases during the training, first a "learning phase" in which the Fisher Information of the weights increases as the network learns from the data, followed by a "consolidation" or "compression" phase in which the Fisher Information decreases and stabilizes. Sensitivity
to critical-period-inducing deficits is maximal exactly when the Fisher Information peaks.A layer-wise analysis of the network's effective connectivity shows that, in the tasks and deficits we consider, the hierarchy of low-level and high-level features in the training data is a key aspect behind the observed phenomena. In particular
, our experiments suggest that the existence of critical periods in deep neural networks depends on the inability of the network to change its effective connectivity pattern in order to process different information (in response to deficit removal). We call this
phenomenon, which is not mediated by any external factors, a loss of the "Information Plasticity" of the network.
Critical periods have thus far been considered an exclusively biological phenomenon.
At the same time, the analysis of DNNs has focused on asymptotic properties and neglected the initial transient behavior.
To the best of our knowledge, we are the first to show that artificial neural networks exhibit critical period phenomena, and to highlight the critical role of the transient in determining the asymptotic performance of the network.
Inspired by the role of synaptic connectivity in modulating critical periods, we introduce the use of Fisher Information to study this initial phase.
We show that the initial sensitivity to deficits closely follows changes in the FIM, both global, as the network first rapidly increases and then decreases the amount of stored information, and layer-wise, as the network "reorganizes" its effective connectivity in order to optimally process information.Our work naturally relates to the extensive literature on critical periods in biology.
Despite artificial networks being an extremely reductionist approximation of neuronal networks, they exhibit behaviors that are qualitatively similar to the critical periods observed in human and animal models.
Our information analysis shows that the initial rapid memorization phase is followed by a loss of Information Plasticity which, counterintuitively, further improves the performance.
On the other hand, when combined with the analysis of BID0 this suggests that a "forgetting" phase may be desirable, or even necessary, in order to learn robust, nuisance-invariant representations.The existence of two distinct phases of training has been observed and discussed by BID28 , although their analysis builds on the (Shannon) information of the activations, rather than the (Fisher) information in the weights.
On a multi-layer perceptron (MLP), BID28 empirically link the two phases to a sudden increase in the gradients' covariance.
It may be tempting to compare these results with our Fisher Information analysis.
However, it must be noted that the FIM is computed using the gradients with respect to the model prediction, not to the ground truth label, leading to important qualitative differences.
In Figure 6 , we show that the covariance and norm of the gradients exhibit no clear trends during training with and without deficits, and, therefore, unlike the FIM, do not correlate with the sensitivity to critical periods.
However, Published as a conference paper at ICLR 2019 a connection between our FIM analysis and the information in the activations can be established based on the work of BID0 , which shows that the FIM of the weights can be used to bound the information in the activations.
In fact, we may intuitively expect that pruning of connections naturally leads to loss of information in the corresponding activations.
Thus, our analysis corroborates and expands on some of the claims of BID28 , while using an independent framework.Aside from being more closely related to the deficit sensitivity during critical periods, Fisher's Information also has a number of technical advantages: Its diagonal is simple to estimate, even on modern state-of-the-art architectures and compelling datasets, and it is less sensitive to the choice estimator of mutual information, avoiding some of the common criticisms to the use of information quantities in the analysis of deep learning models.
Finally, the FIM allows us to probe fine changes in the effective connectivity across the layers of the network FIG5 ), which are not visible in BID28 .A
complete analysis of the activations should account not only for the amount of information (both task-and nuisance-related), but also for its accessibility, e.g., how easily task-related information can be extracted by a linear classifier. Following
a similar idea, BID24 aim to study the layer-wise, or "spatial" (but not temporal) evolution of the simplicity of the representation by performing a principal component analysis (PCA) of a radial basis function (RBF) kernel embedding of each layer representation. They show
that, on a multi-layer perceptron, task-relevant information increasingly concentrate on the first principal components of the representation's embedding, implying that they become more easily "accessible" layer after layer, while nuisance information (when it is codified at all) is encoded in the remaining components. In our work
we instead focus on the temporal evolution of the weights. However, it
's important to notice that a network with simpler weights (as measured by the FIM) also requires a simpler smooth representation (as measured, e.g., by the RBF embedding) in order to operate properly, since it needs to be resistant to perturbations of the weights. Thus our analysis
is wholly compatible with the intuitions of BID24 . It would also be
interesting to study the joint spatio-temporal evolution of the network using both frameworks at once.One advantage of focusing on the information of the weights rather than on the activations, or behavior of the network, is to have a readout of the "effective connectivity" during critical periods, which can be compared to similar readouts in animals. In fact, "behavioral
" readouts upon deficit removal, both in artificial and neuronal networks, can potentially be confounded by deficit-coping changes at different levels of the visual pathways BID4 BID16 . On the other hand,
deficits in deprived animals are mirrored by abnormalities in the circuitry of the visual pathways, which we characterize in DNNs using the FIM to study its "effective connectivity", i.e., the connections that are actually employed by the network to solve the task. Sensitivity to critical
periods and the trace of the Fisher Information peak at the same epochs, in accord with the evidence that skill development and critical periods in neuronal networks are modulated by changes (generally experience-dependent) in synaptic plasticity BID16 BID10 . Our layer-wise analysis
of the Fisher Information FIG5 ) also shows that visual deficits reinforce higher layers to the detriment of intermediate layers, leaving low-level layers virtually untouched. If the deficit is removed
after the critical period ends, the network is not able to reverse these effects. Although the two systems
are radically different, a similar response can be found in the visual pathways of animal models: Lower levels (e.g., retina, lateral geniculate nucleus) and higher-level visual areas (e.g., V2 and post-V2) show little remodeling upon deprivation, while most changes happen in different layers of V1 BID34 BID9 ).An insightful interpretation
of critical periods in animal models was proposed by BID16 : The initial connections of neuronal networks are unstable and easily modified (highly plastic), but as more "samples" are observed, they change and reach a more stable configuration which is difficult to modify. Learning can, however, still
happen within the newly created connectivity pattern. This is largely compatible with
our findings: Sensitivity to critical-period-inducing deficits peaks when connections are remodeled (Figure 4, Left) , and different connectivity profiles are observed in networks trained with and without a deficit ( FIG5 ). Moreover, high-level deficits such
as imageflipping and label permutation, which do not require restructuring of the network's connections in order to be corrected, do not exhibit a critical period.
Our goal in this paper is not so much to investigate the human (or animal) brain through artificial networks, as to understand fundamental information processing phenomena, both in their biological or artificial implementations.
It is also not our goal to suggest that, since they both exhibit critical periods, DNNs are necessarily a valid model of neurobiological information processing, although recent work has emphasized this aspect.
We engage in an "Artificial Neuroscience" exercise in part to address a technological need to develop "explainable" artificial intelligence systems whose behavior can be understood and predicted.
While traditionally well-understood mathematical models were used by neuroscientists to study biological phenomena, information processing in modern artificial networks is often just as poorly understood as in biology, so we chose to exploit well-known biological phenomena as probes to study information processing in artificial networks.Conversely, it would also be interesting to explore ways to test whether biological networks prune connections as a consequences of a loss of Information Plasticity, rather than as a cause.
The mechanisms underlying network reconfiguration during learning and development might be an evolutionary outcome obtained under the pressure of fundamental information processing phenomena.
DISPLAYFORM0 In order to avoid interferences between the annealing scheme and the architecture, in these experiments we fix the learning rate to 0.001.The Fully Connected network used for the MNIST experiments has hidden layers of size [2500, 2000, 1500, 1000, 500] .
All hidden layers use batch normalization followed by ReLU activations.
We fix the learning rate to 0.005.
Weight decay is not used.
We use data augmentation with random translations up to 4 pixels and random horizontal flipping.
For MNIST, we pad the images with zeros to bring them to size 32 × 32.

assistant: text: Sensory deficits in early training phases can lead to irreversible performance loss in both artificial and neuronal networks, suggesting information phenomena as the common cause, and point to the importance of the initial transient and forgetting.