Recently proposed activation functions for neural networks such as rectified linear (ReL (Glorot et al., 2011)), maxout (Goodfellow et al., 2013a) and LWTA (Srivastava et al., 2013) are quite unlike sigmoidal activation functions. These functions depart from the conventional wisdom in that they are not continuously differentiable (and sometimes non-continuous) and are piecewise linear. Nevertheless, many researchers have found that such networks can be trained faster and better than sigmoidal networks, and they are increasingly in use for learning from large and complex datasets (Krizhevsky et al., 2012; Zeiler et al., 2013). Past research has shown observational evidence that such networks have beneficial properties such as not requiring unsupervised training for weight initialization (Glorot et al., 2011), better gradient flow (Goodfellow et al., 2013a) and mitigation of catastrophic forgetting (Srivastava et al., 2013; Goodfellow et al., 2014). Recently, the expressive power of deep networks with such functions has been theoretically analyzed (Pascanu et al., 2013). However, we are far from a complete understanding of their behavior and advantages over sigmoidal networks, especially during learning. This paper sheds additional light on the properties of such networks by interpreting them as models of models.
A common theme among the ReL, maxout and LWTA activation functions is that they are locally competitive. Maxout and LWTA utilize explicit competition between units in small groups within a layer, while in the case of the rectified linear function, the weighted input sum competes with a fixed value of 0. Networks with such functions are often trained with the dropout regularization technique (Hinton et al., 2012) for improved generalization.
We start from the observation that in locally competitive networks, a subnetwork of units has non-zero activations for each input pattern. Instead of treating a neural network as a complex function approximator, the expressive power of the network can be interpreted to be coming from its ability to activate different subsets of linear units for different patterns. We hypothesize that the network acts as a model that can switch between “submodels” (subnetworks) such that similar submodels respond to similar patterns
. As evidence of this behavior, we analyze the activated subnetworks for a large subset of a dataset (which is not used for training) and show that the subnetworks activated for different examples exhibit a structure consistent with our hypothesis. These observations provide a unified explanation for improved credit assignment in locally competitive networks during training, which is believed to be the main reason for their success. Our new point of view suggests a link between these networks and competitive learning approaches of the past decades. We also show that a simple encoding of which units in a layer are activated for a given example (its subnetwork) can be used to represent the example for retrieval tasks. Experiments on MNIST, CIFAR-10, CIFAR-100 and the ImageNet dataset show that promising results are obtained for datasets of varying size and complexity.
2 Locally Competitive Neural Networks
Neural networks with activation functions like rectified linear, maxout and LWTA are locally competitive. This means that local competition among units in the network decides which parts of it get activated or trained for a particular input example. For each unit, the total input or presynaptic activation is first computed as , where
is the vector of inputs to the unit,is a trainable weight vector, and is a trainable bias. For the rectified linear function, the output or postsynaptic activation of each unit is simply , which can be interpreted as competition with a fixed value of 0. For LWTA, the units in a layer are considered to be divided into blocks of a fixed size. Then the output of each unit is where is an indicator which is 1 if the unit has the maximum in its group and 0 otherwise. In maxout, the inputs from a few units compete using a operation, and the block output is the maximum among the units111In our terminology, the terms unit and block correspond to the terms filter and units in Goodfellow et al. (2013a).. A maxout block can also be interpreted as an LWTA block with shared outgoing weights among the units. A comparison of the 3 activation functions is shown in Figure 2.
In each of the three cases, there is a local gating mechanism which allows non-zero activations (and errors during training) to propagate only through part of the network, i.e. a subnetwork. Consider the activation of a neural network with rectified linear units (ReLUs) in a single hidden layer. For each input pattern, the subset of units with non-zero activations in the hidden layer form a subnetwork, and an examination of the subnetworks activated for several examples shows that a large number of different subnetworks are activated (Figure 2). The result of training the network can interpreted in the following way: when training a single network with a local gating mechanism, a large number of linear subnetworks are trained on the dataset such that different examples are gated to different subnetworks, each getting trained to produce the desired output. At test time, the system generalizes in the sense that the appropriate subnetwork for a given example is activated.
3 Subnetwork Analysis
network layer which lacks any discernable structure. (b) shows submasks from a trained network layer, showing clearly demarcated clusters relevant to the supervised learning task. ‘Mistakes’ made by the network can also be observed, such as mistaking ‘4’s for ‘9’s.
This section investigates how the model of models that is implemented though local competition self-organizes due to training. In order to visualize the organization of subnetworks as a result of training, they are encoded as bit strings called submasks. For the input pattern , the submask , where is the number of units in the full network, represents the corresponding subnetwork by having a 0 in position , , if the corresponding unit has zero activation, and 1 otherwise. The submasks uniquely and compactly encode each subnetwork in a format that is amenable to analysis through clustering, and, as we show in Section 4.2, facilitates efficient data retrieval.
In what follows, the subnetworks that emerge during training are first visualized using the t-SNE (Van der Maaten & Hinton, 2008) algorithm. This dimensionality reduction technique enables a good visualization of the relationship between submasks for several examples in a dataset by preserving the local structure. Later in this section, we examine the evolution of subnetworks during training, and show that the submasks obtained from a trained network can directly be used for classification using a simple nearest neighbors approach. All experiments in this section are performed on the MNIST (LeCun et al., 1998) dataset. This familiar dataset was chosen because it is relatively easy, and therefore provides a tractable setting in which to verify the repeatability of our results. Larger, more interesting datasets are used in section 4 to demonstrate the utility of techniques developed in this section for classification and retrieval.
3.1 Visualization through Dimensionality Reduction
For visualizing the relationship between submasks for a large number of input patterns, we trained multiple networks with different activation functions on the MNIST training set, stopping when the error on a validation set did not improve. The submasks for the entire test set (10K examples) were then extracted and visualized using t-SNE. Since the competition between subnetworks is local and not global, subsets of units in deeper (closer to the output) layers are activated based on information extracted in the shallow layers. Therefore, like unit activations, submasks from deeper layers are expected to be better related to the task since deeper layers code for higher level abstractions. For this reason, we use only submasks extracted from the penultimate network layers in this paper, which considerably reduces the size of submasks to consider.
Figure 2(b) shows a 2D visualization of the submasks from a 3 hidden layer ReL network. Each submask is a bitstring of length 1000 (the size of the network’s penultimate layer). Ten distinct clusters are present corresponding to the ten MNIST classes. It is remarkable that, irrespective of the actual activation values, the subnetworks which are active for the testing examples can be used to visually predict class memberships based on their similarity to each other. The visualization confirms that the subnetworks active for examples of the same class are much more similar to each other compared to the ones activated for the examples of different classes.
Visualization of submasks from the same layer of a randomly initialized network does not show any structure (Figure 2(a)), but we observed some structure for the untrained first hidden layer (Appendix A.1). For trained networks, similar clustering is observed in the submasks from shallow layers in the network, though the clusters appear to be less separated and tight. The visualization also shows many instances where the network makes mistakes. The submasks for some examples lie in the cluster of submasks for the wrong class, indicating that the ‘wrong’ subnetwork was selected for these examples. The experiments in the next sections show that the organization of subnetworks is indicative of the classification performance of the full network.
Other locally competitive activation functions such as LWTA and maxout result in similar clustering of submasks (visualizations included in Appendix A.1). For LWTA layers, the submasks can be directly constructed from the activations because there is no subsampling when going from presynaptic to postsynaptic activations, and it is reasonable to expect a subnetwork organization similar to that of ReL layers. Indeed, in a limited qualitative analysis, it has been shown previously (Srivastava et al., 2013) that in trained LWTA nets there are more units in common between subnetworks for examples of the same class than those for different class examples.
For maxout layers, the situation is trickier at a first glance. The unit activations get pooled before being propagated to the next layer, so it is possible that the maximum activation value plays a much more important role than the identity of the winning units. However, using the same basic principle of credit assignment to subnetworks, we can construct submasks from maxout layers by binarizing the unit activations such that only the units producing the maximum activation are represented by a 1. Separation of subnetworks is necessary to gain the advantages of local competition during learning, and the visualization of the generated submasks produces results similar to those for ReLU and LWTA (included in AppendixA.1).
3.2 Behavior during Training
In order to measure how the subnetworks evolve over the course of training, the submasks of each sample in the training set were recorded at each epoch. Figure 4 characterizes the change in the subnets over time by counting the number of input patterns for which a unit flips from being on to being off, or vice-versa, from one epoch to the next. The curve in the figure shows the fraction of patterns for which an inter-epoch flip occurred, averaged across all units in the network. Higher values indicate that the assignment of subnets to patterns is not stable. The batch size for this experiment was 100, which means that each pass over the training set consists of 500 weight updates. For the run shown, the average fraction of flips starts at 0.2, but falls quickly below 0.05 and keeps falling as training proceeds, indicating that, the assignment of subnetworks to individual examples stabilizes quickly. In this case, after a brief (3 epochs) transient period, a fine-tuning period follows where the assigned subnetworks keep getting trained on their corresponding examples with little re-assignment.
3.3 Evaluating Submasks
Since the visualization of submasks for the test set shows task-relevant structure, it is natural to ask: how well can the submask represent the data that produced it? If the submasks for similar examples are similar, perhaps they can be used as data descriptors for tasks such as similarity-based retrieval. Sparse binary codes enable efficient storage and retrieval for large and complex datasets due to which learning to produce them is an active research area (Gong et al., 2013; Masci et al., 2014b, a; Grauman & Fergus, 2013). This would make representative submasks very attractive since no explicit training for retrieval would be required to generate them.
To evaluate if examples producing similar binary codes are indeed similar, we train locally competitive networks for classification and use a simple nearest neighbors (
NN) algorithm for classifying data using the generated submasks. This approach is a simple way to examine the amount of information contained in the submasks (without utilizing the actual activation values).
We trained networks with fully connected layers on the MNIST training set, and selected the value of with the lowest validation error to perform classification on the test set. Results are shown in Table 1. In each case, the NN classification results are close to the classification result obtained using the network’s softmax layer. If we use the (non-pooled) unit activations from the maxout network instead of submasks for NN classification, we obtain 121 errors.
Submasks can also be obtained from convolutional layers. Using a convolutional maxout network, we obtained 52 errors on the MNIST test set when we reproduced the model from Goodfellow et al. (2013a). Since the penultimate layer in this model is convolutional, the submasks were constructed using the presynaptic unit activations from this layer for all convolutional maps. Visualization of these submasks showed similar structure to that obtained from fully connected layers, NN classification on the submasks resulted in 65 errors. As seen before, for a well-trained network the NN performance is close to the performance of the network’s softmax layer.
3.4 Effect of Dropout
The dropout (Hinton et al., 2012) regularization technique has proven to be very useful and efficient at improving generalization for large models, and is often used in combination with locally competitive activation functions (Krizhevsky et al., 2012; Goodfellow et al., 2013a; Zeiler et al., 2013). We found that networks which were trained with dropout (and thus produced lower test set error) also yielded better submasks in terms of NN classification performance. To observe the effect of dropout in more detail, we trained a 3 hidden layer network with 800 ReLUs in each hidden layer without dropout on MNIST starting from 5 different initializations until the validation set error did not improve. The networks were then trained again from the same initialization with dropout until the validation error matched or fell below the lowest validation error from the non-dropout case. In both cases, minibatch gradient descent with momentum was used for training the networks. A comparison of NN classification error for the dropout and non-dropout cases showed that when dropout training is stopped at a point when validation error is similar to a no-dropout network, the submasks from both cases give similar results, but as dropout training continues (lowers validation set error), the submasks yield improved results. This supports the interpretation of dropout as a regularization technique which prevents “co-adaptation of feature detectors” (units) (Hinton et al., 2012), leading to better representation of data by the subnetworks. Another way to look at this effect can be that dropout improves generalization by injecting noise in the organization of subnetworks, making them more robust.
4 Experimental Results
The following experiments apply the methods described in the previous section to more challenging benchmark problems: CIFAR-10, CIFAR-100, and ImageNet. For the CIFAR experiments, we used the models described in Goodfellow et al. (2013a)
since they use locally competitive activations (maxout), are trained with dropout, and good hyperparameter settings for them are available(Goodfellow et al., 2013b). We report the classification error on the test set obtained using the softmax output layer, as well NN classification on the penultimate layer unit activations and submasks. The best value of is obtained using a validation set, though we found that = 5 with distance weighting usually worked well.
4.1 Cifar-10 & Cifar-100
CIFAR-10 and CIFAR-100 are datasets of 3232 color images of 10 classes. The results obtained on the test sets for these datasets are summarized in Table 2. We find that when comparing nearest neighbor classification performance with submasks to unit activation values, we lose an accuracy of 1.25% on the CIFAR-10 dataset, and 2.26% on the CIFAR-100 dataset on average. Figure 4(a) shows the 2-D visualization of the test set submasks for CIFAR-10. Some classes can be seen to have highly representative submasks, while confusion between classes in the lower half is observed. The clusters of subnetworks are not as well-separated as in the case of MNIST, reflecting the relatively worse classification performance of the full network. Submask visualization for CIFAR-100 (Figure 4(b)) reflects the high error rate for this dataset. Although any visualization with 100 classes can be hard to interpret, many small clusters of submasks can still be observed.
|CIFAR-10||9.94 0.31%||9.63 0.21%||10.11 0.16%||11.36 0.22%|
|CIFAR-100||34.49 0.22%||37.54 0.14%||41.37 0.26%||43.63 0.18%|
The results of NN classification and t-SNE visualization using submasks on small datasets of varying complexities show that the submasks contain substantial information relevant for image classification. In this section, the utility of the submasks obtained for a large convolutional network trained on the ImageNet Large Scale Visual Recognition Challenge 2012 (ILSVRC-2012) (Deng et al., 2012) dataset is evaluated.
Our results show that submasks retain a large amount of information on this difficult large scale task, while greatly improving storage efficiency. For instance, 4096-dimensional submasks for the full ILSVRC-2012 training set can be stored in about 0.5 GB. Our experiments also indicate that submasks obtained from a better trained network result in better performance (Table 3). Krizhevsky et al. (2012) suggested that the activations from a trained convolutional network can be compressed to binary codes using auto-encoders. We show here that the submasks can be directly utilized for efficient retrieval of data based on high level similarity even though no pair-wise loss was used during training.
We compare to DiffHash, a supervised similarity-preserving hashing approach proposed by Strecha et al. (2012), trained on the non-binarized features from the network. Supervision is represented in terms of similar and dissimilar pairs of points, for which a ground-truth similarity measure is known, i.e. sharing the same class or not. While it is beyond the scope of this paper to provide an exhaustive comparison or to introduce a new approach to supervised hashing, we nevertheless show very competitive performance w.r.t. a dedicated algorithm devised for this task. Precision-recall curves are shown in Figure 6 while Table 4 reports results for mean average precision; , where indicates the relevance of a result at a given rank , the precision at , and the number of retrieved results. DiffHash learns a linear projection, which is one of the reason we decided to use it to limit impact of supervision. Thus we attribute the small performance gap to the input features already being very discriminative which left little room for improvement. For the purpose of this comparison, we did not investigate more sophisticated techniques which would have steered the focus to conventional hashing approaches. Sample retrieval results for examples from the ILSVRC-2012 dataset are shown in Figure 7.
Training a system of many networks on a dataset such that they specialize to solve simpler tasks can be quite difficult without combining them into a single network with locally competitive units. Without such local competition, one needs to have a global gating mechanism as in Jacobs et al. (1991)
. The training algorithm and the objective function also need modifications such that competition between networks is encouraged, and the system becomes hard to train. On the other hand, a locally competitive neural network can behave like a model composed of many subnetworks, and massive sharing of parameters between subnetworks enables better training. Stochastic gradient descent can be used to minimize the desired loss function, and the implementation is so simple that one does not even realize that a model of models is being trained.
Figure 4 suggests that during optimization, the subnetworks get organized during an early transient phase such that subnetworks responding to similar examples have more parameters in common than those responding to dissimilar examples. This allows for better training of subnetworks due to reduced interference from dissimilar examples and shared parameters for similar examples. In the later fine-tuning phase, the parameters of subnetworks get adjusted to improve classification and much less re-assignment of subnetworks is needed. In this way, the gating mechanism induced by locally competitive activation functions accomplishes the purpose of global competition efficiently and no modifications to the error function are required.
We believe that due to above advantages locally competitive networks have allowed easier and faster training on complex pattern recognition tasks compared to networks with sigmoidal or similar activation functions. These findings provide indirect evidence that low interference between subnetworks is a beneficial property for training large networks. The nature of organization of subnetworks is reminiscent of the data manifold hypothesis for classification(Rifai et al., 2011). Just like data points of different classes are expected to concentrate along sub-manifolds, we expect that the organization of subnetworks that respond to the data points reflects the data manifold being modeled.
An important take-away from these results is the unifying theme between locally competitive architectures, which is related to past work on competitive learning. Insights from past literature on this topic may be utilized to develop improved learning algorithms and neural architectures. This paper, to the best of our knowledge, is the first to show that useful binary data descriptors can be obtained directly from a neural network trained for classification without any additional training
. These descriptors are not just results of a thresholding trick or unique to a particular activation function, but arise as a direct result of the credit assignment process. Our experiments on datasets of increasing complexity show that when the network performance improves, the performance gap to submask-based classification closes. This suggests that in the near future, as training techniques continue to advance and yield lower errors on larger datasets, submasks will perform as well as activation values for retrieval and transfer learning tasks. Importantly, these binary representations will always be far more efficient for storage and retrieval than continuous activation vectors.
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Appendix A Supplementary Materials
a.1 Extra Visualizations
Appendix B Dataset Descriptions
b.1 CIFAR-10 and CIFAR-100
CIFAR-10 is a dataset of 3232 color images of 10 classes split into a training set of size 50,000 and testing set of size 10,000 (6000 images per class) (Krizhevsky & Hinton, 2009). CIFAR-100 is a similar dataset of color images but with 100 classes and 600 images per class, making it more challenging. The models from Goodfellow et al. (2013a) for these dataset utilize preprocessing using global contrast normalization and ZCA whitening as well as data augmentation using translational and horizontal reflections.
b.2 ImageNet (ILSVRC-2012)
ILSVRC-2012 is a dataset of over a million natural images split into 1000 classes. An implementation of the network in Krizhevsky et al. (2012), with some minor differences (Donahue et al., 2013), is available publicly. For the experiments in this section, the penultimate-layer activations obtained using this model were downloaded from CloudCV Batra et al. (2013). The activations were obtained using the center-only option, meaning that only the activations for the central, 224224 crop of each image were used.
For each validation set example, 100 examples from the training set with the closest submasks were weighted by the inverse of the distance, then the classes with top-1 or top-5 weighted sums were returned as predictions.
Appendix C Note on Sigmoidal Networks
In this paper we focused on improving our understanding of neural networks with locally competitive activation functions. We also obtained binary codes for efficient retrieval directly from neural networks trained for classification, but this was not the primary aim of our study. When this is the aim, we note here that it possible to use sigmoidal activation functions to obtain binary codes by thresholding the activation values after supervised or unsupervised (Salakhutdinov & Hinton, 2009) training. However it should be noted that:
The thresholding is somewhat arbitrary and the best threshold needs to be selected by trying various values. For locally competitive networks, the binarization is natural and inherent to the nature of credit assignment in these networks.
Since sigmoidal networks are hard and slow to train, the approach of thresholding their activations is impractical for large datasets which are common for retrieval tasks. Locally competitive networks have been crucial for the successful application of neural networks to such datasets.