1 Introduction
Unsupervised representation learning is a fundamental problem in machine learning. Intuitively, one aims to learn a function
which maps the data into some, usually lowerdimensional, spacewhere one can solve some (generally a priori unknown) target supervised tasks more efficiently, i.e. with fewer labels. In contrast to supervised and semisupervised learning, the learner has access
only to unlabeled data. Even though the task seems illposed as there is no natural objective one should optimize, by leveraging domain knowledge this approach can be successfully applied to a variety of problem areas, including image (Kolesnikov et al., 2019; van den Oord et al., 2018; Hénaff et al., 2019; Tian et al., 2019; Hjelm et al., 2019; Bachman et al., 2019) and video classification (Wang and Gupta, 2015; Sun et al., 2019) and natural language understanding (van den Oord et al., 2018; Peters et al., 2018; Devlin et al., 2019).Recently, there has been a revival of approaches inspired by the InfoMax principle (Linsker, 1988): We choose a representation
maximizing the mutual information (MI) between the input and its representation, possibly subject to some structural constraints. MI measures the amount of information obtained about a random variable
by observing some other random variable . Formally, the MI between and , with joint density and marginal densities and , is defined as the Kullback–Leibler (KL) divergence between the joint and the product of the marginals(1) 
The fundamental properties of MI are well understood and have been extensively studied (see e.g. Kraskov et al. (2004)). Firstly, MI is invariant under reparametrization of the variables — namely, if and are homeomorphisms (i.e. smooth invertible maps), then . Secondly, estimating MI in highdimensional spaces is a notoriously difficult task, and in practice one often maximizes a tractable lower bound on the quantity (Poole et al., 2019). Nonetheless, any distributionfree highconfidence lower bound on entropy requires a sample size exponential in the size of the bound (McAllester and Statos, 2018).
Despite these fundamental challenges, several recent works have demonstrated promising empirical results in representation learning using MI maximization (van den Oord et al., 2018; Hénaff et al., 2019; Tian et al., 2019; Hjelm et al., 2019; Bachman et al., 2019; Sun et al., 2019). In this work we argue, and provide empirical evidence, that the success of these methods might be only loosely attributed to the properties of MI. In fact, we show that maximizing tighter bounds on MI can result in worse representations. In addition, we establish a connection to deep metric learning and argue that this interpretation may be a plausible explanation of the success of the recently introduced methods.^{1}^{1}1The code for running the experiments and visualizing the results is available at https://github.com/googleresearch/googleresearch/tree/master/mutual_information_representation_learning.
2 Background and Related Work
Recent progress and the InfoMax principle While promising results in other domains have been presented in the literature, we will focus on unsupervised image representation learning techniques that have achieved stateoftheart performance on image classification tasks (Hénaff et al., 2019; Tian et al., 2019; Bachman et al., 2019). The usual problem setup dates back at least to Becker and Hinton (1992) and can conceptually be described as follows: For a given image , let and be different, possibly overlapping views of , for instance the top and bottom halves of the image. These are encoded using encoders and respectively, and the MI between the two representations and is maximized,
(2) 
where is a samplebased estimator of the true MI and the function classes and can be used to specify structural constraints on the encoders. While not explicitly reflected in (2), note that and can often share parameters. Furthermore, it can be shown that ,^{2}^{2}2Follows by multiple applications of the data processing inequality (see Proposition 1 in Appendix A). hence the objective in (2) can be seen as a lower bound on the InfoMax objective (Linsker, 1988).
Practical advantages of multiview formulations There are two main advantages in using (2) rather than the original InfoMax objective. First, the MI has to be estimated only between the learned representations of the two views, which typically lie on a much lowerdimensional space than the one where the original data lives. Second, it gives us plenty of modeling flexibility, as the two views can be chosen to capture completely different aspects and modalities of the data, for example:

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In DeepInfoMax (Hjelm et al., 2019; Bachman et al., 2019), extracts global features from the entire image and local features from image patches ( and
correspond to the same neural network applied to the same input with features extracted on different layers).

Contrastive predictive coding (CPC) (van den Oord et al., 2018; Hénaff et al., 2019) incorporates a sequential component of the data. Concretely, one extracts a sequence of patches from an image in some fixed order, maps each patch using an encoder, aggregates the resulting features of the first
patches into a context vector, and maximizes the MI between the context and features extracted from the patch at position
. In (2), would thus correspond to the first patches and to the patch at location .
Other approaches, such as those presented by Hu et al. (2017) and Ji et al. (2018), can be similarly subsumed under the same objective.
Lower bounds on MI As evident from (2), another critical choice is the MI estimator . Given the fundamental limitations of MI estimation (McAllester and Statos, 2018), recent work has focused on deriving lower bounds on MI (Barber and Agakov, 2003; Belghazi et al., 2018; Poole et al., 2019)
. Intuitively, these bounds are based on the following idea: If a classifier can accurately distinguish between samples drawn from the joint
and those drawn from the product of marginals , then and have a high mutual information.We will focus on two such estimators, which are most commonly used in the representation learning literature. The first of them, termed InfoNCE (van den Oord et al., 2018), is defined as
(3) 
where the expectation is over independent samples
from the joint distribution
(Poole et al., 2019). In other words, for each , one has to predict which of the samples it was jointly drawn with. This is done using a critic function that ideally assigns high values when are drawn from the joint, and low values when they are drawn from the product of marginals. The second estimator is based on the variational form of the KL divergence due to Nguyen, Wainwright, and Jordan (NWJ) (Nguyen et al., 2010) and takes the form(4) 
For detailed derivations we refer the reader to (Ruderman et al., 2012; Poole et al., 2019). Note that these bounds hold for any critic and when used in (2) one jointly maximizes over and in practice. Furthermore, it can be shown that (3) is maximized by and (4) by (Poole et al., 2019). Common choices for include bilinear critics (van den Oord et al., 2018; Hénaff et al., 2019; Tian et al., 2019), separable critics (Bachman et al., 2019), and concatenated critics (Hjelm et al., 2019) (here
are typically shallow multilayer perceptrons (MLPs)). When applying these estimators to solve (
2), the line between the critic and the encoders can be blurry. For example, one can train with an inner product critic , but extract features from an intermediate layer of , in which case the top layers of form a separable critic. Nevertheless, this boundary is crucial for the interplay between MI estimation and the interpretation of the learned representations.3 Biases in approximate information maximization
It is folklore knowledge that maximizing MI does not necessarily lead to useful representations. Already Linsker (1988) talks in his seminal work about constraints, while a manifestation of the problem in clustering approaches using MI criteria has been brought up by Bridle et al. (1992) and subsequently addressed using regularization by Krause et al. (2010). To what can we then attribute the recent success of methods building on the principles of MI maximization? We will argue that their connection to the InfoMax principle might be very loose. Namely, we will show that they behave counterintuitively if one equates them to mutual information maximization, and that the performance of these methods depends strongly on the bias that is encoded not only in the encoders, but also on the actual form of the used estimators.

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We first consider encoders which are bijective by design. Even though the true MI is maximized for any choice of model parameters, the representation quality (measured by downstream linear classification accuracy) improves during training. Furthermore, there exist invertible encoders for which the representation quality is worse than using raw pixels, despite also maximizing MI.

We next consider encoders that can model both invertible and noninvertible functions. When the encoder can be noninvertible, but is initialized to be invertible, still biases the encoders to be very illconditioned and hard to invert.

For and , highercapacity critics admit tighter bounds on MI. We demonstrate that simple critics yielding loose bounds can lead to better representations than highcapacity critics.

Finally, we optimize the estimators to the same MI lowerbound value with different encoder architectures and show that the representation quality is impacted more by the choice of the architecture, than the estimator.
As a consequence, we argue that the success of these methods and the way they are instantiated in practice is only loosely connected to MI. Then, in Section 4 we provide an alternative explanation of the success of recent methods through a connection to classic triplet losses from metric learning.
Setup Our goal is to provide a minimal set of easily reproducible empirical experiments to understand the role of MI estimators, critic and encoder architectures when learning representations via the objective (2). To this end, we consider a simple setup of learning a representation of the top half of MNIST handwritten digit images. This setup has been used in deep canonical correlation analysis papers (Andrew et al., 2013), where the target is to maximize the correlation between the representations. Following the widely adopted downstream linear evaluation protocol (Kolesnikov et al., 2019; van den Oord et al., 2018; Hénaff et al., 2019; Tian et al., 2019; Hjelm et al., 2019; Bachman et al., 2019), we train a linear classifier^{3}^{3}3Using SAGA (Defazio et al., 2014), as implemented in scikitlearn (Pedregosa et al., 2011). for digit classification on the learned representation using all available training labels. To learn the representation we instantiate (2) and split each input MNIST image into two parts, the top part of the image corresponding to , and the bottom part, , corresponding to , respectively. We train , , and using the Adam optimizer (Kingma and Ba, 2015), and use as the representation for the linear evaluation. Unless stated otherwise, we use a bilinear critic (we investigate its effect in a separate ablation study), set the batch size to and the learning rate to .^{4}^{4}4Note that is upperbounded as (van den Oord et al., 2018). We experimented with batch sizes up to and obtained consistent results aligned with the stated conclusions. Throughout, values and downstream classification accuracies are averaged over runs and reported on the testing set (we did not observe large gaps between training and testing values). As a common baseline, we rely on a linear classifier in pixel space on , which obtains a testing accuracy of about . For comparison, a simple MLP or ConvNet architecture achieves about (see Section 3.3 for details).
3.1 Large MI is not predictive of downstream performance
We start by investigating the behavior of and when and are parameterized to be always invertible. Hence, for any choice of the encoder parameters, the MI is constant, i.e. for all . This means that if we could exactly compute the MI, any parameter choice would be a global maximizer and thus the gradients would vanish.^{5}^{5}5In the context of continuous distributions and invertible representation functions the InfoMax objective might be infinite. One solution is to maximize the entropy of the representation instead (Bell and Sejnowski, 1995). However, here the MI between the two views is finite as the two halves are not deterministic functions of each another. However, as we will empirically show, the estimators we consider are biased and prefer those settings which yield representations useful for the downstream classification task.
Maximized MI and improved downstream performance We model and using the invertible RealNVP architecture (Dinh et al., 2016)
. We use a total of 30 coupling layers, and each of them computes the shift using a separate MLP with two ReLU hidden layers, each with 512 units.
Figure 1 shows the testing value of and the testing accuracy on the classification task. Despite the fact that MI is maximized by any instantiation of and , and downstream accuracy increase during training, implying that the estimators provide gradient feedback leading to a representation useful for linear classification. This confirms our hypothesis that the estimator biases the encoders towards solutions suitable to solve the downstream linear classification task.
The previous experiment demonstrated that among many invertible encoders, all of which are globally optimal MI maximizers, some give rise to improved linear classification performance over raw pixels, and maximizing and yields such encoders. Next we demonstrate that for the same invertible encoder architecture there are model parameters for which linear classification performance is significantly worse than using raw pixels, despite also being globally optimal MI maximizers.
Maximized MI and worsened downstream performance The goal is to learn a (bijective) representation maximizing MI such that the optimal linear classifier performs poorly; we achieve this by jointly training a representation and classifier in an adversarial fashion (a separate classifier is trained for the evaluation), without using an MI estimator. Intuitively, we will train the encoder to make the classification task for the linear layer as hard as possible. The experimental details are presented in Appendix B. Figure 0(c) shows the result of one such training run, displaying the loss of a separately trained classifier on top of the frozen representation. At the beginning of training the network is initialized to be close to the identity mapping, and as such achieves the baseline classification accuracy corresponding to raw pixels. All points beyond this correspond to invertible feature maps with worse classification performance, despite still achieving globally maximal MI.
Alternatively, the following thought experiment would yield the same conclusion: Using a lossless compression algorithm (e.g. PNG) for and also satisfies . Yet, performing linear classification on the raw compressed bit stream will likely lead to worse performance than the baseline in pixel space.
We next investigate the behavior of the model if we use a network architecture that can model both invertible and noninvertible functions. We would like to understand whether prefers the network to remain bijective, thus maximizing the true MI, or to ignore part of the input signal, which can be beneficial for representation learning.
Bias towards hardtoinvert encoders We use an MLP architecture with hidden layers of the same dimension as the input, and with a skip connection added to each layer (hence by setting all weights to
the network becomes the identity function). As quantifying invertibility is hard, we analyze the condition number, i.e. the ratio between the largest and the smallest singular value, of the Jacobian of
: By the implicit function theorem, the function is invertible if the Jacobian is nonsingular.^{6}^{6}6Mathematically, is invertible as long as the condition number of the Jacobian is finite. Numerically, inversion becomes harder as the condition number increases. However, the data itself might lie on a lowdimensional manifold, so that having a singular Jacobian is not necessarily indicative of losing invertibility on the support of the data distribution. To ensure the support of the data distribution covers the complete input space, we corrupt and in a coupled way by adding to each the same 392dimensional random vector, whose coordinates are sampled (independently of) from a normal with standard deviation
(the standard deviation of the pixels themselves is 0.3). Hence, noninvertible encoders do not maximize . ^{7}^{7}7This would not necessarily be true if the noise were added in an uncoupled manner, e.g. by drawing it independently for and , as the MI between the two noise vectors is in that case. As a reference point, the linear classification accuracy from pixels drops to about 84% due to the added noise.In Figure 2 we can see that the value and the downstream accuracy both increase during training, as before. Moreover, even though is initialized very close to the identity function (which maximizes the true MI), the condition number of its Jacobian evaluated at inputs randomly sampled from the datadistribution steadily deteriorates over time, suggesting that in practice (i.e. numerically) inverting the model becomes increasingly hard. It therefore seems that the bounds we consider favor hardtoinvert encoders, which heavily attenuate part of the noise (as the support of the noise is the entire input space), over well conditioned encoders (such as the identity function at initialization), which preserve the noise and hence the entropy of the data well.
3.2 Higher capacity critics can lead to worse downstream performance
In the previous section we have established that MI and downstream performance are only loosely connected. Clearly, maximizing MI is not sufficient to learn good representations and there is a nontrivial interplay between the architectures of the encoder, critic, and the underlying estimators. In this section, we will focus on how one of these factors, namely the critic architecture, impacts the quality of the learned representation. Notably, we will show that more powerful critics, which can only improve the MI lower bounds, can result in worse representations.
We do so by studying the function class of the critic , which has a profound impact on the learned representation. Recall that it determines how the estimators such as and distinguish between samples from the joint distribution and the product of the marginals , and thereby determines the tightness on the lower bound. A higher capacity critic should allow for a tighter lowerbound on MI (Belghazi et al., 2018). Furthermore, in the context of representation learning where is instantiated as a neural network, the critic provides gradient feedback to and and thereby shapes the learned representation. In this section, we investigate the effect of the critic architecture on downstream classification performance.
Looser bounds with simpler critics can lead to better representations We compare three critic architectures, a bilinear critic, a separable critic ( are MLPs with a single hidden layer with units and ReLU activations, followed by a linear layer with units; comprising k parameters in total) and an MLP critic with a single hidden layer with units and ReLU activations, applied to the concatenated input (k trainable parameters). Further, we use identical MLP architectures for and with two hidden layers comprising units each, and a third linear layer mapping to a dimensional feature space.
Figure 3 shows the downstream testing accuracy and the testing value as a function of the iteration. It can be seen that for both lower bounds, representations trained with the MLP critic barely outperform the baseline on pixel space, whereas the same lower bounds with bilinear and separable critics clearly lead to a higher accuracy than the baseline. While the testing value is close to the theoretically achievable maximum value for all critics, the testing value is higher for the MLP critic than for the separable and bilinear critics, resulting in a tighter bound on the MI. However, despite achieving the smallest testing value, the simple bilinear critic leads to a better downstream performance than the highercapacity separable and MLP critics.
A similar phenomenon was observed in the context of variational autoencoders (VAEs)
(Kingma and Welling, 2014), where one maximizes a lower bound on the data likelihood: Looser bounds often yield better inference models, i.e. latent representations (Rainforth et al., 2018).3.3 Encoder architecture can be more important than the specific estimator
We will now show that the encoder architecture is a critical design choice and we will investigate its effect on the learned representation. We consider the same MLP architecture (k parameters) as in Section 3.2, as well as a ConvNet architecture comprising two convolution layers (with a
kernel, stride of
, ReLU activations, and and channels, respectively; k parameters), followed by spatial average pooling and a fully connected layer. Before the average pooling operation we apply layer normalization (Ba et al., 2016)which greatly reduces the variance of
.^{8}^{8}8LayerNorm avoids the possibility of information leakage within minibatches that can be induced through batch normalization, potentially leading to poor performance
(Hénaff et al., 2019). To ensure that both network architectures achieve the same lower bound on the MI, we minimize instead of solving (2), for two different values .Figure 4 shows the downstream testing accuracy as a function of the training iteration. It can be seen in the testing loss curves in Appendix F that for both architectures and estimators the objective value after k iterations matches the target (i.e., ) which implies that they achieve the same lowerbound on the MI. Despite matching lower bounds, ConvNet encoders lead to clearly superior classification accuracy, for both and . Note that, in contrast, the MLP and ConvNet architectures trained endtoend in supervised fashion both achieve essentially the same testing accuracy of about .
In the context of VAEs, Alemi et al. (2018) similarly observed that models achieving the same evidence lower bound value can lead to vastly different representations depending on the employed encoder architecture, and do not necessarily capture useful information about the data (Tschannen et al., 2018; Blau and Michaeli, 2019).
4 Connection to deep metric learning and triplet losses
In the previous section we empirically demonstrated that there is a disconnect between MI maximiszation and representation quality. However, many recent works have applied the approach to obtain stateoftheart results in practice. We provide some insight on this conundrum by connecting the estimator to a popular triplet (plet) loss known in the deep metric learning community.
The metric learning view Given sets of triplets, namely an anchor point , a positive instance , and a negative instance , the goal is to learn a representation such that the distances (i.e., ) between and is smaller than the distance between and , for each triplet. In the supervised setting, the positive instances are usually sampled from the same class, while the negative instances are sampled from any other class. A major focus in deep metric learning is how to perform (semi)hard positive mining — we want to present nontrivial triplets to the learning algorithm which become more challenging as improves. Natural extensions to the unsupervised setting can be obtained by exploiting the structure present in the input data, namely spatial (e.g. patches from the same image should be closer than patches from different images) and temporal information (temporally close video frames should be encoded closer than the ones which are further away in time) (Hoffer and Ailon, 2015).
Connection to InfoNCE The InfoNCE objective can be rewritten as follows:
The derivation is presented in Appendix C. In the particular case that and take value in the same space and is constrained to be of the form , for some function , this coincides (up to constants and change of sign) with the expectation of the multiclass Kpair loss proposed in (Sohn, 2016, Eqn. (7)):
(5) 
Representation learning by maximizing using a symmetric separable critic and an encoder shared across views is thus equivalent to metric learning based on (5). When using different encoders for different views and asymmetric critics as employed by CPC, DeepInfoMax, and CMC one recovers asymmetric variants of (5), see, e.g. (Yu et al., 2017; Zhang et al., 2019). As a result, one can view (5) as learning encoders with a parameterless inner product critic, for which the MI lowerbound is very weak in general.
There are (at least) two immediate benefits of viewing recent representation learning methods based on MI estimators through the lens of metric learning. Firstly, in the MI view, using inner product or bilinear critic functions is suboptimal since the critic should ideally be as flexible as possible in order to reduce the gap between the lower bound and the true MI. In the metric learning view, the inner product critic corresponds to a simple metric on the embedding space. The metric learning view seems hence in better accordance with the observations from Section 3.2 than the MI view. Secondly, it elucidates the importance of appropriately choosing the negative samples, which is indeed a critical component in deep metric learning based on triplet losses (Norouzi et al., 2012; Schroff et al., 2015).
InfoNCE and the importance of negative sampling The negative sample mining issue also manifests itself in MIbased contrastive losses. In fact, while InfoNCE is a lower bound on MI if the negative samples are drawn from the true marginal distribution (Poole et al., 2019), i.e.
we show that if the negative samples are drawn in a dependent fashion (corresponding to the being drawn identically but not independently), the estimator is in general neither a lower nor an upper bound on the true mutual information . We prove this in Appendix D and present empirical evidence here. Let , where and are twodimensional Gaussians. We generate batches of data where each is sampled independently for each element of the batch, but is sampled only once per batch. As such, has the same marginal distribution for each , but the elements of the batch are not independent. Although we do not treat it theoretically, we also display results of the same experiment using the estimator. The experimental details are presented in Appendix E. We observe in Figure 4c that when using noni.i.d. samples both the and values are larger than the true MI, and that when i.i.d. samples are used, both are lower bounds on the true MI. Hence, the connection to MI under improper negative sampling is no longer clear and might vanish completely.
Notwithstanding this fundamental problem, negative sampling strategy is often treated as a design choice. In Hénaff et al. (2019), CPC is applied to images by partitioning the input image into patches. Then, mutual information (estimated by InfoNCE) between representations of patches and a context summarizing several patches that are vertically above or below in the same image is minimized. Negative samples are obtained by patches from different images as well as patches from the same image, violating the independence assumption. Similarly, van den Oord et al. (2018) learn representations of speech using samples from a variety of speakers. It was found that using utterances from the same speaker as negative samples is more effective, whereas the “proper” negative samples should be drawn from an appropriate mixture of utterances from all speakers.
A common observation is that increasing the number of negative examples helps in practice (Hjelm et al., 2019; Tian et al., 2019; Bachman et al., 2019). Indeed, Ma and Collins (2018) show that is consistent for any number of negative samples^{9}^{9}9Under some technical conditions., and Poole et al. (2019) show that the signaltonoise ratio increases with the number of negative samples. On the other hand, (Arora et al., 2019) have demonstrated, both theoretically and empirically, that increasing the number of negative samples does not necessarily help, and can even deteriorate the performance. The intricacies of negative sampling hence remain a key research challenge.
5 Conclusion
Is MI maximization a good objective for learning good representations in an unsupervised fashion? Possibly, but it is clearly not sufficient. In this work we have demonstrated that, under the common linear evaluation protocol, maximizing lower bounds on MI as done in modern incarnations of the InfoMax principle can result in bad representations. We have revealed that the commonly used estimators have strong inductive biases and—perhaps surprisingly—looser bounds can lead to better representations. Furthermore, we have demonstrated that the connection of recent approaches to MI maximization might vanish if negative samples are not drawn independently (as done by some approaches in the literature). As a result, it is unclear whether the connection to MI is a sufficient (or necessary) component for designing powerful unsupervised representation learning algorithms. We propose that the success of these recent methods could be explained through the view of tripletbased metric learning and that leveraging advances in that domain might lead to further improvements.
In terms of future work, one possible direction to explore is going beyond the widely used linear evaluation protocol. While it was shown that learning good representations under this protocol can lead to reduced sample complexity for downstream tasks (Arora et al., 2019), some recent works (Bachman et al., 2019; Tian et al., 2019) report marginal improvements in terms of the downstream performance under a nonlinear regime. We stress that a highlynonlinear evaluation framework may result in better downstream performance, but it defeats the purpose of training transferable data efficient representations. Secondly, given the fundamental issues with the current estimators, one may investigate other notions of divergence between and , for example using the Wasserstein distance instead (Ozair et al., 2019), which could enable a natural approach for enforcing smoothness in the resulting estimators.
Acknowledgments
We would like to thank Alex Alemi, Ben Poole, Olivier Bachem, and Alexey Dosovitskiy for inspiring discussions and comments on the manuscript. We are grateful for the general support and discussions from other members of Google Brain team in Zurich.
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Appendix
Appendix A Relation between (2) and the InfoMax objective
Proposition 1.
Let be a random variable and define and be arbitrary functions of . Then .
Proof.
Follows by two applications of the data processing inequality, which states that for random variables , and satisfying the Markov relation , the inequality holds.
The first step is to observe that , and satisfy the relation , which is Markov equivalent to . It therefore follows that . The second step is to observe that and therefore .
Combining the two inequalities yields , as required. ∎
Appendix B Experiment details: Adversarially trained encoder (Section 3.1)
In the following, we present the details for training the invertible model from Section 3.1 adversarially. We model with the same RealNVP architecture as in the first experiment, and do not model . On top of we add a linear layer mapping to
outputs (i.e. logits). The parameters of the linear layer trained by minimizing the crossentropy loss with respect to the true label of
from which is derived. Conversely, the parameters of the encoder are trained to minimize the crossentropy loss with respect to a uniform probability vector over all 10 classes. We use the Adam optimizer with a learning rate of for the parameters of the classifier and for the parameters of the encoder, and perform classifier optimization steps per encoder step. Furthermore, in a warmup phase we train the classifier for k iterations before alternating between classifer and encoder steps.Appendix C Connection between metric learning and InfoNCE
can be rewritten as follows:
Appendix D InfoNCE under noni.i.d. sampling
The proof that InfoNCE is a lower bound on mutual information presented in [Poole et al., 2019] makes crucial use of the assumption that the negative samples are drawn from the true marginal distribution. We briefly review this proof to highlight the importance of the negative sampling distribution. Their proof starts from the NWJ lower bound of the KL divergence, namely that for any function the following lower bound holds [Nguyen et al., 2010, Nowozin et al., 2016]:
(6) 
Suppose that are i.i.d. draws from and write . Then, for any we have that . We thus have
where the equality follows from the assumption that the are i.i.d. and the inequality is (6) applied to . In particular, taking yields
(7) 
This is then averaged over the samples , in which case the third term above cancels with the constant (all occurences of in the last term of (7) can be replaced with thanks to being identically distributed), yielding the familiar lower bound:
(8) 
The point in this proof that makes use of the i.i.d. assumption of the negative samples is in the equality , which allowed us to leverage multiple samples when estimating the mutual information between two variables. If instead the negative samples are drawn in a dependent fashion (corresponding to the being drawn identically but not independently), we have , though the remainder of the proof still holds, resulting in
Therefore the resulting estimator is neither a lower nor an upper bound on the true mutual information .
Appendix E Experiment details: Noni.i.d. sampling (Section 4)
Recall that . We use and , where
Batches of data are obtained as where each is sampled independently for each element of the batch, but is sampled only once per batch. The true mutual information can be calculated analytically since is jointly Gaussian with known covariance matrix : For two univariate random variables that are jointly Gaussian with covariance the MI can be written as
This can be derived using the decomposition and the analytic expression for the entropy of a Gaussian.
We compare the same setting trained using i.i.d. sampled pairs as a baseline. We parametrize the critic as a MLP with 5 hidden layers, each with 10 units and ReLU activations, followed by a linear layer and maximize using these noni.i.d. samples with batch size 128. Note that if a batch size of is used, the bound always holds. We used sufficiently large so that to avoid trivially lower bounding the true MI.
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