NeurIPS 2019
Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
Paper ID:2072
Title:A Normative Theory for Causal Inference and Bayes Factor Computation in Neural Circuits

Reviewer 1

After reading the Author Feedback: I thank the authors for the useful explanations and comments, in particular with respect to novelty and comparison to related work, which I encourage them to include in the revised paper. Overall, I think this is an interesting contribution but still somewhat niche (e.g., it applies only to circular variables), and somewhat incremental with respect to recent similar work; and empirical evidence is only suggestive. Nevertheless, there are enough novel theoretical contributions to potentially deserve publication in NeurIPS; so I confirm my score. Summary: This paper proposes how neurons could implement perceptual causal inference (that is, decide between integration or segregation of two sensory cues from distinct modalities, such as vision and vestibular sense) via population coding and simple linear operations, at least for cues living in a circular space (e.g., heading direction). The major difficulty here is the computation of the Bayes factor so as to decide which model (integration or segregation) best describes the data. The authors show that by assuming that the brain infers stimulus reliability as well as stimulus direction, and via some approximations (e.g., Laplace), all the computations become tractable. Interestingly, the theory requires that in order to compute the Bayes factor, the brain should have neurons and anti-neurons tuned to opposite directions (from different modalities) which act simultaneously; which is compatible with experimental findings. Originality: Medium. The proposal for computing Bayes factors is novel as far as I know, but there is some overlap with recent work which is not fully addressed in the paper (see detailed comments). However, the mathematical techniques used here could lead to further developments of population codes. Quality: High. The problem is interesting, and the analysis and quality of the work is high throughout. Clarity: The paper is well-structured; the text is well-written and the mathematical content well-explained, and the images provide useful information. Significance: This is a potentially interesting contribution as it would potentially explain some experimental findings -- on the other hand, there seems to be considerable overlap with previous literature, which would make this paper still technically interesting for how causal inference is implemented, but somewhat incremental. Major comments: This is a generally thorough and well-written paper, with a considerable amount of additional material to expand upon the main text. My main concern with the work is that the authors should better clarify in the manuscript what their original contributions are, in particular with respect to prior work: - Zhang et al. (2016), which is cited in passing in the current submission, but not expanded upon; - Most importantly, Zhang et al. (2018), which is not cited in the current submission, and with which there seems to be a certain amount of overlap. The same work seems also to have been recently published in eLife (Zhang et al., 2019), but note that the preprint has been available online at least since November 2018. Second, it is not completely clear from the paper why the authors' proposal succeeds where previous work has failed (in particular, reference [15]). The key point, also stressed by the authors, seems to be that, in this submission, the stimulus reliability R is treated explicitly as part of the inference (see lines 99-106), whereas it "was treated as a 'nuisance' parameter in the previous studies". However, being a nuisance parameter merely means that it gets marginalized away, and this study too eventually has to perform a marginalization (Eq. 14), although on Lambda (the *observed* spike count), which is likely the key difference. Can the authors elaborate on why their approach is successful (as opposed to the previous ones)? In short, it seems that the paper would strongly benefit from an extended "Related work" section which highlights the novelty and strengths of the current submission. Minor comments: line 39: Note that the Bayes factor is the ratio of the *posteriors* over models -- which is equivalent to the marginal likelihood ratio for an equal prior over models, but this (while being a common choice) is still a specific case. Figure 1: Panel A uses a non-standard depiction for latent vs. observed variables in graphical models. A fairly standard depiction in statistics and machine learning is to use "filled gray" nodes for observed variables, and solid-circle (white) nodes for latent variables (although if the authors are familiar with another convention, that's fine too). Besides this negligible point, the figure is very clear and well-made. line 71: w_int is a parameter vector, not a single parameter, and it includes both s and R. A uniform distribution over s is non-problematic, since it is a naturally bounded dimension (uniform over the circle), but I reckon that you assume a *bounded* uniform distribution over R (and not an improper prior); as explained later (Eq. 10). Perhaps here just add that you use a "bounded uniform" distribution, to avoid ambiguity. I am stressing this apparently innocuous detail because the choice of bounds (more in general, the choice of prior) is *crucial* for the computation of the marginal likelihood; an arbitrary large interval for R can arbitrarily penalize the more complex model. Typos: The paper is generally clear and very well-written, but there is a bunch of typos here and there, such as occasionally missing articles ("the"), or redundant ones. Given the otherwise high quality of the writing, I recommend to double-check the paper and supplementary information for spelling and grammar. I am pointing out a few of these errors here, but there is likely much more to fix. line 15: amendable --> amenable line 24: "often are hierarchy with latent variables" --> unclear phrasing line 75: "the prior of two models" --> "the prior of the two models" line 83: function over --> function of line 84: the width of tuning function --> the width of the tuning function line 85: the reliability of stimulus --> the stimulus reliability line 137: "In the below, we presented how" --> "In the section below, we present how" line 161: "of two models" --> "of the two models" line 166: "In above equation" --> "In the above equation" line 171: "When the cues are from the same object, their consensus is similar with themselves statistically" --> rephrase better line 175: "normalizing constant" --> "normalizing constants" line 183: "two red vectors" --> "the two red vectors" line 184: "two likelihood ratios" --> "the two likelihood ratios" line 185: "the opposite means" --> "opposite means" line 187: "irrelevant with" --> "irrelevant of" line 191: "two cues’ consensus to explain cues" --> "the two cues’ consensus to explain the cues" line 191: "explaining power" --> "explanatory power" line 218: "opposite with" --> "opposite to" line 219: "reveals" --> "reveal" line 227: "stimlus" --> "stimulus" line 231: "with the equal number" --> "with equal number" line 246: "addresses causal inference" --> "addresses how causal inference" Supplementary Material: line 30: Anglogy --> Analogy line 33: with first kind --> of first kind line 90: logritham --> logarithm line 92, 110 (and perhaps others): maximum-a-posterior --> maximum-a-posteriori line 108: Taking logarithm --> Taking the logarithm line 118: Combining above results --> Combining the above results line 126: requires maginalize --> requires to maginalize line 131: "is presented in the below" --> "is presented below" line 134: through omitting --> by omitting line 144: "it is a mixture of Gaussian" --> it is not a mixture, it's the product of two distributions line 181: consine --> cosine line 183: should satisfies --> should satisfy References: Zhang, W. H., Wang, H., Wong, K. M., & Wu, S. (2016). “Congruent” and “opposite” neurons: sisters for multisensory integration and segregation. In Advances in Neural Information Processing Systems (pp. 3180-3188). Zhang, W. H., Wang, H., Chen, A., Gu, Y., Lee, T. S., Wong, K. M., & Wu, S. (2018). Concurrent Multisensory Integration and Segregation with Complementary Congruent and Opposite Neurons. bioRxiv, 471490. Zhang, W. H., Wang, H., Chen, A., Gu, Y., Lee, T. S., Wong, K. M., & Wu, S. (2019). Complementary congruent and opposite neurons achieve concurrent multisensory integration and segregation. eLife, 8, e43753.

Reviewer 2

This paper proposes a Bayesian model for how neural circuits decide whether observed stimulus features arose from a common stimulus (i.e., under a cue integration model) or whether observations arose from unrelated stimuli (under a cue segregation model). The idea is that a neural circuit can perform model selection by computing Bayes factors, which favour an integration model when the disparity between cues is small. The computation of Bayes factors is implemented by two populations of neurons: a population of "congruent" neurons and a population of "opposite" neurons. Such populations have been observed experimentally, and the model provides a parsimonious explanation of the experimental data. The main contribution of the paper is a derivation of the Bayes factor for a population of Poisson-spiking neurons using a number of analytically convenient assumptions and approximations, and a computational model illustrating how a component of the Bayes factor can be implemented in neural networks. The modelling work is interesting and to a high standard, and I believe provides novel insight (both analytic and geometric) into the conditions under which integration vs segregation is favoured. The authors suggest that their work provides the first rigorous formulation of how the opposite neurons could play a central role in the encoding of Bayes factors. One drawback of the work, as the authors acknowledge, is that they only provide a network model that calculates one component of the Bayes factor, and it is unclear how the remaining components (the Occam factors) could be calculated by the network. I found that this point was not sufficiently addressed until the final sentence of the discussion. This is at odds with the abstract, which seems to imply that the authors had a complete implementation. The authors could be more transparent about this. Also, at times the writing was rather unclear, occasionally with words seemingly mistakenly omitted (e.g. on line 35); the paper could benefit from further proof reading.

Reviewer 3

The authors describe a way to compute Bayes factors that could be implemented by neurons to carry out model/causal inference. My primary concern with this paper is that while the authors describe a plausible inference rule for neuronal computation, they do not actually present any empirical evidence to support their claim. The authors argue that the proposed inference method may be implemented by "opposite neurons". While I do agree that "opposite neurons" support the plausibility of the authors' arguments, I feel that for a NeurIPS submission stronger evidence would be warranted. As such, I think this is a decent paper with some interesting ideas, but that is not sufficiently comprehensive for NeurIPS. On a side note, the authors appear to equate Bayes factor computation with causal inference. I would recommend a more careful wording here. In my opinion, computing Bayes factors may -- but does not need to be -- one step in a causal inference algorithm. Update after rebuttal/discussion: The authors and other reviewers have convinced me that there is value in this work without strong empirical evidence. I have raised my score from 4 to 6.