Paper:
Attentional Pooling for Action Recognition
We introduce a simple yet surprisingly powerful model to incorporate attention in action recognition and human object interaction tasks. Our proposed attention module can be trained with or without extra supervision, and gives a sizable boost in accuracy while keeping the network size and computational cost nearly the same. It leads to significant improvements over state of the art base architecture on three standard action recognition benchmarks across still images and videos, and establishes new state of the art on MPII dataset with 12.5% relative improvement. We also perform an extensive analysis of our attention module both empirically and analytically. In terms of the latter, we introduce a novel derivation of bottom-up and top-down attention as low-rank approximations of bilinear pooling methods (typically used for fine-grained classification). From this perspective, our attention formulation suggests a novel characterization of action recognition as a fine-grained recognition problem.

Review:
The authors propose to use a low rank approximation of second order pooling features for action recognition. They show results on three compelling vision benchmarks for action recognition and show improvements over the baseline. 
Pros:
(+) The proposed method is simple and can be applied on top of any network architecture
(+) Clear improvement over baseline 
Cons:
(-) Unclear novelty over well known second order pooling approaches
(-) Fair comparison with other attention based models is missing
It is unclear to me how the proposed approach is novel compared to other low rank second order pooling methods, typically used for fine grained recognition (e.g. Lin et al, ICCV2015), semantic segmentation etc. While the authors do a decent job walking us through the linear algebra and interpreting the variables used, the final approach is merely a classifier on second order features.
In addition, the authors make design choices that are not clearly justified. For multi class recognition, the authors do not justify the choice for a class agnoistic b but for class-specific a. These experiments should be added in order to prove the effectiveness of the proposed design. 
The authors claim that they get rid of the box detection step present at other approaches such as R*CNN or Mallya & Lazebnik. However, they do not discuss how their approach would generalize to instance-specific action recognition. An image can contain many people who perform different actions. In this case, the action labels are not assigned at the frame level, but at an instance level. R*CNN was designed for instance-level recognition, thus the necessity of the person box detection step. The authors should discuss how the proposed approach can handle this more generic setup of instance-level labels? This is a limitation of the current approach, rather than a benefit as implied in the Related Work section.
The authors fail to provide a fair comparison with competing approaches. The authors use a ResNet101 or an Inception-V2 architecture for their base network. They show improvement over their own non-attentive baseline, using the same architecture. However, the competing approaches, such as R*CNN or Mallya & Lazebnik all use a VGG16 network. This makes comparison with these approaches and the current approach unfair and inconclusive on MPII and HICO dataset. On HMDB, the improvement over the TSN BN-inception method is small while comparisons with other methods is unclear due to the varying underlying base architectures.