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Title: Why do These Match? Explaining the Behavior of Image Similarity Models

Paper content:
Explaining a deep learning model can help users understand its behavior and allow researchers to discern its shortcomings.
Recent work has primarily focused on explaining models for tasks like image classification or visual question answering.
In this paper, we introduce an explanation approach for image similarity models, where a model's output is a score measuring the similarity of two inputs rather than a classification.
In this task, an explanation depends on both of the input images, so standard methods do not apply.
We propose an explanation method that pairs a saliency map identifying important image regions with an attribute that best explains the match.
We find that our explanations provide additional information not typically captured by saliency maps alone, and can also improve performance on the classic task of attribute recognition.
Our approach's ability to generalize is demonstrated on two datasets from diverse domains, Polyvore Outfits and Animals with Attributes 2.
Many problems in artificial intelligence that require reasoning about complex relationships can be solved by learning some feature embedding to measure similarity between images and/or other modalities such as text.
Examples of these tasks include scoring fashion compatibility (Han et al., 2017b; Hsiao & Grauman, 2018; Vasileva et al., 2018) , image retrieval (Kiapour et al., 2015; Radenovi et al., 2018; Yelamarthi et al., 2018) , or zero-shot recognition (Bansal et al., 2018; Li et al., 2018b; Wang et al., 2018) .
Reasoning about the behavior of similarity models can aid researchers in identifying potential improvements, show where two images differ for anomaly detection, promote diversity in fashion recommendation by ensuring different traits are most prominent in the top results, or simply help users understand the model's predictions which can build trust (Teach & Shortliffe, 1981) .
However, prior work on producing explanations for neural networks has primarily focused on explaining classification models (e.g. (Fong & Vedaldi, 2017; Nguyen et al., 2016; Petsiuk et al., 2018; Ribeiro et al., 2016; Selvaraju et al., 2017; Zeiler & Fergus, 2014) ) and does not directly apply to similarity models.
Given a single input image, such methods produce a saliency map which identifies pixels that played a significant role towards a particular class prediction (see Figure 1a for an example).
On the other hand, a similarity model requires at least two images to produce a score.
The interaction between both images defines which features are more important, so replacing just one of the images can result in identifying different salient traits.
Another limitation of existing work is that the saliency alone may be insufficient as an explanation of (dis)similarity.
For image pairs where similarity is determined by the presence or absence of an object, a saliency map may be enough to understand model behavior.
However, when we consider the image pair in Figure 1b , highlighting the necklace as the region that contributes most to the similarity score is reasonable, but uninformative given that there are no other objects in the image.
Instead, what is important is the fact that the necklace shares a similar color with the ring.
Whether these attributes or salient parts are a better fit as an explanation is not determined by the image domain (i.e. attributes for e-commerce imagery vs. saliency for natural imagery), but instead by the images themselves.
For example, an image can be matched as formal-wear because of a shirt's collar (salient part), while two images of animals can match because both have stripes (attribute).
Guided by this intuition, we introduce Salient Attributes for Network Explanation (SANE).
Our approach generates a saliency map to explain a model's similarity score, paired with an attribute explanation that identifies important image properties.
SANE is a "black box" method, meaning it Existing explanation methods focus on image classification problems (left), whereas we explore explanations for image similarity models (right).
We pair a saliency map, which identifies important image regions, but often provides little useful information, with an attribute (e.g., golden), which is more human-interpretable and, thus, a better explanation than saliency alone.
can explain any network architecture and only needs to measure changes to a similarity score when provided with different inputs.
Unlike a standard classifier, which simply predicts the most likely attributes for a given image, our explanation method predicts which attributes are important for the similarity score predicted by a model.
Predictions are made for each image in a pair, and allowed to be non-symmetric, e.g., the explanation for why the ring in Figure 1b matches the necklace may be that it contains "black", even though the explanation for why the necklace matches the ring could be that it is "golden."
A different similarity model may also result in different attributes being deemed important for the same pair of images.
SANE combines three major components: an attribute predictor, a prior on the suitability of each attribute as an explanation, and a saliency map generator.
Our underlying assumption is that at least one of the attributes present in each image should be able to explain the similarity score assigned to the pair.
Given an input image, the attribute predictor outputs a confidence score and activation map for each attribute, while the saliency map generator produces regions important for the match.
During training, SANE encourages overlap between the similarity saliency and attribute activation.
At test time, we rank attributes as explanations for an image pair based on a weighted sum of this attribute-saliency map matching score, the explanation suitability prior of the attribute, and the likelihood that the attribute is present in the image.
Although we evaluate only the top-ranked attribute in our experiments, in practice more than one attribute could be used to explain a similarity score.
We find that using saliency maps as supervision for the attribute activation maps during training not only improves the attribute-saliency matching, resulting in better attribute explanations, but also boosts attribute recognition performance using standard metrics like average precision.
We evaluate several candidate saliency map generation methods which are primarily adaptations of "black box" approaches that do not rely on a particular model architecture or require access to network parameters to produce a saliency map (Fong & Vedaldi, 2017; Petsiuk et al., 2018; Ribeiro et al., 2016; Zeiler & Fergus, 2014) .
These methods generally identify important regions by measuring a change in the output class score resulting from some perturbation of the input image.
Similarity models, however, typically rely on a learned embedding space to reason about relationships between images, where proximity between points or the lack thereof indicates some degree of correspondence.
An explanation system for embedding models must, therefore, consider how distances between embedded points, and thus their similarity, change based on perturbing one or both of the input images.
We explore two strategies for adapting these approaches to our task.
First, we manipulate just a single image (the one we wish to produce an explanation for) while keeping the other image fixed.
Second, we manipulate both images to allow for more complex interactions between the pair.
See Section 3.2 for additional details and discussion on the ramifications of this choice.
Our paper makes the following contributions:
1) we provide the the first quantitative study of explaining the behavior of image similarity models;
2) we propose a novel explanation approach that combines saliency maps and attributes;
3) we validate our method with metrics designed to link our explanations to model performance, and find that it produces more informative explanations than adaptations of prior work to this task and also improves attribute recognition performance.
In this paper we introduced SANE, a method of explaining an image similarity model's behavior by identifying attributes that were important to the similarity score paired with saliency maps indicating import image regions.
We confirmed humans believe our explanations are useful for explaining a model's behavior, which could help build trust, to supplement automatic metrics.
In future work we believe closely integrating the saliency generator and attribute explanation model, enabling each component to take advantage of the predictions of the other, would help improve performance.

assistant: text: The paper proposes an explanation mechanism that pairs the typical saliency map regions together with attributes for similarity matching deep neural networks.