In a recent blog, I described an experiment to train a deep neural network to distinguish between photographs of Vincenzo Nibali and Alejandro Valverde, using a very small data set of images. In the conclusion, I suggested that the network was probably basing its decisions more on the colours of the riders’ kit rather than on facial recognition. This article investigates what the network was actually “looking at”, in order to understand better how it was making decisions.
The issues of accountability and bias were among the topics discussed at the last NIPS conference. As machine learning algorithms are adopted across industry, it is important for companies to be able to explain how conclusions are reached. In many instances, it is not acceptable simply to rely on an impenetrable black box. AI researchers and developers need to be able to explain what is going on inside their models, in order to justify decisions taken. In doing so, some worrying instances of bias have been revealed in the selection of data used to train the algorithms.
I went back to my rider recognition model and used an approach called “Class Activation Maps” to identify which parts of the images accounted for the network’s choice of rider. Making use of the code provided in lesson 7 of the course offered by fast.ai, I took advantage of my existing small set of training, validation and test images of the two famous cyclists. Starting with a pre-trained version of ResNet34, the idea was to replace the last two layers with four new ones, the crucial one being a convolutional layer with two outputs, matching the number of cyclists in the classification task. The two outputs of this layer were 7×7 matrix representations of the relevant image.
The final predictions of the model came from a softmax of a flattened average pooling of these 7×7 representations. The softmax output gave the probabilities of Nibali and Valverde respectively. Since there was no learning beyond the final convolution, the activations of the two 7×7 matrices represented the “Nibali-ness” and “Valverde-ness” of the image. This could be displayed as a heat map on top of the image.
Examples are shown below for the validation set of 10 images of Nibali followed by 10 of Valverde. The yellow patch of the heat map highlights the part of the image that led to the prediction displayed above each image. Nine out of ten were correct for Nibali and six for Valverde.
The heat maps were very helpful in understanding the model’s decision making process. It seemed that for Nibali, his face and helmet were important, with some attention paid to the upper part of his blue Astana kit. In contrast, the network did a very good job at identifying the M on Valverde’s Moviestar kit. It was interesting to note that the network succeeded in spotting that Nibali was wearing a Specialized helmet whereas Valverde had a Catlike design. Three errors arose in the photos of his face, which was mistaken for Nibali’s. In fact, any picture of a face led to a prediction of Nibali, as demonstrated by the cropped image below that was used for training.
Why should that be? Looking back at the training set, it turned out that, by chance, there were far more mugshots of Nibali, while there were more photos of Valverde riding his bike, with his face obscured by sunglasses. This was an example of unintentional bias in the training data, providing a very useful lesson.
The final set of pictures shows the predictions made on the out-of-sample test set. All the predictions are correct, except the first one, where the model failed to spot the green M on Valverde’s chest and mistook the blurred background for Nibali. Otherwise the results confirmed that the network looked at Nibali’s face, the rider’s helmet or Valverde’s kit. It also remembered seeing an image of Nibali holding the Giro trophy in the training set.
In conclusion, Class Activation Maps provide a useful way of visualising the activations of hidden laters in a deep neural network. This can go some way to accounting for the decisions that appear in the output. The approach can also help identify unintentional bias in the training set.