  Convolutional Neural Networks (CNNs) are frequently and successfully used in
medical prediction tasks. They are often used in combination with transfer
learning, leading to improved performance when training data for the task are
scarce. The resulting models are highly complex and typically do not provide
any insight into their predictive mechanisms, motivating the field of
'explainable' artificial intelligence (XAI). However, previous studies have
rarely quantitatively evaluated the 'explanation performance' of XAI methods
against ground-truth data, and transfer learning and its influence on objective
measures of explanation performance has not been investigated. Here, we propose
a benchmark dataset that allows for quantifying explanation performance in a
realistic magnetic resonance imaging (MRI) classification task. We employ this
benchmark to understand the influence of transfer learning on the quality of
explanations. Experimental results show that popular XAI methods applied to the
same underlying model differ vastly in performance, even when considering only
correctly classified examples. We further observe that explanation performance
strongly depends on the task used for pre-training and the number of CNN layers
pre-trained. These results hold after correcting for a substantial correlation
between explanation and classification performance.
