  Language models retain a significant amount of world knowledge from their
pre-training stage. This allows knowledgeable models to be applied to
knowledge-intensive tasks prevalent in information retrieval, such as ranking
or question answering. Understanding how and which factual information is
acquired by our models is necessary to build responsible models. However,
limited work has been done to understand the effect of pre-training tasks on
the amount of knowledge captured and forgotten by language models during
pre-training. Building a better understanding of knowledge acquisition is the
goal of this paper. Therefore, we utilize a selection of pre-training tasks to
infuse knowledge into our model. In the following steps, we test the model's
knowledge retention by measuring its ability to answer factual questions. Our
experiments show that masking entities and principled masking of correlated
spans based on pointwise mutual information lead to more factual knowledge
being retained than masking random tokens. Our findings demonstrate that, like
the ability to perform a task, the (factual) knowledge acquired from being
trained on that task is forgotten when a model is trained to perform another
task (catastrophic forgetting) and how to prevent this phenomenon. To foster
reproducibility, the code, as well as the data used in this paper, are openly
available.
