Process models constitute essential tools in business process management and software engineering for representing and managing real-world business processes. Hence, the proper comprehension of these models is crucial for enabling an effective and efficient communication among stakeholders. While several studies have examined factors affecting process model comprehension, such as the used modeling notation or process complexity, there is a lack of in-depth research on the cognitive processes important to comprehend process models deeper. This systematic literature review explores the cognitive mechanisms underlying process model comprehension by integrating insights from relevant disciplines such as cognitive neuroscience and psychology. Key areas of cognition include perception, attention, memory, language, problem solving, emotion, and metacognition. The review was conducted following the well-established Kitchenham methodology and included an extensive search in the following digital libraries (Web of Science, IEEE Xplore, ScienceDirect, ACM Digital Library, PubMed, and Springer-Link). By analyzing 47 studies, the literature review identifies gaps in current research, emphasizing the need for further investigation into these cognitive processes to improve model comprehensibility.