To reuse existing digital learning resources, a teacher has to perform a complex and creative in order to adapt them to the students' needs and fit their cognitive abilities. Nevertheless, metadata do not generally describe the cognitive facets of a digital learning resource. We give here a partial account of a trace-based system in its current state of development to index digital learning resources according to the metadata describing their "cognitive features". By "cognitive feature" we mean the cognitive activities (e.g., reading, listening, body interactions, etc.) associated with different message formats (e.g., text, audio, interactive animations, etc.).