The order, but not the structure, of cross-domain learning influences memory consolidation

Everyday activities often require learning sequences that necessitate the involvement of both the declarative and the procedural memory domains. Previous research has shown that a learning structure that is common across tasks from different domains can improve learning and resistance to interference. However, it remains unknown whether such shared learning structure can enhance longer-term memory retention. To address this question, forty-eight healthy adults participated in a pre-registered study in which they learned both an object sequence task (declarative learning) and a motor sequence task (procedural learning) in two separate sessions separated by 4h. Participants were assigned to either an associated group, where the two tasks shared a common learning structure - that consisted of a specific mapping between finger movements and object categories across learning sessions - or an unassociated group with no such cross-domain shared structure. Memory retention was assessed with a 24h retest session on both tasks. Contrary to our predictions, a shared higher-order structure between tasks from different domains did not enhance memory retention. Exploratory follow-up analyses revealed that the order the tasks were learned (i.e., object or motor first), rather than their structural overlap, influenced performance. Specifically, learning the motor task before the object task impaired the consolidation of the object task irrespective of whether the tasks shared a common learning structure or not. This effect was unidirectional as learning the object task before the motor task had no effect on the consolidation of the motor task. Altogether, the current findings suggest that the order of cross-domain learning experiences rather than their structure influences memory consolidation.