How the size of the to-be-learned material influences the encoding and later retrieval of associative memories: A pupillometric assessment

Pupillometry have recently added valuable insights about the cognitive and possible neurobiological processes underlying episodic memory. Most of the studies, however, investigated recognition memory, which only partially relies on cue-driven recollection, the hallmark feature of episodic memory. Here we measured pupil size during a paired associate learning task, where participants encoded word-pairs, and after a short delay they took part in a cued recall. Importantly, we manipulated the size of the learning set: participants either learnt two, four or eight word-pairs in a row. As expected, increasing set size resulted in larger forgetting, assumingly as a consequence of weaker memory strength for the word-pairs. Our results show an important difference between pupil size changes observed during encoding and retrieval. During retrieval, the pupil instantly begun to dilate, as a sign of increased processing load accompanying the retrieval of the target memory. Importantly, large set size was also associated with larger pupil dilation during retrieval. This supports the notion that pupil dilation can be regarded as a marker of memory strength. In contrast, during encoding, pupil dilation decreased with increasing amount of encoded information, which might be due to the overuse of attentional resources. Furthermore, we also found that serial position during encoding modulated subsequent memory effects: for the first three serial positions, successful recall was predicted by larger pupil dilation during encoding, whereas such subsequent memory effect was absent for later serial positions. These results suggest that the amount of information independently modulates pupil dilation during encoding and retrieval, and support the assumption that pupillometric investigation of paired associate learning could be an informative way to investigate the cognitive and neurobiological processes of episodic memory.