Memory Strength Differently Affects Pupil Dilation during Recognition and Cued-Recall

Beside its well-known link to mental effort, pupil dilation (PD) can be also informative regarding the cognitive and neurobiological background of episodic memory. It has been shown, for example, that the magnitude of PD during retrieval signals the strength of a memory trace. Importantly, however, this link might be mediated by the specific memory test. In the case of direct and automatic forms of memory retrieval (e.g. recognition memory), the magnitude of retrieval-related PD might reflect the aggregate strength of the memory trace. In contrast, in more effortfull, indirect forms of retrieval (e.g.cued-recall), PD might be related to mental effort, and thus higher memory strength might be associated with less mental effort and consequently attenuated PD. In our study, we tested these predictions by probing the cue-target associations by either a recognition or by a cued-recall task and simultaneously measuring the PD triggered by the retrieval process. Memory strength was manipulated by encoding the cue-target associations either once or twice. As predicted, our results showed that higher memory strength was associated with attenuated PD in the cued-recall task. In contrast, memory strength did not predict PD in our recognition memory task, which suggest that PD in recognition memory might be associated with the recollection-familiarty dichotomy and not memory strength per se. Our results show that the type of the specific memory test influences how the strength of a memory trace can be assessed using pupillometry.