USING OPM-MEG TO ASSESS RETRIEVAL DYNAMICS OF HOLISTIC EPISODIC EVENTS

Recalling your last birthday, you can remember who attended, where it was, and what gifts you received. This ability to retrieve all elements of multi-element events (e.g., A-B-C triplets) is called holistic retrieval and can be measured behaviourally with retrieval dependency – the probability that individual elements are (un)successfully retrieved together (Horner & Burgess, 2013). fMRI has shown that holistic retrieval is accompanied by the reinstatement of cortical activity for targets (e.g., cue-A, retrieve-B) and nontargets (element-C; Horner et al., 2015). However, the temporal dynamics of holistic retrieval are unknown. We used multivariate pattern analysis of OPM-MEG to temporally track event-element reinstatement of ‘holistic’ event representations. Participants studied events composed of coloured images of person-location-object triplets appearing either simultaneously in ‘closed-loops’ (where integration occurs) or as two overlapping pairwise associations in ‘open-loops’ (where integration does not occur). To-be-encoded images were first shown individually for training element-wise Linear Discriminant Analysis classifiers. Participants’ memory was tested on all pairwise associations. Test trials showed one event element as a cue paired with a prompt requiring retrieval of another associated element (e.g., cueing a specific person and asking ‘where?’ to retrieve the associated location). Participants then chose the target among three foils. Classifiers decoded brain patterns during cue presentation to assess reinstatement. As expected, in open-loops targets were reinstated more than nontargets ~500-800-ms. In closed-loops, they did not differ initially but diverged ~1,000-ms. Data suggest that holistic retrieval drives the early reinstatement of targets and nontargets, which are then excluded from later retrieval processing.