Out and about: Subsequent memory effect captured in a natural outdoor environment with smartphone EEG

Spatiotemporal context plays an important role in episodic memory. While temporal context effects have been frequently studied in the laboratory, ecologically valid spatial context manipulations are difficult to implement in stationary conditions. We investigated whether the neural correlates of successful encoding (subsequent memory effect) can be captured in a real-world environment. An off-the-shelf Android smartphone was used for wireless mobile EEG acquisition and stimulus presentation. Participants encoded single words, each of which was presented at a different location on a university campus. Locations were approximately 10-12 m away from each other, half of them with striking features (landmarks) nearby. We predicted landmarks would improve recall performance. After a first free recall task of verbal stimuli indoors, participants performed a subsequent recall outdoors, in which words and locations were recalled. As predicted, significantly more words presented at landmark locations as well as significantly more landmark than nonlandmark locations were recalled. ERP analysis yielded a larger posterior positive deflection during encoding for hits compared to misses in the 400-800 ms interval. Likewise, time-frequency analysis revealed a significant difference during encoding for hits compared to misses in the form of stronger alpha (200-300 ms) and theta (300-400 ms) power increases. Our results confirm that a vibrant spatial context is beneficial in episodic memory processing and that the underlying neural correlates can be captured with unobtrusive smartphone EEG technology. The advent of mobile EEG technology promises to unveil the relevance of natural physical activity and natural environments on memory.