MEG frequency tagging reveals a grid-like code during attentional movements.

Grid-cells firing fields tile the environment with a 6-fold periodicity during both locomotion and visual exploration. Here, we tested, in humans, whether movements of covert attention elicit grid-like coding using frequency tagging. Participants observed visual trajectories presented sequentially at fixed rate, allowing different spatial periodicities (e.g., 4-, 6-, and 8-fold) to have corresponding temporal periodicities (e.g., 1, 1.5, and 2 Hz), thus resulting in distinct spectral responses. We found a higher response for the (grid-like) 6-fold periodicity and localized this effect in medial-temporal sources. In a control experiment featuring the same temporal periodicity but lacking spatial structure, the 6-fold effect did not emerge, suggesting its dependency on spatial movements of attention. We report evidence that grid-like signals in the human medial-temporal lobe can be elicited by covert attentional movements and suggest that attentional coding may provide a suitable mechanism to support the activation of cognitive maps during conceptual navigation.

Spatiotemporal properties of the neural representation of conceptual content for words and pictures - an MEG study.

The entwined nature of perceptual and conceptual processes renders an understanding of the interplay between perceptual recognition and conceptual access a continuing challenge. Here, to disentangle perceptual and conceptual processing in the brain, we combine magnetoencephalography (MEG), picture and word presentation and representational similarity analysis (RSA). We replicate previous findings of early and robust sensitivity to semantic distances between objects presented as pictures and show earlier (~105 â€‹msec), but not later, representations can be accounted for by contemporary computer models of visual similarity (AlexNet). Conceptual content for word stimuli is reliably present in two temporal clusters, the first ranging from 230 to 335 â€‹msec, the second from 360 to â€‹585 msec. The time-course of picture induced semantic content and the spatial location of conceptual representation were highly convergent, and the spatial distribution of both differed from that of words. While this may reflect differences in picture and word induced conceptual access, this underscores potential confounds in visual perceptual and conceptual processing. On the other hand, using the stringent criterion that neural and conceptual spaces must align, the robust representation of semantic content by 230-240 msec for visually unconfounded word stimuli significantly advances estimates of the timeline of semantic access and its orthographic and lexical precursors.