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Poster Session C: Friday, August 15, 2:00 – 5:00 pm, de Brug & E‑Hall

The Role of Landmarks and Travel Pauses in the Generation of Hexadirectional fMRI Signals during Spatial Navigation

Siyuan Mei¹, Ziwei Wei, Martin Stemmler, Andreas Herz², Xiaoli Chen; ¹University of Munich, Ludwig-Maximilians-Universität München, ²Ludwig-Maximilians-Universität München

Presenter: Siyuan Mei

As humans navigate in physical, conceptual, or even social domains, functional magnetic resonance imaging (fMRI) signals from the hippocampal formation vary with movement directions, fluctuating in strength with a periodicity of 60 degrees. While grid cells are believed to underlie these hexadirectional fMRI signals, the exact mechanism linking the two is unclear. Three hypotheses have been proposed: conjunctive grid-by-head-direction cells, repetition suppression, and nonlinear transformations of grid-cell activity into BOLD signals. We aim to combine theoretical analysis and fMRI experiments and identify key design parameters for distinguishing among the three hypotheses. Two critical factors emerge from our analysis: the presence of landmarks as a precondition, and pauses between linear path segments as a way to distinguish among the hypotheses. First, we modeled the trajectories represented by grid-cell activity using an extended Kalman filter fitted to behavioral data. Under all three hypotheses, hexadirectional signals emerge when landmarks are present, reflecting the landmarks' role in providing correct directional information. With correct directions, the theoretical analysis further predicts that the pauses measurably increase the hexadirectional strength under the repetition suppression hypothesis, diminish it under the nonlinearity hypothesis, and have no effect under the conjunctive grid-by-head-direction cell hypothesis. We are conducting experiments to examine the predictions.

Topic Area: Memory, Spatial Cognition & Skill Learning

Extended Abstract: Full Text PDF