Humans navigating the real world were shown by Graves et al. (2023) to represent their location, heading direction, and walking speed in local field potentials (LFP) recorded from the hippocampus with chronic brain implants, on a linear track. The source's discussion raises unresolved mechanistic questions: whether real-world navigation coding is theta-mediated or whether delta-band activity also contributes given that clinical hardware filters may attenuate delta; what motor, proprioceptive, sensory-cue, and task-demand signals explain why real-world coding appears in theta while virtual-navigation coding appears in delta; whether the multiplexed codes within single LFP channels arise from distinct neural populations or from joint representation by shared populations; what drives the session-to-session instability of location codes, potentially hippocampal remapping; and what mechanism underlies the inverse relationship between direction and speed coding. Because the study tested a single linear track and retested only four patients longitudinally, it further remains open whether the codes generalize to two-dimensional environments and whether the stability findings hold in larger samples. Later work (Seeber et al. 2025 and related mobile intracranial-EEG studies) partially addresses two-dimensional position coding and delta contributions, but the cellular substrate, remapping drivers, direction-speed interaction, whether delta contributes under clinical filter settings, and larger-sample stability remain open.

Background

The hippocampus, a medial temporal lobe structure essential for spatial memory, contains neurons with strong spatial tuning in rodents: place cells that fire at specific locations, head-direction cells that fire when the animal faces a particular direction, and speed cells whose firing tracks running speed. The local field potential (LFP), the summed electrical activity of local neural populations recorded by intracranial electroencephalography (iEEG), can also carry these signals; in rodents, position can be reconstructed from hippocampal LFP almost as well as from spiking. Human studies of hippocampal navigation coding have historically been confined to virtual navigation, because hospital-based iEEG in epilepsy patients keeps them stationary; a widely replicated finding from that literature is that spatial coding in virtual navigation manifests mainly in the delta band (roughly 1-4 Hz) rather than the theta band (roughly 4-12 Hz) prominent in freely moving rodents. A line of mobile-recording work then showed that theta in the human medial temporal lobe tracks real ambulatory movement, boundaries, and distances (Aghajan et al. 2017; Stangl et al. 2021; Bohbot et al. 2017), and new wireless/ambulatory implant technology (Topalovic et al. 2020) made fully mobile deep recordings feasible. Graves et al. (2023, bioRxiv) recorded hippocampal LFP from ambulatory epilepsy patients with chronic clinical neurostimulator implants (a responsive neurostimulation, RNS, device whose electrode placement and filter settings are fixed for clinical purposes) while they walked a real-world linear track. They found that heading direction and location were decodable from spectral patterns in the theta band (4-12 Hz in their analysis; delta defined as 1-4 Hz), speed from broadband activity, that many single channels multiplexed several of these codes, that direction and speed coding generalized across sessions days-to-months apart in the four retested patients while location coding did not (a possible signature of remapping, the reorganization of hippocampal spatial representations across sessions or contexts), and that direction and speed coding strength were negatively correlated across channels, hinting at competition between the two codes. Human single-neuron navigation data exist only for stationary/virtual paradigms (Ekstrom et al. 2003 and subsequent work), so no real-world human cellular data are available.

Problem Statement

Determine the neural mechanisms and the generality of the spatial codes that the human hippocampus expresses during real-world navigation, as consolidated from the open issues stated by Graves et al. (2023) and from the generality limits of that study's design (a single linear track, four longitudinally retested patients). The single overarching objective is to explain, at the levels of spectral mechanism, cellular substrate, and circuit interaction, how location, direction, and speed are coded in human hippocampal recordings during real ambulatory navigation, and how far those codes generalize. The single objective has the following seven facets, which are aspects of one integrated account rather than interchangeable or optional alternatives: (1) the spectral basis — whether real-world navigation coding is primarily theta-mediated or whether delta-band activity also contributes, given that the clinical implant recording filters, fixed for clinical purposes, may attenuate delta (1-4 Hz) activity; (2) the origin of the frequency difference between real-world (theta) and virtual (delta) navigation, i.e., disentangling the contributions of motor and proprioceptive signals, sensory-cue availability, and task demands; (3) the cellular basis of the multiplexing of location, direction, and speed codes within single LFP channels — whether distinct neural populations underlie each code or a shared population jointly represents multiple variables, potentially as distinct dynamical modes; single-unit or multi-unit recordings during real-world human navigation that could settle this directly do not yet exist; (4) the factors driving the session-to-session instability of location coding, i.e., whether it is hippocampal remapping and what roles sensory conflict, hippocampal subregion (for example CA1 versus CA3), and individual differences play in humans; (5) the mechanism of the inverse relationship between direction and speed coding strength across channels, such as resource allocation or shared-circuit competition; (6) whether the location, direction, and speed codes established on a linear track generalize to two-dimensional environments (position/theta coding has partial support, direction and speed coding are untested); and (7) whether the longitudinal stability findings hold in samples larger than the four retested patients. A resolution is the single integrated account of the mechanisms and their generality that addresses all seven facets jointly; an account that covers only a subset of the facets does not resolve the task. Facets whose decisive observations have not yet been made — the real-world single-unit recordings of (3), direction and speed coding in the two-dimensional environments of (6), and the larger samples of (7) — are satisfied by reasoned mechanistic inference and by specifying the decisive experiments, not only by measured results.

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