Grid-like representations in rodents and models

Hosted by Loreen Hertäg

External speaker

Noam Almog

Position: PhD student, Kavli Institute for Systems Neuroscience

Title: The human grid-like signal from a roden grid cell perspective

Since their discovery in 2005 (Hafting et al.), rodent grid cells and their 6-symmetric spatial firing pattern are one of the most studied phenomena in neuroscience. A study published in 2010 (Doeller et al.) described a 6-symmetric activity pattern in the fMRI BOLD signal with respect to the movement direction of a human subject in VR, now known as the hexadirectional signal. With both patterns being 6-symmetric and ostensibly occurring in homologous regions of the cortex, it is easy to assume that individual grid cell activity will naturally lead to a population hexadirectional signal detectable in fMRI. However, on closer inspection this hypothetical relationship is not so straightforward. In this work we use geometric deduction, rodent data, and neural simulation to investigate the underpinnings of this hypothetical relationship. We also look for evidence of this relationship in rodent data, and what mechanisms need to be present for a grid cell-based hexadirectional signal to be detectable in a noisy neural population measurement such as fMRI BOLD.

Local Berlin speaker

Ikhwan Bin Khalid

Position: PhD student, AG Kempter, Institute for Theoretical Neuroscience

Title: Quantitative modeling of the emergence of macroscopic grid-like representations

Grid cells are neurons in the entorhinal cortex that are thought to perform computations in support of spatial navigation. As direct recordings of grid cells from the human brain are only rarely possible, functional magnetic resonance imaging (fMRI) studies proposed and described an indirect measure of entorhinal grid-cell activity, which is quantified as a hexadirectional modulation of fMRI activity as a function of the subject’s movement direction through a virtual environment. However, the contributing role of the aggregated activity of grid cells to this modulation remains unclear. Our research addresses the unresolved question concerning the origin of hexadirectional modulation of activity in the entorhinal cortex, as observed in fMRI, iEEG, and MEG studies (e.g. Doeller et al., Nature, 2010; Staudigl et al., Curr Biol, 2018; Convertino et al., Brain, 2023). Here, we explored three hypotheses through both numerical simulations and analytical calculations: head-direction tuning (conjunctive grid by head-direction cell hypothesis); firing-rate adaptation (repetition suppression hypothesis); or a bias towards a certain grid phase offset (structure-function mapping hypothesis). Our findings indicate that, in principle, all three hypotheses can account for hexadirectional modulation of sum grid-cell activity in ideal conditions. However, when including grid-cell properties found in the literature, our simulations most strongly support the conjunctive grid by head-direction cell hypothesis. In contrast, our simulations do not support the structure-function mapping hypothesis. With respect to the repetition-suppression hypothesis, our simulations are insufficient to substantiate or refute it, and further experiments on the adaptation properties of single grid cells are required.

 

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This event is part of the PostDoc Network Speaker Series.

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