Biology subjects

Stemmler, M. B.

Publications and source records attributed to Stemmler, M. B..

3 recordsLinked to original sources

bioRxiv2026

On the neural origin of hexadirectional fMRI modulations

When human beings navigate through an environment, a conceptual space, memories or even imagined spaces, fMRI signals in select brain regions vary with movement direction, alternating in strength with a periodicity of 60 degrees. At the cellular level, certain neurons called grid-cells form a hexagonal lattice representation of physical space. Given that the axes of the lattices align across neurons, the concerted directional and spatial firing of populations of these neurons may form the basis of the hexadirectional modulation of the fMRI signal. However, the exact link between the two remains unclear. Multiple mechanisms have been proposed that could couple neuronal activity to the fMRI signal. These include (1) preferential neuronal discharge when the direction of motion coincides with the lattice axes, (2) repetition suppression when multiple fields are traversed in succession, or alternatively, (3) a nonlinear transduction step. Here, we propose targeted experiments that can distinguish these mechanisms, for which we assess the expected effect sizes via simulations. In particular, by introducing calibrated pauses between linear path segments, it is possible to differentiate the repetition-suppression hypothesis from the nonlinear transduction model. By additionally varying the subjects movement speed and path lengths, the proposed protocol can distinguish conjunctive neuronal tuning (in which neurons respond to both speed and location or direction and location) from the other hypotheses. Crucially, if either the repetition-suppression or the nonlinear-transduction hypothesis holds, then the scale of the lattices will play a role. According to our analysis, hexadirectional signals should emerge only when a linear path segment is longer than half of the spatial lattice period. As grid cells with different scales are grouped into distinct modules along the dorso-ventral axis of the entorhinal cortex, this may make it possible to non-invasively measure the hierarchy of grid scales in the human brain.

neuroscience↗

bioRxiv2025

A multi-ring shifter network computes head direction in zebrafish

From insects to fish to mammals, many species have an internal compass: a set of recurrently connected neurons that combine motor feedback, vestibular signals, and external cues to compute the animals heading direction. Whether the underlying mechanism is universal across different species is unresolved. In Drosophila, for instance, the central complex contains three connected, yet anatomically separate, functional neuron rings. Two rings receive countervailing velocity signals that shift neuronal activity bumps around all three rings. An alternative, classical continuous-attractor model invokes only one ring but requires velocity-modulated synaptic connections. A single-ring compass has been discovered in the anterior hindbrain of zebrafish. Using theory and experiments, we find, however, that three rings, including two shifter rings, are intermingled on the same anatomical scaffold. Zebrafish and Drosophila, therefore, use the same basic compass mechanism, suggestive of convergent evolution.

neuroscience↗

bioRxiv2019

Spike afterpotentials shape the in-vivo burst activity of principal cells in medial entorhinal cortex

Principal neurons in rodent medial entorhinal cortex (MEC) generate high-frequency bursts during natural behavior. While in vitro studies point to potential mechanisms that could support such burst sequences, it remains unclear whether these mechanisms are effective under in-vivo conditions. In this study, we focused on the membrane-potential dynamics immediately following action potentials, as measured in whole-cell recordings from male mice running in virtual corridors (Domnisoru et al., 2013). These afterpotentials consisted either of a hyperpolarization, an extended ramp-like shoulder, or a depolarization reminiscent of depolarizing afterpotentials (DAPs) recorded in vitro in MEC stellate and pyramidal neurons. Next, we correlated the afterpotentials with the cells propensity to fire bursts. All DAP cells with known location resided in Layer II, generated bursts, and their inter-spike intervals (ISIs) were typically between five and fifteen milliseconds. The ISI distributions of Layer-II cells without DAPs peaked sharply at around four milliseconds and varied only minimally across that group. This dichotomy in burst behavior is explained by cell-group-specific DAP dynamics. The same two groups of bursting neurons also emerged when we clustered extracellular spike-train autocorrelations measured in real two-dimensional arenas (Latuske et al., 2015). No difference in the spatial coding properties of the grid cells across all three groups was discernible. Layer III neurons were only sparsely bursting and had no DAPs. As various mechanisms for modulating the ion-channels underlying DAPs exist, our results suggest that the temporal features of MEC activity can be altered while maintaining the cells spatial tuning characteristics. Significance StatementDepolarizing afterpotentials (DAPs) are frequently observed in principal neurons from slice preparations of rodent medial entorhinal cortex (MEC), but their functional role in vivo is unknown. Analyzing whole-cell data from mice running on virtual tracks, we show that DAPs do occur during behavior. Cells with prominent DAPs are found in Layer II; their inter-spike intervals reflect DAP time-scales. In contrast, neither the rarely bursting cells in Layer III, nor the high-frequency bursters in Layer II, have a DAP. Extracellular recordings from mice exploring real two-dimensional arenas demonstrate that grid cells within these three groups have rather similar spatial coding properties. We conclude that DAPs shape the temporal but not the spatial response characteristics of principal neurons in MEC. Author contributionsAll authors designed research. DEC, CF, and JN performed research and analyzed data (equal contribution). AVMH wrote and edited the paper with support from MS and the other authors.

neuroscience↗