Biology subjects

Yebra, M.

Publications and source records attributed to Yebra, M..

10 recordsLinked to original sources

bioRxiv2026

Microstimulation of the human dopaminergic midbrain causally reshapes value-based decision-making and happiness

Midbrain circuits are thought to govern reward-guided behavior and momentary happiness, yet causal evidence in humans remains scarce. Here, we show that focal microstimulation of the substantia nigra pars compacta alters both decision making and momentary happiness in patients undergoing awake deep-brain stimulation surgery for Parkinsons disease. During a gambling task with interleaved momentary happiness ratings, stimulation delivered at outcome reduced risk-seeking in the loss domain without increasing overall gambling probability. Stimulation improved choice efficiency, guiding behavior toward gambles with higher expected value and thereby increasing cumulative earnings. Computational modeling revealed selective changes in loss processing, with reduced loss weighting and more linear loss utility, while gain processing remained unchanged. In parallel, stimulation did not tonically elevate happiness but selectively amplified outcome-related happiness changes, scaling with decision value and reward prediction errors. These results provide causal evidence that localized perturbation of human midbrain circuits reshapes valuation, choice, and momentary well-being.

neuroscience↗

bioRxiv2025

Forests and woodlands resistant to drought revealed in remotely sensed foliar moisture content using probabilistic models

Foliar moisture content (FMC) is of primary importance in many biological and ecological processes of forests and woodlands, including herbivory and fire dynamics. The role of FMC, which is the mass of moisture on a dry matter basis, in such processes is increasingly important due to changes in length and severity of climate extremes, like drought. Here we studied the sensitivity of forest and woodland FMC to climatic water balance, i.e. Standardised Precipitation and Evapotranspiration Index (SPEI), across five landscapes in temperate and sub-tropical south-eastern Australia. To do this we used copula-probabilistic modelling, including consideration of drought (i.e. SPEI) length. We used an FMC dataset over eight years of Sentinel-2 satellite reflectance (2015-2023), based on the inversion of a radiative transfer model at 20m resolution. We found the spatial variation of FMC within landscapes to be large and that the spatial variation in response to drought was very large, particularly within the more arid landscapes. The areas of lesser response to extreme drought are revealed to be drought resistant forests and woodlands that are potential climate refugia for animals, and natural breaks in the contiguity of fire fuels.

ecology↗

bioRxiv2025

Diel foliar moisture content recovery time occurs soon after midnight in Eucalyptus species of koala (Phascolarctos cinereus) habitats

Many herbivores are reliant on the foliar moisture content (FMC, the water mass as a percentage of the dry matter mass), of feed tree leaves, which may vary through the diel cycle. This includes koalas (Phascolarctos cinereus), who have heightened reliance on FMC during high temperatures. However, it is unknown how, and to what degree, FMC varies through the 24-hour period. In this experiment we measured live leaf FMC every three hours over the diel period by calibrating a near-infrared spectroscopy (NIRS) model from leaf-surface reflectance, for tree species associated with koala habitat. Coinciding with reflectance measurements, trees in a glasshouse were exposed to normal and extreme climatic conditions including heatwaves and a drought, while collecting concurrent climate variables and soil moisture content. Our analysis of FMC timeseries of glasshouse trees found the diel range of leaf FMC was 23 % dry weight (DW, i.e. 19 % fresh weight) on average, between a minimum and maximum diel range of 10 %DW and 46 %DW, as predicted by an accurate spectral regression model (r2=0.96, RMSE=9.29 %DW). Overall, FMC minima occurred at 13:30 and FMC had recovered to initial levels (7:30) by 1:30 the following morning, though near-recovery levels were reached earlier by 19:30. This result shows that during a single day and night koala tree species may vary the proportion of leaf moisture by about one fifth, on average, and the timing of canopy moisture recovery may discourage foraging in the evening on temporarily dry species, during hot conditions.

ecology↗

bioRxiv2025

Order-Selective Cells Tile Temporal Space and Predict Order Memory in Humans

Remembering the temporal order of events is critical for episodic memory, allowing us to link individual events into sequences. While the medial temporal lobe and prefrontal cortex are essential for this process, the underlying neural mechanisms remain poorly understood. Here we characterized the representation of order information at the level of single neurons and field potentials recorded from human neurosurgical patients watching naturalistic videos of everyday events and later recalling the order and content of the events depicted. We found order-selective cells (OSCs) in the human hippocampus, amygdala, and orbitofrontal cortex that responded selectively to specific event orders, independent of event content or absolute time. OSCs exhibited transient theta phase precession following their preferred order during both memory encoding and retrieval, the strength of which predicted participants order memory accuracy. During retrieval, OSC spike timing relative to theta varied with the relative position of their preferred event within the recalled event sequence, enabling selective retrieval of relevant events. These findings reveal a neural substrate for representing, encoding into and retrieving from memory absolute and relative ordinal relationships between discrete events. OSCs tile temporal space into discrete ordinal positions, thereby weaving episodic experiences into coherent temporal narratives.

neuroscience↗

bioRxiv2025

Hippocampal stimulation reveals causal role of persistent neural activity in human working memory

Working memory (WM) enables the temporary maintenance and manipulation of information, supporting flexible, goal-directed behavior. While converging evidence suggests that persistent activity in the hippocampus and other areas of the brain contributes to WM storage, this link has not been examined by manipulating neural activity experimentally. Here, we combined simultaneous intracranial single-neuron recordings with focal electrical stimulation in the human hippocampus to test the role of memoranda-selective persistent neural activity for WM. Thirty patients with implanted hybrid depth electrodes performed a WM task with images as memoranda. Electrical stimulation (2 s, 50 Hz, 1 mA) was delivered to the hippocampus during the maintenance period on a subset of trials. Behaviorally, stimulation impaired WM performance, increasing response times especially in low-load conditions. Neuronally, stimulation reduced memoranda-selective activity in hippocampus and ventral temporal cortex (VTC), thereby disrupting content-specific neural representations of WM content. The extent of stimulation-induced change in firing rate mediated trial-by-trial impaired WM-related behavior, linking disrupted neural selectivity to impaired WM. At the population level, stimulation shifted neural trajectories farther from attractor states, consistent with degraded mnemonic fidelity. Together, these data provide evidence that selective persistent activity of individual neurons in hippocampus and VTC supports WM maintenance in humans. Our results demonstrate that hippocampal stimulation disrupts both single-neuron coding and population-level attractor stability, linking cellular mechanisms to behavior and highlighting the contributions of persistent activity to WM maintenance.

neuroscience↗

bioRxiv2025

Monitoring Forest Foliar Moisture Using Sentinel-2 Reflectance and Radiative Transfer Model Inversion

Foliar moisture content (FMC) of forests canopies is important for understanding biodiversity, including animal habitat quality for forest-dependent fauna and forest fire risk. In this study, we modelled spatiotemporal variation in FMC using optical remote sensing. We inverted the PROSPECT and GEOSAIL Radiative Transfer Models (RTM) on Sentinel-2 satellite reflectance data of 20 m ground resolution to retrieve FMC, and then predicted FMC at sub-continental scales using a random forest (RF) regression emulation of the RTM model. An RTM look-up-table, from earlier work, was filtered by ecological criteria from 24 sites sampled along precipitation and canopy cover gradients and was inverted by using the spectral angle, as a merit function. The RF emulator allowed efficient computation and presentation of an extensive and fine-scale forest FMC data cube of interest in animal, fire and plant ecology. Our RTM-based, predictions of forest and woodland FMC had a root mean square error (RMSE) of 19.9% of dry matter content and explained more than 60% of variance (r2 = 0.62). This represents an improvement over previous models using reflectance data of coarser spatial resolution, particularly in terms of explained variance (r2=0.17 and RMSE=32%). The emulator model achieved similar performance to the RTM with slightly larger error (RMSE = 21.77%) and smaller explained variance (r2 = 0.54). Overall, both models performed best in forests, and woodlands of moderate to high canopy density (> [~]0.75 LAI). This study demonstrates that FMC can be monitored at spatial resolutions that allow intra- or inter-landscape patterns to be resolved surpassing previous capabilities. Index TermsFoliar moisture content, FMC, remote sensing, radiative-transfer, forest, inversion, Sentinel-2, emulator, koala

ecology↗

bioRxiv2023

Hippocampal Theta Phase Precession Supports Memory Formation and Retrieval of Naturalistic Experience in Humans

Linking different experiences together is a key aspect of episodic memory. A potential neural mechanism for linking sequential events over time is phase precession, which causes neurons to fire progressively earlier in time relative to theta-frequency local field potential oscillations. However, no direct link between phase precession and behaviorally assessed memory encoding or retrieval success has been established. We recorded the activity of single neurons and local field potentials in the human medial temporal lobe (MTL) while participants encoded and retrieved memories of movie clips. Transient brief theta bouts and theta phase precession were observed following cognitive boundaries during movie watching as well as following stimulus onset during memory retrieval. Phase precession was dynamic, with different neurons exhibiting phase precession in different task periods. The strength of phase precession provided information about memory encoding and retrieval success that was not available in firing rates, thereby linking the temporal code established by phase precession to behaviorally assessed memory strength. These data reveal phase precession during non spatial memory in humans and provide direct neural evidence for a functional role of phase precession in episodic memory.

neuroscience↗

bioRxiv2021

A gradient of electrophysiological novelty responses along the human hippocampal long axis.

The hippocampus is implicated in novelty detection, thought to be important for regulating entry of information into long-term memory. Whether electrophysiological responses to novelty differ along the human hippocampal long axis is currently unknown. By recording from electrodes implanted longitudinally in the hippocampus of epilepsy patients, here we show a gradual increase of theta frequency oscillatory power from anterior to posterior in response to unexpected stimuli, superimposed on novelty responses common to all long axis portions. Intracranial event-related potentials (iERPs) were larger for unexpected vs. expected stimuli and demonstrated a polarity inversion between the hippocampal head (HH) and body (HB). We observed stronger theta coherence between HH and hippocampal tail (HT) than between HB and HT, similarly for expected and unexpected stimuli. This was accompanied by theta and alpha traveling waves with surprisingly variable direction of travel characterized by a [~]180{degrees} phase lag between hippocampal poles. Interestingly, this phase lag showed a pronounced phase offset between anterior and middle (HH-HB) hippocampal portions coinciding anatomically with a drop in theta coherence and the novelty iERP polarity inversion. Our findings indicate common response properties along the hippocampal long axis to unexpected stimuli, as well as a multifaceted, non-uniform engagement along the long axis for novelty processing.

neuroscience↗

bioRxiv2021

Cognitive boundary signals in the human medial temporal lobe shape episodic memory representation

While experience unfolds continuously, memories are organized as a set of discrete events that bind together the "where", "when", and "what" of episodic memory. This segmentation of continuous experience is thought to be facilitated by the detection of salient environmental or cognitive events. However, the underlying neural mechanisms and how such segmentation shapes episodic memory representations remain unclear. We recorded from single neurons in the human medial temporal lobe while subjects watched videos with different types of embedded boundaries and were subsequently evaluated for memories of the video contents. Here we show neurons that signal the presence of cognitive boundaries between subevents from the same episode and neurons that detect the abstract separation between different episodes. The firing rate and spike timing of these boundary-responsive neurons were predictive of later memory retrieval accuracy. At the population level, abrupt neural state changes following boundaries predicted enhanced memory strength but impaired order memory, capturing the behavioral tradeoff subjects exhibited when recalling episodic content versus temporal order. Successful retrieval was associated with reinstatement of the neural state present following boundaries, indicating that boundaries structure memory search. These findings reveal a neuronal substrate for detecting cognitive boundaries and show that cognitive boundary signals facilitate the mnemonic organization of continuous experience as a set of discrete episodic events.

neuroscience↗

bioRxiv2017

Action boosts episodic memory encoding in humans via engagement of a noradrenergic system.

We are constantly interacting with our environment whilst we encode memories. However, how actions influence memory formation remains poorly understood. Goal-directed movement engages the locus coeruleus (LC), the main source of noradrenaline in the brain. Noradrenaline is also known to enhance episodic encoding, suggesting that action could improve memory via LC engagement. Here we demonstrate, across seven experiments, that action (Go-response) enhances episodic encoding for stimuli unrelated to the action itself, compared to action inhibition (NoGo). Supporting a noradrenergic mechanism underlying this enhancement, functional magnetic resonance imaging, and pupil diameter as a proxy measure for LC-noradrenaline transmission, indicate increased encoding-related LC activity during action. A final experiment confirmed a novel prediction derived from these data that emotionally aversive stimuli, which recruit the noradrenergic system, modulate the mnemonic advantage conferred by Go-responses relative to neutral stimuli. We therefore provide converging evidence that action boosts episodic memory encoding via a noradrenergic mechanism.

neuroscience↗