arXiv subjects

David Hofmann

Publications and source records attributed to David Hofmann.

6 recordsLinked to original sources

arXiv2025

Scientific Machine Learning of Chaotic Systems Learns Reduced-Order Equations for Neural Populations

Extracting interpretable mathematical models from complex dynamical systems is difficult, especially for chaotic dynamics observed with noisy experimental data. We present PEM-UDE, a method that combines prediction-error methodology with universal differential equations to discover governing equations from limited, noise-corrupted observations. Prediction-error feedback smooths the chaotic optimization problem; for noise-free data generated within the model class, it preserves the data-consistent zero-loss set, whereas noise and model misspecification introduce a gain-dependent stability-bias trade-off. Preservation of the zero-loss set is not a guarantee of unique structural identifiability. We test the method on two benchmark chaotic systems, the Rossler attractor and a real electrical circuit, and recover the correct functional forms even when one observed dimension contains noise of five times the signal magnitude. The method also accepts prior knowledge of the system as an initial functional form, which we use to learn neural circuit equations that account for sparse connectivity, a feature missing from conventional neural mass models. Applied to a population of Izhikevich neurons, PEM-UDE yields a multi-scale neural mass model that ties single-neuron parameters to macroscopic network dynamics and predicts a relationship between connection density, dominant oscillation frequency, and synchrony. We test these predictions against three intracranial recording datasets from rat and human cortices. For the neuroscience application, the learned equations are a reduced-order closure for a specified simulated Izhikevich network family; the experimental recordings provide an indirect consistency check of predicted frequency and synchrony trends, not a direct fit of the equations to recordings.

cs.LG↗

arXiv2025

Identifying Electronic Doorway States in the Secondary Electron Emission from Layered Materials

We investigate the secondary low-energy electron emission induced by inelastic electron scattering from graphene and layered materials thereof. By applying a coincidence detection of the primary scattered and the emitted secondary electron we unravel pronounced resonance features otherwise overshadowed by the largely structureless secondary electron energy distribution. Supported by density functional theory calculations we show that these structures are the signature of prominent Feshbach resonances above the vacuum threshold which originate from interlayer states acting as a doorway state for electron emission. Remarkably, some of these doorway states open up only for samples with more than five layers.

cond-mat.mes-hall↗

arXiv2020

Inferring phenomenological models of first passage processes

Biochemical processes in cells are governed by complex networks of many chemical species interacting stochastically in diverse ways and on different time scales. Constructing microscopically accurate models of such networks is often infeasible. Instead, here we propose a systematic framework for building phenomenological models of such networks from experimental data, focusing on accurately approximating the time it takes to complete the process, the First Passage (FP) time. Our phenomenological models are mixtures of Gamma distributions, which have a natural biophysical interpretation. The complexity of the models is adapted automatically to account for the amount of available data and its temporal resolution. The framework can be used for predicting the behavior of various FP systems under varying external conditions. To demonstrate the utility of the approach, we build models for the distribution of inter-spike intervals of a morphologically complex neuron, a Purkinje cell, from experimental and simulated data. We demonstrate that the developed models can not only fit the data but also make nontrivial predictions. We demonstrate that our coarse-grained models provide constraints on more mechanistically accurate models of the involved phenomena.

q-bio.QM↗

arXiv2018

Ultrafast population coding and axo-somatic compartmentalization

Cortical neurons in the fluctuation driven regime can realize ultrafast population encoding. The underlying biophysical mechanisms, however, are not well understood. Reducing the sharpness of the action potential onset can impair ultrafast population encoding, but it is not clear whether a sharp action potential onset is sufficient for ultrafast population encoding. One hypothesis proposes that the sharp action potential onset is caused by the electrotonic separation of the site of action potential initiation from the soma, and that this spatial separation also results in ultrafast population encoding. Here we examined this hypothesis by studying the linear response properties of model neurons with a defined initiation site. We find that placing the initiation site at different axonal positions has only a weak impact on the linear response function of the model. It fails to generate the ultrafast response and high bandwidth that is observed in cortical neurons. Furthermore, the high frequency regime of the linear response function of this model is insensitive to correlation times of the input current contradicting empirical evidence. When we increase the voltage sensitivity of sodium channels at the initiation site, the two empirically observed phenomena can be recovered. We provide an explanation for the dissociation of sharp action potential onset and ultrafast response. By investigating varying soma sizes, we furthermore highlight the effect of neuron morphology on the linear response. Our results show that a sharp onset of action potentials is not sufficient for the ultrafast response. In the light of recent reports of activity-dependent repositioning of the axon initial segment, our study predicts that a more distal initiation site can lead to an increased sharpness of the somatic waveform but it does not affect the linear response of a population of neurons.

q-bio.NC↗

arXiv2017

Chance, long tails, and inference: a non-Gaussian, Bayesian theory of vocal learning in songbirds

Traditional theories of sensorimotor learning posit that animals use sensory error signals to find the optimal motor command in the face of Gaussian sensory and motor noise. However, most such theories cannot explain common behavioral observations, for example that smaller sensory errors are more readily corrected than larger errors and that large abrupt (but not gradually introduced) errors lead to weak learning. Here we propose a new theory of sensorimotor learning that explains these observations. The theory posits that the animal learns an entire probability distribution of motor commands rather than trying to arrive at a single optimal command, and that learning arises via Bayesian inference when new sensory information becomes available. We test this theory using data from a songbird, the Bengalese finch, that is adapting the pitch (fundamental frequency) of its song following perturbations of auditory feedback using miniature headphones. We observe the distribution of the sung pitches to have long, non-Gaussian tails, which, within our theory, explains the observed dynamics of learning. Further, the theory makes surprising predictions about the dynamics of the shape of the pitch distribution, which we confirm experimentally.

q-bio.NC↗

arXiv2017

Reverse-engineering biological networks from large data sets

Much of contemporary systems biology owes its success to the abstraction of a network, the idea that diverse kinds of molecular, cellular, and organismal species and interactions can be modeled as relational nodes and edges in a graph of dependencies. Since the advent of high-throughput data acquisition technologies in fields such as genomics, metabolomics, and neuroscience, the automated inference and reconstruction of such interaction networks directly from large sets of activation data, commonly known as reverse-engineering, has become a routine procedure. Whereas early attempts at network reverse-engineering focused predominantly on producing maps of system architectures with minimal predictive modeling, reconstructions now play instrumental roles in answering questions about the statistics and dynamics of the underlying systems they represent. Many of these predictions have clinical relevance, suggesting novel paradigms for drug discovery and disease treatment. While other reviews focus predominantly on the details and effectiveness of individual network inference algorithms, here we examine the emerging field as a whole. We first summarize several key application areas in which inferred networks have made successful predictions. We then outline the two major classes of reverse-engineering methodologies, emphasizing that the type of prediction that one aims to make dictates the algorithms one should employ. We conclude by discussing whether recent breakthroughs justify the computational costs of large-scale reverse-engineering sufficiently to admit it as a mainstay in the quantitative analysis of living systems.

q-bio.MN↗