En face retinal imaging | EPFL Graph Search

Retinal imaging is routinely used to provide information about vision health. Recent studies showed that retinal images can also provide information on general health (e.g. stress level), brain pathologies and attention. Depending on the target information, different imaging systems can be chosen and optimized. This thesis will focus on two distinct challenges in ophthalmology. The first one regards a see-through wearable ophthalmoscope for applications such as health monitoring and eye tracking. Such a system uses a scanning 2D micromirror that results in a compact confocal scanning laser ophthalmoscope. Another key feature of such a system is the use of a holographic plate on which a diffraction grating has been recorded. This allows the light from the laser beam to be diffracted to the eye without altering the patientâs field of view. Such a system is proposed and analyzed with simulations and with a proof of concept setup. The second challenge regards in-vivo imaging of retinal microstructures. Indeed, with the current state-of-the-art technology, it is still not possible to observe the morphology of transparent cellular structures in the top layers of the living retina with high contrast in a time frame relevant for use in patients. Such a possibility is appealing since neuronal layers of the retina could be used to obtain information on degenerative diseases of both retina and brain. In this thesis, it is shown how asymmetrical illumination of the retina results in phase contrast and how this can be used for fast imaging modalities of cellular structures. The technique is here detailed with a theoretical model, numerical simulations and ex-vivo experimental validations.

EEG correlates of active visual search during simulated driving: An exploratory study

Ricardo Andres Chavarriaga Lozano, Lucian Andrei Gheorghe, Hadrien Marc Renold

Brain responses to visual stimuli can provide information about visual recognition processes. Several studies have shown stimulus-dependent modulation of the evoked neural responses after gaze shifts (i.e. eye fixation related potentials, EFRP) depending on the relevance of the fixated object. However these studies are typically performed on still images under constrained conditions. Here we extend this approach to study overt visual attention during a simulated driving task. Simultaneous analysis of eye-tracking and electroencephalography data revealed similar patterns than those previously reported. However, natural visual exploration yielded shorter fixations, which imposes constraints in the analysis of the elicited brain responses. Nevertheless, we found significant differences between EFRPs corresponding to target objects or non-object stimuli. These results suggest the possibility of decoding such information during driving, allowing better understanding of how drivers process the environmental information.

2014

Uncovering the neural substrates of communication between the brain and ventral nerve cord in Drosophila melanogaster (italic)

Florian Aymanns

Despite a long history of research in motor control, the exact mechanism of how the brain communicates with the the invertebrate ventral nerve cord (VNC) and the vertebrate spinal cord on a single neuron basis remains largely elusive.Drosophila melanogaster is an ideal model organism to address the role of individual neurons, thanks to its stereotyped nervous system.Descending neurons (DNs), interneurons in the brain that project to the invertebrate ventral nerve cord or vertebrate spinal cord, have been shown to elicit specific behaviors upon optogenetic activation in Drosophila.These results suggest that behavior is controlled by sparse sets of command neurons, each eliciting a particular behavior.However, it remains to be seen how many DNs are involved in the control of naturalistic behaviors and what other information they might encode.In order to avoid commands leading to physically impossible or destabilizing actions, the brain has to be aware of the current behavior state.Ascending neurons (ANs), interneurons in the invertebrate ventral nerve cord or vertebrate spinal cord projecting to the brain, are likely conveying this information to the brain.To which degree of fidelity and in what way ANs encode behavior state remains unclear.To address these questions, we developed a dissection approach that allows functional imaging of ANs and DNs in the cervical connective.This dissection gives us optical access to the cervical connective and parts of the VNC in behaving adult Drosophila such that a two-photon fluorescence microscope can be used to image neural activity via calcium indicators.We began by imaging a new library of sparse split GAL4 driver lines targeting ANs.Our recordings from 247 genetically identifiable ANs revealed neurons encoding walking, resting, turning, eye grooming, and foreleg movements.Anatomical characterization of these ANs showed that they predominantly project to two brain regions: the anterior ventrolateral protocerebrum (AVLP) and the gnathal ganglion (GNG).Our results suggest that ANs encoding the motion of the animal with respect to the surrounding environment (self-motion) predominantly project to the AVLP, an integrative sensory hub. ANs projecting to the GNG mostly encode discrete actions.We then proceeded to image populations of DNs.The majority of DNs we recorded encoded walking behavior, with smaller fractions encoding resting and head grooming.Some of these neurons are likely to drive walking, but we also identified neurons encoding walking speed and turning.This suggests that a core set of DNs drives a behavior whose features can be modulated by additional DNs.Besides encoding behaviors, some DNs encode sensory information, including the presence of odors and deflection of the antennae.

EPFL2022

Methods for acute and long-term imaging of the ventral nerve cord in behaving adult Drosophila

Laura Joan Hermans

Understanding how neural circuits remodel and adapt to animal behavior is a central theme in the field of Neuroscience. One strategy to reach this goal is to repeatedly record the same animal's neural circuits and observe how they adapt when facing different challenges. Research has primarily focused on identifying neural circuits and connectivity maps within the brains of model organisms. Nevertheless, it has become evident that functional recordings are essential to have a complete understanding of how neural circuits generate behaviors. Numerous techniques have been introduced to record activity from brain circuits in model animals spanning from monkeys to insects. However, imaging the spinal cord in mammals or the ventral nerve cord in insects remains an open challenge due to the complexity of obtaining optical access without impacting the animal's behavior and damaging neural tissues. As a result, we have very limited information on how neural circuits in the spinal cord generate behaviors and how they adapt to different experiences such as injury or aging. Drosophila is a perfect model organism for carrying out this line of research thanks to its small size, genetically tractable nervous system, and behavioral diversity. The results are relevant to human physiology and behavior as many mechanisms have homologs in mammals. What is missing is a technological advancement that allows recording of behavior, structural changes in the circuits, and spatiotemporal dynamics of neural activity on the same animal.This thesis addresses this gap and introduces two novel methods for in vivo functional recordings of the fly's ventral nerve cord (VNC). The first one gains short-term access to the fly's VNC using a novel dissection preparation. The second one enables long-term functional recordings of the fly's VNC thanks to a toolkit combining four microengineered devices: a surgical arm, a V-shape implant, an optically transparent and numbered window and a remounting stage. The toolkit is suitable for both male and female flies and provides optical access to the VNC for at least one month. Additionally, the long-term toolkit does not impact flies' behaviors compared to intact flies and can be used to image both sparse and large populations of neural activity in flies' cervical connective and VNC. The toolkit was tested in two proof-of-concept (POC) applications. In the first POC, the toolkit monitored the degradation of sensory neurons in the VNC over weeks following limb amputation. In the second POC, the toolkit monitored global changes in neural dynamics following ingestion of a high-concentration caffeine solution. In conclusion, this thesis introduces new techniques for functional recordings of the fly's cervical connective and VNC over short and long time scales. They enable optical access to the fly's cervical connective that can be leveraged to study the content of information transmitted between the brain and the VNC while the fly is behaving. The two POCs demonstrate that the novel technology is ideally suited for long-term experiments and could be applied to a wide range of studies. For instance, the toolkit could be used to investigate how behaviors are encoded in the fly's VNC and how they adapt over time to experiences such as disease, aging, drug intake and learning.

EPFL2022

Uncovering the neural substrates of communication between the brain and ventral nerve cord in Drosophila melanogaster (italic)

Florian Aymanns

EPFL2022

Methods for acute and long-term imaging of the ventral nerve cord in behaving adult Drosophila

Laura Joan Hermans

EPFL2022