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All-optical approach to the study of neuronal coding principles in sensory perception

Periodic Report Summary 2 - S1PERCEP (All-optical approach to the study of neuronal coding principles in sensory perception.)

INTRODUCTION.
A fundamental hypothesis of neuroscience is that our brain perceives the world through coordinated neuronal activity. Two lines of evidence support this hypothesis. On the one hand, many studies have shown that sensory stimuli can generate reliable and reproducible patterns of activity in ensembles of neurons throughout sensory pathways – from the sensory periphery to the cerebral cortex. Conversely, direct stimulation of neurons in sensory brain areas has been shown to enhance, modify, or even generate perceptually dependent behaviours. Nevertheless, whether a rate or temporal code underlie this neuronal activity remains hotly debated. A large number of studies argue for a rate code in which the average firing rate over a large population of neurons carries information. Such a rate code is robust against noise and coarse electrical manipulations of spike rate in primary motor and sensory cortices correlate with behavioural effects. However, cortical activity is sparse and thus information might be also carried by the precise temporal order of neuronal activity. Many studies demonstrate stimulus-specific spiking precision at millisecond timescales in various sensory systems. Such a temporal code would be more energy efficient, permit higher information content when used in relation to ongoing cortical oscillations and allow for learning rules based on timing-dependent synaptic plasticity.
The aim of this project was to address this key question of whether primary sensory cortices represent stimulus features through a rate or temporal coding scheme, or, in fact, a combination of both.

PROJECT OBJECTIVES.

OBJECTIVE 1: IMPLEMENT A NOVEL HOLOGRAPHIC TWO-PHOTON MICROSCOPE FOR OPTICAL IMAGING AND STIMULATION.
We aimed to develop and validate a novel, all-optical approach for simultaneous observation and manipulation of neuronal population activity in-vivo. We planned to confirm that a two-photon microscope equipped with a spatial light modulator (SLM) is capable of imaging and selectively photo-activating genetically defined neuronal populations in all three spatial dimensions.

OBJECTIVE 2: DETERMINE THE BEHAVIOURAL IMPORTANCE OF RATE VS. TEMPORAL NEURONAL CODES FOR SENSORY PERCEPTION.
We planned to measure the neuronal activity in layer 2/3 of the mouse barrel cortex during a sensory discrimination task using a genetically encoded calcium ion reporter in order to identify the populations of neurons that are active for different sensory stimuli. We planned to then use holographic photo-stimulation to perturb the neuronal activity in response to sensory stimulation. We intended to change either the overall firing rate or the temporal order and fidelity of spiking activity in these populations of neurons and monitor the effect of these manipulations on task performance.

OBJECTIVE 3: QUANTIFY THE EFFECTIVENESS OF RATE VS. TEMPORAL CODES FOR SENSORY PERCEPTION.
After establishing which perturbations to rate and temporal codes have the most significant effects on behaviour, we planned to study the underlying mechanisms in more detail by optically manipulating neuronal population activity in layer 2/3 during sensory stimulation. We planned to monitor the effect on electrically recorded spiking in layer 5, which is the output signal then translated into behaviour. We intended to then compare layer 5 output in response to changing the numbers of active neurons, their average firing rate, or their precise spike timing.

WORK PERFORMED SINCE THE BEGINNING OF THE PROJECT.
Since the beginning of the project, we have successfully installed the two-photon imaging system and started the setting up the SLM-based photo-stimulation. We obtained concurrent electrical and imaging recordings from GCaMP6-expressing cells in acute brain slices for calibration. We were also successful in setting up electrophysiology in vivo and head-fixed imaging during behavioural paradigms such as sensory discrimination tasks.

MAIN RESULTS ACHIEVED SO FAR.
We have achieved progress on the imaging engineering part, the cellular physiology part, and the behavioural testing part of the project. In comparatively very short time, the CIG enabled us to establish a three advanced research paradigms in a nascent laboratory. Using single-photon stimulation, our results suggest a differential involvement of different subtypes of inhibitory interneurons in controlling the propagation of information packets related to sensory perception.

EXPECTED FINAL RESULTS AND THEIR POTENTIAL IMPACT AND USE.
Our results show that information about sensory stimuli and behavioural outputs are encoded in a highly redundant and robust manner that is unaffected by small perturbations using SLM-based optogenetic photostimulation. These findings will significantly inform future experiments aimed at causally investigating the neuronal codes underlying sensory perception.