Neural correlates of neighboring vibrissae discrimination in the primary somatosensory cortex

Gardères, Pierre-Marie; Kampa, Björn Michael (Thesis advisor); Feldmeyer, Dirk (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Understanding the neural code, i.e. how information is mapped to the spatiotemporal features of brain activity, is one of the main goals in system neuroscience. From the experimenters’ perspective, there are essentially two entry points to the problem, at the interface between the brain and the external world. We can test whether the neural activity maps either the inputs (entering sensory information) or outputs (motor activity) of the individual. In this thesis, I use the whisker primary somatosensory cortex (wS1) as a model of neocortical circuitry. In particular, I focus on the neural code associated with tactile stimuli and their perception.In a first set of experiment, I question the properties of the neural code for passive tactile inputs in layer 2/3. It is known that a sparse code is used in this structure of the neocortex, meaning that a small fraction of neurons reliably respond to the same whisker stimulus over trials, during weeks and months. However, the functional properties of highly responsive neurons are generally studied in vivo with little access to their morphological and electrophysiological properties. We overcome this problem using an opto-tagging strategy that allows us to identify and label highly responsive neurons in vivo. These same neurons can be later retrieved in ex vivo slices, to have their morphological and electrophysiological properties precisely characterized. Our results show that highly whisker responsive neurons do not display increased excitability. Instead, their increased level of responsiveness could be due to a higher rate of high amplitude excitatory inputs. Functional analysis carried in vivo suggests that highly responsive neurons might constitute "Hub cells", recipients of converging inputs and strongly interconnected.In a second study, I will question the role of the cortical layer 2/3 evoked activity on perception during a tactile driven behavior. It is known that sensorimotor transformation is distributed over a network of sensory and non-sensory areas, but the role of the primary sensory areas remains unclear. I developed a new task of neighboring whisker frequency discrimination and characterized both the neuronal and perceptual ability of mice in the task. We find that multi-whisker suppression, (a dominant computation in cortical layer 2/3), allows the comparison of frequencies across the range of stimuli tested and reduces the variability of sensory response, which is computationally beneficial for readout mechanisms. Besides, we find that behavioral engagement is associated with selective up-modulation of sensory gain of single whisker signals. In addition, I show that alternated optogenetic silencing of the two whiskers' home barrels results in selective biases of the sensory perception, thus causally relating the whisker representation in wS1 to the animal percept of whisker stimulation intensity. However, signals relating to choice were weak and their neuronal representation overlapped little with the sensory encoding of frequencies. Therefore our results illustrate the role of wS1 in forming a reliable sensory substrate for whisker frequency discrimination and in relaying it to downstream areas from which most behavioral variability must arise.