Rodents rely mostly on their whiskers to explore their environment and a large part of their primary somatosensory cortex (S1) is dedicated to whisker perception. Each whisker is represented by a single column within the contralateral S1. This pattern of columns is organized similarly to the whisker pattern on the snout. And because of those distinct features, it is also referred to as the ‘barrel cortex’. Chapter 2 provides an extensive overview of the anatomical characteristics of the whisker system. The neuronal pathways of the whisker system are well known and the process of somatosensation is less complicated than, for example, the process of vision. The advantage of somatosensation over vision is that it is easy to generate a simple tactile stimulus, just by moving a single whisker. However, exactly how the neurons within the somatosensory system use the action potentials they generate to perceive this simple stimulus is not known. This thesis therefore aims to determine the different roles of different cell types in somatosensation, using an electrophysiological approach in both anaesthetized and awake rats.
It is possible to record from cells and find out whether or not it fires action potentials in response to whisker movement, but these results do not show how the brain actually uses its action potentials. To find a causal relationship between neuronal firing and its effect on perception, we can stimulate the cell and then measure the effect of that stimulation on perception. To find out if the rat’s perception gets biased during stimulation of a single cell, we used a behavioural task (chapter 3). The rat was trained to report movement of a single whisker by licking sugared water from a tube. A fast whisker movement is easy to detect for the rat, but when velocity decreases, it gets more difficult for the rat to detect the whisker movement. We slowly decreased the whisker velocity down to a value which the rat could only detect half of the time. We then stimulated a single cell in the C1 barrel column while simultaneously moving the corresponding whisker. Using this behavioural task, we wanted to determine whether stimulation of a single cell can bias whisker perception. For long, it was suspected that interneurons have an inhibitory effect on behaviour and perception. Our results show that stimulation of a single fast-spiking putative interneuron or a highly sensitive whisker responsive cell can actually enhance perception of a simple whisker movement. When stimulating a single non-responsive pyramidal cell, no perceptual bias was detected. These results suggest that animals decode neural activity in a detection task using a selective subset of neurons in the barrel cortex.
Because of the reciprocal connections between S1 and primary motor cortex and because it is known that stimulation of multiple cells in barrel cortex can lead to whisker movement, we wanted to determine which cell types play a role in this process. Stimulating one cell at a time can teach us about the differences between different cells. In chapter 4, we used nanostimulation to explore whether single cell stimulation in barrel cortex can elicit a whisker movement in anaesthetized and awake rats. All but three whiskers were clipped. Only the principle whisker and its two adjacent whiskers were kept intact. We then stimulated a single cell in the barrel column corresponding to the principle whisker to find out if the whiskers were moving during stimulation of the cell. Several different cell types in layer 5 of barrel cortex were explored, but we had to conclude that single-cell stimulation in barrel cortex is not sufficient to elicit whisker movements, despite the fact that microstimulation of multiple cells in the same location did elicit whisker movements.