Dynamics In Primary Somatosensory Cortex: A Role For SI In The Processing Of Tactile Information

Traditional views of primary somatosensory cortex (SI) and its role in the processing of tactile information have limited its function as a dynamic component in the somatosensory pathway. Here I present evidence that the response in SI to stimuli at a given skin site is systematically modified with changes in the stimulus parameters and displays considerable dynamics. Optical intrinsic signal (OIS) imaging was used to study the responses (in vivo) evoked by 25 Hz (flutter) vertical skin displacement stimuli to the forelimb of squirrel monkey and cat. Responses to electrical stimulation were also measured in rat sensorimotor cortical slices using OIS imaging and local field potential (LFP) recordings. Results indicate that, contrary to traditional views, the intensive but not spatial attributes of the SI response are modified by increases in stimulus amplitude. Increasing the duration of flutter stimulation evokes increases in response magnitude in cortical regions near to the maximally responding center and simultaneous decreases in surrounding cortical regions; the net effect of this is the spatial sharpening of the SI response during prolonged stimulation. The distribution of decreased absorbance in surrounding cortex was non-uniform, indicating the possibility of stronger intracortical inhibition along the proximodistal axis of the body representation. Cortical slices in the sagital and coronal planes of the rat somatosensory cortex demonstrated a similar anisotropy in the distribution and impact of GABAergic inhibition on the horizontal spread of activity, and lend support to the idea that the non-uniformity observed in vivo may contain functional relevance. Bilateral stimulation of both forelimbs demonstrated that, although input to SI has been traditionally regarded as exclusively contralateral, not only can the response to an ipsilateral stimulus be measured in SI, but when stimulation is applied bilaterally the spatiotemporal characteristics of the evoked response cannot be accounted for by the responses of either stimulus alone or by the linear summation of the pair. All of these results taken together present a strong case for the necessity of strong dynamics in SI and the role of SI as an important site of cortical information processing in the somatosensory pathway

Sensory mechanisms involved in obtaining frictional information for perception and grip force adjustment during object manipulation

Sensory signals informing about frictional properties of a surface are used both for perception to experience material properties and for motor control to be able to handle objects using adequate manipulative forces. There are fundamental differences between these two purposes and scenarios, how sensory information typically is obtained. This thesis aims to explore the mechanisms involved in the perception of frictional properties of the touched surfaces under conditions relevant for object manipulation. Firstly, I show that in the passive touch condition, when the surface is brought in contact with immobilised finger, humans are unable to use existing friction-related mechanical cues and perceptually associate them with frictional properties. However, a submillimeter range lateral movement significantly improved the subject's ability to evaluate the frictional properties of two otherwise identical surfaces. It is demonstrated that partial slips within the contact area and fingertip tissue deformation create very potent sensory stimuli, enabling tactile afferents to signal friction-dependent mechanical effects translating into slipperiness (friction) perception. Further, I demonstrate that natural movement kinematics facilitate the development of such small skin displacements within the contact area and may play a central role in enabling the perception of surface slipperiness and adjusting grip force to friction when manipulating objects. This demonstrates intimate interdependence between the motor and sensory systems. This work significantly extends our understanding of fundamental tactile sensory processes involved in friction signaling in the context of motor control and dexterous object manipulation tasks. This knowledge and discovered friction sensing principles may assist in designing haptic rendering devices and artificial tactile sensors as well as associated control algorithms to be used in robotic grippers and hand prostheses

Cortical Diagnostics: Measuring Brain Health through Somatosensation

Over the past several years, a number of unique quantitative tactile based sensory testing methods were designed with the intent of obtaining objective metrics that would be sensitive to alterations in cortical information processing. The design of these tasks was based on information obtained from neurophysiological studies of the nonhuman primate (NHP) cerebral sensory cortical response to a variety of modes of natural skin stimulation, and these NHP studies typically exhibit characteristics of cortical modularity, or cortical-cortical dynamics that occur between adjacent and near-adjacent assemblies of cortical neurons. The initial goal of these studies was to demonstrate cortical correlates of perception by comparing observations of stimulus evoked activity in primary somatosensory cortex of non-human primates, and a secondary goal was to demonstrate that these measures of sensory perception were altered in a predictable fashion with neurological insult. To date, observations consistent with systemic cortical alterations have been made in individuals with neurotrauma (concussion/TBI, stroke), neurodevelopmental disorders (Autism, ADHD, Tourette's, OCD) and chronic pain (migraine, fibromyalgia, VVS, TMJD, carpal tunnel syndrome). One unifying theme of these findings is the role that cortical modularity plays in sensory information processing and that when cortical modularity is disrupted, significant quantifiable deficits in sensory information processing can be detected.Doctor of Philosoph