The Representation of Multimodal Tactile Sensations in the Human Somatosensory System

Citation

Rosenthal, Isabelle Anna (2023) The Representation of Multimodal Tactile Sensations in the Human Somatosensory System. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yxtg-sr73. https://resolver.caltech.edu/CaltechTHESIS:05162023-172436741

Abstract

The sense of touch is critical to executing basic motor tasks and generating a feeling of embodiment. To construct touch percepts, the brain integrates information from tactile mechanoreceptors with inputs from other senses and top-down variables such as attention and task context. In this thesis, we investigate how these factors influence neural activity within the somatosensory system at different stages of tactile processing, using electrophysiological and behavioral data from a human tetraplegic participant implanted with microelectrode arrays. First, we find that neural responses to imagined touches of different types are decodable in the primary somatosensory cortex, ventral premotor cortex, and the supra-marginal gyrus, and these responses remain stable over many months. Following this analysis, the primary somatosensory cortex is explored in greater depth to better characterize early-stage cortical tactile processing. Touches to the arm and finger are examined during a passive task, in a variety of conditions including visually observed physical touches, physical touches without vision, and visual touches without physical contact. Analysis of the two touch locations suggests that touch encoding in primary somatosensory cortex may be less rigid than in the classical topographic view. Additionally, this experiment uncovers a modulatory effect of vision in the primary somatosensory cortex when it is paired with a physical touch, but no effect of vision alone. Finally, we investigate how visual information impacts artificial tactile sensations, which can be elicited using intra-cortical microstimulation to the primary somatosensory cortex. The ability to elicit reliable, naturalistic artificial touch sensations is vital to the implementation of a tactile brain-machine interface, which would benefit patients with spinal cord injury and others with somatosensory impairments. We find that visual information biases the qualitative percept of artificial stimulation towards an interpretation that is visually plausible. The temporal binding window between vision and stimulation is found to be larger when visual information is biologically relevant, suggesting that the brain’s ability to causally relate artificial stimulation to visual cues depends on visual context. Additionally, recordings from the primary somatosensory cortex indicate that visual information relevant to artificial stimulation is represented across contexts, during an active task. The effect of task on the responsiveness of the primary somatosensory cortex to visual information points to a role of attention in mediating early cortical tactile processing. In combination, the findings presented in this thesis provide insight into the basic neuroscience of how tactile experiences are constructed by the brain, suggesting that early tactile processing is influenced by multisensory, contextual factors. These findings also have clinical applications to developing a brain-machine interface capable of providing naturalistic sensations within a complex real world environment.

Thesis Files