Date of Degree

9-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Psychology

Advisor

Tony Ro

Committee Members

Tatiana Emmanouil

Robert Duncan

Jonathan Levitt

Axel Cleeremans

Subject Categories

Cognitive Neuroscience

Keywords

Conscious Perception, Somatosensory Cortex, Touch

Abstract

Despite our everyday reliance on touch, from manipulating tools to dressing ourselves, relatively little is known about the neural correlates of tactile perception. As with other modalities, our conscious, reportable experiences of touch can dissociate from the physical tactile stimulation processed by the skin. In unconscious touch perception, tactile stimuli can be processed and guide our behavior without an accompanying conscious percept. For example, we may swat away a mosquito without having consciously registered its presence on our skin. In tactile illusions, conscious tactile experiences occur without a corresponding tactile stimulus. Multisensory tactile illusions arise when stimulation of a different modality influences conscious tactile perception. Using converging neuroscientific methods, we characterized the neural mechanisms underlying these two types of dissociations in touch perception. In a first experiment, we assessed the role of primary somatosensory cortex (S1) in conscious and unconscious touch perception using transcranial magnetic stimulation (TMS). We demonstrated for the very first time the existence of TMS-induced numbsense, whereby the disruption of S1 suppressed tactile awareness but left unconscious localization of touch above-chance. In a second experiment, we assessed the role of early somatosensory activity in a visually induced tactile illusion. We recorded electroencephalographic (EEG) activity and fast-signal optical imaging over the somatosensory cortex and found activations in S1 and S2 starting 128 ms after visual stimulus presentation associated with this illusion. These findings imply the involvement of early somatosensory representations in a multisensory illusion of touch. In a follow-up experiment, we explored the roles of S1 and of the PPC, a multimodal structure known to participate in the integration of visual and tactile signals, in this visually induced tactile illusion. Unexpectedly, stimulating S1 did not reduce visually induced tactile illusions, suggesting that these may rely on other somatosensory processes that can compensate for S1 suppression. Stimulating the PPC 140 ms after visual stimulus presentation caused a significant decrease in the visual facilitation of tactile sensitivity, likely due to an increase in visually induced tactile illusions. This demonstrated the role of the PPC in improving tactile sensitivity during visual tactile multisensory integration, and suggests it may also influence visually induced tactile illusions. In our last experiment, we used a psychophysical approach to understanding the behavioral mappings between auditory and tactile perception in sound-touch synesthesia, in which individuals experience consistent and reliable sound-induced tactile illusions. We found that sound frequency was strongly correlated with the location of synesthetic tactile illusions on the body, suggesting the involvement of early, somatotopically-organized somatosensory areas. Together, our results support the critical importance of early somatosensory brain areas in both unconscious and illusory touch perception, as well as in multisensory integration. These results provide an important insight into the neural mechanisms supporting the subjective aspects of touch perception.

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