Tactile perception across the human foot sole as examined by the firing characteristics of cutaneous afferents and mechanical properties of the skin
This dissertation presents four complementary experiments that investigated distinct aspects of tactile perception across the human foot sole. The firing characteristics of primary cutaneous afferents in response to light touch and vibration was established, and the influence of the mechanical properties of the skin examined. This work contributes to a large body of electrophysiology and psychophysical literature that established the neural mechanisms of tactile perception in the hand. This earlier work demonstrated that tactile sensibility in the hand arises from four classes of specialized mechanoreceptor afferents in the glabrous skin. Each class preferentially responds to distinct features of mechanical stimuli, which enables the transmission of a broad range of tactile, somatosensory feedback to the central nervous system. Cutaneous afferent feedback provides both exteroceptive information about the interaction of the body with objects in the environment, and proprioceptive information about joint position. The glabrous skin on the hands and feet purportedly contain the same four classes of cutaneous mechanoreceptor afferents; however little work has investigated tactile perception or cutaneous afferent firing characteristics in the foot sole. This dissertation investigated the ability of foot sole cutaneous afferents to respond to exteroceptive, light touch and vibration stimuli, the influence of skin mechanics, and the relationship with tactile perception. The results from this dissertation can be summarized in five main conclusions. First, skin hardness and thickness can be significantly different across the foot sole, and this has an influence on monofilament (MF), but not vibration perception threshold (VPT). Second, fast adapting (FA) but not slowly adapting (SA) afferents are shown to mediate MF perceptual threshold across the foot sole. Third, skin hardness can influence the firing thresholds of FA afferents, which is thought to account for elevated monofilament perceptual thresholds at harder foot sole sites. Fourth, each cutaneous afferent class is uniquely tuned to vibration stimuli, however there is a limited capacity to isolate firing from individual afferent classes, with the exception of FA type II afferents at low amplitude and high frequency vibrations. Fifth, foot sole vibration perception may be mediated by two neural mechanisms; a low frequency range where a combination of firing across afferent classes contribute to perception, and a high frequency range mediated by FAII afferent activity. The findings from the four experiments in this dissertation help to establish the capacity of foot sole cutaneous afferents to deliver tactile feedback to the central nervous system, and contributes to a restructuring of the assumptions about the contributions of the different cutaneous afferent classes to tactile sensitivity testing. This work enhances the understanding of the foot sole as a sensory structure, and provides a foundation for the development of facilitatory insoles and other tactile enhancement interventions.