Dynamic shifts in tactile localization after stroke

Plasticity in somatosensory maps has been observed after brain damage and differential stimulation. These studies demonstrate that the relationship between tactile stimulation and activation in primary somatosensory maps is labile. However, few studies have examined the perceptual consequences of somatosensory reorganization in humans. Repeated tactile stimulation can result in rapid changes in both somatosensory map topography and tactile perception in neurologically-intact individuals. Given evidence for rapid activity-dependent plasticity of somatosensory maps, along with the need to incorporate higher-order representations of the body to localize touch, we hypothesized that tactile localization could be shifted based on the location of previous stimulation in brain-damaged individuals. Therefore, we instructed two individuals with left primary somatosensory damage to localize suprathreshold tactile stimuli presented to the hand only, or to the hand preceded by forearm stimulation. On hand only trials, we found systematic mislocalization of tactile sensation, likely due to a mismapping from primary somatosensory maps to a higher-order representation of body size and shape. However, when preceded by forearm stimulation, localization judgments of hand stimuli shifted proximally on the contralesional, but not ipsilesional, limb. In an additional experiment, we presented stimuli to the medial segment of the contralesional fingers, and found that localization judgments were shifted in the direction of prior stimulation on the distal or proximal segment of the same finger. Our results, consistent with decreased inhibition in sensory cortex after stroke, may reflect rapid changes in primary somatosensory cortex combined with a continued mismapping from sensory maps to higher-order body representations.