Inter-hemispheric integration of tactile-motor responses across body parts

Luigi Tamà ̈; Longo, Matthew R. · 2015 · OpenAlex-citations

DOI: 10.3389/fnhum.2015.00345

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Summary

This study investigates whether the crossed-uncrossed difference (CUD) in tactile-motor reaction times varies depending on the body part stimulated. The CUD reflects the time required for interhemispheric transfer of sensorimotor information, typically mediated by the corpus callosum (CC). While previous research established that callosal connections are denser for proximal body regions (near the midline) and sparser for distal extremities, it remained unknown if this anatomical variation modulates the efficiency of tactile information transfer. The authors hypothesized that stimuli presented on distal regions with sparse callosal connections would produce a larger CUD due to less efficient interhemispheric transfer compared to proximal regions. To test this, twenty-eight participants performed a simple detection task where they responded as quickly as possible with either their left or right foot to unpredictable tactile stimuli delivered to the middle finger, forearm, or forehead. The experimental design controlled for stimulus-response compatibility by using distant stimulation sites (upper body) and response effectors (feet). Stimulus intensity was standardized across body parts using pressure sensors. Participants completed 1,200 trials across randomized blocks, with reaction times recorded to calculate the CUD for each body part and response foot. The results demonstrated a significant main effect of body part on the magnitude of the CUD. The CUD was largest for the fingers (approximately 2.6 ms) and forearm (approximately 1.8 ms), while it was significantly smaller for the forehead (approximately 0.9 ms). The CUD for the fingers was significantly different from zero, whereas the values for the forearm and forehead did not reach statistical significance. Additionally, a significant asymmetry was observed based on the response foot: a positive CUD was present when participants responded with the left foot, but not when they responded with the right foot. These findings indicate that interhemispheric transfer of tactile stimuli is modulated by the density of callosal connections associated with the stimulated body region. The larger CUD for distal body parts supports the hypothesis that sparse callosal connections result in slower or less efficient information transfer. The study confirms that the tactile CUD is not invariant across the body, unlike visual CUD which remains stable regardless of stimulus eccentricity. Furthermore, the observed left-foot response asymmetry aligns with previous literature suggesting faster signal transfer from the right to the left hemisphere. This work provides evidence that sensory channels, in addition to motor channels, contribute to the interhemispheric integration measured by the CUD.

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• multisensory crossmodal