Abstract

When two repeating stimuli activate the same neuronal population, a decreased overall neural response is observed. This neurophysiological response is detectable by functional magnetic resonance imaging, and has been termed fMRI adaptation. Following this logic, when two tactile events are repeated on exactly the same region of skin, all neurons that have a strictly somatotopic response should reduce their activity. Here we used fMRI adaptation to address the issue of reference frames for touch. In particular, we asked if there exist neuronal populations that adapt to stimulation that repeats over distinct regions of skin when some other aspect of spatial coding of touch is identical (e.g., homologous finger stimulated between the two hands). We examined this question for SI and SII, measuring the blood-oxygen level-dependent (BOLD) effect to touches delivered in sequence within or between hands, to homologous or non-homologous fingers. The results showed stronger adaptation (i.e., a lower BOLD effect) at the level of SI when stimulation repeated over same (i.e., homologous) in comparison to different (i.e., non-homologous) fingers, regardless of whether stimulation occurred within or between hands. This pattern of results is less clear in SII. These results suggest a bilateral representation for touch in human SI, in agreement with neurophysiological data on monkeys.