The Contribution of Primary and Secondary Somatosensory Cortices to the Representation of Body Parts and Body Sides: An fMRI Adaptation Study

While the somatosensory homunculus is a classically used description of the way somatosensory inputs are processed in the brain, the actual contribution of primary (SI) and secondary (SII) somatosensory cortices to the spatial coding of touch remains poorly understood. We studied adaptation of the fMRI BOLD response in the somatosensory cortex by delivering pairs of vibrotactile stimuli to the finger tips of the index and middle fingers. The second stimulus (test) was always administered to the left index finger, while the first stimulus (adaptor) was delivered either to the same or to a different (middle) finger of the right or left hand. The overall BOLD response evoked by the stimulation was primarily contralateral in SI and was more bilateral in SII. However, our fMRI adaptation approach also revealed that both somatosensory cortices were sensitive to ipsilateral as well as to contralateral inputs. SI and SII adapted more when the stimulation repeated over homologous than non-homologous fingers, showing a distinction between different fingers. Most importantly, for both somatosensory cortices this finger-specific adaptation occurred irrespective of whether the tactile stimulus was delivered to the same or to different hands. This result implies integration of contralateral and ipsilateral somatosensory inputs in SI, as well as in SII. These findings suggest that SI is more than simply a relay for sensory information, and that both SI and SII contribute to the spatial coding of touch by discriminating between body parts (fingers) and by integrating the somatosensory input from the two sides of the body (hands).