On the Organisation of the SII human somatosensory cortices: preliminary results with fMRI and electrical peripheral nerve Stimulation

Direct electrical Stimulation of the exposed human brain during neurosurgery of drug-resistant epilepsies firstly demonstrated the presence of stimulus-evoked paresthesis when the stimulator tip was located on the superior bank of the Sylvian fissure and/or nearby convexial cortex [1]. This cortical area has been named since secondary somatosensory (SII); it shows a relatively rough topographical organization, is adjacent to primary auditory cortex and receives sensory inputs from both ipsiand cotralateral hemibodies [2,3]. In fact, animal studies showed that receptive fields of the majority of SII neurons are bilateral; this probably provides an early Integration of somatosensory Information from both sides of the body. Such an hypothesis explains why in monkeys bilateral SII lesions affect severely their ability to learn from tactile Stimuli, äs well äs their intermanual transfer of somatosensory Information [4,5]. Electromagnetic brain responses to sensory Stimuli in intact and awake humans, consistently demonstrated the existence of generator sources bilaterally located adjacent to the Sylvian fissure [6]. Functional Magnetic Resonance is a non-invasive functional imaging technique able to identify an eventrelated increase in oxygen consumption in discrete brain regions with a very good spatial resolution. Aim of the present study was to investigate the ability of fMRI to localize the SII areas, and possibly reveal their somatotpic organization, in healthy humans tbllowing upper and lower Hmb sensory Stimulation.