Site Specificity of Changes in Cortical Oxyhaemoglobin Concentration Induced by Water Immersion

Our previous studies have shown that water immersion (WI) changes sensorimotor processing and cortical excitability in the sensorimotor regions of the brain. The present study examined the site specificity of the brain activation during WI using functional near infrared spectroscopy (fNIRS). Cortical oxyhaemoglobin (O₂Hb) levels in the anterior and posterior parts of the supplementary motor area (pre-SMA and SMA), primary motor cortex (M1), primary somatosensory cortex (S1), and posterior parietal cortex (PPC) were recorded using fNIRS (OMM-3000; Shimadzu Co.) before, during, and after WI in nine healthy participants. The cortical O₂Hb levels in SMA, M1, S1, and PPC significantly increased during the WI and increased gradually along with the filling of the WI tank. These changes were not seen in the pre-SMA. The results show that WI-induced increases in cortical O₂Hb levels are at least somewhat site specific: there was little brain activation in response to somatosensory input in the pre-SMA, but robust activation in other areas.