Cortical activation induced by intraoral stimulation with water in humans

Studies of gustatory processing frequently utilize water as a control stimu lus. However, the neural representations of intraoral stimulation with wate r have received little attention. We report a series of positron emission t omography studies involving intraoral stimulation with deionized distilled water. Attempting to taste water produced large, bilateral activations in i nsular opercular, Rolandic and cerebellar cortices relative to resting with eyes closed or 'smelling' odorless air. The magnitude and volume of activa tion was substantially reduced when tasting water was contrasted with volun tary swallowing. This indicates that much of the activity induced by water reflects intraoral somatosensory or motor processing. Nevertheless, portion s of the insula, operculum, post-central gyrus and cerebellum remained sign ificantly activated in the contrast between 'tasting' water and swallowing. This activity appears to represent a specific neural correlate of fluid st imulation, and may reflect aspects of trigeminal, gustatory or thermal codi ng. These findings emphasize the large volume of cortex dedicated to intrao ral processing, and highlight the importance of controlling for nongustator y factors in studies of gustation.