Cerebral Cortical Processing of Saliva and Water Bolus Swallowing

Objectives: Functional brain imaging has substantiated the role of the cerebral cortex in swallowing, corroborating electrophysiologic studies in awake primates. However, neuroimaging studies have reported some discrepant findings, possibly related to task-related motion artifact or variable imaging modalities, swallowing tasks, and participant demographics. The present study identified swallow-related brain activation that is common across brain-imaging studies.

Methods: A systematic literature review was conducted using: (1) PubMed and ISI Web of Science keywords, (2) references in identified papers, (3) papers citing identified studies, and (4) recent swallowing papers. A voxel-wise meta-analysis, based on activation likelihood estimation (ALE), was then performed on studies that: (1) included water swallowing, voluntary saliva swallowing, or both, and (2) reported results in stereotaxic coordinates. Of 29 studies identified, 10 met these criteria. Differences between the ALE maps for water and saliva swallowing were computed by subtracting the voxel-wise ALE values for the two swallowing conditions.

Results: Voluntary water swallowing activated the bilateral sensorimotor cortex, right inferior parietal lobule, and right anterior insula. Voluntary saliva swallowing activated the bilateral cingulate gyrus, right supplementary motor area, left precentral and postcentral gyri, right precentral gyrus, and right posterior insula. Clusters with significantly higher ALE values for water, than saliva, swallowing corresponded to the right inferior parietal lobule, right postcentral gyrus, and right anterior insula. Clusters with significantly higher ALE values for saliva swallowing included the right supplementary motor area, bilateral anterior cingulate gyrus, and bilateral precentral gyrus.

Conclusions: This quantitative neuroimaging meta-analysis indicates that saliva and water swallowing activate overlapping but distinct neural networks. Water bolus swallowing activates the right inferior parietal cortex, possibly reflecting sensory processing. Saliva swallowing more strongly activates premotor areas that are crucial for movement initiation and control. These findings suggest fundamental differences in the neural control of water swallowing and voluntary saliva swallowing.