Functional brain imaging of swallowing: an activation likelihood estimation meta-analysis

Background: The neuroanatomy and functional significance of swallowing-related networks in humans are not entirely clear. Activation likelihood estimation (ALE), a novel quantitative meta-analysis technique, was employed to pool the results of functional brain imaging studies for the investigation of the neural control of swallowing (Sörös et al. 2008).

Methods: Studies that were included in the meta-analysis 1) examined water swallowing, saliva swallowing, or both, and 2) reported brain activation as coordinates in standard space. Using these criteria, a systematic literature search identified 7 studies that examined water swallowing, and 5 studies of saliva swallowing. An ALE meta-analysis of these studies was performed with GingerALE (http://brainmap.org).

Results: For water swallowing, clusters with high activation likelihood were found in the bilateral sensorimotor cortex (Fig.1; clusters 1, 2), right inferior parietal lobule (cluster 3), and right anterior insula (cluster 6). For saliva swallowing, highest ALE values were found in the bilateral cingulate gyrus (Fig.2; clusters 1b, 1c), the right supplementary motor area (cluster 1a), left pre- and postcentral gyrus (clusters 2a, 2b), and right posterior insula (cluster 3). A between-condition meta-analysis revealed clusters with higher activation likelihood for water than for saliva swallowing in the right inferior parietal lobule (cluster 1), right postcentral gyrus (cluster 2), and right anterior insula (cluster 3). Clusters with higher activation likelihood for saliva than for water swallowing were found in the right supplementary motor area (Fig.3; cluster 6c), bilateral anterior cingulate gyrus (clusters 6a, 6b), and bilateral precentral gyrus (clusters 7, 8).

Conclusions: This meta-analysis emphasizes the distributed and partly overlapping cortical networks involved in the control of water and saliva swallowing. Water swallowing is associated with right inferior parietal activation, likely reflecting the sensory processing of intraoral water stimulation. Saliva swallowing more strongly involves premotor areas, which are crucial for the initiation and control of movements.

Reference: Sörös P, Inamoto Y, Martin R. Functional brain imaging of swallowing: An activation likelihood estimation meta-analysis. Hum Brain Mapp 2008 (in press).