Timing of cortical activation during spontaneous swallowing

Saliva accumulation in the oropharynx generates an automatic pattern of swallowing in the brainstem in animals. Previous fMRI studies have found that spontaneous saliva and water swallows in humans evoked activation following swallow onset in both precentral motor and postcentral somatosensory cortical regions. Using event-related averaging of continuous functional near infrared spectroscopy (fNIRS), we examined cortical hemodynamic responses (HDR) from 5 s before to 35 s after spontaneous reflexive saliva swallow onset in the lateral postcentral somatosensory and precentral motor regions in both hemispheres in healthy volunteers. Three HDR changes from baseline were detected. First, the onset of HDR occurred 2 s before swallow onset in the left postcentral somatosensory area and 0.67 s before swallow onset in the right postcentral somatosensory area. Second, an early HDR peak amplitude occurred 3-4 s after swallow onset in all four regions. Z scores relative to baseline pre-swallow cortical activity levels averaged 20 and 22.7 s in the right and left somatosensory regions and 10 and 15.8 s in left and right motor areas, respectively. Finally, a late HDR peak occurring between 22 and 23 s after swallow onset in the somatosensory regions and 17-19 s in the motor areas likely resulted from esophageal peristalsis. Overall, cortical activation timing relative to swallow onsets showed activation began before the pharyngeal phase of swallowing in the somatosensory areas. This indicates that somatosensory triggering of swallowing occurs not only in the brainstem but also in the cortex for reflexive saliva swallowing in awake humans.