Temporal resolution in swallowing neural control: the way forward.

Investigation of the neurophysiologic control of swallowing presents the researcher with a fascinating and challenging journey through the varied terrains of neuromotor control. A unique task by virtue of its complexity, the swallowing process is engaged both volitionally and reflexively, presumably driven by both cortical and subcortical neural structures, and is complicated by the nature of swallowing which is frustratingly inconsistent across such contexts as arousal, attention, diet level and task. Early researchers have extensively defined the imperative role of brainstem central pattern generators in the creation of the hard-wired swallowing motor plan (Miller, 1982; Jean, 1984, 2001), perhaps almost to the exclusion of recognition of cortical involvement. The pendulum has perhaps now swung too far in the other direction. More recent trends in swallowing research have heavily emphasised in situ cortical activation during swallowing using brain imaging techniques. Studies utilizing functional magnetic resonance imaging (fMRI) have revealed that many regions of the cortex are activated during the swallowing process, with major contributions from the primary motor and somatosensory cortices, the supplementary motor area (SMA), cingulate gyrus and anterior insula (Mosier et al., 1999a,b; Hamdy et al. 1999; Kern et al., 2001; Martin et al., 2001, 2007; Mosier and Bereznaya, 2001). Increased activations at these sites are generally reported to be bilateral, with hemispheric dominance evident in most studies. Which hemisphere is ‘swallowing dominant’ remains unclear from studies to date (Hamdy et al., 1999; Mosier et al., 1999a,b; Martin et al., 2007), with implications that task, gender or individual variation may contribute hemispheric dominance. But even with many skilled researchers developing creative paradigms to tease out cortical contributions, this research is fraught with difficulty and to date has been confined to evaluation of the entire complex swallowing process, thereby yielding spastically precise but functionally non-specific information regarding cortical localisation. With the pharyngeal swallow contained in a temporal envelope of approximately 800–1200 ms, and significant movement artifact generated by lingual and pharyngeal muscular structures, the temporal resolution of fMRI struggles to capture the intricacies of swallowing neural control. Nonaka et al. (2009) are to be lauded for their contribution to the task of dissecting the neural substrates of specific temporal components of swallowing. By evaluating only pre-motor brain activity, very specific information regarding neural control for motor planning can be harvested with precise temporal specificity and with management of extra-cranial lingual artifact that has plagued