Decoding human swallowing via electroencephalography: a state-of-the-art review

Abstract

Swallowing and swallowing disorders have garnered continuing interest over the past several decades. Electroencephalography (EEG) is an inexpensive and non-invasive procedure with very high temporal resolution which enables analysis of short and fast swallowing events, as well as an analysis of the organizational and behavioral aspects of cortical motor preparation, swallowing execution and swallowing regulation. EEG is a powerful technique which can be used alone or in combination with other techniques for monitoring swallowing, detection of swallowing motor imagery for diagnostic or biofeedback purposes, or to modulate and measure the effects of swallowing rehabilitation. This paper provides a review of the existing literature which has deployed EEG in the investigation of oropharyngeal swallowing, smell, taste and texture related to swallowing, cortical pre-motor activation in swallowing, and swallowing motor imagery detection. Furthermore, this paper provides a brief review of the different modalities of brain imaging techniques used to study swallowing brain activities, as well as the EEG components of interest for studies on swallowing and on swallowing motor imagery. Lastly, this paper provides directions for future swallowing investigations using EEG.

Figure 1

Four swallowing phases: (a) oral preparatory phase; (b) oral transit phase; (c) pharyngeal phase; (d) esophageal phase.

Figure 2

An illustrative sample of ERP activation across different time ranges/windows (i.e., the three vertical rectangular boxes shown exist between roughly 100–300 ms). Each ERP activation respectively captures the peak of one of the three illustrated EEG signals. As ERP are quick voltages which are generated in the cerebral cortex due to some event or swallowing-related stimulus, they are very useful for identifying brain regions involved in the summation of fast and short duration events such as sensory stimulation (e.g. taste and smell).

Figure 3

An illustrative sample of MRCP contains two components: Bereishafts potential (BP) which occurs 1.5s before swallowing onset, and negative slope which occurs after swallowing onset. Besides identifying brain regions involved in swallowing, MRCP allows distinguishing between cortical motor preparation (BP), cortical control of swallowing execution (swallowing onset), and cortical swallowing regulation (negative slope).

Figure 4

An illustrative scheme of the dual-tree complex wavelet transform. x(t) is an input signal. The first pair of wavelets (Level 1) are offset from each others by one half. Another pair of wavelets (Level 2) are forming an approximate Hilbert transform pair.