Sour taste increases swallowing and prolongs hemodynamic responses in the cortical swallowing network

Abstract

Sour stimuli have been shown to upregulate swallowing in patients and in healthy volunteers. However, such changes may be dependent on taste-induced increases in salivary flow. Other mechanisms include genetic taster status (Bartoshuk LM, Duffy VB, Green BG, Hoffman HJ, Ko CW, Lucchina LA, Weiffenbach JM. Physiol Behav 82: 109-114, 2004) and differences between sour and other tastes. We investigated the effects of taste on swallowing frequency and cortical activation in the swallowing network and whether taster status affected responses. Three-milliliter boluses of sour, sour with slow infusion, sweet, water, and water with infusion were compared on swallowing frequency and hemodynamic responses. The sour conditions increased swallowing frequency, whereas sweet and water did not. Changes in cortical oxygenated hemoglobin (hemodynamic responses) measured by functional near-infrared spectroscopy were averaged over 30 trials for each condition per participant in the right and left motor cortex, S1 and supplementary motor area for 30 s following bolus onset. Motion artifact in the hemodynamic response occurred 0-2 s after bolus onset, when the majority of swallows occurred. The peak hemodynamic response 2-7 s after bolus onset did not differ by taste, hemisphere, or cortical location. The mean hemodynamic response 17-22 s after bolus onset was highest in the motor regions of both hemispheres, and greater in the sour and infusion condition than in the water condition. Genetic taster status did not alter changes in swallowing frequency or hemodynamic response. As sour taste significantly increased swallowing and cortical activation equally with and without slow infusion, increases in the cortical swallowing were due to sour taste.

Saliva Secretion and Swallowing—The Impact of Different Types of Food and Drink on Subsequent Intake

“…The finding that citrus juice eased the swallowing of food ingested after its intake was in agreement with a study by Mulheren (2016), which showed that sour products increased swallowing frequency after intake [16]. This was also seen in our results, where there was an increase in the ease of swallowing the cracker after most of the test products.…”

Genetic Taster Status as a Mediator of Neural Activity and Swallowing Mechanics in Healthy Adults

“…A third consideration involves the extant neuroscience literature regarding taste-related sensation, or rather the gaps that exist in our current understanding of the complex underlying networks. Investigators using various stimuli and neuroimaging technologies have identified a range of brain regions and timing patterns that appear to be involved in the processing of taste stimuli generally (Babaei et al, 2010;Humbert and Joel, 2012), and specifically with regard to GTS (Bembich et al, 2010;Eldeghaidy et al, 2011;Mulheren et al, 2016). While all of these contributions are valuable, a clear picture of the neural pathways for processing of the multimodal inputs associated with taste stimulation, much less one that accounts specifically for GTS, has yet to emerge.…”

“…Eldeghaidy et al (2011) further investigated other brain regions that may be differentially activated based on GTS, and reported significant positive correlations between blood oxygen level-dependent (BOLD) responses and GTS in primary and secondary somatosensory cortices (SI, SII) the anterior cingulate cortex, and the anterior-, mid-, and posterior-insula after varied concentrations of isoviscous and isosweet fat emulsions. A functional near infrared spectroscopy (fNIRS) study by Mulheren et al (2016) found no significant differences in hemodynamic activity among GTS groups in the sensorimotor cortices using sweet and sour stimuli. Similar to the neuroimaging literature, the evidence of an effect of GTS on swallowing biomechanics is also mixed.…”

Guide to Enhancing Swallowing Initiation: Insights from Findings in Healthy Subjects and Dysphagic Patients

“…Among taste qualities, use of sour bolus such as carbonated [ 60 ] and citric acid liquids (e.g., lemon juice) [ 61 , 62 ] are the popular approaches for the treatment of dysphagia, and the studies reported therapeutic potential of using such stimuli in dysphagia rehabilitation. Mulheren et al showed that a sour bolus (0.08 M citric acid) induces larger response in the cerebral cortex as well as increase in the frequency of swallowing to a greater degree than water and sweet stimuli in 15 healthy subjects [ 63 ]. Recently, Elshukri et al tested the effects of 5-ml liquid bolus swallows of carbonated, citric acid, and still water on volitional swallowing performance and corticobulbar excitability evaluated by pharyngeal MEPs in healthy adults [ 64 •].…”