The study, published in Journal of Texture Studies, tested the salivary response of human volunteers to a number of stimuli that have been reported to produce ‘mouthwatering’ sensations, such as food associated imagery, or feelings of hunger.
The authors explained that the concept of ‘mouthwatering’ is often used to promote or sell a food product, however “the physiological basis of this response is unclear.”
“Unlike animals, and in particular Pavlov's dogs, humans are not able to salivate at the thought of food … [However] in agreement with several studies there was no statistical increase in … salivary flow rates in response to visual images of food,” said the authors, led by Dr Guy Carpenter from the Salivary Research Unit, at Kings College London Dental Institute, UK.
“One potential criticism of purely visual images of food is that the brain is not convinced that it will receive the food and therefore, no mouthwatering would occur … Certainly during these experiments, none of the subjects said they experienced a ‘mouthwatering’ sensation while viewing the images,” added the authors.
Dr Carpenter and colleagues explained that the “‘mouthwatering sensation’ is a common phenomenon thought to occur … at the suggestion of food.”
Famously the Russian researcher Ivan Pavlov conditioned dogs to salivate at the sound of a bell, as the dogs had learned to associate the sound with food. However, the authors said that there are few reports of similar ‘conditioning’ in humans.
“Previous studies conflict as to whether humans have a conditioned reflex to the sight or thought of food despite being well established in other animals,” said the researchers
“Despite its perceived universal occurrence, there appears to be little physiological basis for the phenomena,” they added.
It has been suggested the sensation is an increased secretion of saliva released from glands, whilst others believe that it is purely the perception, or a greater awareness, of saliva that is already present in the mouth, explained the researchers.
The new study investigated whether a “psychic” salivary reflex exists, by comparing major salivary gland responses to visual stimuli and the handling of foods to other stimuli known to cause saliva secretion.
By using a computer screen to present the images rather than using food, the authors explained that all other possible stimuli, such as any olfactory component, was removed.
Analysis of saliva samples indicated differences between resting and stimulated (olfactory, gustatory and masticatory stimuli), however the researchers found “no obvious differences between resting and visual stimuli.”
Carpenter and co-workers found also found there was no effect from feelings of hunger (assessed by testing pre and post meal).
In a further experiment actual foods (rather than images of foods) were presented to subjects as the stimulus, which led to an increased salivary flow rate.
The authors said that although subjects did not taste the foods, “they were certainly aware of food-related smells” during the exposure periods. They added it was obvious to consider this was due to olfactory stimulation – a stimulus known to cause secretion.
“Handling food elicited a small, but measurable, increase in saliva flow,” confirmed Carpenter and co-workers.
“It is concluded that no true reflex secretion occurs in response to the thought or handling of food but that small amounts of saliva ejected into the mouth, possibly by muscle activity, is likely to be the cause of the mouthwatering sensation,” said the researchers.
Source: Journal of Texture Studies
Published online ahead of print, doi: 10.1111/j.1745-4603.2011.00290.x
“Is the mouthwatering sensation a true salivary reflex?”
Authors: Y. Ilangakoon, G.H. Carpenter