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Assignment #2 - Article Review:

Brain mechanisms underlying flavour and appetite.
By Edmund T. Rolls

http//www.ncbi.nlm.nih.gov/pmc/articles/PMC1642694/pdf/rstb20061852.pdf/?tool=pmcentrez

This article is a review of several papers with the agenda to address three main areas:

• the evaluation of neuronal processing of taste and smell
• the relationship between 'pleasantness' of taste and smell and the brain
• the mechanistic link between these, appetite and food consumption

Neurophysiological studies in non-human primates was examined as supporting evidence to research using functional magnetic resonance imaging (fMRI) in humans. Discussed first are those of the neurophysiological studies of which findings are based on the degree of neuronal stimulation before and after feeding to satiety.

Taste processing in the primate brain:

The pathways involved in taste processing involves the Primary Taste Cortex which includes the anterior insula and adjoining frontal operculum; and the Secondary Taste Cortex which includes the caudolateral orbitofrontal cortex.

The primary taste cortex houses:

• Taste neurons - responding to sweet, sour, bitter, salt and umami stimuli
• Somatosensory neurons - responding to viscosity, fat texture, temperature and capsaicin stimuli
• Combination neurons - responding to both taste and texture

The secondary taste cortex houses:

• Neurons with high specificity - responding to a specific stimuli
  

The processing of taste requires the stimulation of many neurons c0rrelating to the five protypical tastes noted above but also neurons that respond to water and astringencies such as that of tannic acid.

The pleasantness of the taste of food was characterized as a reward value and is found to stimulate neurons in the secondary taste cortex but not the primary taste cortex. When a person is hungry, thereby motivated to eat, the food is considered pleasant tasting hence rewarding. However if the food is eaten to satiety then it is no longer pleasant and is considered 'hedonically neutral'.

The ability of a neuron to respond to a stimulus corresponds to feeding to satiety; this applies specifically to the food being consumed. Monkeys were fed glucose to satiety at which point the neurons had decreased in responsiveness to glucose but not to fruit juice. A similar study on humans reported that the glucose source was not longer pleasant at satiety but the flavour intensity was undimished.

The represenation of flavour: convergence of olfactory and taste inputs

Within the orbitofrontal cortex 112 neurons responsed, corresponding to unimodal stimuli; taste 34%, olfactory 13%, visual 21%, but were found closely arranged. Some of these neurons also demonstrated multimodal responses; taste and visual stimuli 13%, taste and olfactory 13%, olfactory and visual 5%. It is suspected that the multimodal responses are resultant of the close proximity of the neurons in the brain, passing signals between them.

Flavour is defined in the article as, 'a representation which is evoked best by a combination of gustatory and olfactory input' evidenced by the convergence of modalities in the obitofrontal cortex.

Presentation of pleasantness in the brain

Olfactory neuronal responses in the orbitofrontal cortex demonstrated the link with visual and taste stimuli by firing in response to the sight and taste of specific foods and then diminishing upon satiety of that food; connecting behaviour response to hunger. When this was compared with pleasantness, they suggest that the firing of the neurons prior to consumption is a representation of a hedonic response and therefore links pleasantness to the odour of the food to be consumed.

A human study was done in which the food was chewed but not swallowed and the neurons again diminshed in response to satiety of the taste. Subjects stated that the intesity of flavour was unchanged but the pleasantness was gone suggesting that neuronal activity is olfactory related.

Sight, texture and temperature

When visual reinforcement of food is altered the neuronal responses in the orbitofrontal cortex are altered, linking food choices with vision and prediction of flavour. This area of the brain, as mentioned previously, is an active area of convergence of somatosensory input. This is evidenced by connections made between taste and texture. An experiment done by altering food consistancy; addition of methyl cellulose or gelatin or by pureeing displayed changes in the neurophysical state. Similar results were demonstrated for temperature of food.

Studies done to evaluate fat in the mouth revealed a correlation to texture but not chemical sensitivity. The study showed neurons respond to cream and milk but also paraffin oil and silicone oil. A convergence was observed in orbitofrontal neuronal activity in relation to texture and taste, each having their own specific neurons.

Sight of Food

Neuronal activation in the orbitofrontal cortex was shown to reverse very quickly when visual stimulus was altered such that it no longer respresented food. The amygdala region of the brain also shows neuronal response to food-visual stimuli but is not altered when the stimuli is dissociated from food. This further exemplifies the hedonic relationship to the secondary taste cortex.

fMRI Studies in Human

The results presented here support those presented above; however, was specific to human subjects.

Critique

Some observations in this paper were vague; for example the connection between sight, texture and temperature . Its was noted that the neuronal responses to altered texture and temperature existed but it wasn't clear as to whether these responses were positive or negative. I would have liked to learn more about the connection between food texture adversions and the neuronal activation.

Over all I found this paper enlightening, particularly the connections made between the dimishing pleasantness, taste and olfactory responses when a particular food is consumed to satiety.

The organization of the paper could have combined the neurophysiolocal study with the fMRI study as opposed to presenting one and then the other, it resulted in a sense of redundancy.

The images were very effective in illustrating proximity of the various neurons to each other, complimenting the literature with regards to convergence of unimodal sensory input.

This paper certainly highlighted the functional complexity of sensory pathways involved behind the relatively simplistic structural organization of the taste buds.