Cognitive Science: An Introduction/Gustation

The information

• Differentiation between taste and flavour
• What information in the environment is involved in taste?
  • Essentially, potential nutrients or toxins (e.g., carbohydrates, Na+, L-glutamate, L-amino acids etc...)
• What is transduced? (water-soluble molecular compounds interact with proteins on the apical tips of taste buds)
  • Although chemosensory cells in the gut respond to some of the same chemicals that stimulate taste receptor cells, they likely do not contribute to conscious perception of flavour

Overview

The gustatory system is the sensory system responsible for the sense of taste. The primary flavors detectable by humans are sweet, sour, bitter, salty, fatty and umami. Umami is a Japanese word for “savory” which was recently added to the list of our basic tastes. Taste perception differs within each person, and contributes to food choices.  The sense of taste contributed to evolutionary survival by testing the foods before they were eaten. Taste is used to sense if food is nourishing or harmful to ingest. It can also signals internal needs such as hunger and thirst and the nutritional qualities of food. Certain taste indicators can indicate which foods to avoid or consume. Bitter or sour taste suggests that something might be poisonous, while sweet and salty are a sign of food rich in calories or nutrients.   The ability to taste begins with the tongue which is covered with tastebuds called papillae. Taste buds can be found in the walls and the grooves surrounding the papillae. Taste buds contain fine microscopic hairs called microvilli. When these microvilli encounter a substance in the mouth these small hairs send messages to the brain determining if something is sweet, salty, sour, bitter, or umami. These messages from the taste buds are sent to the brain through cranial nerves. The sensory information of taste is sent to the gustatory cortex of the brain, which lies within the cerebral cortex responsible for thinking, perceiving, producing and understanding language. The most interconnected senses are gustation and olfaction. Olfactory sensory receptors in the upper part of the nose contain distinct cells that send messages to the brain which allow us to smell. Food releases chemicals that travel into the nose that then prompt the sensory receptors. Both taste and smell then work together to produce the perception of taste . 

Gustatory Cortex

The gustatory cortex refers to the brain structure which is responsible for the perception of taste. It consists of two substructures, these being the anterior insula located on the insular lobe and the frontal operculum and dorsal insula on the frontal lobe. It has been recorded that there is a secondary gustatory area in the brain as taste neurons have been found in primates within the precentral opercular area. The pathway to the gustatory cortex begins with the receptors in the oral cavity known as the taste buds. The taste cells within each taste bud are innervated with primary sensory axons which are connected to the facial nerve (7th cranial nerve), glossopharyngeal nerve (9th cranial nerve), and vagus nerve (10th cranial nerve). All other parts of the oral cavity which contain taste buds are the posterior tongue, the pharynx, larynx and epiglottis. These are all innervated by the glossopharyngeal and vagus nerves. Taste cells transmits an electric signal through their axon to the rostral division of the nucleus tractus solitarius of the medulla upon an action process when a taste stimulus is detected. These then project to the ventral posterior complex of the thalamus until they reach the ventral posterior medial nucleus. From there, this nucleus casts to the cortex.

Taste change

Various factors play a role in perceiving tastes differently. The medical term of a change in the sense of taste is “dysgeusia” while a total loss of taste is called “ageusia”. Taste buds are replaced with a new one every 10 to 14 days. The regeneration of taste buds slows down around the age of 50 years causing a blander taste in food. Other factors can also change the taste of foods, such as antibiotics and medication for lowering cholesterol and blood pressure. A compromised sense of smell can also change the way food is interpreted, which can lead to eating stronger tasting, more savory food. Taste can also change due to pregnancy. Over 90% of women experience some alteration of taste during pregnancy. Particularly in the second and third trimester, foods with higher salt content were consumed more. Taste preference that did not change during pregnancy was that of sour and bitterness. Changes in taste may also be caused by the increase in sex hormones estrogen and progesterone.

Brand Names Affect Taste

People are known to often close their eyes while tasting food or drink. This can help focus attention on the fine nuances sensory information. While the eyes are open, outside factors can influence how taste is perceived, such as social context and even a brand name. One study looked at the taste ratings of identical turkey portions given to participants. In this study, one turkey was cooked and portions were given to participant on two plates. The sole difference between these two plates was a label placed behind the plates of turkey. One was labeled with a well-know brand name and the other with an unknown brand name. It was found that even though both samples were identical the participants rated the well-know label to be better tasting. When given the option to label the turkey as tasting identical, participants they still indicated the brand name to be better-tasting. This study revealed that human taste can be influenced by external factors. Research indicates that consumers experience leading brands and generic brands differently, solely because of their perceived status. Another study found that when presented with generic brands and leading brands in a blind taste test, participants sensed no differences in taste. The sense of taste is influenced by social perception and external cues. 

Artificial Flavor

A multi-billion dollar industry exists in what is called flavor engineering, where chemists manufacture various taste experiences by blending aromatic chemicals, oils, botanical extracts, and other essences. Flavor chemists focus on improving exiting taste rather than creating new ones with the mindset to create foods that consumers crave. In the making of the flavor formulas, there is an average of 2,000 chemicals and 500 natural flavors and can take up to 80 tries to get the desired flavor.

Sensation in the human gustatory system

• Description of taste buds ("tongue map" discredited (Lindemann, 1999)
  • Differentiated epithelial cells, constantly regenerate
• Summary of major taste receptors involved
  • Type I (degrade of absorb neurotransmitters)
  • Type II (Recepetors)
  • Type III (Presynaptic cells)

Perception in the human gustatory system

• Summary of components involved in flavour perception (gustatory, olfactory, somatosensory (visual, tactile))
• Summary of flow of information after hair cell stimulation (on taste receptor cells)
  • 3-14 sensory ganglion neurons (including those for pain, tactile information and thermal information beyond simple gustatory information) innervate each taste bud (Chaudhari, 2010)
  • Gustatory stimuli initiate a cascade of signals that eventually stimulate gustatory afferent nerve fibres
  • Information travels to hindbrain
  • Description of two theories of how taste receptor codes are translated to codes for taste perception ("labeled line" vs. combinatorial system)
• Sensory integration in the brain

There are high-level detectors in the mind whose job it is to identify certain characteristics of what we eat. Psychologist Debra Lieberman suggests that we have high-level detectors for things that are good for us, such as sugar, protein, and salt, as well as for things bad for us, such as toxins and pathogens. However, detection of these things do not come completely from the gustatory system. They get input from visual, olfactory, and tactile sensations, as well as top-down beliefs about what we're eating. The emotion of disgust can result from the detection of consuming pathogens or toxins.