Before we can actually perceive something from our environment. we need to be able to sense it. Sensation doesn’t necessarily always translate into perception within the mind. This translation from sensation to perception depends on the context of the sensation. The context of a particular sensation may be our emotional state at the time, our memories associated with the sense, our health status, what occurs as a consequence of the sensation amongst other things, or what happens to occur while we’re sensing that particular sensation. It is this context which influences how we perceive various sensations.
Each sense is mediated through its own receptor and specialist organ. It is at this point that the external stimulus is transduced into something which can be encoded into electrochemical signals along our neuronal cells which then relay to the brain where they are interpreted to allow the body to respond, and potentially become consciously appreciated so a behaviour, memory, emotion or thought may be generated in response. Our senses provide us with the raw material with which we construct our reality.
For Vision , our eyes detect electromagnetic radiation, To hear, structures deep within our ears detect vibrations in the air around us, touch is the sensation of physical movements and pressure on our skin, smell (olfaction) interprets airborne chemicals and taste detects soluble chemicals in objects we are interested in ingesting.
Taste and smell are referred to as the chemical senses as they respond to specific molecules. As we all know, Taste is detected through taste buds on the tongue. Taste buds are housed within papillae. The 5 basic tastes are Sweet, sour, salt, bitter and umami. Each of these tastes is detected by a different type of taste bud, expressing different cell surface receptors on its taste receptor cells. These 5 different tastes are important in their own right, as they signal to the brain what the content of the food is, and this enables us to make a decision as to whether or not something is worth eating.
- Sweet tastes are generated from the presence of sugars, sucrose, glucose.
- Sour tastes are generated from the presence of acid (chemically, the hydrogen ion).
- Salty tastes arise from sodium.
- The bitter taste is a response to toxins. Hence why we don’t much like bitter.
- And umami is the taste of amino acids, such as glutamate. Important for sensing protein content.
There is some evidence to suggest that there may even be tastes designated for ‘metallic’ and ‘fatty’ taste, though this is not as yet conclusively proven.
The structure of tastebuds. Picture adapted from ‘The Receptors and Cells for Mammalian Taste’ by Chandrashekar et al, 2006, Nature.
Taste is sensed at the tongue and this is transmitted to the brain via the glossopharyngeal and facial nerves, These nerves enter the brain at the brain stem where the information is transmitted to various regions of the cortex. The integration and processing of sensory input from the tongue mainly occurs in the gustatory cortex, which is housed around the Insula and frontal operculum region of the temporal lobe. (The processing of gustatory stimuli may not be exclusive to the gustatory cortex, there may be a more diffuse component to it). The somatosensory cortex will interpret tactile information. This is important concerning the texture of the food, and will integrate with information from other sensory modalities to construct a final percept of the food. The gustatory cortex then projects information to the limbic system and the orbitofrontal cortex. The limbic system will be involved in the emotional and memory aspect of the processing of the taste, helping to generate a preference or aversion to the food, based on previous experiences, or social context. The orbitofrontal cortex may play a role in conscious appreciation of the food.
The gustatory pathway
The innate responses to umami and sweet are generally positive, there is an innate ‘good taste’ associated with foods which come across as sweet or umami. The evolutionary reason being sugars are an immediate and useful energy source. Umami, indicating amino acids (the building blocks of proteins), signals protein content - also vital biomolecules.
Bitter and sour have innate aversive responses. Bitter taste is a response to toxins in food. We have evolved to recognise certain classes of compounds as potentially hazardous. Sour is aversive because the internal environment of the body must strike a delicate pH balance. Adding acid to this could potentially damage the digestive system, or indeed alter the pH of the blood, leading to disruptions in homeostasis.
Salt is an interesting one. Our ability to taste salt is an important regulatory mechanism for the body to control its salt levels. Salt is vital for neuronal function, amongst other functions in the digestive system and basic cellular homeostasis. So too little salt isn’t good, but too much salt would also cause many problems. Acceptable levels of salt appear to interact with a specific salt sensing receptor, called the epithelial sodium channel (ENaC). This generally provides a positive, attractive response, at least as has been demonstrated in mice. There are other, less characterised salt sensing cells and receptors, which were shown, by Oka et al, 2013, to activate neuronal pathways associated with bitter and sour responses, making these receptors intricately tied with producing an aversive response to salt. These findings demonstrate that the differentiation of a positive salty taste and aversive salty taste appears to occur on the molecular level on the tongue.
It is also important to note that recent studies have shown that there is no taste map on the tongue. Different tastes can be sensed anywhere, and it appears that some tastebuds may even respond to tastes other than the ones they’re designated for, if the molecules are present in high enough concentrations.
If the chemical signature of a piece of food comes back as being something which is not worth eating, then we may respond with aversion. This would be a basic explanation of how some kinds of taste aversion are generated.
For humans, this picture becomes a little more complicated due to differences in learning behaviour and intelligence. We actually enjoy some sour and salty and bitter things. In addition to the primal responses we can essentially learn to like or dislike something.
The mechanisms underlying perception of taste are poorly understood, but a general framework may work along the lines of context of tasting food. If peers express a dislike for a food, when one is first coming in contact with a food, this may influence a dislike for that food. Furthermore, knowledge of the benefits of the food may add a ‘top-down’ input of control over what one tolerates.
Our preference for food is regarded as a conditioned behaviour. The taste of the food is part of conditioning this preference. Additionally, the aroma and the texture of the food contribute to our overall perception of food. Generating a preference or aversion to a food involves more than just the specific taste, but shifts in preference may occur in response to different nutritional needs and differing social context of eating food. How the brain builds a conscious response to taste is immensely complex, and we’re only at the tip of the iceberg when it comes to understanding how this occurs.
An excellent post that ought to get more reads!