Sensory Malfunction: Taste
Taste is the least important of the five standard senses, and to lose it is to have the inability to detect sweet, sour, bitter, or salty substances. All other attributes of food, from it's firmness, flakiness, temperature, texture, detailed flavour are functions of other senses. The same goes for anything else we might put in our mouths. Any sensation which is not sweet, sour, bitter, or salty is not part of taste. Detailed flavours are part of smell, whilst the firmness, the feeling of presence in your mouth, the coolness , or heat, these are part of touch.
In VR taste sensory data is still in its early stages, and in fact only one taste replicating sensor system has been developed to date. Partially this is because taste is so limited - it relies of touch and smell for the full experience - and partially, it is because taste is so hard to replicate. Thus, nearly all virtual sensory systems at this time, can really be said to be ageusic (lacking taste).
Taste is sensation produced by a stimulus applied to the gustatory nerve endings in the tongue. Some scientists indicate the existence of a fifth taste, described as savoury, based partially on the structure of those nerve endings. The gustatory nerves convert the tastes into electrical signals, which are sent as nerve impulses to the brain, where they are interpreted.
Any step save the last, can in theory be bypassed, and taste will still take place. So long as the nerve input to the brain is intact, even without a physical tongue, you will still be able to taste, with full acuity.
There are many things that can go wrong with taste systems, and the medical world has a plethora of terms borrowed from Latin for them.
The Latin word for the sense of taste, is geusia. Taking this term, geusia, and adding other phrases to it, to prefix it, gives us a range of conditions.
The table below, shows some of the prefixes that can be applied to this word, for all the things that can go wrong with this sense.
From these, we can create the following descriptors:
That gives us nine conditions, eight of which are malfunctions of the sense. If we look at each, in detail, we can see how each has something to offer the effort to replicate normal taste.
The term ageusia refers to the complete lack of the sense of taste. It is frequently caused by tissue damage to the nerves that support the tongue.
The lingual nerve passes taste for the front two-thirds of the tongue and the glossopharyngeal nerve passes taste for the back third of the tongue. If the lingual nerve is damaged or impaired, this leads to a metallic taste for the rest of the tongue.
Additionally, any damage to the taste buds themselves which renders them non-functional, will also cause ageusia. Irritiation, or long term tobacco abuse are prime examples of this.
In short, ageusia occurs when there is no data flowing from the tongue to the brain. The only real way to combat it, is to repair the damage, or use artificial stimulation, to bypass the damaged tissue, and feed new signals into the brain.
Cacogeusia is interesting. Instead of a loss of taste, or a distortion of taste when chewing food, there is a permanent, vile taste in the mouth, that does not go away. Cacogeusia occurs independent of any drug taste, food taste, liquid, or in fact any outside stimuli upon the taste receptors.
This would actually seem to be the exact opposite of ageusia. Rather than a complete lack of signal from the tongue to the brain, cacogeusia could almost be described as all five taste signals, permanently on and mixing.
Heterogesia is another interesting one. It occurs when the ability to differentiate between tastes disappears. For example, sweet and sour taste the same, or bitter and salty, or savoury and sour.
As it is eminently unlikely that one taste bud type would spontaneously transform into another type, the most likely explanations are thus:
Hypergeusia is a hypersensitivity of the sense of taste. Every taste is accurate, but profoundly magnified far beyond what it should be - impossibly sweet, deeply sour, pillar of salt.
The cause of hypergeusia is once again, most likely a malfunction in the signal propagation from the tongue to the brain. It is extremely unlikely that each taste bud has a direct conduit to the brain. As the structure of the tongue itself shows, nerve ganglion cells group tastebuds together then group those groups, and group again and again before transmitting a single, combined pulse back.
Like a logic gate, the groupings and combinings transmit a signal on only if they receive a signal from a taste bud, and a stronger signal if they receive signals from more than one:
Should this mechanism break down, as it does in cacoguesia, when the 'gates' become locked on transmitting a signal all the time, for all taste buds. With hypergeusia, they are not transmitting all the time. However, when one source taste bug transmits a nerve pulse, the transmission relays react as if all were transmitting, sending a stronger and stronger pulse back until the brain is informed all taste buds are detecting their fill of sweet, or sour, salty, or any other.
Hypogeusia is the opposite to hypergeusia, and rather than a hypersensitivity to taste, tastes are intensely muted, present, but very very weak.
The cause of hypogeusia is very likely to be the same as hypergeusia. It is most likely a malfunction in the signal propogation from the tongue to the brain. Once again, we look to the nerve ganglion cells which group tastebuds together before transmitting a single, combined pulse back.
As before, an error in the hierarchy would be the cause. However, this time, instead of sending a combined signal, when multiple are received, at one or more levels, it acts like an OR gate instead, in which it does not matter how many signals are sent, only one makes it through - at the same level.
Phantogeusia is where it gets really interesting. Phantogeusia is the creation of phantom tastes. It occurs where the taste system just generates a salty or sweet, or savoury taste in the mouth, of varying strengths, without anything actually in the mouth to trigger it.
It might come about through misfiring nerves, sending signals where there is no source, or it might be linked to memory in the brain, where the brain itself is creating those taste signals.
Whatever the reason, the net result is, under phgantogeusia, food tastes can be recreated, and mapped accurately, without the food present, or residual aftertastes.
Phantogeusia, hypogeusia, hypergeusia, heterageusia, cacogeusia and agusia all offer potential research paths towards the creation of SimStim for taste.
SimStim means Simulated Stimulation. Literally the recreation of natural sensory input completely, flawlessly, and artificially, through jacking in or hijacking the natural nerve pathways and overlaying new signals upon the old.
All these disruptions to geusia offer potential to help unlock the nerve signal pathways and their codes. To allow us to ultimately, one day, create a simulation ... that has taste.