"By convention sweet is sweet, bitter is bitter, hot is hot, cold is cold, colour is colour; but in truth there are only atoms and the void."
This is a translation of the writing of Democritus of Abdera, a Greek philosopher who lived from about 460 BCE to 370 BCE. Only fragments of his writing remain as he was often ridiculed and dismissed by his successors.
Yet it is an incredibly insightful statement and easily thousands of years ahead of its time.
Democritus came up with the theory of atoms long before modern science revitalized the concept. He even went so far as to describe how atoms could join together through a hook-and-eye mechanism and why acids taste sour - they had pointy atoms which poked the tongue.
That, of course, is not the reason acids taste sour. Rather our sense of a sour taste is the result of molecular and ionic interactions.
The free proton associated with an acidic substance interacts with channels in the taste buds lining the tongue resulting in an influx of protons along while the un-dissociated acid is transported across the cell membrane acidifying the cytosol. The resulting change in internal pH triggers the cell to send signals to the brain through a complex pathway involving a number of internal structures.
In the brain, the signal is then combined with other signals from other taste buds and when or if a particular threshold is reached, an impulse is sent to the higher brain. Note that any given taste bud could falsely respond to an external stimulant so we need a number of taste buds to fire simultaneously to verify something sour is actually being detected.
In the higher brain, the sour taste is blended with information from the olfactory receptors and the visual cortex.
If the final determination is the food is in fact sour, the brain needs to then decide if the sour taste is unpleasant or acceptable. After all, a small amount of lemon juice on food or in water might be quite pleasant whereas sucking on a lemon might induce a negative response. In the latter case, we visually demonstrate the unpleasantness with a sour face.
But all of this involves atoms interacting with atoms. The initial response by the taste buds is to the presence of molecules and protons. The signal sent down neuronal membranes is transported by sodium and potassium ions. The interaction between nerve endings is modulated by neurotransmitters. The memories of previous sour tastes - which we call up to determine if the present sour taste is unpleasant or not - are carried by molecules.
It is the whole symphony of thousands or millions of neurons exchanging molecular moieties which ultimate results in our response to the stimulus. Billions of chemical compounds engaging with one another generate our sense of taste.
Or, as Democritus might have put it, "Sour is sour; but in truth there are only atoms and the void."
Yet ask anyone what sour tastes like and they can provide a response. They know what the sensation is like. Depending upon their own personality, they might even enjoy very sour food.
I had a friend who liked to eat lemons, skin and all.
The sour taste is a construct of our minds. To demonstrate this, we can even cause someone to respond to a sour taste when they are not actually tasting anything at all. Just the sight of someone sucking on a lemon can cause people to scrunch up their face. The thought of the taste produces the same response as actually eating the lemon in the first place.
Similarly, other tastes can be mimicked. One of the more interesting simple science fair experiments is to make lemon Jello cubes using an ice cube tray but colour the cubes with food dyes to give red, purple and blue cubes.
When volunteers eat the cubes and are asked to describe the taste, they invariably say "strawberry, grape and blueberry" even though they are tasting lemon.
What is equally interesting is doing the same experiment with the same volunteers but blindfolded in which case, they declare the cubes to be lemon Jello.
These sorts of experiments help to elucidate the connection of the mind with the brain. The latter is the gelatinous mass of interacting neurons and chemical compounds within our skulls. The former is our understanding of what all of those interactions mean.
Of course, that is an incredibly simplistic way of describing the mind-body problem. There is so much more to consider. How is our mind actually connected to our bodies? How do we sense our limbs and such? Where do we get our sense of time? And is it immutable?
But those questions are for next week's column. For now, the one thing we do know is there are atoms and the void.
