What our eyes, ears, mouth, nose, and skin register is made into a coherent image of the outside world by our brain. Every sense is handled by its own part of the brain. Researchers discovered these parts are far more interconnected than we had assumed. Especially the part dedicated to vision is influencing the other senses a great deal.
We hear a car, we smell coffee, and we see a cat. Our senses seem to function rather clear-cut. You see with your eyes, you hear with your ears, you taste with your mouth, you smell with your nose, and you feel by means of your skin.
But eyes, ears, mouth, nose, and skin are merely tools. They only receive signals from the outside world. The real seeing, hearing, tasting, smelling, and feeling happens in our brain. That is where a coherent whole is being formed of all this incoming information to create an image of the world that surrounds us, of the environment in which we live.
Canadian researchers discovered our senses are not such separated systems as they appear to be. One sense influences the other. Vision in particular plays an important part. In the course of evolution, vision has become ever more important to humans. This is contrary to what happened in many other animals that see rather poorly, but have developed a much better sense of hearing or smelling, like for instance the dog.
The researchers wanted to know more about the influence one sense exerts on another. They made a group of volunteers undergo transcranial magnetic stimulation. During a TMS procedure, a coil of copper wire is placed near the head of the person receiving the treatment.
The coil is connected to a pulse generator that delivers electric current to the coil. This generates a magnetic field that enters through the skull into the brain. There, it creates small electric currents that activate brain cells in the region of the brain just under the coil.
TMS is a noninvasive method used to stimulate small regions of the brain. It is used as an experimental treatment for neurological problems such as migraine, stroke, Parkinson’s disease, and depression.
During the Canadian study, via TMS the different parts of the brain involved in the senses were stimulated. The researchers were very surprised to see that the volunteers had a better sense of smell while stimulating the vision part of the brain. Stimulation of the audition brain area did not have the same effect. According to the researchers, the area of the brain dedicated to vision apparently plays a part in connecting information coming from the other senses as well.
Sensory information is entering our brain in different ways. Our eyes catch light, our ears process vibrations. Odour and taste molecules activate cells in mouth and nose. Our sense of touch depends on changes in pressure and temperature which are registered by our skin.
All these different forms of information activate networks of cells. The information goes from cell to cell in the form of electric currents and finally ends up in the brain’s cortex.
What we see is being send to the visual cortex, what we hear to the auditory cortex, what we smell to the olfactory cortex, what we taste to the gustatory cortex, and what we feel to the somatosensory cortex. All these areas are located closely together and brain cells in one part of the cortex also have contacts with brain cells in the neighbouring part.
The connections between brain cells are very flexible. Depending on how frequently cells are being activated, their connections become stronger or weaker. This is a process that continues throughout your life.
In people who are blind for instance, the brain cells within the visual cortex are out of a job since no information is entering via the eyes. The idle cells are confiscated by the somatosensory cortex, where our sense of touch is coordinated. That’s why blind people in general have more sensitive fingers than seeing people. After all, they have far more brain cells available for their tactile sense.
A similar process takes place in the brains of deaf people. Unused cells of their auditory cortex are incorporated into networks of the visual cortex. This rearranging of cells can take place in very limited time, as experiments with volunteers wearing a blindfold for a certain period have shown.
In some people, brain cells have too many connections. They suffer from a condition called synaesthesia, in which stimulation of one sense also arouses sensation in another. This sensory phenomenon results in cross-sense experiences, such as colour-hearing, colour-tasting, taste-hearing, and smell-vision. Grapheme-colour synaesthesia, in which an individual’s perception of numbers and letters is associated with the experience of colours, is the most common form of the condition.
Synaesthesia has to do with the abnormal merging of neural networks. During the growth of a baby in the womb, brain cells make contacts in abundance. Stimulated by nutrients present in the brain and not hindered by information from the exterior, the brain cells keep on chattering with as many neighbours as they encounter. It is as if they are practicing for the time they will be functioning in the outside world. They end up with far too many, often useless contacts.
After birth, most of these connections will be broken again. Fine-tuning takes place influenced by information that enters the brain from outside. With the maturation of the brain, extensive pruning happens and stronger connections form between the cells in one sensory system, sharpening and separating the senses. This process continues throughout life, but is going on rather rigorously in the first years of one’s existence.
Of course from time to time something goes wrong in this fine-tuning process. Connections that should have been broken may continue to exist. This may be the explanation for synaesthesia. At about three months of age, all children probably are synesthetes. In adults who suffer synaesthesia, the pruning process apparently hasn’t been finished completely. Brain cells involved in the processing of letters or numbers for example have unusual connections with cells that are active in the processing of colours. Or cells from taste networks communicate simultaneously with cells in sound networks.
Synaesthesia is a harmless, but, for the person involved, very interesting abnormality. It used to be considered a very rare condition, but now it has been shown to be rather common, appearing in maybe as much as about 1 in every 200 persons. Synaesthesia runs in families and hence is at least partly genetically induced.
The condition is much more common among people in creative professions, like artists and writers. Apparently, creative and abstract thinking is linked to cells with an above normal number of connections in certain parts of the brain. Famous people known to be synesthetes are among others the composers and musicians Franz Liszt, Duke Ellington, Frank Zappa and Lady Gaga, the Greek mathematician Pythagoras, the actress Marilyn Monroe, and the Russian writer Vladimir Nabokov.
But essentially we all have some form of synaesthesia when for example we experience high musical notes as clear and low notes as dark. Or when we feel blue, see red or are green with envy.
Three wise monkeys © Jeffrey Collingwood – Fotolia.com
Cortex images © Center for Life Science – Fotolia.com