Synesthesia is a form of perception in which sensory impressions are crossed. So, in one example, people will experience colour as a musical tone. Higher level impressions can be crossed as well so alphabetic letters can be experienced with colours. So, for example, the letter ‘B’ may be experienced with a shade of pink. It had long been thought that synesthesia was a rare condition. As such, it did not receive much research interest. However current research has found that at least one percent of the population exhibit synestesic perception. It is not a rare condition. However people who experience it are surprised to find that others do not share their experience. They learn to keep silent about their perceptions.
Current research has shown that synesthesia arises because of connections between centres of perception. So using the colour/letter example above, a brain scan will show the region concerned with colour will be activated when the letter ‘B’ is presented to the subject. It was thought that these cross connections were unusual but it has been found by research that they are a standard part of the structure of the brain. In subjects without synesthesia, these connections are inactive but they are present. A research question arises as to why thee interconnections exist if their functioning is suppressed.