Some people experience a crossover in senses. Known as synesthesia, it can manifest as "seeing" music in colors, or "tasting" words. Typically thought to be fairly rare in the general population, scientists have found that the number of people who have this sensory cross-wiring in the brain may actually be much higher.

A new paper, published in the journal Consciousness and Cognition, has found that at least one-fifth of people tested experienced a sensation in which they can hear what they see. The researchers discovered that as these people see flashes of light or visual movements, it can correlate with them hearing certain sounds that aren’t actually there, such as a buzzing, that are real enough to distract them from external noises.

Conducting the tests on 40 people who had no previous reports of synesthesia, the researchers found that 22 percent of the participants seemed to be able to ‘hear’ noises when shown flashes on a screen. They suspect that the noises sound like generic background noise, and so have gone previously unnoticed.

If you want to take part in the research being carried out to investage the extent of this form of synesthesia in the general population, you take take part here. 

The researchers think that there could be an explanation as to why this mild form of synesthesia is seemingly so common in the population, while other more extreme versions are not. They suggest that it is all down to how frequently the two stimuli occur at the same time. Because movement and sound are often correlated in our day-to-day lives, it could be that the link between the two is heavily reinforced, strengthening the connections between the two senses in our brains.

It could also be because in some cases, forging a link between the two senses may actually be of benefit. Those people who were identified as having the visual-auditory link were better able to distinguish between whether or not two flashing patterns of Morse code were the same or different sequences. If people can hear sounds when they see patterns of flashes, it might help them discern the patterns.

When it comes to other forms of synesthesia, however, this reinforcement is not frequently experienced. For example, words and colors do not frequently occur at the same time, and neither do sounds and touch, which is another type of rare synesthesia.

It is becoming increasingly clear that synesthetes may well have the cross-activation hard-wired into their brains, rather than an imbalance of chemicals as has previously been suggested – as demonstrated by the case of a woman with synesthesia who was struck by lightning. After the event, she lost her ability to see colors when she heard music, only for it to return again after a period of time.