Synesthesia is a perception and sensation anomaly in which the stimulation of one sense results in other senses being activated. It also describes when words, letters, numbers, or shapes are accompanied by a sense activation such as scent, a color, or taste.
An example of this would be an individual who sees the number 5 as being the color blue (which is called grapheme-color synesthesia). Synesthesia is described in neurobiology as 'crossed wires' - neurons that are associated with one sense are perhaps located in or near another neuronal area devoted to another sense.
Synesthetes perceive a stimulus and the crossed wiring makes multiple senses react to the stimulus. Individuals who experience this neural phenomenon describe being able to 'hear colors' or 'taste shapes' for example. Not much is known about the etiology of synesthesia but some evidence suggests that it can sometimes be hereditary and that it is more common in females than in males. Some research has suggested that synesthesia may be caused by stronger neural connections between certain parts of the brain. Synesthesia is commonly a durable condition that people have for their entire lives but in some cases acquired synesthesia can be caused by brain injury, brain disease, or stroke.