Synaesthesia, or synesthesia in American English, is a neurological phenomenon wherein the activation of one sense (via a sensory or cognitive pathway) causes the involuntary activation of a second, unrelated sense (via another sensory or cognitive pathway) at the same time. For example, hearing a particular music note may trigger a person with synaesthesia to visualise a specific colour, or they may associate each letter of the alphabet with a specific colour.

Individuals with this condition are referred to as synaesthetes (or synesthetes). In fact, the word synaesthesia itself is a coined derivation of the Greek words syn and aesthesia which mean ‘union’ and ‘sensation’, respectively – collectively implying a union of senses.

It is believed that between 3-5% of the population has some form of synaesthesia, and that incidence is higher among women than men. More often than not, synaesthetes consider their condition to be a gift due to how it ‘enhances’ their sensory experience of the world. Synaesthesia has been associated with augmented mental imagery and subsequently elevated levels of creativity and memory due to the ease at which connections are made between different sensory concepts.

Symptoms

There is a spectrum of possibilities in how synaesthesia may manifest itself due to the numerous sensory combinations that could occur in an individual. The secondary sensation is usually experienced in the mind of the synaesthete. However, in some cases, the synaesthete may feel like the secondary sensation occurs in their peri-personal space (the space surrounding a person’s body within which one can reach or be reached by an external individual or object) or as a projection where the primary sensation occurs.

Most times, an individual may have more than one form of synaesthesia. Some of the known sensory combinations of synaesthesia include:

• grapheme-colour synaesthesia – association of specific colours with letters and numbers; this is probably the most common form of synaesthesia
• chromaesthesia – association of specific colours with sounds (e.g., music notes, car horns); this is also a commonly occurring form of synaesthesia
• auditory-tactile synaesthesia – experience of specific body sensations (e.g., tingling at the back of the neck) with sounds
• lexical-gustatory synaesthesia – experience of specific tastes with different words (not food related)
• spatial sequence synaesthesia – visualisation of numbers or sequences as points in space
• ordinal linguistic personification – association of personalities or genders with ordered sequences (e.g., days of the week, months of the year)
• number form – visualisation of a mental number map associated with the thought of numbers
• mirror-touch synaesthesia – experiencing the same form of touch upon seeing another person being touched

Causes

Although research into synaesthesia has picked up to some extent over the past few decades, there is still limited data available to draw definite conclusions on what exactly causes the condition.

It is widely believed that there is a genetic aspect involved in the incidence of synaesthesia. A family history of synaesthesia does seem to increase the chance of an individual experiencing the same.

The common school of thought is that synaesthesia arises due to a sort of ‘cross-wiring’ in the brain. That is, researchers believe that there might be overlaps between neurons and synapses of one sensory system with those of another. However, it is not clear as yet if these excessive connections occur in synaesthetes alone, or if they are present at birth and then reduce and separate with growth – but does not occur in the case of synaesthetes. The current level of research suggests that synaesthesia originates in the cerebral cortex and limbic system of the brain. However, further research would be required to affirm this.

Diagnosis

In general, the following observations should be an indication of an individual being a synaesthete:

• the experience of secondary sensations has been long-term, usually starting from childhood
• the secondary unrelated experience for a given primary stimulus remains consistent and would produce the same outcome if tested months or years apart (e.g., the same colour is always associated with a particular word or sound)
• the secondary sensation is involuntary and cannot be controlled

In addition, there is a Synaesthesia Battery test available online to support self-diagnosis.

Of course, a formal diagnosis from a medical professional may also be obtained although there are no specific tests for diagnosing synaesthesia.

Cover illustration from Gazette 2.0