Can you imagine being able to hear something that you taste, or taste a sound? People with synesthesia experience things like this everyday. There is a sort of union of senses in their mind, one that links senses that would otherwise be unrelated.
Synesthesia is defined as “the production of a sense impression relating to one sense or part of the body by stimulation of another sense or part of the body.” The most commonly reported form is grapheme, or color synesthesia: letters and numbers are always associated with an inherent color. For example, 8 is always blue, while 9 is green. Another form is number form synesthesia: numbers, dates, months of the year, or days of the week can elicit precise locations in space. These dates can appear as a three-dimensional map, or dates can be described as physically farther away or closer to the synesthete.
Greek philosophers once asked if the color of music was a quantifiable quantity, a question that was later asked again by Isaac Newton, when he proposed that musical and color tones share a common frequency. These were the first instances of interest in “colored hearing”. Medical descriptions, however, did not arise until 1812, in a thesis written by the German physician Sachs. The first empirical survey of colored photisms was conducted by Gustav Fechner in 1876, and Carl Jung referred to “color hearing” in his Symbols of Transformation in 1912. However, because of difficulties in measuring subjective experiences and the rising popularity of behaviorism (making the study of any subjective experience a sort of taboo), the study of synesthesia faded into scientific oblivion somewhere between 1930 and 1980.
Certain regions of the brain are dedicated to specific functions. When these sections interact, or cross-talk, it can account for the many different types of synesthesia. For example, when an individual associates color with graphemes, it would most likely be due to the fact that there is cross-activation in the region associated with grapheme-recognition and the color area (known as V4). An alternative cause for synesthesia is known as diminished feedback. This has to do with a reduction in the amount of inhibition along an already existing feedback pathway, which means that the signals that are feeding back from the later stages of sensory processing can influence the earlier stages, causing an association between the two different stimuli. A final hypothesis about the causes of synesthesia is one that is based on ideasthesia: synesthesia is a phenomenon caused by the extraction of meaning from a stimulus.
There are several more notable forms of synesthesia, along with grapheme-color and number-form synesthesia:
- Chromesthesia: The association of sounds with colors. Everyday sounds, such as cars honking, doors opening and closing, and people talking can trigger seeing certain colors. People can also associate musical notes and keys with colors, which often leads to perfect pitch, since those colors allow them to easily identify notes and keys.
- Number Form: A mental map of numbers will automatically appear whenever someone thinks of numbers.
- Auditory-Tactile Synesthesia: Certain sounds induce sensations in parts of the body. This particular form can either originate at birth or be acquired later on in life, and is one of the least common forms.
- Ordinal Linguistic Personification: Ordered sequences, such as ordinal numbers, days of the week, or alphabetical letters, are associated with different personality types or genders. This form normally occurs alongside other forms, usually grapheme-color synesthesia.
- Mirror-Touch: A rare form where an individual feels the same sensation that another person feels. People with this form tend to have higher levels of empathy compared to the general population, and it may be associated with the mirror neurons in the brain.
- Lexical-Gustatory: Certain tastes are experienced when hearing words. For example, the word basketball may taste like waffles.
Many synesthetes use their conditions to influence creative processes, such as writing, singing, or drawing. Both Pharrel Williams and Lady Gaga are examples of such individuals.