Synesthesia is a mixing of the senses. Synesthetes are people with synesthesia. This trait runs in families, albeit inconsistently in its expression.
Synesthetes consider synesthesia a normal condition until they are surprised to discover otherwise – or, at least, not as pronounced in the general population. Most of our perceptions are a confluence of sensory input.
A degree of synesthesia is inherent in many animals. Consistency from different sensory inputs is the formula for normalcy. Synesthesia somewhat simplifies mental processing via harmonious input from multiple senses.
Higher awareness and focus are prompted when sensory inconsistency arises. A sour note or bitter breeze cajole caution. Danger may be afoot.
Most people agree that soft sounds are dimmer than loud ones, which sound bright by comparison. Sounds have color and vice versa – for instance, a “loud shirt.”
While smell and taste create a unified experience in mammals, there is also interplay between smell and the perception of sound. For instance, smell and hearing converge in the olfactory tubercles of rodents.
In one experiment participants took a whiff of various scents, ranging from apple to violet to wood smoke, and then were asked, after each smell sample, to associate the scent among a selection of musical sounds, played at varying pitches by different instruments: piano, string, woodwind, or brass. Consensus was easily had. Sweet and sour smells are high-pitched. Smoky and woody scents are lower pitched. Berries, such as blackberry and raspberry, are very piano. Vanilla has a mixed timbre of piano and woodwind. Musk is brazenly brassy.
Taste has a similar sonic palette. Sweet and sour tastes hit a high note. Bitter taste resides in a low tone.
Imagination plays a role in our sense of smell. People detect a difference in smell when none exists other than by suggestion.
For taste, such a suggestion may be audible. Participants in an experiment ate the same toffee. One piece was savored while a somber brassy bit of music played, filled with low tones. Another bite of toffee was enjoyed while a higher-pitched piano piece played. The low-pitched toffee was more bitter, it was consistently reported; the high-toned toffee sweeter. The difference in toffee taste was simply that the sound tasted different.
Sight tends to dominate smell for primates. The sight of something affects what we smell, but the opposite is also true. In an experiment using special glasses, participants saw 2 images simultaneously, 1 in each eye: of a marker pen and of a rose, though they were aware of only 1 image at a time.
Binocular rivalry is long known. The mind registers seeing only 1 of the 2 images. More generally, the mind sorts out conflicting sensory inputs to create a coherent reception for itself.
While viewing the images, participants were exposed to odorants that smelled either of roses or marker pens. The participants reported seeing the rose for a longer time when smelling rose scent, with similar bias when smelling the pens. Mixed images and scents were confusingly short by recall.
Unless focus is applied, sensory processing is largely subconscious, as are most mental processes.