Here is a detailed explanation of the neuroscience behind synesthesia, focusing on the specific phenomena of tasting words (lexical-gustatory synesthesia) and seeing sounds (chromesthesia).
Introduction: What is Synesthesia?
Synesthesia (from the Greek syn, meaning "together," and aisthesis, meaning "sensation") is a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
For a synesthete, the brain is hyper-associative. While a neurotypical person hears a C-sharp note and simply processes auditory data, a synesthete might hear the note and simultaneously see a flash of indigo blue. These are not hallucinations or metaphors; they are consistent, repeatable, and perceived as real sensory inputs.
To understand how someone can "taste" a word or "see" a sound, we must look at two primary neuroscientific theories: Cross-Activation and Disinhibited Feedback.
1. The Architecture of the Synesthetic Brain
Neuroimaging studies, such as fMRI (functional Magnetic Resonance Imaging) and DTI (Diffusion Tensor Imaging), have revealed distinct structural and functional differences in the brains of synesthetes compared to non-synesthetes.
A. Hyper-Connectivity (The Structural Basis)
The brain is composed of specialized regions (modules) responsible for different tasks—V4 for color processing, the fusiform gyrus for word recognition, the auditory cortex for sound. In infant brains, these regions are heavily interconnected. As we age, a process called synaptic pruning occurs, cutting away unnecessary connections to make the brain more efficient.
The leading theory posits that in synesthetes, this pruning process is genetically inhibited. As a result, they retain dense "cross-wiring" between sensory regions that are typically segregated in adult brains.
B. The Cross-Activation Theory
Proposed by neuroscientists V.S. Ramachandran and E.M. Hubbard, this theory suggests that when one area of the brain is activated (e.g., the area processing a word), the excess neural connections cause neurons in a neighboring area (e.g., the area processing taste) to fire simultaneously.
2. Lexical-Gustatory Synesthesia: Tasting Words
This is one of the rarest forms of synesthesia (occurring in less than 0.2% of the population). Individuals experience specific tastes or textures in the mouth when hearing, speaking, or reading specific words.
The Neural Mechanism: 1. The Trigger: The synesthete hears the word "basketball." 2. Processing: This auditory input travels to the auditory cortex and then to the anterior insula and the frontal operculum, areas deeply involved in language processing and the physical mechanics of speech. 3. The Cross-Wire: In the brain, the primary gustatory cortex (responsible for taste) is located in the insula, incredibly close to the regions that process speech sounds and word meaning. 4. The Experience: Due to hyper-connectivity between the language centers and the gustatory cortex, the neurons responsible for the sensation of "waffles" are activated by the word "basketball."
Key Insight: The connections are often phonological rather than semantic. For example, the name "Derek" might taste like earwax because of the hard 'D' and 'k' sounds, rather than any emotional association with a person named Derek.
3. Chromesthesia: Seeing Sounds
This is a more common form of synesthesia where sounds (music, voices, or environmental noise) induce the perception of colors and shapes.
The Neural Mechanism: 1. The Trigger: A musical note is played. 2. Processing: The sound enters the Primary Auditory Cortex (in the temporal lobe) for processing pitch and tone. 3. The Cross-Wire: The visual cortex, located at the back of the brain (occipital lobe), contains a specific area called V4, which is specialized for color processing. In chromesthetes, there are dense white matter tracts (neural highways) directly linking the Auditory Cortex and area V4. 4. The Experience: The firing of auditory neurons spills over into V4. The brain interprets this V4 activation as genuine visual input.
Disinhibited Feedback Theory: An alternative or complementary view (proposed by researchers like Cohen and Kadosh) suggests that the connections aren't necessarily new or extra. Instead, the "inhibitory" neurotransmitters that usually stop information from leaking between senses are weaker. In a normal brain, the visual cortex might get a whisper of information from the ears, but the brain suppresses it. In a synesthetic brain, that suppression fails, and the whisper becomes a shout.
4. Characteristics of the Synesthetic Experience
From a neurological standpoint, these experiences share specific traits that differentiate them from imagination or memory:
- Involuntary: It happens automatically. A synesthete cannot "turn off" the taste of a word any more than you can choose not to hear a loud bang.
- Projectors vs. Associators:
- Projectors actually see the colors in external space (e.g., a red triangle floating in front of a speaker's face). This suggests strong activation in the primary visual cortex.
- Associators see the colors in their "mind's eye." This suggests activation in higher-level visual association areas, rather than the primary visual cortex.
- Consistency: If the word "table" tastes like cold milk today, it will taste like cold milk ten years from now. This stability indicates fixed neural pathways rather than fleeting associations.
5. Why Does This Evolutionarily Exist?
Why didn't evolution prune these connections away in everyone? Some neuroscientists believe synesthesia is a benign side effect of a different, advantageous trait: enhanced metaphoric thinking.
The ability to link unrelated concepts (e.g., "a sharp cheese" or "a loud shirt") requires cross-modal communication in the brain. Synesthesia may be the result of a "hyper-connected" gene that, in lower doses, gives humans creativity and the ability to understand metaphors, but in higher doses, results in literal sensory cross-wiring. This explains why synesthesia is reportedly more common among artists, poets, and musicians.
Summary
When a synesthete tastes a word or sees a sound, they are not imagining it. Their reality is fundamentally different because their neural topography is different. Through incomplete synaptic pruning and disinhibited feedback, their sensory cortices—regions that are usually good neighbors keeping to themselves—have knocked down the fences and are constantly talking to one another.