Attention deficit hyperactivity disorder (ADHD) and synesthesia are two fascinating neurological conditions that have captured the interest of researchers and the public alike. Both involve differences in brain wiring and perception that set affected individuals apart from the general population. An intriguing question is whether these two conditions are related – does having synesthesia make a person more likely to have ADHD, or vice versa? Research into this area is shedding new light on the connections between these neurological phenotypes.
Introduction to ADHD and Synesthesia
First, let’s provide some background on what exactly ADHD and synesthesia are:
What is ADHD?
ADHD is a developmental disorder characterized by difficulty paying attention, excessive activity, and trouble controlling impulsive behavior. It is considered a neurodevelopmental disorder meaning it originates in the brain. People with ADHD often have differences in brain anatomy and neurotransmitter systems compared to those without the disorder.
There are three main subtypes of ADHD:
- Predominantly inattentive – difficulty focusing, easily distracted
- Predominantly hyperactive/impulsive – excessive restlessness, difficulty sitting still
- Combined type – a mix of inattentive and hyperactive/impulsive symptoms
ADHD typically appears in childhood and can persist into adulthood. According to the CDC, around 6.1 million children in the U.S. have received an ADHD diagnosis.
What is Synesthesia?
Synesthesia is a neurological condition where stimulation of one sense involuntarily triggers another sense. For example, seeing letters or numbers may trigger the experience of specific colors. Or hearing music may elicit sensations of color or shapes.
There are over 60 different subtypes of synesthesia involving diverse combinations of senses. Some common forms are:
- Grapheme-color – letters/numbers trigger color perception
- Sound-color – sounds trigger color perception
- Number form – numbers visualized in specific patterns
- Lexical-gustatory – words elicit taste sensations
Synesthesia is estimated to affect between 4-5% of people. It runs strongly in families and has a genetic basis.
How are ADHD and Synesthesia Related?
So what do these two fascinating neurological conditions have to do with each other? At first glance they appear quite distinct. However, research has uncovered some striking overlaps that suggest a shared underlying neurological basis.
Higher Rates of Synesthesia Among Those with ADHD
Several studies have found that synesthesia appears to be more prevalent among those diagnosed with ADHD compared to the general population:
| Study | Sample | Main Finding |
|---|---|---|
| Neufeld et al. 2013 | 164 adults with ADHD | Rate of synesthesia 5X higher than controls |
| Van Hoorn et al. 2016 | 285 children with ADHD | 12.6% had synesthesia vs. 4.4% of controls |
| Bouvet et al. 2020 | 62 adults with ADHD | Lifetime rate of synesthesia 32.3% |
Based on these studies, researchers estimate that synesthesia occurs in 5-35% of those with ADHD compared to only ~4% of the general population. This suggests a strong association between the two conditions.
Shared Genetic Basis
Both ADHD and synesthesia have been linked to genetic variations on some of the same genes. For example, both conditions have been tied to differences in genes controlling serotonin and glutamate – two neurotransmitters involved in brain cell communication:
- Variants in the serotonin transporter gene SLC6A4 are implicated in ADHD and associated with synesthesia.
- Variants in glutamate receptor genes (e.g. GRIK2) have been found in individuals with ADHD and certain synesthesia subtypes.
In addition, family studies have uncovered inheritance patterns suggestive of some shared genetic factors.Synesthesia strongly runs in families – between 40-80% of synesthetes have a first-degree relative with synesthesia. Similarly, ADHD has high heritability of around 70-80%. Having a parent or sibling with ADHD significantly increases risk for the disorder.
The overlapping genetics provides biological evidence that ADHD and synesthesia are connected at a fundamental neurological level.
Possible Shared Neurological Mechanisms
Researchers propose two key neurological mechanisms that may drive the association between ADHD and synesthesia:
Enhanced Neural Connectivity
One leading theory is that excess neural connections in the brain underlie both conditions:
- Synesthesia is thought to arise from extra connections between brain regions processing different senses, causing cross-wiring of sensory perceptions.
- ADHD has been linked to greater connectivity between brain networks involved in attention and default mode resting states.
Brain imaging studies have found patterns of enhanced structural and functional connectivity in both synesthetes and those with ADHD. Excess neuronal connections could make the brain hypersensitive to stimuli and prone to sensory overload.
Imbalances in Inhibition vs. Excitation
Another mechanistic theory involves an imbalance between excitatory and inhibitory neurotransmission in the brain:
- Synesthesia may be related to disinhibited feedback between brain areas processing different senses.
- ADHD has been associated with deficits in inhibitory neurotransmitters like GABA that control overexcitation.
In both conditions, inadequate inhibition could lead to hyperactivity of sensory neural circuits. More research is still needed to confirm the neurological links.
Implications of the ADHD-Synesthesia Relationship
The apparent association between ADHD and synesthesia has intriguing implications for our understanding of these conditions:
Insights into Brain Development
The overlap suggests shared developmental mechanisms shape the maturation of brain networks in childhood. Detecting connections between phenotypes provides clues into typical and atypical pathways.
Targeted Treatments
Drugs that address common underlying deficits, like imbalanced excitation/inhibition, could potentially help manage both conditions. For example, treatments that normalize glutamate signaling could benefit certain ADHD and synesthesia subtypes.
Cognitive Enhancements
Synesthesia shows that sensory cross-wiring is not necessarily detrimental. In some cases, it may even enhance certain cognitive abilities. Some synesthetes leverage their condition for superior memory and creative skills. Understanding this phenomenon may uncover new avenues for cognitive augmentation.
Outstanding Questions
While research indicates ADHD and synesthesia are linked, there is still much to uncover about this relationship:
- Are certain subtypes of ADHD or synesthesia more connected?
- What are the precise shared genetic markers?
- How do gender differences factor in?
- What is the neurological basis in brain development trajectories?
- Can insights from one condition guide treatment for the other?
As we fill in these knowledge gaps, the association between these conditions will offer broader lessons about the mysteries of the human brain and mind.
Conclusion
Evidence suggests ADHD and synesthesia frequently co-occur and share underlying genetics and neurobiology. Both conditions involve altered brain connectivity and sensory processing. Further research into their relationship will provide insights into the root causes of each one. It may also uncover novel approaches for diagnosis and treatment that cut across categorical boundaries of neurological disorders. By elucidating the connections between phenotypes, scientists aim to ultimately gain a deeper understanding of the intricacies of brain development in health and disease.