Theories on the Potential Neurological Causes of Synesthesia

Abstract

In the broad scope of neurological conditions, synesthesia is one of the least harmful and even most coveted abnormalities that exists. The condition causes those affected by it (synesthetes) to have a sensory experience in one sensory modality after receiving sensory input in a different modality (Robertson & Sagiv, 2005). Because of the condition’s complete lack of health risks, insufficient research has been completed into the topic and no definitive cause has been identified. One theory as to the cause of synesthesia is that certain neural pathways are not pruned during early development (Hubbard, 2007), while another posits that different, irregular pathways may sprout between usually unrelated brain regions (Rich & Mattingley, 2002).

While these theories focus on the connections between brain regions, the theory that attributes synesthesia to a failure to inhibit feedback within the brain focuses on the level of communication found within synesthete' brains (Hubbard, 2007). This level-of-communication based theory did not attempt to explain any form of synesthesia other than those that involve the visual field and as such, without further research, is hard to consider too carefully. The explanations that were focused on the unusual neural connections were all-encompassing in terms of our senses and with more research, could be very promising in determining the cause of synesthesia.

Potential Neurological Causes of Synesthesia

Synesthesia is a neurological condition in which stimulation of one sense produces a sensory experience in a different modality (Robertson & Sagiv, 2005). Although some report symptoms of synesthesia after taking psychedelic drugs such as LSD or after a self-reported ‘divine intervention’, the neurological condition is referred to as developmental synesthesia as it is not caused by any factors outside the brain (Robertson & Sagiv, 2005). This condition can appear within any of the five senses, but it is most commonly reported by synesthetes as seeing color that is sparked by an unrelated piece of sensory input (Robertson & Sagiv, 2005). The condition does not even have to necessarily spark an experience in a different modality than that of the sensory input to be considered synesthesia. In fact, the most common form of synesthesia is grapheme to color synesthesia which is entirely contained within the ocular sense. Rich & Mattingley (2002) describe this common form of synesthesia as a letter or a digit (a grapheme) causing a synesthete to see a particular color associated with that grapheme.

Synesthesia is a completely harmless condition and is not found within the Diagnostic and Statistical Manual of Psychological Disorders which is commonly used by psychological professionals to diagnose mental illnesses (Robertson & Sagiv, 2005). In reality, most synesthetes consider their condition to be a gift and it has often been cited as a source of great artistic inspiration by artists like popular musicians, Ella ‘Lorde’ Yelich-O’Connor and Maggie Rogers (Geggel, 2017). Since it is not considered a health risk, not much research has been done into this condition and there is still much to learn about synesthesia. In recent years, however, researchers have begun to take an interest in this fascinating condition and have come up with several theories and many possible explanations for the condition, although no decisive answer has been found.

Potential Causes of Synesthesia

The neurological cause of synesthesia is still unknown and many researchers are currently working on finding a definitive difference in the development of synesthete’s brains or a particular brain region to attribute this condition to. Hubbard (2007) posited that synesthesia may develop as a result of incomplete pruning of neural pathways within synesthete’s brains during their early childhood as their brain is developing. During our formative years, neural pathways go through a process known as pruning in which certain synapses within the brain are eliminated because the neurons that they connect are not deemed necessary to be connected to each other any longer. The brain normally does this in order to ensure effectiveness and keep everything working smoothly, but, according to Hubbard (2007), in synesthetes, some of these pathways do not go through this pruning process and some irregular pathways may remain that lead to experiencing the symptoms of synesthesia. For example, the brain region that is most responsible for grapheme recognition is directly adjacent to the part of the brain that deals with color processing and some believe that in some synesthetes, there is a connection between these two areas that was not pruned off in early childhood that now contributes to them seeing colors when viewing certain letters and numbers (Hubbard, 2007). These ‘un-pruned’ connections can exist between any two neurons, however, and can theoretically contribute to any form of synesthesia, not just grapheme to color (Hubbard, 2007).

Rich and Mattingley (2002) hypothesized a similar, yet distinct theory that focused on the sprouting of unusual neural connections and pathways within synesthetes’ brains during development, instead of discussing the potential lack of pruning. These abnormally sprouted connections, much like the ‘unpruned’ connections, then, theoretically, contribute to the symptoms of synesthesia by providing new pathways for sensory input and information to be carried on between parts of the brain that they usually would not interact with (Rich & Mattingley, 2002). Both of these theories targeted atypical connections that may exist within synesthetes’ brains, but did not consider the potential difference in the amount of communication that may be occurring between different regions of their brains as a potential explaining factor of synesthesia.

Another theory as to the possible cause of synesthesia is that there is a failure to inhibit feedback in the visual system in synesthetes’ brains that leads to experiencing the symptoms of the condition (Hubbard, 2007). According to the theory, synesthetes experience a higher level of neural communication back and forth within their visual system because of decreased levels of inhibitory processes in their brain when compared to the brains of non-synesthetes (Hubbard, 2007). There are many possible theories as to which parts of synesthetes’ brains are communicating more with each other according to this explanation, but Hubbard (2007) theorized that the visual system was interacting with a region of the brain, such as the temporo-parietal-occipital junction, that is involved with the input and experiential components of multiple senses. This theory is also supported by the fact that psychedelic drugs like LSD can sometimes cause synesthesia-like symptoms in users (Robertson & Sagiv, 2005). Although there has been little research done on the effects of LSD on the brain because it is an illegal substance, it has been speculated that it may increase feedback within the brain which may possibly be the explanation for why LSD-users report seeing colors in response to letters, numbers, etc. (Hubbard, 2007).

Discussion

Although synesthesia affects a significant portion of the world’s population, there is still no conclusive answer as to which neurological phenomenon (or which combination of multiple) is the explanation for the condition. In recent years, many researchers have done excellent studies into the condition and as a result, there are now several potential plausible explanations for why synesthetes experience things in the unique way that they do. Some of the completed studies, however, although based on brilliant ideas and impressive work, have produced far less convincing results and explanations than others.

For example, the theory that focused on the failure to inhibit feedback and the evidence presented for that being the driving cause of synesthesia is less persuasive than some of the other explanations presented. In Hubbard’s (2007) theory, only an explanation for synesthesia that involves the visual sense is presented. It is posited that synesthetes’ visual sense may have increased communication with certain regions of the brain which then produces increased feedback which may lead to a sensory experience, but it only discusses the visual sense (Hubbard, 2007). In reality, although synesthesia involving the visual field is the most common, the condition can interact within and between any of our five senses and as such, this explanation does not stand up to much scrutiny (Robertson & Sagiv, 2005). If more research into the potential explanation showed that this theory could be applied to the other four senses as well, it would hold much more weight and become a viable explanation for the cause and development of synesthesia.

The other two theories that were discussed, however, did not suffer from this issue of not focusing on the full scope of synesthesia and as such, presented extremely plausible explanations with a lot of good work to back them up. Both the neural pruning theory and the irregular sprouting theory focused on unusual connections in synesthetes’ brains that if present, would explain the abnormal sensory occurrences that they experience. Although providing slightly different takes on the cause of synesthesia, these theories work very nicely together and give each other merit since two separate research studies came up with such similar explanations. More research into both the potential lack of pruning present in the brains of synesthetes and the possible abnormal sprouting of neural pathways should be conducted in order to attempt to ascertain which theory, if either, may transition from conjecture to certainty and be the underlying cause of synesthesia.