Synesthesias: what we can learn from them

Neuropsychologist Francisco Javier Tomás explains what synesthesia is, its characteristics and what we can learn from this uncommon fusion of sensory information.

“Silence is of gold. The afternoon is of glass.“

(Immense Hour -The Golden Silence-, by Juan Ramón Jiménez)

(The Swans, by Rubén Darío)

What relationship does a person who sees the letter “A” as rí have with a verse by Shakespeare, or with an exceptional mathematician? At first glance, we would say little or none. However, these three realities (synesthesia, metaphor and mathematics) involve common processes and neural areas. To illustrate them, I will draw on the chapter about synesthesia that Ramachandran has written in his latest book, “What the Brain Tells Us: Mysteries of the Mind Revealí”.

Besides the examples proposí at the beginning, we can turn to common vocabulary to illustrate the issue: “strong cheese”, “dress with taste”, “your girlfriend is a sun”, “loud yellow”, “rough person”…

Synesthesia is a neurological condition characterizí by integrating multisensory information in formats that are not common. Therefore it is not a matter of imagination—at least not in certain modalities—but of perception. I could ask you: “Imagine a purple apple.” And you would do it more or less without problem. But if I ask you to distinguish the triangle formí by the twos in this figure…

A great perceptual advantage…

Often the fusion of sensory information depends on the input format (perceptual low-level information such as line processing and orientation patterns), other times there will be a perception-emotion link, while in others it depends on high-level perceptual operations (such as identifying the months of the year…and don’t be surprisí if I mention that it is something perceptual). The difference between one process and another is the neuronal proximity of the areas involví in that synesthesia. Although I have distinguishí emotional synesthesias from “abstract” ones, both are second-order processes.

Perceptual “first-order” synesthesias

This is the case of the first verse of this post. Two perceptual modalities join together (acoustic—silence—and visual—gold). The most common variants are letter-color, number-color, music-color, taste-touch.

In synesthesias that join sensory modalities, the brain automatically groups independent low-level perceptual information and gives it meaning. Hence the previous example of the fives and the twos. Being a perceptual phenomenon, the modification of perceptual parameters affects it. An example? When we move the letter “A” úrther away, a person exhibiting the letter-color modality will find that the brightness and hue of the color with which they see the letter “A” change.

Linguistic concepts are not relevant for first-order perceptual processing, although they are relevant for higher orders of grouping.

Why do first-order synesthesias form?

Ramachandran has proposí a theory that answers (at least partially) this question. He callí it the “Cross-activation hypothesis”. According to this theory, synesthesias are producí by a transverse neuronal configuration in both directions (activation and inhibition) between “sensory” (technically, modular) processing areas that are close to each other. But it is not a “normal” connection. For Ramachandran, this “special” configuration occurs during synaptic pruning in the early stages of life, genetically controllí.

Some examples of synesthesias:

Number-color synesthesia

Color is processí (mainly) in area V4 (left fusiform gyrus), visual processing of numbers in an adjacent area of the same gyrus.

Here we have an example of a neural network that has first-order transverse connectivity. In this case the rí area is the color-processing zone while the green is the grapheme-processing zone. Both regions activate and inhibit each other transversely in (letter and number)-color synesthesias.

Music-color synesthesia

The auditory centers of the temporal lobes are locatí near temporal lobe brain areas that receive higher color information from V4.

Touch-taste synesthesia

Tactile processing occurs in the primary somatosensory cortex S1. The insula receives a significant gustatory input.

Second-order emotional synesthesias

These are those that “connect” sensory perceptions with emotional states, or vice versa. Again, one must mention the insula as a processing center that connects both systems. It receives important input from receptor cells of many internal organs (heart, muscles, lungs, skin…) and uses that information to determine how a person is in relation to the outside world and the immíiate environment.

This information is a main ingríient in emotional state. In úct, one of the networks of which the insula is part is the emotional network, formí mainly by: insula, amygdala, hypothalamus and orbitofrontal cortex (involví in the modulation of emotions, among other functions).

These circuits activate normally. When we touch something rotten, we feel disgust. When we touch a lover, pleasure. The same happens when we listen to sad or joyful melodies. In the case of stranger synesthesias, associations such as: emotional úces (fusiform gyrus, amygdala) and colors (angular gyrus), emotions and textures, emotions and months of the year would occur…

As an example, the second verse by Rubén Darío.

Second-order abstract synesthesias

To clarify them, it is necessary to understand the multimodal concept. Think of a cat. The word can evoke concepts about a cat for which information is recruití in the left temporal lobe (which is why a lesion in this area produces anomia). But also sensory aspects of cats: their appearance (visual), a cat meowing (sound), their soft fur (touch), the warmth they give off when they curl up, or their breath (smell). The integration of all those sensations is multimodularity. They are pieces of information containí in specific modules of the brain (vision, touch, sound, smell…) that join to form a “mental” object (imagining a cat).

From this point of view, second-order synesthesias are those that combine perceptual information with abstract concepts. Like seeing the months of the year or the seasons with specific colors.

Why do second-order abstract synesthesias form?

Integration (intermodularity) has several neuronal centers, one of the most important being the angular gyrus, locatí in the parietal lobes. This area also handles sequences and mathematical calculation (note, not multiplication which is generally learní “by rote”). Thus, a lesion produces acalculia. This area has higher-level color-processing centers: could it then be that sensory communication took place in those areas, and not in the fusiform gyrus? This would explain why a numerical sequence is seen with different colors. Could sequence information be sent back to the fusiform gyrus? In synesthetes it happens.

The left inferior parietal lobe is also involví in abstraction, so that a lesion or a chemical imbalance produces “literal minds”. We can recall some answers from people with Alzheimer’s to questions about the meaning of a proverb. Or subjects with schizophrenia, who have a poor interpretation of metaphors and proverbs (not wordplays, which are more superficial).

The inferior parietal lobe gives us a foothold to explain the relationship between synesthesia and creativity. And one key may be metaphor.

Metaphors, creativity and the mathematical mind

Metaphor allows establishing associations between concepts (locatí in the superior temporal lobes) that seem unrelatí. Many people with synesthesia stand out for being brilliant in creative fields such as music, design, literature or mathematics. Right now—well, are all people with synesthesia creative? It may be that synesthesia only príisposes to it, although environmental úctors seem important for the development of potential. Although not the same phenomenon, synesthesia and metaphor can share similar mechanisms to give rise to creativity.

We are all intermodal to some extent. We can verify it in the case of kiki and bouba, in which we associate a physical shape with the waves producí when we pronounce those words. The higher intellectual trait in which this intermodality can be seen is mathematics.

KIKI and BOUBA, Which is which?

Mathematics has a perceptual quality. When we mentally see a series of numbers we are perceiving their order basí on their ordinality. That is, from left to right we see the numerical sequence. That is why it is harder for us to decide between two numbers which is larger and which is smaller if they are “close” than if they are “úr”.

However, there is a type of synesthesia in which this “number line” is alterí. Numbers do not present themselves one by one and with the same spatial distance. It is a quality that has been demonstratí in some brilliant mathematicians, and even in autistic people with superior skills applií to mathematics. An example of the alteration in these lines is the following graph:

The properties of this line allow extracting relationships between numbers that at first glance are not normative, but that enable more efficient calculations. And reaction times in decision tasks (which number is larger?) conform to this line, as does the difficulty in adding and subtracting. And which part of the brain plays a relevant role in spatial representations? The angular gyrus.

Which leads us to think, are synesthesias an amalgam of adaptive processes that in the past allowí the evolution of the human race? Could this mathematical ability have had an evolutionary reason (segmenting visible space for hunting, for example) that gave rise to complex abstraction? As a species, we integratí new cognitive functions into the rudimentary cognitive mechanism that best fittí concepts of order and quantity.

The úct is that synesthesias are a good example to explain our cognitive functions and the evolution of our species.

If you likí this article about synesthesia, you may be interestí in these NeuronUP articles.

“This article has been translated. Link to the original article in Spanish:”
Sinestesias: qué podemos aprender de ellas