Synesthesia: The Direct Cross-Activation Hypothesis

Stella Huggins

Our world boasts a plethora of stimuli to engage with. Without getting too deep down the hole of whether an objective external reality exists, I think it’s fairly uncontroversial to say that we do not perceive all of the world in the same way. An internal world, just as rich, exists within and mediates how we interact with our environment. Higher cognitive processes and brain structures are the culprits of how we feel about the information intake of daily life. Our limbic system is involved in behavioural and emotional responses, while the prefrontal cortex, basal ganglia and thalamus (among other things) regulate our capacity for executive function [1].

It’s easy to accept that individual differences exist at these levels. What is harder to conceptualise is the individual differences in sensory experiences. Nevertheless, multiple fields of psychology have done so, beginning with Francis Galton’s observations in the 19th century. Galton dubbed the experience of sensory blending as synesthesia [2]. A fascinating and broad condition, synesthesia has captured the curiosity of neuroscience in particular, as researchers map and locate brain regions where activity crosses over. Numerous contemporary artists have the condition — Lorde, Beyonce, Kanye West and Billie Eilish, to name a few. It’s understandable that synesthesia would be a condition that’s conducive to a life dedicated to art. However, it’s not just famous creatives that have the condition — some studies report the prevalence of grapheme-colour synesthesia for example, to be between 0.8% and 2.8% of the population [3].

Information that ‘should’ affect just one of your sensory organs (if sensory experiences can even be considered so isolated), in synesthetes, evokes a joint experience [4]. An enormous variety of synesthesia sub-categories exist, so that several sensory modalities are able to be combined. For example, someone may see the number two as always being pink. They may taste caramel when they hear the word ‘town’. They may smell freshly cut grass when they hear a violin being played. The associations between senses and specific sensory inputs aren’t thought to be pathological, nor is the occurrence of the associations in the first place.

The structures that build an individual’s perceived reality are two-fold. Primary sense organs (the eyes, ears, tongue, nose and skin) exist purely to receive information about what’s going on outside ourselves. It is the brain that interprets this information in its multiple forms into meaningful and interpretable signals off of which we function[5]. The process of information travelling from the exterior world, through sensory circuits from the primary sense organs, is just the beginning of perception. It is important to note that synesthesia is not considered to be a condition of bodily dysfunction. It is not an issue with the ability of the eyes to take in information, or the capacity of your taste buds to taste accurately. The condition is postulated to be a cognitive quirk that is involuntary, with stable associations across time [6].

As is often the case for psychological conditions, an enormous amount of hypotheses, each with their own nuances and levels of evidence, exist to try to explain the cause of synesthesia. Here, I will discuss the direct cross-activation hypothesis, as well as more broad hypotheses about brain function and development that tie into the cross-activation hypothesis [7].

Direct cross-activation refers to the idea that the messages intended for one section of the brain activate tissue in neighbouring regions. Cell firing in cortical tissue has electrical outputs, and this output can travel. Excitation can be simply described as a signal [8]. This hypothesis relies on the idea that the electrical signals moving from one cortical area affect another in such a way that it produces a consistent mixing of sensory experiences for the individual. It is true that the human brain experiences large amounts of cortical folding [9]. This essentially means that brain regions are very close to one another, and neuropsychologists, in general, are hesitant to prescribe one function to one brain region. This makes finding the cause of synesthesia even more complex. Brain function is already considered to be highly collaborative, rather than a delegation of tasks to specific tissue — so how do we isolate function and prescribe meaning to it?

In some researchers' eyes, the proximity of brain regions, particularly in grapheme-colour synesthesia, holds significance. An experience of some synesthetes with this subtype of the condition is associating numbers with colours; for example, ‘2’ may always take on a pink hue. Despite the hesitance to categorically prescribe tasks to tissue, there are particular brain areas that are known to be correlated with specific processing tasks. This is significant when considering the cross-activation hypothesis. Intriguingly, the visual-word form area (VWFA) or ‘grapheme area’ is directly adjacent to the V4 colour processing area [10]. V4 describes the third cortical area in the ventral stream, and, according to the widely accepted hypothesis of two-stream visual processing, this stream carries information about object forms from the primary visual cortex to the temporal lobe [11].

The cross-activation hypothesis ties neatly into other postulated causes at this point: for example, the broad idea of neuroplasticity hosts many nooks and crannies for cross-activation to tuck nicely into. It’s well in line with the widely accepted concept of neuroplasticity [12]. Neuroplasticity is something of a buzzword for the field and there is considerable evidence that we possess huge amounts of connections and neurons at birth. Think of tasks or behaviours like strengthening a muscle or a connection in the brain — as I do more of one thing, it becomes stronger. Acording to this school of thought, we grow and strengthen connections for some activities, and lose the strength for others. This is referred to as neural pruning [13] — the process of getting rid of connections that aren’t being used (yes, if you don’t use it, you really do lose it).

It has been suggested that abnormalities in the pruning process where excess synapses aren’t eliminated as per usual could lead to hyper-connectivity between regions [14]. In the case of grapheme-colour synesthesia, connectivity occurs in the fusiform gyrus, an area responsible for object identification [15]. The cross-activation hypothesis comes in when you consider the evidence that synesthesia is genetic [16]. This suggests that maybe synesthesia could have something to do with pruning in infancy: maybe these individuals simply have more connections in their brains. With more connections comes more potential for cross-activation.

Another take on the neuroplasticity route is the neonatal theory [17], which asserts that we are all synesthetes at birth, and the experience of synesthesia is actually a failure in modality separation. Modularity theory explains that the mind separates cognitive processes into ‘modules’, each with its own distinct properties and abilities [18].

All of this seems like an enormous amount of background information for a simple claim: that excitation in one region of the brain, due to proximity to another, causes excitation in that region, and thus regions intermingle, causing a dual-sensory experience. A somewhat unsatisfying conclusion lies in amongst the competing theories; we aren’t sure of what causes synesthesia. What we do know is a variety of experiences with the condition exist: from mildly bothersome, to not troublesome at all, to advantageous. Synesthesia represents what we already knew about the world — everybody’s experience is unbelievably varied. It’s the way these differences are framed that dictates how positively or negatively somebody relates to their perceived internal and external world.

References

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