Imagine a world where the word “friend” tastes like fresh lemons, the letter G may be a vibrant purple, and a D# note from a piano may be a strange colored line extending from left to right somewhere in your visual space. Synesthesia is a unique way of perception where one attribute of a stimulus (i.e. shape, sounds, or meaning) may lead to the conscious experience of an additional attribute. The triggered attribute may lie in the same sensory system, like the color associated with the presence of the letter G. While these kinds of perception may seem foreign to many of us reading this, more and more evidence is coming to light that we all begin life with this perceptual network characteristic of synesthesia.
In order to understand how theories of infant synesthesia came to be, it is important to understand a little bit about the causes of synesthesia. Though conclusive causes are not known, there are many consistent neural abnormalities that have been observed in individuals with synesthesia. All of these abnormalities reflect differences in connectivity relative to the neurotypical brain. Many of the structural differences in the brains of synesthetes are assumed to reflect extra connections or an absence of inhibition relative to controls and it is these proposed mechanisms that are most enlightening when considering synesthesia in infants.
In the most simplistic view of neurodevelopment, during gestation and infancy there are two primary goals in the brain. The first is the production of new neurons, which involves everything from neural migration (the movement of neurons throughout the brain) to synaptogenesis (the creation of new synapses to facilitate neuronal communication). The second is pruning, an incredibly important part of neurodevelopment, designed to eliminate extra neurons and synaptic connections in order to increase the efficiency of neuronal transmissions.
It is this pruning process that is the key to understanding why infants are thought to posses this synesthetic perception of the world.
Pruning begins in the earliest embryonic state and continues until about two years of age. When we consider the importance of this process, the timeline along which it occurs, and the proposed causes for synesthesia, the connections begin to appear. Synesthesia is thought to be caused by an excess of connections or lack of inhibition along certain sensory pathways in the brain. During the first two years of life we are working to prune these connections so that they are as efficient as possible, but until we have completed this process, it is not unreasonable to think that we may have an excess of connections of a lack of inhibition on some of these multisensory pathways. When taking this all into consideration, theories of infant synesthesia seem to make more sense. As babies, before we have been able to successfully optimize our neuronal connections, the presence of an excessive and less precise network may in fact mean that all babies can taste color or see music.
Santos, E., & Noggle, C. A. (2011). Synaptic Pruning. In Encyclopedia of Child Behavior and Development (pp. 1464-1465).
Springer, U. S., Ward, J. (2013).Synesthesia. Annual review of psychology, 64, 49-75.
Deroy, O., & Spence, C. (2013). Are we all born synaesthetic? Examining the neonatal synaesthesia hypothesis. Neuroscience & Biobehavioral Reviews, 37(7), 1240-1253.