Synesthesia is when a stimulus not only induces the typical percept, but it also induces another automatic percept that is often in a different sensory modality (Spector & Maurer, 2009). For example, a particular sound may induce color. The pitch or tone of this sound may induce a more specific color. A stimulus that triggers the synesthesia is called the inducer. The modality in which the synesthesia is encountered is called the concurrent (Ward & Simner, 2003). There are at least 54 different types of synesthesia (Spector & Maurer, 2009), mixing different aspects of our senses. Synesthesia can manifest its behavioral markers in children as young as six years old. There are over 170,000 grapheme-color synesthetes ages 0-17 in the UK and over 930,000 grapheme-color synesthetes ages 0-17 in the USA (Simner et. al., 2009). There are two main developmental theories of synesthesia and this paper will explore both perspectives; the neonatal synesthesia theory and the disinhibited feedback theory.
Developmental Theories of Synesthesia
There are two predominant theories in regards to the developmental origins of synesthesia. One theory is called the neonatal synesthesia theory. It posits that synesthesia arises when the pruning of synapses is not completed between some contiguous brain areas (Spector & Maurer, 2009). Essentially all neonates are synesthetes until the appropriate pruning and apoptosis occurs. The other theory is called the disinhibited feedback theory. This theory posits “synesthesia arises when the reentrant feedback that develops postnatally from higher cortical areas onto lower sensory cortical areas is not strong enough to inhibit effects from connections between primary sensory cortical areas” (Grossenbacher & Lovelace, 2001). Both of these theories and examples of the respective research will be reviewed along with potential limitations and implications.
Neonatal Synesthesia Theory. In normal adults, information from each sensory modality is processed by its respective sensory cortical area. For example, neurons in the visual cortex respond to information from the eyes and neurons in the auditory cortex respond to information from the ears. There are transient connections between these areas that are pruned during development dependent upon experience (Spector & Maurer, 2009). In early development, there is a period of exuberant neural connectivity that is followed by a retraction and reweighting of connections (Wagner & Dobkins, 2011). In infants, this connectivity may enable arbitrary sensory experiences similar to synesthesia. If the connections are not pruned, these connections may remain and the individual may develop synesthesia.
Tzourio-Mazoyer et al. (2002) used a positron emission tomography (PET) to map the brain activity in six 2-month-old human infants while they observed unknown female faces. They recruited six infants who were full term but had suffered hypoxic-ischemic encephalopathy (HIE). Permission was...