Neural basis of synesthesia
In yet another form of synesthesia, called ordinal linguistic personification, either numbers, days of the week, or months of the year evoke personalities.
Recent research has begun to explore the neural basis of these experiences, drawing both on neuroscientific principles and on functional neuroimaging data.
This cross-activation may arise due to a failure of the normal developmental process of pruning, which is one of the key mechanisms of synaptic plasticity, in which connections between brain regions are partially eliminated with development.
Similarly, lexical → gustatory synesthesia may be due to increased connectivity between adjacent regions of the insula in the depths of the lateral sulcus involved in taste processing that lie adjacent to temporal lobe regions involved in auditory processing.
In these cases, specific letters or words evoke colors, suggesting that semantic processing may link otherwise separate sensory experiences.
Research has shown that this phenomenon may be due to abnormal structural or functional connections in the brain, particularly in regions involved in sensory processing and in areas associated with meaning, such as the left fusiform gyrus.
Theories suggest that cross-activation or disinhibition of feedback between adjacent regions (such as those that process color and those involved in letter recognition) contributes to these experiences.
Studies using fMRI and DTI (diffusion tensor imaging) have found increased connectivity between areas associated with the stimulus (e.g., the visual word form area for letters and words) and the sensory experience they evoke (e.g., color processing regions in the visual cortex).
However, these neuroimaging studies do not have the spatial and temporal resolution to distinguish between the pruning and disinhibited feedback theories.
Additionally, as previously mentioned, there are numerous different types of synesthesia conditions that an individual could experience.
[2] For example, an individual with grapheme color synesthesia may consistently perceive the letter "E" as purple, and the number "6" as yellow.
