Inside a Synesthete’s Brain

Synesthesia does not localize to any one spot in the brain. Rather we need to think in terms of neural networks that connect multiple brain regions because synesthesia seems to be the perceptual result of increased cross talk between brain areas.

How might such an increased cross talk come about? Is it from a failure to prune connections at an early age? Or is it cross-wiring present in every one, synesthetes and nonsynesthetes alike, with only syntesthetes having the pleasure of experiencing activity above the threshold for conscious awareness?

Divisions of labor in the brain (a) The brain is roughly divisible into areas specialized for movement, sensing the body parts and internal viscera, various aspects of language, vision, hearing and so on. b) Many subspecializations exist within the sense of vision. As we move father away from primary visual cortex(V1) into higher visual areas, we encounter brain areas specialized for increasingly more complex stimuli, such as detecting motion, houses or human faces. The box diagram summarizes the vast array of data processing undertaken by populations of cells within the larger visual cortex. V1, primary visual cortex; V2, secondary visual cortex, MT, medial temporal; MST, medial superior temporal, PP, posterior parietal, PIT, posterior inferotemporal cortex
Differences between “Crosstalk” in the Normal and Synesthete’s Brain
There are differences between “Crosstalk” in the Normal Brain and the Synesthete’s Brain
Cross Talk in the Normal Brain 
For example in the ventriloquist illusion, the ears hear sound from one direction while the eyes see a moving mouth in a different location. The brain incorrectly concludes that the sound emanates from the mouth location because the sight of moving lips influences where we localize the sound. This illustrates the natural interconnectivity between visual and auditory parts of the brain, which collaborate to generate a single, unified perception. Source: Eagleman DM. 2001. Visual illusions and neurobilogy. Nature Reviews Neuroscience 2(12): 920-926 
We also mentioned the McGurk illusion, in which the sound of /ba/ is perceived as /da/ when it is coupled with visual lip movement associated with /ga/. Again, the McGurk effect indicates that sight and sound cues rapidly combine at an early processing stage, even before any decisions are made about phoneme or word categories. Source: McGurk H, Macdonald J.1976. Hearing lips ands eeing voices. Nature 264:746-748, Schwartz j, Robert-Ribes, J, Escudier JP. 1998, p. 319 in R Campbell, B Dodd, Dk Burham(eds). 
Studies using fMRI confirm that seen speech influences the perception of heard speech at a very early processing stage . Source ( Calvert GA et al 1997. Activation of auditory cortex during silent lipreading)  Also supporting the conclusion that sight and sound are quickly coupled is the “auditory driving illusion” in which the steady rate of a flicking light appears faster or slower depending on whether accompanying beeps are presented at a faster or slower rate. For example : facial expressions can modify the perceived emotion in the voice of a speaker even when facial expression is not consciously perceived. And its not only sight and sound that are connected: in an example linking touch and vision, a sudden touch on one hand can temporarily improve your vision near that hand due to feedback from multi sensory areas.
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