The neurological basis: synesthesia and the brain

The following models have been proposed as a neurological basis for synesthesia. Research is ongoing, as none of them have yet been conclusively proven as the definitive explanation.

1. The cross-activation (or neural pruning) theory (Ramachandran and Hubbard, 2001) suggests that all individuals are born with an excess of working connections between brain areas (they are "born synesthetes", as it were), but in the non-synesthete majority the neural connections required for synesthetic processing are pruned away during the first few months of life.

Psychophysical investigations into the neural basis of synaesthesia, V. S. Ramachandran and E. M. Hubbard, 2001

(After 10 years, they wrote the following: The cross-activation theory at 10. Hubbard, E.M., Brang, D. & Ramachandran, V.S., 2011)

2. The disinhibited feedback theory (Neufeld et al., 2012) puts forward the idea that there is nothing that distinguishes a synesthete brain from a normal brain but instead synesthethic sensations arise via disinhibited feedback: excess activity between the levels of the sensory hierarchy or concurrent pathways because of a disinhibition of normally-occurring feedback signals.

Disinhibited feedback as a cause of synesthesia: evidence from a functional connectivity study on auditory-visual synesthetes. Neufeld, J. et al., 2012.

3. The stochastic resonance model (Lalwani and Brang, 2019) suggests that rather than being due to either of the two previous models, a simple change in levels of neural noise in the sensory systems can lead to the experience of synaesthesia (both acquired and developmental forms).

Stochastic resonance model of synaesthesia, Poortata Lalwani and David Brang, 2019

For more general descriptions and further reading on this subject:

Chapter 9 of the book Wednesday is Indigo Blue by Richard E. Cytowic and David M. Eagleman, "Inside a Synesthete's Brain" (2009)

Synesthesia: opening the doors of perception. Dartmouth Undergraduate Journal of Science (2010)

Why synesthesia occurs: Neurowiki (2013)

The section Hypotheses from the classic literature (paragraphs 12-23) of the excellent research article More than a condition: an examination of synaesthesia as a key cognitive factor in the processing of reality and in its literary and pictorial renditions, in the scientific review Interfaces, 36 | 2015, pp.29-55. Séverine Letalleur-Sommer, University of Paris-Ouest la Défense (2015).

The Stochastic Resonance Model study (theory no. 3 above) includes a description of all three models mentioned here (2019)

Synaesthesia and autism: Different developmental outcomes from overlapping mechanisms? Tessa M. van Leeuwen, Janina Neufeld, James Hughes & Jamie Ward, in Cognitive Neuropsychology Volume 37 Issue 7-8 (2020)

This page last updated: 6 February 2022

The neurological basis for synaesthesia