The spatial extension synaesthesia has been studied by several early authors, and reviewed by Richard E. Cytowic in his book Synaesthesia: Union of the Senses3. It’s characterized, as the name indicates, by an ability to spatially extend one percept belonging to a given sensory modality, e.g. touch; which is induced by a percept belonging to yet another sensory modality, e.g. vision. The projection of the percept takes place in the peripersonal space of the synaesthete, rather than being in his mind’s eye. Here, we’ll be interested in a particular case of pharmacological synaesthesia6 in which the spatial extension of touch is experienced as an outstretching of the own bodily boundaries in mescal users9. Their limbs assumed a spiral form echoing the spiral rotation of the visual stimulus that induced such perception.
The main goal of the present work is to produce a comparative study amongst what is known about the spatial extension of touch pharmacological synaesthesia, and the most recent studies demonstrating that the human body schema is liable to be induced into rapid plasticity processes even on non-synaesthetes, healthy subjects. More specifically, we’ll review recent studies targeting the Rubber Hand Illusion (RHi) 2,5,8,11,13 and the Out-of-Body Experiences (OBE)4,10.
In a second moment, we’ll also speculate on the putative neural substrates of this specific type of synaesthesia, highlighting recent literature that reveals the multisensory role of traditionally considered unimodal visual structures, which indeed are actively involved in the processing of correlated tactile information1,7,12.
We would like to learn if and how all those phenomena can help to better understand one-another, defining its putative phenomenological resemblances, as well as potential common neural substrates.


Reference List

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