Our latest research into training-induced synaesthesia has now been published in Neuropsychologia. In this new study, led by Nicholas Rothen with me, Dan Bor, and David Schwartzman, we investigate the phenomenological, behavioural, and - crucially - the neuronal changes that accompany extensive cognitive and perceptual training of synaesthesia-like experiences.
In grapheme-colour synaesthesia, people have additional 'colour experiences' when they perceive achromatic (black) letters (graphemes). These additional experiences provide a unique window into the brain mechanisms underlying conscious perception. But natural-born synaesthetes are relatively rare and not so easy to study.
In a previous study, published in 2014, we wondered whether non-synaesthetic adults could be trained, over several weeks, to develop synaesthesia-like experiences (phenomenology). We found that they could. After 9 weeks of intensive reinforcement of letter-colour associations, most of our participants were reporting vivid synaesthesia-like experiences, even outside the lab - when walking around town or around our campus.
Our new study used online training over 5 weeks, and this time we also measured neurophysiological properties of the visual cortex both before and after training. We found the same phenomenological results: people reliably developed synaesthesia-like experiences and exhibited the classic behavioural signs of synaesthesia (e.g., difficulties judging the colour of letters when the real colour conflicted with the 'synaesthetic' colour). Critically, we also found evidence in changes in the visual brain, compared to control experiments where no synaesthesia-like experience was developed. Training-induced synaesthesia led to an increased 'excitability' in the visual cortex, revealed both by lowered phosphene thresholds under transcranial magnetic stimulation, and by increased electrical responses (visual evoked potentials) when our participants were shown flickering checkerboard stimuli.
Altogether, our results provide new evidence for specific neuronal (cortical) changes, following training-induced acquisition of synaesthesia-like phenomenology, that are characteristic of genuine synaesthesia. Our data reveal a dramatic plasticity in human visual perception, expressed through a coordinated set of behavioural, neurophysiological, and phenomenological changes.
Rothen, N., Schwartzman, D., Bor, D., and Seth, A.K. (2018). Coordinated neural, behavioural, and phenomenological changes in training-induced synesthetic experience. Neuropsychologia
Bor, D., Rothen, N., Schwartzman, D., Clayton, S., and Seth, A.K. (2014). Adults can be trained to acquire synaesthetic experiences. Scientific Reports 4:7089.