Psychophysical investigations into the neural basis of synaesthesia

Abstract

We studied two otherwise normal, synaesthetic subjects who`saw' a speci¢c colour every time they saw a speci¢c number or letter. We conducted four experiments in order to show that this was a genuine perceptual experience rather than merely a memory association. (i) The synaesthetically induced colours could lead to perceptual grouping, even though the inducing numerals or letters did not.(ii) Synaesthetically induced colours were not experienced if the graphemes were presented peripherally. (iii) Roman numerals were ine¡ective: the actual number grapheme was required. (iv) If two graphemes were alternated the induced colours were also seen in alternation. However, colours were no longer experienced if the graphemes were alternated at more than 4 Hz. We propose that grapheme colour synaesthesia arises from`cross-wiring' between the`colour centre' (area V4 or V8) and the`number area', both of which lie in the fusiform gyrus. We also suggest a similar explanation for the representation of metaphors in the brain: hence, the higher incidence of synaesthesia among artists and poets.

Binding, spatial attention and perceptual awareness

“…Some have argued that the automaticity and consistency of the synaesthetic experience represent direct connections between cortical feature maps 13 , perhaps through synaptic connections that fail to undergo normal synaptic pruning during development 70,71 . This explanation of synaesthesia has been favoured by various investigators 72 , and is consistent with findings from behavioural experiments suggesting that synaesthetic binding occurs before attention 72,73 .…”

“…In addition, other studies began to be reported showing that visual search in an array of grey-scale letters or digits was easier for synaesthetes than for non-synaesthetes, presumably because the induced synaesthetic colours helped to segregate the target from the distractors before attention was engaged. However, in both of these cases, the distractors were letters or digits that also induced a colour 72,73 . This means that when a distractor was attended it would induce its colour and might be more easily rejected, speeding the time to find the target.…”

“…Functional imaging data of C's brain have not yet been reported, so we do not know whether parietal activation will appear (it is present in AD; Hubbard, E., personal communication), but C might represent a synaesthete who confirms the hypothesis that synaesthetic binding can be a result of direct physiological connections between specialized cortical feature maps. There are many stages in neural processing where synaesthesia could arise 72 . One suggestion has been that re-entrant connections send an extra signal to sensory systems that results in synaesthesia 75 .…”

Grapheme-color synesthetes show peculiarities in their emotional brain: cortical and subcortical evidence from VBM analysis of 3D-T1 and DTI data

“…Some authors suggest that the existence of a structural basis is necessary to understand the functional dynamics of synesthesia; this anatomical extraconnectivity may be the result of a heritable defective pruning (Ramachandran and Hubbard 2001a). Indeed, this incomplete apoptosis could contribute to the maintenance of intrinsic intersensory pathways that exists in early childhood (Spector and Maurer 2009).…”

“…This idea is supported by (a) the early associative theories which rely on the usefulness and pleasantness of the experience (Calkins 1895) (b) the claims about the emotional component of synesthesia (Cytowic and Eagleman 2009) (c) the idea that some types of synesthesia are emotionally mediated (Ward 2004) (d) the emotional displeasure provoked by incongruently colored graphemes (Ramachandran and Hubbard 2001b) (e) the behavioral evidence about synesthetically elicited affective reactions in grapheme-color synesthetes (Callejas et al 2007) (f) the hypotheses about the hyperconnectivity between limbic and extra-striate regions (Ramachandran and Hubbard 2001a) (g) the neuroimaging evidence of unusual activity in the retrosplenial cortex (Nunn et al 2002;Weiss et al 2001) and the insula (Niccolai et al 2012;Sperling et al 2006;Specht and Laeng 2011) (h) the neuroanatomical evidence of variations in the cingulate gyrus, orbitofrontal cortex, insula (Jancke et al 2009) and the retrosplenial cortex (Hupé et al 2011) and, last, but not least, (i) the synesthetes' accounts of their own experience. Despite the relevance of these claims, at present emotion is still not fully integrated in explanatory neurocognitive models.…”