Synesthesia hearing to see

Question

Can a synesthetic person, also known as a synaesthete, see sounds even when that person is actually blind? If so how does the brain interpret a picture without a vision?

Answer 1

In order for your 2 questions to be answered I have taken each question in turn.

Can a synaesthete see sounds even when that person is actually blind?

According to Steven & Blakemore (2004), there seems to be 2 forms of visual synaesthesia which blind people can experience. That is Coloured-Braille synaesthesia, and Coloured-hearing synaesthesia for letters, numbers, and words (including time-related words) This is also known as Chromesthesia (see also this Wikipedia Article). All quotes below are from Steven & Blakemore's study article, and there is one point of note with the study.

The synaesthesia described by our subjects is most likely idiopathic (ie of early onset, as opposed to acquired after blindness) because all of them report having had synaesthesia for as long as they can remember (ie from prior to the onset of blindness). Furthermore, three of the six subjects have one or more first-degree relatives who are sighted and have synaesthesia (suggesting a genetic aetiology for the phenomenon)

I haven't come across any study where people blind from birth can experience synaesthesia, yet it doesn't mean it cannot happen. The question then of course would be how the colours could be relayed to someone studying the case as the person blind from birth would not have any colour references to go by.

Coloured-Braille synaesthesia

This is where the blind person sees colours whilst reading Braile.

One subject, JF, reports experiencing coloured Braille. For JF, the tactile sensation of each Braille character immediately evokes a synaesthetic impression of an array of tiny coloured dots, "like an LED display", corresponding to the Braille character. However, he does not see colours when he feels other textures or objects with his fingers, nor when he touches dots arranged in shapes or spatial arrangements not related to Braille.

Coloured-hearing synaesthesia (also known as *Chromesthesia* or sound-to-colour synesthesia

This is where heard sounds automatically and involuntarily evoke an experience of colour.

All six of our subjects have coloured hearing for words relating to time (throughout this paper, 'time' refers to days of the week and months of the year, but not to hours, minutes, or seconds). Some have coloured hearing for all words and many have coloured hearing for spoken letters and numbers. Furthermore, several subjects also see these percepts as vague shapes, forming spatially localised patterns in their "mind's eye".

Coloured-hearing synaesthesia seems to be different in each of the cases in Steven & Blakemore's study, including how with JF,

When JF moves his eyes or head from left to right, his colours remain stable in space, but if he rotates his whole body, the array of colours moves with him. Hence the synaesthetic phantoms are body-centred, not oculocentric or head-centred.

I would recommend reading this study article as I found it interesting.

How does the brain interpret a picture without a vision?

It seems that synaesthetes see colours because the visual cortex is stimulated by the sensation which triggered it. (Steven & Blakemore, 2004)

In a single-subject fMRI study, Aleman and colleagues (2001) reported significant activation of human striate cortex when a synaesthete experienced illusory colours in response to hearing words. An earlier positron emission tomography (PET) study, however, had shown activation of visual-association cortex but not primary visual cortex in coloured-word synaesthesia (Paulesu et al 1995). Nunn et al (2002) recently supported this conclusion with their finding that cortical areas thought to be involved specifically in colour analysis (V4/V8), but not earlier visual areas, are activated when coloured-hearing synaesthetes listen to spoken words. Despite some differences in these results, it seems clear that parts of the visual cortex are activated when synaesthetes experience spurious visual sensations.

As long as there is no damage to the visual cortex, if other conditions are set for it within the brain connections, all that is needed is the correct stimulation within the visual cortex for colours to be seen.

Sadato et al (1996), using PET, demonstrated that the visual cortex is activated when (non-synaesthetic) late-blind subjects read Braille, and others have reported activity in visual areas during non-Braille tactile discrimination in the blind (Büchel et al 1998; Burton et al 2002). Not only was primary visual cortex activated during Braille reading in the blind, but extrastriate cortex was also activated, including areas in the fusiform gyrus with Talairach coordinates corresponding to V4, which normally processes colour vision in sighted subjects (Burton 2003; Burton et al 2002). Again, this unusual visual cortical activation appears to be functionally significant, since TMS of the primary visual cortex has been shown to disrupt the ability to read Braille (Cohen et al 1997; note that TMS of V4 is not currently feasible), and it might somehow account for the fact that tactile acuity is enhanced in blindness (Goldreich and Kanics 2003)

References

Steven M. S., & Blakemore C. (2004). Visual synaesthesia in the blind. Perception 33(7), 855-68
PMID: 15460512 DOI: 10.1068/p5160
Free PDF available from semanticscholar.org

References in quotes from Steven & Blakemore (2004)

Aleman A, Rutten G-J M, Sitskoorn M M, Dautzenberg G, Ramsey N F, 2001 "Activation of striate cortex in the absence of visual stimulation: an fMRI study of synesthesia" NeuroReport 12 2827—2830

Büchel C, Price C, Frackowiak R S, Friston K, 1998 "Different activation patterns in the visual cortex of late and congenitally blind subjects" Brain 121 409—419

Burton H, Snyder A Z, Conturo T E, Akbudak E, Ollinger J M, Raichle M E, 2002 "Adaptive changes in early and late blind: a fMRI study of Braille reading" Journal of Neurophysiology 87 589—607

Burton H, 2003 "Visual cortex activity in early and late blind people" Journal of Neuroscience 23 4005—4011

Cohen L G, Celnik P, Pascual-Leone A, Corwell B, Faiz L, Dambrosia J, Honda M, Sadato N, Gerloff C, Catala M D, Hallett M, 1997 ``Functional relevance of cross-modal plasticity in blind humans" Nature 389 180—183

Goldreich D, Kanics I M, 2003 "Tactile acuity is enhanced in blindness" Journal of Neuroscience 23 3439—3445

Nunn J A, Gregory L J, Brammer M, Williams S C R, Parslow D M, Morgan M J, Morris R G, Bullmore E T, Baron-Cohen S, Gray J A, 2002 "Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words" Nature Neuroscience 5 371—375

Paulesu E, Harrison J, Baron-Cohen S, Watson J D, Goldstein L, Heather J, Frackowiak R S, Frith C D, 1995 "The physiology of coloured hearing: A Positron Emission Tomography activation study of coloured-word synaesthesia" Brain 118 661—676

Sadato N, Pascual-Leone A, Grafman J, Ibanez V, Deiber M P, Dold G, Hallett M, 1996 "Activation of primary visual cortex by Braille reading in blind subjects" Nature 380 562—568