The physiology of coloured hearing A PET activation study of colour-word synaesthesia

Abstract

In a small proportion of the normal population, stimulation in one modality can lead to perceptual experience in another, a phenomenon known as synaesthesia. In the most common form of synaesthesia, hearing a word can result in the experience of colour. We have used the technique of PET, which detects brain activity as changes of regional cerebral blood flow (rCBF), to study the physiology of colour-word synaesthesia in a group of six synaesthete women. During rCBF measurements synaesthetes and six controls were blindfolded and were presented with spoken words or pure tones. Auditory word, but not tone, stimulation triggered synaesthesia in synaesthetes. In both groups word stimulation compared with tone stimulation activated the classical language areas of the perisylvian regions. In synaesthetes, a number of additional visual associative areas, including the posterior inferior temporal cortex and the parieto-occipital junctions, were activated. The former has been implicated in the integration of colour with shape and in verbal tasks which require attention to visual features of objects to which words refer. Synaesthetes also showed activations in the right prefrontal cortex, insula and superior temporal gyrus. By contrast, no significant activity was detected in relatively lower visual areas, including areas V1, V2 and V4. These results suggest that colour-word synaesthesia may result from the activity of brain areas concerned with language and visual feature integration. In the case of colour-word synaesthesia, conscious visual experience appears to occur without activation of the primary visual cortex.

Does Binding of Synesthetic Color to the Evoking Grapheme Require Attention?

“…In the case of visual search it appears that synesthesia allows subjects to guide search based on synesthetic color, but as Palmeri et al point out, it does not 'pop-out' as real color does. Ramachandran and Hubbard (2001a) showed that a form defined by distributed items among distractors was difficult to find for non-synesthetes but not for synesthetes. However, they too included target and distractors that both induced color.…”

“…Stroop-like paradigms have been used to study synesthesia in the lab (e.g., Odgaard et al, 1999;Bergfeld-Mills et al, 1999;Dixon et al, 2000). Others have used variants of visual search paradigms in synesthetes to address the perceptual reality of color-grapheme synesthesia (Palmeri et al, 2002;Ramachandran and Hubbard, 2001a). These studies have demonstrated that synesthesia is a genuine perceptual phenomenon (rather than mnemonic, associative, or metaphoric) by showing that synesthetic colors facilitate search in what are objectively monochromatic displays containing letters or digits.…”

“…This study focuses on the role of attention in this type of synesthesia. A recurring theme in discussions of synesthesia is that some form of cross-activation between one brain area and another may be present (e.g., Baron-Cohen et al, 1993;Ramachandran and Hubbard, 2001a). If indeed graphemic-chromatic synesthesia is the result of direct communication between ventrally located color and word-form areas synesthesia may not rely on visual attention or require parietal input 2 .…”

Neurophysiology of synesthesia

“…In the first study of synesthesia, Paulesu et al [9] used positron emission tomography (PET) to determine whether color selective areas of the cortex were active when auditory word → color synesthetes reported seeing colors. Subjects were presented with blocks of either pure tones or single words.…”

“…In spatial-sequence, or number → space synesthesia, numbers, months of the year, and/or days of the week elicit precise locations in space such as a three-dimensional view of a year as a map [5-7]. "Colored hearing" which includes auditory word → color and music → color synesthesia [8][9][10] involve linkages that are truly cross-modal, and are often considered the paradigmatic examples of synesthesia, despite being less common than the above mentioned forms.…”