Some people experience an intermingling of the senses, known as synaesthesia, in which certain shapes become combined or “bound with” certain colors, or that certain colors are strongly associated with certain sounds. Of course, in healthy normal adults, color and shape become bound together only when visible shapes really are visibly colored – a process which is itself still a mystery of neuroscience. Yet similar mechanisms may be involved in both these kinds of “binding,” but are merely more active in synaesthetes than in normal adults.
Based on Patient RM, we know that binding between color and shape relies on the parietal lobes – without them, the sense of space dissolves and the visual world disintegrates into individual and unconnected sensations. If the parietal lobes are important for binding, one might expect them to also be critical for synaesthetes tightly integrated sensations. To test this idea, Esterman et al temporarily disabled the parietal lobes of two synaesthetes who reliably associated particular letters with particular colors.
The subjects first identified the RGB values which were mentally most strongly associated with each of the alphabet’s 26 letters. Then, for each of many trials, the synaesthetes had to indicate whether a colored letter was actually red, green or yellow. Critically, some of these letters were colored in a way that was consistent with each individual’s synaesthesia, others in a way that was inconsistent with each individual’s synaesthesia, and yet other letters or symbols which were colored but not mentally associated with any color for the subjects (such as “#”).
Replicating previous work with synaesthetes, these subjects showed interference from their mental sensation of color when it conflicted with the actual color of the letters. That is, subjects were slightly slower (less than 60 msec) to name the color of a letter when it mismatched their synaesthetic experience than when the letter was not associated with a color. These kinds of subtle, consistent differences can be difficult to fake, suggesting that the two subjects were indeed synesthetic.
But even more interesting is what happened when the experimenters disabled the subjects’ parietal lobes.
The experimenters targeted each subject’s posterior parietal cortex with strong, repetitive magnetic stimulation, which is thought to temporarily “scramble” the activity in that region of the brain. The technique, known as “rTMS,” is thought by many to deactivate the targeted region of neural tissue, which in this case was the intraparietal sulcus and the transverse occipital sulcus.
When rTMS was applied to their right parietal lobe, these subjects no longer showed the pattern described above. That is, they were no slower to name the color of a letter when it conflicted with their synaesthetic experience as when the letter had no color association. rTMS had no such effect when applied to other regions, including the left parietal cortex and the V1 area of the occipital lobe.
So the binding of colors and shapes in normal healthy adults seems to involve the same brain area that’s involved in synaesthesia. Of course, it’s unlikely that just one area is involved in synaesthesia; like most cognitive phenomena, it is likely a complex product of interactions among multiple brain regions.
Many questions remain – would the same be true of “associator” type synaesthetics, who associate their synaesthetic experiences with more internal than external sensations? What role might other areas, including the frontal cortex, have in synaesthetic experience? And finally, do synesthetes have altered attentional mechanisms, given the notable attentional deficits of those with Balint’s syndrome?
Tomorrow’s post will cover the hypothesized role of attention in binding, to gain insight onto the cognitive mechanisms that work to integrate our sensations, whether only sparsely (as in Balint’s) or over-zealously (as in synesthesia).