SUBJECTIVE experience poses a major problem for neuroscientists and philosophers alike, and the relationship between them and brain function is particularly puzzling. How can I know that my perception of the colour red is the same as yours, when my experience of the colour occupies a private mental world to which nobody else has access? How is the sensory information from an object transformed into an experience that enters conscious awareness? The neural mechanisms involved are like a black box, whose inner workings are a complete mystery.
In synaesthesia, the information entering one sensory system gives rise to sensations in another sensory modality. Letters can evoke colours, for example, and movements can evoke sounds. These extraordinary additional sensations therefore offer a unique opportunity to investigate how the subjective experiences of healthy people are related to brain function. Dutch psychologists now report that different types of synaesthetic experiences are associated with different brain mechanisms, providing a rare glimpse into the workings of the black box.
One of the best characterized forms of synaesthesia is grapheme-colour synaesthesia, in which seeing or hearing a particular letter or number evokes the experience of a particular colour. One well-known grapheme-colour synaesthete is the Nobel Prize-winning physicist Richard Feynman, who said, “When I see equations, I see the letters in colours.” Feynman was a “projector” synaesthete – he automatically perceived the evoked colours in the external space, or “outside world”. The quality of the synaesthetic experience can, however, differ markedly between individuals: in “associator” synaesthetes, the evoked colour is perceived internally, or “in the mind’s eye”.
Projector and associator synaesthetes are most easily distinguished from one another on the basis of their own subjective reports, but the differences between their experiences can also be measured objectively. One study showed that projectors and associators can easily be distinguished from one another by their performance on various types of Stroop tests, in which a colour word is printed in a colour that differs from the meaning of the word (e.g. RED and BLUE). When asked to name the printed colour, the discrepancy causes longer response times and more errors, because we can read words more quickly than we can name colours. Projectors take longer to respond than associators, because their synaesthetic colour experience results in more interference during the task.
Romke Rouwe and H. Steven Scholte of the University of Amsterdam now report that these different synaesthetic experiences are associated with distinct underlying brain mechanisms, and that there also are structural differences between the brains of projector and associator synaesthetes. They recruited 16 projector and 26 associator grapheme-colour synaesthetes, placed them in a brain scanner, and showed them letters and numbers to evoke synaesthetic experiences. 42 non-synaesthetic controls were also recruited, so that the researchers could look for differences in brain structure and activation patterns between the synaesthetes and controls, as well as differences between the projectors and associators.
The synaesthetes were found to have greater gray matter density than the non-synaesthetes in the superior posterior parietal lobe (above), and this was found to be independent of the exact nature of the synaesthetic experience. The superior parietal lobe is a region of the visual cortex which is known to be selectively activated by colours. Previous studies have shown that activity in this part of the brain is increased in synaesthesia, and that disrupting its activity diminishes the intensity of synaesthetic experiences. The superior parietal lobe is therefore closely associated with synaesthesia, and is thought to integrate (or “bind”) the neural representations of colour and shape so that sensations of colour are evoked by letters or numbers.
The scans also revealed structural and functional differences between the projectors and associators. Projectors were found to have increased gray matter density in the visual and auditory regions of the brain. Increased activation of these areas was also observed in the projectors but not in the associators or controls. This may account for the subjective experiences of projector synaesthetes – activation of the brain areas areas involved in perceiving real objects causes letters and numbers to evoke the vivid experience of colours in external space.
By contrast, associators were found to have increased gray matter in, and greater activation of, the hippocampus and surrounding areas, which are known to be critical for the formation of autobiographical, semantic and spatial memories. The synaesthetic experiences of associators, then, which are purely internal, are apparently similar to memory retrieval – letters and numbers merely evoke recollections of the experience of colours, rather than vivid impressions of the colours themselves.
Thus, the brain mechanisms underlying the experiences of projector and associator grapheme-colour synaesthetes are partly shared – both projectors and associators exhibit structural and functional differences in the superior parietal lobe when compared to non-synaesthetes. But the subjective experiences of projectors are very different from those of associators, even though the sensory stimuli stimuli that evoke the experiences are exactly the same. This is because of differences in the underlying brain mechanisms, and the properties of the experiences are wholly consistent with the observed brain activity patterns: projectors actually see colour impressions due to visual cortical activation, but associators do not because the stimuli do not evoke activity in the visual brain areas.
An interesting question raised by these findings is whether or not the observed differences between projectors and associators extend to other forms of synaesthesia. Time-space synaesthetes, for example, experience units of time as occupying specific locations around the body, and in many cases, the passage of time is associated with changes in the perspective from which this visual representation is seen. Are there also individual differences in the experiences of time-space synaesthetes and, if so, are they too associated with different underlying brain mechanisms? Further research may provide some answers.
- The genetics of synaesthesia
- The neuropsychology of synaesthesia
- Tactile-emotion synaesthesia
- The sound of dots moving: A new form of synaesthesia
- The cognitive benefits of time-space synaesthesia
Rouw, R., & Scholte, H. (2010). Neural Basis of Individual Differences in Synesthetic Experiences. J. Neurosci. 30: 6205-6213. DOI: 10.1523/JNEUROSCI.3444-09.2010.
Dixon, M. J., et al. (2004). Not all synaesthetes are created equal: Projector versus associator synaesthetes. Cog. Aff. Behav. Neurosci. 4: 335-343. [PDF]