The researchers said their findings lend support to the hypothesis that the condition is due to cross-activation between adjacent brain areas involved in perceiving shapes and colors. Some synesthetes report seeing colors when listening to music, or feeling tactile shapes while tasting food. This cross-activation might develop, they theorize, by a failure of the "pruning" of neural connections between the areas in the developing brain.
The rare condition called synesthesia--in which people's sensory perceptual circuitry seems to be miswired--was long dismissed as an oddity not worthy of scientific study. Now, however, researchers such as Hubbard and his colleagues are using the condition to gain insights into the neural basis of perception.
In their experiments with synesthetes who report seeing colors when they view numbers or letters, the researchers first sought to determine whether synesthetes really see the colors.
In one such experiment, they presented six synesthetes with patterns of black letters or numbers--known as "graphemes"--on a white background. They chose those graphemes that the synesthetes reported elicited specific colors. They designed the experiment so that if the synesthetes really were seeing the colors, that color perception would help them distinguish shapes such as triangles or squares formed by the graphemes. In another experiment, the researchers found that synesthetic color helped the synesthetes pick out specific numbers or letters in a crowded display.
The researchers found that the synesthetic colors really did help the synesthetes distinguish the shapes or gra phemes, compared to normal control subjects who were tested on the same patterns. However, the experiments with both the synesthetes and the controls also revealed that the synesthetic colors were not as effective as real colors in such tasks.
In fMRI scans, the researchers found that the synesthetes showed greater activation in a color-perception region of the cortex when viewing graphemes, compared to normal control subjects. The researchers found that the strength of this activation influences the strength of the synesthetic colors. In fMRI, harmless radio waves and magnetic fields are used to map regions of higher blood flow in the brain, which reflects higher activity in those regions.
Importantly, the researchers found evidence suggesting that synesthetes may be quite different from one another, which the researchers said "has profound implications for the studies of synesthesia that group together data from multiple synesthetes and treat them as if they all come from a homogeneous population.
"The use of single case studies in synesthesia is also of concern because the results obtained with one synesthete may not generalize to other synesthetes.
The researchers concluded that "Our results suggest that synesthetic colors lead to improved behavioral performance in a manner similar to real colors. Because this study uses both psychophysical and neuroimaging measures in the same subjects in the study of synesthesia, we are able to examine specific aspects of the synesthetic experience that previous studies have not been able to address."
Edward M. Hubbard, A. Cyrus Arman, Vilayanur S. Ramachandran, and Geoffrey M. Boynton: "Individual Differences among Grapheme-Color Synesthetes: Brain-Behavior Correlations"