Abstract
We tested the perceptual reality of synaesthesia in two phases. During the first phase, four color-grapheme synaesthetes reported on the synaesthetically induced colors of the letters A-Z and the numbers 0–9 in two sessions. We computed consistency within and between the two sessions and identified letters and numbers whose colors are consistently chosen, taking into account the strength of the synaesthetic association and the ability of the computer display to adequately represent the synaesthetic color.
In the second phase, we pitted grouping by proximity against grouping by similarity, using letters that induced colors either strongly or weakly. We measured the strength of grouping by proximity and grouping by similarity. Synaesthetes are more likely to group a dot pattern according to the similarity of synaesthetic colors than by proximity. This effect is greater for alphanumeric symbols that induced colors strongly than for alphanumeric symbols that induced them weakly. For control subjects the alphanumeric symbols had no effect on perceptual organization and the difference between strong and weak symbols was absent.
The results show that synaesthetes choose induced colors with high consistency and that synaesthetically induced colors affect perceptual organization. Since these results are based on precise quantitative methods, they constitute evidence that synaesthesia is a genuine perceptual phenomenon.