Components of Attention in Synesthesia

One of the most common forms of synesthesia is between colors and graphemes (Colizoli, Murre, & Rouw, 2012). Numerous studies have investigated different aspects of attention and synesthesia, e.g. effects of Stroop-like interference by colors that are incongruent with the synesthetic experience. Here we attempt to isolate how specific components of attention are affected by grapheme-color synesthesia. Eight carefully screened healthy participants with synesthesia reported the letters in briefly presented, post-masked arrays of letters and digits. On half the trials, the letters and digits were presented in colors congruent with the synesthetic experience. On the other half of the trials, the letters and digits were presented in colors that were incongruent with the synesthetic experience. Components of attention were estimated separately for congruent and incongruent trials by fitting the data to a mathematical model based on A Theory of Visual Attention (Bundesen, 1990). It has previously been demonstrated that color experiences in observers with synesthesia are very stable over time, and that the color experience seems to be an integrate part of the processing of letters, in for example grapheme-color synesthesia (Mattingley, 2009). Results from the present experiment show that synesthesia affects both speed of processing (C) and the number of objects that can be retained in visual short-term memory (K). Participants were faster at encoding characters that were colored congruently with their synesthesia. In addition, the capacity of the visual short-term memory increased slightly in the congruent compared to the incongruent condition. Interestingly, congruent trials compared to incongruent trials did not seem to afford benefits to attentional selectivity (α), nor did they affect the threshold for visual perception (t0). The results, therefore, indicate that synesthesia relates to a specific subset of attentional components.