Idiosyncratic spatial representations of the days of the week in individuals without synesthesia

People with spatial sequence synesthesia have clear and stable mental images of spatial arrays of numbers, days of the week, and months. The shapes of the arrays vary by individual. We conducted an experiment to estimate the shapes of spatial arrays in people without synesthesia. We presented a pair of Japanese characters that represent the days of the week on a display, and asked participants without synesthesia to state which day they thought came later by pressing a key that was congruent with the spatial position of the day shown. We then estimated the shape of the spatial array in the participants' mental space (mental weekday lines) from the distribution of the reaction times. We found that these 'estimated mental weekday lines' differed by individual, and that the shapes (straight or bent lines, U-shapes, zigzags, or triangles) were similar to the spatial arrays found in individuals with synesthesia. We then conducted simulations using a self-organizing map and found that differences in the shapes of the spatial arrays compared with those in individuals with synesthesia could be explained by the inputs to the network. These results support the hypothesis that interactions between spatial and ordinal/cardinal representations in both synesthetes and non-synesthetes develop under the influence of self-organizing learning.