Effects of non-symbolic numerical information suggest the existence of magnitude-space synesthesia

In number-space synesthesia, numbers are visualized in spatially defined arrays. In a recent study (Gertner et al. in Cortex, doi: 10.1016/j.cortex.2012. 03.019, 2012), we found that the size congruency effect (SiCE) for physical judgments (i.e., comparing numbers' physical sizes while ignoring their numerical values, for example, [InlineMediaObject not available: see fulltext.] 8) was modulated by the spatial position of the presented numbers. Surprisingly, we found that the neutral condition, which is comprise solely of physical sizes (e.g., [InlineMediaObject not available: see fulltext.] 3), was affected as well. This pattern gave rise to the idea that number-space synesthesia might entail not only discrete, ordered, meaningful symbols (i.e., Arabic numbers) but also continuous non-symbolic magnitudes (i.e., sizes, length, luminance, etc.). We tested this idea by assessing the performance of two number-space synesthetes and 12 matched controls in 3 comparative judgment tasks involving symbolic and non-symbolic stimuli: (1) Arabic numbers, (2) dot clusters, and (3) sizes of squares. The spatial position of the presented stimuli was manipulated to be compatible or incompatible with respect to the synesthetic number-space perceptions. Results revealed that for synesthetes, but not for controls, non-symbolic magnitudes (dot clusters) as well as symbolic magnitudes (i.e., Arabic numbers) interacted with space. Our study suggests that number-space synesthetes might have a general magnitude-space association that is not restricted to concrete symbolic stimuli. These findings support recent theories on the perception and evaluation of sizes in numerical cognition.