Cognitive Advantages and Individual Differences in Sequence-Space Synaesthesia

Synaesthesia is a condition in which certain stimuli produce sensations that the general population typically does not experience. One variant is sequence-space synaesthesia in which people tend to perceive sequences (e.g., numbers, months, days, and letters) as forms that occupy actual space around them. This study aimed to test whether sequence-space synaesthetes have advantages in visuo-spatial skills such performing 2D and 3D mental rotation tasks. Previous studies asking this question have produced mixed results with some showing mental rotation advantages (Simner, Mayo & Spiller, 2009; Brang, Miller, McQuire, Ramachandran & Coulson, 2013) and some that do not (Rizza & Price, 2012). We also looked at a range of individual differences within synaesthetes that might have caused these differences across studies; specifically, effects of education, visual imagery, nature of forms (2D or 3D representation of sequences), number of forms (e.g., for months, days, numbers), and tendency to project sequences into space versus the mind’s eye only. Additionally, we sought to see if synaesthetes had better recall for events. We found that synaesthetes do show enhanced spatial abilities in 3D mental rotation as well as in memory for events. Furthermore, those synaesthetes that project their sequences into space were more accurate on the 3D mental rotation task compared to those who see sequences in the mind’s eye only. We also found that synaesthetes self-report higher visual imagery. The ability to better mentally rotate objects may stem from the ability to project and manipulate sequences, an ability not seen in those synaesthetes maintaining their forms just within their mind. Although effects of individual differences were discovered in mental rotation, none appeared to significantly influence event memory. Overall, our data support previous studies showing superior abilities in sequence-space synaesthesia in imagery (Price, 2009), mental rotation (Simner et al., 2009; Brang et al., 2013) and event memory (Simner et al., 2009) and suggest that any prior conflicts across studies (at least for mental rotation) may have arisen from individual differences in the synaesthetes recruited for testing