Do sequence-space synaesthetes have better spatial imagery skills? Yes, but there are individual differences

People with sequence-space synaesthesia perceive sequences (e.g. numbers, months, letters) as spatially extended forms. Here, we ask whether sequence-space synaesthetes have advantages in visuo-spatial skills such as mental rotation. Previous studies addressing this question have produced mixed results with some showing mental rotation advantages (Simner et al. in Cortex 45:1246-1260, 2009; Brang et al. in Cogn Process, 2013), but one that did not (Rizza and Price in Cogn Process 13:299-303, 2012). We tested this hypothesis again with a new group of sequence-space synaesthetes, and we also tested a range of individual differences that might have caused this conflict across previous studies. Specifically, we tested: years of education, visual imagery ability, nature of forms (2D or 3D representation of sequences), number of forms (e.g. for months, days, numbers), and tendency to project sequences into external space versus the mind's eye. We found yet again that synaesthetes had enhanced abilities in mental rotation compared to controls, but that one individual difference in synaesthetes (the ability to project forms into space) was especially linked to performance. We also found that synaesthetes self-reported higher visual imagery than controls (Price in Cortex 45:1229-1245, 2009; Mann et al. in Conscious Cognit 18:619-627, 2009; Rizza and Price 2012). Overall, our data support previous studies showing superior imagery reports (Price 2009) and mental rotation (Simner et al. 2009; Brang et al. 2013) in sequence-space synaesthetes, and we suggest that one previous failure to replicate (Rizza and Price 2012) might be explained by individual differences among synaesthetes recruited for testing.