Participant groups
1 = sss_syn
2 = control

1 = male
2 = female
3 = other

1 = none
2 = schooled to 16 years
3 = schooled to 17-19 years
4 = undergraduate
5 = postgraduate
6 = other

whether synaesthesia status was fully objectively verified (i.e. passed synaesthesia test and questionnaire)
0 = no
1 = yes