All synesthetes described having forms for several additional sequences (e.g., months, letters, and days of the week) see more and 3 out of 6 also reported having color associations for a few of these forms. The control group consisted of undergraduate students who were matched to the synesthetes for gender (all females), age (24.4 years old, SD = .7) handedness (all right-handed) and field of study (social sciences). All participants were unaware of the experiment’s purpose. They all gave their informed consent and the experiment was approved by local ethics committee. A stimulus
display consisted of two Arabic digits, presented on a computer screen, printed in bold “Arial” font. The digits could appear either to
the left and right (horizontal version) or at the top and bottom (vertical version) of the center of a screen, separated by 1 cm. There were 12 possible mixed pairs (1-2, 3-4, http://www.selleckchem.com/erk.html 6-7, 8-9, 1-3, 2-4, 6-8, 7-9, 1-6, 2-7, 3-8, 4-9), 8 possible same pairs (1-1, 2-2, 3-3, 4-4, 6-6, 7-7, 8-8, 9-9) and 2 possible font sizes (22 and 30). In line with the classic numerical Stroop task (Henik and Tzelgov, 1982), physical size (i.e., font size) and semantic magnitude (i.e., numerical value) were manipulated orthogonally to create 3 congruency levels: congruent (e.g., 3 5), incongruent (e.g., 3 5) and neutral (e.g., 3 3 and 3 5 for physical and numerical blocks, respectively). In for addition, digit spatial location was controlled as well. Thus, each pair could appear compatibly (left-to-right or bottom-to-top) or incompatibly (right-to-left or top-to-bottom) with the numbers’ position on the synesthetic number form. In accordance with the synesthetes’ number forms, there were two versions of the same task: a horizontal one and a vertical one. The synesthetes performed the version that corresponded to their number form, whereas controls performed both versions in two different sessions approximately 2 months apart. The vertical task was always carried out first3.Each task consisted of 2 blocks in which participants were asked to make
a comparative judgment regarding the numbers’ physical size (physical blocks) and 2 blocks in which they were asked to make a comparative judgment regarding the numbers’ numerical value (numerical blocks). The order of the blocks (2 physical and 2 numerical) was counterbalanced between participants. In each block, pairs of digits (1–9) were presented in a randomized order. Each digit was paired with itself or with a different digit that was numerically larger or smaller (by 1, 2 or 5 units), and appeared twice in 2 different physical sizes (i.e., dimension congruency) and in 2 different spatial locations (i.e., number-line compatibility). An entire block was composed of 144 trials; 48 congruent trials (12 different pairs × 2 different locations on the screen × 2 repetitions), 48 neutral trials and 48 incongruent trials.