Design and procedure
Participants did a speed discrimination task in a self-paced experiment using the “method of single stimulus”39. In the unimodal conditions, a speed drawn from a set of seven (20, 40, 50, 60, 70, 80 & 100°/s) was presented and subjects were asked to report whether they perceived the speed as faster or slower than the average of all previously presented speeds. This paradigm has been used in previous speed discrimination studies37 to determine the point of subjective equality (PSE) and discrimination threshold and requires only half as much stimulus exposure as two-alternative forced-choice methods and produces better discrimination thresholds38. In effect, the method is a two-alternative comparison involving the current stimulus and an internal standard of mean speed.
We examined unimodal and crossmodal conditions separately in two sessions over two days in a counterbalanced order. In the unimodal conditions, every trial in an experimental block consisted of only auditory or visual motion. The seven velocities from the stimulus set were tested for both leftward and rightward directions, each repeated 20 times for a total of 280 trials per block. Subjects completed two blocks for each modality, alternating between audition and vision, and took breaks between each block. The order of presentation within a block (direction and speed) was completely randomised, as shown in Figure 1d, so that direction was unpredictable. This meant that consecutive directions could occur in the same direction (congruent) or opposite direction (incongruent). When analysing unimodal motion priming (Figure 3), only congruent data was used.
In the crossmodal condition, auditory and visual trials were interleaved in a balanced alternation (either A, V, A, V … or V, A, V, A …). The crossmodal condition contained no bimodal trials, only interleaved unimodal trials, and subjects again reported whether the current stimulus moved faster or slower than the average of the set of all stimuli. The velocities tested were as above (20, 40, 50, 70, 80 & 100°/s), in leftward and rightward directions, each repeated 10 times per block (240 trials) and subjects completed two blocks with a break in between. Again, direction of motion (leftward or rightward) was randomised to ensure unpredictability. When analysing crossmodal motion priming (A, V or V, A) we analysed separately for congruent priming (same direction: Figure 4) or incongruent (opposite directions: Figure 5).