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).