Vernier acuity was measured using two-bar vertical vernier stimuli, which were defined either by static luminance contrast dots
(Fig. 1A)or by the relative motion of randomly placed dots
(Fig. 1B) . The static luminance-defined stimuli were composed of small dot elements (17 sec arc-square pixels), which were randomly placed within the bars. The bars were 10 min arc wide and 15 min arc high and were separated vertically by 4 min arc. The dots were white (100 cd/m
2) and were presented on a black (0.5 cd/m
2) background. For the motion-defined stimuli, both the bars and the background were covered with random dots of equivalent density (4593 dots per deg
2) throughout a central 1.23° square patch. A relatively high dot density was chosen, as motion-defined vernier acuity performance has been shown not to be limited by sampling for dot densities of this magnitude.
25 When the dots were stationary, the vernier target was invisible. The target was made visible by the motion of the dots within the bars relative to that of the background. The background dots moved upward at 1.25 deg/sec, while the dots within the bars moved downward at the same speed. The motion was created by the use of an eight-frame motion sequence, in which each frame was displayed for 57 ms, resulting in the total stimulus duration being 456 ms for the motion-defined vernier targets. The stimulus duration was identical for the static vernier targets. The density of the dots was equivalent for the static and motion-defined vernier stimuli; hence, the static stimulus is simply the vertical bars from the motion-defined case displayed without motion. For each stimulus presentation, the random dot texture was recreated to avoid any systematic influence on performance caused by the positioning of the random dot elements. We recognize that the static stimulus was broader and of lower perceived contrast than would provide optimal vernier acuity thresholds.
25 We wanted to maintain as much similarity as possible between the static luminance and motion-defined vernier stimuli, to ensure that if the migraine group displayed deficits on one of the vernier tasks alone, that performance could not have been influenced by simple differences in stimulus attributes such as dot density.
Thresholds were determined with a two-interval, forced-choice (2IFC) procedure. In the test interval, a vernier target was shown with a variable degree of horizontal displacement between the upper and lower bars, whereas the control interval showed a target with no horizontal offset. Subjects were required to choose the interval in which the stimuli were horizontally displaced. The offset was randomly chosen on each presentation to be either left or right. The amount of horizontal displacement was varied according to a staircase strategy, where the offset was reduced if three consecutive correct responses were made and increased for every incorrect response. This three-down, one-up design results in the staircase converging on the 79% correct performance level.
30 Two staircases were interleaved, and each started with an offset of 2 min arc for the static condition and 6 min arc for the motion condition. The initial staircase step size was 4 pixels (each pixel was 5.7 sec arc when viewed at 10 meters), which was halved on the first two reversals. The staircases terminated after six reversals, with the result of each staircase being determined as the average of the final four reversals. The mean of the two interleaved thresholds was taken as the threshold estimate for each task.