(A) Linear dependence between saccade peak velocity and saccade amplitude on a single participant; the regression line is superimposed to the data cloud. (B) Data are shown for single participants (gray lines) and average mean regression (black line). (C) Linear dependence between saccade peak velocity and saccade amplitude on a single representative participant. The gray continuous line represents the linear fit, whereas the gray dotted line represents the line orthogonal to the fit (see
Equation 2). Gray points represent trials greater than or equal to the orthogonal line (assigned to the faster/larger group; see
Equation 3), whereas black points represent trials less than the orthogonal line (assigned to the slower/smaller group; see
Equation 3). (D) The perisaccadic discrimination performance as a function of mask onset time interval—presaccadic interval, (−100, 0] ms; postsaccadic interval, (0, 100] ms—and combined amplitude × peak velocity (faster/larger group and slower/smaller group) on the present data set (17 participants). (E) The same analysis was applied to an independent data set (De Pisapia et al.,
2010;
n = 9). Again, we observed a significant influence of combined amplitude × peak velocity only for the presaccadic interval. Asterisks represent intervals in which subjects' accuracy statistically differed from chance level: *
p < 0.05, **
p < 0.01, Bonferroni-corrected paired two-sample
t test. Dashed lines represent saccade onset time (0 on the
x-axis) and baseline performance level of the two-alternative forced-choice task (0.5 on the
y-axis). Error bars represent 1
SEM. (F) The plot shows the bootstrapped one-sample
t statistic for the faster/larger – slower/smaller difference in the presaccadic interval (−100, 0] across the two data sets (26 participants) with a 99% bootstrapped confidence interval (10,000 repetitions with resample). Three split criteria were tested: median amplitude, median peak velocity, and combined (amplitude × peak velocity) split. Only the test in the combined (amplitude × peak velocity) split condition was significantly different from the null distribution (horizontal line at
t = 0).