<p>Each data point represents the mean of 15 participants (at least 133 RTs); error bars show ±SEM. RTs formed statistically homogeneous groups for each contrast level. While the RTs (filled circles) were not significantly different from the shortest mean RT (370 ms for 10% and 317 ms for 90%), RTs signed open circles were not significantly different from the longest mean RTs (495 ms for 10% and 373 ms for 90% contrast), except 7.3 arc min at 10% contrast. Solid black curves show best fit 2<sup>nd</sup> order polynomial functions (R<sup>2</sup> = 0.867, min. value = 16.3 arc min, equation = 186 * <i>x</i><sup>2</sup> − 450 * <i>x</i> + 654 or 10% and R<sup>2</sup> = 0.833, min. value = 20.2 arc min, equation = 79 * <i>x</i><sup>2</sup> − 206 * <i>x</i> + 464 for 90% contrast).</p
