Displacement thresholds in central and peripheral vision during tracking

In this study 26 observers were presented a series of forced choice, paired comparison trials in which a long, thin, collimated, horizontally oriented (horizon) line moved downward through 12 angles ranging from 0.6-60' of arc on one of the two trials. They had to judge which stimulus had moved in each pair. The displacements were produced by combinations of five stimulus displacement durations (0.25, 0.5, 1,2, and 4 s) and four stimulus angular velocities (0.042, 0.083, 0.167, and 0.25 degrees X s-1 for a total of 20 conditions. It was found that increasing stimulus duration produced significantly more correct displacement judgments. These results are consistent with the hypothesis that threshold is based on a constant stimulus velocity over this range of durations and velocities. However, since fixation remained on the stimulus during its smooth displacement, a corollary neural discharge associated with the voluntary rotation of the eyes may have been a major contribution to this judgment.