Abstract
Purpose: When the contrast of successive displays is reversed in an apparent motion stimulus, the perceived direction is reversed. We investigated if contrast reversal affects speed perception relative to regular phi motion. Methods: Ten normal subjects (age range: 18–40years) were included. Two types of motion stimuli were used: regular phi and reverse phi, the only difference between the two being alternating contrast polarity in the latter condition. Random dot kinematograms (RDK) with 500 dots were presented sequentially in a central 8×8deg window with a limited dot lifetime of 3 frames. The temporal interval was 16.7ms. Standard speed used was 18 deg/s and the test speeds were +/−25%, +/−30%, and 60%. These speeds resulted in the spatial offsets of 0.2deg to 0.5deg. The inter-stimulus interval between the intervals was 250ms. On a 3-alternative forced choice paradigm, the subjects responded to the fastest motion (first/second) or equality. The direction of perceived motion was the same on both intervals. The interval containing the standard speed and the motion type was randomized. Stimulus duration was 0.5s or 1s. Each trial was presented 10 times randomly and hit rates were calculated. The speed discrimination thresholds were calculated at 66.7% probability and compared between the motion types. Results and Conclusion: Subjects confirmed perceiving a reversed direction on a reverse phi stimulus using single RDK. Speed discrimination threshold for regular phi and reverse phi was 12.4deg/s and 13.8deg/s. There was no significant difference between the two threshold values (p=.13). This suggests a similar neural mechanism for both motion types. However, when the perceived speed of reverse phi and regular phi was compared, reverse phi was perceived as being faster about 88% of times when the actual speed of the two motion types was the same, despite randomizing the order of the presentation.