Abstract
Purpose :
When the contrast of successive frames is reversed in an apparent motion stimulus, the perceived direction is reversed. This is believed to be due to the inhibition of neurons tuned to the direction of physical displacement. In this observational study, we tested the inhibition hypothesis using transparent motion stimuli.
Methods :
Ten normal subjects in 18-40 years were included. Transparent motion stimuli consisted of 2 random dot kinematogram (RDK) moving in opposite directions with 100% coherence resulting in a percept of 2 sets of dots moving in opposite directions. Two types of motion stimuli were used: regular phi and reverse phi, the only difference between the two being alternating contrast polarity in reverse phi. Number of dots varied from 500 to 2500 in a 10x10deg window with a limited dot lifetime of 3 frames. Temporal interval was 16.7ms and the spatial offset varied from 0.1 to 0.3deg. Stimulus duration varied from 1 to 3s. Subjects responded to the direction of motion in a 4-alternative forced choice (right or left diagonal, right-left or up-down). Number of dots, spatial offset, motion direction and stimulus duration varied in each trial presented 10 times and hit rates were calculated. From the literature, expected results are transparency motion on regular phi, and orthogonal direction on reverse phi condition because of inhibition of the neurons tuned to the stimulus directions, for e.g. perceiving up-down direction for a stimulus moving right-left.
Results :
Subjects confirmed perceiving a reversed direction for a reverse phi stimulus using single RDK. In transparent motion experiment, subjects reported perceiving motion along the direction of stimulus displacement for both motion conditions; 100% hit rate for regular phi and 0% hit rate for orthogonal direction on reverse phi condition. There was no difference in the hit rates with spatial offset and stimulus duration. When dot density increased to 2000 dots in reverse phi condition, hit rate for orthogonal direction increased to 10%, however, the subjects reported difficulty in judging the motion direction at higher dot density.
Conclusions :
In reverse phi motion, transparency motion was perceived rather than an orthogonal motion. This suggests two possible conclusions: 1) there is no inhibition caused by a reverse phi motion on neurons tuned to the direction of physical displacement, 2) if at all any inhibition was present, it was not enough to elicit an orthogonal motion.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.