Abstract
Purpose :
When the contrast of successive frames is reversed in apparent motion stimuli, the perceived motion direction is reversed. Literature on sensitivity of reverse phi motion is limited and variable. In this observational study, we compared the spatiotemporal (ST) tuning and sensitivity of reverse phi and regular phi motion for both central and peripheral fixation.
Methods :
Clinically normal subjects (18-40 years) were included. Random dot kinematogram (RDK) with 500 dots in a 10x10deg aperture was used. In the first experiment, the temporal interval was varied from 16.7 to 66ms in multiples of 16.7ms and the spatial offset was varied from 0.1 to 0.6 deg in steps of 0.1 achieved by varying the dot speed from 6 to 35 deg/sec respectively. Regular phi stimuli had white dots of luminance 140cd/m2 while reverse phi had alternating contrast polarity at every displacement. 100% coherent RDK was presented on a gray background for 500ms at center and 10deg inferior to central fixation. Each spatial and temporal level was repeated randomly and hit rates were calculated. In the second experiment, motion coherence was varied from 0 to 100% with spatial and temporal offsets being 0.2deg and 16.7ms. Coherence threshold at 63% probability was compared between both motion types at central and peripheral fixation. In all experiments, subjects responded to the direction of motion (25% guess rate). Correct response for regular phi is the direction of physical displacement whereas for reverse phi, it is the opposite direction. No error feedback was provided.
Results :
Ten subjects (6 males) participated. Hit rate of motion direction was reduced by >50% at temporal intervals >50ms for regular phi and >33ms for reverse phi at all spatial offsets for both fixation conditions and appeared as flicker. There was no significant difference in the ST tuning between center and periphery except in reverse phi condition where forward motion was seen at low spatial offsets at 33ms temporal interval for central fixation. The motion coherence threshold was significantly higher for reverse phi (34±5%) than regular phi (18±3%) at central fixation (p=.013) and no statistical significance in threshold was found between the center and periphery.
Conclusions :
The ST tuning of both motion types was statistically different. Motion sensitivity is weaker for reverse phi than regular phi at the center. This may suggest a possible inefficiency in the correlation of dots of opposite contrasts.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.