PHANTOM CONTOUR ILLUSION

Title PHANTOM CONTOUR ILLUSION
Author, Co-Author Kathryn Weed, John Flanagan
Topic
Year
1992
Day
Sunday
Program Number
1:15 pm
Room
Ireland B
Affiliation
Abstract A new letter test has been developed using a phantom contour illusion to give a low spatial, high temporal frequency stimulus incorporating a recognition task. The 'illusion' is generated using a phase-reversing, textured border of black and white dots. The ten standard Sloane optotypes were used and were defined from a random dot presentation by ensuring that the dots inside the edge of the letter were out of phase with the dots outside the letter. Contrast threshold at a fixed frequency and dot was defined by establishing the 55% threshold level using a logit establishing the 55% threshold level using a logit function. The aim of this study was to establish the phase-difference contrast threshold and repeatability of the phantom contour illusion letter test. Twenty clinically normal subjects (Mean Age 23.7 + 2.2 years) were assessed using randomly presented 3/60 letters on screens of 4000 or 1000 random dots. The dots had a random size between 4 and 14 minutes of arc. Phase-reversal of 30Hz was used for both dot densities. In addition 15Hz and 7.5Hz were investigated for the 1000 dot stimulus. Repeatability was assessed by 2 further tests using the 4000 dot, 30Hz stimulus on a subgroup of 10 subjects. The average contrast threshold at 30Hz was 12.0% for 4000 and 40.4% for 1000 dots. At 15Hz and 7.5Hz the 1000 dot threshold were 12.8% and 8.0% respectively. The coefficient of repeatability gave a mean of the differences of -0.86 and limits of agreement of 1.33.
Affiliation of Co-Authors
Outline