Frequency doubling illusion: detection vs. form resolution.
OBJECTIVE
To investigate the difference between detection and form resolution thresholds for the frequency doubling (FD) illusion.


METHODS
The right eyes of 20 normal healthy subjects (10 female; age range, 18 to 30 years; mean age, 24.05 years; SD, 1.39) were examined. Vertically oriented FD stimuli were presented at fixation and 10 degrees nasally using a dual temporal alternate forced choice method of constant stimuli (2AFC MOCS) to estimate the thresholds for detection and form resolution. Additional strategies for threshold estimation (yes/no, modified rapid estimation by binary search) were used to determine the detection threshold. The effect of subject instruction on the FD threshold was also examined. The test-retest characteristics were investigated by determining the coefficient of repeatability.


RESULTS
Detection thresholds using 2AFC MOCS were 0.86 +/- 0.20 (+/-1 SE) Michelson contrast percentage (MC%) at fixation and 0.84 +/- 0.21 at 10 degrees. Form resolution thresholds using 2AFC MOCS were 1.08 +/- 0.23 MC% at fixation and 1.04 +/- 0.16 at 10 degrees . These thresholds were found to be significantly different at fixation (p = 0.004) and 10 degrees (p = 0.005). No difference was found between threshold estimation strategies, but subject instruction had a significant effect (fixation: 2.41 +/- 0.52 MC%, p < 0.001; 10 degrees : 2.19 +/- 0.48 MC%, p < 0.001).


CONCLUSIONS
Detection thresholds were significantly lower than form resolution thresholds for the FD stimulus. This result is in agreement with classic FD studies and illustrates that the perception of flicker precedes the perception of the FD illusion at threshold.
Quaid
Patrick T
PT
School of Optometry, University of Waterloo, Ontario, Canada. ptquaid@sciborg.uwaterloo.ca
Simpson
Trefford L
TL
Flanagan
John G
JG
D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D009903:Optical Illusions; D010775:Photic Stimulation / Q000379:methods*; D015203:Reproducibility of Results; D012684:Sensory Thresholds; D014785:Vision, Ocular