Mechanisms Underlying Frequency-Doubling Technology (FDT) in Glaucoma

Abstract

 

Purpose

 

The mechanism of the frequency doubling illusion (FDI) and FDT remain uncertain. FDT perimetry such as the Humphrey Matrix FDT (Carl Zeiss Meditec) uses flickering gratings that presumably elicit the FDI for enhanced detection of glaucoma, but has not definitely been shown superior to the current reference standard, SITA-SAP. Patients supposedly perceive the FDI in the test, but this is not verified. To better understand FDT, we present a study comparing the perception of the FDI in normal subjects and subjects with glaucoma.

 

Methods

 

We examined 8 normal subjects (19-31 years) and 8 subjects with mild to moderate primary open-angle glaucoma (26-86 years). Experiments consisted of a monocular two-alternative forced-choice task. A high-contrast counterphase grating of varied spatial and temporal frequencies was briefly presented, followed by 2 static matching stimuli at veridical and doubled spatial frequencies. The task was to choose the one best resembling the counterphase grating. Static stimuli were viewed centrally and presented in a random order.

 

Results

 

Average psychometric functions are shown in Fig 1. Perception of the FDI by normal subjects shows little dependence on spatial frequency; subjects with glaucoma show some dependence. Strikingly, subjects with glaucoma perceive the FDI at much higher temporal frequencies than normal subjects. Although our data are preliminary, this result seems significant, as the mean frequencies are approximately 2 standard deviations apart.

 

Conclusions

 

Our results imply that individuals with glaucoma are less subject to the FDI than normal subjects, as they maintain “correct” perception over a broader temporal frequency range. However, perception of the FDI in general tends to be accompanied by enhanced contrast perception. This suggests that the diminished perception of the FDI by patients with glaucoma results from impaired contrast threshold relative to normal subjects; this may be the mechanism for glaucoma detection by FDT. This interesting observation merits further investigation, with future observations including age-matched controls and careful perimetric testing of the FDI itself in search of early diagnosis of glaucoma.

 

 

Psychometric functions averaged for normal subjects (A) and subjects with glaucoma (B). Lighter shades represent a higher probability of seeing the illusion. Solid lines represent 0.75 probability of seeing the illusion; dashed lines represent ±1 SD.

 

Keywords: 642 perimetry