Abstract
Purpose :
Retinal birefringence scanning (RBS) is a method of detecting central fixation (CF). With it, binocular eye alignment is declared when both eyes are fixating at the same time on a small target. Central fixation is assumed when the spectral power of the scanning signal returned from the retina is above a certain threshold for a characteristic frequency, or a combination of frequencies. So far, this has been done for each eye separately, and binocular eye alignment was declared when both eyes pass the same threshold. However, due to optical hardware asymmetries and/or the presence of certain instrumental noise, device-to-device variability, etc., applying a threshold-based decision-making may become imprecise. The methods proposed here were developed to avoid absolute measurements of spectral power.
Methods :
The magnitude-squared coherence (MSC) between two time-domain signals x(t) and y(t) is a real-valued function that is defined as:
MSCxy(f) = │Gxy(f)│2 / [Gxx(f)Gyy(f)]
where Gxy(f) is the cross-spectral density between x and y, and Gxx(f) and Gyy(f) are the autospectral densities of x and y, respectively. The MSC is a measure of similarity in the frequency content of two signals. In this case, the two signals are frequency powers for the right eye and the left eye, respectively. The MSCxy(f) is in the range [0…1]. For ideal spectral linkage between the two signals x(t) and y(t), the MSCxy→1.
The Spectral Correlation Coefficient (SCC) treats the signals x and y as the spectral powers (for RE and LE, respectively) for a certain CF-characteristic frequency (or a combination of frequencies).
Results :
The test subject was asked to look at a presented central target during acquisitions 0-50, 100-150, 200-250, and 300-350. For the remaining times, the subject was looking 1.5° away, in different directions (at 3, 6, 9 and 12 o’clock on the rim of the 3 deg scanning circle encircling the central target). MSCxy(f) proved to be steadily above 0.15 for CF.
High SCC calculated from the spectral traces indicates good spectral similarity, hence good binocular eye alignment, while a lower SCC is a sign of spectral dissociation, and most likely binocular eye misalignment. Experiments indicate that a reasonable threshold for binocular eye alignment would be around SCC=0.8.
Conclusions :
Both the MSC and the SCC worked well and were able to identify binocular alignment during CF.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.