Measures of spectral similarities for the detection of eye alignment during retinal birefringence scanning

Retinal birefringence scanning is a well-established method for detecting central fixation. Using this technique, binocular eye alignment is confirmed when both eyes simultaneously fixate on a small target. Central fixation is identified when the spectral power of the scanning signal returned from the retina exceeds a certain threshold at a characteristic frequency, or a combination of frequencies. Traditionally, this assessment is performed separately for each eye, with binocular fixation declared when both pass the same threshold. However, factors such as hardware asymmetries, pupil diameter variability, retinal reflectivity differences, or suboptimal eye positioning within the device's exit pupil can introduce inaccuracies in threshold-based decision-making. This pilot study explores cross-spectral methods to mitigate amplitude imbalances and improve reliability.

This research examines spectral similarities between the signals from both eyes, to establish a more robust identification of eye alignment, independent of amplitude asymmetry. Two primary techniques are proposed and tested: magnitude-squared coherence and the spectral correlation coefficient, both of which quantify spectral linkage between the eyes.

Magnitude-squared coherence reliably identifies eye alignment even in systems with significant signal imbalances, providing a continuous trace from which an alignment threshold can easily be determined. The spectral correlation coefficient, while computationally faster, has a limited time resolution. Additionally, spectral traces can be re-balanced using a linear fit, enhancing visualization. An algorithm for detecting the misaligned eye is also introduced, with potential clinical relevance pending validation.

The proposed spectral-domain techniques offer reliable measures of signal similarity for detecting eye alignment. These findings have the potential to significantly enhance the precision of decision-making in ophthalmic diagnostic devices utilizing retinal birefringence scanning. Of particular importance is their application in pediatric vision screeners, which play a crucial role in detecting strabismus (misaligned eyes) and amblyopia ("lazy eye").