Considering the Effect of Neural Adaptation in the Estimation of Haidinger's Brush Perceptual Threshold

This study aimed to investigate the role of neural adaptation in the perception of Haidinger's brushes, an entoptic phenomenon enabling the perception of polarization by the human visual system. The objective was to consider the rotational speed of the pattern in combination with the polarization level of the input light, to identify normal values in a cohort of healthy individuals.

An experimental setup was designed and assembled using two LED sources, one of which was filtered by a rotating linear polarizer with adjustable speed. The relative intensity of the LEDs could be changed to tune the polarization level. A cohort of 37 healthy individuals underwent monocular testing at seven polarization levels. For each value, descending and ascending limit tests were conducted to determine the rotational speed thresholds.

The rotational speed threshold decreases linearly with the logarithm of the polarization level. All participants could perceive the pattern down to at least 20% polarization, with a dramatic drop in the number of subjects at 10% and 5% polarization levels. The estimated average polarization threshold was 9.7 ± 1.2%. Lower initial rotational thresholds at maximum contrast corresponded with lower polarization thresholds.

The study confirms the crucial role of rotational speed in the perception of Haidinger's brushes, especially at low polarization levels. As a normal pattern perception correlates with macular pigment distribution and density, those findings are essential for the development and calibration of screening devices based on Haidinger's brush perception for the early detection of maculopathy and other macular diseases.