Structured light enhanced entoptic stimuli for vision science applications

Abstract

The dichroic macular pigment in the Henle fiber layer in the fovea enables humans to perceive entoptic phenomena when viewing polarized blue light. In the standard case of linearly polarized stimuli, a faint bowtie-like pattern known as the Haidinger's brush appears in the central point of fixation. As the shape and clarity of the perceived signal is directly related to the health of the macula, Haidinger's brush has been used as a diagnostic marker in studies of early stage macular degeneration and central field visual dysfunction. However, due to the weak nature of the perceived signal the perception of the Haidinger's brush has not been integrated with modern clinical methods. Recent attempts have been made to increase the strength of the perceived signal by employing structured light with spatially varying polarization profiles. Here we review the advancements with the structured light stimuli and describe the current challenges and future prospects.

Keywords: Haidinger's brush; age-related macular degeneration; entoptic phenomena; macular pigment; structured light.

Figure 1

Examples of phase and polarization profiles of structured light stimuli (top) and the corresponding entoptic profiles that a participant with a healthy macula would observe (bottom). The clarity of the entoptic profiles is proportional to macular pigment density which typically peaks at the central point of vision and decreases with eccentricity. The first column depicts a horizontally polarized light stimulus and the Haidinger's brush. Introducing structured light techniques to prepare stimulus with polarization coupled orbital angular momentum (OAM) states allows us to induce a wide variety of entoptic patterns. The second column depicts the scenario where the stimulus with OAM = 2 is used to match the structure of the Henle fibers thereby inducing a monotone entoptic pattern. The third column depicts the use of higher OAM numbers to induce stronger stimulus with higher numbers of azimuthal fringes. The last column depicts the use of a radial state coupled to an OAM = 2 state that induces entoptic profiles with radially varying fringes.

Figure 2

The working principle of the studies with structured light stimuli. A spatial light modulator (SLM) creates an arbitrary polarization state with spatial resolution limited by its pixel size (modern values around ≈ 3 μm by 3 μm). Given the versatility of the SLM, one can introduce arbitrary obstructions, such as the depicted example which removes the central region in order to test the participant's peripheral vision. Optics components (not shown) are used to project the state from the location of the SLM to the participant's retina, thereby removing propagation effects. The size of the obstruction can be varied according to the participant's feedback, and a threshold value for eccentricity can be obtained through a standard staircase method.