Polarization perception in humans: on the origin of and relationship between Maxwell’s spot and Haidinger’s brushes

Abstract

Under specific conditions of illumination and polarization, differential absorption of light by macular pigments is perceived as the entoptic phenomena of Maxwell's spot (MS) or Haidinger's brushes (HB). To simulate MS and HB, an existing computational model of polarization-dependent properties of the human macula was extended by incorporating neuronal adaptation to stabilized retinal images. The model predicted that polarized light modifies the appearance of MS leading to the perception of a novel phenomenon. the model also predicted a correlation between the observed diameters of MS and HB. Predictions were tested psychophysically in human observers, whose measured differences in the diameters of each entoptic phenomenon generated with depolarized and linearly polarized light were consistent with the model simulations. These findings support a common origin of each phenomenon, and are relevant to the clinical use of polarization stimuli in detecting and monitoring human eye disorders, including macular degeneration. We conclude: (i) MS and HB both result from differential light absorption through a radial diattenuator, compatible with the arrangement of macular pigments in Henle fibres; (ii) the morphology of MS is dependent on the degree of linear polarization; (iii) perceptual differences between MS and HB result from different states of neural adaptation. The human macula is the retinal area anatomically and functionally optimised for high visual acuity. Centred on the visual axis of the eye, it derives its name (macula lutea = yellow spot) from its high concentration of plant-derived xanthophyll carotenoid pigments (i.e. macular pigments). Macular pigments are thought to protect against the damaging effects of high-energy visible wavelengths (380-500 nm) by acting as violet/blue light filters and free-radical scavengers 1-3. Additional vision-related functions include the reduction of chromatic aberration 4 and light scatter 5. It is generally thought that low concentrations of macular pigment predispose to eye disease, particularly age-related macular degeneration 6-9. The high concentrations of yellow pigments within the human central macula result in a reduction of up to 80% of high-energy visible light reaching the photoreceptor outer segments 10. Nonetheless, under normal viewing conditions, variations in pigment concentration across the macula do not manifest as perceptual differences in colour and/or luminance. This is because adaptive mechanisms function to negate the perception of unchanging retinal images (e.g. the Troxler effect) 11,12. Inhomogeneities in retinal anatomy and function become visible when adaptive mechanisms are disrupted, such as in the case of Maxwell's spot (MS) 13. This entoptic phenomenon results from a change in illumination of macular photoreceptors achieved, for example, by alternately viewing uniformly illuminated fields of visible light that are either absorbed (wavelengths 380-520 nm, blue) or transmitted (adapting wavelengths 520-700 nm, e.g. yellow) by macular pigment....

The Clinical Application of Polarization Pattern Perception

“…It is known that the macular structures underlying the mechanism of human polarization perception are radially symmetric. 2 , 16 Any link between polarization stimulus symmetry preference and the symmetry of macular architecture requires further investigation, particularly because macular disease such as age-related macular degeneration disrupts the macular architecture.…”

The Differential Contribution of Macular Pigments and Foveal Anatomy to the Perception of Maxwell’s Spot and Haidinger’s Brushes

“…Experiment 1 required measurement of the horizontal width of MS and HB, as detailed elsewhere [ 6 ]. In brief, the apparatus consisted of a diffused unpolarized light source of red (633 nm), green (519 nm) and blue (456 nm) light emitting diodes (LED), viewed through a Maxwellian system with a micrometer eyepiece.…”

“…Maxwell also noted similarities with the Haidinger’s brush (HB) entoptic phenomenon described several years previously [ 4 ]. The latter is seen transiently as a faint hour-glass-like pattern in central vision when viewing a uniform linearly polarized light field containing blue wavelengths [ 4 , 5 , 6 ].…”

“…The perceived size of MS in a small number of individuals was correlated with macular pigment distribution data derived from digital colour fundus images [ 13 ]. We recently proposed a computational model that links MS and HB as emerging from a common mechanism of differential absorption by radially symmetric deattenuating elements within the foveolar retina [ 6 ]: the principle absorbing component was assumed to be macular pigment molecules, with an overall orientation preference dictated by radial structural elements within the Henle fibre layer of the fovea.…”

“…The aim of this study was to investigate the factors influencing the appearance of HB and MS and their reported variations, with particular reference to macular pigment and foveal anatomy. Given a reliable method for quantifying both entoptic phenomena [ 6 ], together with accurate in vivo methods for determining macular pigment distribution (DWAF) and retinal structure (OCT), the factors giving rise to MS and HB can now be investigated in greater detail than previously possible.…”