Size of the foveal blue scotoma related to the shape of the foveal pit but not to macular pigment

• A high inter-individual variability was found for the foveal blue scotoma.
• The foveal blue scotoma is related to the geometry of the fovea.
• The steeper the foveal pit, the larger the foveal blue scotoma.
• Maxwell’s spot and macular pigment are unrelated to the foveal blue scotoma.

When the eye is covered with a filter that transmits light below 480 nm and a blue field is observed on a computer screen that is modulated in brightness at about 1 Hz, the fovea is perceived as small irregular dark spot. It was proposed that the “foveal blue scotoma” results from the lack of S-cones in the foveal center. The foveal blue scotoma is highly variable among subjects. Possible factors responsible for the variability include differences in S-cone distribution, in foveal shape, and in macular pigment distribution. Nine young adult subjects were instructed to draw their foveal blue scotomas on a clear foil that was attached in front of the computer screen. The geometry of their foveal pit was measured in OCT images in two dimensions. Macular pigment distribution was measured in fundus camera images. Finally, blue scotomas were compared with Maxwell’s spot which was visualized with a dichroic filter and is commonly assumed to reflect the macular pigment distribution. The diameters of the foveal blue scotomas varied from 15.8 to 76.4 arcmin in the right eyes and 15.5 to 84.7 arcmin in the left and were highly correlated in both eyes. It was found that the steeper the foveal slopes and the narrower the foveal pit, the larger the foveal blue scotoma. There was no correlation between foveal blue scotoma and macular pigment distribution or Maxwell’s spot. The results are therefore in line with the assumption that the foveal blue scotoma is a consequence of the lack of S-cones in the foveal center. Unlike the foveal blue scotoma, Maxwell’s spot is based on macular pigment as previously proposed.