Origin of the foveal granular pattern in entoptic viewing

To investigate the controversial origin of the foveal granular pattern at the center of the entoptic Purkinje vessel shadows. Both phenomena may be vividly elicited by oscillating a focused spot of light across the scleral surface of the eye in a circumferential direction.

The site and pattern of oscillation of the light spot were varied and were correlated with the appearance of the foveal granular pattern. Movement of the granular pattern relative to a foveal afterimage was also observed.

Oscillation of the light in a meridional direction abolishes the granular pattern. Oscillating illumination through the central pupil can elicit Purkinje vessel shadows but not the characteristic foveal granular pattern. With transscleral illumination, the granular pattern oscillates with an "against" motion with respect to the motion of the Purkinje vessel shadows and with a "with" motion with respect to apparent motion of an afterimage serving as a fixed anatomic reference, and it is displaced from the center of the foveal avascular zone in the visual direction away from the source of illumination.

These observations strongly suggest that the foveal granular pattern is a random moire pattern produced by spatial aliasing as the striated light pattern cast by the parafoveal nerve fiber elements sweeps over the photoreceptors in the form of a faint, high-spatial-frequency, irregular grating. An anatomic section of the fovea reveals the necessary geometry for production of such striated patterns, and the Nyquist frequency for the foveal photoreceptor mosaic supports the spatial aliasing effect. Also, a grating moving over a stationary random dot background demonstrates the origin of the "against" motion that is characteristic of the foveal granular pattern.