Outer plexiform layer receptive fields as underlying factors of the Hermann grid illusion

The Hermann grid was first described and discussed by the physiologist Ludimar Hermann in 1870. It is composed of white horizontal and vertical bars on a black background [1]. Subjects perceive black or gray smudges at the intersections of white bars when looking at the grid. This effect was discussed by Baumgartner who proposed a theory related to ganglion cell receptive fields to explain the appearance of the smudges [2]. Since then, various versions of the Hermann grid have been experimented on which demonstrate that the theory breaks down due to inconstant effects [3]. However, Baumgartner´s original arguments do not involve outer-plexiform layer (OPL) receptive fields and how they affect the receptive fields at the ganglion cell layer. Hence, this paper will present a verification of the Hermann grid effect by using a model of the retina that includes both outer and inner plexiform layers and will address the question of the role of the OPL in generating the Hermann grid illusion. The paper also discusses how these findings may have a direct impact on the design of retinal prostheses.