title: Selective stimulation of penumbral cones reveals perception in the shadow of retinal blood vessels.
abstract: In 1819, Johann Purkinje described how a moving light source that displaces the shadow of the retinal blood vessels to adjacent cones can produce the entopic percept of a branching tree. Here, we describe a novel method for producing a similar percept. We used a device that mixes 56 narrowband primaries under computer control, in conjunction with the method of silent substitution, to present observers with a spectral modulation that selectively targeted penumbral cones in the shadow of the retinal blood vessels. Such a modulation elicits a clear Purkinje-tree percept. We show that the percept is specific to penumbral L and M cone stimulation and is not produced by selective penumbral S cone stimulation. The Purkinje-tree percept was strongest at 16 Hz and fell off at lower (8 Hz) and higher (32 Hz) temporal frequencies. Selective stimulation of open-field cones that are not in shadow, with penumbral cones silenced, also produced the percept, but it was not seen when penumbral and open-field cones were modulated together. This indicates the need for spatial contrast between penumbral and open-field cones to create the Purkinje-tree percept. Our observation provides a new means for studying the response of retinally stabilized images and demonstrates that penumbral cones can support spatial vision. Further, the result illustrates a way in which silent substitution techniques can fail to be silent. We show that inadvertent penumbral cone stimulation can accompany melanopsin-directed modulations that are designed only to silence open-field cones. This in turn can result in visual responses that might be mistaken as melanopsin-driven.
authors.affiliation: Department of Psychology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America.
authors.forename: Manuel
authors.initials: M
authors.lastname: Spitschan
authors.affiliation: Department of Neurology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America.
authors.forename: Geoffrey K
authors.initials: GK
authors.lastname: Aguirre
authors.affiliation: Department of Psychology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America.
authors.forename: David H
authors.initials: DH
authors.lastname: Brainard
mesh_terms: D000328:Adult; D003118:Color Perception; D006801:Humans; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D012171:Retinal Vessels / Q000033:anatomy & histology