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Abstract:
The retinal vasculature lies between the cornea and the retina and consists of a fine network of blood vessels which filter the light impinging on the retina, and cast shadows on the retinal mosaic. The light seen by cones in the partial shadow of the blood vessels is filtered according to the transmittance properties of hemoglobin. The entoptic visualization of the retinal blood vessels has been known since at least J. E. Purkinje (1823) and is typically evoked using a moving light source which shifts the shadow of the blood vessels to cones adjacent to the penumbral and umbral cones, thus rendering the so-called Purkinje tree visible. Here, we used the method of silent substitution using a multi-primary (56 primaries) spectral illumination system (OneLight Spectra Digital Light Engine) to produce a spectral modulation which silenced L, M, S cones and melanopsin under photopic conditions and selectively stimulated penumbral L and M cones. This modulation elicits a strong and vivid percept of one's own Purkinje tree. We find that this modulation produces BOLD fMRI responses in primary visual cortex, and that it elicits a considerably stronger amplitude than full-field L and M cone modulation matched in contrast. The V1 response peaks at a modulation frequency of 16 Hz and is presumably attributable to the pronounced spatial structure in the observed percept. We note that inadvertent penumbral cone stimulation could confound silent substitution paradigms directed towards other photoreceptor classes. Support: NIH R01 EY020516, R01 EY10016, P30 EY001583; Deutscher Akademischer Austauschdienst. Disclosure: A patent on the alternative photoreceptor isolation technique and its applications has been filed by the University of Pennsylvania with G.K.A., D.H.B., and M.S. as inventors.
