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F.H. Zaidi, R.G. Foster, S.N. Peirson, K. Wulff, K. Gregory–Evans, M.J. Moseley; Visual Perception in a Profoundly ‘Blind’ Human. Are the Image–Forming and Non–Image–Forming Photoreceptor Systems Functionally Distinct? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5710.
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It has been asserted that the image and non–image forming components of the primate visual system are not functionally distinct and that the latter may contribute to aspects of visual perception (Dacey et al., Nature 2005;433:749–54). To examine this notion we studied a subject who appeared to possess intact retino–pretectal (RPT) and retino–hypothalamic (RHT) projections but lacked functional rod and cone photoreceptors.
SC is an 85 year–old woman with long–standing (50+ years) cone–rod dystrophy who reports a complete inability to perceive light. OCT and photopic and scotopic electroretinograms are consistent with a total lack of functional rods or cones. Nonetheless, the subject appears to have a normally entrained sleep–wake cycle as monitored objectively by wrist actigraphy – an observation suggestive of an intact RHT and the presence of intrinsically photosensitive retinal ganglion cells. We recorded pupillary responses to light as a function of wavelength (n=8, 420–580nm) and intensity and accordingly determined irradiance–response relations and action spectroscopy. The subject also undertook a two–alternative forced choice (2AFC) psychophysical procedure to quantify their clinical and self–reported blindness using spectrally identical stimuli to those presented during pupillometry.
Despite lacking rod and cone function, the subject demonstrated spectrally tuned pupillary responses to monochromatic visual stimuli with a maximum sensitivity at 478nm (R2=0.854). Although the subject is unable to perceive light from sources in her everyday environment, in the 2AFC procedure she was able to report, at a level significantly above chance (Sign test, p=0.05), the presence of a 481nm light stimulus. No other wavelength tested was able to elicit above chance detection.
In a patient in whom all anatomical, functional and electrophysiological findings suggest a total lack of image–forming (i.e. rod and cone) photoreceptors, we were able to elicit a spectrally tuned pupillary response with a peak sensitivity close to that which a psychophysical procedure indicated that the patient could detect. We posit that this remarkable correlate between the pupillary and perceptual and systems in a nominally blind individual provides evidence that the non–image forming component of the human visual system is not functionally distinct from the image–forming component.
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