Rhodopsin density and visual threshold in retinitis pigmentosa.

It is sometimes helpful to investigate the etiology of lesions of the visual path centripetally, starting at the periphery. This ap­ proach enabled Carr and Ripps to show that Oguchi's disease cannot be due to a failure in visual pigment kinetics, and Carr and as­ sociateshave similarly established that le­ sions causing congenital nyctalopia are situ­ ated on the central side of the receptor outer limbs. These workers used f undus reflectometry, a technique derived from ophthalmoscopy in which the usual white light is re­ placed by various monochromatic lights, and a radiometric device is substituted for the practitioner's eye. The method is based on two properties of visual pigments, the colored light-absorbing materials contained in the outer limbs of the rods and cones. As the present report is only concerned with the rod pigment in man, namely rhodopsin, it will be unnecessary to refer to the cones. The first of the properties referred to is the strong preferential light absorption for blue-green light. Over 20% of such light incident on the human retina is absorbed by the rod pigment: other wave­ lengths are absorbed to a significantly smaller extent—e.g., red light is absorbed hardly at all. It follows that absorption by the pigment, and, therefore, also changes in, and variations from, the norm are gauged best with blue-green light, undiluted by any other wavelength. The second property rele­ vant to fundus reflectometry is that rhodop­ sin, when exposed to light, changes its mo­ lecular structure: its absorption of bluegreen light is reduced and the pigment is