The long term objectives of this study are twofold: to complete our identification of the carotenoids that comprise the human macular pigment and the turtle cone oil droplet pigments, and to increase our understanding of temporal processing in the human visual system. The macular pigment reportedly provides protection of retinal tissue against light induced lesions as well as age related loss of blue cones. Its absolute identity, however, awaits characterization by electron impact mass spectrometry, as planned for this study. To accomplish this, relatively large amounts of pigment will be purified by solid phase extraction, size exclusion chromatography and reversed-phase HPLC. A project related to the pigment's polarizing properties - a study of corneal birefringence - will also be undertaken. The turtle cone pigments, which are an integral part of the animal's color vision system, are possibly zeaxanthin and fatty acid esters of astaxanthin. For the identification of the latter we will attempt to deesterify the compounds and characterize the products by HPLC, UV-vis spectrometry and mass spectrometry. Temporal processing in the long, middle and short wavelength color mechanisms will be investigated psychophysically by an adaptation of the increment threshold technique. This will be used to determine the gains and phase shifts of the mechanisms, as functions of input frequency and amplitude of sinusoidal signal. A probe, in the form of a brief flash, will be superimposed at the center of a bright, sinusoidally modulated background field and adjusted in intensity until it is only just visible. The probe will be superimposed at different, precisely controlled instants during this modulation. Any change in the threshold intensity of the probe muse be caused by a change in the sensitivity of the system in response to the input modulation. These measurements will provide directly an output waveform. Isolation of a particular color mechanism will be accomplished with an appropriate choice of wavelength for the modulated field and probe, and with the inclusion of a superimposed steady field of suitable spectral composition to selectively depress the sensitivity of the other 2 mechanisms. Information gained from this study will be an important addition to that gained by other techniques such as flicker fusion or impulse response. The phase response of the color mechanisms will be of particular interest since no other known technique can provide this information directly. In relation to health, there are a number of studies which indicate that temporal processing is affected by age. This is not surprising in view of the known age related loss of certain retinal cells, which are almost certainly implicated in the temporal processing system.
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