A method of determining the functional integrity of the visual system (including the photoreceptive elements, the retino-hypothalamic tracts and their connections to the suprachiasmatic nuclei of the hypothalamus) of visually imparied human beings. The method includes measuring the melatonin content in blood samples taken before, during and after application of a stimulus (e.g. light) to the subject's retinas, and comparing the measured melatonin content to determine if the melatonin content was suppressed during application of the stimulus.
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