Light exerts influences on many physiological and behavioural functions in humans. These
functions can be described as image-forming (IF) and non-image forming (NIF) visual processes, both
originating in the retina of the eye. Image-forming refers to vision; the process of detecting and distinguishing
shapes and colour of objects. Non-image forming refers to detecting level of light intensity or
brightness of ambient space, which affects basal physiology such as cycles of rest and activity or the
endocrine system. Rod and cone photoreceptors in the outer retinal layer are most important for imageforming
vision, while non-image forming functions depend upon additional input from the photopigment
melanopsin, which is expressed in retinal ganglion cells (RGC) that makes these cells photosensitive
(pRGC). Projections of these pRGCs convey light-induced electrical impulses to a number of brain
regions. Visual acuity and colour contrast naturally diminishes with age but dementia often has major
effects on the visual processing systems, which impact on the quality of life. The ability of humans to
manipulate their light exposure has the immediate potential to either create problems with human physiology
(as in shift workers) or to compensate physiological disadvantages (of IF and NIF visual impairment).
This mini-review describes the impact of aging on the function of the eye with respect to nonimage
forming effects of light, summarises light intervention studies for sleep and neuropsychiatric
symptoms and considers implications from photoreceptor-weighted light intensities for biologically effective
light intervention and lighting solutions for patients with dementia.
Keywords: Sleep, circadian, melanopsin, retina, visual impairment, photoreception, Alzheimer, cognitive impairment.
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