Melanopsin Vision: Sensation and Perception Through Intrinsically Photosensitive Retinal Ganglion Cells
Intrinsically photosensitive retinal ganglion cells (ipRGGs) are the most recently discovered photoreceptor class in the human retina. This Element integrates new knowledge and perspectives from visual neuroscience, psychology, sleep science and architecture to discuss how melanopsin-mediated ipRGC functions can be measured and their circuits manipulated. It reveals contemporary and emerging lighting technologies as powerful tools to set mind, brain and behaviour.
Life has been evolving on this planet for some 3.5 billion years. For a good portion of that time (depending, for example, on atmospheric conditions), life has been exposed to the regular and alternating pattern of light and dark caused by the Earth’s 24-hour rotation on its axis as it orbits the sun. It is perhaps unsurprising then that light is one of the most powerful drivers of behaviour – light influences the way that we think, feel, and act.
The study of these effects of light has a long and rich history that is rooted in medicine. The ancient Greek physician Hippocrates built a solarium and prescribed sunbaths to manage a variety of disorders. The Roman scholar Aulus Cornelius Celsus recommended that sufferers of sickness or melancholy (depression) live in light-filled houses, especially in winter. More recently, Florence Nightingale argued that ‘Where there is sun, there is thought’, and that hospital wards should be brightly lit, ideally by sunlight. Contemporary medicine now recommends light exposure as a first-line treatment against both seasonal and non-seasonal depressions.
Our understanding of the detection of light is often discussed in relation to an aspect of perception known as ‘image-forming’ vision mediated via the rods and three cone photoreceptor classes and their classical post-receptoral pathways. Image-forming vision includes the sensory and perceptual aspects of visual experience such as colour, form, or motion, usually discussed in the context of the neurotypical individual. However, lighting also drives diverse aspects of the human experience through setting physiology, arousal, cognition, and mood; responses that are classified as ‘non-image-forming’. While these non-image-forming pathways can drive conscious awareness, many of these responses occur over timescales that are much longer than the momentary changes to which our visual perceptual awareness is tuned. This requires a mechanism with a fundamentally distinct temporal tuning to that of the classical visual pathways.
The modern study of non-image-forming vision is grounded in the scientific method and draws strongly from the fields of neuroscience, sleep and circadian sciences, and experimental and applied psychology. Its study has undergone a recent renaissance, where modern psychophysical and neuroscience methods have converged to identify the specialized visual circuits that serve non-image-forming vision and that originate in the retina of the eye. This fifth human photoreceptor class is located in the inner retina and termed the intrinsically photosensitive retinal ganglion cells (ipRGCs). Phototransduction initiated by the intrinsic melanopsin photopigment expressed by ipRGCs was initially shown to have a unique, characteristic temporal response: a slow onset followed by a sustained depolarization that is maintained even after the stimulating light is switched off. In addition to their unique intrinsic photoresponse, ipRGCs extrinsically mediate signals originating in outer retinal rod and cone photoreceptors. The ipRGCs therefore possess temporal characteristics suited to sensing both transient changes in light but also day-length changes.
These non-image-forming pathways project to over a dozen diverse efferent brain targets, and in this Element we evaluate the current state of knowledge for these functional melanopsin pathways that set pupil size, perceptual vision, circadian rhythms and sleep/wake transitions, and arousal, mood, and cognition. We focus on delineating findings in primates (including humans) from those of other model organisms. Indeed, these non-image-forming signals appear fundamentally entwined with the human condition and we discuss lightscapes that not only serve image-forming vision, but that target non-image-forming physiology to positively modify health and behaviour. Physiologically targeted electric light sources have future applications as ‘photoceuticals’, with therapeutic effects analogous to those of pharmaceuticals and designed with similar considerations concerning disease specificity, dosage, and timing. Given the new developments in the understanding of ipRGCs and their image-forming and non-image-forming projections, we provide a contemporary account of the importance of light and melanopsin function for brain, mind, and behaviour.
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https://www.cambridge.org/core/elements/melanopsin-vision/25D01A0152E76F068B54E0CE351C62B8#
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