Human evolution; how our eyes see light intensity (Introduction)

by David Turell , Monday, August 19, 2019, 00:24 (1922 days ago) @ David Turell

A toally new cell function is described:

https://www.sciencedaily.com/releases/2019/08/190818101642.htm

"Scientists are shedding new light on the importance of light-sensing cells in the retina that process visual information. The researchers isolated the functions of melanopsin cells and demonstrated their crucial role in the perception of visual environment. This ushers in a new understanding of the biology of the eye and how visual information is processed.

"The back of the human eye is lined with the retina, a layer of various types of cells, called photoreceptors, that respond to different amounts of light. The cells that process a lot of light are called cones and those that process lower levels of light are named rods.
Up until recently, researchers have thought that when light struck the retina, rods and cones were the only two kinds of cells that react. Recent discoveries have revealed an entirely new type of cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs contain melanopsin, a photopigment that is sensitive to light. While it has been established that ipRGCs are involved in keeping the brain's internal clock in sync with changes in daylight, their importance in the detection of the amount of light had not yet been well understood.

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"In the study, the authors showed how cones and melanopsin combine to allow the perception of brightness. In order to better assess the contribution of melanopsin to the detection of light, the melanopsin's signals were isolated from cones and rods. This separation allowed for more accurate observation of the melanopsin signal alone. Visual stimuli were carefully designed and positioned in order to specifically stimulate the light-sensitive chemical. Also, the researchers used tracking software to measure study participants' pupil diameters under each visual stimulus. This served as a way to determine the relationship between brightness perception and the actual visual stimulus intensity on the retina.

"The researchers were able to show that the varying brightness levels of an image that was perceived is a sum of the melanopsin response and the response that is generated by the cones. The former is a linear readout and the latter is not. The results also show that melanopsin is not a minor contributor in brightness perception. Rather, it is a crucial player in brightness perception."

Comment: The retina is shown to be even more complex in this newly recognized cell with its very special light sensing protein. Not by chance.