We examined the contributions of rh odopsin and melanopsin to human brightness estimation under dim lighting. Absolute brightness magnitudes were estimated for full-field, rhodopsin-, or melanopsin-equated narrowband lights (lambda(max) : 462, 499, 525 nm). Our data show that in scotopic illumination (-5.1 to -3.9 log mu Watts.cm(-2)), the perceptual brightness estimates of rhodopic irradiance-equated conditions are independent of their corresponding melanopic irradiance, whereas brightness estimates with melanopic irradiance-equated conditions increase with increasing rhodopic irradiance. In mesopic illumination (-3.4 to -1.9 log mu Watts.cm(-2)), the brightness estimates with both lighting conditions increase with increasing rhodopic or melanopic irradiances. Rhodopsin activation therefore entirely signals scotopic brightness perception and plateaus in mesopic illumination where intrinsic melanopsin contributions become first evident. We infer that all photoreceptor signals are transmitted to higher visual centers for representing scene brightness in scotopic and mesopic illumination through both conventional and melanopsin ganglion cell pathways. (C) 2020 Optical Society of America
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