Contrast sensitivity and functional aspects of cone, rod, and melanopsin vision in humans

The sensitivity of the visual system is best defined by its response to contrast. To this end, the visual system can be decomposed into pathways with unique contrast responses to the spatial and temporal content of the stimulus. In this chapter, we evaluate the contrast sensitivity of the individual mechanisms in the eye through the lens of silent substitution - a technique that exploits a high degree of spectral control to precisely set the stimulus contrast presented to each of the five photoreceptor classes. With the discovery of a fifth (melanopsin) photoreceptor in the human eye, new methodologies have been developed to independently probe the contribution of each of the photoreceptor classes to the functional aspects of vision. We consider how the seminal experiments - which were often conducted using fewer than five spectrally independent primaries - could introduce uncontrolled splatter contrast that may be detected by the nominally silenced photoreceptors and thereby confound the interpretation of the contrast response function. This chapter studies the contrast sensitivity of the cone photoreceptors through the canonical, cone-mediated visual pathways. In these cone-mediated pathways, achromatic vision is signaled in the magnocellular (MC) pathway, with a weaker achromatic signal carried by the parvocellular (PC) pathways, and chromatic vision is separated into a red-green mechanism mediated via the PC pathway and a blue-yellow mechanism via the koniocellular (KC) pathway. Cortical mechanisms integrate and analyze these retinogeniculate signals to form our visual percepts. We discuss how silent substitution can be used to reveal rod- and melanopsin-mediated vision from beneath the more sensitive cone-mediated contrast responses. This measurement of the photoreceptor-directed visual contrast sensitivity is important for detecting and monitoring the progression of ophthalmic and neurodegenerative disease, and we provide an overview of how these diseases change each visual pathway's contrast sensitivity.