UC Berkeley

The human visual system is constrained by its physiological implementation. The imperfect optics of the cornea and lens blur the light arriving at the retina. The inherent spectral sensitivity functions of the retina's three cone photoreceptor types give rise to the allowable gamut of colors that humans can perceive. The mosaic-like tiling arrangement of those cone cells in the retina dictates the spatial sampling characteristics of the eye, limiting the resolution with which one can perceive fine spatial detail. Even under fixation, the eye's constant motion causes the retinal image to shift continuously across the photoreceptor mosaic. Together, these consequences of human biology impose constraints on visual perception under natural viewing conditions. However, through recent advances in retinal imaging and stimulation technology, we bypass these constraints through unnaturally precise spatial control of light at the retinal interface, allowing for manipulation of the inputs to the visual system at the level of individual photoreceptors. By gaining control of cone activations at the cellular scale, we push past the spectral and spatial boundaries of the human visual system. This work explores color and spatial vision beyond the physiological restrictions of typical viewing conditions. Chapter 1 provides background on those restrictions and the optical instrument used to make such experiments possible through simultaneous imaging, eye-tracking, and stimulation. Each subsequent chapter outlines the application of this technology to perform psychophysical experiments which probe the human color and spatial vision system outside of its typical operating conditions. Chapter 2 studies color perception of wavelengths outside the visible spectrum by eliciting 2-photon absorption of infrared light in photoreceptor cells. Chapter 3 shows humans a color with a saturation beyond the boundary of the human color gamut using targeted stimulation of individual cone photoreceptors. Chapter 4 tests the spatial limits to vision, imposing emulated photoreceptor loss on the retina and finding where visual acuity breaks down as a result. These experiments offer insight into the way color and spatial vision arise from the signals delivered to the retina, by enacting control over the input while simultaneously measuring the resulting sensory output.