At and beyond the neural limits: visual psychophysics using an adaptive-optics visual stimulator

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The optics of all of our eyes are to some degree imperfect, even those of individuals with "20/20 vision". These imperfection produce small, often imperceptible, aberrations in the images that we see and limits both image quality and visual acuity.New technology first developed for astronomy, called "adaptive optics" or AO for short, can correct those aberrations and produce nearly perfect, aberration-free images in the eye. Most AO relies on the use of a flexible mirror that can be deformed in a controlled away. The optical aberrations present in the eye are measured and the inverse of those distortions are applied to the mirror, thus correcting the image quality and overcoming the optical aberrations that usually limit image quality in the eye.AO instruments can be used in two ways in visual science: either to look into the eye or to look out of it. Looking into the eye, instruments that image the retina at the back of the eye can be improved by AO to the extent that individual cells can be resolved. Looking out of the eye, the resolution of a visual image projected onto to retina can be increased by AO to beyond normal visual acuity, producing so called "super-vision". The instrument that we will design and build will be a user-friendly AO visual stimulator with which we can do both of these things. The device will include a digital light projector-normally used to project images in cinemas-that will project super high resolution images directly onto the retina. The primary purpose of the instrument is to investigate the underlying properties of the eye and brain beyond the normal limits imposed by the optics of the eye. Experiments will be conducted to test how well observer are able to see by asking them to make judgments about what they can see. Once the device is constructed, we will embark on a series of standard measurements of human visual performance. Rather than trying to measure the response to complex visual scenes, we will measure an observer's sensitivity to simple spatial patterns made up of periodic patterns of light and dark. These simple "sinusoidal" patterns are the building blocks from which we can predict the responses to any complex stimuli. To fully characterise the vast space of possible visual scenes, the simple sinusoidal patterns will be systematically varied from coarse to fine. For each pattern, we will determine how much contrast the observer needs to just see the pattern. From these measurements, we derive the spatial contrast sensitivity function (or spatial CSF) that defines the visual performance of the human observer. The importance of the AO system is that we can use it to measure spatial CSFs without the measurements being limited by the optics of the eye. Moreover, we can make those measurements for chromatic stimuli, achromatic stimuli and stimuli detected by single classes of light-sensitive detectors. By using different stimuli, we can investigate different neural pathways in the visual system.The AO stimulator will also allow us to investigate human vision beyond the usual neural limits of the system. Visual patterns produced by the AO stimulator that are too fine to be seen directly (and are therefore "invisible") can be seen indirectly due to interactions between pairs of patterns or due to changes in apparent intensity when a pattern is turned on and off. Using these patterns we will be able to investigate the underlying properties of the neurons in the visual system. Again, we can make measurements using chromatic stimuli, achromatic stimuli and stimuli detected by single classes of light-sensitive detectors. The indirect detection of these "invisible" patterns allows us to probe the inner working of the pathways answer specific questions about how the retina works.