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Nicole Putnam, Pavan Tiruveedhula, Austin Roorda; Measurement of stimulus contrast and fading under natural and manipulated motion trajectories. Invest. Ophthalmol. Vis. Sci. 2013;54(15):573.
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High-resolution imaging with Adaptive Optics Scanning Laser Ophthalmoscopy (AOSLO) combined with real-time image stabilization and targeted stimulus delivery allows simultaneous imaging and psychophysical testing. We utilize this capability to investigate the role of small fixational eye movements, or the lack thereof, on stimulus contrast and fading.
Image stabilization and targeted stimulus delivery with AOSLO facilitates the presentation of any stimulus motion including natural, stabilized, amplified, rotated, or independent trajectories. We first investigated the threshold for detection of a small grating that increased in contrast over the course of a 6 second trial at the fovea and 1 degree eccentricity. Subjects performed a 2 AFC judgment of grating orientation as soon as they were able to make a judgment. We recorded the contrast level at the time the decision was made under natural, stabilized and manipulated eye motion conditions. We then looked at the time-course of image fading for small stimuli near the fovea. Subjects fixated the corner of the imaging raster and a small 100% contrast grating appeared at a random location within the field. Subjects pressed one button when the stimulus was no longer visible and pressed another if the 9 second trial concluded with no image fading. We measured the time the stimulus was visible and the percentage of trials where the stimulus was visible for the entire trial for natural, stabilized, and manipulated eye motion conditions.
We find that the contrast required to detect a stimulus that was ramping up in contrast over a 6 second trial was the highest under stabilized conditions at both locations tested. The conditions under which thresholds were smallest, however, depended on the retinal location. At the fovea, the thresholds were at a minimum for natural stimulus motion whereas at 1 deg, the minimum threshold occurred with amplified motion (double the retinal slip). For the fading experiments, we found that the time required for an image to fade was shortest under stabilized conditions and that there were no instances where the stimulus did not fade in the 9-second trials for three subjects.
Our results suggest that the sensitivity of foveal cones is optimal for natural image motion caused by fixation. However, as close as 1 degree, contrast sensitivity is improved when the retinal image motion is amplified.
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