All ETDs from UAB

Advisory Committee Chair

Lawrence C Sincich

Advisory Committee Members

Christine A Curcio

Paul D Gamlin

Timothy W Kraft

Yuhua Zhang

Document Type

Dissertation

Date of Award

2016

Degree Name by School

Doctor of Philosophy (PhD) School of Optometry

Abstract

To perceive a spot of light at photopic threshold, a minimum number of photons must be absorbed by cone photoreceptors. While many factors affecting psychophysical threshold have been studied, few have been examined at the single-cone level. Here we reveal how variation in synaptic weighting and signal integration among cones in the human retina occurs in vivo. Using an adaptive optics scanning laser ophthalmoscope (AOSLO) equipped with stimulation capabilities, we targeted cone-sized stimuli to single cones. Perceptual increment thresholds were measured from individual cones and cone pairs in 7 subjects using a Bayesian-based staircase method of threshold estimation. In 42 of 99 cone pairs, thresholds differed between cones (p < 0.05), and differences in threshold as small as 14% were detectable. To determine if the observed variability was real, we examined several factors that could lead to threshold differences. We found that thresholds were consistent over multiple days, that cone reflectivity in AOSLO images was unrelated to threshold (n = 494 tests; p = 0.19), and that stimulus delivery errors were not a cause (average error = 0.2 arcmin; R2 = 8 x— 10-5). To investigate how cone signals combine, individual cones in pairs were stimulated simultaneously and analyzed with respect to their signal summation. All pairs exhibited summation, with 17/99 pairs manifesting linear summation, and 42/99 integrating signals according to a two-detector model. The nature of signal integration was not correlated with eccentricity, but was related to inter-cone distance (most linear pairs were located within 1.5 cone spacings of one another), suggesting bipolar cells as the site of cone signal summation. Using longitudinal imaging, we found that some cones were persistently poorly reflective. To test if these were functional, 10 dark cones from 5 subjects were targeted for threshold testing. All 10 dark cones had thresholds comparable with those from normally reflective cones measured concurrently (p = 0.49), indicating that low cone reflectivity is not a reliable indicator of cone sensitivity in normal retinas. These results should be considered in the clinical setting, where microperimetry and AOSLO imaging are used to assess visual function in patients with retinal disease.

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