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Larry Thibos, Norberto Lopez-Gil, Arthur Bradley, Renfeng Xu; Absolute cone thresholds for detecting black holes. Invest. Ophthalmol. Vis. Sci. 2018;59(9):4048.
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© ARVO (1962-2015); The Authors (2016-present)
Although important visual stimuli (e.g. printed text, opaque objects silhouetted against the sky) are darker than the background, classical psychophysics (e.g. Ricco’s law of spatial summation) has concentrated on detecting light stimuli. Our study was designed to determine the minimum background illuminance needed for foveal detection of the absence of light in small, high contrast, black targets as a function of target size.
With refractive errors corrected by trial lenses, three presbyopic subjects (age 69, 63 and 50 yr) viewed stimuli monocularly through a 5mm diameter artificial pupil. A red filter absorbing wavelengths < 610nm was used to exclude rod vision. The visual target was a black circular disk of variable diameter (2.5 to 20 arc minutes) centered on a uniformly-illuminated diffuse-reflector (2x2 degrees of visual angle) augmented with perifoveal fixation aids. The black disk was created by a hole in the reflector. Light passing through the hole was absorbed by a light trap with reflectance < 10-3. After dark adaptation, subjects adjusted illumination of the background reflector so the dark hole was just detectable. Five threshold measurements were taken for each of 7 target sizes. To assess the role of optical factors in setting visual threshold, wavefront aberrations were measured and optical point-spread functions and retinal images were calculated using Fourier optics theory.
For all three observers, absolute thresholds of retinal illuminance for detecting black targets varied inversely with target area for disks less than 10 arcmin in diameter. For targets larger than 10 arc minutes, threshold remained constant at about 0.3 Trolands, which was also the absolute threshold for detecting light spots that were 10 arcmin or larger in diameter. These results are consistent with Ricco’s law of spatial summation, with optical factors accounting for approximately half of the measured size of Ricco’s area at absolute threshold.
A black hole of a critical size (10 arcmin) or larger is detectable if background light surrounding the black hole is detectable. This result suggests that spatial summation and sensitivity to local luminance decrements is the same as for increments even though neural signals for darkness and lightness may be carried preferentially by OFF and ON visual pathways, respectively.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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