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Visual Psychophysics / Physiological Optics - ARVO

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<strong>ARVO</strong> 2013 Annual Meeting Abstracts by Scientific Section/Group – <strong>Visual</strong> <strong>Psychophysics</strong> / <strong>Physiological</strong> <strong>Optics</strong>Purpose: The luminance flicker pathway is typically conceived of asa simple additive combination of M- and L-cone signals. As wereported before (<strong>ARVO</strong>, 2001), this simple picture is complicated bythe existence of multiple cone signals with different delays and signs,which can be revealed by most conditions of chromatic adaptation.Here, we extend these measurements and model the results in termsof an inhibitory surround network.Methods: Cone delays were measured by a flicker photometriccancellation technique. Human subjects varied the relative delaybetween two superimposed flickering lights (and their relativemodulation) in order to minimize or null the perception of flicker.One flickering light was either an M-cone or an L-cone isolatingstimulus, and the other was a “standard” light that was of the samewavelength as the background. Measurements were made as afunction of temporal frequency from 2.5 to about 25 Hz on a series ofadapting backgrounds with wavelengths between 410 and 658 nmand at several background intensities.Results: Under most conditions, sizeable delays or advances by asmuch as 33 ms have to be introduced between the M- or L-coneflickering light and the standard in order to null the perception offlicker, particularly when M-cone flicker is used. In general, theresults are consistent with the L- and M-cone signals being made upof faster components combined with delayed ones of the same oropposite sign. For M-cone signals, the delayed components can belarger than the fast, so that they predominate.Conclusions: These results challenge the idea that the luminancechannel is made up of only additive M- and L-cone inputs. However,these complexities may simply reflect the fact that the inhibitorysurround of each cone has both L- and M-cone inputs, and that thereare typically more L-cone inputs than M. We model the surround as anetwork of direct and indirect inhibitory lateral connections acrosswhich each step in the network inverts and delays the signal. Thus,the effective sign and overall delay of the slow signal depend uponthe relative strengths of the inhibitory feedback at each step and uponthe number of steps.Commercial Relationships: Andrew Stockman, NoneSupport: BBSRC410 Spatial Vision, <strong>Visual</strong> <strong>Psychophysics</strong> and Aging IIWednesday, May 08, 2013 8:30 AM-10:15 AMTCC LL 4/5 Paper SessionProgram #/Board # Range: 4063-4068Organizing Section: <strong>Visual</strong> <strong>Psychophysics</strong> / <strong>Physiological</strong> <strong>Optics</strong>Program Number: 4063Presentation Time: 8:30 AM - 8:45 AMTwo-point detection and appearance in the absence of higherorder aberrationsHeidi J. Hofer, Darren E. Koenig. College of Optometry, Universityof Houston, Houston, TX.Purpose: We investigated the impact of the cone mosaic onspatiochromatic reconstruction of two-point stimuli by measuringdetection and appearance with stimuli made smaller than retinalcones with adaptive optics. We sought to determine whetherdetection and perception of these points are independent near thefovea, as has been previously hypothesized, and if not the extent ofsummation for detection and color. We also investigated whethertwo-point resolution is consistent with a photoreceptor limit underthese conditions.Methods: We performed two experiments in which subjects viewed550nm two-point stimuli through a 6mm pupil with adaptive opticsaberration correction. Stimuli were viewed at ~1 degree eccentricityunder a rod bleaching protocol to isolate cone responses. In oneexperiment two subjects were shown either one or two spots (25 ms,0.5 arcmin estimated retinal full-width at half maximum (fwhm)) ofdifferent intensities and reported the number seen and colorappearance. Separation was varied from 1.5 to 15 arcmin. In thesecond experiment two subjects were shown either one or two dim,yet suprathrehsold, spots (15 ms, 0.5 and 1.1 arcmin estimated fwhm)and reported whether they believed one or two spots were present.Separation was varied from 0.5 to 12 arcmin.Results: Detection thresholds in the first experiment were consistentwith complete spatial summation up to 3-4 arcmin. Detection, but notcolor appearance, was independent for separations of 8 arcmin ormore and consistent with at least 15 arcmin of spatial uncertainty. Fortwo-point stimuli color appearance was highly variable, as for singlepoints, but also highly coupled. Both points were reported to have thesame color appearance on over 97% of trials, regardless ofseparation. Resolution thresholds were similar in both experiments,2.5-5 arcmin, with a slight tendency towards lower thresholds withthe larger spots.Conclusions: Cone detection at 1 degree is consistent with completespatial summation over an extent including at least a cone and itsnearest neighbors, creating challenges in recovering ‘single cone’sensations from the responses to tiny stimuli. Despite this extent,two-point resolution is roughly consistent with expectations based ona photoreceptor limit. The large variation, but lack of independence,in the color appearance of the two-point stimuli, implies nonlocalityin the color reconstruction mechanisms.Commercial Relationships: Heidi J. Hofer, None; Darren E.Koenig, NoneSupport: ROI EY019069, P30 EY07551Program Number: 4064Presentation Time: 8:45 AM - 9:00 AMA Comparison of Acuity and Cone Density in the TemporalRetinaNancy J. Coletta 1 , Toco Y. Chui 2 , Ann E. Elsner 2 . 1 Vision Science,New England College of Optometry, Boston, MA; 2 School ofOptometry, Indiana University, Bloomington, IN.Purpose: Cone spacing has long been thought to set a limit on visualacuity. While parafoveal acuity and cone density decrease at parallelrates with increasing myopia (Coletta and Watson, Vis Res, 2006;Chui et al., IOVS, 2008), parafoveal acuity has been reported toexceed the limit estimated from histological measures of conespacing (Williams and Coletta, JOSA A, 1987). Recent advances inretinal imaging allow a comparison of acuity and cone spacing in thesame eyes. We compared parafoveal acuity to cone density in severalyoung adult individuals to determine whether acuity is matched to thelimit estimated from cone density.Methods: Measurements were made on five subjects with an averageage of 28 years +/- 4.30 s.d., average spherical equivalent refractiveerror of -1.93 D +/- 3.32 s.d. and average axial length of 24.48 mm+/- 1.38 s.d. (IOL Master, Zeiss Meditec). Acuity was measured onthe temporal retinal meridian using 543 nm laser interference fringesat a retinal illumination of 100 td; fringes were displayed in a 1 degpatch at 2 deg and a 2 deg patch at 7 deg eccentricity. Acuity wasestimated at the 75% correct level for a 2AFC vertical/horizontaldiscrimination task. Cone densities in the temporal retinal meridianwere assessed using a second generation Adaptive <strong>Optics</strong> ScanningLaser Ophthalmoscope. Densities were measured at a nominal 630and 2070 microns from the foveal center with custom software(Matlab, Mathworks). Axial lengths were used to correct the conesampling positions and retinal area, as well as to convert conedensities to Nyquist frequencies (NF) in units of c/deg.©2013, Copyright by the Association for Research in Vision and Ophthalmology, Inc., all rights reserved. Go to iovs.org to access the version of record. For permissionto reproduce any abstract, contact the <strong>ARVO</strong> Office at arvo@arvo.org.

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