Spatial summation in the dark-adapted fovea
Koenig, Darren 1970-
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Purpose: Application of adaptive optics to vision science creates the potential to probe the link between the retinal mosaic and visual perception. However, cellular level interrogation requires small, threshold stimuli and an implicit detection model. Unfortunately the parameters governing detection at cone threshold are not well known. For insight into detection and perception at the cellular level we investigated the absolute detection threshold, and the extent and strategy of spatial summation in the dark-adapted fovea. Appearance reports were also monitored to investigate the relationship between detection and perception. Methods: 1) Absolute thresholds for small (~1' full width at half maximum – FWHM, and 27' diameter), brief (34 ms), monochromatic (550 and 490 nm) spot stimuli were measured and compared for cone (fovea) and rod (11° eccentricity) vision using rating scale and 2-alternative, forced-choice paradigms. 2) To investigate the potential for combined measures, thresholds and color appearance were measured in isolation or simultaneously for brief, foveal, monochromatic (580 nm) point stimuli. 3) To determine if the detection strategy is optimally tailored to match stimulus size, thresholds and color appearance were measured with different size monochromatic (550 nm) foveal stimuli presented either in blocks or randomly intermixed. 4) To constrain the spatial summation extent and strategy, and determine the impact of foveal mosaic granularity on detection, thresholds and color appearance as a function of stimulus size were measured with adaptive optics corrected stimuli. Results: 1) Cone thresholds with lenient detection criteria were on average 203 ± 38 photons at the cornea, and ~0.47 log units lower than previously reported. Unlike rod thresholds, cone thresholds were less responsive to lenient detection criteria and were therefore not solely limited by Poisson noise, but rather also limited by uncertainty. 2) Detection was not significantly impacted by color reports, and color reports were not significantly different whether or not detection certainty was also reported. Thresholds were actually lower for most subjects when simultaneously reporting detection certainty and appearance, suggesting color appearance information is informative to stimulus presence at threshold. 3) Thresholds and color reports were not significantly different between the blocked and intermixed conditions, suggesting a similar detection strategy was used whether or not a priori stimulus size information was available. Comparison of psychophysical results to detection model results suggested two possible detection strategies: complete spatial summation within ~8-14' diameter fixed-sized units or independent combination across multiple-sized units. The latter was more consistent with appearance reports for small stimuli. 4) Results with adaptive optics stimuli revealed cone mosaic granularity did not negatively impact detection of stimuli at least one cone diameter FWHM. Comparison of area-threshold behavior to detection model results suggested detection is mediated by multiple-sized units up to 4-10ꞌ in diameter rather than a single size unit. Conclusions: Comparison of independent and simultaneous measurements of color appearance and detection certainty suggest color appearance is informative to stimulus presence at threshold, and color and detection can be simultaneously measured without either being negatively impacted. Thus models developed to describe appearance can incorporate typical cone detection parameters. However, unlike rod thresholds, limited cone threshold response to lenient detection criteria suggests cone thresholds are not limited solely by Poisson noise and cannot be described or modeled as such. Appearance and detection results as a function of stimulus size indicate subjects do not use optimal, stimulus-matching spatial summation at detection threshold; nor are they limited to using independent cones, as is often assumed. Investigations with adaptive optics stimuli indicate that in the central fovea the retinal mosaic granularity does not negatively impact detection, and that detection is mediated by multiple-sized summation units up to at least ~4-10' in diameter. A minimum summation extent of ~2-3 cones cannot be ruled out given the relative stimulus sizes used.