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Sangeetha Metlapally, Jianliang Tong, Humza Tahir, Clifton M Schor; Evidence that Accommodative Microfluctuations provide Odd-Error Cues to Accommodation and Insights into the Potential Spatial Cues. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3772. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The goal was to revisit an important, yet unproven theory that accommodative microfluctuations may provide a mechanism to extract a directional signal for accommodation, and investigate some spatial cues that might be used by the system.
We presented step changes in optical vergence randomized in direction and magnitude while subjects (n=3) looked at a polychromatic Maltese spoke stimulus through a Badal system monocularly in a Maxwellian view system. An artificial 4 mm aperture was imaged at the entrance pupil of the eye. The stimulus was static or varying smoothly in image quality at a combined compound frequency to add uncorrelated external noise to natural microfluctuations as follows. In one condition, the flickering stimulus simulated defocus fluctuations (0.5 D at 0.5 Hz; 0.25 D at 2 Hz), and in the other, contrast fluctuations (+0.15 at 0.5 Hz, -0.15 at 2 Hz; mean contrast level of 0.7). Wavefront aberrations were collected using a custom Shack-Hartmann aberrometer. Changes in the defocus term with step changes in optical vergence were analyzed. Percentage correct responses for 1, 2 and 3 D stimuli were analyzed using set criteria for latency, velocity and persistence of the response. D-prime analysis was carried out to assess the strength of the stimulus at eliciting the correct response under each of the conditions.
Stimuli that were varied to simulate uncorrelated defocus fluctuations significantly impaired the ability to extract odd-error information from the reduction seen in percent correct responses compared to the static stimulus. Stimuli fluctuating only in contrast did not affect the responses, suggesting that they did not affect detectability of the correct direction in which to accommodate.
Since uncorrelated defocus fluctuations affect the detectability of the correct direction for accommodation, but not contrast fluctuations, it is possible that that the spatial cue is the variation in contrast at certain mid-high spatial frequencies. Also critical for a sharp percept is phase alignment, which is distorted by defocus fluctuations and irremediable, misleading the system in this condition. The contrast gradient is highest at the edge of the stimulus and the background and so it is here that the system may compute the variations in contrast at low vs high spatial frequencies, where the signal is largest.
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