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Elie J. de Lestrange, Chris Dainty; Phase Signature In The Spatio-temporal Dynamics Of Human Faces Identification. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3605.
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Visual perception is a complex process that comprises both spatial and temporal components . This work investigates visual spatio-temporal mechanisms/dynamics, and how they can modulate the appearance of optically degraded images.
A custom-built adaptive optics visual simulator was employed to control the ocular aberration. Two young subjects were asked to look at a near diffraction-limited target, over a fixed 5mm pupil diameter. The target was a representation of a human face of size 30 arc minutes at the retina, presented on a monochrome microdisplay. This type of stimulus is known to invoke a spatio-temporal response from the visual system  that involves specialized processes and pathways in the brain. Time-varying optical aberrations were computer-generated using Fourier techniques. The subject’s task was to equate the appearance of two consecutively displayed side-by-side images, and characterized by two distinct exposure durations: one the reference (an almost static image of about 1s duration); and one the test image flashed at varying exposure (either 0.1s or 0.2s exposure duration).In the first test, both the target and the reference were administered the same filter characteristic: either a simple defocus blur (of a quarter wavelength), or the subject’s own aberration. The reference filter was scaled in the frequency domain by varying the size of increments in steps of 20nm of rms wavefront error.In an adaptive staircase forced-two-choice task, the subject selected the sharpest stimuli and adjusted the scaling of the spatial filter as to minimize the just-perceivable difference between images.In order to investigate phase dependency of the spatial shift of perceived frequency, the static stimulus of the natural wavefront aberration of the subject was compared to four of its rotational transformations. The change of distribution of separable images yields an estimation of the spatial frequency dynamic for varying time.
We found a systematic shift of the perceived frequency under the simple blur matching conditions. The equality point between images was obtained at an increment of about /10 and /20 of rms wavefront error (at the exposure duration 0.1s and 0.2s respectively) of the static target of reference, for both subjects. This would correspond to a reduction in the apparent frequency of the static stimulus at 0.1s by a factor of 0.7 and 0.8.
Further investigations are currently ongoing to understand the dynamics of spatial frequency interaction in the context of optically degraded stimuli.
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