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Rigmor Baraas, Craig Aaen-Stockdale, Stuart Gilson; Age-related variation in foveal, parafoveal and peripheral spatial suppression. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4065.
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© ARVO (1962-2015); The Authors (2016-present)
The study investigated motion-related spatial suppression across different age groups and across different eccentricities, with the stimuli scaled according to the size of MT receptive fields. The aim was to test three alternative hypotheses: If spatial suppression (1) is a perceptual correlate of center-surround antagonism in cortical visual area V5 then the levels of spatial suppression should be constant at all eccentricities; (2) reduces with increasing age, as a result of a generalized weakening of inhibition then suppression should be lower for older observers, but constant at all eccentricities; (3) is related to contrast-related changes in receptive field size at a retinal level then suppression should vary with eccentricity and possibly vary with cone density.
Eleven normal, healthy subjects (7 females) with no known ocular abnormalities, aged 21-71 yrs participated in the study. Their left eye was patched and they fixated a fixation cross presented on the screen at eccentricities of 0, 2, 5 or 10 deg, or a back-lit fixation point at 25 or 40 deg. The stimulus was a 2 Hz drifting sinusoidal grating with a peak contrast of 92%. Spatial frequency was varied with eccentricity to keep it close to peak contrast sensitivity. Stimulus duration was varied to derive a duration threshold. High-resolution images of the cone mosaic were obtained with the Kongsberg Adaptive Optics Ophthalmoscope II.
Older observers showed zero suppression for foveal vision (0-2 deg), gradual increase in suppression in the parafovea (5-10 deg eccentricity) and weaker suppression in the periphery (25-40 deg). Contrary to this, young observers showed strong suppression for foveal and parafoveal vision (0-5 deg) with a gradual decline in suppression from 10 deg. There was between-individual variation in both spatial suppression and cone density. Initial analyses indicate that lower levels of suppression correlate with lower cone count.
The strength of suppression, in both young and old observers, varied with eccentricity, despite the scaling of the stimulus with average MT receptive field size. The results for older observers imply that weaker suppression is not a generalizable result. The results suggest that spatial suppression may be the result of low-level stimulus characteristics and structural variation in the retina rather than a direct result of center-surround antagonism in MT.
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