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Zhong-Lin Lu, Jiawei Zhou, Lin Li, Pan Zhang, Jie Xi, Yifeng Zhou, Chang-Bing Huang; Unmasking potential brain plasticity in amblyopic vision. Invest. Ophthalmol. Vis. Sci. 2014;55(13):814.
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© ARVO (1962-2015); The Authors (2016-present)
With abnormal visual cortical development, amblyopia is generally acknowledged as a spatial vision deficit in high spatial frequencies. The deficit may however reflect high internal noise rather than absence of cortical representation of high spatial frequency signal in amblyopia. To bypass the noise limitation, we measured supratheshold tilt aftereffects (TAE) following adaption to gratings of perceptually resolvable and unresolvable frequencies in subjects with amblyopia.
Six anisometropic amblyopes (average age: 22.3±2.6 yrs; 2 females) participated in the study. Performance in grating orientation identification (tilted +15° or -15° from horizontal) was measured in eight spatial frequencies. The spatial frequencies of the adapting sinewave gratings were chosen for each observer based on performance in the orientation task. In general, 1 or 2 easily resolvable, 1 resolvable, and 2 to 3 unresolvable spatial frequencies were chosen for each observer. Observers were asked to judge the orientation of a 4 c/d test grating (clockwise or counter-clockwise from horizontal) following adaptation. Five test orientations were used in each adaptation condition based on results from pilot studies. TAE thresholds were estimated from these measurements.
Adapting to gratings of resolvable spatial frequencies yielded an average TAE threshold of 1.35 ± 0.14° (Mean ± S.E.), consistent with previous reports in normal observers (He & MacLeod, 2001). What is surprising is that adapting to gratings of unresolvable spatial frequencies still yielded considerable TAE with an average effect size of 0.48 ± 0.07°. Although the TAE thresholds at unresolvable spatial frequencies were smaller than those at the resolvable spatial frequencies (t(28)=5.35, p <0.01), all the TAE thresholds were significantly greater than 0 (p<0.05). Averaged across observers, the ratio of the cutoff spatial frequencies in the TAE and traditional orientation identification tasks is 1.53 (± 0.08).
Neural connections in the amblyopic cortex, at least in V1, may have profoundly developed to represent high spatial frequency information. The demonstration of extant neural connections for high spatial frequencies therefore unmasks improvement potentials and provides a strong theoretical basis for developing new therapies for amblyopia. Our paradigm may also serve as a non-invasive probe to diagnose the status of neural connections in other clinical conditions.
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