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Nihong Chen, Fang Fang; Two perceptual consequences of orientation discrimination learning and their distinct time courses. Journal of Vision 2011;11(11):1020. doi: 10.1167/11.11.1020.
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Perceptual learning can improve sensory discrimination and the improvement is long-lasting. Here we show that orientation learning can not only improve orientation sensitivity, but also affect our judgement of absolute orientation. Eight subjects were trained to discriminate orientation at 15°. After six daily training sessions (over 8000 trials practice), as expected, subjects showed a significant orientation-specific improvement at the trained orientation. Before and after training, we also measured their subjective vertical orientation. Interestingly, training was able to render the subjective vertical orientation perceived to shift towards the trained orientation, which resembled the well-known tilt aftereffect. The change of subjective vertical orientation after training was highly specific to the trained retinal location. It was also dependent on the trained orientation since data from other eight subjects showed that training at 75° had little effect on their subjective vertical orientation. Regarding the time course of these two perceptual consequences, although orientation discrimination improvement was completely preserved two months after training, the change of subjective vertical orientation substantially diminished. In addition, we conducted an exposure experiment that was identical to the 15° orientation discrimination experiment except that subjects did a simple fixation task. Although subjects' orientation sensitivity remained the same, their subjective vertical orientation significantly shifted towards the exposed orientation (15°). These results demonstrated a clear dissociation between orientation discrimination improvement and subjective orientation change, which compels us to re-evaluate the role of the early visual cortex in orientation discrimination learning. Although learning (even exposure) may have changed the orientation coding in the early visual cortex, which altered the subjective vertical orientation, the orientation coding change might have nothing to do with the perceptual learning of orientation discrimination. The neural substrate of orientation discrimination learning could be in high-level cortical areas (e.g. decision making areas).
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