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Rui Wang, Jie Wang, Shu-Han Luo, Cong Yu, Wu Li; Orientation discrimination and learning may not rely on direct sensory inputs from orientation detectors. Journal of Vision 2012;12(9):1141. doi: 10.1167/12.9.1141.
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It is assumed that the brain relies on orientation detectors in the early visual cortex for orientation discrimination. Accordingly, perceptual learning of orientation discrimination is often interpreted as refined readout of inputs from these orientation detectors at a later decision stage. However, recent studies demonstrated that orientation learning can transfer completely across the hemispheres and to an orthogonal orientation, suggesting that orientation learning could occur in high-order brain areas. Therefore, there exists the possibility that, instead of direct readout of inputs from the orientation detectors, the high-level decision stage may rely on orientation inputs from later stages of brain processing.
To test this hypothesis, we trained human observers to discriminate the explicit orientation of gratings, putatively detected by V1 neurons; or the implicit orientation of mirror-imaged dot patterns with a single axis of symmetry, unlikely detectable in V1 but most likely encoded by higher cortical areas. We compared the mutual transfer of learning between these two distinct stimuli. Learning in orientation discrimination of the symmetric dot patterns transferred completely to the gratings. In contrast, learning in orientation discrimination of the gratings transferred only partially to the dot patterns; but subsequent exposure to the same-oriented dot patterns in a suprathreshold irrelevant task ("which of the two patterns contains more dots?") (a Training-Plus-Exposure technique, Zhang et al., J. Neuroscience, 2010) further markedly reduced orientation thresholds for the dot patterns, achieving a complete learning transfer from the gratings to the dot patterns.
The complete learning transfer, especially from the implicit orientation of symmetric axis to explicit orientation of gratings, is unpredicted if orientation discrimination and its learning rely on direct readout of inputs from early orientation detectors. Our findings pose the challenge of understanding the neural computations for fine orientation discrimination and its further refinement through perceptual learning in later cortical processing.
Meeting abstract presented at VSS 2012
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