Non-retinotopic perceptual learning
Perceptual learning is usually specific for the encoded stimuli. For example, improvements in performance for a certain stimulus do not transfer if the stimulus is rotated by 90 degree or is presented at a different location. These findings are usually taken as evidence that orientation-specific and retinotopic synaptic changes underlie perceptual learning. To test this hypothesis, we used a recently developed non-retinotopic masking paradigm. An offset vernier was followed by nine consecutive pairs of non-offset flanking lines arranged along a circular trajectory, giving rise to the percept of two diverging and turning motion streams. Critically, the vernier itself was invisible but its offset was perceived at the last line of the stream, which had a different orientation and location than the vernier. For 4,000 training trials, sixteen observers attended to one motion stream and indicated the perceived offset direction (left vs. right). Performance improved significantly. In addition, we measured performance for various other stimulus configurations before and after training. Our results show that the improvement in performance is specific for the orientation and location of the perceived offset of the last line of the attended motion stream but not for the actual orientation and location of the invisible vernier. Hence, orientation-specific, retinotopic synaptic changes cannot explain our results. We propose instead that perceptual learning involves changes in non-retinotopic, attentional readout processes.