Abstract: Adults’ ability to discriminate some basic features can be improved through training, which is referred to as perceptual learning. During recent years, perceptual learning has been investigated extensively in various visual tasks. In particular, studies on visual perceptual learning have reported mirror-transfer symmetrical about the vertical line. The present study aims to further investigate whether mirror-transfer symmetrical about the 45° diagonal line exists.
The two-alternative forced-choice paradigm was applied to grating orientation discrimination task: two gratings of different orientations were presented successively, and subjects were required to decide whether the orientation of the second grating (test orientation) was clockwise or counterclockwise to that of the first one (reference orientation). There were three reference orientations, 15°, 45°, and 75° (15° and 75 ° are mirror orientations of each other about the 45° diagonal line). 11 subjects were trained with reference orientation of 15° or 75° for several days, and received threshold tests on all the three reference orientations both before and after the training. The two-up-one-down staircase procedure was adopted in threshold tests and training.
The results showed that the threshold of trained orientation declined substantially after training. Such learning effect did not transfer to the 45° orientation which was 30° away from the trained orientation, but almost completely transferred to the mirror orientation which was 60° away. These results indicated that mirror–transfer symmetrical about the 45° diagonal line exists in visual perceptual learning of orientation discrimination.
Diagonal mirror-transfer found in the present study suggests that visual perceptual learning of grating orientation discrimination might involve the intermediate visual cortex, which is characterized by both orientation selectivity and diagonal mirror transferability

Key words: visual perceptual learning, orientation discrimination, diagonal, mirrortransfer, symmetrical