PURPOSE: Although visual perceptual learning is often location specific, learning can transfer to a new location if the new location is additionally trained with an irrelevant task (Xiao et al., 2008). Here we characterized this double training effect on Vernier learning and its transfer at various levels of external noise. 
METHODS: A 2-Gabor Vernier was imbedded in five levels of white noise (rms contrast 0%～29%) and centered in a visual quadrant at 5o eccentricity.
RESULTS: When training was at zero noise, Vernier thresholds were reduced at both zero and high noise at the trained location, but only at high noise at a diagonal quadrant location. However, if orientation discrimination with a Gabor at zero noise was also practiced at the diagonal location, Vernier thresholds at the diagonal location were improved at all noise levels. When training was at high noise, Vernier thresholds were only reduced at high noise at both trained and diagonal locations. Additional orientation training at zero noise, but not at high noise, at a diagonal location reduced Vernier thresholds at lower noise levels at both locations.
CONCLUSIONS: Fitting TvC functions of Vernier learning revealed that: 1) Training at zero noise enhances the sensitivities of perceptual mechanisms, which can transfer to untrained location with double training. 2) Training Vernier at high noise reduces the effect of external noise only, but additional orientation training at zero noise at a different location enhances the sensitivities to Vernier differences at lower noise levels at both locations. These results suggest that training a fine discrimination task enhances the sensitivity of perceptual mechanisms, which appears to be a high-level cognitive process.