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Nicholas Van Horn, Alexander Petrov; Perceptual learning of visual motion: The role of the spatial frequency of the carrier. Journal of Vision 2009;9(8):886. doi: 10.1167/9.8.886.
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Motion-sensitive neurons in the middle temporal (MT) cortex inherit their basic spatial tuning from their inputs from earlier visual areas. In particular, the tuning for spatial frequency is not very different than that in areas V1 and V2, though MT neurons are somewhat more broadly tuned (Britten, 2004). We investigated how the spatial frequency of the motion carrier affected the d' of direction discrimination and the perceptual learning thereof.
Method: The stimuli were filtered-noise textures moving at 10 degrees/sec for 400 msec. Each patch was filtered with a radial bandpass filter with a peak frequency of either 1 (Low) or 4 (High) cycles/deg. Twenty-five observers discriminated motion directions that differed by either 7 degrees (Easy) or 4 degrees (Difficult) in a block. All observers were pre-tested on Day 1 and post-tested on Day 6 on all four combinations of frequency and difficulty. On Days 2 through 5, participants trained exclusively on one pair of features, counterbalanced across four groups.
Results: The average d' improved in all groups: EasyHigh from 1.67 to 2.91, EasyLow from 1.43 to 2.56, DifficultHigh from 1.11 to 1.81, and DifficultLow from 0.89 to 1.50. The learning effects transferred well across spatial frequencies. As expected, the Easy groups performed better than the Difficult groups. Additionally, the High frequency groups performed better than their Low frequency counterparts for both difficulties.
Conclusion: The transfer of learning across the spatial frequency of the carrier suggests plasticity site(s) broadly tuned for spatial frequency, such as areas LIP and/or MST. Area MT, which is relatively narrowly tuned, seems to remain unchanged. This agrees with recent physiological evidence of training-induced changes in LIP but not MT (Law & Gold, 2008). The MT involvement can account for the main effect of spatial frequency, which did not interact with the practice effects.
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