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Tsung-Ren Huang, Takeo Watanabe; Association of perceptual learning to reduce spatial crowding with shrinkage of receptive fields. Journal of Vision 2011;11(11):1006. doi: 10.1167/11.11.1006.
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© ARVO (1962-2015); The Authors (2016-present)
The difficulty of identifying crowded letters can be mitigated through training (Chung, Vision Research, 2007). The underlying mechanism for such perceptual improvement has yet to be clarified. Here we report that training enhanced contrast sensitivities in association with a higher ability to identify high-contrast yet crowded letters. In our experiment, participants (n = 6) were trained on an English letter identification task in which a target letter (~2° × 2° in size) that was 10° away from the central fixation in the lower visual field was flanked by two other English letters of the same size and contrast with a small amount of inter-letter spacing (0.8 times the width of ‘x’). In 6 days of training on this crowding task (100 trails × 10 blocks per day), the letter identification accuracy steadily improved (~15% on average) and such improvement transferred to conditions of different inter-letter spacings. More importantly, the contrast sensitivity functions were measured at locations that were also 10° away from the central fixation using 2° × 2° strip patches for both upper and lower visual fields. With perceptual learning obtained after 6 days of training, significantly enhanced contrast sensitivities to a certain high spatial frequencies range were observed for the trained location (i.e., peripheral 10° in the lower visual field) but not untrained location (i.e., peripheral 10° in the upper visual field). Given that visual neurons' contrast sensitivities to harmonic stimuli are dependent upon their receptive field sizes (Wandell, Foundations of Vision, 1995), the observed enhanced sensitivities to high spatial frequencies are in accord with the hypothesis that learning to reduce crowing is associated with neurons' receptive field shrinkage at the trained location. Such receptive field plasticity may also explain why action video games (Li et al., Nature Neuroscience, 2009) among other perceptual tasks unexpectedly improve contrast sensitivities.
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