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Ramakrishna Chakravarthi, Patrick Cavanagh; Temporal properties of the polarity effect in crowding. Journal of Vision 2005;5(8):998. doi: 10.1167/5.8.998.
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The identification of a peripheral target is severely impaired by flanking distracters, but the extent of this crowding effect is reduced if the target contrast is opposite to that of the distracters (e. g., a white target flanked by black distracters, Kooi, Toet, Levi, & Tripathy, 1994). We explored the temporal properties of this polarity advantage by rapidly flickering target and distracter contrasts during each trial. In the out-of-phase condition, the target polarity was the opposite of the distracter polarity on each frame (white target among black distracters and then black among white). In the in-phase condition, target and distracters had the same polarity and again alternated on each frame (white among white, then black among black). At low temporal rates, the contrasts of the target and distracters were visible in each frame and were seen to reverse polarity from frame to frame. At higher rates, however, the letters were seen not as individually light or dark but all seemed to be flickering equivalently: the individual states and phase relations were no longer apparent. We varied the target-distracter spacing across trials and estimated the critical spacing that gave 62.5% performance from the resulting psychometric functions. We found the polarity effect at low frequencies: the critical spacing was as much as 59% smaller in the out-of-phase condition (i. e., showing less crowding) than in the in-phase condition. However, at frequencies of 7.5 Hz and higher, the polarity effect was lost suggesting that the instantaneous target and distracter polarities were no longer available to the mechanism responsible for crowding effects. It is known that attention has a coarse temporal resolution (6 – 8 Hz) whereas lateral masking operates at much higher frequencies. The findings of this study suggest that attention plays a role in the phenomenon of crowding.
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