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Ming Zhang, Yang Zhang, Sheng He; Attention capture by an invisible flicker not in the middle of gamma range. Journal of Vision 2010;10(7):118. doi: 10.1167/10.7.118.
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It was recently reported that a subliminalflicker could trigger attentional selection at the target location (Bauer et al, PNAS 2009). They specifically attributed this effect to the middle of gamma range (50Hz) since they failed to find such effect with flickers slower than 35-Hz. However, it is possible that rendering a flicker signal subliminal by lowering its contrast resulted in more severe loss of effective neural contrast for a 30 Hz flicker than a 50 Hz flicker. To test this possibility, we used flicker contrast reduction combined with spatial crowding to render a flicker signal subliminal. This approach allowed a subliminal flicker to maintain substantial contrast. Specifically, a 30 Hz flicker Gabor patch and a non-flickering control were presented one to the left and one to the right of the fixation, both surrounded by four static Gabor patches. Subjects performed at chance level in 2AFC task detecting which side had the flickering signal. However, in a modified Posner cueing paradigm, subjects responded faster to a probe target presented at the 30 Hz subliminal flicker location than the control location. In a follow-up experiment, when the probe target appeared at the non-flickering control location in 80% of the trials and subjects were instructed to use this information to direct their attention, the side with the subliminal flicker still showed a benefit effect. Together these results show that a subliminal flicker can capture spatial attention and the flicker frequency does not need to be in the middle of the gamma range.
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