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Yee Joon Kim, Marcia Grabowecky, Ken A. Paller, Satoru Suzuki; The different properties of object-based and spatial attention revealed by ssveps. Journal of Vision 2005;5(8):1029. doi: 10.1167/5.8.1029.
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© ARVO (1962-2015); The Authors (2016-present)
We investigated how the average SSVEP power spectra in stimulus-driven frequency bands varied with stimulus location (overlapping or separated stimuli), contrast, frequency, and attention. Blocks where Os attended to overlapping stimuli (“object-based”) or to spatially separated stimuli were counterbalanced. For object-based attention, the central stimuli (8 radial arms, arranged as a + and x) were flickered at different frequencies. For spatial attention, the bull's eye stimulus in each visual hemifield was flickered at a different frequency. A counter-phase flicker (black-white against mid-gray) was used to prevent afterimages. Os maintained central fixation for the duration of each 4.8-second trial and attended to the +/x or the left/right stimulus throughout a trial. Contrast was varied randomly within each block of trials. Across blocks, stimulus frequency and attention conditions were counterbalanced. Spatial attention boosted amplitudes of SSVEPs within the same areas as were activated by the stimuli. In contrast, object-based attention modulated responses in areas that differed from those activated by the stimuli. Specifically, attending to 16.7 Hz or 12.5 Hz components boosted activation in largely complementary areas even when the same shape was attended. These results suggest that attention mechanisms may use topographically-segregated frequency-coding to support object-based attention; attention appears to segregate different frequencies into separate cell assemblies. The effect of spatial attention clearly demonstrated response gain in contrast response functions for all subjects. In contrast, object-based attention produced individual differences. For example, Os who demonstrated topographically-segregated frequency-coding of the attentional modulation showed a response-gain pattern. In the absence of this topographical segregation, attentional modulation of the contrast response function was better characterized by contrast gain.
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