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Yee-Joon Kim, Krishnakumar Muthu, Marcia Grabowecky, Ken A. Paller, Satoru Suzuki; Effects of stimulus contrast and attention on steady-state visual evoked potentials.. Journal of Vision 2004;4(8):790. doi: 10.1167/4.8.790.
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Contrast response functions are fundamental to understanding vision because they reflect initial stimulus strength in cortical visual processing. Contrast response has been measured both neurophysiologically and with fMRI. Steady-state visual evoked potentials (SSVEPs) reflect activity of visual cortical neurons that is phase-locked to flickering stimuli. The evoked SSVEPs are approximately sinusoidal and in this case have a fundamental frequency twice the driving frequency (with on and off responses). This is a promising technique for studying basic vision, because it allows us to track, with high temporal resolution, neural responses to multiple stimuli that are present concurrently and may overlap spatially. We investigated how the average SSVEP power spectra in the driven frequency bands varied with stimulus contrast, frequency, duration, and attention. The 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. Observers maintained central fixation for the duration of each 12-second trial and either distributed their attention or attended to the stimulus on either the left or right side throughout a block. Contrast was varied randomly within a block of trials. Across blocks, stimulus frequency, duration and attention conditions were counterbalanced. The contrast response functions obtained with SSVEPs were averaged from 15 occipito-parietal electrodes and were fit with Naka-Rushton functions. These functions were qualitatively similar to those obtained previously with fMRI, and were similar for 12.5 Hz and 16.7 Hz flicker, but weaker for 25 Hz flicker. In comparison, the averaged band power from 6 frontal electrodes did not change with contrast. Additional experiments revealed how focused attention affected contrast response functions, and how SSVEP responses are correlated with awareness of a visual stimulus.
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