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Yee Joon Kim, Marcia Grabowecky, Ken A. Paller, Satoru Suzuki; Selective lateralization of steady state visual evoked potentials at the second harmonic. Journal of Vision 2008;8(6):1002. doi: 10.1167/8.6.1002.
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We previously reported that the steady-state visual evoked potentials (SSVEPs) elicited by lateralized flickering stimuli were topographically fractionated into bilateral 1st-harmonic (1f) responses and contralateral 2nd-harmonic (2f) responses (VSS 2007). The current study investigated whether this harmonic-based topographic segregation generalized to a broader range of frequencies. We used flicker frequencies of 6.25 Hz, 8.33 Hz, 12.5 Hz, 16.7 Hz, and 25 Hz, inducing 1f and 2f responses ranging from theta to gamma frequencies. The bilateral topography of 1f responses generalized to all frequencies. We confirmed that the bilateral-contralateral topographic segregation of 1f and 2f responses was harmonic (rather than frequency) dependent. Specifically, the 1f elicited by the 16.67 Hz flicker and the 2f elicited by the 8.33 Hz flicker both had the same response frequency of 16.67 Hz; similarly, the 1f elicited by the 25 Hz flicker and the 2f elicited by the 12.5 Hz flicker had the same response frequency of 25 Hz. In both cases, we demonstrated a bilateral 1f and contralateral 2f, confirming the harmonic dependence of SSVEP lateralization. Interestingly, the degree of lateralization (contralateral minus ipsilateral) of 2f responses exhibited an inverted-U-shaped frequency dependence, broadly peaking around 25 Hz (12.5 Hz stimulus flicker), showing no lateralization at the lowest, 12.5 Hz (6.25 Hz stimulus flicker), and highest, 50 Hz (25 Hz stimulus flicker), frequencies. This frequency dependence of lateralization cannot be attributed to the frequency dependence of 2f responses which monotonically decreased with increasing flicker frequency. Combined with our previous finding that only the contralateral 2f responses were substantially modulated by visual spatial attention, the obtained frequency dependence of lateralized 2f responses suggests that top-down feedback for visual spatial attention may selectively utilize an ∼8–34 Hz range of frequencies.
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