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Cerebral Cortex Advance Access published online on October 23, 2006
Cerebral Cortex, doi:10.1093/cercor/bhl098
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© The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Article

Topography of Cortical Activation Differs for Fundamental and Harmonic Frequencies of the Steady-State Visual-Evoked Responses. An EEG and PET H215O Study

M.A. Pastor 1 *, M. Valencia 1, J. Artieda 1, M. Alegre 1, and J.C. Masdeu 1

1 Department of the Neurological Sciences, Center for Applied Medical Research, University of Navarra School of Medicine and the Clínica Universitaria de Navarra, Pamplona, Spain

* To whom correspondence should be addressed.
M.A. Pastor, E-mail: mapastor{at}unav.es


  Abstract

In humans, visual flicker stimuli of graded frequency (2-90 Hz) elicit an electroencephalographic (EEG) steady-state visual-evoked response (SSVER) with the same fundamental frequency as the stimulus and, in addition, a series of harmonic responses. The fundamental component of the SSVER is generated by increased synaptic activity in primary visual cortex (V1). We set out to determine the cortical origin of the harmonic responses in humans. For this purpose, we recorded the SSVERs at 5 different frequencies (5, 10, 15, 25, and 40 Hz) and measured regional cerebral blood flow (rCBF) with positron emission tomography-H215O at rest and during visual stimulation at the same frequencies. The rCBF contrast weighted by the amplitude of the SSVERs first harmonics showed activation of a swath of cortex perpendicular to V1, including mostly the inferior half of the parieto-occipital sulcus. This area overlapped minimally with the primary visual cortex activated by the fundamental frequency. A different method, estimating EEG cortical source current density with low-resolution brain electromagnetic tomography, gave the same results. Our finding suggests that the inferior portion of the banks of the parieto-occipital sulci contains association visual cortex involved in the processing of stimuli that can be as simple as a flickering light source.

Keywords: LORETA; parieto-occipital sulcus; positron emission tomography; regional cerebral blood flow; steady-state visual-evoked potentials; visual cortex.
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