Topography of Cortical Activation Differs for Fundamental and Harmonic Frequencies of the Steady-State Visual-Evoked Responses. An EEG and PET H15 2 O Study

摘要

In humans, visual flicker stimuli of graded frequency (2--90 Hz) elicitan electroencephalographic (EEG) steady-state visual-evoked response(SSVER) with the same fundamental frequency as the stimulusand, in addition, a series of harmonic responses. The fundamentalcomponent of the SSVER is generated by increased synaptic activityin primary visual cortex (V1). We set out to determine the corticalorigin of the harmonic responses in humans. For this purpose, werecorded the SSVERs at 5 different frequencies (5, 10, 15, 25, and 40Hz) and measured regional cerebral blood flow (rCBF) with positronemission tomography-H152 O at rest and during visual stimulation atthe same frequencies. The rCBF contrast weighted by the amplitudeof the SSVERs first harmonics showed activation of a swath of cortexperpendicular to V1, including mostly the inferior half of the parietooccipitalsulcus. This area overlapped minimally with the primaryvisual cortex activated by the fundamental frequency. A differentmethod, estimating EEG cortical source current density with lowresolutionbrain electromagnetic tomography, gave the same results.Our finding suggests that the inferior portion of the banks of theparieto-occipital sulci contains association visual cortex involved inthe procparieto-occipital sulcus,