Cerebral activity during visual stimulation: a positron emission tomography and functional magnetic resonance imaging study.

Stimulation of cerebral areas induces a regional increase in blood flow and metabolism. Positron emission tomography (PET) is an established procedure to localize cerebral regions of enhanced activity. Exposure to a radioactive indicator and limited spatial and temporal resolution are disadvantages of this method as compared with other imaging techniques, but anatomical orientation can be improved by matching PET images with high-resolution magnetic resonance imaging (MRI) scans. Recently, functional MRI (fMRI) has arisen as an alternative method. This procedure presumably detects changes in the paramagnetic properties of hemoglobin, depending on its oxygenation state, as well as an increased regional blood flow in activated cerebral areas. These structures can be visualized using sensitive scanning techniques and appear with bright signal intensities. Visual stimulation was performed with the help of a high-resolution color VDU for PET registration and of an LCD video projector for fMRI (1.5 T). Hemifield stimulation as well as subtraction between images of flickering and stable random dot stimuli showed a preferential activation of the primary visual cortex. In addition, the first MRI results obtained during stimulation with moving gratings are demonstrated; hereby, preferentially extrastriate regions presumably responsible for motion detection were activated.