Cortical Control of Smooth Pursuit Eye Movements: Sensory and Motor Components. An fMRI study

Smooth pursuit eye movements (SP) serve to keep a moving object in our fovea, the place of highest visual acuity. For this purpose, the SP system must detect the object moving across our retina (sensory input), process its velocity, and prepare a motor command signal being sent to the eye muscles (motor output), which enables the eyes to follow the object. The purpose of this study is to identify which cortical regions process the sensory input information, which process the motor output signals, and which are related to both processes (possibly sensorimotor transformation sites). For this purpose we measured cortical activation related to pure visual (sensory), pure oculomotor, and combined visuo-oculomotor stimulation. A central fixation dot and a second dot, 4° below the center dot, were presented in darkness. Subjects always fixated the center dot. Moving the lower dot sinusoidally in the horizontal plane yielded a visual (sensory) stimulation. Moving both dots while subjects pursued the upper dot yielded oculomotor (but almost no sensory) stimulation. Moving only the upper dot while subjects tracked this dot yielded oculomotor plus sensory stimulation. Eye movements were measured with an infrared light technique (MR-Eyetracker), cortical activation with a 3T Siemens Trio scanner. Twelve healthy subjects participated in this study. SP gain was close to one, indicating that subjects followed the moving dot accurately. Analysis of saccadic eye movements showed that small saccades occurred during both fixation (rest) periods and pursuit (stimulation) periods, such that these saccades do not add to the cortical activation investigated here. Cortical activation was found in areas V5/V5A, precuneus, intraparietal sulcus, SEF, FEF, cingulate gyrus. Visual stimulation activated mainly occipito-parietal regions V5/V5A, precuneus and the intraparietal sulcus, while oculomotor stimulation activated predominantly the frontal regions FEF and SEF and the cingulate gyrus. This was confirmed by the combined visuo-oculomotor stimulation which activated all these regions. Parts of V5/V5A and the intraparietal sulcus seem to be involved in both sensory and motor processing. Our data show that the occipito-parietal regions are processing the sensory input signals of pursuit eye movements, while the FEF and SEF are related to the motor output signal. Parts of V5/V5A and the intraparietal sulcus are possible sites for sensorimotor transformation