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Zusammenfassung:
A fundamental problem of neuroscience is being able to understand the input-output relationship of early sensory areas. Central to this understanding is the notion of the receptive field (RF). Previous electrophysiological studies in the visual system of primates have demonstrated that receptive fields do not have fixed properties but are dynamically changing as a function of the stimulation conditions or behavioral tasks. In the human, aggregate (voxel-based) receptive field sizes (pRF) of early visual cortex were estimated by fMRI using standard retinotopic stimuli (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008) but it would be desirable to obtain more detailed estimates of RF spatio-temporal properties. In this study, we have used flickering checkerboard stimuli (white noise) in order to measure the spatio-temporal response properties of population RFs by fMRI. Blood oxygen level dependent (BOLD) measurements were performed both in anesthetized macaques (4.7 Tesla vertical scanner) and awake-fixating human subjects (3 Tesla Siemens Trio). We found that the BOLD-signal amplitude in early visual cortex changes as a function of the checker-size of the stimulus and this can provide estimates of the central excitatory integration area as well as the surround suppression of the aggregate RFs. The estimates of the size of the central portion of the RFs were found to increase with eccentricity within each visual area as well as from lower to higher visual areas. The results were comparable to pRF-size estimates derived from additional experiments with classical retinotopic stimuli (expanding rings, rotating wedges, moving horizontal and vertical bars). Current work focuses on evaluating how RF parameter estimates change as a function of the temporal frequency and contrast of the stimuli. We believe that this method can provide robust estimates of RF parameters which can be used in longitudinal studies for the assessment of cortical reorganization and plasticity in patients suffering from retinal and cortical lesions.
