fMR-adaptation: a tool for studying the functional properties of human cortical neurons

The invariant properties of human cortical neurons cannot be studied direct ly by fMRI due to its limited spatial resolution. One voxel obtained from a fMRI scan contains several hundred thousands neurons. Therefore, the fMRI signal may average out a heterogeneous group of highly selective neurons. H ere, we present a novel experimental paradigm for fMRI, functional magnetic resonance-adaptation (fMR-A), that enables to tag specific neuronal popula tions within an area and investigate their functional properties. This appr oach contrasts with conventional mapping methods that measure the averaged activity of a region. The application of fMR-A to study the functional prop erties of cortical neurons proceeds in two stages: First, the neuronal popu lation is adapted by repeated presentation of a single stimulus. Second, so me property of the stimulus is varied and the recovery from adaptation is a ssessed. If the signal remains adapted, it will indicate that the neurons a re invariant to that attribute. However, if the fMRI signal will recover fr om the adapted state it would imply that the neurons are sensitive to the p roperty that was varied. Here, an application of fMR-A for studying the inv ariant properties of high-order object areas (lateral occipital complex - L OG) to changes in object size, position, illumination and rotation is prese nted. The results show that LOC is less sensitive to changes in object size and position compared to changes of illumination and viewpoint. fMR-A can be extended to other neuronal systems in which adaptation is manifested and can be used with event-related paradigms as well. By manipulating experime ntal parameters and testing recovery from adaptation it should be possible to gain insight into the functional properties of cortical neurons which ar e beyond the spatial resolution limits imposed by conventional fMRI. (C) 20 01 Published by Elsevier Science B.V.