4 - Functional Mapping in the Cat Primary Visual Cortex Using High Magnetic Fields

Abstract

This chapter describes the recent advances in functional magnetic resonance imaging (fMRI) enabling the functional organization of the cat visual cortex to be studied at “columnar” level, thereby approximating the spatial resolution of multielectrode and optical imaging techniques, but without their limitations. It reviews the imaging principles and techniques of this new method and presents the first MRI studies of the functional activity in cat primary visual cortex at columnar resolution. The results from the imaging studies on cat primary visual cortex clearly indicate that the functional specificity of blood oxygenation level-dependent (BOLD) at columnar scale depends highly on the temporal dynamics of the underlying signals. The delayed positive BOLD changes are clearly indicative of the pattern of the overall activation per se in the visual cortex, but are less suited to discriminate between the active and inactive columns, as they are more diffused and less specific to the individual stimulus properties. The early decrease of BOLD signals can most likely be attributed to the regional increase of deoxyhemoglobin after neuronal stimulation. In principle, the capability of BOLD-fMRI to label functional columns in vivo may be applicable also in humans and monkeys.