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Abstract

In the macaque the extrastriate area V5/MT is located within the dorsal half and on the posterior bank of the superior temporal sulcus. Neurons in V5/MT show directional tuning to moving stimuli. Furthermore, these neurons are organized in a retinotopic fashion with those responding to stimuli located at the center of gaze being more lateral and ventral within V5/MT (Gattass and Gross, 1981). We have now extended the technique of combined electrical microstimulation and fMRI from the striate cortex (Tolias et al., 2005) to this well-studied extrastriate region to further probe this technique as a tool to map the functional connectivity of the brain. We electrically stimulated V5/MT in the anaesthetized macaque in a 4.7T scanner with biphasic charge-balanced pulses (up to 1mA and 200us pulse width per phase) and evoked BOLD responses consistently in a number of brain areas known to be directly connected to V5/MT. BOLD responses were observed in ipsilateral V2, V3, V4, V4t, PO, MST, in the anterior and posterior banks of the IPS (corresponding to LIP) and in the superior colliculus. Two types of projection patterns could be discerned by stimulation of either the peripheral or the foveal retinal representation of area V5/MT. The latter showed activation of regions located on the lateral surface of the occipital cortex while the former showed activity in mesial occiptio-parietal cortex. BOLD responses were surprisingly rather difficult to evoke in V1 with our current stimulation paradigms. V1 responses were largely seen in peripheral V1 regions. This could indicate that the evoked BOLD responses are dominated by orthodromic vs antidromic pathway activation, however, electrical stimulation of the pulvinar in contrast to V5/MT evoked excellent BOLD responses in V1. Since the V1-pulvinar connectivity is mainly feedforeward, this then proves that antidromic pathway activation is well detected by our method. Hence the different activation patterns that we observe in V1 after MT/V5 and pulvinar stimulation are rather related to the different pathways characteristics, possibly related to differences in the type of synaptic connectivity. Thus microstimulation combined with fMRI may well prove to be a novel technique suited to reveal different characteristics of the brains functional connectivity.