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Wim VanDuffel, Ph.D.

(MGH)

The topography of causal interactions between the FEF and occipital cortex: a combined fMRI-microstimulation study in awake behaving monkeys

There is ample evidence from electrophysiology, functional imaging and psychophysics that visual representations and perception are enhanced at (c)overtly attended locations. Moreover, a clear correlation has been suggested between 'increased' neuronal activity at attended locations and corresponding benefits at behavioral level. However, the neural mechanisms that produce these effects are poorly understood. Two possible mechanisms have been suggested: a bottom-up mechanism in which salient stimuli in the visual scene automatically 'attract' attention without the need of endogenous signals. Secondly, it is widely recognized that voluntary (hence endogenous) attentional 'source' signals emerge in higher level areas within a fronto-parietal network and that feedback signals from these areas modulate sensory signals in visual cortex. However, to date there is very little evidence proving a causal link between activity in the frontal (or parietal) cortex and modulation of visually driven activity in occipital cortex. In this presentation, I will show that microstimulation of the FEF modulates visually-driven fMRI activity in occipital cortex in a very specific topographic manner. Surprisingly, our results suggest that artificially increased FEF output boost stimulus representations primarily by enhancing fMRI activity of weakly visually-driven neurons, as opposed to increasing activity of neurons that were already strongly activated (in the absence of FEF-microstimulation).