Abstract
Almost all cortical areas in the vertebrate brain take part in recurrent connections through the subcortical basal ganglia (BG) nuclei, through parallel inhibitory and excitatory loops. It has been suggested that these circuits can modulate our reactions to external events such that appropriate reactions are chosen from many available options, thereby imposing volitional control over behavior. The saccade system is an excellent model system to study cortico-BG interactions. In this study two possible pathways were investigated that might regulate automaticity of eye movements in the human brain; the cortico-tectal pathway, running directly between the frontal eye fields (FEF) and superior colliculus (SC) and the cortico-striatal pathway from the FEF to the SC involving the caudate nucleus (CN) in the BG. In an event-related functional magnetic resonance imaging (fMRI) paradigm participants made pro- and anti-saccades. A diffusion tensor imaging (DTI) scan was made for reconstruction of white matter tracts between the FEF, CN and SC. DTI fiber tracts were used to divide both the left and right FEF into two sub-areas, projecting to either ipsilateral SC or CN. For each of these FEF zones an event-related fMRI timecourse was extracted. In general activity in the FEF was larger for anti-saccades. This increase in activity was lateralized with respect to anti-saccade direction in FEF zones connected to the SC but not for zones only connected to the CN. These findings suggest that activity along the contralateral FEF–SC projection is responsible for directly generating anti-saccades, whereas the pathway through the BG might merely have a gating function withholding or allowing a pro-saccade.
Highlights
When we react to events in our direct environment, the proper reaction often depends on the context of the situation
The non-frontal eye fields (FEF) clusters were removed from the maps to create FEF masks
This study used a combination of functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) to disentangle the role of the parallel projections between the FEF and superior colliculus (SC) in regulating saccades
Summary
When we react to events in our direct environment, the proper reaction often depends on the context of the situation. When a cup falls from the table, we would normally grab it automatically. Single-neuron recordings on non-human primates suggest that higher cortical areas control purposeful behavior, whereas subcortical areas are responsible for automatic reactions. Multiple parallel excitatory and inhibitory loops, from a large number of cortical areas through the basal ganglia (BG), modulate cortical activity and might subserve many functions such as eye and bodily movements and even higher cognitive processes (Alexander and Crutcher, 1990). The neurophysiology underlying the ability to select the proper reaction in a certain context has often been studied using eye movements as a model system. The pro- and anti-saccade paradigm requires participants to either look toward or away from an emerging target and has proven to be an ideal behavioral paradigm to study our control over response selection (Munoz and Everling, 2004)
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