Topography and lateralization in frontal eye fields during covert spatial orienting and visual search revealed by fMRI

To what extent are the frontal eye fields (FEFs) topographically organized and do they show hemispheric lateralization (i.e. contralateral preference) during spatial orienting and visual search? Search tasks spanning the whole visual field revealed activity prominently within the right FEF (Donner et al. 2000). In fact orienting tasks separating between hemifields revealed topographic maps and suggested contralateral preference in both FEFs (Hagler & Sereno 2005). However, it is unclear, whether topography and lateralization effects are specific for orienting or if they can also be found during search.

We mapped FEF activations during both spatial orienting and visual search for distinct hemifields (upper, lower, right, left). Using event-related fMRI and a high resolution sequence (voxel-size=2*2*3mm, slices=32) subjects monitored a circular array with 12 positions. An initial cue (3, 6 or 9s) indicated the hemifield, to which subjects (N=10) covertly shifted and maintained their attention. After the cue, 12 stimuli appeared and subjects indicated the presence of a target within the cued hemifield by covert search. This design separated the orienting (cue) from the search period.

We found topographic maps in the FEF and parietal subregions (AIPS, PIPS, IPTO) during orienting. However, during search, topographies were only found in IPTO.

Interestingly, during orienting different types of activation could be distinguished between subjects: Three subjects showed a contralateral preference (CP) in both FEFs, i.e. that the left FEF was mainly activated by orienting to the right hemifield and vice versa. In four subjects, this CP was only found in the left FEF while the right FEF is equally activated by both hemifields. One subject showed CP only in the right FEF. These types of lateralization are compared to those in parietal subregions and are found to be consistent in 6 subjects, i.e. subjects who showed lateralized activity in both FEFs also showed lateralized activity in bilateral parietal regions. In contrast, during search, these types of lateralization could not be obtained.

We conclude, that lateralization effects in the FEF vary broadly between different processes (orienting vs. search) and individual subjects. The view, that FEFs must have a contralateral preference should be reconsidered.