Distractor-evoked deviation in saccade direction suggests an asymmetric representation of the upper and lower visual fields on oculomotor maps.

The programming of rapid eye movements or "saccades" involves a large collection of neural substrates. The subcortical oculomotor center - the superior colliculus (SC) - contains a topographical motor map that encodes saccade vectors. Using a visual distractor task, the present study examined a classic model of the SC motor map, which assumes a symmetrical representation of the upper visual field (UVF) and lower visual field (LVF). Visual distractors are known to attract or repel the saccade trajectory, depending on their angular distance from the target. In the present study, the distractor (if presented) was placed at a location that mirrored the target in the opposite visual field (upper or lower). The symmetrical SC model predicts equivalent directional deviations for saccades into the UVF and LVF. The results, however, showed that the directional deviations evoked by visual distractors were much stronger for saccades directed to the LVF. We argue that this observation is consistent with the recent neurophysiological finding that the LVF is relatively under-represented, as compared to the UVF, in the SC and possibly in other oculomotor centers. We conclude the paper with a suggested revision to the SC model.