Asymmetric responses to temporal versus nasal hemifield stimulation in the human superior colliculus

Abstract

Normal subjects show a temporal hemifield advantage in visual orientating tasks, with a reduction in saccade latency when orienting to cues presented in the temporal (versus nasal) hemifield. Hemianopic patients with geniculostriate lesions show this effect for cues presented in their blind field, while newborns (who have yet to develop functioning geniculostriate pathways) show an exaggerated effect. These lines of evidence suggest that the temporal hemifield advantage is mediated via extrageniculate pathways, specifically the retinotectal pathway involving the superior colliculus. We therefore examined temporal/nasal field asymmetries in the human visual system using fMRI at high spatial resolution (voxel size - 1.5mm3) accompanied by conventional retinotopic mapping in eight right-eye dominant subjects who viewed monocular checkerboard stimuli. The superior colliculi consistently showed significantly greater activity for temporal compared to nasal field stimulation, while LGN and V1 showed no temporal/nasal asymmetry. This provides direct evidence that the extrageniculate pathway in humans has a bias towards the temporal visual field, and may represent the neural basis for the temporal field advantage seen in visual orientating tasks.