Fiber pathways of cortical areas mediating smooth pursuit eye movements in monkeys

Recent studies in rhesus monkeys and human patients have increased our understanding of the role of various cortical regions in the generation of smooth pursuit eye movements. Little is known, though, about the cerebral white matter pathways mediating smooth pursuit. In this paper, we describe both the corticocortical and corticosubcortical projections from areas in the monkey brain known to be involved in smooth pursuit. The corticocortical pathways within one hemisphere consist of a sequence of arcuate fiber bundles interconnecting (1) striate cortex with an area in occipitoparietal cortex called the middle temporal (MT) area, (2) area MT with two areas in parietal cortex called the medial superior temporal (MST) and posterior parietal (PP) areas, and (3) area MST with area PP. An interhemispheric pathway interconnecting areas MT and MST consists of fibers sequentially passing through the tapetum, major forceps, and the splenium of the corpus callosum. The corticosubcortical pathway from area MST to the pontine nuclei and accessory optic system consists of fibers sequentially running through the internal sagittal stratum, the retrolenticular part of the internal sagittal stratum, and the cerebral peduncle. Based on the effects of lesions on pursuit eye movements, these corticocortical and corticosubcortical projections can be divided into sensory, motor, and attentional/spatial systems. Evidence from clinical studies suggests that homologous systems exist in the human cerebrum.