Cortical control of saccades

Abstract

Saccades may be reflexive, externally triggered by a visual target (reflexive visually guided saccade) or a noise (reflexive auditory saccade), suddenly occurring on the peripheral retina or in the immediate environment, respectively. Saccades may also be intentional, either externally triggered by a visual target already present on the peripheral retina for a period of time (volitional visually-guided saccade), or internally triggered towards a target just seen previously (memory-guided saccade) or predicted (predictive saccade). Four main cerebral areas appear to be involved in the control of saccades: the frontal eye field (FEF), located on the precentral sulcus at the level of the middle frontal gyrus; the prefrontal cortex (PFC) or area 46 of Brodmann, located just anteriorly to the FEF; the supplementary motor area (SMA), located anteriorly to the precentral sulcus in the superior frontal gyrus; the posterior parietal cortex (PPC), namely the superior part of the angular gyrus (area 39 of Brodmann). Reflexive visually guided saccades are triggered both by the PPC and the FEF, though the former would appear to predominate in the normal state. These saccades, when not required, are probably inhibited by the PFC. Memory-guided saccades could be controlled successively by the visual cortex (reception), the PPC (visuospatial integration), the PFC (memorization and decision) and the FEF (triggering). Sequences of memory-guided saccades include an extra relay in the SMA, between the PFC and the FEF. Predictive saccades could be controlled by the FEF. Finally, all saccades are triggered through two cortical poles, the FEF and the PPC. The parietal pole, which is mainly responsible for the triggering of reflexive saccades, could specialize in the exploration of the present environment. The frontal pole, which is mainly involved in the triggering of intentional saccades, could explore an environment in the immediate past or future.