Modulation of saccadic intrusions by exogenous and endogenous attention.

Primary gaze fixation in healthy individuals is frequently interrupted by microsaccades and saccadic intrusions (SI). The neural systems responsible for the control of attention and eye movements are believed to overlap and in line with this, the behaviour of microsaccades appears to be affected by exogenous and endogenous attention shifts. In the current work we wished to establish whether SI would also be influenced by attention in order to provide evidence that SI and microsaccades exhibit similar behaviour and further investigate the extent of overlap between attention and eye movement systems. Twelve participants performed a cue-target task where they were cued exogenously or endogenously and had to respond to the appearance of a peripheral target with either a button press or saccade. Our results replicate earlier microsaccade research, indicating that SI are also influenced by exogenous and endogenous attention. In all conditions, SI frequency initially decreased following the cue, then rose to a maximum before falling to below baseline levels. Following the exogenous cue, SI were more frequently directed away from the cue as predicted by inhibition of return. Additionally, SI direction following the endogenous cue was biased towards the cue for the saccadic response mode only, suggesting that the degree to which the eye movement and attention systems overlap depends on whether an eye movement is required. In summary, our findings indicate that SI characteristics are modulated by exogenous and endogenous attention and in a similar way to microsaccades, suggesting that SI and microsaccades may lie on a continuum of fixational instabilities. Furthermore, as with microsaccades, SI are likely to provide additional insights into the relationship between attention and the oculomotor systems.