How global is the global effect? The spatial characteristics of saccade averaging

When a target and a distractor are presented in close proximity, an eye movement will generally land in between these two elements. This is known as the 'global effect' and has been claimed to be a reflection of the averaged saccade programs towards both locations. The aim of the present study was to systematically investigate whether there is only a limited area in the saccade map in which saccade averaging occurs. To this end, we examined various distances between target and distractor in two experiments and investigated whether the majority of eye movements landed in between the target and the distractor. Results indicated that the endpoint distribution was unimodal for distances up to 35° (in polar coordinates), with saccades generally landing in between the target and the distractor. When the distance was higher than 45°, the saccade endpoint distribution was predominantly bimodal, with saccades landing either on the target or on the distractor. The decrease in saccade averaging was linear until almost no averaging saccades were observed for the longest distances. As saccades landing in between target and distractor reflect a weak, or absent, top-down signal, the present study indicated that top-down information is unable to strongly influence the oculomotor system when target and distractor are presented in close proximity. In this situation, the resulting eye movement is determined by the weighted average of saccade vectors present in a restricted region in the motor map.