Averaging is not everything: the saccade global effect weakens with increasing stimulus size

Area of Science:

• Neuroscience
• Cognitive Psychology
• Vision Science

Background:

• The global effect describes how saccade endpoints average between two aligned visual elements.
• This phenomenon is often explained by the center of gravity account, relating endpoint to relative element saliency.

Purpose of the Study:

• To investigate the influence of stimulus size on the global effect and saccade averaging.
• To test the prediction that saccade endpoints land intermediately regardless of stimulus size when elements are equidistant and equally salient.

Main Methods:

• Participants executed eye movements to simultaneously presented, equally-sized visual elements.
• Stimulus size was manipulated across conditions to assess its impact on saccade endpoints.
• A control experiment examined saccade endpoint distribution with single stimuli of varying sizes.

Main Results:

• The global effect (saccade averaging) was strongest with smaller stimuli and weaker with larger stimuli.
• Saccade endpoint distributions were broader for smaller stimuli when only one element was presented.
• A tendency to saccade towards a single element increased with stimulus size.

Conclusions:

• Perfect saccade averaging is not a default oculomotor response and is influenced by stimulus size.
• Increased uncertainty in localizing smaller stimuli may lead to merging of representations, affecting saccade targeting.
• The findings challenge a simple center of gravity model and suggest size-dependent mechanisms in visual target selection for eye movements.