On the limited effect of stimulus boundaries on saccade metrics

How the endpoint of saccadic eye movements is determined out of many potential peripheral locations is a crucial issue in the field of vision. Models of saccade generation account for this seemingly selective process in terms of competitive interactions between populations of neurons that encode respectively for different saccade amplitudes and directions. However, these models do not specify which visual stimulus properties other than the relative location of the stimuli are involved and how these properties contribute to ultimately determine a single saccade endpoint. We addressed this issue by contrasting the respective contributions of the 2-D spatial extent of the stimuli and the location of their boundaries in a global-effect paradigm. Participants were presented a to-be-looked-at peripheral target stimulus with or without a less eccentric visually invariant distractor. The extent of the target stimulus was manipulated in either one or two dimensions, such that targets differed either by their 2-D spatial extent (small, medium, or large circle) or the location of their boundaries (circle vs. horizontal or vertical ellipse of medium size). Results showed that the distractor deviated the eyes away from the target with the deviation varying with the 2-D spatial extent of the target but not the location of its boundaries. This finding suggests that the spatial distribution of luminance contrast and/or the number of elementary features that compose the stimuli prevails over visual boundaries in specifying the saccade endpoint. Implications for models of saccade generation are discussed.