Shapes, surfaces and saccades

Saccadic localization of spatially extended objects requires the computation of a single saccadic landing position. What representation of the target guides saccades? Saccades were examined for various targets composed of dots to determine whether landing position corresponded to the center-of-gravity (average location) of the dots, the center-of-area of the shape, or the symmetric axis. Targets were composed of dots configured as outline drawings of circles, ellipses, cardioids, wiggly lines, or amorphous blobs. In some cases, dot spacing was varied, extraneous dot clusters were superimposed, or different distributions of dots inside the boundary were added. Quasi-random dot clusters without a well-defined contour were also studied. Instructions were to look at the target as a whole, and keep latency long enough to avoid compromising accuracy. Saccades landed with a high level of precision (S.D.s 7-10% of target eccentricity) near the center-of-area of the target shape, rather than at the center-of-gravity of the dots or on the symmetric axis. Landing position was unaffected by the spacing of dots along the boundary, the addition of dots within the boundary, or the addition of the extraneous dot clusters. When the target was a cluster of quasi-random dots, saccades landed closer to the center-of-area of the implied surface than to the average location of the dots. Overall, the positions of individual dots were important only insofar as the dots affected overall target shape. The results show that a representation of target shape guides saccades, rather than a more primitive representation of individual elements within the attended region.