THE SPATIAL SIGNAL FOR SACCADIC EYE-MOVEMENTS EMPHASIZES VISUAL BOUNDARIES

By investigating the visual processing involved when saccades are made to newly appearing targets, we show that this processing is significa ntly nonlinear and that texture boundary information predominates. We used the global, or center-of-gravity, effect whereby a saccadic eye m ovement directed to a target consisting of a pair of elements has an a mplitude intermediate between that of saccades directed to the individ ual elements. We measured the effect using target elements with differ ent visual characteristics, including phase-reversal checkerboard targ ets that had the same space-average luminance as the background. The c ontribution to the center-of-gravity calculation was used to measure r elative salience. We found that positive and negative contrast element s contribute almost equal weightings. Thus, salience, assessed in this way, is a highly nonlinear function of luminance. The salience of che ckerboard targets was found to decrease as check size was decreased an d increase as the overall size of the target was increased. Checkerboa rds with an empty center were as effective as were full checkerboards, showing the importance of boundaries in the salience signal.