Saccades to sounds: effects of tracking illusory visual stimuli

In 10 normal human subjects, we studied the accuracy of memory-guided saccades made to the remembered locations of visual targets and sounds. During the time of stimulus presentation, subjects were smoothly tracking a projected laser spot that was moving horizontally across a tangent screen, sinusoidally +/-15 degrees at 0.25 Hz. In one set of experiments, the laser spot moved across a 40 degrees x 28 degrees random dot display that moved synchronously in the vertical plane; this induced a strong illusion that the trajectory of the laser spot was diagonal (variant of Duncker illusion). In control experiments, the laser spot moved across the same display, which was stationary. The visual targets and speakers were at six locations (range +/-15 degrees ) in the horizontal plane. Saccades made to the remembered locations of targets presented during background motion (illusion) were significantly (P < 0.05) more inaccurate than with the background stationary (control) in 9 of 10 subjects for lights and in 6 of 10 subjects for sounds. As a group, the median change in errors due to the Duncker illusion was approximately 2.5 times greater for visual compared with acoustic targets (P < 0.001). These findings are consistent with electrophysiological studies which have shown that neurons in the primate lateral intraparietal area (LIP) may respond to both visual and auditory targets and these neurons are also influenced by the Duncker illusion during programming of memory-guided saccades.