Alternative mode of presentation of Kanizsa figures sheds new light on the chronometry of the mechanisms underlying the perception of illusory figures.

ABSTRACT

The mechanisms responsible for the perception of illusory modal figures are usually studied by presenting entire Kanizsa figures at stimulus onset. However, with this mode of presentation, the brain activity generated by the inducers (the 'pacmen') is difficult to differentiate from the activity underlying the perception of the illusory figure. Therefore, in addition to this usual presentation mode, we used an alternative presentation mode. Inducer disks remained permanently on the screen and the illusory figure was induced by just removing the notches from the disks. The results support the heuristic value of this alternative mode of presentation. The P1 deflection of the visual evoked potentials (VEPs) was found to be greater for the illusory modal figure than for its control and for an amodal figure. This modulation is one of the earliest direct evidences for a low-level processing of illusory forms in the human brain. Meanwhile, larger N1s were obtained for the control figures than for the illusory figures in the notch mode of presentation. While this new type of N1 modulation could shed some light on the stage of processing indexed by this deflection, several propositions are put forward to account for the P1 and N1 variations found.