On the functional significance of the P1 and N1 effects to illusory figures in the notch mode of presentation

Abstract

The processing of Kanizsa figures have classically been studied by flashing the full "pacmen" inducers at stimulus onset. A recent study, however, has shown that it is advantageous to present illusory figures in the "notch" mode of presentation, that is by leaving the round inducers on screen at all times and by removing the inward-oriented notches delineating the illusory figure at stimulus onset. Indeed, using the notch mode of presentation, novel P1 and N1 effects have been found when comparing visual potentials (VEPs) evoked by an illusory figure and the VEPs to a control figure whose onset corresponds to the removal of outward-oriented notches, which prevents their integration into one delineated form. In Experiment 1, we replicated these findings, the illusory figure was found to evoke a larger P1 and a smaller N1 than its control. In Experiment 2, real grey squares were placed over the notches so that one condition, that with inward-oriented notches, shows a large central grey square and the other condition, that with outward-oriented notches, shows four unconnected smaller grey squares. In response to these "real" figures, no P1 effect was found but a N1 effect comparable to the one obtained with illusory figures was observed. Taken together, these results suggest that the P1 effect observed with illusory figures is likely specific to the processing of the illusory features of the figures. Conversely, the fact that the N1 effect was also obtained with real figures indicates that this effect may be due to more global processes related to depth segmentation or surface/object perception.

Figure 1. The notch mode of presentation.

Notches appear at onset and disappear at offset; full black disks remain on screen between trials. In half of the trials, notches were oriented inward so as to delineate a salient illusory square. In the other half of the trials, the control condition, notches were oriented outwards.

Figure 2. Results of Experiment 1.

(A) Identification of the left-sided electrodes used in the analyses. (B) Grand averaged VEPs (n = 15) elicited by the illusory square (dark blue) and the control figure (light blue). The black arrowhead identifies the P1 and the white arrowhead, the N1. The subtraction between the amplitudes of the two VEPs is also presented (thin gray line) to illustrate the magnitudes of the figure effect across the entire epoch. (C) Mean voltage maps illustrating the topographic scalp distribution of the VEP difference (subtractions) averaged within the time-windows of 70 to 130 ms (P1), 130 to 200 ms (N1), and 200 to 260 ms.

Figure 3. Results of Experiment 2.

(A) Grand averaged VEPs (n = 15) elicited by the central square (dark red) and the four squares (light red) accompanied with the subtraction data (thin gray line). (B) Mean voltage maps illustrating the topographic scalp distribution of the VEP difference (subtractions) averaged within the time-windows of 70 to 130 ms (P1), 130 to 200 ms (N1), and 200 to 260 ms.

Figure 4. Mean amplitudes of the figure effect (illusory square vs. control figure in Experiment 1 and central square vs. peripheral squares in Experiment 2) for the four subsets of electrodes.