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

Brodeur M, Bacon BA, Renoult L, Prévost M, Lepage M, Debruille JB - PLoS ONE (2008)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychiatry, Douglas Mental Health McGill University Institute, Montreal, Quebec, Canada.

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.

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Related in: MedlinePlus

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.
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pone-0003505-g002: 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.

Mentions: As it can be seen by looking at the waveforms (Figure 2), the P1 and N1 differences were difficult to separate because they occurred in adjacent time-windows. More importantly, the VEP differences seen in the N1 time window were not limited to the N1 deflection. They were part of a larger effect that lasted until the end of the epoch. Two strategies were employed to distinguish the P1, the N1 and the subsequent VEPs. The first was used to circumvent the fact that the magnitude of the VEP effect is sometimes difficult to estimate visually, particularly over the slopes of the deflections. It consisted of the simple subtraction of the VEPs to the control figure from the VEPs to the illusory square. The second strategy was to add a time-window (2 levels) factor to the previously described ANOVA. It was conducted to verify whether the scalp distribution of the figure effect within the P1 and the N1 time-windows differed significantly. The time-windows, 70 to 130 ms for the P1 and 130 to 200 ms for the N1, were those used to identify the P1 and N1 peaks. The same analysis was conducted between the N1 and a third time-window including the averaged VEPs within 200 to 260 ms. It could thus be verified whether the figure effect occurring at the peak of the N1 was distributed over the scalp like the effect following the peak and reaching its maximal amplitude after 200 ms (see the subtractions in Figure 2).


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

Brodeur M, Bacon BA, Renoult L, Prévost M, Lepage M, Debruille JB - PLoS ONE (2008)

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.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2567430&req=5

pone-0003505-g002: 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.
Mentions: As it can be seen by looking at the waveforms (Figure 2), the P1 and N1 differences were difficult to separate because they occurred in adjacent time-windows. More importantly, the VEP differences seen in the N1 time window were not limited to the N1 deflection. They were part of a larger effect that lasted until the end of the epoch. Two strategies were employed to distinguish the P1, the N1 and the subsequent VEPs. The first was used to circumvent the fact that the magnitude of the VEP effect is sometimes difficult to estimate visually, particularly over the slopes of the deflections. It consisted of the simple subtraction of the VEPs to the control figure from the VEPs to the illusory square. The second strategy was to add a time-window (2 levels) factor to the previously described ANOVA. It was conducted to verify whether the scalp distribution of the figure effect within the P1 and the N1 time-windows differed significantly. The time-windows, 70 to 130 ms for the P1 and 130 to 200 ms for the N1, were those used to identify the P1 and N1 peaks. The same analysis was conducted between the N1 and a third time-window including the averaged VEPs within 200 to 260 ms. It could thus be verified whether the figure effect occurring at the peak of the N1 was distributed over the scalp like the effect following the peak and reaching its maximal amplitude after 200 ms (see the subtractions in Figure 2).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychiatry, Douglas Mental Health McGill University Institute, Montreal, Quebec, Canada.

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.

Show MeSH
Related in: MedlinePlus