Event-related potentials

Neural signature of Rotation. A cluster-based permutation analysis for the comparison of activity of trials on which “Rotation yes – Target no” was reported in contrast to trials on which “Rotation no – Target no” was reported, yielded a significant positive cluster (Cluster 1, test statistic = 7086, p < .001).

Cluster 1 covered the time range 230-553 ms after target onset. The spatial and temporal extent as well as the subdivision of Cluster 1 in five time ranges is depicted in an electrode-time matrix of Figure D1A (Appendix IV). This matrix visualized the activity differences of the two perceptions for the cells which were part of Cluster 1. The visualization of the results of Cluster 1 by ERPs and topographies (Figure 5.2) was separated into the five time ranges presented in the electrode-time matrix of Figure D1A (Appendix IV).

ERPs were averaged across electrodes defining Cluster 1 in the respective time range (Figure 5.2A). The amplitude of ERPs elicited by the perception of “Rotation yes – Target no” were higher compared to the amplitude of ERPs elicited by the perception of “Rotation no – Target no”. Topographies visualized differences between EPRs elicited by the two perceptions, “Rotation yes – Target no” and “Rotation no – Target no”, averaged across the time range of the particular subdivision (Figure 5.2B). The first subdivision of Cluster 1 covered the time range 230 – 295 ms and showed a large spatial distribution. The second time range (296 – 365 ms) depicted activity differences at fronto-parietal electrodes. The third time

5 | Neurophysiological evidence for the multidimensionality of metacontrast masking

range (366 – 438 ms) showed a lateralization on the right hemisphere in the area of fronto-parietal electrodes. A large spatial distribution was found at 439 – 516 ms, whereas activity differences of the fifth time range (517 – 553 ms) were limited to occipital-temporal electrodes over the left hemisphere.

Specificity of the neural signature of Rotation. In a second step a subsequent cluster-based permutation analysis examined whether Cluster 1 resulted also in a significant effect for the perception of a Target before Mask. The analysis yielded a significant effect (test statistic = 3672, p = .005). The amplitude of ERPs elicited by trials on which “Rotation no - Target yes” was reported exceeded the amplitude of ERPs elicited by trials on which

“Rotation no - Target no” was reported (Figure 5.2A). However, the activity difference between these two answers was smaller, compared to the original contrast of Cluster 1 between the answers “Rotation yes – Target no” and “Rotation no – Target no”, as also reflected in the topography maps (Figure 5.2C), especially in time ranges three, four and five.

In addition, an inverse effect was found with stronger activity for trials on which “Rotation no – Target no” was reported compared to trials on which “Rotation no – Target yes” was reported, mainly in the first time range (230 – 295 ms) at occipital electrodes and in the last time range (517 – 553 ms) at occipital-temporal electrodes.

The third analysis examined whether a difference between the perception of a Rotation and the perception of a Target before Mask could be found for Cluster 1. A cluster-based permutation analysis was performed by contrasting trials on which “Rotation yes - Target no”

was reported with trials on which “Rotation no - Target yes” was reported. The analysis yielded a significant effect (test statistic = 2628, p = .035). ERPs (Figure 5.2A) and topography maps (Figure 5.2D) showed a stronger activity for trials on which “Rotation yes - Target no” was perceived compared to trials on which “Rotation no - Target yes” was perceived. The strongest activity difference was visible at the latest time range (517 – 553 ms).

⁵¹⁷

– 553 ms Figure 5.2. Each column shows one of the five time ranges of Cluster 1: Contrast of the conditions “Rotation yes – Target no” and “Rotation no – Target no”.A Target-locked, grand averaged ERPs for three conditions: “Rotation yes – Target no” (red, solid line), “Rotation no – Target yes” (black, solid line), “Rotation no – Target no” (black, dashed line), averaged across electrodes defining Cluster 1 in the particular time range (marked by gray bars and displayed below the timeline). B-D Rows depict topography maps averaged across the particular time range. Squares indicate electrodes, not belonging to Cluster 1 (unfilled), belonging to Cluster 1 in any time range (black, filled) and belonging to Cluster 1 in the particular time range (black, filled with a white dot). Electrodes not belonging to Cluster 1 were masked.B Contrast between “Rotation yes – Target no” and “Rotation no – Target no”. C Contrast between “Rotation no – Target yes” and “Rotation no – Target no”. D Contrast between “Rotation yes – Target no” and “Rotation no – Target yes”.

439 – 516 ms366 – 438 ms296 – 365 ms230 – 295 ms

A B Rotation yes - Target no minus Rotation no - Target no C Rotation no - Target yes minus Rotation no - Target no D Rotation yes - Target no minus Rotation no - Target yes

5 | Neurophysiological evidence for the multidimensionality of metacontrast masking

777 – 836 ms 5.3. Each column shows one of the five time ranges of Cluster 2: Contrast of the conditions “Rotation no – Target yes” and “Rotation no – Target no”. A get-locked, grand averaged ERPs for three conditions: “Rotation yes – Target no” (red, solid line), “Rotation no – Target yes” (black, solid line), “Rotation no Target no” (black, dashed line), averaged across electrodes defining Cluster 2 in the particular time range (marked by gray bars and displayed below the eline). B-D Each row depicts topography maps averaged across the particular time range. Squares indicate electrodes, not belonging to Cluster 2 (unfilled), onging to Cluster 2 in any time range (black, filled) and belonging to Cluster 2 in the particular time range (black, filled with a white dot). Electrodes not onging to Cluster 2 were masked. B Contrast between “Rotation no – Target yes” m “Rotation no – Target no”. C Contrast between “Rotation yes – Target nd “Rotation no – Target no”. D Contrast between “Rotation no – Target yes” and “Rotation yes – Target no”.

724 – 776 ms654 – 723 ms607 – 653 ms525 – 606 ms

Rotation no – Target yes minus Rotation no – Target no otation yes – Target no minus otation no – Target no Rotation no – Target yes minus otation yes – Target no

Neural signature of Target before Mask. A cluster-based permutation analysis, comparing the activity of trials on which “Rotation no – Target yes” was reported with trials on which “Rotation no – Target no” was reported, yielded a significant negative cluster (Cluster 2, test statistic = -6528, p < .001). Cluster 2 covered the time range between 525 – 836 ms after target onset, as visualized in the electrode-time matrix (Figure D1B, Appendix IV). ERPs elicited by trials on which participants reported to perceive “Rotation no – Target yes” showed more negative amplitudes than ERPs elicited by trials on which “Rotation no – Target no” was reported (Figure 5.3A). Therefore, Cluster 2 which described the neural signature of a Target before Mask yielded less activity in conditions on which a Target before Mask was perceived compared to conditions on which a Target before Mask was not perceived. Accordingly, the topographies showed a negative activity differences (Figure 5.3B). The first (525 – 606 ms), second (607 – 653 ms) and third time range (654 – 723 ms) of Cluster 2 showed a large spatial distribution, with only slight differences in the respective topographies. The activity in the fourth time range (724 – 776 ms) was limited to occipital electrodes, similar to the fifth time range (777 – 836 ms) which showed activity differences at occipital-temporal electrodes over the right hemisphere.

Specificity of the neural signature of Target before Mask. The second subsequent cluster-based permutation analysis examined whether Cluster 2, found for the perception of a Target before Mask, also yielded a significant effect for the perception of a Rotation. This analysis did not yield a significant effect (test statistic = -769, p = .30), indicating that trials on which a Rotation was not perceived did not yield a significant lower activity compared to trials on which a Rotation was perceived. Descriptive, a difference between ERPs of the conditions “Rotation yes – Target no” and “Rotation no – Target no” were found in the last time range (777 – 836 ms) (Figure 5.3A). The topography maps (Figure 5.3C) showed for the first two time ranges slightly positive activity differences over the left hemisphere as well as slightly negative activity differences over the right hemisphere. Also the third (654 – 723 ms) and fourth time range (724 – 776 ms) showed an unsystematic pattern of activity differences, with some electrodes showing slightly positive and others showing slightly negative activity differences.

The third subsequent cluster-based permutation analysis examined whether significant activity differences between the perception of a Rotation and a Target before Mask could be found for Cluster 2. A significant difference of activity was found between trials on which

“Rotation no – Target yes” was perceived compared to trials on which “Rotation yes – Target no” was perceived (test statistic= -5470, p = .004). ERPs elicited by the perception of a Target

5 | Neurophysiological evidence for the multidimensionality of metacontrast masking

before Mask showed more negative amplitudes than ERPs elicited by the perception of a Rotation (Figure 5.3A). In accordance, topography maps depicted activity differences between those two perceptions, which decreased with increasing time range (Figure 5.3D).

5.5 Discussion