## 4 Experimental Results
### Event-Related Potentials (ERPs)  
The neural fluctuation over the perception of images is shown in Figure 5, which show the averaged brain signals in the time domain, across all epochs, for channel P3. For an overview of all channels, see Figure A11 in Appendix F. Even though, the experiment was performed with 3 different blocks of visual stimulation, each having a different stimulation duration (block 1:1000 ms; block 2:750 ms; block 3:500 ms), the grand average responses are similar, see Figure A10 in Appendix F. Hence, the ERPs are visually investigated considering all blocks together. In more detail, Figure 5, shows the temporal evolution of the grand average ERP responses, complemented with their spatial evolution. The grey horizontal line marks the start of the trial period (0 ms) which lasts for 500 ms minimum, followed by the relaxation period (750 ms). When looking at the brain responses over time, we can easily observe 2 distinct groups of strong peaks, one at 300-600 ms, representing the well-known P200 and P300 components and another group later around 800-1200 ms, as an effect to the visual response over the non-informal grey image within the relaxation period, indicating prolonged cognition, with both groups of peaks being more prominent for the parietal area at P3 and P4 channels, which is responsible for cognitive reasoning. We observe the same latency and duration of the averaged ERP peaks for all 3 types of images perception, with differences in amplitude, such as:  
1. a slight decreased N200 peak for Uni images (at 250 ms), relating to visual perception [73];
2. a gradually increased amplitude already from the P200 (350 ms), as response to an increase in image complexity perception from Uni to Nat and Frac images perceptions, observed also spatially (see scalp plots in Figure 5) with increased activity in the parietal area (P3, P4 channels), highest for fractal images perception (3 \(\mu\)V);
3. even higher amplitude for the P300 (at 450 ms) towards 4 \(\mu\)V in the parietal area, as compared to P200.
4. the second group of peaks around 800-1100 ms, with similar amplitude and spatial distribution for P200 in the parietal area (at 850 ms, appearing 350 ms after the grey image presentation), followed by another peak (950 ms, 450 ms after the grey image presentation) with gradually increased amplitude for Uni, Nat and Frac images of up to (2-3 \(\mu\)V), focused in the right parietal area. This relates to an extended reasoning process, since participants stated that they were unintentionally still thinking over the image complexity, even after the stimulus interval, even though they were requested to only relax in the relaxation period.  
Moreover, the ERPs discriminations are highlighted by the signed \(r^{2}\) measure, as shown in Figure 6 for channel P3, and class groups (Nat-Uni, Frac-Nat) where a value of zero of the signed \(r^{2}\) indicates no correlation between the classes and a positive value indicates that the amplitude was larger for the first class in the group than for the second class and vice versa for negative values. We observe higher correlation over trials in time in the parietal area for Nat-Uni discriminability (Figure 6a) of up to \(4\times 10^{-3}\) within 300-400 ms, 800-900 ms, and 1150-1250 ms intervals, while for 500-600 ms the distribution is viewed across the entire brain (see scalp plots in Figure 6a). Similarly, the Frac-Nat discriminability in Figure 6b show higher parietal differences for the 300-400 ms, 500-600 ms, and 1150-1250 ms intervals. The signed \(r^{2}\) differences are higher between Nat and Uni, compared to Frac and Nat images perceptions.