## 5 Discussion, Conclusions and Future Work
In more detail, from the observers' point of view, half of the participants considered the structure of the natural images as more complex compared to the fractal ones, which might indicate more entangled structures for the natural images case. More, since the complexity decision was freely-open to participants, the thought processes and criteria used play an important role in reasoning, which may activate different parts of the brain, therefore grouping participants further based on reasoning will be a good choice. The findings are enforced by the fact that even there is no difference between the number of occurrences of the presented natural and fractal images, and similar rangein complexity degree according to CFD, a noticeable difference is still observed in the brain signal representations. Regardless of the cause that triggered the neural responses differentiation, good classification discrimination between Nat, Frac and the reference Uni are obtained. Despite the fact that the combined temporal-spectral classification did not provide higher performances, the complementary information can be taken into account by adapting the classifier on each participant, to the most discriminative features either in time or frequency, to take advantage of the physio-anatomical differences between individuals and different thought processes which produce distinct neural effects. Further, we plan to investigate in more depth the perception over the complexity degrees, to see whether different complexity levels within images can induce distinct brain responses. This should be investigated not only on grand average, but also within each participant, since each individual has its own perception over complexity, as noticed in this study. For a more in-depth view over perception, the future studies should consider more participants for an expansive overview (at least 15, as proved statistically [78]) and more channels to better capture the spatial activity in detail. Considering hardware, a system with active electrodes will be advantageous to better filter the noise and have better signal conductance, while targeting practical applications the WiFi property have to be kept and dry electrodes would be better suited [79]. Considering timing, maybe a longer Inter-Stimuli Interval (ISI) should be considered for the future studies to capture all reasoning processes steps, while extended thought processes over complexity prolonged also in the relaxation period, as seen in this study. Further the neural fluctuations in an overt scenario [80], where participants can freely scan an image, will be interesting to be investigated. For pointing out the fractal complexity itself, in a natural-synthetic textural scenario, integrating also fractal natural images and similar synthetic generated textures, will be a good comparison, to have similar complexity ranges, fractal structures and textural variations. Here, we considered natural non-fractal images for the moment, due to the limited availability of a standardized diverse data base of natural fractal images with same image captured conditions (e.g., light, angle, distance to object). For the future, we consider creating our own dataset. The higher activity in the later interval even after the end of the presented image, suggests a complex and a prolonged continuous background process which may also indicate a delay in decision making. This aspect should be captured in the trial-by-trial variability and separately between subjects by looking over the latency and duration of the potentials. If this is the case, ERPs should elicit late positivity with high amplitude [81] and ongoing negative ERP over the prefrontal cortex suggesting indecisiveness [82]. Since natural images represent recognizable structures and objects, as compared to unidentified forms for the synthetic fractal images, it might infer different reasoning mechanisms. In neuroscience, it is suggested that the sensory cortex may have adapted to statistical regularities and therefore automatically relax, reducing attention [83].