In visual fractals the same patterns repeat at different scales. For illustrations of fractals, see https://en.wikipedia.org/wiki/Fractal#fractals_in_nature.
Taylor and Spehar report that seeing moderately complex fractals reduces stress: “Humans are continually exposed to the rich visual complexity generated by the repetition of fractal patterns at different size scales. Fractals are prevalent in natural scenery [for example]. . . . we . . . investigate the powerful significance of fractals for the human visual system. In particular, we propose that fractals with midrange complexity (D = 1.3–1.5 measured on a scale between D = 1.1 for low complexity and D = 1.9 for high complexity) play a unique role in our visual experiences because the visual system has adapted to these prevalent natural patterns. . . . the visual system processes mid-D fractals with relative ease. This fluency optimizes the observer’s capabilities (such as enhanced attention and pattern recognition) and generates an aesthetic experience accompanied by a reduction in the observer’s physiological stress levels.”
Richard Taylor and Branka Spehar. 2016. “Fractal Fluency: An Intimate Relationship Between the Brain and Processing of Fractal Stimuli.” In A. Di Ieva (ed.) The Fractal Geometry of the Brain, Springer: New York, pp. 485-496.