Investigating the Physiological Role of Fractal Pupil Oscillations.

Fractals are formed by patterns that repeat across multiple size scales. They are found in natural structures (for example, trees, clouds, and mountains) and have also been generated by artists and mathematicians. Previously, the authors have shown that pupil size oscillates over time in a fractal manner when people view mathematically-generated fractal images. However, it was unclear if these fractal oscillations were induced by fractal variations in luminance as the eye scanned the fractal images or if these oscillations were instead a signature of a more general physiological response to viewing fractal patterns. In particular, pupil size is a well-established measure of relaxation, and previous skin conductance and EEG measurements have shown that specific fractal images induce relaxation. Here, we expand on the original study by including a larger range of types of viewed images, including distorted fractal images, non-fractal images, and uniform grayscale images. We show that fractal pupil oscillations are not limited to images displaying fractal luminance variations. We observe small changes in the oscillations' fractal dimension when the fractal dimension of the image is varied and identify a relationship between these two fractal dimensions and pupil size that is consistent with the oscillations serving as a novel indicator of viewer relaxation.