Proceedings of the International Workshop on the Scientific approach to the Acheiropoietos Images, ENEA Frascati, Italy, 4-6 May 2010 Sight and brain: an introduction to the visually misleading images Daniele Murra , Paolo Di Lazzaro ENEA, UTAPRAD, Research Center of Frascati, via Enrico Fermi 45, 00044 Frascati (Rome, Italy) daniele.murra@enea.it Abstract The visual perception of the reality is a complex process involving eyes and brain. The result of this process is an individual response to the external stimulus which, in some cases, can differ person-to-person and, most important, can give a false representation of the reality. In this paper we discuss some aspects of the visual perception, focusing our attention to shapes and colors recognition. We also present a brief introduction to the physiology of the vision and a discussion about the potentially misleading use of the modern techniques for elaborating images. The link beteween the visual perception and the acheiropoietos images is commented. Keywords: visual perception, optical illusions, image processing, pareidolia 1. INTRODUCTION The visual perception is a very complex process which mainly involves the eye and the brain. The former behaves as an electronic device that collects the data while the latter elaborates the information. The result of this process is what we call “perception” and it is a subjective sensation, which can be different person- to-person. Sometimes, the aspect of many images can be modified by our brain, depending on many reasons linked to the internal mechanisms of our mind. The state of mind, the past experience, the particular context in which the object is inserted act as a filter that transforms the objective image in a personal sensation. In this paper, starting from a brief introduction on the basics of the human vision, we demonstrate how much is easy to be deceived by our senses thus achieving a wrong or, at least, a debatable conclusion. 2. THE HUMAN VISION The eye is the main organ of sight. It takes light from an external subject and sends electrical pulse to the brain [1]. The human eye is an excellent detector, and in spite of the impressive development of the electronic technology, it is still largely better than the most advanced CCD cameras available today [2]. The active region of the eye is the retina, a membrane that lies on its back and where the crystalline lens let the light rays converge to form an image. Unlike electronic devices, where there are three detectors per pixel (one for each primary color), on the retina there are two distinct detectors, namely the cones and the rods. The cones are sensitive to the colors and they are subdivided into three categories (red-, green- and blue- sensitive) similarly to the electronic pixel. On the contrary, the rods are sensitive only to the luminance and they are particularly active to low levels of light. So, the cones are responsible for the day-light vision (and they distinguish colors), while the rods are responsible for the vision in the darkness (and they distinguish shapes). Figure 1 shows the section of a human eye and an electron microscope image of the retina, where cones and rods are well visible. Figure 1. a) Schematic of the section of the human eye. b) A scanning microscope photo of the retina, where the rods (thin and long) and the cones (thicker and shorter, marked by arrows) are visible. a) b)