Anthropomorphic Visual Sensors

The tentative of reproducing a biological eye with artificial electronic devices has always had a major drawback. In fact there is an important difference between how a human eye sees the world and how a standard video camera does: while the common visual sensors generally have constant resolution on each part of the image, in their biological counterparts the picture elements are arranged in order to get a very high amount of information in the central part of the field of view (the so called fovea) and a gradually decreasing density of photoreceptors while approaching the borders of the receptive field. There is a practical motivation behind this evolutional choice: a human being needs both a high resolution in order to distinguish between the small details of a particular object for fine movements (the human eye has actually a maximum resolution of about 1/60 degrees) and, at the same time, a large enough field of view (i.e., about 150 degrees horizontally and about 120 vertically for the human eye) so to have a sufficient perception of the surrounding environment. With a constant resolution array of sensors, this two constraints would have increased the total number of photoreceptors to an incredibly high value, and the consequence of this would have been the need of other unrealistic features such as an optic nerve having a diameter of few centimeters (the actual human optic nerve diameter is about 1.5 mm), in order to transfer this amount of data, and a much bigger brain (weighting about 2300 kg, compared to about 1.4 kg of our brain) in order to process all this information, not considering the huge power requirements of such a big brain. Since a zooming capability would have implied a renounce to the simultaneity of the two features, the evolution has answered to the question on how to optimally arrange a given number of photoreceptors over a finite small surface. A lot of different eyes evolved with the disposition of the photoreceptors adapted to the particular niche. Examples of this diversity can be found in the eyes of insects (see, for example, [1] for a review) and in those of some birds that have two foveal regions to allow simultaneous flying and hunting ([2, 3]). In the human eye (Fig. 1.1) we have a very high density of cones (the color sensitive photoreceptors) in the central part of the retina and a decreasing density when we move towards the periphery. The second kind of receptors (the rods, sensitive to luminance), are absent in fovea, but they have a similar spatial distribution. In fact the cone density in the foveola (the central part of the fovea) is estimated at about 150–180,000 cones/mm2 (see Fig. 1.1). Towards the retinal periphery, cone density decreases from 6000 cones/mm2 at a distance of 1.5 mm from the fovea to 2500 cells/mm2 close to the ora serrata (the extremity of the optic part of the retina, marking the limits of the percipient portion of the membrane). Rod density peaks at 150,000 rods/mm2 at a distance of about 3–5 mm from the foveola. Cone diameter increases from the center (3.3 m at a distance of 40 m from the foveola) towards the periphery (about 10 m). Rod diameter increases from 3 m at the area with the highest rod density to 5.5 m in the periphery [4]. Since this sensor arrangement has been proven by the evolution to be an efficient one, we tried to investigate how this higher efficiency could be translated in the world of artificial vision. From the visual processing point of view we asked on one hand whether the morphology of the visual sensor facilitates particular sensorimotor coordination strategies, and on the other, how vision determines and shapes the acquisition of behaviors that are not necessarily purely visual in nature. Also in this case we must note that eyes and motor behaviors coevolved: it does not make sense to have a fovea if the eyes cannot be swiftly moved over possible regions of interest (active vision). Humans developed a sophisticated oculomotor apparatus that includes saccadic movements, smooth tracking, vergence, and various combinations of retinal and extra-retinal signals to maintain