Form Perception
The purpose of this chapter is to explain the various ways to account for the capability to perceive form. The chapter includes a description of the notion of contours and a review of the factors influencing the perception of it. Then, the fundamental phy
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Form Perception
We could say that we live in a world where our retinas are constantly assaulted from everywhere. Thousands of potential stimuli in the immediate environment may reach our eyes at any moment. These various stimuli result from the interactions between the surface properties and those coming from light sources (intensities and wavelengths). Moreover, our environment is sometimes stable, sometimes not; sometimes, things are moving and sometimes, we are moving. Therefore, there is constantly an incredible variety of stimuli on the retina. Nevertheless, we extract from all this information something intelligible; moreover, this task is completed without effort. This remarkable efficiency is made possible by the functioning of some basic mechanisms described below.
6.1
Perception of Contours
We can extract a shape in the environment because it provides brightness variations. These variations are such that there are boundaries between objects. We know that there is somewhere a given object because we perceive delimitation, or an edge, between this object and its surroundings. We call this edge a contour. This contour could be considered the elementary unit of form perception. To fully realize the importance of a contour, just think about what happens during a snowstorm. When there is too much blowing snow, it becomes no longer possible to see anything, even when you try to keep your eyes open, because the field is evenly lit (in German, this phenomenon is referred to as a ganzfeld—i.e., complete field). If you want to experience a ganzfeld without waiting for the next snowstorm, simply try the following activity. Take two white plastic spoons or even the two halves of a white ping-pong ball, and draw a small but clearly visible colored line on the inside of the spoons or half balls. Then, just make sure to completely cover the eyes with spoons or half balls so that no light can enter. Keep your eyes open while fixating the inside line and avoid any eye movements.
© Springer International Publishing Switzerland 2016 S. Grondin, Psychology of Perception, DOI 10.1007/978-3-319-31791-5_6
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Form Perception
You need to maintain this fixation activity for several seconds so that the line remains at the same place on the retina. What happens after a few seconds (less than a minute)? If there was no movement of your eyes, the line disappears. For perceiving form, even just a single line, it takes brightness variations between this form and its environment. We clearly see the line at first, but eventually lose sight if we prevent the visual system from restoring the perception of a contour. In fact, the image never remains stable for long on the retina. The image on the retina keeps moving because there are always small eye movements called microsaccades. These small involuntary eye movements create variations in time on the receptors of the retina. What the experiment with the spoons or half balls teaches us is not only that it takes contour perception to see but also that it is necessary, for avoid
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