ABSTRACT

In Chapter 1 we introduced the example of the dog guiding a blind human across the road. The guide dog must perceive its environment geocentrically in order to behave in the manner it does; it must respond to the edge of the pavement, the width of the road, and to the approaching vehicles in much the same way as a sighted human would. Both guide dogs and sighted humans respond to objects in terms of their visual locations and dimensions in threedimensional space, rather than the projective aspects of these (the locations and dimensions on the retina). That is, objects are normally seen as having constant dimensions despite changes in the retinal projections of their sizes and shapes. This is called perceptual constancy, and it is the prerequisite of more complex perception like recognition.