Throughout this book we have emphasised the applied, functional character of vision. Vision exists in humans and other animals because it is an aid to survival. The properties of vision are those needed to carry out significant activities such as seeking food, shelter, and sex, so that individuals can survive and reproduce. In the case of humans, significant activities are not necessarily restricted to the primary demands of survival and reproduction. We may seek a variety of sources of visual stimulation, many of which are cultural products nowadays. Some of these, like painted pictures, may be as old as the human race itself. Others, such as photographic or television images, are more recent. Writing and printed text are visual stimuli of particular complexity, because the shapes bear little visual relationship to the objects or meanings they convey.
Human survival seems to depend on intellectual as well as nutritional sustenance, and in the case of the former pictures appear to satisfy a universal appetite. The use of created patterns of visual stimulation as representations of other things is one of the most characteristic features of modern culture. Of course, pictures may sometimes be abstract, and appreciated for the shapes they contain, but it is very hard not to attribute meaning into anything we look at. Pictures can be both objects in themselves (a surface with marks on it like a painting or computer screen) and representations of objects. One consequence of this is that pictures are likely to be more complex stimuli to process than solid objects.
How might animals, such as our guide dog, respond to pictures? What does a dog see when it is looking at a television programme? Little experimental evidence exists, despite a plethora of anecdote. We might expect a response on the basis of stimulus generalisation; that is, in terms of the extent to which a pictorial pattern has common features with the thing it represents. Certainly, pictorial stimuli are often used to study animal vision, but these pictures are usually quite abstract, and the animal learns to associate a particular visual feature (such as the number of dots or the orientation or colour of lines) with a particular response. It is much harder to know if an animal's perception of a pictorial representation in any way resembles ours. There has been much research on the possibility of animal language (a different type of