Evolutionary Approach: Perceptual Adaptations

What is Perception For?

Because philosophy was such an important part of the history of perception, it is often said that the purpose of perception is to gain knowledge. But long before there were philosophers, people had to move about in space without bumping into things, decide what is edible, swallow food without choking, and find their way home. Thus, perception is first of all for action. For example, we are much better at moving about in space and avoiding objects than we are at estimating our distance from those objects.

We could avoid collision with objects by estimating our distance from them and our speed of approach. Instead, we do something much simpler: Because the image on the retina grows at a rapidly increasing rate as we approach an object at a constant speed, we are able to avoid collision by stopping when the rate of expansion reaches a threshold. This simple method is used by pilots landing a helicopter, pelicans diving for fish, and babies blinking at looming objects.

Perception and Fitnes

An evolutionary approach leads us to consider the energy requirements of perception. We note that more than half of the human brain is devoted to perception, and the brain consumes about 25% of our metabolic resources. Thus, perception must make a great contribution to our fitness; otherwise, we would spend that energy on some other resource, such as muscles. Humans are properly [Page 406]considered perceptual generalists, in that our senses respond to many kinds of information. Nevertheless, we devote metabolic resources to those sources of information that are most useful to us, and ignore others. Thus, we do not respond to ultraviolet light and are relatively poor at echo-location, which is judging objects and their distances by sounds reflected from those objects. Similarly, sessile animals (those that don't move), such as sponges, lack eyes altogether. A remarkable example of this principle involves sexual dimorphism, where with at least one animal, the insect stylops, the winged male has eyes, but the parasitic female is sightless.

Sensory Ecology

Consideration of the sources of energy available in the world enables us to understand what the senses were designed to do. Vision, for example, responds to electromagnetic energy, which is abundant in the daytime, travels extremely fast in a straight line, and is little affected by air. Still, we use only a small part of the electromagnetic spectrum. Wavelengths longer or shorter than the visible range travel through objects, and so would be less useful. In addition, shorter wavelengths tend to be filtered out by the atmosphere, so there is not much available to be used. Further, shorter wavelengths, such as ultraviolet, X-rays, and gamma rays, tend to be damaging to tissue. So it is not surprising that the eye contains pigments that filter out near-visible ultraviolet rays, which we could otherwise see.

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