3.3 How we perceive things
3.3.1 The human perceptual system
In order to survive, all living things have evolved some sort of ability to sense or perceive the world around them. Even the humble amoeba is sensitive to light. Complex animals have intricate perceptual systems that respond to many different features of their environment – insects, despite their impressive eyes, are most sensitive to trails of chemicals; bats are blind to light but responsive to sonar pulses; dogs and pigs depend more on smell than vision for sensing the world.
We were all taught at school that humans have five senses: vision, hearing, smell, taste and touch (though some talk of balance as a sixth sense). Of these, vision and hearing are probably the ones we most depend on. The human visual system detects a narrow range of electromagnetic radiation we call light using sensitive nerve cells in the eye. Our auditory (hearing) system responds to the vibrations of the air which we recognise as sound. Our other senses also use specialised nerve cells to detect physical contact with the skin surface (touch), or concentrations of certain chemicals around us (smell and taste).