Vision and hearing
Seeing and hearing begin with specialised receptors that turn light and sound into brain signals.
The eye
Light enters through the cornea and pupil, is focused by the lens, and lands on the retina. The retina contains rods, which work in dim light and detect black and white, and cones, which detect colour and fine detail in bright light and cluster in the fovea. The optic nerve carries signals to the brain; where it leaves the eye there are no receptors, creating a blind spot.
Colour vision
The trichromatic theory says colour vision depends on three types of cones, sensitive to roughly red, green, and blue light. The opponent-process theory says colour is coded in opposing pairs, such as red-green and blue-yellow, which explains afterimages. Both theories are correct at different stages of the visual system.
The ear
Sound waves are collected by the outer ear, vibrate the eardrum, and are amplified by three tiny bones in the middle ear. In the inner ear, the fluid-filled cochlea contains hair cells that transduce vibrations into neural signals. Pitch depends on frequency and loudness on amplitude; the vestibular system in the inner ear also helps with balance.
Flashcard
Retinal receptors for dim light and black-and-white vision
Check your understanding
After staring at a red image and then looking at a white wall, you see a green afterimage. Which theory best explains this?