Signal transduction, in a sensory processing sense, is the conversion of energy into a neural signal. It occurs in receptor cells located in sensory organs such as the ears, eyes, and hands. Receptor cells are responsive to certain types of energy, but not others. In the cochlea (inner ear) hair cells located in the basilar membrane have stereocilia, which are hair-like structures that touch the tectorial membrane. Sound vibration causes hair displacement and opens mechanically gated ion channels, which causes the cells to depolarize and release neurotransmitters. These cells do not fire action potentials. There are four different types of touch receptors: pain, touch, vibration, and stretch. These can be found subcutaneously all around the body. Each touch receptor type has a distinct pathway to the brain. The visual system detects both brightness and contrast. Photoreceptors perform signal transduction. There are two types of photoreceptors: scotopic (rods), which work in dim light, and photopic (cones), which govern vision of colors. The visual pathway crosses sides at the optic chiasm, so the right visual field is processed in the left occipital lobe, and vice versa.