Hearing is mostly a mechanical sense. It converts physical movement into the electrical impulses that comprise the brain's language, transforming these vibrations into what we perceive as the world of sound. Hearing also involves the other senses, especially smell and taste, which help determine what is heard. Finally, hearing involves mental processes too, such as perception and memory. Hearing can be divided into two main categories: auditory and vestibular. The auditory system responds to sounds, while the vestibular system detects movement. Both systems work together to provide accurate location information for moving objects by using the inner ear. Humans can hear sounds from 20 Hz to 20 kHz, which covers the range of human hearing. Men are able to hear higher frequencies than women because of greater bone density in their skulls. People with normal hearing can hear differences between notes played simultaneously on a piano. Those who are deaf have no ability to detect sound above 15 dB(A). People who are blind rely solely on the tactile (touching) senses to navigate around their environment. However, some individuals are born with vision and are therefore able to see colors and shapes. These people are called congenitally blind. They may use other senses such as hearing or touch in addition to vision to guide them through life. The most common cause of blindness is age-related macular degeneration.