More advanced visual prosthetics can be divided into three major groups. First, there are the devices that use either ultrasound or a camera to sample the environment ahead of an individual and render the results into either a series of sounds or a tactile display. This process is known as sensory substitution because the sense of hearing or touch is substituted for that of sight. From this substitution users are supposed to be able to discern the shape and proximity of objects in their path. The second major form is retinal enhancement. These sensors supplement functions of the retina by stimulating it with electrical signals, which are then converted to nerve impulses and transferred through the optic nerve to the brain. The third major category of visual prosthetic is a digital camera that samples an image and stimulates the brain directly with electrical signals, either by penetrating into or placing electrodes on the surface of the optic nerve or the visual cortex.