The visual sensing system in humans is very well developed. The make-up of visual stimuli is well understood,
and display technology can produce extremely expressive stimuli, whether iconic, textual, or graphical. However,
one aspect that limits its general usage is the need for the user to attend to (i.e., look at) the visual
stimulus. If the user is looking in a different direction, or is preoccupied with another visual task, stimuli
can be missed. Contrast this with the audio channel, which can receive stimuli from any direction. A positive
aspect of vision is that the stimulus is external to the body, so display of the stimuli is non-intrusive. In
contrast, the olfactory and gustatory senses require stimuli to enter the body in order to be sensed. Visual
stimuli are generated by combining pixels of varying hue, saturation, and intensity. The external nature of the
stimulus allows both public and private displays to be used, depending on the target application. There is a
range of pathologies that can affect visual acuity. The more-common cases of near- and farsightedness are
routinely corrected optically. Aging can also degrade the visual sense.