When most people think of "virtual reality", they probably imagine someone with a bulky helmet on, flailing her arms wildly at nonexistent objects. She might also be wearing tactile gloves. This is the type of virtual reality that was popular at shopping mall arcades in the early 1990s. When one of these machines showed up in the mall of my childhood hometown, it was considerably more expensive than any other video game, and yet the line (consisting mainly of teenagers) snaked throughout the mall. I never tried one of these machines (the line was too long and besides, I didn't have the pocket money) but I remember standing there watching the ceiling-mounted CRT screens that showed onlookers what the helmeted players were seeing. There were two "games": one was some sort of motorcycle simulation, and the other seemed to simply be a rather blank-looking "world" in which a person could walk around and view various 3D objects generated by the computer. The graphics were fairly primitive, consisting of simple shapes and perspective planes. Yet the idea that you could be walking while standing still, and looking at objects that didn't really exist, was quite exhilirating for those accustomed to the white-bread tranquility of deepest suburbia.

The history of virtual reality interweaves with the history of the computer itself. The 1950s and 60s saw computers employed as vast data-crunching engines; few imagined that the future held computers to be something of a practice arena for real life. A critical, but seemingly obvious necessity in the development of virtual reality was the idea of the interface -- a means for humans to interact in real-time with the electronics within the computer. Virtual reality represents the ultimate abstraction layer between man and machine; the very presence of the machine is muted by the interface. Our minds "know" that what we are experiencing is not real, yet our senses react in the same manner as they would to an actual event or situation. This concept explains why one of the first applications of VR-like technology was that of the flight simulator. If a pilot can learn how to control and maneuver an aircraft while s/he is safely on the ground, liability is reduced and the pilot has the opportunity to acquire real-life skills. Flight simulators give pilots the luxury of learning from their mistakes, mistakes that might be fatal in an actual airplane.

From the beginning, the difference between looking at a screen and interacting with a virtual environment has been a matter of immersion. Virtual reality systems started out as bulky head-mounted displays (HMDs), such as the 1968 monstrosity known as Ivan Sutherland's Sword of Damocles (which sounds like a good name for a heavy metal band). It was constructed by a Harvard student and incorporated a large mechanical sensing arm. Simulation hardware and software, then, have been in existence for quite some time. The term Virtual Reality was not coined until 1989, by computer scientist Jaron Lanier. Lanier used "virtual reality" to describe any interactive three-dimensional environment generated by a computer. Nowadays, most virtual reality systems are still mounted on helmets or headbands, but they are much smaller and lighter than their predecessors. LCD screens have been put to good use in virtual reality technology, owing to their lightness and relatively low power requirements.

In order to successfully create the illusion of a three-dimensional world-within- a-machine, it is important to understand the way human vision works. When we examine an object, our eyes move over its surface. A three-dimensional object can have one shape when viewed in a particular plane, and an entirely different shape when viewed in another. Virtual reality needs to "know" what the user's eyes are doing, and react accordingly. The process of gathering and responding to information about the position of a person's eyes and head is called tracking. The most basic method of tracking involves simply determining the position of the head. The virtual reality effect garnered from this method, however, can be somewhat distorted because people aren't necessarily looking exactly where their head is pointing. A more realistic effect results when the more accurate (but more expensive) method of eye tracking is used.

There are two main types of eye tracking: limbus tracking and video tracking. Limbus tracking employs an infrared LED whose beam is bounced off the surface of the eye. The reflections from the LED are picked up by a photodetector, and the eye's position can be determined by the intensity of the reflected light. Video tracking, on the other hand, acquires images of the eye and uses these images to determine the center of the pupil. Once again, however, an LED is reflected off the eye, resulting in a highlight. The computer uses the vector between the pupil and the corneal reflection to determine the angle of the eye at a given moment.

In addition to tracking a user's head and eye movements, it is also important for a VR system to provide adequate field of view and frame rate. In a sophisticated simulation, one might also want to consider auditory and tactile as well as visual effects. By using the same position tracking technology, a computer can determine how to make sounds seem to emanate from objects in the virtual environment, and how to make ambient sound seem realistic. Specialized virtual reality gear might also include gloves, boots or other clothing that applies force feedback to create the illusion of actually touching objects or experiencing vibrations. I'm sure the Internet porn empire is simply drooling, waiting for the full-body VR suits to make their debut.

A recent intriguing, if unanticipated, application of virtual reality technology has been in the world of psychiatry. Patients with severe phobias can now be exposed to their worst fear...be it of heights or of snakes... in a safe and less threatening manner. Aversion therapy has been around for years, but only recently have computers been called upon to create the stimuli. Another, perhaps even more innovative use of VR in the psychiatric world is that of using it to "teach" schizophrenics the difference between fantasy and reality. Common hallucinations, such as shadowy figures or "hearing voices" can be simulated; the patient then has to figure out whether this stimuli is being internally or externally generated.

In the world of entertainment, virtual reality is everywhere. From the literary works of William Gibson and Neal Stephenson to the much-hyped film The Matrix, the idea of a computer-generated universe has captured the collective imagination. In many science-fiction examples of virtual reality, technologies such as direct neural interface are used heavily. We don't yet have the ability to put a chip in a person's head that will create a perfectly realistic virtual environment, but I am almost certain someday we will.