Many times, people ask why is eye tracking useful beyond the obvious applications of facilitating research and providing user interface for people with disabilities.
One interesting application is in using eye tracking to minimize head tracking latency. Consider the following graph:
The graph shows the position of an eye (black line) and the position of the head (red line), provided in degrees over time. Let's look at some areas of this graph:
- From about 6.5 seconds to 6.7 seconds, we see rapid eye movement from about -5 to +25 degrees. During this period, the head did not move.
- From 6.7 to about 7 seconds, we see the head moving and, in the same time, the eye moving in the opposite direction. Notice that the sum of the head position and eye position is approximately constant throughout this period.
- From 7 to 8 seconds, both the eye and the head are stationary
- From 8 to about 8.2 seconds, the eye moves in the opposite direction
- From 8.2 to about 8.5 seconds, the head follows and the eye reverses direction
What is happening? It turns out that eye movements precede head movements. When the body wants to look in a certain direction, the eye jumps ahead (to a direction over +20 or under -20 relative to the 'straight ahead' position). Then, the head starts to follow while the eye compensates with a movement to the other direction so that the gaze direction (head orientation + eye orientation) stays the same. During the head movement, the eye remains locked on the same target.
Why is this useful? It is useful because significant eye movements signal the intent to move the head. If we wish to minimize the latency of sensing head movements, it might be very useful to use the hints provided by the eye. It shows us that a large movement is coming, and we can use this information in anticipating head movement, and thus reduce latency.