Six extraocular muscles control rotations of each eye around three axes. Eye movements can be classified in several different ways. I categorize them into gaze-shifting, fixational, and gaze-stabilizing eye movements.
Figure 2.2: A cut-away illustration of the outer, middle, and inner ear, revealing the vestibular system [adapted from Martini (1998)].
2.2.3.1 Gaze-Shifting Eye Movements
Gaze-shifting eye movements enable people to track moving objects or look at different objects.
Pursuit is the voluntary tracking of a visual target. The purpose of pursuit is to stabilize a target on the fovea, in order to provide maximum resolution, and to prevent motion blur, as the brain is too slow to process foveal images moving faster than a few degrees per second. It is important to note that observers still perceive movement even when pursuing a single subject-relative cue (no other objects are visible), due to efference copy (Section 2.2.4).
Saccades are fast voluntary or involuntary movements of the eye that allow different parts of the scene to fall on the fovea. Saccades are the fastest moving external part of the body with speeds up to 1000◦/s (Bridgeman et al., 1994). Most saccades are about 50ms in duration (Hallett, 1986).
Saccadic suppression greatly reduces vision during and just before saccades, effectively blinding observers. Although observers do not consciously notice this loss of vision, events can occur during these saccades and observers will not notice. A scene can rotate by 8–20% of eye rotation during these saccades without observers noticing (Wallach, 1987). If eye tracking were available in a VE, then the system could perhaps (for the purposes of redirected walking) move the scene during saccades without users perceiving motion.
Vergence is the simultaneous rotation of both eyes in opposite directions in order to obtain or maintain binocular vision for objects at different depths. Convergence rotates the eyes towards each other. Divergence rotates them away from each other.
2.2.3.2 Fixational Eye Movements
Fixational eye movements enable people to maintain vision when holding the head still and looking in a single direction. These small movements keep rods and cones from becoming bleached. Humans do not consciously notice these small and involuntary eye movements, but without such eye movements the visual scene would fade into nothingness.
Small and quick movements of the eyes can be classified asmicrotremors (less than one minute of arc at 30-100 Hz) andmicrosaccades (about five minutes of arc at variable rates) (Hallett, 1986).
Ocular drift is slow movement of the eye, and the eye may drift as much as a degree without the observer noticing (May and Badcock, 2002). Involuntary drifts during attempts at steady fixation have a median extent of 2.5 arc minutes and have a speed of about 4 arc minutes per second (Hallett, 1986).
In the dark, the drift rate is faster. Ocular drift plays an important part in the autokinetic illusion described in Section 2.2.7.6. As discussed there, this may play an important role in judgments of position constancy.
2.2.3.3 Gaze-Stabilizing Eye Movements
Gaze-stabilizing eye movements enable people to see objects clearly even as their heads move.
Retinal image slip is movement of the retina relative to a visual stimulus being viewed (Stoffregen et al., 2002). The greatest potential source of retinal image slip is due to rotation of the head (Robinson, 1986). Two mechanisms stabilize gaze direction as
the head moves—thevestibular-ocular reflex (VOR) and theoptokinetic reflex (OKR). The VOR rotates the eyes as a function of vestibular input and occurs even in the dark with no visual stimuli. Eye rotations due to the VOR can reach smooth speeds up to 500 ◦/s (Hallett, 1986).
The OKR stabilizes retinal gaze direction as a function of visual input from the entire retina. If uniform motion of the visual scene occurs on the retina, then the eyes reflexively rotate to compensate. Eye rotations due to OKR can reach smooth speeds up to 80◦/s (Hallett, 1986). VOR is not perfect, and the OKR corrects for residual error.
Gain is the ratio of the velocity of eye rotation divided by the velocity of head rotation (Hallett, 1986; Draper, 1998). Gain due to the VOR alone (i.e., in the dark) is approximately 0.7. If the observer imagines a stable target in the world, gain increases to 0.95. If the observer imagines a target that turns with the head, then gain is suppressed to 0.3 to 0.5. If one adds OKR by providing a stabilized visual target, gain is close to 1 over a wide range of frequencies.
Gain also depends on the distance to the visual target being viewed. For an object at an infinite distance, the eyes are looking straight ahead in parallel. In this case, gain is ideally equal to one so that the target image remains on the fovea. For a closer target, eye rotation must be greater than head rotation for the image to remain on the fovea. These differences in gains are because the axis of the rotation of the head is different than the axis of rotation for the eyes. The differences in gain can quickly be demonstrated by comparing eye movements while looking at a finger held in front of the eyes versus an object further in the distance.
Nystagmus is a rhythmic and involuntary rotation of the eyes (Howard, 1986a). Researchers typically discuss nystagmus caused by rotating the observer continuously at a constant angular velocity. The eyes rotate to stabilize gaze direction. This rotation is called the slow phase of nystagmus. As the eyes reach their maximum amount of rotation relative to the head, a saccade snaps the eyes back to looking straight ahead relative to the head. This rotation is called the fast phase of nystagmus. This pattern repeats, resulting in a rhythmic movement of the eyes.
Pendular nystagmus occurs when one rotates her head back and forth at a fixed frequency. This results in an always-changing slow phase with no fast phase.
Little is known about how users rotate their eyes while turning their heads when latency-induced scene motion occurs in an HMD. Subjects in one latency experiment (Ellis et al., 2004) claimed to concentrate on a single feature in the moving visual field.
However, this was anecdotal evidence and could not be verified. A user’s eye gaze might remain stationary in space (because of the VOR) when looking for scene motion, resulting in retinal image slip, or might follow the scene (because of OKR), resulting in no retinal image slip. I postulate this varies by the amount of head motion, the type of task, the subject, etc. Eye tracking would allow an investigator to study this in detail.