Adaptation

Humans must be able to adapt to new situations in order to survive. People must not only adapt to an ever-changing environment, but also adapt to internal changes such as neural processing times and spacing between the eyes, both of which change over a lifetime. When studying perception in virtual environments, investigators must be aware of adaptation, as it may confound measurements.

Negative aftereffects are changes in perception of the original stimulus after the adapting

stimulus has been removed. These negative aftereffects provide the most common measure of adaptation (Cunningham et al., 2001).

2.2.9.1 Sensory and Perceptual Adaptation

Adaptation can be divided into two categories: sensory adaptation and perceptual adaptation (Wallach, 1987).

Sensory adaptation alters a person’s sensitivity to detect a stimulus. Sensitivity increases

or decreases over time—one stops detecting or starts detecting the original stimulus after a period of constant stimulus intensity. Dark adaptation is an example of sensory adaptation.

Perceptual adaptation alters a person’s perceptual processes. Welch (1986) defines

perceptual adaptation to be “a semipermanent change of perception or perceptual-motor

coordination that serves to reduce or eliminate a registered discrepancy between or within sensory modalities or the errors in behavior induced by this discrepancy”. VOR adaptation is an example of perceptual adaptation.

2.2.9.2 Light and Dark Adaptation

The sensitivity of the eye changes by as much as six orders of magnitude, depending on the lighting of the environment. The cones reach maximum dark adaptation in approximately 10 minutes after initiation of dark adaptation. The rods reach maximum dark adaptation in approximately 30 minutes after initiation of dark adaptation. Complete light adaptation occurs within five minutes after initiation of light adaptation.

Rods are relatively insensitive to red light. Thus, if red lighting is used, rods will dark adapt whereas cones will maintain high acuity.

Dark adaptation causes a person’s perception of stimuli to be delayed By presenting a relatively large and bright scene between trials, I keep subjects light-adapted. Alternately only red stimuli could be used to maintain dark adaptation.

2.2.9.3 Position-Constancy Adaptation

The compensation process that keeps the environment stable during head rotation can be altered by perceptual adaptation. Wallach (1987) calls this adaptation to be “adaptation to constancy of visual direction”, and I call it position-constancy adaptation. This adaptation corrects for the effect of eyeglasses that cause the stationary environment to move optically during head movements.

Wallach and Kravitz (1965) had subjects wear minifying glasses that caused the world to seemingly move with the direction of head turns. They found that over time subjects’ perceived motion of the environment during head turns subsided, resulting in apparent position constancy. After adaptation and removal of the optical device, subjects reported a negative aftereffect such that the real world seemed to move against the direction of their

head turns. Draper (1998) found similar adaptation for subjects in HMDs when the rendered field of view was intentionally modified to be different from the true field of view of the HMD. People can learn to perceive position constancy for different displacement ratios if there is a cue (e.g., glasses on the head or scuba-diver face masks (Welch, 1986)).

Position-constancy adaptation could be due to vestibular adaptation, eye-movement adaptation, or visual-field adaptation.

If vestibular adaptation is a cause, then sounds should be perceived to move in the same way the visual scene appears to move after position-constancy adaptation. Wallach and Kravitz (1968) tested if this was the case, but sounds remained spatially fixed after position- constancy adaptation. Thus, vestibular adaptation is not at work here.

If eye-movement adaptation is a cause, then the observer should adapt when tracking a small target, with a displacement ratio different than zero, even with a surrounding stationary pattern (with a displacement ratio equal to zero). Wallach and Canal (1976) indeed found this to be the case.

If visual-field adaptation is a cause, then the observer should adapt when when keeping their gaze stabilized on a stable target (with a displacement ratio equal to zero), even when when a surrounding visual pattern moves with some displacement ratio. Wallach and Canal (1976) indeed found this to be the case.

Thus, the position-constancy adaptation process is due to both eye-movement adaptation, or visual-field adaptation, but not due to vestibular adaptation.

2.2.9.4 Temporal Adaptation

Cunningham et al. (2001) provided behavioral evidence that humans can adapt to a new intersensory temporal relationship caused by delayed visual feedback. A virtual airplane was displayed moving downward with constant velocity on a standard monitor. Subjects attempted to navigate through an obstacle field by moving a mouse that controlled only left/right movement. Subjects first performed the task in a pre-test with a visual latency of 35 ms. Subjects were then trained to perform the same task with 200 ms of additional latency introduced into the system. Finally, subjects performed the task in a post-test with the original minimum latency of 35ms. The subjects performed much worse in the post-test than in the pre-test.

Toward the end of training, with 235msof visual latency, several subjects spontaneously reported that visual and haptic feedback seemed simultaneous. All subjects showed very strong negative aftereffects. In fact, when the latency was removed, some subjects reported the visual stimulus seemed to move before the hand that controlled the visual stimulus, i.e., a reverse of causality occurred, where effect seemed to occur before cause!

that sensorimotor adaptation to latency requires exposure to the consequences of the discrepancy. Subjects in previous studies were able to reduce discrepancies by slowing down their movements when latency was present, whereas in this study subjects were not allowed to slow the constant downward velocity of the airplane.

These results suggest HMD users may adapt to latency, thereby changing latency thresholds over time. If this is the case, the adaptation could cause latency thresholds to vary over time. I present reference scenes with zero effective latency between test scenes containing some latency to prevent latency adaptation from occurring. I also randomly vary latency between trials so that subjects do not have a constant latency to adapt to.