Short-term vestibulo-ocular adaptation: influence of context

Sensorimotor adaptation to destabilizing dynamics in weakly electric fish

Humans and other animals can readily learn to compensate for changes in the dynamics of movement. Such changes can result from an injury or changes in the weight of carried objects. These changes in dynamics can lead not only to reduced performance but also to dramatic instabilities. We evaluated the impacts of compensatory changes in control policies in relation to stability and robustness in Eigenmannia virescens, a species of weakly electric fish. We discovered that these fish retune their sensorimotor control system in response to experimentally generated destabilizing dynamics. Specifically, we used an augmented reality system to manipulate sensory feedback during an image stabilization task in which a fish maintained its position within a refuge. The augmented reality system measured the fish's movements in real time. These movements were passed through a high-pass filter and multiplied by a gain factor before being fed back to the refuge motion. We adjusted the gain factor to gradually destabilize the fish's sensorimotor loop. The fish retuned their sensorimotor control system to compensate for the experimentally induced destabilizing dynamics. This retuning was partially maintained when the augmented reality feedback was abruptly removed. The compensatory changes in sensorimotor control improved tracking performance as well as control-theoretic measures of robustness, including reduced sensitivity to disturbances and improved phase margins.

Does spectacle use lead to vestibular suppression?

BACKGROUND

Laboratory experiments indicate that changes in retinal image size result in adaptive recalibration or suppression of the vestibulo-ocular reflex. Myopia correction with spectacles or contact lenses also leads to retinal image size changes, and may bring about similar vestibulo-ocular reflex alterations.

METHODS

A hypothesis-generating preliminary investigation was conducted. In this cross-sectional study, findings of electronystagmography including bithermal caloric testing were compared between 17 volunteer myopes using spectacles or contact lenses and 17 volunteer emmetropes (with no refractive error).

RESULTS

Bilateral hypoactive caloric responses were demonstrated in 6 of 11 spectacle users, in 1 of 6 contact lens users and in 1 of 17 emmetropes. Hypoactive caloric responses were significantly more likely in spectacle users than in emmetropes (p < 0.01; relative risk = 9.3).

CONCLUSION

A significant proportion of myopes using spectacles have vestibulo-ocular reflex suppression, as demonstrated by the caloric test. This has implications for the interpretation of electronystagmography and videonystagmography results, and highlights spectacle use as a possible cause of vestibular impairment. Further corroboration of these findings is warranted, with more precise and direct vestibulo-ocular reflex tests such as rotational tests and the head impulse test.

Vergence-dependent adaptation of the vestibulo-ocular reflex

The gain of the vestibulo-ocular reflex (VOR) normally depends on the distance between the subject and the visual target, but it remains uncertain whether vergence angle can be linked to changes in VOR gain through a process of context-dependent adaptation. In this study, we examined this question with an adaptation paradigm that modified the normal relationship between vergence angle and retinal image motion. Subjects were rotated sinusoidally while they viewed an optokinetic (OKN) stimulus through either diverging or converging prisms. In three subjects the diverging prisms were worn while the OKN stimulus moved out of phase with the head, and the converging prisms were worn when the OKN stimulus moved in-phase with the head. The relationship between the vergence angle and OKN stimulus was reversed in the fourth subject. After 2 h of training, the VOR gain at the two vergence angles changed significantly in all of the subjects, evidenced by the two different VOR gains that could be immediately accessed by switching between the diverged and converged conditions. The results demonstrate that subjects can learn to use vergence angle as the contextual cue that retrieves adaptive changes in the angular VOR.

Vestibular findings in professional divers

The purpose of the present study was to investigate possible inner ear changes related to professional diving, by the documentation of auditory and vestibular function in 13 asymptomatic professional divers and 12 nondiver controls. A higher average pure tone hearing threshold, although of no clinical significance, was found in the study group (8.53 +/- 4.85 versus 6.67 +/- 3.54 dB hearing level, p = .04). In the vestibular evaluation, the smooth harmonic acceleration test phase leads for 0.01, 0.02, and 0.04 Hz were significantly lower in the divers (0.01 Hz, 38.46 degrees +/- 7.15 degrees versus 45.83 degrees +/- 9.02 degrees, p = .02; 0.02 Hz, 21.08 degrees +/- 5.19 +/- versus 25.17 degrees +/- 5.78 degrees, p = .05: 0.04 Hz, 12.38 degrees +/- 3.69 degrees versus 14.25 degrees +/- 3.14 degrees, p = .05). We suggest that the lower smooth harmonic acceleration phase values found in the professional divers, reflecting longer vestibulo-ocular reflex primary time constants and enhancement of the velocity storage mechanism, are the result of a habituation process that augments the low-frequency response of the canal-ocular system.

Functional operation of the balance system in daily activities

This article presents the anatomy and physiology of the balance system focusing on the daily use of the system in normal individuals to accomplish the major goals of the system. These goals include perceptions of movement and orientation, control of eye movements for clear visualization of the world, and control of upright stance during static and dynamic tasks. When discussing various pathologies and the assessment and treatment options available, the information provided sets the foundation on which the remainder of the articles are built.