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PubMedDate not supplied

Effect of incisions in the brainstem commissural network on the short-term vestibulo-ocular adaptation of the cat.

This study was intended to test the adaptive plasticity of the vestibulo-ocular reflex before and after either a midsagittal or parasagittal incision in the brainstem. Eye movements were measured with the electromagnetic search coil technique during the vestibulo-ocular reflex (VORD) in the dark, the optokinetic reflex (OKN), and the visuo-vestibular adaptive training procedure. Two types of visual-vestibular combined stimulation were applied by means of low frequency stimuli (0.05 to 0.10 Hz). In order to increase or decrease the VORD gain, the optokinetic drum was oscillated either 180 degrees out-of-phase or in-phase with the vestibular stimulus turntable. This "training" procedure was applied for 4 hours. Initial measurements of the VORD were normal with a mean gain value of 0.92 +/- 0.08. After 4 hours of "training" with the out-of-phase condition (180 degrees), VORD gain reached mean values of 1.33 +/- 0.11 (n = 6 cats). In the in-phase combination, the mean VORD gain decreased from 1.0 to 0.63 +/- 0.02 (n = 2 cats). No significant change of VORD phase was found in any of the cats. Midsagittal or parasagittal pontomedullary brainstem incisions were performed in 4 cats. Recovery of the VOR was tested on the 2nd, 7th, and 30th day after operation. After the 30th day, recovery of the VORD gain stabilized at about 66% of the initial preoperative value. At this stage of the recovery, the optokinetic response (OKN) of the midsagittal-lesioned cats was practically normal: in the parasagittal-lesioned cats, the postoperative OKN responses were asymmetric. After stabilization of recovery, lesioned cats were trained with the same adaptation procedure. Although the direct effect of the visuo-vestibular combined stimulation during the training was still operative in all lesioned cats, the adaptive plasticity was completely abolished by the lesions. These results suggest that the commissural brainstem network may play a crucial role in the acquisition of the forced VOR adaptation.

Adaptation, Ocular↗
PubMed1998

Short-term vestibulo-ocular adaptation: influence of context.

A number of mechanisms and strategies are used to help an individual compensate for loss of labyrinthine function. One important example is the ability to produce a preplanned motor response that anticipates the motion of the head and so compensates for it. Closely tied to this phenomenon is the gating, in or out, of a learned response on the basis of the context in which it must occur. This issue is particularly relevant to designing programs of physical therapy that optimize performance for natural behavior. Here we discuss a model of short-term vestibulo-ocular adaptation-adjustment of vestibulo-ocular phase (timing)-and how it can be used to study context-dependent vestibulo-ocular learning. We will show how vestibulo-ocular phase can be adjusted by selectively altering the common velocity-to-position ocular motor neural integrator for one type of eye movement (vestibular) and not for another (saccades), or for one type of head movement (sinusoidal) and not for another (step). These results are another example of the remarkable flexibility of the vestibulo-ocular adaptive mechanism and further show that the fundamental process of integration for eye movements can be modified according to the pattern of afferent information.

Adaptation, Physiological↗
PubMed1995

Cerebellar nitric oxide is necessary for vestibulo-ocular reflex adaptation, a sensorimotor model of learning.

1. Nitric oxide (NO) production in the nervous system has been implicated in cellular mechanisms of learning and memory. Our study investigates an in vivo sensorimotor model of learning. It demonstrates that a localized vestibulocerebellar injection of the NO synthase inhibitor, L-NG-monomethyl-arginine (L-NMMA), which specifically blocks NO production, inhibited the acquisition of adaptive vestibulo-ocular reflex (VOR) gain increases but not gain decreases in the goldfish. 2. Restoration of NO production by concomitant administration of L-arginine (the substrate for NO synthase) and L-NMMA suppressed the inhibitory effect of L-NMMA on adaptive gain increases. 3. This effect of L-NMMA was stereospecific because injection of D-NMMA did not suppress adaptive VOR gain increases. 4. Injection of L-NMMA after VOR adaptation had no effect on retention, failing to alter the postadaptive recovery after a VOR gain increase. 5. In conclusion, acquisition of adaptive VOR gain increases are affected by cerebellar NO inhibition. However, because gain decreases are not, they may involve either non-NO cerebellar or extracerebellar mechanisms. In addition, different processes for acquisition and retention of gain increases may be operating, because inhibition of cerebellar NO affects the acquisition but not the retention phase.

Adaptation, Ocular↗
PubMed1994

Short-term vestibulo-ocular reflex adaptation in humans. I. Effect on the ocular motor velocity-to-position neural integrator.

We investigated the effect of short-term vestibulo-ocular reflex (VOR) adaptation in normal human subjects on the dynamic properties of the velocity-to-position ocular motor integrator that holds positions of gaze. Subjects sat in a sinusoidally rotating chair surrounded by an optokinetic nystagmus drum. The movement of the visual surround (drum) was manipulated relative to the chair to produce an increase (x 1.7 viewing), decrease (x 0.5, x 0 viewing), or reversal (x (-2.5) viewing) of VOR gain. Before and after 1 h of training, VOR gain and gaze-holding after eccentric saccades in darkness were measured. Depending on the training paradigm, eccentric saccades could be followed by centrifugal drift (after x 0.5 viewing), implying an unstable integrator, or by centripetal drift [after x 1.7 or x (-2.5) viewing], implying a leaky integrator. The changes in the neural integrator appear to be context specific, so that when the VOR was tested in non-training head orientations, both the adaptive change in VOR gain and the changes in the neural integrator were much smaller. The changes in VOR gain were on the order of 10% and the induced drift velocities were several degrees per second at 20 deg eccentric positions in the orbit. We propose that (1) the changes in the dynamic properties of the neural integrator reflect an attempt to modify the phase (timing) relationships of the VOR and (2) the relative directions of retinal slip and eye velocity during head rotation determine whether the integrator becomes unstable (and introduces more phase lag) or leaky (and introduces less phase lag).

Adaptation, Physiological↗
PubMed1992

Disconjugate ocular motor adaptation in rhesus monkey.

We report a model for inducing disconjugate, orbital-position dependent, ocular motor adaptation in the rhesus monkey. Animals wore a combination of laterally-displacing prisms placed in front of one eye calling for a discrete change in ocular alignment when the eyes reached particular orbital positions. After wearing the prism combination the animals developed adaptive changes both in static alignment during fixation and in dynamic alignment during eye movements. These changes persisted with only one eye viewing and so became independent of the immediate presence of disparity cues. There were, however, imperfections in the adaptive responses; the changes in the innervation were gradual across the prism edge, not abrupt as required. This finding may reflect inherent limitations in the capability for disconjugate adaptation.

Adaptation, Ocular↗
PubMed1976

A study of dark adaptation in ocular hypertensives.

Previous studies of dark adaptation in glaucoma have been hampered by lack of standardization and poor definition of clinical categories. A new method of testing has been used on ocular hypertensives. A modified Goldmann/Weekers adaptometer and an 11 degree centrally fixated test patch were used to obtain two dark adaptation threshold curves for each subject -- one for blue-green and one for yellow. Results indicate that ocular hypertensives with no field loss and normal fundi have impaired normal subjects. The differences in the thresholds for blue-green were especially noticeable with the blue-green showing the greater difference. The 2.8 minute difference in mean curve cross-over time between normal and ocular hypertensive was also found to be significant. It appears that ocular hypertensives have impaired rod and cone functions, as well as deficient rod-cone interaction.

Animals↗
PubMed2003

Torsional and horizontal vestibular ocular reflex adaptation: three-dimensional eye movement analysis.

This study used visual-vestibular conflict to effect short-term torsional and horizontal adaptation of the vestibulo-ocular reflex (VOR). Seven normal subjects underwent sinusoidal whole-body rotation about the earth-vertical axis for 40 min (+/- 37 degrees/s, 0.3 Hz) while viewing a stationary radial pattern fixed to the chair (x0 viewing). During adaptation and testing in darkness, the head was pitched either up or down 35 degrees to excite both the horizontal and torsional VOR. The eyes were kept close to zero orbital elevation. Eye movements were recorded with a dual search coil in a three-field magnetic system. VOR gain was determined by averaging peak eye velocity from ten cycles of chair oscillation in complete darkness. The gain of the angular horizontal VOR (response to rotation about the head rostral-caudal axis) was significantly reduced after training in both head orientations. Angular torsional VOR gain (head rotation about the naso-occipital axis) was reduced in both head orientations, but this reached statistical significance only in the head down position. These results suggest that torsional and horizontal VOR gain adaptation, even when elicited together, may be subject to different influences depending upon head orientation. Differences between head up and down could be due to the relatively greater contribution of the horizontal semicircular canals with nose-down pitch. Alternatively, different VOR-adaptation processes could depend on the usual association of the head down posture to near viewing, in which case the torsional VOR is relatively suppressed.

Adaptation, Physiological↗