Recovery of the vestibulocolic reflex after aminoglycoside ototoxicity in domestic chickens.
Avian auditory and vestibular hair cells regenerate after damage by ototoxic drugs, but until recently there was little evidence that regenerated vestibular hair cells function normally. In an earlier study we showed that the vestibuloocular reflex (VOR) is eliminated with aminoglycoside antibiotic treatment and recovers as hair cells regenerate. The VOR, which stabilizes the eye in the head, is an open-loop system that is thought to depend largely on regularly firing afferents. Recovery of the VOR is highly correlated with the regeneration of type I hair cells. In contrast, the vestibulocolic reflex (VCR), which stabilizes the head in space, is a closed-loop, negative-feedback system that seems to depend more on irregularly firing afferent input and is thought to be subserved by different circuitry than the VOR. We examined whether this different reflex also of vestibular origin would show similar recovery after hair cell regeneration. Lesions of the vestibular hair cells of 10-day-old chicks were created by a 5-day course of streptomycin sulfate. One day after completion of streptomycin treatment there was no measurable VCR gain, and total hair cell density was approximately 35% of that in untreated, age-matched controls. At 2 wk postlesion there was significant recovery of the VCR; at this time two subjects showed VCR gains within the range of control chicks. At 3 wk postlesion all subjects showed VCR gains and phase shifts within the normal range. These data show that the VCR recovers before the VOR. Unlike VOR gain, recovering VCR gain correlates equally well with the density of regenerating type I and type II vestibular hair cells, except at high frequencies. Several factors other than hair cell regeneration, such as length of stereocilia, reafferentation of hair cells, and compensation involving central neural pathways, may be involved in behavioral recovery. Our data suggest that one or more of these factors differentially affect the recovery of these two vestibular reflexes. (+info)
Microstimulation of the lateral wall of the intraparietal sulcus compared with the frontal eye field during oculomotor tasks.
We compared the effects of intracortical microstimulation (ICMS) of the lateral wall of the intraparietal sulcus (LIP) with those of ICMS of the frontal eye field (FEF) on monkeys performing oculomotor tasks. When ICMS was applied during a task that involved fixation, contraversive saccades evoked in the LIP and FEF appeared similar. When ICMS was applied to the FEF at the onset of voluntary saccades, the evoked saccades collided with the ongoing voluntary saccade so that the trajectory of voluntary saccade was compensated by the stimulus. Thus the resultant saccade was redirected and came close to the endpoint of saccades evoked from the fixation point before the start of voluntary saccade. In contrast, when ICMS was applied to the LIP at the onset of voluntary saccades, the resultant saccade followed a trajectory that was different from that evoked from the FEF. In that case, the colliding saccades were redirected toward an endpoint that was close to the endpoint of saccades evoked when animals were already fixating at the target of the voluntary saccade. This finding suggests that the colliding saccade was directed toward an endpoint calculated with reference to the target of the voluntary saccade. We hypothesize that, shortly before initiation of voluntary saccades, a dynamic process occurs in the LIP so that the reference point for calculating the saccade target shifts from the fixation point to the target of a voluntary saccade. Such predictive updating of reference points seems useful for immediate reprogramming of upcoming saccades that can occur in rapid succession. (+info)
Oculomotor tracking in two dimensions.
Results from studies of oculomotor tracking in one dimension have indicated that saccades are driven primarily by errors in position, whereas smooth pursuit movements are driven primarily by errors in velocity. To test whether this result generalizes to two-dimensional tracking, we asked subjects to track a target that moved initially in a straight line then changed direction. We found that the general premise does indeed hold true; however, the study of oculomotor tracking in two dimensions provides additional insight. The first saccade was directed slightly in advance of target location at saccade onset. Thus its direction was related primarily to angular positional error. The direction of the smooth pursuit movement after the saccade was related linearly to the direction of target motion with an average slope of 0.8. Furthermore the magnitude and direction of smooth pursuit velocity did not change abruptly; consequently the direction of smooth pursuit appeared to rotate smoothly over time. (+info)
Effect of reversible inactivation of macaque lateral intraparietal area on visual and memory saccades.
Previous studies from our laboratory identified a parietal eye field in the primate lateral intraparietal sulcus, the lateral intraparietal area (area LIP). Here we further explore the role of area LIP in processing saccadic eye movements by observing the effects of reversible inactivation of this area. One to 2 microl of muscimol (8 mg/ml) were injected at locations where saccade-related activities were recorded for each lesion experiment. After the muscimol injection we observed in two macaque monkeys consistent effects on both the metrics and dynamics of saccadic eye movements at many injection sites. These effects usually took place within 10-30 min and disappeared after 5-6 h in most cases and certainly when tested the next day. After muscimol injection memory saccades directed toward the contralesional and upper space became hypometric, and in one monkey those to the ipsilesional space were slightly but significantly hypermetric. In some cases, the scatter of the end points of memory saccades was also increased. On the other hand, the metrics of visual saccades remained relatively intact. Latency for both visual and memory saccades toward the contralesional space was increased and in many cases displayed a higher variance after muscimol lesion. At many injection sites we also observed an increase of latency for visual and memory saccades toward the upper space. The peak velocities for memory saccades toward the contralesional space were decreased after muscimol injection. The peak velocities of visual saccades were not significantly different from those of the controls. The duration of saccadic eye movements either to the ipsilesional or contralesional space remained relatively the same for both visual and memory saccades. Overall these results demonstrated that we were able to selectively inactivate area LIP and observe effects on saccadic eye movements. Together with our previous recording studies these results futher support the view that area LIP plays a direct role in processing incoming sensory information to program saccadic eye movements. The results are consistent with our unit recording data and microstimulation studies, which suggest that area LIP represents contralateral space and also has a bias for the upper visual field. (+info)
Isodirectional tuning of adjacent interneurons and pyramidal cells during working memory: evidence for microcolumnar organization in PFC.
Studies on the cellular mechanisms of working memory demonstrated that neurons in dorsolateral prefrontal cortex (dPFC) exhibit directionally tuned activity during an oculomotor delayed response. To determine the particular contributions of pyramidal cells and interneurons to spatial tuning in dPFC, we examined both individually and in pairs the tuning properties of regular-spiking (RS) and fast-spiking (FS) units that represent putative pyramidal cells and interneurons, respectively. Our main finding is that FS units possess spatially tuned sensory, motor, and delay activity (i. e., "memory fields") similar to those found in RS units. Furthermore, when recorded simultaneously at the same site, the majority of neighboring neurons, whether FS or RS, displayed isodirectional tuning, i.e., they shared very similar tuning angles for the sensory and delay phases of the task. As the trial entered the response phase of the task, many FS units shifted their direction of tuning and became cross-directional to adjacent RS units by the end of the trial. These results establish that a large part of inhibition in prefrontal cortex is spatially oriented rather than being untuned and simply regulating the threshold response of pyramidal cell output. Moreover, the isodirectional tuning between adjacent neurons supports a functional microcolumnar organization in dPFC for spatial memory fields similar to that found in other areas of cortex for sensory receptive fields. (+info)
MR imaging of Dejerine-Sottas disease.
We report the MR findings in two patients with clinically and histologically proved Dejerine-Sottas disease. One patient had spinal involvement with multiple thickened and clumped nerve roots of the cauda equina; the second had multiple enlarged and enhancing cranial nerves. Although these findings are not specific for Dejerine-Sottas disease, they are suggestive of the diagnosis, which is further corroborated with history and confirmed with sural nerve biopsy and laboratory studies. (+info)
Stereotactic radiosurgery for cavernous sinus cavernous hemangioma--case report.
A 40-year-old female presented with cavernous sinus cavernous hemangioma manifesting as left abducens and trigeminal nerve pareses. Magnetic resonance imaging revealed a left cavernous sinus tumor. The tumor was partially removed. Histological examination of the specimen confirmed cavernous hemangioma. Radiosurgery was performed using the gamma knife. The tumor markedly decreased in size after radiosurgery and morbidity was avoided. Cavernous sinus cavernous hemangiomas may be difficult to treat surgically due to intraoperative bleeding and cranial nerve injury. Stereotactic radiosurgery can be used either as an adjunct treatment to craniotomy, or as the primary treatment for small cavernous sinus cavernous hemangioma. (+info)
Model for the translational vestibuloocular reflex (VOR).
The function of the translational vestibuloocular reflex (tVOR) and the angular vestibuloocular reflex (aVOR) is to stabilize images on the retina during translational and rotational motion, respectively. It has generally been assumed that these two reflexes differ in their central processing because they differ significantly in their primary afferent behavior and characteristics at the motor level. So far, models of the tVOR have focused on the type of processing that the primary afferent signal must undergo before reaching the neural integrator. Here, we propose a model that does not require any prefiltering. It is known that the eye plant requires signals in phase with velocity and position. We propose that the velocity signal is obtained directly from the neural integrator, whereas the position signal is obtained directly from the primary afferents synapsing onto the oculomotor nuclei. This design proved sufficient to simulate eye movements in response to translational motion. (+info)