Effects of initial eye position on saccade-related behavior of abducens nucleus neurons in the primate.

Previous work suggests that when the eye starts at different orbital initial positions (IPs), the saccade control system is faced with significant nonlinearities. Here we studied the effects of IP on saccade-related firing of monkey abducens neurons by either isolating saccade variables behaviorally or applying a multiple linear regression analysis. Over a 50 degrees range of IPs, we could select 10 degrees horizontal saccades with identical velocity profiles, which would require identical control signals in a linear system. The bursts accompanying ipsiversive saccades for IPs above the threshold for steady firing were quite similar. The excess burst rate above steady firing was either constant or decreased with ipsiversive IP, and both the number of excess spikes in the burst and burst duration were nearly constant. However, for ipsiversive saccades from IPs below threshold, both peak burst rate (6.82 +/- 1.38 spikes.s(-1).deg(-1)) and burst duration (0.67 +/- 0.28 ms/deg) increased substantially with ipsiversive IPs. Moreover, the pause associated with contraversive saccades shortened considerably with ipsiversive IPs (mean 1.2 ms/deg). This pattern of results for pauses and for bursts below threshold suggests the presence of a significant nonlinearity. Abducting saccades are produced by the net force of agonist lateral rectus (LR) and antagonist medial rectus (MR) muscles. We suggest that the decreasing force in the MR muscle with IPs in the abducting direction requires a more vigorous burst in LR motoneurons, which appears to be generated by a combination of saturating and nonsaturating burst commands and the recruitment of additional abducens neurons.

Vertical vestibular responses to head impulses are symmetric in downbeat nystagmus.

BACKGROUND: Downbeat nystagmus (DBN), a frequent ocular motor sign often caused by cerebellar lesions, is a fixational nystagmus with fast phases directed downward. Its precise etiology is not known. One hypothesis is that it is caused by a central imbalance of pathways of the vertical vestibulo-ocular reflex (VOR). Such an imbalance would cause not only ocular drift but also deficient and asymmetric vertical VOR responses. OBJECTIVE: To test this hypothesis, the authors analyzed the functionally relevant VOR responses to head impulses in the roll, pitch, and yaw planes. METHODS: Head and eye movements were measured with the search-coil method in 10 patients with DBN and 10 age-matched control subjects. RESULTS: Analysis revealed no gain difference in patients with DBN compared with control subjects. Specifically, upward and downward VOR responses in patients with DBN in the pitch plane were symmetric. CONCLUSIONS: These findings do not support the hypothesis of a vertical VOR imbalance and put into question the view that DBN is a central vestibular syndrome in the sense of vestibular dysfunction. Although DBN possibly involves vestibulocerebellar pathways, in the patients that we studied, DBN did not affect the immediate VOR responses in the high-frequency range that corresponds to natural head movements.

[Nocardia farcinica: life-threatening chorioiditis under systemic immunosuppression]. / Nocardia farcinica: Schwere Chorioiditis mit lebensbedrohlicher Generalisierung unter systemischer Immunsuppression.

BACKGROUND: Immunosuppressed patients occasionally suffer from a multifocal infection with Nocardia. It is important to distinguish Nocardia farcinica from Nocardia asteroides, because of different sensitivity against antibiotics. PATIENT AND METHODS: A 40-year-old patient with polycystic renal disease successfully underwent a kidney transplantation without complications. Immunosuppression consisted of: corticosteroids, azathioprin and ciclosporin A. Ten weeks later he developed acute choroiditis with consecutive retinal detachment and neovascular glaucoma in one eye. In addition, 14 weeks after transplantation CT scans revealed multiple cerebral abscesses. RESULTS: In the course of the disease Nocardia farcinica (N. f.) was identified by cerebral stereotactic biopsy of a cerebral lesion, histological examination of the enucleated globe and sputum culture. Histologically filamentous, eosinophilic organisms were found. Microbiology identified aerobic actinomycetes in cultures and Nocardia farcinica by PCR. Therapeutically the combination of vancomycin, ampicillin, and sulbactam was successful. CONCLUSION: In immunosuppressed patients Nocardia farcinica can become life-threatening. One of the first manifestations may be a choroiditis. Infection of the respiratory tract followed by hematogenous spread is the common way of systemic nocardiosis. Biopsy followed by identification of species by PCR is recommended because of the specific therapeutic strategies associated with each species.