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•Our study suggests that the fight against the emergence of multidrug-resistant bacteria can entail collateral damage, for both patients and the health care system.

Au cours de l’épidémie de COVID-19, un nombre inhabituel de cas d’engelures a été rapporté en Europe. L’hypothèse d’un lien entre ces engelures et la COVID a été rapidement avancée. Cependant, aucune étude prospective explorant de façon approfondie ces cas d’engelure n’a été réalisée.

Binocular and monocular gain of optokinetic nystagmus (OKN), OKN dynamics, vestibulo-ocular reflex (VOR) and VOR adaptation were measured in 5 normal cats and in 5 cats which underwent bilateral visual cortical lesions involving the 17–18 complex at least 4 months before testing. We observed longterm deficits after bilateral lesions involving area 17 and variable parts of area 18 but failed to observe deficits after 18–19 lesions. These deficits were limited to the OKN gain and the build-up time constant of OKN; the VOR and the optokinetic after-nystagmus (OKAN) time constant were within normal limits. Our results suggest that areas 17–18 operate in parallel to control the encoding of retinal slip velocity at the level of the nucleus of the optic tract (NOT) and the accessory optic system (AOS), which are known to represent the initial stage of the optokinetic pathways.

The horizontal optokinetic reflex (OKN) was studied in cat, monkey and man under conditions of steady or stroboscopic illumination. In all species, there was an abrupt decrease in OKN gain for a given spatial displacement of the stimulus between two consecutive stroboscopic flashes. The upper limit of spatial displacement which preserved optimal OKN gain was independent of stimulus velocity and flash frequency. The value of this limit differed in the three species studied. In the cat, OKN gain was affected when the spatial displacement between two stimuli exceeded 0.55° of visual angle. In monkey and man, these limits were 1.48° and 2.87°, respectively. When human subjects were asked to voluntary track the stimulus, the limit value reached 4.3°. This result is discussed in the context of the evolution of the smooth pursuit system and its contribution to optokinetic response.

The aim of this study was to evaluate the functional significance of similarities observed in the anatomy and the physiology of cat and ferret visual systems. Optokinetic nystagmus (OKN) in response to movement of the entire visual field, and optokinetic after nystagmus (OKAN) were measured in 8 ferrets with binocular stimulation. A shift of the beating field in the same direction as the fast phase of eye movements was observed both in ferret and cat. The absence of a fast rise in slow phase velocity (SPV) and similarities in the time constant to reach the steady state OKN gain, using step velocity stimuli are noted. As in the cat, primary OKAN was observed with a gradual decrease in its SPV. Following termination of stimulation, no sudden fall in SPV was noted for either species. However, for the ferret, the decrease was more rapid. With monocular stimulation, small differences were observed in OKN gain when responses to temporonasal and nasotemporal directions of the stimulus were compared in the two species. In contrast, the ferret displays a OKN gain which is approximatively twice that of the cat at stimulus velocities of 100°/sec. Even at 200°/sec., visual movement still induces a discernable OKN response (gain.0.07). Secondary OKAN, always present in the cat, was observed in only 43% of ferret records. Taken together with other considerations, these findings recommend the ferret as an alternative to the cat for the study of OKN and of other visuo-motor capacities in carnivores.

Summary: Cats were submitted to repeated step stimulations either vestibular or optokinetic. Regardless of which of the two stimuli was used, dynamic modifications were observed in both vestibulo-ocular response and optokinetic after-nystagmus (OKAN). The progressive changes in post-rotational nystagmus and OKAN were quantified by measuring the duration of their primary phase. A parallel evolution of these two parameters was found. When repeated unidirectional vestibular stimulations were used, the same asymmetry was induced in both vestibuloocular responses and OKAN. These results support the hypothesis that the vestibulo-ocular and the optokinetic reflex share a common velocity storage mechanism, although alternative hypotheses cannot be excluded.

In order to clarify the problem of which stimulus parameters affect vestibular habituation, a group of cats was submitted to repeated velocity steps involving changes in either the step amplitude or the interval between two consecutive steps. In the first two experiments, the protocol was the same as in a previous study which used steps of 160°/s separated by 60 s, except that the steps were of 80°/s and 16°/s. In the remaining experiments the step amplitude was kept constant (160°/s) and the interstimulus interval was changed: each step was delivered either immediately after the reversal of the nystagmus elicited by the preceding step (only a few beats in the reversed direction were allowed to occur) or immediately before (no beats in the reversed direction). Vestibular habituation was found to occur in both experiments of the first series. Nevertheless, the marked initial suppression of the response, that was reported as one aspect of vestibular habituation to steps of 160°/s, was clearly present when steps of 80°/s were used, but was not as clear when the step amplitude was reduced to 16°/s. The experiments of the second series showed that a typical vestibular habituation still occurs when steps are delivered just after the reversal of nystagmus. On the contrary, no habituation was observed when steps preceded the reversal of nystagmus. It was concluded that the presence of an anticompensatory phase is interpreted by the central nervous system as a sign that the response evoked in a reflex way is functionally meaningless or even detrimental. An habituation process is then started to suppress the response.

1.Unilateral habituation of the vestibuloocular reflex was produced in adult cats stimulated by repeated unidirectional velocity steps (vestibular training) or by a continuously moving visual surround (optokinetic training). — 2. Unidirectional vestibular training produced a strong asymmetry of vestibuloocular responses (VOR). Responses to velocity steps applied to the “trained” labyrinth were decreased both in gain and in time-constant. This effect generalized to responses to sinusoidal oscillations (0.03 Hz to 0.1 Hz), i.e. to a stimulus not used during training. — No spontaneous nystagmus was ever observed in spite of the dynamic VOR asymmetry. — 3. Unilateral vestibular habituation produced by vestibular training appeared to be a long-lasting phenomenon. It was still present 10 days after the end of training. — 4. Optokinetic responses were not affected by vestibular training. — 5. Unidirectional optokinetic training produced an increase in the slow phase velocity of optokinetic nystagmus (OKN) by about 25% in both directions. This effect did not persist for more than a few minutes. A marked spontaneous nystagmus was recorded in the dark after each session of optokinetic training, with a slow phase in the direction opposite to the previous OKN. — 6. VOR in response to velocity steps and to sinusoidal oscillations were decreased unilaterally after optokinetic training. This effect was of short duration, however, and disappeared within the interval between training sessions. This lack of retention contrasted with the prolonged effect of vestibular training.

Cats reared in stroboscopic illumination (strobe reared cats) have been found to have abnormal eye movements. Visual and vestibular evoked compensatory eye movements were inefficient. Vestibuloocular reflex in the dark had a maximum gain of 0.6 (1.0 in normal animals). Optokinetic nystagmus had a mean gain which approached unity only at stimulus velocities around 7 °/s (up to 30 °/s in normal animals). The asymmetry of the Optokinetic nystagmus resulting from monocular stimulation was more pronounced in strobe reared cat than in normal animals. Interaction between vestibulo-ocular reflex and Optokinetic nystagmus to give adequate compensatory eye movements was absent in strobe reared cats: visual suppression of vestibulo-ocular reflex was absent when the animal was rotated in an illuminated environment which remained stationary with respect to the head. Optokinetic nystagmus failed to improve the gain of the vestibulo-ocular reflex when the animal was rotated in a normally lit environment. The deprived animals showed no signs of recovery after 5 months exposure to normal lighting.

Two-second cathodal current pulses were applied at one-minute intervals at a point external to the round window in the ear of each albino rat subject. Responses were recorded in the vestibular nerve ganglion, the vestibular nuclei (single units), or in the eye movements (search coil recording method) of anaesthetized, decerebrated, or alert rats. The unit responses to the galvanic stimuli were characterized and compared with responses to galvanic and rotational stimuli reported in the literature. The main focus of the study, however, was effects of stimulus repetition. In both the vestibular nerve and vestibular nuclei recordings, the responses of many units were substantially larger or smaller at the end of a 13-pulse stimulus train than at the beginning. In the vestibular nuclei, but not in the nerve, there was a slight bias towards a decrease in response magnitude, with 10/88 units showing decreases great enough to be considered as reflecting an habituation process. In contrast, the eye movement responses showed more consistent response decrements, especially in the alert condition, but also in the other conditions (none of the unit recordings were done in alert rats). It is concluded that some of the modifications underlying habituation of the vestibuloocular reflex probably occur in portions of the neuronal reflex pathways that are downstream from the vestibular nuclei.

Non-cerebellar afferents from visual relays to the vestibular nuclei (VN) of the cat have been re-evaluated with the use of the horseradish peroxidase technique. From our data it can be concluded that: (1) A monosynaptic projection from the nucleus reticularis tegmenti pontis to the VN can be excluded. (2) Monosynaptic projections from the superior colliculus and some of the pretectal nuclei (nucleus of the optic tract, olivary pretectal nucleus) to the nucleus prepositus hypoglossi may constitute polysynaptic visual afferents to the VN, which would account for the residual visual sensitivity of the VN neurons after cerebellar or inferior olivary lesions.

Summary: A photographic technique was used to study the evolution of lateral head-tilt following hemilabyrinthectomy in adult cats. Animals were maintained post-operatively in normally lit conditions (LM cats), in total darkness (DM cats), or in stroboscopic light.In LM cats, the head tilt peaked at 45 degrees (with the lesionned side down) on the second post-operative day, and decreased to about 0 degree within about 10 days. This evolution was followed by rebounds of head-tilt to larger angles before a stable compensated head position could be maintained (approximately at the end of the third post-operative month).In DM cats, the head remained tilted by a large angle throughout the duration of the dark period. Re-exposure to light was followed by a rapid decrease of head-tilt.In stroboscopic light, the evolution of head-tilt was found to be closely similar to that in the normally lit condition.Finally, when put back in the dark at a late post-operative stage, already compensated animals were found to loose their symmetrical head position, and to re-acquire a strong head tilt. This effect resumed on re-exposure to light.It is inferred that static visual input is a necessary condition for compensation of the postural deficits of hemi-labyrinthectomy in the cat. Maintenance of a stable head posture also depends upon continuous availability of visual input.

Summary:

Abstract:  The effect of exposure to repeated angular velocity steps about the earth-vertical axis on the vertical vestibulo-ocular reflex (VOR) during onside pitch rotation was investigated in normal cats. By contrast with the VOR in the horizontal plane, the amplitude and duration of the vertical VOR did not progressively decrease throughout the repetition of velocity steps alternated in both directions. Instead, the amplitude of VOR decreased by about 40% during the very first trials in naive cats and then stayed unchanged with repeated stimuli. Habituation of the amplitude of the vertical VOR was observed when the velocity steps were always directed in the same direction. However, the duration of the vertical VOR did not show any signs of habituation. The habituation of the amplitude of the vertical VOR during unidirectional training was due to the progressive development of an initial inhibition of the VOR. This initial inhibition appeared much earlier during the bidirectional protocol, and was presumably responsible for the larger reduction in VOR amplitude observed during the very first session. These results support the model of two distinct mechanisms for VOR habituation, one producing an increasing inhibition of nystagmus, and the other depressing the response duration, and suggest that only the first mechanism is generated during repeated stimulation in the vertical plane. The low-frequency information provided by the velocity storage mechanism during onside pitch rotation, when the otoliths are positioned so they do not signal head tilt relative to gravity, could prevent a decrease in the overall response by the second mechanism.

Bilateral surgical lesions of the flocculus or the nodulo-uvular lobes were performed in the cat. Effects of these lesions on optokinetic and optokinetic afternystagmus (OKAN), vestibulo-ocular reflex (VOR), visual suppression, and adaptation and habituation of VOR were studied using an identical experimental protocol. After flocculectomy, all these functions were impaired, except for habituation. Long-term postoperative recordings only revealed a recovery of the suppression of VOR, suggesting a limited contribution of the flocculus to this function. After nodulo-uvulectomy, only habituation and OKAN were modified. When the lesion was restricted to part of the uvula, OKAN duration was decreased. For other lesions involving the uvula together with the nodulus and/or the lobules VII-VIII, OKAN duration was increased. Habituation was lost after destruction of the nodulo-uvular lobes. When this latter structure was damaged, the retention component of habituation was selectively impaired, sparing the acquisition. Additional lesions outside the vestibulocerebellum appeared necessary to suppress the two components. Comparison of results obtained after flocculectomy and after nodulouvulectomy confirms and extends to nonprimate species the concept of a “differential control” of adaptation and habituation by distinct vestibulocerebellar structures.