Effects of accessory optic system lesions on vestibulo-ocular and optokinetic reflexes in the cat

Summary Horizontal vestibulo-ocular reflex (VOR) and optokinetic nystagmus (OKN) were studied before and after lesions within the accessory optic system (AOS) in the cat. Post-lesion retinal input to the AOS was evaluated using the autoradiographic technique. Unilateral lesion of the lateral terminal nucleus of the AOS (LTN) and the resulting retinal deafferentation of the medial terminal nucleus of the AOS (MTN) induced a spontaneous nystagmus in the dark whose slow phase was directed ipsilaterally to the lesion. VOR gain was reduced for both directions with a maximal decrease for stimulation directed ipsilaterally to the lesion. OKN gain obtained for both directions of binocular stimulation was decreased, mainly when the stimulus was directed contralaterally to the lesion. After two postoperative weeks, spontaneous nystagmus disappeared and the VOR symmetry recovered simultaneously. A symmetrical OKN was only observed after one month. In spite of the known visual selectivity for vertical direction in LTN-MTN cells, the results of this study support a functional involvement of these nuclei in horizontal VOR and OKN.Supported by C.N.R.S. (ATP N 8115)     

Cholinergic modulation of optokinetic and vestibulo-ocular responses: a study with microinjections in the flocculus of the rabbit

Summary In spite of a large body of histochemical evidence for a cholinergic system in the cerebellum, particularly in lobules IX and X, the physiological role of such a system has remained obscure. In view of the important role of these same lobules in the control of the vestibulo-ocular (VOR) and optokinetic (OKR) responses, we tested the effect of microinjections of cholinergic (ant)agonists in the flocculus of the rabbit on these reflexes. Very marked effects were found. Bilateral floccular injection of the aspecific cholinergic agonist carbachol raised the gain of the OKR by about 0.46 above the baseline values, while the gain of the VOR in darkness was raised by about 0.14. These effects were statistically significant and persisted for several hours. Similar, but smaller effects were obtained after injection of eserine, an inhibitor of acetylcholinesterase. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine. Microinjections of the nicotinic blocker mecamylamine reduced the gain of the VOR and OKR, although these effects did not reach statistical significance. The muscarinic blocker atropine significantly reduced the gain of the OKR, but not of the VOR. The present results argue strongly for an important physiological role of the cholinergic system in the cerebellum. Specifically, acetylcholine appears to be involved in the modulation of oculomotor reflexes through the flocculus.     

Involvement of GABAergic mechanisms in the optokinetic nystagmus of the frog

Summary Frog optokinetic nystagmus (OKN) was studied before and after an intravitreal injection of picrotoxin, a specific non-competitive GABA antagonist. In monocular vision, the OKN displayed a directional asymmetry favouring the Temporal-Nasal (T-N) stimulation. In that case, the nystagmus extinction frequency (NEF) is low, about 2 frames/s. In binocular vision, the OKN is symmetrical with a facilitation of performances compared to monocular vision (NEF = 3 frames/s). In monocular as in binocular vision, an intravitreal injection of picrotoxin (between 1×10^–4 and 5×10^–3 M) provoked the disappearance of the injected eye OKN and a spectacular facilitation in the performances of the intact eye, with the appearance of a N-T component and the increase of the NEF value reaching 7 or 11.5 frames/s according to the experimental conditions. This contralateral facilitation was no longer observed after the optic nerve of the injected eye had been cut, indicating that such a facilitation can only be explained by alterations of a central process triggered by the visual input. It is concluded that GABAergic mechanisms might be responsible for the inhibition of the N-T component in the frog OKN and might be involved in the control of the power of temporal resolution in this animal.     

Unilateral cholinergic stimulation of the rabbit's cerebellar flocculus: asymmetric effects on optokinetic responses

Summary In previous work, we have demonstrated an acceleration of the buildup of slow-phase velocity of optokinetic nystagmus (OKN) after bilateral floccular injection of the aselective cholinergic agonist carbachol (Tan and Collewijn 1991; Tan et al. 1992a). In the present study we investigated the effects of unilateral floccular injections of carbachol. Such unilateral injections specifically enhanced the buildup of OKN slow-phase velocity in the direction toward the injected flocculus (ipsiversive). During binocular optokinetic stimulation, this enhancement was expressed in the motion of both eyes. Acceleration of the eye contralateral to the injected flocculus increased from 1 to about 2°/s², while the acceleration of the ipsilateral eye increased from 1 to about 1.5°/s². In contrast, buildup of contraversive OKN was unchanged. No changes were found in the steady-state OKN and optokinetic afternystagmus (OKAN). Monocular optokinetic stimulation was only effective in the nasal direction, and the effects of unilateral injection of carbachol were disconjugate. Ipsiversive OKN was enhanced only in the contralateral, seeing eye, while the response of the ipsilateral, covered eye was unchanged. We hypothesize that the directionally specific effect of unilateral cholinergic floccular stimulation on OKN is due to enhancement of predominantly the excitatory phase of modulation of the Purkinje cell's simple-spike activity by carbachol, without a marked effect of carbachol on the inhibitory phase of simple-spike modulation.     

Effects of lesions of the nucleus of the optic tract on optokinetic nystagmus and after-nystagmus in the monkey

Summary 1. The nucleus of the optic tract (NOT) and the dorsal terminal nucleus (DTN) of the accessory optic system were lesioned electrolytically or with kainic acid in rhesus monkeys. When lesions involved NOT and DTN, peak velocities of optokinetic nystagmus (OKN) with slow phases toward the side of the lesion were reduced, and optokinetic after-nystagmus (OKAN) was reduced or abolished. The jump in slow phase eye velocity at the onset of OKN was smaller in most animals, but was not lost. Initially, there was spontaneous nystagmus with contralateral slow phases. OKN and OKAN with contralateral slow phases were unaffected. 2. Damage to adjacent regions had no effect on OKN or OKAN with two exceptions: 1. A vascular lesion in the MRF, medial to NOT and adjacent to the central gray matter, caused a transient loss of the initial jump in OKN. The slow rise in slow phase velocity was prolonged, but the gain of OKAN was unaffected. There was no effect after a kainic acid lesion in this region in another animal. 2. Lesions of the fiber tract in the pulvinar that inputs to the brachium of the superior colliculus caused a transient reduction in the buildup and peak velocity of OKN and OKAN. 3. In terms of a previous model (Cohen et al. 1977; Waespe et al. 1983), the findings suggest that the indirect pathway that activates the velocity storage integrator in the vestibular system to produce the slow rise in ipsilateral OKN and OKAN, lies in NOT and DTN. Activity for the rapid rise in OKN, carried in the direct pathway, is probably transmitted to the pontine nuclei and flocculus via an anatomically separate fiber path-way that lies in the MRF. A fiber tract in the pulvinar that inputs to the brachium of the superior colliculus appears to carry activity related to retinal slip from the visual cortex to NOT and DTN.Abbreviations used in Figures BIC brachium of the inferior colliculus - BSC brachium of the superior colliculus - C caudate nucleus - CG central gray - CL Centralis lateralis - dbc decussation of the brachium conjunctivum - DTN dorsal terminal nucleus of the accessory optic system - IC inferior colliculus - Hb habenular nucleus - hc habenular commissure - LD lateralis dorsalis - LGn lateral geniculate nucleus - MD medialis dorsalis - MGn medial geniculate nucleus - MLF median longitudinal fasciculus - MRF mesencephalic reticular formation - cMRF central mesencephalic reticular formation - NL nucleus limitans - NLL nucleus of the lateral lemniscus - NOT nucleus of the optic tract - PB parabigeminal nucleus - pc posterior commissure - Pi pineal gland - PON pretectal olivary nucleus - Pt pretectum - Pulv pulvinar - R nucleus reticularis - RN red nucleus - RpN raphe nucleus - RTP nucleus reticularis tegmenti pontis - SC superior colliculus - SCpit superior cerebellar peduncle - VPL ventralis postero-lateralis - VPM ventralis posteromedialis - III oculomotor nucleus - IV trochlear nucleus - IVn trochlear nerve - Vm mesencephalic trigeminal nucleus     

A comparison of the horizontal and vertical optokinetic reflexes of the rabbit

Summary The horizontal and vertical monocular optokinetic reflexes of the rabbit were measured under closed-loop and open-loop conditions. A random noise, optokinetic stimulus subtending 70×70 deg was presented to the left eye of rabbits placed in front of a rear projection tangent screen. The position of the right eye (nonstimulated) was measured using an infrared light projection technique. During open-loop optokinetic stimulation the eye position signal was fed back to sum with a time-integrated velocity command signal driving the optokinetic stimulus. The dynamics of eye movements evoked by horizontal and vertical optokinetic stimulation were different. Horizontally evoked eye movements never exceeded a deviation of 15 deg before being interrupted by resetting saccades, which returned the eye past the primary position. By contrast, vertical eye deviations greater than 20 deg were often maintained for intervals exceeding 10 s without resetting. The closed-loop gain of optokinetically evoked horizontal eye movements was higher for monocular posterior-anterior optokinetic stimulation than for anterior-posterior stimulation. The vertical optokinetic gain for up-down stimulation was slightly greater than the gain for down-up stimulation. The vertical up-down, open-loop optokinetic gain was greater than the down-up gain over a range of retinal slip velocities of 0.5–5.0 deg/s. Measurement of the horizontal vestibulo-ocular reflex during simultaneous horizontal optokinetic stimulation demonstrated that visual and vestibular information combine linearly to produce reflex eye movements. These data suggest that the higher gain of the horizontal optokinetic reflex may compensate in part for the reduced gain of the horizontal vestibulo-ocular reflex at lower angular accelerations of the head. An equivalent vertical optokinetic gain would be obviated by the contribution of the utricular otoliths to the vertical vestibulo-ocular reflex at low frequencies of head movement.This research was supported by the National Institutes of Health Grant EY00848 and the Oregon Lions Sight and Hearing Foundation     

Receptive field organization of climbing fiber afferents responding to optokinetic stimulation in the cerebellar nodulus and flocculus of the pigmented rabbit

Summary and conclusions Under anesthesia with N₂O (70%) and halothane (2–4%), complex spikes of Purkinje cells were extracellularly recorded in the nodulus and flocculus of immobilized pigmented rabbits. Optokinetic stimulation (OKS) was delivered to each eye as repetitive movements of a random dot pattern. The visual field of each eye was divided into anterior, central and posterior fields at axes 45° and 135° along the horizon. With OKS of the ipsilateral eye, the preferred direction of complex spike responses was: (1) forward (F) in all visual fields (F response), (2) upward (U) in both the anterior and central visual fields but downward (D) in the posterior visual field (U response), or (3) no response (N) in any of the visual fields (N response). With OKS of the contralateral eye, the preferred direction was: (1) backward (B) in both the anterior and central visual fields but N in the posterior visual field (B response), (2) U in the anterior but D in both the central and posterior visual fields (D response), or (3) N in all visual fields. Purkinje cells were classified into five categories in terms of the complex spike responses to OKS to the central visual fields of the ipsi-/contralateral eyes: F/B, F/N, U/D, U/N and N/D types. In cells with ipsi-F and/or contra-B responses, OKS delivered above the horizon induced F and/or B responses, but OKS below the horizon induced no response. In cells with contra-D response, OKS both above and below the horizon induced D responses. Cells with ipsi-U responses showed unusually complex direction selectivity: in the anterior, central and posterior fields, the preferred direction was U, U and D, respectively, above the horizon, as opposed to N, D and D below the horizon. In the nodulus, F/B and F/N type cells were localized in the ventral lamella within two distinct longitudinal zones about 0.5–1.5 and 2.5–3.5 mm from the midline, while U/D, U/N and N/D type cells were found in both the ventral and dorsal lamellae within a longitudinal zone about 1.5–2.5 mm from the midline. In the flocculus, the locations of F/N (or F/B), U/N and N/D type cells roughly corresponded to H, anterior V and R zones, respectively, as previously reported on the basis of the direction of eye movements induced by microstimulation. The receptive field of visual climbing fiber inputs to the nodulus and flocculus is organized such that complex spike activity is best modulated with retinal image slips caused by head rotation around the axis of either the horizontal (F/B, F/N types) or the anterior canal (U/D, U/N, N/D types) on the ipsilateral side.     

Effects of early monocular deprivation on development of cortico-geniculate projections in the cat

Summary In 16 cats monocularly deprived from 2 to 3 weeks of age, we studied 53 striate cortical cells which were identified as projecting to the dorsal lateral geniculate nucleus (LGN) on the basis of antidromic activation from LGN and of histological localization within cortical layer VI. As in the normal cat, these cortico-geniculate cells could be classified as slow, intermediate or fast, according to their axonal conduction velocities. The sampling ratio of the slow cells (mostly unresponsive to visual stimuli) was much higher than normal. On the other hand, the ratio of the intermediate (one half were simple cells) and fast cells (all except one were complex cells) was significantly lower than the norm. Also, the average axonal conduction velocities of the complex and simple cells were significantly slower than normal. These results suggest that normal maturation of cortico-geniculate cells, particularly fast and intermediate ones, is retarded or arrested by monocular visual deprivation.Supported by a grant from the Ministry of Education of Japan     

Effects of cholinergic neuromodulation in cerebellar flocculus on transparent motion processing in the rabbit

The way a rabbit moves its eyes in response to a stimulus consisting of two moving random dot patterns largely depends on the relative luminances of the two patterns. Concurrent rotation of the animal enhances the response to the visual pattern that represents the same head movement as the vestibular stimulation. In this paper we investigate the role that the flocculus plays in this behaviour. We injected the non-selective acetylcholine agonist carbachol into the flocculus. These injections are known to increase the gain of the optokinetic reflex, but have a smaller effect on the vestibulo-ocular reflex. We investigated the effect on the oculomotor response to (vestibulo-) transparent stimuli, where one pattern oscillated sinusoidally and the other pattern was stable with respect to the head. We found that the injections caused a higher response gain at a lower luminance of the oscillating pattern. Furthermore the influence of concurrent vestibular stimulation decreased. These findings agree with a role of the flocculus that is downstream of the visual normalisation, but upstream of the visual-vestibular interaction.     

Nature of optokinetic response and zonal organization of climbing fiber afferents in the vestibulocerebellum of the pigmented rabbit

Summary In pigmented rabbits anesthetized with N₂O (70%) and halothane (2–4%), Purkinje cells were extracellularly recorded in the nodulus. Large field (60°×60°) optokinetic stimulation (OKS) with constant velocity was delivered to either the ipsi- or contralateral eye, and the direction and velocity selectivities of complex spike responses were examined. To ipsilateral OKS (n = 181), the preferred direction was forward (F, n = 72), upward (U, n = 38) or downward (D, n = 10), while the remaining cells (n = 61) showed no response (N). To contralateral OKS (n = 117), the preferred direction was backward (B, n = 22), upward (U, n = 7) or downward (D, n = 22), while the rest (n = 66) showed N. Cells tested with both eyes (n = 95) fell into 8 categories based on the preferred direction to ipsi- and contralateral OKS: (1) ipsi-F and contra-B (F/B type, n = 20), (2) ipsi-F but contra-N (F/N type, n = 12), (3) ipsi-U and contra-D (U/D type, n = 15), (4) ipsi-U but contra-N (U/N type, n = 13), (5) ipsi-N but contra-D (N/D type, n = 1), (6) ipsi-D but contra-N (D/N type, n = 5), (7) ipsi-N but contra-U (N/U type, n = 6), and (8) N to both eyes (N/N type, n = 23). The optimum velocity was in the range 0.1–2.0°/s for all cells responsive to OKS. In the ventral lamella, four medio-laterally aligned zones were demonstrated. In the most medial zone (0–0.5 mm from the midline), the majority of cells showed ipsi-N or contra-N responses. In the second zone (0.5–1.5 mm), most cells preferred ipsi-F or contra-B directions. In the third zone (1.5–2.5 mm), most cells preferred ipsi-U or contra-D directions. In the most lateral zone (2.5–3.5 mm), most cells preferred ipsi-F or contra-B directions. In the dorsal lamella, a longitudinal zone characterized with cells preferring ipsi-U or contra-D directions was found about 1.5–2.5 mm from the midline. This zone seemed to be the continuation of the third zone in the ventral lamella. Cells preferring ipsi-D or contra-U directions were scattered in the medial half of both the dorsal and ventral lamellae. Most cells responsive to electrical stimulation of the contralateral optic tract (OT) preferred the ipsi-F direction and were localized in the second and the most lateral zones of the ventral lamella. As for cells activated by a climbing fiber with a branching axon to the flocculus, no characteristic feature was found in terms of the preferred direction to OKS, except that none of the cells preferring ipsi-D or contra-U directions were activated by such branching climbing fibers. The results indicate that the nodulus consists of at least four functionally distinct zones in terms of direction selectivity of visual climbing fiber afferents.     

Effects of prolonged weightlessness on horizontal and vertical optokinetic nystagmus and optokinetic after-nystagmus in humans

Horizontal and vertical optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) provided by a partial-field, binocular optokinetic stimulator were recorded in one astronaut before, during, and after a 25-day space flight. A ground-based study was performed on six control subjects. During the flight experiment, performed on flight days 5, 18, 19, and 21, the subject either had their feet attached to the deck or was free-floating. Vertical OKN gain only slightly increased in weightlessness compared with ground data, but the center of interest (CI) during vertical OKN, evaluated by the eye position in the saggital plane at the end of the fast phases relative to the straight-ahead direction, was found to be significantly changed during long-term exposure to weightlessness. The horizontal CI showed very little change in-flight, but the gain was increased. The time constant for the astronaut was small for vertical OKAN, but there was an increase in slow-phase velocity (SPV) by the end of the flight, which returned to normal postflight. These results partly confirm the data obtained during head-tilt studies on the ground and are in accordance with the hypothesis of a gravity-dependent control of vertical gaze direction during orientation reflexes.     

Long-term deficits in otolith, canal and optokinetic ocular reflexes of pigmented rats after unilateral vestibular nerve section

 Static and dynamic otolith, horizontal vestibular and optokinetic ocular reflexes were investigated in pigmented rats 1–6 and more months after unilateral vestibular nerve (UVN) section. Evoked responses were compared with published data from control rats studied under identical conditions. Static lateral tilt of UVN rats in the light evoked a vertical deviation in static eye position that was as large as in controls. In darkness, the evoked responses in UVN rats 6 months after the lesion were consistently smaller than in controls. Linear horizontal acceleration in darkness evoked vertical and torsional response components in UVN rats that were parallel-shifted towards lower gains and larger phase lags. Off-vertical axis rotation on a platform provoked responses that differed markedly from those recorded in intact rats with respect to the bias velocity component. These results suggest a permanent deficiency in the static and dynamic otolith-ocular reflex performance of UVN rats. Ocular responses to horizontal table velocity steps in darkness exhibited a direction-specific asymmetry in UVN rats. Step responses evoked by acceleration towards the intact side were larger in gain and longer in duration than responses evoked by acceleration towards the operated side. When compared with control data, responses to either side were reduced in UVN rats and the velocity store mechanism was barely activated by velocity steps towards the operated side. Responses evoked by horizontal optokinetic stimulation with constant pattern velocities were below control values in either direction. Slow-phase eye velocity saturated at much lower values than in intact rats, particularly during pattern motion towards the intact side. The duration of the optokinetic afternystagmus was asymmetrically reduced with respect to control data. Practically identical reductions in duration were found for vestibulo-ocular responses in the opposite directions. Behaving animals exhibited no obvious impairment in their spontaneous locomotory or exploratory activities. However, each UVN rat was impaired, even 2 years after the lesion, in its postural reaction to being lifted by the tail in the air. This observation suggests the presence of a permanent deficit in static and dynamic otolith-spinal reflexes that may be substituted on the ground by proprioceptive inputs. Received: 26 February 1997 / Accepted: 2 July 1997     

Effects of neonatal monocular enucleation on the number of GAD-positive puncta in rat visual cortex

Summary Rats that had one eye removed on the day of birth were examined at various postnatal ages with immunocytochemical methods to determine the effect on the development of the GABAergic axonal plexus in the visual cortex. The monocular segment of visual cortex contralateral to the enucleated orbit had 20–30% fewer GABAergic axon terminals than the monocular segment of visual cortex contralateral to the normal eye. Other cortical areas did not show any significant changes. These findings suggest that sensory deprivation of the visual cortex interferes with the normal development of GABAergic neurons.     

Effects of static tilt about the roll axis on horizontal and vertical optokinetic nystagmus and optokinetic after-nystagmus in humans

Summary Horizontal and vertical OKN and OKAN were recorded in four conditions using the EOG technique. Instructions to subjects were aimed at obtaining a look type OKN. Two optokinetic stimulators, a stationary sphere and a binocular portable model, were compared with the subject in the upright condition. Three posture orientations, upright, 90° roll (horizontal), and upside-down, were then compared using the portable stimulator to determine the effect of roll-axis tilt on OKN at three velocities and on OKAN. Vertical OKN asymmetry was found to increase in the 90° roll position and to tend toward a reversal in the upside-down position. The time constant of vertical OKAN with slow phase up increased in both the 90° roll and upside-down positions. And finally, cross-coupled vertical eye movements during and after horizontal OKN were clearly observed. These results confirm the data obtained in monkeys, and are in accordance with the hypothesis of a three-dimensional organization of the velocity storage mechanism.     

Variability in the effects of monocular deprivation on the optokinetic reflex of the non-deprived eye in the cat

Summary 1. Six cats monocularly deprived by eye lid closure within the first week after birth showed the same deficits in the optokinetic reflex (OKR) when tested through the deprived eye as adults irrespective of whether the deprivation period was 6, 24 or 36 months. Closed loop gain (eye velocity/ stimulus velocity) during temporo-nasal stimulus movement was below 0.8 and approached zero at stimulus velocities above 20°/s. Naso-temporal stimulus movement was ineffective in eliciting OKR gain higher than 0.1 at velocities above 10°/s. 2. Different optokinetic deficits were found when the non-deprived eye was tested. In 3 cats OKR gain of the non-deprived eye was reduced with temporally directed stimulus movement when compared to normal whereas the gain of nasal OKR was uneffected. In these cats only monocular cells could be found in the nucleus of the optic tract (NOT), a pretectal cell aggregation involved in the optokinetic reflex pathway. In the other 3 cats the OKR of the non-deprived eye was not different from normal and could be elicited almost equally well in both directions. In these cats binocular cells were found in the NOT ipsilateral to the non-deprived eye. Again duration (6, 24 or 36 months) of monocular deprivation had no influence on this dichotomy. 3. In a cat with asymmetric OKR of the non-deprived eye, the removal of the visual cortex ipsilateral to the non-deprived eye produced a small but significant gain decrease for temporal OKR of the non-deprived eye but no change when the deprived eye was tested. Visual cortex lesion ipsilateral to the deprived eye in the same cat had also no effect on the deprived eye's performance but reduced nasal OKR gain for the non-deprived eye at high velocities. 4. The effects induced by long term monocular deprivation were not reversed after intensively forcing the use of the deprived eye by closing the non-deprived eye. Also enucleation of the deprived eye had no effect on the gain of the non-deprived eye. 5. These optokinetic deficits are discussed in relation to functional changes in the NOT.     

Single unit activity in the nucleus of the basal optic root (nBOR) during optokinetic,vestibular and visuo-vestibular stimulations in the alert pigeon (Columbia livia)

Summary Extracellular recordings were performed in the nucleus of the basal optic root (nBOR) of alert pigeons during optokinetic nystagmus (OKN), vestibulo-ocular reflex (VOR) and combined visuo-vestibular stimulation. Cell identification was assessed either by histological control or by electrophysiological testing (antidromic response to vestibulo-cerebellar or oculomotor complex stimulation). 1) OKN was induced in 8 directions by a binocular stimulation. During the fast phase of OKN, optokinetic after nystagmus (OKAN) or reversed OKAN, most cells showed an inhibition which varied in magnitude independent of the direction of stimulation. A few cells however showed a phasic discharge for some OKN directions. 2) During the slow phase of OKN induced by a binocular stimulation, cells displayed either a tonic activation or a more or less strong inhibition according to the direction of the OKN. Cells were classified in 4 groups, according to their degree of directional specificity. The best OKN direction (slow phase) for maximal cell activation was upwards and naso-upwards, and next to best, nasotemporal and downwards. Maximal cell inhibition occurred during downward, and for some cells during upward, directions. 3) During OKN induced by stimulating the eye contralateral to the recorded nBOR, cell responses resembled those obtained during binocular stimulation, but, during ipsilaterally induced OKN, the cells lost their directional specificity. As a result of binocular integration, neuronal activation seems to originate from contralateral input whereas cell inhibition would mainly come from ipsilateral input. 4) During sinusoidal optokinetic stimulation induced in the temporo downward-naso upward axis, cells showed a more or less marked modulation (according to their directional selectivity) that was closely in phase with the stimulation velocity, and therefore probably with retinal slip. 5) nBOR cells appeared generally unaffected during both the slow phase and the fast phase of the VOR. However, some cells showed a slight but irregular modulation which might imply a weak vestibular input. During visuo-vestibular stimulation, the response resembled that obtained with sinusoidal optokinetic stimulation but the fast phase inhibition was often strengthened in the downwards direction (fast phase).     

Vestibular nuclei activity in the alert monkey during suppression of vestibular and optokinetic nystagmus

Summary Single neurons were recorded in the vestibular nuclei of monkeys trained to suppress nystagmus by visual fixation during vestibular or optokinetic stimulation. During optokinetic nystagmus vestibular nuclei neurons exhibit frequency changes. With the suppression of optokinetic nystagmus this neuronal activity on average is attenuated by 40% at stimulus velocities of 40 °/s. At a stimulus velocity of 5 °/s responses are, under both conditions, close to threshold. For steps in velocity, suppression of vestibular nystagmus shortens the time constants of the decay of neuronal activity from 15–35 s to 5–9 s, while the amplitude of the response remains unchanged. The results are discussed in relation to current models of visual-vestibular interaction. These models use a feedback mechanism which normally operates during vestibular and optokinetic nystagmus. Nystagmus suppression interrupts this feedback loop.Supported by the Swiss National Foundation for Scientific Research (SNF 3.233.77) and the Deutsche Forschungsgemeinschaft (U.W. Buettner, Bue 379/2)     

Effects of early monocular deprivation on response properties and afferents of nucleus of the optic tract in the ferret

Summary Effects of early monocular deprivation on visual response properties of neurons in the nucleus of the optic tract (NOT) were studied in six adult ferrets. Retinal input to NOT was investigated by orthodromic electrical stimulation of optic chiasm and optic nerves. Electrical stimulation of the ipsilateral primary visual cortex was applied to reveal the presence of a cortical pathway to NOT. All 75 neurons studied in the NOT displayed the typical strongly direction-specific response to horizontal stimulus motion; they were activated by ipsiversively directed motion (i.e. motion towards the recorded hemisphere) similar to NOT-cells in animals with normal visual experience. When tested binocularly most of the NOT-cells preferred velocities of 10 or 20 deg/s, revealing no significant difference from animals reared with normal binocular experience. The most pronounced effect of monocular deprivation was observed on ocular dominance: In the hemisphere contralateral to the non-deprived eye, NOT-cells were almost exclusively driven through the contralateral eye. In the hemisphere contralateral to the deprived eye, three of the six animals studied showed a marked dominance of the ipsilateral, non-deprived eye. In the other three animals, most neurons were binocularly activated, but over all they were significantly more strongly activated by the ipsilateral eye than found in normal animals. In four animals, dependence of ocular dominance on stimulus velocity was tested in the NOT contralateral to the deprived eye. In one of them, neurons were almost exclusively driven by the ipsilateral, non-deprived eye, irrespective of stimulus velocity. In the other animals, ocular dominance shifted from contralateral to ipsilateral with increasing stimulus velocities. Electrical stimulation of the optic chiasm revealed a mean latency of 5.53 ± 0.48 ms. In both hemispheres, NOT-units could only be activated by stimulation of the contralateral optic nerve. Thus, no significant difference in the retinofugal conduction velocities from the deprived and the normal nerve could be detected. Of 52 cells studied, 28 (= 54%) could be activated by stimulation of primary visual cortex, mean latency being 3.9± 1.7 ms. No significant difference in the percentage of cortically excitable cells between the two hemispheres as well as compared to normal animals was found (contralateral to the deprived eye: 67%, contralateral to the non-deprived eye: 53%). Therefore, cortical projections to NOT seem not to be affected by monocular deprivation. The effects of monocular deprivation in the ferret NOT, especially on ocular dominance and cortical input, are compared to the results previously described for the cat.     

Effect of neonatal unilateral eye enucleation in the chick

Following unilateral eye enucleation in the chick at or soon after hatching studies by autoradiography and degeneration methods failed to reveal any anomalous fibre projection from the remaining eye. The lack of sprouting of fibres was discussed.     

Post-rotatory nystagmus and optokinetic after-nystagmus in the rabbit linear rather than exponential decay

Summary The decay of the slow phase velocity of post-rotatory (PRN) and optokinetic (OKAN) afternystagmus as a function of time was measured in Dutch rabbits after stimulation with velocity steps of 30, 60, and 150 °/s. The decays fitted linear functions very well, but only poorly exponential ones. Typical decay rates were 2–5 °/s², with apparent time constants (defined by decay to 37% of initial velocity) in the order of 10–20 s. Within one animal, the decays of OKAN and PRN with similar initial velocities were indistinguishable. With sinusoidal oscillation, the time constant of the vestibulo-ocular reflex — estimated from phase lead — was only 2–3 s, and probably similar to the cupular time constant. In general, time constants increased when eye velocities increased. This indicates that the vestibulo-ocular reflex of the rabbit behaves as a non-linear system. A velocity storage system with a constant discharge rate is postulated as a main non-linear element. This would introduce a linear decay of velocity as well as a threshold for velocity. This storage system would be common to both vestibulo-ocular and optokinetic reflexes.