Endemic Lagos bat virus infection in Eidolon helvum.

Phylogenetic analyses suggest lyssaviruses, including Rabies virus, originated from bats. However, the role of bats in the maintenance, transmission and evolution of lyssaviruses is poorly understood. A number of genetically diverse lyssaviruses are present in Africa, including Lagos bat virus (LBV). A high seroprevalence of antibodies against LBV was detected in Eidolon helvum bats. Longitudinal seroprevalence and age-specific seroprevalence data were analysed and capture-mark-recapture (CMR) analysis used to follow 98 bats over 18 months. These data demonstrate endemic infection, with evidence of horizontal transmission, and force of infection was estimated for differing age categories. The CMR analysis found survival probabilities of seronegative and seropositive bats were not significantly different. The lack of increased mortality in seropositive animals suggests infection is not causing disease after extended incubation. These key findings point towards acute transmission of bat lyssaviruses in adapted bat hosts that occurs at a far higher rate than the occurrence of disease.

Body mass index and common mental disorders: exploring the shape of the association and its moderation by age, gender and education.

BACKGROUND: Obesity is known to be associated with increased prevalence of common mental disorders (for example, depression and anxiety), and there is evidence of age and gender differences in this relationship. However, categorisation of body mass index (BMI) and age has limited our ability to understand the nature of these differences. This study used continuous values of BMI and age to explore the shape of the association between common mental disorders and BMI and whether it varied with age, gender and education. METHOD: The analysis used cross-sectional data on 7043 adults from the English 2007 Adult Psychiatric Morbidity Survey. Common mental disorders were assessed using the revised Clinical Interview Schedule (CIS-R). Cubic splines allowed BMI and age to have non-linear effects in the logistic regression analysis. RESULTS: BMI was strongly associated with the presence of common mental disorders, and there was clear evidence that this association varied with gender and age. In young women the probability of having a disorder increased as BMI increased, whereas in young men the relationship was U-shaped-probabilities were higher for both underweight and obese men. These associations diminished in older age groups, particularly when potential confounders such as physical health were taken into account. There was no evidence that the relationship varied with education. CONCLUSIONS: Age and gender differences must be taken into account when investigating the link between BMI or obesity and common mental disorders. Furthermore, results of studies that categorise BMI may be highly sensitive to the width of the 'normal weight' reference category.

Spatial distribution and trends of total mercury in waters of the Great Lakes and connecting channels using an improved sampling technique.

Environment Canada recently developed a clean method suitable for sampling trace levels of metals in surface waters. The results of sampling for total mercury in the Laurentian Great Lakes between 2003 and 2009 give a unique basin-wide perspective of concentrations of this important contaminant and represent improved knowledge of mercury in the region. Results indicate that concentrations of total mercury in the offshore regions of the lakes were within a relatively narrow range from about 0.3 to 0.8 ng/L. The highest concentrations were observed in the western basin of Lake Erie and concentrations then declined towards the east. Compared to the offshore, higher levels were observed at some nearshore locations, particularly in lakes Erie and Ontario. The longer-term temporal record of mercury in Niagara River suspended sediments indicates an approximate 30% decrease in equivalent water concentrations since 1986.

Context contingent signal processing in the cerebellar flocculus and ventral paraflocculus during gaze saccades.

The vestibuloocular reflex (VOR) functions to stabilize gaze when the head moves. The flocculus region (FLR) of the cerebellar cortex, which includes the flocculus and ventral paraflocculus, plays an essential role in modifying signal processing in VOR pathways so that images of interest remain stable on the retina. In squirrel monkeys, the firing rate of most FLR Pk cells is modulated during VOR eye movements evoked by passive movement of the head. In this study, the responses of 48 FLR Purkinje cells, the firing rates of which were strongly modulated during VOR evoked by passive whole body rotation or passive head-on-trunk rotation, were compared to the responses generated during compensatory VOR eye movements evoked by the active head movements of eye-head saccades. Most (42/48) of the Purkinje cells were insensitive to eye-head saccade-related VOR eye movements. A few (6/48) generated bursts of spikes during saccade-related VOR but only during on-direction eye movements. Considered as a population FLR Pk cells were <5% as responsive to the saccade-related VOR as they were to the VOR evoked by passive head movements. The observations suggest that the FLR has little influence on signal processing in VOR pathways during eye-head saccade-related VOR eye movements. We conclude that the image-stabilizing signals generated by the FLR are highly dependent on the behavioral context and are called on primarily when external forces unrelated to self-generated eye and head movements are the cause of image instability.

Current concepts of vestibular nucleus function: transformation of vestibular signals in the vestibular nuclei.

The vestibular nerve sends signals to the brain that code the movement and position of the head in space. These signals are used for a variety of functions, including the control of reflex and voluntary movements and the construction of a sense of self-motion. In order to carry out these functions, sensory vestibular signals need to be transformed in a variety of ways. Transformations are thought to occur at an early stage of sensory processing in the brain, and in many cases are apparent in the responses of neurons in the vestibular nuclei that receive direct inputs from the vestibular nerve. Several specific examples of sensory transformation in the vestibular nuclei are presented, and current hypotheses about the mechanisms that are used to produce the transformations are discussed.

The neurophysiological substrate for the cervico-ocular reflex in the squirrel monkey.

Passive rotation of the trunk with respect to the head evoked cervico-ocular reflex (COR) eye movements in squirrel monkeys. The amplitude of the reflex varied both within and between animals, but the eye movements were always in the same direction as trunk rotation. In the dark, the COR typically had a gain of 0.3-0.4. When animals fixated earth-stationary targets during low-frequency passive neck rotation or actively tracked moving visual targets with head movements, the COR was suppressed. The COR and vestibulo-ocular reflex (VOR) summed during passive head-on-trunk rotation producing compensatory eye movements whose gain was greater than 1.0. The firing behavior of VOR-related vestibular neurons and cerebellar flocculus Purkinje cells was studied during the COR. Passive neck rotation produced changes in firing rate related to neck position and/or neck velocity in both position-vestibular-pause neurons and eye-head-vestibular neurons, although the latter neurons were much more sensitive to the COR than the former. The neck rotation signals were reduced or reversed in direction when the COR was suppressed. Flocculus Purkinje cells were relatively insensitive to COR eye movements. However, when the COR was suppressed, their firing rate was modulated by neck rotation. These neck rotation signals summed with ocular pursuit signals when the head was used to pursue targets. We suggest that the neural substrate that produces the COR includes central VOR pathways, and that the flocculus plays an important role in suppressing the reflex when it would cause relative movement of a visual target on the retina.

Role of the cerebellar flocculus region in cancellation of the VOR during passive whole body rotation.

A series of studies were carried out to investigate the role of the cerebellar flocculus and ventral paraflocculus in the ability to voluntarily cancel the vestibuloocular reflex (VOR). Squirrel monkeys were trained to pursue moving visual targets and to fixate a head stationary or earth stationary target during passive whole body rotation (WBR). The firing behavior of 187 horizontal eye movement-related Purkinje (Pk) cells in the flocculus region was recorded during smooth pursuit eye movements and during WBR. Half of the Pk cells encountered were eye velocity Pk cells whose firing rates were related to eye movements during smooth pursuit and WBR. Their sensitivity to eye velocity during WBR was reduced when a visual target was not present, and their response to unpredictable steps in WBR was delayed by 80-100 ms, which suggests that eye movement sensitivity depended on visual feedback. They were insensitive to WBR when the VOR was canceled. The other half of the Purkinje cells encountered were sensitive to eye velocity during pursuit and to head velocity during VOR cancellation. They resembled the gaze velocity Pk cells previously described in rhesus monkeys. The head velocity signal tended to be less than half as large as the eye velocity-related signal and was observable at a short ( approximately 40 ms) latency when the head was unpredictably accelerated during ongoing VOR cancellation. Gaze and eye velocity type Pk cells were found to be intermixed throughout the ventral paraflocculus and flocculus. Most gaze velocity Pk cells (76%) were sensitive to ipsilateral eye and head velocity, but nearly half (48%) of the eye velocity Pk cells were sensitive to contralateral eye velocity. Thus the output of flocculus region is modified in two ways during cancellation of the VOR. Signals related to both ipsilateral and contralateral eye velocity are removed, and in approximately half of the cells a relatively weak head velocity signal is added. Unilateral injections of muscimol into the flocculus region had little effect on the gain of the VOR evoked either in the presence or absence of visual targets. However, ocular pursuit velocity and the ability to suppress the VOR by fixating a head stationary target were reduced by approximately 50%. These observations suggest that the flocculus region is an essential part of the neural substrate for both visual feedback-dependent and nonvisual mechanisms for canceling the VOR during passive head movements.

Role of the cerebellar flocculus region in the coordination of eye and head movements during gaze pursuit.

The contribution of the flocculus region of the cerebellum to horizontal gaze pursuit was studied in squirrel monkeys. When the head was free to move, the monkeys pursued targets with a combination of smooth eye and head movements; with the majority of the gaze velocity produced by smooth tracking head movements. In the accompanying study we reported that the flocculus region was necessary for cancellation of the vestibuloocular reflex (VOR) evoked by passive whole body rotation. The question addressed in this study was whether the flocculus region of the cerebellum also plays a role in canceling the VOR produced by active head movements during gaze pursuit. The firing behavior of 121 Purkinje (Pk) cells that were sensitive to horizontal smooth pursuit eye movements was studied. The sample included 66 eye velocity Pk cells and 55 gaze velocity Pk cells. All of the cells remained sensitive to smooth pursuit eye movements during combined eye and head tracking. Eye velocity Pk cells were insensitive to smooth pursuit head movements. Gaze velocity Pk cells were nearly as sensitive to active smooth pursuit head movements as they were passive whole body rotation; but they were less than half as sensitive ( approximately 43%) to smooth pursuit head movements as they were to smooth pursuit eye movements. Considered as a whole, the Pk cells in the flocculus region of the cerebellar cortex were <20% as sensitive to smooth pursuit head movements as they were to smooth pursuit eye movements, which suggests that this region does not produce signals sufficient to cancel the VOR during smooth head tracking. The comparative effect of injections of muscimol into the flocculus region on smooth pursuit eye and head movements was studied in two monkeys. Muscimol inactivation of the flocculus region profoundly affected smooth pursuit eye movements but had little effect on smooth pursuit head movements or on smooth tracking of visual targets when the head was free to move. We conclude that the signals produced by flocculus region Pk cells are neither necessary nor sufficient to cancel the VOR during gaze pursuit.

Neck proprioceptive inputs to primate vestibular nucleus neurons.

The contribution of neck proprioceptive signals to signal processing in the vestibular nucleus was studied by recording responses of secondary horizontal canal-related neurons to neck rotation in the squirrel monkey. Responses evoked by passive neck rotation while the head was held stationary in space were compared with responses evoked by passive whole body rotation and by forced rotation of the head on the trunk. Most neurons (76%; 45/59) were sensitive to neck rotation. The nature and strength of neck proprioceptive inputs varied and usually combined linearly with vestibular inputs. In most cases (94%), the direction of the neck proprioceptive input was "antagonistic" or "reciprocal" with respect to vestibular sensitivity and, consequently, reduced the vestibular response during head-on-trunk rotation. Different types of vestibular neurons received different types of proprioceptive input. Neurons whose firing behavior was related to eye position (position-vestibular-pause neurons and position-vestibular neurons) were often sensitive to the position of the head with respect to the trunk. The sensitivity to head position was usually in the same direction as the neuron's eye position sensitivity. Non-eye-movement related neurons and eye-head-velocity neurons exhibited the strongest sensitivity to passive neck rotation and had signals that were best related to neck velocity. The results suggest that neck proprioceptive inputs play an important role in shaping the output of the primate vestibular nucleus and its contribution to posture, gaze and perception.

Sensory processing in the vestibular nuclei during active head movements.

Many secondary vestibular neurons are sensitive to head on trunk rotation during reflex-induced and voluntary head movements. During passive whole body rotation the interaction of head on trunk signals related to the vestibulo-collic reflex with vestibular signals increases the rotational gain of many secondary vestibular neurons, including many that project to the spinal cord. In some units, the sensitivity to head on trunk and vestibular input is matched and the resulting interaction produces an output that is related to the trunk velocity in space. In other units the head on trunk inputs are stronger and the resulting interaction produces an output that is larger during the reflex. During voluntary head movements, inputs related to head on trunk movement combine destructively with vestibular signals, and often cancel the sensory reafferent consequences of self-generated movements. Cancellation of sensory vestibular signals was observed in all of the antidromically identified secondary vestibulospinal units, even though many of these units were not significantly affected by reflexive head on trunk movements. The results imply that the inputs to vestibular neurons related to head on trunk rotation during reflexive and voluntary movements arise from different sources. We suggest that the relative strength of reflexive head on trunk input to different vestibular neurons might reflect the different functional roles they have in controlling the posture of the neck and body.

Firing behavior of vestibular neurons during active and passive head movements: vestibulo-spinal and other non-eye-movement related neurons.

The firing behavior of 51 non-eye movement related central vestibular neurons that were sensitive to passive head rotation in the plane of the horizontal semicircular canal was studied in three squirrel monkeys whose heads were free to move in the horizontal plane. Unit sensitivity to active head movements during spontaneous gaze saccades was compared with sensitivity to passive head rotation. Most units (29/35 tested) were activated at monosynaptic latencies following electrical stimulation of the ipsilateral vestibular nerve. Nine were vestibulo-spinal units that were antidromically activated following electrical stimulation of the ventromedial funiculi of the spinal cord at C1. All of the units were less sensitive to active head movements than to passive whole body rotation. In the majority of cells (37/51, 73%), including all nine identified vestibulo-spinal units, the vestibular signals related to active head movements were canceled. The remaining units (n = 14, 27%) were sensitive to active head movements, but their responses were attenuated by 20-75%. Most units were nearly as sensitive to passive head-on-trunk rotation as they were to whole body rotation; this suggests that vestibular signals related to active head movements were cancelled primarily by subtraction of a head movement efference copy signal. The sensitivity of most units to passive whole body rotation was unchanged during gaze saccades. A fundamental feature of sensory processing is the ability to distinguish between self-generated and externally induced sensory events. Our observations suggest that the distinction is made at an early stage of processing in the vestibular system.

Integration of vestibular and head movement signals in the vestibular nuclei during whole-body rotation.

Single-unit recordings were obtained from 107 horizontal semicircular canal-related central vestibular neurons in three alert squirrel monkeys during passive sinusoidal whole-body rotation (WBR) while the head was free to move in the yaw plane (2.3 Hz, 20 degrees /s). Most of the units were identified as secondary vestibular neurons by electrical stimulation of the ipsilateral vestibular nerve (61/80 tested). Both non-eye-movement (n = 52) and eye-movement-related (n = 55) units were studied. Unit responses recorded when the head was free to move were compared with responses recorded when the head was restrained from moving. WBR in the absence of a visual target evoked a compensatory vestibulocollic reflex (VCR) that effectively reduced the head velocity in space by an average of 33 +/- 14%. In 73 units, the compensatory head movements were sufficiently large to permit the effect of the VCR on vestibular signal processing to be assessed quantitatively. The VCR affected the rotational responses of different vestibular neurons in different ways. Approximately one-half of the units (34/73, 47%) had responses that decreased as head velocity decreased. However, the responses of many other units (24/73) showed little change. These cells had signals that were better correlated with trunk velocity than with head velocity. The remaining units had responses that were significantly larger (15/73, 21%) when the VCR produced a decrease in head velocity. Eye-movement-related units tended to have rotational responses that were correlated with head velocity. On the other hand, non-eye-movement units tended to have rotational responses that were better correlated with trunk velocity. We conclude that sensory vestibular signals are transformed from head-in-space coordinates to trunk-in-space coordinates on many secondary vestibular neurons in the vestibular nuclei by the addition of inputs related to head rotation on the trunk. This coordinate transformation is presumably important for controlling postural reflexes and constructing a central percept of body orientation and movement in space.

Contribution of the cerebellar flocculus to gaze control during active head movements.

The flocculus and ventral paraflocculus are adjacent regions of the cerebellar cortex that are essential for controlling smooth pursuit eye movements and for altering the performance of the vestibulo-ocular reflex (VOR). The question addressed in this study is whether these regions of the cerebellum are more globally involved in controlling gaze, regardless of whether eye or active head movements are used to pursue moving visual targets. Single-unit recordings were obtained from Purkinje (Pk) cells in the floccular region of squirrel monkeys that were trained to fixate and pursue small visual targets. Cell firing rate was recorded during smooth pursuit eye movements, cancellation of the VOR, combined eye-head pursuit, and spontaneous gaze shifts in the absence of targets. Pk cells were found to be much less sensitive to gaze velocity during combined eye-head pursuit than during ocular pursuit. They were not sensitive to gaze or head velocity during gaze saccades. Temporary inactivation of the floccular region by muscimol injection compromised ocular pursuit but had little effect on the ability of monkeys to pursue visual targets with head movements or to cancel the VOR during active head movements. Thus the signals produced by Pk cells in the floccular region are necessary for controlling smooth pursuit eye movements but not for coordinating gaze during active head movements. The results imply that individual functional modules in the cerebellar cortex are less involved in the global organization and coordination of movements than with parametric control of movements produced by a specific part of the body.

Signal processing related to the vestibulo-ocular reflex during combined angular rotation and linear translation of the head.

The contributions of vestibular nerve afferents and central vestibular pathways to the angular (AVOR) and linear (LVOR) vestibulo-ocular reflex were studied in squirrel monkeys during fixation of near and far targets. Irregular vestibular afferents did not appear to be necessary for the LVOR, since when they were selectively silenced with galvanic currents the LVOR was essentially unaffected during both far- and near-target viewing. The linear translation signals generated by secondary AVOR neurons in the vestibular nuclei were, on average, in phase with head velocity, inversely related to viewing distance, and were nearly as strong as AVOR-related signals. We suggest that spatial-temporal transformation of linear head translation signals to angular eye velocity commands is accomplished primarily by the addition of viewing distance multiplied, centrally integrated, otolith regular afferent signals to angular VOR pathways.

Effects of viewing distance on the responses of horizontal canal-related secondary vestibular neurons during angular head rotation.

Effects of viewing distance on the responses of horizontal canal-related secondary vestibular neurons during angular head rotation. The eye movements generated by the horizontal canal-related angular vestibuloocular reflex (AVOR) depend on the distance of the image from the head and the axis of head rotation. The effects of viewing distance on the responses of 105 horizontal canal-related central vestibular neurons were examined in two squirrel monkeys that were trained to fixate small, earth-stationary targets at different distances (10 and 150 cm) from their eyes. The majority of these cells (77/105) were identified as secondary vestibular neurons by synaptic activation following electrical stimulation of the vestibular nerve. All of the viewing distance-sensitive units were also sensitive to eye movements in the absence of head movements. Some classes of eye movement-related vestibular units were more sensitive to viewing distance than others. For example, the average increase in rotational gain (discharge rate/head velocity) of position-vestibular-pause units was 20%, whereas the gain increase of eye-head-velocity units was 44%. The concomitant change in gain of the AVOR was 11%. Near viewing responses of units phase lagged the responses they generated during far target viewing by 6-25 degrees. A similar phase lag was not observed in either the near AVOR eye movements or in the firing behavior of burst-position units in the vestibular nuclei whose firing behavior was only related to eye movements. The viewing distance-related increase in the evoked eye movements and in the rotational gain of all unit classes declined progressively as stimulus frequency increased from 0.7 to 4.0 Hz. When monkeys canceled their VOR by fixating head-stationary targets, the responses recorded during near and far target viewing were comparable. However, the viewing distance-related response changes exhibited by central units were not directly attributable to the eye movement signals they generated. Subtraction of static eye position signals reduced, but did not abolish viewing distance gain changes in most units. Smooth pursuit eye velocity sensitivity and viewing distance sensitivity were not well correlated. We conclude that the central premotor pathways that mediate the AVOR also mediate viewing distance-related changes in the reflex. Because irregular vestibular nerve afferents are necessary for viewing distance-related gain changes in the AVOR, we suggest that a central estimate of viewing distance is used to parametrically modify vestibular afferent inputs to secondary vestibuloocular reflex pathways.

Effects of viewing distance on the responses of vestibular neurons to combined angular and linear vestibular stimulation.

Effects of viewing distance on the responses of vestibular neurons to combined angular and linear vestibular stimulation. The firing behavior of 59 horizontal canal-related secondary vestibular neurons was studied in alert squirrel monkeys during the combined angular and linear vestibuloocular reflex (CVOR). The CVOR was evoked by positioning the animal's head 20 cm in front of, or behind, the axis of rotation during whole body rotation (0.7, 1.9, and 4.0 Hz). The effect of viewing distance was studied by having the monkeys fixate small targets that were either near (10 cm) or far (1.3-1.7 m) from the eyes. Most units (50/59) were sensitive to eye movements and were monosynaptically activated after electrical stimulation of the vestibular nerve (51/56 tested). The responses of eye movement-related units were significantly affected by viewing distance. The viewing distance-related change in response gain of many eye-head-velocity and burst-position units was comparable with the change in eye movement gain. On the other hand, position-vestibular-pause units were approximately half as sensitive to changes in viewing distance as were eye movements. The sensitivity of units to the linear vestibuloocular reflex (LVOR) was estimated by subtraction of angular vestibuloocular reflex (AVOR)-related responses recorded with the head in the center of the axis of rotation from CVOR responses. During far target viewing, unit sensitivity to linear translation was small, but during near target viewing the firing rate of many units was strongly modulated. The LVOR responses and viewing distance-related LVOR responses of most units were nearly in phase with linear head velocity. The signals generated by secondary vestibular units during voluntary cancellation of the AVOR and CVOR were comparable. However, unit sensitivity to linear translation and angular rotation were not well correlated either during far or near target viewing. Unit LVOR responses were also not well correlated with their sensitivity to smooth pursuit eye movements or their sensitivity to viewing distance during the AVOR. On the other hand there was a significant correlation between static eye position sensitivity and sensitivity to viewing distance. We conclude that secondary horizontal canal-related vestibuloocular pathways are an important part of the premotor neural substrate that produces the LVOR. The otolith sensory signals that appear on these pathways have been spatially and temporally transformed to match the angular eye movement commands required to stabilize images at different distances. We suggest that this transformation may be performed by the circuits related to temporal integration of the LVOR.

Contribution of vestibular nerve irregular afferents to viewing distance-related changes in the vestibulo-ocular reflex.

The contribution of irregular vestibular afferents to viewing distance-related changes in the angular vestibulo-ocular reflex (AVOR) and combined angular and linear VOR (CVOR) was studied in squirrel monkeys trained to fixate earth-stationary targets that were near (10 cm) and distant (90-170 cm) from their eyes. Perilymphatic anodal galvanic currents were used to reversibly silence irregular vestibular afferents for periods of 4-5 s during the AVOR and CVOR evoked by 0.5- to 4-Hz sinusoidal rotations (6-20 degrees/s peak velocity) or 250-400 degrees/s2 acceleration steps. The direction and magnitude of linear translation were changed by positioning the monkeys at different distances off the axis of turntable rotation. The effects of irregular afferent galvanic ablation (GA) on viewing distance-related changes in the AVOR were studied in four animals. Viewing distance-related changes in the AVOR could not always be evoked and were frequently small in amplitude. GA reduced viewing distance-related change in the AVOR by an average of 64% when it was present. Thus vestibular irregular afferents appear to play an important and necessary role in viewing distance-related changes in the AVOR - on those occasions when the changes occur. Viewing distance-related changes in the CVOR were large and reliably evoked. GA had very little effect on the gain or phase of viewing distance-related changes in the CVOR, although the viewing distance-related CVOR responses of individual central vestibular neurons were affected. We conclude that irregular afferents probably contribute to central signal processing related to both the AVOR and the CVOR, but the signals carried by these afferents are only essential for viewing distance-related changes in AVOR.

Viewing distance related sensory processing in the ascending tract of deiters vestibulo-ocular reflex pathway.

The firing behavior of seven antidromically identified ascending tract of Deiters (ATD) neurons was recorded in one alert squirrel monkey trained to pursue moving targets and to fixate visual targets at different distances from the head during whole body rotation. 2. ATD cells generated signals related to contralateral horizontal smooth pursuit eye movements and to ipsilateral angular and linear head velocity. Most ATD neurons reversed the direction of their response to head rotation when the vestibulo-ocular reflex was canceled by fixation of a head stationary target. 3. ATD unit gains in respect to linear head velocity increased dramatically (> 4x) when a near, earth stationary target (10 cm from the eyes) was fixated, compared to the response recorded during fixation of a far target (130 to 170 cm from the eyes). Since the viewing distance related changes in the responses of ATD neurons closely parallel the changes in the responses of the eyes, the ATD appears to be an important premotor pathway for producing viewing distance related changes in the gain of the vestibulo-ocular reflex.

Contributions of regularly and irregularly discharging vestibular-nerve inputs to the discharge of central vestibular neurons in the alert squirrel monkey.

The discharge of neurons in the vestibular nuclei was recorded in alert squirrel monkeys while they were being sinusoidally rotated at 2 Hz. Type I position-vestibular-pause (PVP I) and vestibular-only (V I) neurons, as well as a smaller number of other type I and type II eye-plus-vestibular neurons were studied. Many of the neurons were monosynaptically related to the ipsilateral vestibular nerve. Eye-position and vestibular components of the rotation response were separated by multiple regression. Anodal currents, simultaneously delivered to both ears, were used to eliminate the head-rotation signals of irregularly discharging (I) vestibular-nerve afferents, presumably without affecting the corresponding signals of regularly discharging (R) afferents. R and I inputs to individual central neurons were determined by comparing rotation responses with and without the anodal currents. The bilateral currents, while reducing the background discharge of all types of neurons, did not affect the mean vestibular gain or phase calculated from a population of PVP I neurons or from a mixed population consisting of all type I units. From this result, it is concluded that I inputs are canceled at the level of secondary neurons. The cancellation may explain why the ablating currents do not affect the gain and phase of the vestibulo-ocular reflex. While cancellation was nearly perfect on a population basis, it was less so in individual neurons. For some neurons, the ablating currents decreased vestibular gain, while for other neurons the vestibular gain was increased. The former neurons are interpreted as receiving a net excitatory (I-EXC) I input, the latter neurons, a net inhibitory (I-INH) input. When compared with the corresponding R inputs, the I inputs were usually small and phase advanced. Phase advances were larger for I-EXC than for I-INH inputs. The sign and magnitude of the I inputs were unrelated to other discharge properties of individual neurons, including discharge regularity and the phase of vestibular responses measured in the absence of the ablating currents. Unilateral currents were used to assess the efficacy of ipsilateral and contralateral pathways. Ipsilateral pathways were responsible for almost all of the effects seen with bilateral currents. The results suggest that the vestibular signals carried by central neurons, even by those neurons receiving a monosynaptic vestibular-nerve input, are modified by polysynaptic pathways.