A non-visual mechanism for voluntary cancellation of the vestibulo-ocular reflex

Summary Squirrel monkeys were trained to cancel their vestibulo-ocular reflex (VOR) by fixating a visual target that was head stationary during passive vestibular stimulation. The monkeys were seated on a vestibular turntable, and their heads were restrained. A small visual target (0.2°) was projected from the vestibular turntable onto a tangent screen. The monkeys' ability to suppress their VOR by fixating a head stationary target while the turntable was moving was compared to their ability to pursue the target when it was moved in the same manner.Squirrel monkeys were better able to suppress their VOR when the turntable was moved at high velocities than they were able to pursue targets that were moving at high velocities. The gaze velocity gain during VOR cancellation began to decrease when the head velocity was above 80°/s, and was greater than 0.6 when the head velocity was above 150°/s. However, gaze velocity gain during smooth pursuit decreased significantly when the target velocity was greater than 60°/s, and was less than 0.4 when the target velocity was 150°/s or more.The latency of VOR suppression was significantly shorter than the latency of smooth pursuit while the monkey was cancelling its VOR. When an unpredictable step change in head acceleration was generated while the monkey was cancelling its VOR, the VOR evoked by the head acceleration step began to be suppressed shortly after the initiation of the step ( 30 ms). On the other hand, the latency of the smooth pursuit eye movement elicited when the visual target was accelerated in the same manner during VOR cancellation was 100 ms. The comparison between these two results suggests that the monkeys did not use visual information related to target motion to suppress their VOR at an early latency.The monkeys' ability to suppress the VOR evoked by an unexpected change in head acceleration depended on the size of the head acceleration step. The VOR evoked by unexpected step changes in head acceleration was progressively less suppressed at an early latency as the size of the acceleration step increased, and was not suppressed at an early latency when the step change in head acceleration was greater than 500°/s².During smooth pursuit eye movements, unexpected step changes in head acceleration evoked a VOR that was suppressed at an early latency ( 50 ms) if the head movement was in the same direction as the ongoing smooth pursuit eye movement. The amount of early VOR suppression increased as the pursuit eye velocity increased.We conclude that squirrel monkeys utilize a fast, non-visual mechanism for cancelling their VOR while they are fixating a visual target and their head is moving. This non-visual mechanism appears to be turned on when the head is moving and the monkey is fixating a head stationary target. The mechanism probably utilizes a voluntarily gated vestibular signal to cancel the signals in VOR pathways at the level of the extraocular motorneurons. Although the VOR cancellation mechanism is not capable of completely suppressing the VOR evoked by large unexpected changes in head acceleration, we suggest that it is capable of suppressing the VOR generated by most voluntary head movements during combined eye and head gaze pursuit and that the function of this gated VOR cancellation system is to extend the range and accuracy of eye-head tracking movements.     

Gaze shifts evoked by stimulation of the superior colliculus in the head-free cat conform to the motor map but also depend on stimulus strength and fixation activity

In our previous paper we demonstrated that electrical microstimulation of the fixation area at the rostral pole of the cat superior colliculus (SC) elicits no gaze movement but, rather, transiently suppresses eye-head gaze saccades. In this paper, we investigated the more caudal region of the SC and its interaction with the fixation area. In the alert head-free cat, supra-threshold stimulation in the anterior portion of the SC but outside the fixation area evoked small saccadic shifts of gaze consisting mainly of an eye movement, the head's contribution being small. Stimulating more posteriorly elicited large gaze saccades consisting of an ocular saccade combined with a rapid head movement. At these latter stimulation sites, craniocentric (goal-directed) eye movements were evoked when the cat's head was restrained. The amplitude of eye-head gaze saccades elicited at a particular stimulation site increased with stimulus duration, current strength, and pulse rate, until a constant or unit value was reached. The peak velocity of gaze shifts depended on both pulse rate and current strength. The movement direction was not affected by stimulus parameters. The unit gaze vector evoked, in the head-free condition, by stimulating one collicular site was similar to that coded by efferent neurons recorded at that site, thereby indicating a retinotopically coded gaze error representation on the collicular motor map which is not revealed by stimulating the head-fixed animal. Evoked gaze saccades were found to be influenced by fixation behavior. The amplitude of evoked gaze shifts was reduced if stimulation occurred when the hungry animal fixated a food target. Electrical activation of the collicular fixation area was found to mimic well the effects of natural fixation on evoked gaze shifts. Taken together, our results support the view that the overall distribution and level of collicular activity contributes to the encoding of the metrics of gaze saccades. We suggest that the combined levels of activity at the site being stimulated and at the fixation area influence the amplitude of evoked gaze saccades through competition. When stimulation is at low intensities, fixation-related activity reduces the amplitude of evoked gaze saccades. At high activation levels, the site being stimulated dominates and the gaze vector is specified only by that site's collicular output neurons, from which arises the close correspondence between the unit-evoked gaze saccades and the neurally coded gaze vector at that site.     

Neck muscle responses to stimulation of monkey superior colliculus. I. Topography and manipulation of stimulation parameters

The role of the primate superior colliculus (SC) in orienting head movements was studied by recording electromyographic (EMG) activity from multiple neck muscles following electrical stimulation of the SC. Combining SC stimulation with neck EMG recordings provides an objective and sensitive measure of the SC drive onto neck muscle motoneurons, particularly in relation to evoked gaze shifts. In this paper, we address how neck EMG responses to SC stimulation in head-restrained monkeys depend on the rostrocaudal, mediolateral, and dorsoventral location of the stimulating electrode within the SC and vary with manipulations of the eye position prior to stimulation onset and changes in stimulation current and duration. Stimulation predominantly evoked EMG responses on the muscles obliquus capitis inferior, rectus capitis posterior major, and splenius capitis. These responses became larger in magnitude and shorter in onset latency for progressively more caudal stimulation locations, consistent with turning the head. However, evoked responses persisted even for more rostral stimulation locations usually not associated with head movements. Manipulating initial eye position revealed that the magnitude of evoked responses became stronger as the eyes attained positions contralateral to the side of stimulation, consistent with a summation between a generic command evoked by SC stimulation and the influence of eye position on tonic neck EMG. Manipulating stimulation current and duration revealed that the relationship between gaze shifts and evoked EMG responses is not obligatory: short-duration (<20 ms) or low-current stimulation evoked neck EMG responses in the absence of gaze shifts. However, long-duration stimulation (>150 ms) occasionally revealed a transient neck EMG response aligned on the onset of sequential gaze shifts. We conclude that the SC drive to neck muscle motoneurons is far more widespread than traditionally supposed and is relayed through intervening elements which may or may not be activated in association with gaze shifts.     

Latency and accuracy of saccades to somatosensory targets

Purpose: To examine the accuracy and latency of reflexive saccades to vibratory stimulation of the fingertips made by normal human subjects and to compare the findings to those of visually guided saccades. Methods: Eye movements were recorded using infrared oculography. Stimuli were presented via an array of audiometric bone vibrator transducers driven at 250 Hz and positioned at eye level in a darkened room. Target locations were at 0 and ±5, 10, and 15 deg. Visual stimuli were green LEDs. Saccades were analysed interactively off-line and latency and amplitude measured for both types of saccade. Results: Saccades to tactile stimuli had longer latencies and showed less accurate final eye positions than those to equivalent visual targets; they were unaffected by subject age. Error magnitude for the tactile saccades increased monotonically with increasing target eccentricity; because the fingers remained in a fixed position throughout the testing, this also meant that error was lowest for the thumb and increased with progression outwards towards the ring finger. Visually guided refixations were accurate and differed less than 0.2 deg across target locations. Subject age had no effect on performance. Conclusions: Human subjects may make relatively accurate refixations to tactile targets in the absence of visual cues, but only to certain locations/fingers; others were localised very poorly. Further studies are needed to determine whether finger selection or target location is the primary determinant of accuracy in this task.This research was supported by the Australian Research Council, Canberra, Australia     

Eye position modulates the electromyographic responses of neck muscles to electrical stimulation of the superior colliculus in the alert cat

Rapid gaze shifts are often accomplished with coordinated movements of the eyes and head, the relative amplitude of which depends on the starting position of the eyes. The size of gaze shifts is determined by the superior colliculus (SC) but additional processing in the lower brain stem is needed to determine the relative contributions of eye and head components. Models of eye–head coordination often assume that the strength of the command sent to the head controllers is modified by a signal indicative of the eye position. Evidence in favor of this hypothesis has been recently obtained in a study of phasic electromyographic (EMG) responses to stimulation of the SC in head-restrained monkeys (Corneil et al. in J Neurophysiol 88:2000–2018, 2002b). Bearing in mind that the patterns of eye–head coordination are not the same in all species and because the eye position sensitivity of phasic EMG responses has not been systematically investigated in cats, in the present study we used cats to address this issue. We stimulated electrically the intermediate and deep layers of the caudal SC in alert cats and recorded the EMG responses of neck muscles with horizontal and vertical pulling directions. Our data demonstrate that phasic, short latency EMG responses can be modulated by the eye position such that they increase as the eye occupies more and more eccentric positions in the pulling direction of the muscle tested. However, the influence of the eye position is rather modest, typically accounting for only 10–50% of the variance of EMG response amplitude. Responses evoked from several SC sites were not modulated by the eye position.     

Compensatory head and eye movements in the frog and their contribution to stabilization of gaze

Summary Compensatory head movements, recorded in unrestrained frogs, were compared to compensatory eye movements recorded from animals that had their head fixed. Movements were evoked by oscillating the animal in the dark (vestibular stimulation) or in the light in front of an earth-fixed, patterned visual background (combined stimulation) or by rotating vertical black and white bars (optokinetic stimulation) around the stationary animal. Oscillations occurred in the horizontal plane at frequencies between 0.025 and 0.5 Hz. Gain and phase values of head and eye movements, relative to stimulus movements were calculated.Evoked eye movements were limited in amplitude to 3–6 °, increasing with the size of the animal. Head movements were limited to ±30–40 °. Resetting fast-phases of both head and eyes were very rarely observed during sinusoidal stimulation and no eye movements were recorded in the absence of intended head movements.Vestibularly evoked head movements exhibited a frequency-dependent threshold that was not observed for vestibulo-ocular responses. Above threshold, the gain of evoked head responses increased and reached a frequency-dependent plateau at which the system behaved approximately linearly. Within the linear range, gain of vestibularly evoked responses increased with frequency (from 0.04 at 0.025 Hz to 0.75 at 0.5 Hz) and phase lead decreased (from about 80 ° to 0 °). Vestibularly evoked eye movements similarly increased in gain from 0.05 to 0.56 and decreased in phase lead from about 56 ° to 10 ° over the same frequency range.Optokinetically evoked head and eye movements had their highest gains (about 0.8 and 0.5) at low constant velocities ( 1–4 °/s) or frequencies ( 0.025 Hz). At higher constant velocities or frequencies the gain dropped. The phase lag increased from close to zero (at 0.025 Hz) to about 60 ° for the head and to about 20 ° for the eye movements (at 0.25 Hz). These phase lags are explained by reaction times of the evoked movements of about 600 ms (head) and 200 ms (eyes).Combined stimulation evoked compensatory head movements with gain and phase values that were frequency-independent in the linear range. Head movements compensated for about 80–90% of the imposed gaze shift with a small phase lag (0–10 °). Evoked eye movements were found to be large enough in amplitude and fast enough in time to enable a frog to stabilize its gaze exclusively with slow phase compensatory movements for a large variety of frequency and amplitude combinations. The two motor systems controlling movements of the head and the eye are matched in such a way that the non-linearities of the evoked eye movements can compensate for the non-linearities of the evoked head movements.Supported by a grant from Deutsche Forschungsgemeinschaft (Pr. 158/2) and Swiss National Science Foundation (3.505.79 and 3.616.80)     

Recovery from conduction failure in optic axons spared by lesions in the rat

Summary A chronic preparation has been developed to determine the physiological sequelae of a lesion of optic axons, and to correlate these with the optic innervation of the superior colliculus (SC). During deep pentobarbital anesthesia: (1) Vigorous small-field (2–5°) multi-unit responses were recorded from microelectrodes in the lower stratum griseum superficiale (SGS) and upper stratum opticum (SO), evoked by the motion of a 40° black edge. (2) The maps of the retinal projection, obtained without immobilization of the eye, were regular in about 1/3 of cases and only slightly irregular in most of the remainder. (3) There was a rhythmic (10–20/s), retina-dependent firing of spike bursts in visually responsive SC. At a lighter level of anesthesia this bursting reduced or disappeared, and coincidentally there was a rapid enlargement of the multi-unit receptive fields (MU-RFs). It is suggested that GABA-based inhibitory mechanisms were involved in producing both the rhythmic bursting and the smaller-field MU-RFs. At either anesthetic level the movement of a 5° disc produced 20–65° MU-RFs. Vertical lesions which severed part of the visual pathway reduced and, if large enough, abolished responses even to a strong testing stimulus that normally produced a response from an area of visual field larger than the range of eye movement. The region of SC affected was always medial or lateral, and sites within the still-responsive area had normally small MU-RFs. Repeated mappings over the following hours to days revealed a variable recovery of response; this recovery always occurred within the silenced zone of SC nearest to the still-responsive area. Ultimately two discrete zones, responsive and silent, persisted. Mapping the SC, at 500 m intervals, involved about 25 microelectrode penetrations. The number of sites silenced immediately postlesion was 1–24. Response returned at 1–15 of these, corresponding to 4–60% of the entire SC. Most of the recoveries occurred by 2 h postlesion, but occasionally an additional 1–3 sites recovered during the following 2–8 days. Recovered MU-RFs were small, and appropriately located within the map. Intraocular horseradish peroxidase (HRP) accumulated in severed optic axons but was transported with no obvious hindrance in spared axons alongside the lesion. Within the SC a sparse optic innervation extended beyond the responsive area, into about 0.5 mm of the region which remained silent after the recoveries were completed. The recoveries are likely to be based on the reversal of conduction failure in a population of mechanically distorted but intact optic axons, presumably very near to those severed by the lesion. The medial or lateral localization of silencing produced by the lesions, and the characteristic siting of the recoveries at the border between responsive and silent SC, support the interpretation that the pathways studied had some degree of medial-to-lateral topographic ordering. The experimental protocol established by these studies should help in the identification of mechanisms responsible for the more delayed recoveries that have been reported after similar lesions.     

Modification of saccadic eye movements by GABA-related substances. I. Effect of muscimol and bicuculline in monkey superior colliculus

Our previous observations led to the hypothesis that cells in the substantia nigra pars reticulata (SNr) tonically inhibit saccade-related cells in the intermediate layers of the superior colliculus (SC). Before saccades to visual or remembered targets, cells in SNr briefly reduce that inhibition, allowing a burst of spikes of SC cells that, in turn, leads to the initiation of a saccadic eye movement. Since this inhibition is likely to be mediated by gamma-aminobutyric acid (GABA), we tested this hypothesis by injecting a GABA agonist (muscimol) or a GABA antagonist (bicuculline) into the superior colliculus and measured the effects on saccadic eye movements made to visual or remembered targets. An injection of muscimol selectively suppressed saccades to the movement field of the cells near the injection site. The affected area expanded over time, thus suggesting the diffusion of muscimol in the SC; the area never included the other hemifield, suggesting that the diffusion was limited to one SC. One of the monkeys became unable to make any saccades to the affected area. Saccades to visual targets following injection of muscimol had longer latency and slightly shorter amplitudes that were corrected by subsequent saccades. The most striking change was a decrease in the peak velocity of the saccade, frequently to less than half the preinjection value. Saccades to remembered targets following injection of muscimol also showed an increase in latency and decrease in velocity, but in addition, showed a striking decrease in the accuracy of the saccades. The trajectories of saccades became distorted as if they were deflected away from the affected area. After muscimol injection, the area over which spontaneous eye movements were made shifted toward the side ipsilateral to the injection. Saccades toward the contralateral side were less frequent and slower. In nystagmus, which developed later, the slow phase was toward the contralateral side. In contrast to muscimol, injection of bicuculline facilitated the initiation of saccades. Injection was followed almost immediately by stereotyped and apparently irrepressible saccades made toward the center of the movement field of the SC cells at the injection site. The monkeys became unable to fixate during the tasks; the fixation was interrupted by saccadic jerks made to the affected area of the visual field and then back to the fixation point.(ABSTRACT TRUNCATED AT 400 WORDS)     

Not looking while leaping: the linkage of blinking and saccadic gaze shifts

Many vertebrates generate blinks as a component of saccadic gaze shifts. We investigated the nature of this linkage between saccades and blinking in normal humans. Activation of the orbicularis oculi, the lid closing muscle, EMG occurred with 97% of saccadic gaze shifts larger than 33°. The blinks typically began simultaneously with the initiation of head and/or eye movement. To minimize the possibility that the blinks accompanying saccadic gaze shifts were reflex blinks evoked by the wind rushing across the cornea and eyelashes as the head and eyes turned, the subjects made saccadic head turns with their eyes closed. In this condition, orbicularis oculi EMG activity occurred with all head turns greater than 17° in amplitude and the EMG activity began an average of 39.3 ms before the start of the head movement. Thus, one component of the command for large saccadic gaze shifts appears to be a blink. We call these blinks gaze-evoked blinks. The linkage between saccadic gaze shifts and blinking is reciprocal. Evoking a reflex blink prior to initiating a voluntary saccadic gaze shift dramatically reduces the latency of the initiation of the head movement.     

Development of intertectal neuronal connections in Xenopus: The effects of contralateral transposition of the eye and of eye removal

The development of intertectal neruonal connections has been investigated in Xenopus laevis. Contralateral eye grafts and enucleations were performed in embryos and the resultant visual projections to the optic tecta were mapped electrophysiologically after metamorphosis. In enucleated animals the ipsilateral projections were found to be normally organised retinotopically but consisted of visual units with abnormally large multi-unit receptive fields. In 10 animals with contralaterally grafted eyes a normal ipsilateral projection had developed from the abnormal eye and an abnormal projection from the normal eye, to produce congruent maps via the two eyes to one tectum. All the maps in these animals were retinotopically organised. In another 11 animals the ipsilateral projection from the operated eye was fragmentary or absent, while that from the unoperated eye resembled the pattern found after enucleation. Retinotopically abnormal contralateral projections had developed in 5 animals of this group. These results suggest that prefunctional specification determines the initial development of diffuse intertectal visual connections but these may be modified by a process of binocular interaction in the presence of a normal primary contralateral input.     

Smooth eye movements evoked by electrical stimulation of the cat's superior colliculus

Head-fixed gaze shifts were evoked by electrical stimulation of the deeper layers of the cat superior colliculus (SC). After a short latency, saccades were triggered with kinematics similar to those of visually guided saccades. When electrical stimulation was maintained for more than 150–200 ms, postsaccadic smooth eye movements (SEMs) were observed. These movements were characterized by a period of approximately constant velocity following the evoked saccade. Depending on electrode position, a single saccade followed by a slow displacement or a staircase of saccades interspersed by SEMs were evoked. Mean velocity decreased with increasing deviation of the eye in the orbit in the direction of the movement. In the situation where a single evoked saccade was followed by a smooth movement, the duration of the latter depended on the duration of the stimulation train. In the situation where evoked saccades converged towards a restricted region of the visual field (goal-directed or craniocentric saccades), the SEMs were directed towards the centre of this region and their mean velocity decreased as the eye approached the goal. The direction of induced SEMs depended on the site of stimulation, as is the case for saccadic eye movements, and was not modified by stimulation parameters (place code). On the other hand, mean velocity of the movements depended on the site of stimulation and on the frequency and intensity of the current (rate code), as reported for saccades in the cat. The kinematics of these postsaccadic SEMs are similar to the kinematics of slow, postsaccadic correction observed during visually triggered gaze shifts of the alert cat. These results support the hypothesis that the SC is not exclusively implicated in the control of fast refixation of gaze but also in controlling postsaccadic conjugate slow eye movements in the cat.     

Disconnection of parietal and occipital access to the saccadic oculomotor system

Summary The experiment explored the networks through which signals arising from visual areas of cortex control saccadic eye movements. Electrical stimulation of the inferior parietal and the occipital cortex (here termed the posterior eye fields) normally evokes saccadic eye movements. We replicated previous reports that these evoked eye movements ceased after large tectal ablations. This initial finding suggested that the posterior eye fields depended on a single route of access to the saccade generator, one descending through the superior colliculus (SC). On closer examination, the critical lesion appeared to be one which removed the SC and cut efferents from the frontal eye field (FEE) coursing nearby. Subsequently we confirmed that eye movements evoked from the posterior eye fields ceased after cooling the SC, or cutting its efferents- but only when one of these procedures was combined with FEF ablation. Thus, visual signals from the occipital and inferior parietal cortex have more than one, but perhaps only two routes of access to the oculomotor system. One passes through the superior colliculus, the other through the frontal eye field. Ancillary experiments revealed that inferior parietal and FEF ablations, alone or combined, do not disrupt saccades evoked from the occipital lobe. Striate and prestriate areas can therefore use their own direct input to the SC or to the basal ganglia to drive saccadic eye movements.Abbreviations for Nuclei C interstitial, of Cajal - CL central lateral - CM centromedianum - D Darkschewitsch - HL lateral habenula - IC inferior colliculus - LD lateral dorsal - LP lateral posterior nucl - MD medial dorsal - MG medial geniculate - MR midbrain reticular formation - NPA nuc. of the pretectal area - NPC nuc. of the posterior commissure - PC posterior commissure - PF parafascicular - PI inferior pulvinar - PL lateral pulvinar - PM medial pulvinar - PO oral pulvinar - R red - SG suprageniculate - SL sublentiform - SN substantia nigra - VPL ventral posterolateral - VPM ventral posteromedial - ZI zona incerta - III oculomotor - IV trochlear Supported by grants NIH EY02941 and EY04005, NSF BNS8603915     

Phenotypic differences between induced and spontaneous 2μ-plasmid-free segregants of Saccharomyces cerevisiae

Summary The maximum specific growth rates (_max) of 2 -plasmid-free ([cir°]) segregants of three haploid and one diploid strain of Saccharomyces cerevisiae have been determined and compared with the _max of their 2 -plasmid-containing ([cir ⁺]) progenitors. Two classes of [cir°] strains have been examined: those induced by transformation with a 2 -based recombinant plasmid according to the method of Dobson et al. (1980) and those isolated as spontaneous [cir°] segregants from glucose-limited continuous cultures. The _max of the spontaneous [cir°] segregants was not found to differ significantly from that of their [cir ⁺] parents. In all cases, however, the induced [cir°] strains had a _max which was significantly less than that of their [cir ⁺] counterparts. This effect was particularly marked in the case of the diploid strain where a 34% reduction in _max was observed. The implications of these results are discussed in terms of the effect of the transformation process on host yeast cells.     

How the frontal eye field can impose a saccade goal on superior colliculus neurons.

Saccades were electrically evoked from the frontal eye field (FEF) of two trained monkeys while saccade-cells were recorded from the intermediate layers of the superior colliculus (SC). We found that FEF microstimulation, eliciting saccades of a given vector, excited SC saccade-cells encoding the same vector and inhibited all others. Such a mechanism can prevent competing commands from arising simultaneously in different structures.     

Functional coupling of the stabilizing eye and head reflexes during horizontal and vertical linear motion in the cat

Summary The otolith contribution and otolith-visual interaction in eye and head stabilization were investigated in alert cats submitted to sinusoidal linear accelerations in three defined directions of space: up-down (Z motion), left-right (Y motion), and forward-back (X motion). Otolith stimulation alone was performed in total darkness with stimulus frequency varying from 0.05 to 1.39 Hz at a constant half peak-to-peak amplitude of 0.145 m (corresponding acceleration range 0.0014–1.13 g) Optokinetic stimuli were provided by sinusoidally moving a pseudorandom visual pattern in the Z and Y directions, using a similar half peak-to-peak amplitude (0.145 m, i.e., 16.1°) in the 0.025–1.39 Hz frequency domain (corresponding velocity range 2.5°–141°/s). Congruent otolith-visual interaction (costimulation, CS) was produced by moving the cat in front of the earth-stationary visual pattern, while conflicting interaction was obtained by suppressing all visual motion cues during linear motion (visual stabilization method, VS, with cat and visual pattern moving together, in phase). Electromyographic (EMG) activity of antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles as well as horizontal and vertical eye movements (electrooculography, EOG) were recorded in these different experimental conditions. Results showed that otolith-neck (ONR) and otolith-ocular (OOR) responses were produced during pure otolith stimulation with relatively weak stimuli (0.036 g) in all directions tested. Both EMG and EOG response gain slightly increased, while response phase lead decreased (with respect to stimulus velocity) as stimulus frequency increased in the range 0.25–1.39 Hz. Otolith contribution to compensatory eye and neck responses increased with stimulus frequency, leading to EMG and EOG responses, which oppose the imposed displacement more and more. But the otolith system alone remained unable to produce perfect compensatory responses, even at the highest frequency tested. In contrast, optokinetic stimuli in the Z and Y directions evoked consistent and compensatory eye movement responses (OKR) in a lower frequency range (0.025–0.25 Hz). Increasing stimulus frequency induced strong gain reduction and phase lag. Oculo-neck coupling or eye-head synergy was found during optokinetic stimulation in the Z and Y directions. It was characterized by bilateral activation of neck extensors and flexors during upward and downward eye movements, respectively, and by ipsilateral activation of neck muscles during horizontal eye movements. These visually-induced neck responses seemed related to eye velocity signals. Dynamic properties of neck and eye responses were significantly improved when both inputs were combined (CS). Near perfect compensatory eye movement and neck muscle responses closely related to stimulus velocity were observed over all frequencies tested, in the three directions defined. The present study indicates that eye-head coordination processes during linear motion are mainly dependent on the visual system at low frequencies (below 0.25 Hz), with close functional coupling of OKR and eye-head synergy. The otolith system basically works at higher stimulus frequencies and triggers Synergist OOR and ONR. However, both sensorimotor subsystems combine their dynamic properties to provide better eyehead coordination in an extended frequency range and, as evidenced under VS condition, visual and otolith inputs also contribute to eye and neck responses at high and low frequency, respectively. These general laws on functional coupling of the eye and head stabilizing reflexes during linear motion are valid in the three directions tested, even though the relative weight of visual and otolith inputs may vary according to motion direction and/or kinematics.     

Frontal lobe lesions in man cause difficulties in suppressing reflexive glances and in generating goal-directed saccades

Summary The frontal eye field (FEF) and superior colliculus (SC) are thought to form two parallel systems for generating saccadic eye movements. The SC is thought classically to mediate reflex-like orienting movements. Thus it can be hypothesized that the FEF exerts a higher level control on a visual grasp reflex. To test this hypothesis we have studied the saccades of patients who have had discrete unilateral removals of frontal lobe tissue for the relief of intractable epilepsy. The responses of these patients were compared to those of normal subjects and patients with unilateral temporal lobe removals. Two tasks were used. In the first task the subject was instructed to look in the direction of a visual cue that appeared unexpectedly 12° to the left or right of a central fixation point (FP), in order to identify a patterned target that appeared 200 ms or more later. In the second anti-saccade task the subject was required to look not at the location of the cue but in the opposite direction, an equal distance from FP where after 200 ms or more the patterned target appeared. Three major observations have emerged from the present study. (a) Most frontal patients, with lesions involving both the dorsolateral and mesial cortex had long term difficulties in suppressing disallowed glances to visual stimuli that suddenly appeared in peripheral vision. (b) In such patients, saccades that were eventually directed away from the cue and towards the target were nearly always triggered by the appearance of the target itself irrespective of whether or not the anti-saccade was preceded by a disallowed glance. Those eye movements away from the cue were only rarely generated spontaneously across the blank screen during the cue-target time interval. (c) The latency of these visually-triggered saccades was very short (80–140 ms) compared to that of the correct saccades (170–200 ms) to the cue when the cue and target were on the same side, thereby suggesting that the structures removed in these patients normally trigger saccades after considerable computations have already been performed. The results support the view that the frontal lobes, particularly the dorsolateral region which contains the FEF and possibly the supplementary motor area contribute to the generation of complex saccadic eye-movement behaviour. More specifically, they appear to aid in suppressing unwanted reflex-like oculomotor activity and in triggering the appropriate volitional movements when the goal for the movement is known but not yet visible.     

Minimal synaptic delay in the saccadic output pathway of the superior colliculus studied in awake monkey

The synaptic organization of the saccade-related neuronal circuit between the superior colliculus (SC) and the brainstem saccade generator was examined in an awake monkey using a saccadic, midflight electrical-stimulation method. When microstimulation (50–100 A, single pulse) was applied to the SC during a saccade, a small, conjugate contraversive eye movement was evoked with latencies much shorter than those obtained by conventional stimulation. Our results may be explained by the tonic inhibition of premotor burst neurons (BNs) by omnipause neurons that ceases during saccades to allow BNs to burst. Thus, during saccades, signals originating from the SC can be transmitted to motoneurons and seen in the saccade trajectory. Based on this hypothesis, we estimated the number of synapses intervening between the SC and motoneurons by applying midflight stimulation to the SC, the BN area, and the abducens nucleus. Eye position signals were electronically differentiated to produce eye velocity to aid in detecting small changes. The mean latencies of the stimulus-evoked eye movements were: 7.9±1.0 ms (SD; ipsilateral eye) and 7.8±0.9 ms (SD; contralateral eye) for SC stimulation; 4.8±0.5 ms (SD; ipsilateral eye) and 5.1±0.7 ms (SD; contralateral eye) for BN stimulation; and 3.6±0.4 ms (SD; ipsilateral eye) and 5.2±0.8 ms (SD; contralateral eye) for abducens nucleus stimulation. The time difference between SC- and BN-evoked eye movements (about 3 ms) was consistent with a disynaptic connection from the SC to the premotor BNs.     

Visuotopic information conveyed by each eye to the opossum's superior colliculus

Summary The uniocular visual field representations on the superior colliculus (SC), as estimated from multiunit response field centres about the horizontal meridian, were compared in midpontine pretrigeminal opossums (Didelphis marsupialis aurita Wied 1826). Recordings from the rostral pole (RP) and its caudal neighbour, the direct binocular region (DBR), as defined by Rocha-Miranda et al. (1978), were distinguished by the histological control. The results showed that while the hemifield contralateral to the recording site was well represented on the DBR by both eyes, the ipsilateral hemifield was generously represented at the RP only by the contralateral eye. At the RP the ipsilateral eye usually conveyed information about the vertical meridian, bringing about an expanded representation of the central visual space. Distinct patterns of representation were also recognized on graphs which relate recording sites along the AP axis of the SC with the azimuths of response field centres. The representation of the vertical reference meridian upon this axis on an oculocentric system was estimated from the DBR data and localized in the RP, at about 500 m from the rostral end, for the ipsilateral eye (Vo') and in the DBR, at about 800 m for the other eye (Vo). Similarly, plots of the magnification factor against the AP collicular axis indicated different strategies of representation for each eye. At the segment between 500 and 800 m on this axis the magnification factors of the ipsilateral eye were usually much higher than those of the other eye. Furthermore, horizontal disparities between field centres were shown to have distinct distributions along the AP axis within the RP and DBR regions. At the latter a constant crossed disparity value (median=5.3°) was present along the AP axis while at the former greater variability and higher central disparity values were detected. An argument is developed based on this data to suggest that under our conditions the central binocular axis of the opossum are convergent with respect to the visual axis and their representation centred about the RP/DBR boundary. The different strategy of representation adopted by each eye at these two regions argue against a redundancy in the processing of visual information at the RP and the DBR on the opposite side, both of which bear a representation of the same visual space when considering only the information conveyed by the eye contralateral to the RP. The possible roles of this organization and the relevancy of these findings for studies of plastic rearrangement are discussed.Supported by Financiadora de Estudos e Projetos (FINEP/ FUJB 4.3.83.0540.00), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq — Proc. 40.0220/82; 40.0304/83) and Conselho de Ensino para Graduados da UFRJ (CEPG/ UFRJ)     

Significance of skin temperature changes in surface electromyography

Summary Differing results have been reported concerning the direction and quantity of the electromyogram (EMG) amplitude response to changes in tissue temperature. The EMG signals from the soleus muscle of six healthy human subjects were therefore recorded during dynamic exercise (concentric contractions) at ambient temperatures of 30°C and 14°C. The mean skin temperature above the muscle investigated was 32.9° C and 21.7° C, respectively. The core temperature, estimated by rectal temperature, was unchanged. The cooling of the superficial tissues caused approximately a doubling of the EMG amplitude. For the probability level 0.9 in the amplitude probability distribution function, the average signal level increased from 73 V to 135 V (P=0.02). The average mean power frequency of the EMG signal was reduced from 142 Hz to 83 Hz (P=0.004). The amplitude increase was not due to shivering but other possible explanations are presented. As the changes in T _sk investigated were within the range which may occur normally during the working hours, it was concluded that T _sk should be carefully controlled in vocational EMG studies.     

Pretectal jerk neuron activity during saccadic eye movements and visual stimulations in the cat

Summary The activity of jerk neurons was recorded extracellularly in the pretectum of the awake cat. The characteristic response of jerk neurons was a short, high-frequency burst that occurred after fast movements (jerks) of a large, structured visual stimulus, during saccadic eye movements in the light, and after on or off visual stimulation. Mean burst latency to pure visual jerks was 50 ms, whereas it was 30 ms to saccadic eye movements. Bursts were found to be stereotyped; the highest discharge rate was always at burst onset. Jerk neurons were not selective for stimulus parameters (such as movement amplitude or direction) except that in some neurons a weak correlation between stimulus velocity and discharge frequency was found. During saccades in the dark, clear bursts were only rarely found. In about half of the neurons, however, there was a slight but significant increase in the number of spikes above spontaneous frequency. Visual receptive fields were very large (46° horizontal and 35° vertical extent, on average). Nevertheless, the pretectal jerk neurons showed a rough retinotopic order, which was in accordance with the published retinotopy of the pretectum. Jerk neurons were found throughout the whole superficial pretectum, but preferentially in an area that corresponds to the nucleus of the optic tract (NOT) and the nucleus pretectalis posterior (NPP). Saccades were elicited by electrical stimulations at the sites where jerk neurons were recorded. The direction of the elicited saccades depended strongly on the pretectal stimulation site. A possible role of the jerk neurons as a visuomotor relay to elicit saccades or to modulate perception and attention is discussed.