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L E Mays

Publications and source records attributed to L E Mays.

At least 19 recordsLinked to original sources
PubMed1992

Dynamic properties of medial rectus motoneurons during vergence eye movements.

1. An early study by Keller reported that medial rectus motoneurons display a step change in firing rate during accommodative vergence movements. However, a later study by Mays and Porter reported gradual changes in firing rate during symmetrical vergence movements. Furthermore, subsequent inspection of the activity of individual medial rectus motoneurons during vergence movements indicated transient changes in their firing rate that had not been noted by Mays and Porter. For conjugate eye movements, in addition to a position signal, motoneurons display an eye velocity signal that compensates for the characteristics of the oculomotor plant. This suggested that the transient change in firing rate seen during vergence movements represented a velocity signal. Therefore the present study used single-unit recording techniques in alert rhesus monkeys to examine the dynamic behavior of medial rectus motoneurons during vergence eye movements. 2. The relationship between firing rate and eye velocity was first studied for vergence responses to step changes in binocular disparity and accommodative demand. Inspection of single trials showed that medial rectus motoneurons display transient changes in firing rate during vergence eye movements. To better visualize the dynamic signal during vergence movements, an expected firing rate (eye position multiplied by position sensitivity of the cell plus its baseline firing rate) was subtracted from the actual firing rate to yield a difference firing rate, which was displayed along with the eye velocity trace for individual trials. During all smooth symmetrical vergence movements, the profile of the difference firing rate very closely resembled the velocity profile. 3. To quantify the relationship between eye velocity and firing rate, two approaches were taken. In one, peak eye velocity was plotted against the difference firing rate. This plot yielded a measure of the velocity sensitivity of the cell (prv). In the other, a scatter plot was produced in which horizontal eye velocity throughout the vergence eye movement was plotted against the difference firing rate. This plot yielded a second measure of the velocity sensitivity of the cell (rv). 4. The behavior of 10 cells was studied during both sinusoidal vergence tracking and conjugate smooth pursuit over a range of frequencies from 0.125 to 1.0 Hz. This enabled the frequency sensitivity of the medial rectus motoneurons to be assessed for both types of movements. Both vergence velocity sensitivity and smooth pursuit velocity sensitivity decreased with increasing frequency. This is similar to a finding by Fuchs and co-workers for lateral rectus motoneurons during smooth pursuit eye movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Characteristics of near response cells projecting to the oculomotor nucleus.

1. Previous work has shown neurons just dorsal and lateral to the oculomotor nucleus that increase their firing rate with increases in the angle of ocular convergence. It has been suggested that the output of these midbrain near response cells might provide the vergence command needed by the medial rectus motoneurons. However, lens accommodation ordinarily accompanies convergence, and a subsequent study showed that only about one-half of these midbrain near response cells carried a signal related exclusively to vergence. One hypothesis suggested by this finding is that this subgroup of neurons might have a unique role in providing a "pure" vergence signal to the medial rectus motoneurons. 2. In the present study extracellular recordings were made from midbrain near response cells in monkeys while eye position and lens accommodation were measured. The monkeys viewed targets through an optical system that allowed the accommodative and ocular vergence demands to be manipulated independently. This approach was used to produce a partial dissociation of accommodative and vergence responses, so that an accommodative and vergence coefficient could be determined for each cell, by the use of the following equation FR = R0 + kda x AR + kdv x CR where FR is the firing rate of the near response cell, R0 is the predicted firing rate for a distant target, kda is the (dissociated) accommodation coefficient, AR is the accommodative response, kdv is the (dissociated) vergence coefficient, and CR is the convergence response. 3. The vergence and accommodation coefficients were determined for a large number of midbrain near response cells, including a subset that could be antidromically activated from the medial rectus subdivisions of the oculomotor nucleus. Some near response neurons were found with signals related exclusively to convergence (i.e., kdv greater than 0 and kda = 0), whereas several others had signals related exclusively to lens accommodation (i.e., kda greater than 0 and kdv = 0). The majority of the near response cells had signals related to both responses (i.e., kda not equal to 0 and kdv not equal to 0). Furthermore, the vergence and accommodation coefficients of near response cells appeared to be continuously distributed. Some cells had negative accommodation or vergence coefficients. 4. The 17 near response cells that could be antidromically activated from the oculomotor nucleus presumably provide vergence signals to the medial rectus motoneurons. Although all had positive vergence coefficients, only four of these cells carried signals that were related exclusively to vergence.(ABSTRACT TRUNCATED AT 400 WORDS)

Accommodation, Ocular↗
PubMed1991

Antidromic identification of midbrain near response cells projecting to the oculomotor nucleus.

Medial rectus motoneurons carry both conjugate and vergence eye position signals. Abducens internuclear neurons, whose axons travel in the medial longitudinal fasciculus, provide these motoneurons with the major signal for conjugate eye movements but not for vergence eye movements. A vergence signal appropriate for these motoneurons is seen on the near response cells that are found in the mesencephalic reticular formation within 2 mm of the oculomotor nucleus. The goal of the present study was to determine if midbrain near response cells project to the medial rectus subdivision of the oculomotor nucleus. Near response cells were recorded in two trained rhesus monkeys with ocular search coils. A stimulating electrode was positioned within the medial rectus subdivision of the oculomotor nucleus. Twenty-eight near response cells were found that could be driven by single pulse microstimulation of the ipsilateral medial rectus subdivision. In all cases, antidromic activation was confirmed by collision testing. Attempts to antidromically activate midbrain near response cells from the contralateral medial rectus subdivision were unsuccessful. Most antidromically activated cells had a steady state firing rate proportional to vergence angle. One cell also showed burst activity during the vergence eye movements. Divergence cells were not antidromically activated.

Animals↗
PubMed1991

Trochlear unit activity during ocular convergence.

1. Ocular convergence is usually accompanied by excyclotorsion of the eyes. Furthermore, the magnitude of cyclotorsion is dependent on the elevation of the eyes. The reason for this excyclotorsion during convergence is not understood. 2. Excyclotorsion could be produced by either increased activity in the inferior oblique muscle or decreased activity in the superior oblique muscle. An earlier study indicated that convergence may also be accompanied by a temporal (lateral) translation of the eye. This observation is more consistent with a relaxation of the superior oblique than contraction of the inferior oblique. 3. This hypothesis was tested by recording the activity of 31 neurons in the trochlear nucleus, which contains the superior oblique motoneurons. This was done in alert monkeys that were trained to make both versional and vergence eye movements. In addition, the cyclotorsion associated with convergence was measured in one of these monkeys. 4. A consistent excyclotorsion associated with convergence was observed. Trochlear unit activity decreased during convergence in all cells tested. The magnitude of this decrease was significantly greater than that seen with conjugate adduction. Furthermore, the size of the decrease varied systematically with ocular elevation in a manner that was consistent with earlier measures of cyclotorsion during convergence. 5. These results suggest that the excyclotorsion seen during convergence, and perhaps the lateral translation of the eye, are due to a relaxation of the superior oblique muscle. This relaxation during convergence is greater than that which accompanies similar conjugate movements of the eyes. We hypothesize that this peculiar pattern of muscle innervation has a motor rather than sensory function.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Signal transformations required for the generation of saccadic eye movements.

Chronic unit recording experiments conducted over the past two decades have identified many functional classes of neurons with saccade-related activity that reside in a host of brainstem nuclei. Older models of the saccadic system were based upon the properties of only a few of these functional types of neurons. They described the putative flow of signals through the brainstem circuitry and specified some, but not all, of the signal transformations to be performed. How the necessary computations were performed by neurons was not always explicit. Recent experiments investigating the neural control of saccadic eye movements and modifications of the original models are designed to fill in the details of the broad sketch of saccadic circuitry originally available. This review suggests one strategy for proceeding with this effort. Saccadic command signals observed in the SC require transformation to interface with the burst generators and motoneuron pools innervating the extraocular muscles. Specifying the signal transformations required for this interface should facilitate the design of experiments directed toward an understanding of the functional properties of cells located in nuclei intervening between the SC and the pulse/step circuitry, subsets of neurons that often have no role in models of the saccadic system. In this review, we hypothesize that neurons residing in various tectorecipient brainstem nuclei participate in one or more of the required signal transformations. The pathway from SC to cMRF and PPRF may be involved in the extraction of information about the amplitude and/or velocity of the horizontal component of oblique saccades. The pathway from SC to NRTP and cerebellar vermis may act selectively to generate signals compensating for the presaccadic orbital position. Finally, the activity of LLBNs and MLBs discharging maximally before oblique saccades may form the basis of computations required to match component velocity with overall saccade direction and amplitude. Although the data supporting these speculations are meager at present, such conjectures do form the basis of working hypotheses that can be tested experimentally. We also considered the implications of kinematic constraints, especially Donders' and Listing's laws, for future investigations. Tweed & Vilis (1987, 1990) proposed models specifically designed to handle these constraints. In their models, eye position is represented on four oculomotor channels: three coding the vector components of eye position, and one carrying a signal inversely related to gaze eccentricity and torsion. Yet, other evidence suggest that simpler computations may suffice for the implementation of laws that are only approximately obeyed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Abducens internuclear neurons carry an inappropriate signal for ocular convergence.

1. Single-unit recording studies in alert Rhesus monkeys characterized the vergence signal carried by abducens internuclear neurons. These cells were identified by antidromic activation and the collision of spontaneous with antidromic action potentials. The behavior of abducens internuclear neurons during vergence was compared with that of horizontal burst-tonic fibers in the medial longitudinal fasciculus (MLF) and to that of a large sample of unidentified abducens cells (presumably both motoneurons and internuclear neurons). 2. The results indicate that abducens internuclear neurons and lateral rectus motoneurons behave similarly during vergence eye movements: the majority of both groups of cells decrease their firing rate for convergence eye movements: a minority show no change for vergence. This finding is strongly supported by recordings of horizontal burst-tonic fibers in the MLF, the majority of which decrease their activity significantly for convergence eye movements. 3. These findings indicate that a net inappropriate vergence signal is sent to medial rectus motoneurons via the abducens internuclear pathway. Because medial rectus motoneurons increase their activity appropriately during symmetrical convergence, this inappropriate MLF signal must be overcome by a more potent direct vergence input. 4. Overall, both abducens internuclear neurons and lateral rectus motoneurons decrease their activity for convergence less than would be expected based on their conjugate gain. This implies that some degree of co-contraction of the lateral and medial rectus muscles occurs during convergence eye movements. 5. Some horizontal burst-tonic MLF fibers decrease their activity more for convergence than any recorded abducens neuron. These fibers may arise from cells in the nucleus prepositus hypoglossi or vestibular nuclei.

Abducens Nerve↗
PubMed1989

Lidocaine-induced unilateral internuclear ophthalmoplegia: effects on convergence and conjugate eye movements.

1. To characterize the vergence signal carried by the medial longitudinal fasciculus (MLF), it was subjected to reversible blockade by small injections of 10% lidocaine hydrochloride. The effects of these blockades on both conjugate and vergence eye movements were studied. 2. With this procedure, experimentally induced internuclear ophthalmoplegia (INO) and its effects on conjugate eye movements could be studied acutely, without possible contamination from long-term oculomotor adaptation. In the eye contralateral to the MLF blockade, saccadic and horizontal smooth-pursuit eye movements were normal. Horizontal abducting nystagmus, often seen in patients with INO, was not observed in this eye. 3. As previously reported for INO, profound oculomotor deficits were seen in the eye ipsilateral to the MLF blockade. During maximal blockade, adducting saccades and horizontal smooth-pursuit movements in this eye did not cross the midline. Adducting saccades were reduced in amplitude and peak velocity and showed significantly increased durations. Abducting saccades, which were slightly hypometric, displayed a marked postsaccadic centripetal drift. 4. The eye ipsilateral to the blockade displayed a pronounced, upward, slow drift, whereas the eye contralateral to the blockade showed virtually no drift. Furthermore, although vertical saccades to visual targets remained essentially conjugate, the size of the resetting quick phases in each eye was related to the amplitude of the slow phase movement in that eye. Thus the eye on the affected side displayed large quick phases, whereas the eye on the unaffected side showed only slight movements. On occasion, unilateral downbeating nystagmus was seen. This strongly suggests that the vertical saccade generators for the two eyes can act independently. 5. The effect of MLF blockade on the vergence gain of the eye on the affected side was investigated. As a measure of open-loop vergence gain, the relationship of accommodative convergence to accommodation (AC/A) was measured before, during, and after reversible lidocaine block of the MLF. After taking conjugate deficits into account, the net vergence signal to the eye ipsilateral to the injection was found to increase significantly during the reversible blockade. 6. The most parsimonious explanation for this increased vergence signal is suggested by the accompanying single-unit study. This study showed that abducens internuclear neurons, whose axons course in the MLF, provide medial rectus motoneurons with an appropriate horizontal conjugate eye position signal but an inappropriate vergence signal. Ordinarily, this incorrect vergence signal is overcome by another, more potent, v

Animals↗
PubMed1988

Purkinje cell activity in rats following chronic treatment with harmaline.

Harmaline and related alkaloids produce a fine, generalized motor tremor with a frequency of 8-14 Hz in many mammalian species. The tremor is though to be initiated by the synchronous activation of cells in the inferior olive. Repeated administration of the drug at tremorogenic doses results in the rapid development of tolerance in the rat. Since the generation of cerebellar cyclic 3',5'-guanosine monophosphate by harmaline or apomorphine is reduced in harmaline-tolerant rats, it is possible that the site of tolerance is the olivocerebellar system. The present study used extracellular single unit recording techniques to determine whether harmaline tolerance was associated with changes in the firing patterns of Purkinje cells in the cerebellar vermis of the rat. In non-tolerant animals, the majority (8/13) of Purkinje cells recorded in the vermis responded to harmaline with a rhythmic increase in complex spike rate and a prolonged suppression of simple spikes. In harmaline-tolerant animals, only one cell in 14 could be identified that showed this response. In these animals, a variety of responses not encountered in experimentally naive animals were observed. Since the complex spike activity of Purkinje cells is presumed to reflect the activity of climbing fibers originating in the cells of the inferior olive, the results of the studies reported here support the conclusion that a reduction in the synchronous activation of cells at the olivocerebellar level blocks the appearance of tremor in harmaline-tolerant animals.

Action Potentials↗
PubMed1988

Neuropharmacological correlates of the motor syndrome of the genetically dystonic (dt) rat.

The research program described here has focused primarily on identifying sites of dysfunction in the central nervous system of rat mutants described as dystonic. The evidence strongly favors the position that there is a defect in the cerebellum of the dt rat. At present it seems reasonable to propose as a working hypothesis that there is a defect in the Purkinje cells that renders these neurons less sensitive to the excitatory neurotransmitters released by the climbing and parallel fibers. The finding that an abnormality in GAD activity in the deep cerebellar nuclei is relatively localized when first detected but spreads over time as the motor syndrome intensifies may indicate that there is a progressive decline in the function of the Purkinje cells. The fact that electrophysiological techniques detect a mixture of relatively normal and abnormal Purkinje cell activity in animals with advanced symptoms is consistent with such a proposal. Finding significant abnormalities in the cerebellum of the dt rat does not necessarily mean that this is the site of the primary defect responsible for the motor syndrome seen in these animals. We have failed to detect any signs of dysfunction in the basal ganglia, the presumed locus of a defect in human torsion dystonia. However, our investigations have been limited almost exclusively to the striatum. Thus, the possibility of defects at other sites, such as the globus pallidus or thalamus must be considered. Although we have not yet demonstrated that the abnormalities detected in the cerebellum are causally related to the behavior of the dt rat, the behavioral syndrome is consistent with a cerebellar defect. It has been suggested that the cerebellum is important for the continuing calibration of coordinated motor behavior. Important observations on the effects of cerebellar lesions have come from the study of the oculomotor system. Lesions in the cerebellum have been shown to eliminate the ability to recalibrate the saccadic eye movement system and to destroy the adaptive plasticity of the vestibulo-ocular reflex A cerebellar defect could result in a failure in motor learning or in the calibration of motor systems that must take place as the rat pup masters adult patterns of locomotion. We note that lesions of the climbing fiber system with 3-AP lead to some of the same biochemical effects seen in dt rats but do not produce an identical behavioral syndrome.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine↗
PubMed1988

Spontaneous and harmaline-stimulated Purkinje cell activity in rats with a genetic movement disorder.

The genetically dystonic rat (dt) displays a complex movement disorder in the absence of morphological defects in the nervous system. This mutant is also insensitive to the tremorogenic effects of harmaline. Because harmaline is known to act on the cells of the inferior olive to induce activity at the tremor frequency in the olivocerebellobulbar pathway, this pathway has been investigated as a possible site of a defect in the dt rat. Biochemical studies suggested the presence of abnormalities at the level of the Purkinje cell or its afferent input. Thus, the present study investigated the harmaline response of Purkinje cells in dt rats and unaffected littermate controls with extracellular single-unit recording techniques. The spontaneous, simple spike and complex spike firing rates of dt rats were significantly lower than those of normal littermate controls. In normal rats, 2 responses to systemic harmaline injection were seen. Simple spikes were either completely suppressed for periods of 30-180 min, or were intermittently suppressed, pausing repeatedly for periods of 1-18 sec. Cells that showed complete suppression of simple spike activity also showed increased frequency and rhythmicity of complex spikes. In dt rats, intermittent simple spike responses were seen in a proportion (41%) similar to that in normal rats (53%). However, the proportion of cells showing high-frequency, rhythmic, complex spikes and complete suppression of simple spikes was low in the dt rats in comparison with littermate controls (18 versus 47%). In addition, 41% of the cells from dt rats displayed no change, or an anomalous change, in firing patterns in response to harmaline. Since the rhythmic activation of olivary neurons that results in the rhythmic, complex spike discharge of Purkinje cells is assumed to be responsible for the appearance of harmaline tremor, the failure of the dt rat to display tremor is most likely due to a failure at the olivocerebellar level, rather than at a site efferent to the cerebellum.

Action Potentials↗
PubMed1987

Eye movements induced by pontine stimulation: interaction with visually triggered saccades.

1. Rhesus monkeys were trained to look to brief visual targets presented in an otherwise darkened room. On some trials, after the visual target was extinguished but before a saccade to it could be initiated, the eyes were driven to another orbital position by microstimulation of the paramedian pontine reticular formation. If, as current models of the saccadic system suggest, a copy of the motor command is used as a feedback signal of eye position, failure to compensate for stimulation-induced movements would indicate that stimulation occurred at a site beyond the point from which the eye position signal was derived. 2. Animals compensated for perturbations of eye position induced by stimulation of most pontine sites by making saccades that directed gaze to the position of the visual target. With stimulation at other pontine sites, compensatory saccades did not occur. 3. Pontine stimulation sometimes triggered, prematurely, impending visually directed saccades. The direction and amplitude of the premature movement depended upon the location of the briefly presented visual target. The amplitude of the premature movement was also a function of the interval between the stimulation train and the impending saccade. These data suggest that input signals for the horizontal and vertical pulse/step generators develop gradually during the presaccadic interval. Saccade trigger signals need to be delayed until the formation of these signals is completed. 4. The implications of these findings for models of the saccadic system are discussed. Robinson's local feedback model of the saccadic system can explain compensation for pontine stimulation-induced changes in eye position but cannot easily account for the failure to compensate for perturbations in eye position produced by stimulation at other sites. Modified versions of Robinson's model, which assume that the input signal to the pulse/step generator is the desired displacement of the eye, can account for both compensation and the failure to compensate since two separate neural integrators are employed. However, these models ignore kinematic arguments that commands to the extraocular muscles must specify the absolute position of the eye in the orbit rather than a relative movement from a previous position.

Abducens Nerve↗
PubMed1986

Neural control of vergence eye movements: neurons encoding vergence velocity.

Single-unit recordings were made from midbrain areas in monkeys trained to make both conjugate and disjunctive (vergence) eye movements. Previous work had identified cells with a firing rate proportional to the vergence angle, without regard to the direction of conjugate gaze. The present study describes the activity of neurons that burst for disjunctive eye movements. Convergence burst cells display a discrete burst of activity just before and during convergence eye movements. For most of these cells, the profile of the burst is correlated with instantaneous vergence velocity and the number of spikes in the burst is correlated with the size of the vergence movement. Some of these cells also have a tonic firing rate that is positively correlated with vergence angle (convergence burst-tonic cells). Divergence burst cells have similar properties, except that they fire for divergent and not convergent movements. Divergence burst cells are encountered far less often than convergence burst cells. Both convergence and divergence burst cells were found in an area of the mesencephalic reticular formation just dorsal and lateral to the oculomotor nucleus. Convergence burst cells were also recorded in another more dorsal mesencephalic region, rostral to the superior colliculus. Both of the areas also contain cells that encode vergence angle. Models of the vergence system derived from psychophysical data imply the existence of a vergence integrator, the output of which is vergence angle. Some models also suggest the presence of a parallel element that improves the frequency response of the vergence system, but has no effect on the steady-state behavior of the system. Vergence burst cells would be suitable inputs to a vergence integrator. By providing a vergence velocity signal to motoneurons, they may improve the dynamic response of the vergence system. The behavior of vergence burst cells during vergence movements is similar to that of the medium-lead burst cells during saccades. The proposed roles for vergence velocity cells are analogous to those of the saccadic burst cells. In this respect, the neural organization of the vergence system resembles that of the saccadic system, despite the distinct difference in the kinematics of these two types of eye movements.

Action Potentials↗
PubMed1986

Long- and short-term monocular deprivation in the rhesus monkey: effects on visual fields and optokinetic nystagmus.

Monkeys with 1 eyelid sutured within 2 weeks of birth for 7 or 14 d (short-term monocular deprivation, n = 5) or for 18-26 months (long-term monocular deprivation, n = 5) were tested for visual and oculomotor function at approximately 2 years of age. Long-term monocularly deprived animals were behaviorally blind when visual inputs were restricted to the deprived eye. There was no sparing of the monocular segment of the visual field, and optokinetic nystagmus could not be elicited even with vertical stripes up to 15 degrees in width. These behavioral deficits could not be accounted for by optical or retinal abnormalities. In contrast, short-term monocularly deprived animals displayed normal visual fields and optokinetic nystagmus was driven by both eyes. Slow-phase gain was reduced and directional asymmetries were observed when optokinetic stimulation was restricted to the deprived eye.

Animals↗
PubMed1984

Neural control of vergence eye movements: convergence and divergence neurons in midbrain.

Animals with binocular single vision use disjunctive (vergence) eye movements to align the two eyes on a visual target. Several lines of evidence suggest that conjugate and vergence eye movement commands are generated independently and combined at the medial rectus motoneurons. If this were true, then a pure vergence eye-position signal should exist. This signal would be proportional to the horizontal angle between the eyes (vergence angle), without regard to the direction of conjugate gaze. The purpose of this experiment was to identify and study neurons that carry a pure vergence signal. Extracellular unit recordings were made from midbrain and pontine sites in monkeys trained to track visual targets moving in the horizontal, vertical, and depth (or target vergence) planes. The most commonly encountered neuron that had a vergence signal was the convergence cell. These units had a firing rate that was linearly proportional to the convergence angle; their activity was unaffected by changes in conjugate gaze. Changes in convergence cell activity preceded the change in vergence angle slightly. Convergence cell activity increased for increased convergence regardless of whether the change was in response to purely accommodative or disparity cues. Divergence cells were found far less frequently. These cells were similar to convergence cells except that they decreased their firing rate for increases in convergence. The activity of divergence cells was unaffected by changes in the direction of conjugate gaze. Both convergence and divergence cells were found, intermixed, in the mesencephalic reticular formation must outside the oculomotor nucleus. Most cells with a vergence signal were found within 1-2 mm of the nucleus. These results support the view that conjugate and vergence signals are generated independently and are combined at the extraocular motoneurons. Convergence cells seem ideally suited to provide the vergence signal required by the nearby medial rectus motoneurons.

Accommodation, Ocular↗
PubMed1984

Neural control of vergence eye movements: activity of abducens and oculomotor neurons.

Single-unit recordings were made from neurons with horizontal eye position sensitivity in the oculomotor and abducens nuclei in alert monkeys. The animals were trained to perform a visual tracking task that resulted in conjugate eye movements or symmetrical vergence movements. Scatterplots were obtained for unit firing rate as a function of the position of the ipsilateral eye for both types of movement. The slopes of the linear regression line were computed for conjugate (kc) and vergence movements (kv). Previous recording studies implied that kv should be equal to kc for most, if not all, abducens and oculomotor neurons. Other lines of evidence suggested that kv should be zero for a substantial proportion of abducens neurons. In the abducens nucleus, we found some cells for which kv matched kc, and a few cells with a kv value of zero. However, the majority of abducens units had vergence signals that were neither equal to zero nor to their conjugate signals. Overall, kv/kc was 0.62, and the correlation between kv and kc was not significantly different from zero. Similarly, in the oculomotor nucleus, kv was significantly different from kc for a majority of the cells. A few units had kv values less than or equal to zero, whereas other cells had very high kv values. Overall, the kv/kc for oculomotor units was nearly unity (0.94), and the correlation between kv and kc was 0.31. These results confirm previous reports that most neurons in the abducens and oculomotor nuclei with a horizontal eye position sensitivity carry both conjugate and vergence eye movement signals. We do not find that the relative magnitudes of these signals are closely matched for most neurons. It is more likely that vergence and conjugate signals are matched globally, for an entire nucleus, rather than for individual motoneurons. This view is consistent with the hypothesis that conjugate and vergence signals are generated independently and combined for the first time at the motoneurons. Our results also imply that some motoneurons play a more important role than others in either vergence or conjugate movements.

Abducens Nerve↗
PubMed1980

Saccades are spatially, not retinocentrically, coded.

Most models of the saccadic eye movement system imply that saccades are programmed for a certain distance and direction. Electrical stimulation of the brain was used to move the eyes of monkeys just before saccades to visual targets. Despite the stimulation-induced perturbation, saccades brought gaze to the target locations. This compensation indicates that saccades are coded to direct the eyes to a certain position in the orbit (or in space).

Animals↗
PubMed1980

Movement fields of saccade-related burst neurons in the monkey superior colliculus.

The presaccadic burst of superior colliculus (SC) neurons was examined in detail to determine whether or not information concerning the vector components (amplitude and direction) of a saccade is contained within the burst. Results indicate that the pattern of spike activity originating from a single saccade-related burst neuron in the SC does not encode saccade direction or amplitude. Identical discharges may precede a wide range of saccades. Neither the magnitude, configuration nor timing of the discharge are related in any unique way to the duration of the saccade alone or the amplitude of the saccade alone. Furthermore, it is unlikely that information concerning saccade amplitude or direction is encoded by different types of signals originating from different SC neurons. For different neurons, there is no consistent relationship between the parameters of the burst and the optimal saccade amplitude or direction. It is suggested that the discharge of saccade-related burst neurons of the SC serves as a trigger input to pontine circuitry generating the required saccadic burst signals. Information concerning saccade direction and amplitude is not contained within the trigger signal, but must be extracted from the spatial distribution of SC activity.

Animals↗