Biomedical subjects

H Collewijn

Publications and source records attributed to H Collewijn.

At least 37 records · Page 2Linked to original sources

PubMed1994

Instability of ocular torsion during fixation: cyclovergence is more stable than cycloversion.

We investigated spontaneous variation of binocular torsion. Variation was expressed as SD of torsional eye positions measured over periods up to 32 sec. Subjects viewed a single dot target for periods of 32 sec. In half of the trials a large random-dot background pattern was superimposed on the dot. The movements of both eyes were measured with scleral induction coils. Spontaneous torsional movements were largely conjugate: cyclovergence was much more stable than cycloversion. This difference was not due to roll head movements. Stability of cyclovergence was improved by the background pattern. Although overall stability (SD of position) of cycloversion was unaffected by a background, the background induced or enhanced a small-amplitude torsional nystagmus in 3 out of 4 subjects. We hypothesize that the difference in stability of cycloversion vs cyclovergence reflects the greater importance of torsional retinal correspondence, compared to absolute torsional position. In two subjects we found evidence for the existence of cyclophoria, manifested by systematic shifts in cyclovergence caused by the appearance and disappearance of the background.

Adult↗

PubMed1994

Eye torsion associated with disparity-induced vertical vergence in humans.

Recently, Enright described an unexpected association between disparity-induced vertical vergence and cycloversion (conjugate eye torsion) [Enright (1992) Vision Research, 415, 279]. The present experiments were performed to verify these findings and investigate the nature of this association. We presented subjects with a dichoptic image of concentric circles in which a step in vertical disparity of 1 deg was introduced. After 4 sec the disparity was eliminated. Eye movements were measured with scleral coils. We confirmed Enright's findings in that a left-over right vertical vergence was associated with levo-cycloversion (upper poles towards left shoulder) and vice versa. The size of the cycloversion and the vertical vergence were in the same range. In addition we found that part of the cycloversion response was in the form of a torsional nystagmus and that the relative contribution of the left and right eyes was independent of the horizontal gaze angle. These additional findings are in conflict with the hypothesis, offered by Enright, that the association is caused by a bilateral activity of the superior oblique muscles.

Adult↗

PubMed1994

Eye torsion elicited by oscillating gratings: effects of orientation, wavelength and stationary contours.

We studied binocular cyclorotatory (torsional) eye movements in response to gratings that oscillated sinusoidally in a frontal plane. The square-wave gratings viewed by the right and left eye were presented and controlled separately to induce cycloversion and cyclovergence by oscillation in phase and out of phase. Eye movements were recorded with scleral induction coils. Stimulus oscillation frequency ranged from 0.125 to 1 Hz and the wavelength of the gratings ranged from 0.92 to 25.75 deg of visual angle. Cycloversion and cyclovergence gain were, on average, comparable in magnitude and decreased with increasing oscillation frequency. There was no consistent effect of the wavelength on the magnitude of the responses. In general, responses were considerably higher to gratings that were oriented horizontally than to those oriented vertically. This anisotropy was present both in cycloversion and cyclovergence. It was enhanced in a larger sized stimulus and by presenting stationary, orthogonal contours (mimicking a "shear" movement), but it was not consistently influenced by wavelength. Cyclovergence showed a phase lag, which increased with oscillation frequency but which was independent of wavelength. In contrast, cycloversion showed a slight phase lead which was independent of both oscillation frequency and wavelength.

Adult↗

PubMed1993

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

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

Acetylcholine↗

PubMed1993

Shortening of vestibular nystagmus in response to velocity steps by microinjection of carbachol in the rabbit's cerebellar flocculus.

It has been proposed that a common velocity-storage mechanism is responsible for the prolongation of vestibular nystagmus beyond the duration of the change in firing frequency of primary vestibular fibers in response to a step in velocity, and for the production of optokinetic afternystagmus (OKAN). In a previous study, bilateral injection of the aselective cholinergic agonist carbachol in the flocculus shortened the duration of buildup of optokinetic nystagmus (OKN) and the duration of OKAN, suggesting floccular involvement in velocity storage (Tan et al. 1992). In extension to that study of OKN, the present study assesses the effects of floccular carbachol on vestibular nystagmus in response to velocity steps. Our results show that injection of carbachol shortens the duration of vestibular nystagmus from about 13 to about 8 s; a finding which supports a common velocity-storage mechanism for optokinetic and vestibular signals. We propose that the indistinguishable effects of carbachol on OKAN and vestibular nystagmus are due to modification of the transmission of an oculomotor corollary signal, which has been identified electrophysiologically in the flocculus.

Animals↗

PubMed1993

Three-dimensional organization of optokinetic responses in the rabbit.

1. Three-dimensional rotations of both eyes were measured in alert rabbits during optokinetic stimulation about axes lying in the horizontal plane or about an earth-vertical axis, with either one or both eyes viewing the stimulus. Optokinetic stimulus speed was 2 degrees /s, either continuous or alternating in polarity (triangular stimulus). In addition to the gains of the responses, the orientations of the response axes relative to the stimulus axes were determined. 2. In comparison to the response to constant-speed optokinetic stimulation about the vertical axis, the response to constant-speed optokinetic stimulation about horizontal axes was characterized by the lack of a speed buildup. In many cases, slow phase tracking was good as long as the eye was within the central oculomotor range but deteriorated when eye deviation became more eccentric and fast phases failed to be generated. These features suggest that the optokinetic reflex about horizontal axes functions as a position-control system, rather than as a velocity-control system. 3. Binocular optokinetic stimulation at constant speed (2 degrees/s) about the roll axis (0 degrees azimuth horizontal axis) elicited disconjugate responses. Although the gain of the response was not significantly different in the two eyes (0.38 for downward and 0.44 for upward stimulation), the response axes of the two eyes differed by as much as 51 degrees. 4. Monocular, horizontal axis optokinetic stimulation at constant speed elicited responses that were grossly dissociated between the two eyes. The magnitude of the responses was anisotropic in that it varied with the azimuthal orientation of the stimulus axis; the maximum gain for each eye (0.41 for the seeing and 0.33 for the covered eye) was at 135 degrees azimuth for each eye. The axis orientation and direction (sense of rotation) of the optokinetic stimulus eliciting the maximal response for each eye coincided with the optic flow normally associated with the maximal excitation of the corresponding ipsilateral anterior canal. 5. Binocular, triangular optokinetic stimulation with small excursions (+/- 10 degrees), which avoided the saturation problems of constant-speed stimulation, elicited adequate responses without systematic directional asymmetries. Gain was approximately 0.9 for all stimulus axis orientations in the horizontal plane. 6. During monocular stimulation with triangular stimuli, the response of the seeing eye showed a gain of approximately 0.5 for all orientations of the stimulus axis. In contrast, the covered eye showed anisotropic responses, with a maximum gain of approximately 0.5 during stimulation of the seeing eye about its 45 degree axis.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Muscarinic nature of cholinergic receptors in the cerebellar flocculus involved in the enhancement of the rabbit's optokinetic response.

Intrafloccular micro-injection of the aselective cholinergic agonist carbachol enhances the optokinetic reflex (OKR)17. Histochemical and physiological studies have identified cholinergic receptors of the muscarinic as well as nicotinic type in the cerebellar cortex, and both have been implicated in cholinergic transmission. The present study was undertaken to elucidate the receptor type involved in the control of OKR. For that purpose, effects of injections of the nicotinic N1 agonist DMPP on the OKR and vestibulo-ocular reflex (VOR) were compared with injections of the muscarinic agonist betanechol and the aselective cholinergic agonist carbachol. Injection of betanechol mimicked the enhancement of the OKR by carbachol, while DMPP had no effect. We conclude that muscarinic receptors are involved in the positive modulatory action of the cholinergic system in the cerebellar flocculus.

Animals↗

PubMed1992

Cholinergic and noradrenergic stimulation in the rabbit flocculus have synergistic facilitatory effects on optokinetic responses.

A recent study (Exp. Brain Res., 85 (1991) 475-481) showed that injection of the cholinergic agonist carbachol into the cerebellar flocculi had a pronounced facilitatory effect on the gains of the optokinetic (OKR) and vestibulo-ocular (VOR) reflexes, suggesting a positive modulatory role of the cholinergic system in the flocculus. Because many behavioral and electrophysiological studies throughout the brain have revealed a functional relationship between acetylcholine (ACh) and noradrenaline (NA), the present study was undertaken to compare the effects of floccular injection of the ACh agonist carbachol, the beta-adrenergic agonist isoproterenol and the conjoint injection of both of these substances on the basic gain of the VOR and the OKR. Carbachol and isoproterenol both significantly raised the gain of the OKR, by 0.14 and 0.11 respectively. Neither of the two substances significantly affected the gain of the VOR in light or darkness. Conjoint injection of the same amounts of carbachol and isoproterenol resulted in an increase in the gain of the OKR by 0.29 without significant changes in the gains of the VOR in the light or in darkness. These results suggest a synergistic and positive modulatory role of ACh and NA in the flocculus.

Animals↗

PubMed1992

Enhancement of optokinetic and vestibuloocular responses in the rabbit by cholinergic stimulation of the flocculus.

Bilateral microinjections into the cerebellar flocculus of the rabbit of carbachol, a general cholinergic agonist, profoundly affect vestibuloocular (VOR) and optokinetic (OKR) reflexes. For sinusoidal stimuli (0.15 Hz, 5 deg peak to peak), the gain of the OKR was strongly increased, while the gain of the VOR was moderately increased. These effects were partially mimicked by floccular injection of the acetylcholinesterase inhibitor eserine. Floccular injection of the muscarinic blocker atropine significantly lowered the gain of the OKR. The effects of the nicotinic blocker mecamylamine were not significant. Optokinetic nystagmus (OKN) in response to constant stimulus velocities (1-30 deg/second) showed a markedly accelerated buildup and a shortened optokinetic after-nystagmus (OKAN) after floccular injections of carbachol. The steady-state gain of OKN remained unaffected. None of the described effects occurred after floccular injection of the solvent, saline. It is postulated that cholinergic cerebellar afferents, one probable source of which are the vestibular nuclei, enhance the optokinetic and vestibular modulation of floccular Purkinje cells.

Animals↗

PubMed1992

Foveation dynamics in congenital nystagmus. I: Fixation.

Congenital nystagmus (CN) has been described as a 'fixation' nystagmus implying an inability to fixate a target. However, each cycle of CN contains a target-foveation period during which the eye velocity is at, or near, zero. Prolongation of foveation time, reduction of retinal image velocity and cycle-to-cycle foveation repeatability all contribute to increased visual acuity. We developed several methods to accurately measure the dynamics of foveation in CN; their use is illustrated on an individual with typical idiopathic CN and no afferent defects. During eight 5-second intervals of fixation on a stationary target, the horizontal standard deviation (SD) of the mean foveation position (FPOS) was 12.82 minarc and the SD of foveation velocity was 118.36 minarc/sec. The SD of the means of total eye position and of the non-foveating peak of the CN were 43.17 and 25.32 minarc respectively. The mean foveation-time interval (eye velocity less than or equal to 4 degrees/sec) was 57.27 msec. The SD FPOS for the best 1-second interval (4 successive CN cycles), in a typical 5-second record, was 0.71 minarc. Histograms revealed peaks of eye position at 0 +/- 10 minarc and of eye velocity at 0 +/- 240 minarc/sec. The small vertical component of the CN (16 minarc peak-to-peak) had a SD of 6.56 minarc. A nystagmus foveation function related to visual acuity was derived that was more sensitive than CN intensity. The increased visual acuity resulting from the use of convergence or base-out prisms was due to increased foveation time. Although it might appear that CN is a defect of fixation, this individual with CN had strong fixation reflexes in the sense that he was able to accurately (within 1 minarc) achieve (interbeat) and maintain (intrabeat) target foveation for appreciable periods of time. Our data support the hypothesis that individuals with idiopathic CN do not have a primary disturbance of fixation.

Fixation, Ocular↗

PubMed1992

Foveation dynamics in congenital nystagmus. II: Smooth pursuit.

It has been shown that, during 5 seconds of fixation, an individual with congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (SD = 12.87 minarc) and maintain low retinal slip velocities (SD = 118.36 minarc/sec). Smooth pursuit data from several CN subjects showed that eye velocities during these foveation intervals approximated target velocity. Despite some claims that CN is caused by absent or "reversed" smooth pursuit, those with CN hardly ever experience oscillopsia or exhibit any accompanying symptoms of such deficits in pursuit; they are able to master sports requiring tracking of rapidly moving small objects (e.g. racquetball or handball). We developed and describe several new methods to accurately assess the function of smooth pursuit in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during smooth pursuit to test the hypothesis that eye velocities would match target velocities during these periods. Unity or near-unity instantaneous (beat-to-beat) pursuit gains of both experimenter-moved and subject-moved targets at peak velocities ranging from only a few deg/sec up to 210 degrees/sec were measured. The dynamic neutral zone was found to shift oppositely to target direction by amounts proportional to the increase in target speed. Our methods proved that eye velocity is made to match target velocity during the foveation intervals and support the conclusion that smooth pursuit in individuals with CN is functioning normally in the presence of the CN oscillation. In addition, we hypothesize that the same fixation mechanism that prevents oscillopsia during fixation of stationary targets, also does so during pursuit.

Fovea Centralis↗

PubMed1992

Foveation dynamics in congenital nystagmus. III: Vestibulo-ocular reflex.

It has been shown that, during fixation of a stationary target with a fixed head, an individual with congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (within 13 minarc) and maintain low retinal slip velocities (less than 4 degrees/sec). With the head in motion, vestibuloocular reflex (VOR) data showed eye velocities during these foveation periods that approximation head velocity. Despite some claims that the VOR of CN subjects was deficient or absent, individuals with CN hardly ever complain of oscillopsia or exhibit any of the symptoms that would accompany such deficits in the VOR, whether during simple walking and running or while skiing down a mogul field. We developed and describe several different and unrelated methods to accurately assess the function of the VOR in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during head rotation to test the hypothesis that eye velocities would match head velocities during these periods. At about 1 Hz, horizontal VOR instantaneous (beat-to-beat) gains were 0.96 in the light and 0.94 in the dark while imaging a stationary target. Vertical VOR gains were 1.00 and 0.99 for these two conditions at the same frequency; the CN was horizontal. Also, during the VOR there is a CN neutral-zone shift comparable to that found during smooth pursuit. Our methods demonstrated that gaze velocity was held constant during foveation periods and we conclude that the VOR in this subject is functioning normally in the presence of the CN oscillation. Based on our findings in this and previous studies, we hypothesize that CN may be due to a peripheral instability.

Fovea Centralis↗

PubMed1992

Optokinetic nystagmus in the rabbit and its modulation by bilateral microinjection of carbachol in the cerebellar flocculus.

1. In the alert, pigmented rabbit, eye movements were recorded during optokinetic nystagmus (OKN) and during optokinetic after nystagmus (OKAN). These responses were elicited by steps in surround-velocity ranging from 5-110 degrees/s during binocular as well as monocular viewing. 2. In the baseline condition, OKN showed an approximately linear build-up of eye velocity to a steady-state, followed by a linear decay of eye velocity during OKAN after the lights were turned off. Build-up during binocular viewing was characterized by a constant, maximum eye-acceleration (about 1 degree/s2) for stimulus velocities up to 60 degrees/s. OKAN, instead, was characterized by a fixed duration (about 10 s) for stimulus velocities up to 20 degrees/s. Steady-state eye velocity saturated at about 50 degrees/s. 3. Monocular stimulation in the preferred (nasal) direction elicited a build-up that was on average twice as slow as during binocular stimulation. Steady-state velocity during monocular stimulation saturated at about 20 degrees/s. OKAN was of equal duration as during binocular stimulation. In the non-preferred direction, a very irregular nystagmus was elicited without velocity build-up. The stronger response to binocular stimulation, compared to the responses under monocular viewing condition in either nasal and temporal direction suggests potentiation of the signals of either eye during binocular viewing. 4. OKN and OKAN were re-assessed after intra-floccular micro-injection of the nonselective cholinergic agonist carbachol. In the binocular viewing condition, eye-acceleration during build-up was strongly enhanced from 1 degree/s2 before to 2.5 degrees/s2 after injection. The saturation level of steady-state eye velocity was also increased, from 50 degrees/s before to more than 60 degrees/s after carbachol. The duration of OKAN, however, was shortened from 10 s before to 6 s after injection. The response to monocular stimulation in the preferred direction revealed similar changes. 5. The flocculus appears to be involved in the control of the dynamics of OKN in the rabbit. Cholinergic mechanisms affect the floccular control of the rate at which slow-phase velocity can be built up and the rate of decay of eye velocity during OKAN. Cholinergic stimulation of the flocculus enhances the dynamics of OKN, while velocity storage is shortened.

Animals↗

PubMed1992

Visually induced cycloversion and cyclovergence.

Binocular cyclorotatory (torsional) eye movements in response to visual patterns, which oscillated sinusoidally in the frontal plane, were recorded with scleral induction coils in human subjects. Conjugate cycloversion and disjunctive cyclovergence were directly compared by in-phase and out-of-phase oscillation of the same pattern. Stimulus motion had a frequency of 0.2 Hz and amplitudes of 2-8 deg. Both response types had a similar and low gain (about 0.2 averaged over all subjects). Cycloversion showed no time lag, while cyclovergence lagged by about 600 msec. Non-fusible patterns were effective in eliciting cycloversion, but not cyclovergence. Apart from this, the nature of the pattern (randomly distributed dots, regular rows of dots, horizontal or vertical grating, Julesz stereogram or images with a pictorial significance) had only the slightest effect on the magnitude of the responses.

Adult↗

PubMed1992

Nonconjugate adaptation of human saccades to anisometropic spectacles: meridian-specificity.

Recently it has been demonstrated that saccades become different in size in the two eyes if a subject is adapted to anisometropic spectacles, which provide visual images of different magnitude to the two eyes. These nonconjugate adaptations adequately meet the requirements of those spectacles and, once acquired, they persist (with some reduction) even during monocular viewing. We now demonstrate that such nonconjugate adaptations of saccades can be meridian-specific, if there is a pressure for such meridian-specificity. This pressure was provided by means of a cylindrical spectacle-lens. Adaptations along a vertical, horizontal or oblique meridian did not transfer to the orthogonal meridian. These results demonstrate a capability of saccadic adaptation to deal with calibration problems restricted not only to one eye, but even to one specific plane of muscular action. Our results also suggest that the meridian-specific adaptations of oblique saccades take place at a stage before the decomposition of motor commands into separate horizontal and vertical components. The meridian-specific nonconjugacies were also expressed in smooth-pursuit eye movements. Post-saccadic drift adapted only along the horizontal meridian.

Adaptation, Ocular↗

PubMed1991

Effects of alpha-noradrenergic substances on the optokinetic and vestibulo-ocular responses in the rabbit: a study with systemic and intrafloccular injections.

The effects of microinjection of alpha-noradrenergic agonists and antagonists in the flocculus on the basic gain and adaptibility of vestibulo-ocular and optokinetic responses were investigated. A complementary, previous investigation had shown that the adaptation, but not the basic performance, of compensatory oculomotor responses were markedly influenced by beta-noradrenergic mechanisms in the flocculus. In contrast, the present experiments with bilateral, intrafloccular injections of phenylephrine, prazosin, clonidine and idazoxan failed to reveal any effect of alpha 1- or alpha 2-noradrenergic mechanisms on either basic performance or adaptation of compensatory eye movements. Intravenous administration of clonidine, however, reduced the gain of the optokinetic and vestibulo-ocular responses by about 70 and 50%, respectively, at dosages of 0.07 mg/kg. Recovery from this effect took about 1.5 h. A higher dosage of clonidine (0.7 mg/kg) had a similar, but longer lasting effect, and also markedly increased the frequency of spontaneous saccades. Intravenous administration of phenylephrine did not affect the oculomotor responses. It is concluded that the control of oculomotor responses is not susceptible to alpha-noradrenergic influences at the level of the flocculus, but that alpha 2-agonistic action inhibits these responses through an extra-floccular structure.

Adaptation, Physiological↗

PubMed1991

Control of vergence: gating among disparity inputs by voluntary target selection.

The ability of human beings to shift their vergence between multiple disparity stimuli was investigated. The stimulus was a stereogram consisting of a single bar (1 x 0.3 degree) projected in the center of a larger circular pattern (28 degrees dia) of dots (0.3 degree dia). In the initial condition, the subjects perceived a single bar in the center of a cluster of dots, all lying in a single depth plane. The subjects were instructed to fixate either the bar or one of the dots close to the bar. Stepwise changes of target-vergence of either the bar, or of all dots, or of both configurations in opposite directions, were imposed in a random sequence. Under these conditions, ocular vergence was controlled by the target-vergence of the selected target at all times, even if this implied a loss of binocular fusion for the non-selected target. The effect of target selection per se, without concomitant changes in retinal position of the selected target, was studied in two experienced subjects with stabilized image techniques. The subjects viewed a configuration containing short vertical bars placed at either side of a long vertical bar. The short bars on different sides of the long bar had opposite (crossed or uncrossed) disparities. After stabilization of the configuration while the subject fixated the long bar, subjects attempted to fixate the short bars alternatingly. Due to the stabilization the ensuing eye movements did not affect the retinal positions, and thus the disparity, of the bars. Attempted fixation of a parafoveally viewed, short bar induced vergence responses in the appropriate direction.(ABSTRACT TRUNCATED AT 250 WORDS)

Attention↗

PubMed1991

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

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

Animals↗