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H Collewijn

Publications and source records attributed to H Collewijn.

At least 55 records · Page 3Linked to original sources
PubMed1991

Long-term nonconjugate adaptation of human saccades to anisometropic spectacles.

It is generally believed that saccades follow Hering's law in the sense that they are equally large in the two eyes. We demonstrated that saccades are different in size in the two eyes in 8 habitual wearers of anisometropic spectacles, which have lenses of different refractive powers, and therefore supply each eye with a differently sized visual image. The eye provided with the larger visual image made larger saccades than its fellow eye. This nonconjugate adaptation was almost complete for both horizontal and vertical saccades. Post-saccadic drift was also asymmetrically adapted: it reduced any fixation-disparity present at saccadic offset. The nonconjugate adaptation was also expressed in smooth-pursuit eye movements. In addition, these nonconjugate adaptations were present during monocular viewing, which shows that they were hard-programmed.

Adaptation, Ocular↗
PubMed1991

Short-term nonconjugate adaptation of human saccades to anisometropic spectacles.

It has been demonstrated before that the long-term wearing of anisometropic spectacles may induce nonconjugate adaptations of saccades. Saccades then become different in size in the two eyes. We examined the time-course and the limits of such adaptations of horizontal and vertical saccades during the short-term (1-6 hr) wearing of anisometropic spectacles. After only 1 hr of conditioning to 2 D of anisometropia, the nonconjugate size-adaptations were almost complete along the horizontal meridian. For progressively larger anisometropias (up to 8 D) the adaptative nonconjugacies after 1 hr became also systematically larger. An anisometropia larger than 6 D did not further increase the rate of adaptation during the first 6 hr of conditioning, which suggests that about 6 D of difference in spectacle correction, causing size differences of about 12%, may be the upper limit of the nonconjugate adaptive range of the saccadic subsystem. Post-saccadic drift of horizontal saccades was also adequately changed. In addition, nonconjugate adaptations had developed in smooth-pursuit eye movements. All of these plastic changes persisted during monocular viewing, indicating that the basic programming of these eye movements was changed.

Adaptation, Ocular↗
PubMed1991

Effects of GABAergic and noradrenergic injections into the cerebellar flocculus on vestibulo-ocular reflexes in the rabbit.

The role of the vesitibulo-cerebellum of the rabbit in the control of the vestibulo-ocular response (VOR) and optokinetic response (OKR) reflexes was investigated by bilateral microinjections, into the flocculus, of substances affecting GABAergic or noradrenergic neurotransmission. GABA, the main transmitter through which cerebellar interneurons inhibit Purkinje cells directly or indirectly, acts normally through GABAA receptors (mainly located in the granular layer) and GABAB receptors (predominantly located in the molecular layer). Despite this different distribution, floccular injections of the GABAA agonist muscimol and of the GABAB agonist baclofen had a similar effect, presumably by profound inhibition of Purkinje cells. This effect consisted of a reduction in the gain of the VOR (in darkness and in light) as well as of the OKR by at least 50%. This provides firm evidence that the net effect of normal Purkinje-cell activity in the flocculus is to enhance the VOR and OKR, rather than to inhibit these responses, as is sometimes supposed. Intrafloccular injections of the beta-noradrenergic agonist isoproterenol or the beta-noradrenergic antagonist sotalol did not affect the basic magnitude of the VOR and OKR. However, these substances markedly affected the adaptive processes, which cause the VOR and OKR to change its magnitude when this is no longer adequate in stabilizing the retinal image. By a suitable combination of vestibular and optokinetic stimuli, consistent upward changes in the gain of these reflexes could be reliably and reproducibly induced in uninjected animals. Floccular injections of sotalol impaired these adaptive changes markedly, whereas injections of isoproterenol enhanced the adaptation, particularly of the VOR measured in darkness. These findings strongly suggest that the effectuation of adaptive changes of vestibular, and possibly other, motor control systems is strongly facilitated by the noradrenergic innervation of the flocculus, which is normally provided by the locus coeruleus (LC), by way of the beta-receptor system, although the activity of this system does not directly affect the signal transmission supporting the basic reflexes as such.

Adaptation, Physiological↗
PubMed1991

Changes in VOR adaptation after local injection of beta-noradrenergic agents in the flocculus of rabbits.

Noradrenaline (NA) has been implicated as a neuromodulator in plasticity, presumably facilitating adaptive processes. Since the flocculus receives noradrenergic afferents, and ablation of the flocculus interferes with the normal adaptive changes in the VOR gain, experiments were performed to find out whether bilateral injection of monoaminergic substances into the flocculus of rabbits could modify the adaptive changes of the VOR. The visual world surrounding the rabbit was oscillated in opposite direction to the platform on which the rabbit was mounted, which resulted in an adaptive increase in the VOR gain; this adaptation was measured either in light or in darkness. Floccular injection of the beta-agonist isoproterenol did not greatly affect the adaptation of the VOR measured in light. In darkness, however, the increase in gain after injection of isoproterenol was larger than during normal adaptation. The beta-antagonist sotalol reduced the adaptation of the VOR gain significantly in light as well as in darkness. In a control condition without pressure for adaptation (only intermittent testing of the VOR gain over a period of 2.5 h), the gain of the VOR was not significantly affected by similar injections of beta-adrenergic agents. We conclude that the noradrenergic system facilitates the adaptation of the VOR gain to retinal slip in rabbits without affecting the VOR gain directly. At least part of this influence is exerted through beta-receptors located in the cerebellar flocculus.

Adaptation, Physiological↗
PubMed1990

Injections of beta-noradrenergic substances in the flocculus of rabbits affect adaptation of the VOR gain.

Noradrenaline (NA) has been implicated as a neuromodulator in plasticity, presumably facilitating adaptive processes. Recent experiments by others have suggested a modulatory role of NA in adaptive changes in the vestibulo-ocular reflex (VOR). These experiments showed that general depletion of brain NA resulted in a decreased ability to produce adaptive changes in the VOR gain. In order to identify the specific brain region responsible for these effects, as well as the nature of the adrenoceptors involved, we injected beta-adrenergic substances bilaterally into the flocculus of rabbits. The flocculus is known to receive noradrenergic afferents and, moreover, ablation of the flocculus interferes strongly with the normal adaptive changes in the VOR gain. We injected the beta-agonist isoproterenol and the beta-antagonist sotalol, and compared the adaptive capacity of the rabbits after these injections to that in a situation without injection. The rabbit was oscillated in a direction opposite to the direction of motion of the platform on which the rabbit was mounted, a condition which normally results in an increase in the VOR gain, measured either in light or in darkness. Injection of the beta-agonist did not greatly affect the adaptation of the VOR measured in the light. In darkness, the increase in gain after the injection of isoproterenol was larger than in the non-injection experiments in 9 out of 10 rabbits. The beta-antagonist sotalol reduced the adaptation of the VOR gain significantly in the light, as well as in darkness. In a control condition without pressure for adaptation (only intermittent testing of the VOR gain over a period of 2.5 h), the gain of the VOR either remained unaffected or was only slightly affected by similar injections of beta-adrenergic agents in individual rabbits. For the group as a whole, these effects were insignificant. We conclude from these results that noradrenergic systems facilitate the adaptation of the VOR gain to retinal slip in rabbits, without affecting the VOR gain directly. At least part of this influence is exerted through beta-receptors located in the cerebellar flocculus.

Adaptation, Physiological↗
PubMed1990

New directions for oculomotor research.

This paper reviews major trends in the study of the oculomotor system since Westheimer published his doctoral research on this topic 35 years ago. Westheimer introduced the use of linear system analysis for the study of eye movements, an approach used a great deal by others ever since. Westheimer himself abandoned this approach within a decade, in part, because this kind of analysis becomes ambiguous when predictive properties of oculomotor system performance become prominent. We discuss the implications of ignoring the prominence of predictive eye movements and describe recent evidence for their prevelence and power. This leads us to propose that a new approach to the study of oculomotor performance is required. We also discuss the recent trend to apply the "connectionist" (or "neural network") approach in studies of the oculomotor system, and point out that the "symbolic", rather than the "adaptive", nature of predictive eye movements makes successful extension of these models to oculomotor performance unlikely. Our new approach emphasizes the use of natural stimulation in subjects free from bodily restraints. Accurate measurement of eye, head and torso movements under such conditions has become possible recently and data obtained in this manner has led to the discovery of a number of unexpected characteristics of oculomotor system performance. These developments have encouraged us to abandon the modular view of the oculomotor system, popular since Dodge launched the modern era of oculomotor research in 1903, which postulates five, or more, largely independent "subsystems". We suggest that only two subsystems (a fast saccadic and a somewhat slower smooth) are used to fixate and track a central representation of objects located in three-dimensional space. We show that this two-subsystem approach is consistent with current knowledge of oculomotor system neuroanatomy and neurophysiology.

Adult↗
PubMed1989

Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane.

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil--revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: (a) the target was moved manually by the experimenter; (b) the target was moved manually by the subject; (c) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from 'slow' to 'very fast', corresponding to changes in target vergence from about 10 degrees s-1 to about 100 degrees s-1. The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When 'very fast' changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100 degrees s-1. Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i.e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i.e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the 'slow' and 'medium' conditions (stimulus speeds about 10-40 degrees s-1) tracking was accurate to within 1-2 degrees, irrespective of the way in which the target was moved. In the 'fast' and 'very fast' conditions (stimulus speeds about 40-100 degrees s-1), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1989

Ocular vergence under natural conditions. II. Gaze shifts between real targets differing in distance and direction.

Horizontal binocular eye movements of three subjects were recorded with the scleral sensor coil--revolving magnetic field technique during voluntary shifts of gaze between pairs of stationary, real, continuously visible targets. The target pairs were located either along the median plane (requiring symmetrical vergence), or on either side of the median plane (requiring asymmetrical vergence). Symmetrical vergence was primarily smooth, but it was often assisted by small, disjunctive saccades. Peak vergence speeds were very high; they increased from about 50 degrees s-1 for vergence changes of 5 degrees to between 150 and 200 degrees s-1 for vergence changes of 34 degrees. Differences between convergence and divergence were idiosyncratic. Asymmetrical vergence, requiring a vergence of 11 degrees combined with a version of 45 degrees, was largely saccadic. Unequal saccades mediated virtually all (95%) of the vergence required in the divergent direction, whereas 75% of the vergence required in the convergent direction was mediated by unequal saccades, with the remaining convergence mediated by smooth vergence, following completion of the saccades. Peak divergence speeds during these saccades were very high (180 degrees s-1 for a change of vergence of 11 degrees); much faster than the smooth, symmetrical vergence change of comparable size (14 degrees). Peak convergent saccadic speeds were about 20% lower. This difference in peak speed was caused by an initial, transient divergence, observed at the beginning of all horizontal saccades. The waveform of disjunctive saccades did not have the same shape as the waveform of conjugate saccades of similar size. The smaller saccade of the disjunctive pair was stretched out in time so as to have the same duration as its larger, companion saccade. These results permitted the conclusion that the subsystems controlling saccades and vergence are not independent. Vergence responses were relatively slow and incomplete with monocular viewing, which excluded disparity as a cue. Monocularly stimulated vergence decreased as a function of the increasing presbyopia of our three subjects. Subjects were able to generate some vergence in darkness towards previously seen and remembered targets. Such responses, however, were slow, irregular and evanescent. In conclusion, vergence shifts between targets, which provided all natural cues to distance, were fast and accurate; they appeared adequate to provide effective binocular vision under natural conditions. This result could not have been expected on the basis of previous observations, all of which had been made with severely reduced cues to depth.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1989

Differences in accuracy of human saccades between stationary and jumping targets.

Saccades have traditionally been studied in response to suddenly changing visual stimuli, such as jumping targets. In every-day life, however, most targets are stationary. We studied saccades made in either target condition. Saccadic accuracy was two- to five-fold better with stationary targets than with jumping targets. In addition, both the number of secondary saccades and the total time required to foveate the target were decreased with stationary targets in comparison to jumping targets. We also found that with an illuminated background the total time required to foveate the target was shorter than when the background was dark.

Adult↗
PubMed1989

The vestibulo-ocular reflex: an outdated concept?

Traditionally, the vestibulo-ocular reflex (VOR) is described as a distinct, phylogenetically old oculomotor subsystem, which serves to stabilize gaze direction. It is supposed to act as a stereotyped reflex with definite input-output relations, which can be measured by rotating a subject passively in darkness, and which are kept at an ideal level by adaptive, parametric adjustments. This paper argues that such a view is not realistic: (1) the VOR in darkness does not have an ideal, or even well defined, gain; (2) a fixed, automatic VOR is not appropriate in most behavioural situations, and would need continuous conditioning by other subsystems. As there is no compelling phylogenetic, physiological or anatomical evidence for an independent VOR subsystem, a more fruitful hypothesis may be that vestibular signals are just one of many inputs to a spatial localization process, which computes the relative position (and motion) between the subject and a target of his choice. The VOR in darkness may represent no more than a default operation, based on incomplete information, of this larger, multiple input gaze control system. Likewise, adaptation phenomena of the VOR in darkness may be merely an epiphenomenon of adaptation of gaze control with vision active.

Animals↗
PubMed1989

Depression of the vestibulo-ocular and optokinetic responses by intrafloccular microinjection of GABA-A and GABA-B agonists in the rabbit.

The functional implication of the cerebellar flocculus in regulation of the VOR and OKR gain has mostly been studied by lesion experiments, and the hypotheses derived from these experiments are not always in line with one another. In the present study, a reversible method was used to inhibit floccular Purkinje cells. The GABA-A agonist muscimol or the GABA-B agonist baclofen were bilaterally injected into the flocculus of rabbits, and the effects of these injections on the gain of the VOR and OKR were studied. Both drugs induced a reduction by at least 50% of the gain of the VOR in light and darkness, and of the OKR. Although GABA-A and GABA-B receptors are known to have different cerebellar localizations, muscimol and baclofen injections resulted in quantitatively similar effects. It is suggested that these GABA-agonists cause either direct or indirect inhibition of floccular Purkinje cells, thus reducing modulation of the firing rate of these neurons by afferent mossy and climbing fibers. Because the flocular Purkinje cells act out of phase with the vestibular neurons which drive the oculomotor neurons, a reduced output of floccular Purkinje cells would result in a reduction of the VOR and OKR gain. These experiments provide strong evidence that the cerebellar flocculus has a positive influence on the basic VOR and OKR gain.

Animals↗
PubMed1989

The vestibulo-ocular reflex: is it an independent subsystem?

Traditionally, the vestibulo-ocular reflex (VOR) is described as a distinct, phylogenetically old, oculomotor subsystem, which serves to stabilize gaze direction. It is supposed to act as a stereotyped reflex with definite input-output relations, which can be measured by rotating a subject passively in darkness, and which are kept at a nearly ideal level by adaptive parametric adjustments. This paper argues that such a view of the VOR may be not realistic: 1) the VOR in darkness (especially in humans) does not behave as a well-calibrated system; it has a low and variable gain which can be changed easily, even by purely mental assumptions; 2) a hard-wired VOR does not lead to appropriate eye movements in most natural situations, and would need continuous conditioning by other systems. As there is no compelling physiological or anatomical evidence for an independent VOR, it seems more fruitful to hypothesize that vestibular signals are just one of many inputs to a spatial localization process, which computes the relative motion between a subject and an object of his choice on the basis of all available, relevant information. Instead of a distinct subsystem, vestibulo-ocular responses in darkness may represent nothing more than the (somewhat arbitrary) default performance of this larger gaze-control system, functioning poorly in the absence of complete information.

Animals↗
PubMed1989

Asymmetrical adaptation of human saccades to anisometropic spectacles.

We report the extent to which effective asymmetrical saccadic adaptation was achieved by a myopic subject, who was exposed to "long-term" adaptation as he wore anisometropic corrective spectacles for about 40 years and also the extent of "short-term" adaptation in this subject and two other subjects, who initially made conjugate saccades, when they wore newly fitted anisometropic spectacles for about 8 hr. Two-dimensional binocular eye positions were measured with an accurate and precise revolving magnetic field-sensor coil technique. We found that long-term adaptation of vertical saccades was virtually perfect (almost 100% of the asymmetry introduced by the spectacles was corrected). Long-term adaptation of horizontal saccades was less complete and increased with target separation from about 40% for saccadic amplitudes of 5 degrees to about 75% for amplitudes of 60 degrees. Short-term adaptation of vertical saccades was virtually complete (100%) in one newly fitted subject and only partially complete (40%) in the other two subjects. The persistence of the adaptive asymmetry of saccades during monocular viewing showed that adaptation derived from plasticity in the programming of saccades and not from modification of vergence responses. Without the anisometropic spectacles, 30 min of self-paced, one per second changes in binocular fixation between two targets, which required a version change of 45 degrees in combination with a vergence change of 11 degrees, did not induce any asymmetrical adaptation. This result shows that a specific repeated association of version and vergence eye movements was not sufficient to induce asymmetrical adaptation, leading us to suggest that the transient fixation disparities at saccade-offset might be the necessary stimulus for the asymmetrical saccadic adaptation we observed.

Adaptation, Physiological↗
PubMed1988

Directional asymmetries of human optokinetic nystagmus.

Optokinetic nystagmus in the four principal directions was investigated on the occurrence of directional asymmetries in 7 normal human subjects. Instructions were aimed at obtaining a 'stare' type of OKN. The movement of both eyes was recorded simultaneously with a scleral sensor-coil method. Subjects viewed a full-field random dot pattern rotating at velocities of 9 to 57 deg/s binocularly, as well as monocularly with either eye. Gain was always less than 0.85 and decreased when the pattern velocity increased. Horizontal and vertical nystagmus differed in a number of respects. (1) We found no evidence for an overall asymmetry for rightward or leftward motion. However, human OKN showed a clear preference for upward stimulus motion. Mean gain was ca. 0.15 larger for upward than for downward motion. (2) The decrease of the gain of OKN as a function of increasing stimulus velocity was steeper for vertical than for the horizontal direction. (3) The eyes moved nearly perfectly yoked for vertical pattern movement, irrespective of the viewing conditions. In contrast, during horizontal OKN the gain of the eye tracking in the nasal direction was higher (by about 4%) than the gain of the other eye moving simultaneously in the temporal direction. This difference persisted irrespective of the viewing conditions and appears to be motor, not sensory in origin. In addition, for any direction of the pattern motion a statistically significant increase of the gain occurred when the pattern motion was seen binocularly instead of monocularly with either eye.

Functional Laterality↗
PubMed1988

Oculomotor defects in patients with Huntington's disease and their offspring.

We recorded saccadic, pursuit and fixation eye movements in patients (n = 5) with moderately advanced Huntington's disease (HD), offspring of HD patients (n = 22) and control subjects (n = 15), using the scleral sensor coil technique. Saccadic slowing was seen in all patients, no controls and (marginally) in a few at-risk subjects. Fixational stability was lower in patients than in the other groups; a structured background enhanced the difference and revealed decreased stability in a number of at-risk subjects. Smooth pursuit showed large errors in most patients and several controls but none of the at-risk subjects. Sporadic follow-up data show that at least two of the at-risk subjects developed manifest HD within a few years after passing the oculomotor test with entirely normal results. The material as a whole suggests that oculomotor dysfunction does not develop prior to, but concurrently with and as a part of generalized, progressive deterioration of motor control. The implication is that oculomotor screening of clinically healthy at-risk subjects does not reliably contribute to an earlier diagnosis of future HD.

Adult↗
PubMed1988

Binocular co-ordination of human horizontal saccadic eye movements.

1. The binocular co-ordination of human horizontal saccades was analysed for the first time systematically over the full oculomotor range with a precise and accurate scleral sensor coil technique. Effects of amplitude (1.25-80 deg), direction (adduction vs. abduction and centrifugal vs. centripetal) and eccentricity (symmetrical about primary or between primary and secondary positions) were systematically investigated in three subjects). 2. To minimize extraneous effects of stimulus presentation on the programming of saccades, subjects were instructed to voluntarily change their gaze between two continuously visible targets. These were positioned on an iso-vergence locus, and thus contained no stimulus for disjunctive eye movements. 3. Under these conditions the amplitudes of the primary saccades of the two eyes were remarkably accurate; undershooting of the target by about 0.5 deg (independent of amplitude in the range 10-70 deg) was typical. This finding contrasts with the undershooting by about 10% described in the literature as characteristic for other stimulus conditions. 4. Saccadic peak velocities saturated at a mean asymptotic level of 502 +/- 32 (S.D.) deg/s for saccades of 40 deg and larger. The duration was linearly related to amplitude for saccades up to 50 deg; for saccades of larger sizes the duration increased progressively more steeply. Skewness values (acceleration time as a fraction of total saccadic duration) decreased from about 0.45 for saccades up to 10 deg to about 0.20 for saccades of 50 deg and larger. 5. Binocular saccades showed an abduction-adduction asymmetry and were not well yoked dynamically. The saccades of the abducting eye consistently had a larger size, a higher peak velocity, a shorter duration and were more skewed than the concomitant adducting saccades of the fellow eye. As a result, the eyes diverged transiently by as much as 3 deg during horizontal saccades. 6. Saccades also showed a marked centrifugal-centripetal asymmetry. Peak velocities of saccades towards the primary position were about 10% higher than peak velocities of corresponding centrifugal saccades. 7. These directional asymmetries were the main source of variability in the pool of saccades. In comparison, intra- and intersubject variability was minor in our sample. 8. Post-saccadic drift consisted of a vergence and a version component. The vergence component of this drift was a continuation of the vergence movement occurring during saccades. The version component, generally smaller than the vergence component, was directed towards the target position.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗