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Biomedical subjects

C Kennard

Publications and source records attributed to C Kennard.

At least 109 records · Page 6Linked to original sources
PubMed1989

Abnormalities of nonvisually-guided eye movements in Parkinson's disease.

Rapid eye movements (saccades) were examined in 7 patients with idiopathic Parkinson's disease (PD) and a matched group of normal control subjects. The effect of instructional and stimulus conditions used to elicit saccades was examined using 3 experimental paradigms. Eye movements directly elicited by a novel peripheral target were unimpaired in patients with PD as compared with control subjects. Saccades to a remembered target location, however, were dysmetric in the PD group and showed a characteristic multistepping pattern. The PD impairment was not caused by a loss of information on target location since their final eye position was close to the target at all eccentricities. Peak velocity, duration, and latency did not distinguish between PD patients and controls. These results support the view that for saccades which are not directly elicited by a visual target there is a neural pathway that can be distinguished from structures involved in the generation of visually elicited (or 'reflexive') saccades. The finding that in PD saccades to a remembered target are selectively impaired suggests that structures in the basal ganglia play a crucial role in this alternative pathway.

Aged↗
PubMed1989

Predictive responses in Parkinson's disease: manual keypresses and saccadic eye movements to regular stimulus events.

In a coincidence timing task, Parkinsonian patients and a control group were instructed to synchronize a keypress with the onset of a visual signal which had been preceded by a regular train of warning signals. Although the Parkinsonian group had previously exhibited slower reactions in a conventional simple reaction-time task, they were able to generate predictive responses that fell as close to the target onset as the controls' but showed greater variability. In a second experiment, Parkinsonian patients and controls made saccadic eye movements to a visual target that stepped at regular intervals between two fixed locations. After a few trials all the subjects tended to make predictive saccades that were initiated before the target excursion. However, the Parkinsonian group were slower to develop this strategy and when they did their saccades became considerably more hypometric than those of the controls. Both groups were able to maintain predictive responding even when the visual target disappeared and responses were paced by a buzzer. We concluded that Parkinsonian patients are capable of initiating predictive responses of the eye and the hand, at least in some circumstances, but such responses tend to be inaccurate in execution. This, in turn, may dispose the Parkinsonian patient against predictive movement.

Aged↗
PubMed1989

Relative efficacy of intravenous methylprednisolone and ACTH in the treatment of acute relapse in MS.

To compare the efficacy of high-dose intravenous methylprednisolone with intramuscular ACTH in the treatment of acute relapse in multiple sclerosis, we undertook a double-blind, randomized, controlled study involving 61 patients. There was a marked improvement in both groups in the course of the study, but no difference between them in either the rate of recovery or the final outcome. High-dose IV methylprednisolone is a safe alternative to ACTH in the management of acute relapse in MS.

Acute Disease↗
PubMed1989

Oculomotor abnormalities in diseases of the basal ganglia.

Disorders of eye movements have been described in diseases of the basal ganglia for over a century. Recent neurophysiological and clinical work has greatly clarified the oculomotor role of these structures: their major involvement appears to be in the generation of "voluntary" saccades, and in the suppression of "reflex" saccades. The observed abnormalities of saccadic eye movements in Huntington's (HD) and Parkinson's (PD) diseases conform very well to predictions based on a combination of known disease pathology and recent neurophysiological work. This is not quite the case for other types of eye movement, such as smooth pursuit, largely because the exact role of the basal ganglia in their generation has not been defined neurophysiologically. Several diseases other than HD and PD both involve the basal ganglia and have effects on eye movements; such diseases include progressive supranuclear palsy and Wilson's disease. Unfortunately, the pathological processes in these conditions are too widespread to allow comment on how well oculomotor abnormalities fit in with predictions.

Eye Movements↗
PubMed1989

Unilateral reversal of smooth pursuit and optokinetic nystagmus.

A patient is presented who has had three six-month episodes of oscillopsia over the last eight years. During the episodes, abnormal smooth pursuit and optokinetic nystagmus were noted, but there were no other abnormalities. Oculography during the most recent episode demonstrated that when tracking from right to left, the patient's smooth pursuit was reversed. There are several features of the case which suggest that the aetiology may be a form of congenital nystagmus. A possible alternative mechanism whereby such an abnormality might arise as an acquired lesion is discussed.

Adult↗
PubMed1988

Two distinct deficits of visual tracking caused by unilateral lesions of cerebral cortex in humans.

We studied horizontal visual tracking in 20 patients with unilateral cerebral lesions and in 10 age-matched control subjects. Five patients, all with posterior lesions, showed impaired smooth pursuit of predictable targets moving toward the side of the cerebral lesion. Using nonpredictable step-ramp stimuli, we identified two distinct deficits of visual tracking. The first was a unidirectional deficit of smooth pursuit, for targets moving toward the side of the lesion, in response to stimuli presented into either visual hemifield. The second deficit, identified in a sixth patient who did not show pursuit asymmetry to predictable targets, was a bidirectional inability to estimate the speed of a moving target in the visual hemifield contralateral to the side of the lesion; this caused inaccurate saccades to moving (but not stationary) targets and impaired smooth pursuit initiation. These visual tracking deficits were independent of homonymous hemianopia or hemispatial neglect. These two tracking deficits are similar to those described in rhesus monkeys with lesions of the medial superior temporal and middle temporal visual areas.

Adolescent↗
PubMed1987

Predictive eye saccades are different from visually triggered saccades.

The metrics of eye saccades in response to random and predictable target motion were determined in 8 normal young subjects. The peak velocity of predictive saccades (PS) was significantly reduced when compared with randomly elicited saccades (RS) of equal amplitude. Saccadic gain (initial saccadic amplitude/target amplitude) was also decreased in PS. Saccadic inaccuracy was extremely frequent in PS and, in contrast to RS, it was independent of target amplitude displacement. These stimulus-dependent changes in saccadic characteristics may reflect activation of the saccadic pontine generators by different supranuclear pathways.

Adolescent↗
PubMed1987

Visual experience and the maturation of the ipsilateral visuotectal projection in Xenopus laevis.

The effect of visual deprivation upon the maturation of the ipsilateral visuotectal projection has been studied in Xenopus laevis. This topographically ordered projection is polysynaptic. The first stage involves the retinal projection to the contralateral optic tectum. The tectum projects to the nucleus isthmi on the same side. The final stage is the crossed isthmotectal projection from the nucleus isthmi to the tectum ipsilateral to the eye. The topographic precision of connections at various points in this polysynaptic pathway has been investigated by quantifying single-unit and multi-unit receptive field sizes in the contralateral and ipsilateral visuotectal projections. Observations have been made on normal animals of different ages to plot the normal maturational course of events. The effects of visual deprivation on this maturational process has been studied. Between one week and one year after metamorphosis there is an increase in the precision of connections in both the contralateral and ipsilateral visuotectal projections. Visual deprivation had no effect upon the parameters of the contralateral visuotectal projection. Ipsilateral visuotectal single units in dark-reared animals had normal receptive field sizes. Ipsilateral multi-unit receptive fields in dark-reared animals were considerably larger than in normal animals. It was concluded that the effects of visual deprivation are limited to effects on the crossed isthmotectal component of the intertectal system. In this component, however, visual experience seems to play an important role in the normal development and modification of connections. It is suggested that visual experience is utilized to accommodate changes in the system required to respond to normal changes in interocular geometry that take place with development in Xenopus.

Animals↗
PubMed1987

Residual vision in patients with retrogeniculate lesions of the visual pathways.

Twenty-five subjects have been studied who, as a result of damage to the striate cortex, were 'blind' in extensive areas (scotomata) of the visual fields. Of these 25 subjects, 5 exhibited residual vision in response to transient lights presented within the scotoma, which enabled them to locate the stimuli by hand-reaching or by eye movements; the latter have been measured by electro-oculography. The residual vision underlying their responses was characterized by low flicker-fusion and by sensitivity in detection of movement which increased as target speed was raised. Discrimination for the direction of target movement was poor, but spatial resolution in the discrimination of target displacement was essentially normal. The subjects were unable to recognize or discriminate the spatial structure of targets located within the 'blind' field, and the observed dissociation between spatial discrimination of displacement and pattern is examined in relation to the 'two systems' hypothesis of visual function. There is no obvious correlation between the extent of neuronal damage as revealed by CT scans and the existence of residual vision.

Adult↗
PubMed1987

Omnipause neurons in two cases of opsoclonus associated with oat cell carcinoma of the lung.

Opsoclonus is an involuntary eye movement disorder in which there are chaotic, usually conjugate, multidimensional saccadic eye movements. In this paper 2 cases of opsoclonus are reported, as a paraneoplastic phenomenon in association with oat cell carcinoma of the lung. It has previously been hypothesized that opsoclonus results from dysfunction of a group of premotor neurons in the brainstem called omnipause neurons. We describe the location of these cells in man by homology with animal studies, and describe the light microscopic appearance of these neurons in the 2 cases of opsoclonus. Although these neurons appeared normal it is still possible that their function was disturbed as a result of metabolic or neurotransmitter abnormalities.

Aged↗
PubMed1987

Ocular motor and manual tracking in Parkinson's disease and the effect of treatment.

A quantitative analysis of unpredictable saccadic and smooth pursuit ocular motor and manual tracking was performed in 15 mildly affected, previously untreated subjects suffering from Parkinson's disease and in age-matched normal controls. The patients' ocular saccades were characterised by an increased variability of their latencies and reduced accuracy, with normal velocity profiles apart from saccadic duration. Their smooth pursuit had a decreased velocity gain. Similar abnormalities were found with manual tracking. Clinical improvement with dopaminergic drugs was associated with an improvement of saccadic accuracy and smooth pursuit gain. It is postulated that the ocular motor changes seen in Parkinson's disease are contingent upon functional dopamine levels in the basal ganglia.

Adult↗
PubMed1986

A study of systematic visual perseveration involving central mechanisms.

The subject of this investigation experiences vivid after-images which persist for tens of seconds following stimulation by light insufficiently intense to produce after-images in normal subjects. The after-images occur at all points in the visual field, and have spatial structure closely similar to that of the stimuli which elicit them, the only distortion being the absence of high (greater than or equal to 10 cycles X deg-1) spatial frequencies. The duration of the after image is dependent on the spatial structure and spectral content of the eliciting stimulus, being maximum for low, nonzero spatial frequencies (1-2 cycles X deg-1) and yellow or green wavelengths. The after-images appear 'khaki' or 'mustard-green' in colour, regardless of the spectral composition of the light stimulus, and those produced by flickering or moving targets do not themselves exhibit temporal fluctuations, although movement produces a streak along the path described by the moving object. After six weeks of treatment with the anticonvulsant drug, carbamazepine, the duration of the after-images fell by about 30%, but no further reduction occurred over the following three months. In spite of the prolonged after-images, the subject has normal sensitivity for detection of flicker and movement, but that for detection of a single flash is raised abnormally at low background illumination. Neither CT nor MRI brain scans revealed any abnormality. These responses are discussed in relation to those found in other subjects suffering visual perseveration or palinopsia, and we examine their possible causes.

Adult↗
PubMed1986

Higher control mechanisms of saccadic eye movements.

The higher control of saccadic eye movements depends on a complex interaction between a number of different neural centres. Inputs from the frontal eye field and superior colliculus both project to the brainstem and appear to provide parallel and interacting inputs. The former is more concerned with voluntary shifts of gaze, whereas the latter mediates involuntary reflexive gaze shifts to suddenly appearing visual stimuli. The basal ganglia through the substantia nigra, pars reticularis has direct inputs to the colliculus and appears to have a gating role. These concepts, derived from experimental studies on primates, are discussed in relation to clinical material.

Animals↗
PubMed1985

A species difference between Rana and Xenopus in the occurrence of intertectal neuronal plasticity.

In anuran amphibians, a system of neuronal connections links the two optic tecta and is involved in projections of the binocular visual field to the optic tecta. Electrophysiological studies have shown that in the frog, Xenopus laevis, the pattern of connections may be modified by procedures such as larval rotation of one eye. This modification appears to be effected by visual experience. Workers in other laboratories, however, found no evidence of such a modification in the related frog Rana pipiens. This difference in results may have been due to different rearing conditions and different recording techniques or may reflect a true species difference, in this respect, between Rana and Xenopus. In the present experiments, an attempt was made to distinguish between these possibilities by performing eye rotations in Rana and Xenopus, rearing them as identically as possible and recording from them under identical conditions. It was found that while Xenopus displayed the modification of intertectal connections, Rana did not. It is concluded that the different responses of the intertectal systems to larval eye rotation in Xenopus and Rana reflect a species difference.

Adaptation, Physiological↗
PubMed1985

Predictive ocular motor control in Parkinson's disease.

A comparison was made of predictive eye movements of both the saccadic and pursuit ocular motor systems in parkinsonian patients and aged-matched normal controls. In a predictive task our patients, who were mildly or moderately affected, showed a reduced tendency to make anticipatory saccades compared with normal controls. Although there was some impairment of pursuit during a nonpredictive task as shown by an increased phase lag, a normal amount of improvement took place with a predictive task. This difference between prediction in the saccadic and pursuit system is possibly explained by an increased reliance on a visual input by parkinsonian patients which prevents them making use of verbal instruction to generate anticipatory (eye in advance of target) saccades. Improvement of pursuit with a predictive target track is possible in such patients since normally in these circumstances the target is mainly followed rather than anticipated. The metrics of saccadic and smooth pursuit eye movements (saccadic velocity and pursuit turnover velocity) were normal.

Aged↗
PubMed1985

Respiration and sleep in Parkinson's disease.

Sleep and respiration during sleep were studied in patients with idiopathic Parkinson's disease, patients with Parkinsonism with autonomic disturbance, and normal age and sex matched controls. Patients with idiopathic Parkinson's disease showed significantly reduced REM sleep, and more frequent and prolonged waking throughout the night. Hypoventilation and sleep apnoea did not occur in the idiopathic Parkinson's disease or normal groups, but respiration was disorganised with frequent central and obstructive apnoeas in the autonomic disturbance group. Respiratory rate during non rapid eye movement sleep was similar in the idiopathic Parkinson's disease and normal groups, but patients with idiopathic Parkinson's disease showed tachypnoea awake and during REM sleep.

Adult↗