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Christoph Helmchen

Publications and source records attributed to Christoph Helmchen.

14 recordsLinked to original sources
PubMed2006

Neural activity related to self- versus externally generated painful stimuli reveals distinct differences in the lateral pain system in a parametric fMRI study.

Self-generated sensory stimulation can be distinguished from externally generated stimulation that is otherwise identical. To determine how the brain differentiates external from self-generated noxious stimulation and which structures of the lateral pain system use neural signals to predict the sensory consequences of self-generated painful stimulation, we used functional magnetic resonance imaging to examine healthy human subjects who received thermal-contact stimuli with noxious and non-noxious temperatures on the resting right hand in random order. These stimuli were internally (self-generated) or externally generated. Two additional conditions served as control conditions: to account for stimulus onset uncertainty, acoustic stimuli preceding the same thermal stimuli were used with variable or fixed delays but without any stimulus-eliciting movements. Whereas graded pain-related activity in the insula and secondary somatosensory cortex (SII) was independent of how the stimulus was generated, it was attenuated in the primary somatosensory cortex (SI) during self-generated stimulation. These data agree with recent concepts of the parallel processing of nociceptive signals to the primary and secondary somatosensory cortices. They also suggest that brain areas that encode pain intensity do not distinguish between internally or externally applied noxious stimuli, i.e., this adaptive biological mechanism prevents harm to the individual. The attenuated activation of SI during self-generated painful stimulation might be a result of the predictability of the sensory consequences of the pain-related action.

Adult↗
PubMed2006

Eye-hand coordination in essential tremor.

Patients with essential tremor (ET) or with cerebellar lesions have in common oculomotor abnormalities, with the exception of saccadic eye movements, which do not seem to be involved in ET. Since grasping is prolonged in ET and might be related to saccadic dysmetria, we tested whether simultaneous hand pointing could unmask it. Twelve ET patients and 14 controls performed saccades with and without simultaneous pointing movements to the same targets, and with and without a gap between the disappearance of the fixation point and the appearance of the target. Eye movements were recorded with the magnetic search-coil method, hand movements with an ultrasound-emitting probe. ET patients did not have saccadic dysmetria, and contrary to normal subjects their saccadic latency did not decrease during combined eye-hand movements compared with saccades performed in isolation. Hand movements had a longer duration in ET patients, with decreased peak acceleration, an increased latency of the peak velocity, and peak deceleration. In conclusion, this first study on eye-hand coordination in ET revealed abnormal kinematic changes in the early phase of pointing movements. These changes might be related to cerebellar disease but they are independent of the intention tremor component and saccade performance.

Adult↗
PubMed2006

Age-related changes of vergence under natural viewing conditions.

Vergence eye movements were recorded with the scleral search-coil system in 32 healthy subjects (ages 19-73 years) to characterize the age-related effects on the dynamic parameters of vergence responses to step (transient components) and ramp or sinusoidal targets (sustained components) under natural viewing conditions. Transient vergence showed an age-related increase in latency and decreases in peak velocity and acceleration in the binocular stimulus condition but not in accommodative vergence. Sustained vergence showed no age-related effect in the binocular condition, but there was an age-related decrease in accommodative vergence steady-state velocity and an increase in latency. Age-related changes of the transient and sustained components were very similar to those reported for saccades and smooth pursuit; they thus might support a distinction between a sustained and transient vergence system. Furthermore, such age-related effects have to be taken into account when assessing eye movement disorders in neurodegenerative and cerebrovascular diseases.

Accommodation, Ocular↗
PubMed2005

Palsy of "fast" and "slow" vergence by pontine lesions.

The role of pontine nuclei in vergence eye movements to "step" targets ("fast" vergence) is unknown. Eye movements were recorded in two patients with unilateral pontine infarctions and in 11 healthy controls. In addition to the deficit of "slow" vergence, "fast" vergence was particularly impaired. However, conjugate saccades did not differ from controls, but smooth pursuit eye movements did. The authors conclude that "fast" vergence palsy is not only caused by midbrain but also upper pontine lesions.

Aged↗
PubMed2005

Effect of 3,4-diaminopyridine on the postural control in patients with downbeat nystagmus.

Downbeat nystagmus (DBN) is a common, usually persistent ocular motor sign in vestibulocerebellar midline lesions. Postural imbalance in DBN may increase on lateral gaze when downbeat nystagmus increases. 3,4-Diaminopyridine (3,4-DAP) has been shown to suppress the slow-phase velocity component of downbeat nystagmus and its gravity-dependent component with concomitant improvement of oscillopsia. Because the pharmacological effect is thought to be caused by improvement of the vestibulocerebellar Purkinje cell activity, the effect of 3,4-DAP on the postural control of patients with downbeat nystagmus syndrome was examined. Eye movements were recorded with the video-based Eyelink II system. Postural sway and pathway were assessed by posturography in lateral gaze in the light and on eye closure. Two out of four patients showed an improvement of the area of postural sway by 57% of control (baseline) on eye closure. In contrast, downbeat nystagmus in gaze straight ahead and on lateral gaze did not benefit in these two patients, implying a specific influence of 3,4-DAP on the vestibulocerebellar control of posture. It was concluded that 3,4-DAP may particularly influence the postural performance in patients with downbeat nystagmus.

4-Aminopyridine↗
PubMed2005

Effect of blinks on saccades before smooth-pursuit eye-movement initiation.

The effect of blinks on the saccade frequency before the onset of step ramp smooth pursuit was assessed in healthy controls. Blinks elicited before smooth pursuit onset suppressed saccades in contrast to a control and a gap paradigm. The source of blink associated suppression of saccades remains unclear but is probably not caused by visual signals or the omnipause neurons.

Adult↗
PubMed2004

Blink effects on ongoing smooth pursuit eye movements in humans.

Blinks are known to affect eye movements, e.g., saccades, slow and fast vergence, and saccade-vergence interaction, in two ways: by superimposition of blink-associated eye movements and changes of the central premotor activity in the brainstem. The goal of this study was to determine, for the first time, the effects of trigeminal evoked blinks on ongoing smooth pursuit eye movements which could be related to visual sensory or premotor neuronal changes. This was compared to the effect of a target disappearing for 100-300 ms duration during ongoing smooth pursuit (blank paradigm) in order to control for the visual sensory effects of a blink. Eye and blink movements were recorded in eight healthy subjects with the scleral search coil technique. Blink-associated eye movements during the first 50% of the blink duration were non-linearly superimposed on the smooth pursuit eye movements. Immediately after the blink-associated eye movements, the pursuit velocity slowly decreased by an average of 3.2+/-2.1 degrees /s. This decrease was not dependent on the stimulus direction. The pursuit velocity decrease caused by blinks which occluded the pupil more than 50% could be explained mostly by blanking the visual target. However, small blinks that did not occlude the pupil (<10% of lid closure) also decreased smooth pursuit velocity. Thus, this blink effect on pursuit velocity cannot be explained by blink-associated eye movements or by the blink having blanked the visual input. We propose that part of this effect might either be caused by incomplete visual suppression during blinks and/or a change in the activity of omnipause neurons.

Adult↗
PubMed2004

Vergence deficits in pontine lesions.

Eye movements were recorded with the search coil system in two patients to determine whether lesions of the pontine nuclei selectively impair vergence to ramp targets (slow vergence) or step targets (fast vergence). Whereas conjugate saccades were not different from healthy control subjects, conjugate smooth pursuit eye movements had a reduced gain in horizontal and vertical directions. Slow convergence and divergence were impaired, whereas fast vergence did not differ from that of control subjects. Pontine nuclei appear to be involved in the slow vergence control.

Convergence, Ocular↗
PubMed2004

Differential effects of blinks on horizontal saccade and smooth pursuit initiation in humans.

Blinks executed during eye movements affect kinetic eye movement parameters, e.g., peak velocity of saccades is decreased, their duration is increased, but their amplitude is not altered. This effect is mainly explained by the decreased activity of premotor neurons in the brainstem: omni-pause neurons (OPN) in the nucleus raphe interpositus. Previous studies examined the immediate effect of blinks directly on eye movements but not their effect when they are elicited several hundred milliseconds before the eye movements. In order to address this question we tested blinks elicited before the target onset of saccades and pursuit and compared the results to the gap effect: if a fixation light is extinguished for several hundred milliseconds, the reaction time (latency) for subsequent saccades or smooth pursuit eye movements is decreased. Monocular eye and lid movements were recorded in nine healthy subjects using the scleral search-coil system. Laser stimuli were front-projected onto a tangent screen in front of the subjects. Horizontal step-ramp smooth pursuit of 20 deg/s was elicited in one session, or 5 deg horizontal visually guided saccades in another experimental session. In one-third of the trials (smooth pursuit or saccades) the fixation light was extinguished for 200 ms before stimulus onset (gap condition), and in another third of the trials reflexive blinks were elicited by a short airpuff before the stimulus onset (blink condition). The last third of the trials served as controls (control condition). Stimulus direction and the three conditions were randomized for saccades and smooth pursuit separately. The latency of the step-ramp smooth pursuit in the blink condition was found to be decreased by 10 ms, which was less than in the gap condition (38 ms). However, the initial acceleration and steady-state velocity of smooth pursuit did not differ in the three conditions. In contrast, the latency of the saccades in the gap condition was decreased by 39 ms, but not in the blink condition. Saccade amplitude, peak velocity, and duration were not different in the three conditions. There was also no difference in blink amplitude and duration of pupil occlusion in the blink condition, neither in saccades nor in smooth pursuit. The latency reduction of smooth pursuit, but not of saccades, may neither be explained by the brief pupil occlusion nor by visual suppression, warning signals, or the startle response. Whether the effects are caused by the influence of blinks on OPNs or other premotor structures remains to be tested.

Adult↗
PubMed2003

GABAergic neurons in the rostral mesencephalon of the macaque monkey that control vertical eye movements.

The mesencephalic reticular formation is important for the generation of vertical eye movements, but up until now the location of inhibitory premotor neurons is not known in primates. With tract-tracer methods combined with immunocytochemistry or in situ hybridization, we investigated the location of GABAergic premotor neurons in the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) and interstitial nucleus of Cajal (iC) in macaque monkeys. In the present work, only the premotor pathways of the downward pulling eye muscles, superior oblique (SO) and inferior rectus (IR), were studied. We found that very few, small GABAergic neurons are present in the riMLF, and none of them was found to project to the oculomotor nuclei, suggesting the presence of exclusively excitatory projections from the riMLF to the oculomotor neurons. However, in the iC, medium-sized and large GABAergic neurons were identified projecting contralaterally to the SO and IR motoneurons, and presumably the iC of the other side. These commissural GABAergic projections are well suited to inhibit the SO and IR motoneurons and possibly premotor down-burst-tonic neurons during upward eye movements.

Animals↗
PubMed2003

The role of the fastigial nucleus in saccadic eye oscillations.

For the first time, we provide functional magnetic resonance imaging evidence for a recent hypothesis that saccadic oscillations in opsoclonus may result from a disinhibition of the cerebellar fastigial nuclei. Two patients with severe opsoclonus were examined during fixation in the light and during eye closure and in darkness where opsoclonus disappeared. Their activation during opsoclonus was compared with 10 healthy subjects performing visually guided and self-paced saccades in the light and darkness. In contrast to the control subjects, the patients showed a strong bilateral midline cerebellar activation that involved the deep cerebellar nuclei. This is probably not just a secondary finding in the fastigial nuclei due to the high frequent saccadic activity because there was, concomitantly, no oculomotor vermal activation, which is normally seen in healthy subjects. We propose that cerebellar activation of the fastigial nuclei may cause opsoclonus via their projections to the brainstem saccadic generator.

Adult↗
PubMed2002

Blink effect on slow vergence.

Blinks are known to change the kinematic properties of fast eye movements, probably by changes in the brain stem circuits. To determine whether slow disconjugate (slow vergence) eye movements are affected by blinks under natural viewing conditions, we elicited airpuff-evoked trigeminal blinks randomly during ongoing steady slow vergence eye movements. Lid and binocular eye movements were recorded by the scleral search coil method. Slow vergence eye movements showed a peak of vergence velocity during the final part of the blink, which depends on the stimulus direction. We propose that the direction-specific blink effect on slow vergence may be caused by changes in brain stem premotor circuits.

Adult↗
PubMed2002

The contribution of midbrain circuits in the control of gaze.

The midbrain contains several structures important for the generation of torsional and vertical eye movements including the rostral interstitial nucleus of the MLF (riMLF) and the interstitial nucleus of Cajal (iC). While the riMLF is the immediate premotor structure for the generation of torsional and vertical saccades, the iC is considered a major part of the neural integrator for torsional and vertical eye movements. Experiments in monkeys show that a unilateral inactivation of the riMLF with muscimol leads to spontaneous contralesional torsional nystagmus, whereas an iC inactivation causes ipsilesional torsional nystagmus. In addition, inactivation of either structure leads to a tonic ocular torsion to the contralesional side. While the deficits after a riMLF lesion are thought to result from an imbalance of the saccade generator, a vestibular imbalance probably causes the deficits after an iC lesion. Contralesional and ipsilesional torsional nystagmus is also found in patients with unilateral mesencephalic lesions. A detailed analysis of the lesions from MRI scans shows a preferential involvement of the riMLF for patients with contralesional torsional nystagmus, and a major involvement of iC in cases with ipsilesional torsional nystagmus. Thus, the direction of torsional nystagmus appears to be a valuable topodiagnostic sign for patients with midbrain lesions.

Animals↗