Biomedical subjects

T Eggert

Publications and source records attributed to T Eggert.

At least 37 records · Page 2Linked to original sources

PubMed2000

Nicotine-induced nystagmus: three-dimensional analysis and dependence on head position.

Two- and three-dimensional analyses of nicotine-induced eye movements were performed in 53 subjects to evaluate whether they were primarily of vestibular or ocular motor origin. Nicotine-induced nystagmus was detected in 27 subjects (51%); in 25 of these (93%) it was modulated by otolith input. Three-dimensional analysis of nicotine-induced nystagmus revealed that it violates Listing's law. Taken together, these findings suggest that nicotine induces an imbalance in the vestibulo-ocular reflex.

Adult↗

PubMed2000

Size distortion in spatial neglect.

One of the suggested theories to explain some aspects of neglect in patients with right parietal lesions is the spatial distortion hypothesis. To determine whether a distorted representation of space can account for the performance of neglect patients in visuospatial tasks we asked 27 neglect patients to compare the width of two horizontally aligned bars. The bars were presented symmetric to the body midline or either on the left or right side of egocentric space. The size comparison data are in accordance with the hypothesis of a distorted egocentric representation based on a dynamic remapping of space. The results support the idea that the abnormalities observed in the size comparison tasks are due to a distorted internal representation of the outside world. There are some hints that this distortion could be based on a dynamic mapping of space determined by the distribution of visuospatial attention.

Aged↗

PubMed2000

Learning to write letters: transfer in automated movements indicates modularity of motor programs in human subjects.

Many automatic movements are open-loop, feed-forward motor programs (MP) that are kinematically well characterized by smooth speed and acceleration curves. However, it is unclear whether their internal representation consists of monolithic blocks or subroutines. This question was investigated using a learning paradigm of a writing task. Fifty-nine normal subjects were presented with two similar, but different new letters. Every subject practiced each letter in a series of 60 trials, with the order of letter series randomized. Every session was continuously recorded by a digitizing tablet. Using kinematic analysis, we measured the number of vertical acceleration peaks as an indication of the number of corrective movements (COM). Since COM declined as automatization was approached, we could quantitatively infer progress in motor learning under natural learning conditions. In the case of modular storage of MP, transfer in-between letters was expected due to the re-use of pre-learned motor subroutines. Statistical analysis showed that the exponential model described the data much better than the linear model (residual error: P<0.88 and P<0.00001, respectively), as expected for a learning paradigm. There was no difference between letters per se (P<0.77). Motor improvement differed significantly (P<0.02) between the first and the second series; there was a much greater reduction of COM in the second series (50.1 vs. 41.1%). This difference can be logically ascribed to transfer, indicating that automated movements are stored in motor subroutines.

Biomechanical Phenomena↗

PubMed2000

["Inner perilymph fistula" of the anterior semicircular canal. A new disease picture with recurrent attacks of vertigo].

In 1998 Minor et al. described a new variant of perilymphatic fistula: the "superior canal dehiscence syndrome". This syndrome is clinically characterized by recurrent attacks of vertigo and oscillopsia induced by loud noises or stimuli that result in changes in intracranial or middle ear pressure. It is caused by a dehiscence of bone overlying the superior (anterior) semicircular canal. Due to this dehiscence, a third, mobile window (in addition to the round and oval windows) is formed, and changes in pressure are pathologically transduced to the anterior semicircular canal. Although the superior canal dehiscence syndrome is not a rare condition, no other cases have yet been reported. Therefore, we describe a typical patient who suffered for many years from recurrent attacks of vertigo and oscillopsia induced by coughing and Valsalva's maneuvers. High resolution temporal bone CT scan showed a defect in the bone overlying the left anterior semicircular canal. Three-dimensional eye movement recordings using the search coil technique and subsequent vector analysis demonstrated that the eye movements were induced by excitation of the left anterior semicircular canal. We conclude that superior canal dehiscence syndrome is an important differential diagnosis in patients suffering from symptoms of a perilymphatic fistula, especially since it can be successfully treated by "plugging" of the affected semicircular canal. Such patients are thus spared unnecessary surgery of the middle ear.

Diagnosis, Differential↗

PubMed2000

Spatial and temporal aspects of eye-hand coordination across different tasks.

The way in which saccadic eye movements are elicited influences their latency and accuracy. Accordingly, different tasks elicit different types of saccades. Using the tasks steps, gap, memory, scanning and antisaccade, we analyzed combined eye and hand movements to determine whether both motor systems share control strategies. Errors and latencies were measured to examine whether changes in eye motor behavior are reflected in hand motor behavior. Directional and variable errors of eye and hand changed differently according to the tasks. Moreover, errors of the two systems did not correlate for any of the tasks investigated. Contrary to errors, mean latencies of eye movements were organized in the same pattern as hand movements. A correlation of latencies indicates that both motor systems rely on common information to initiate movement. Temporal coupling was stronger for intentional tasks than for reflexive tasks.

Adult↗

PubMed2000

The role of the attention focus in the visual information processing underlying saccadic adaptation.

Three experiments were performed to determine how an error signal for driving saccadic adaptation is derived from visual information processing. The first experiment demonstrated that an intrasaccadic displacement of a visual background does not influence saccadic adaptation when a small foveal target is used. The second experiment showed that when a different type of target, a 4.8 deg annulus, is used an intrasaccadic background shift influences the adaptive process. The third experiment showed that the size of the saccade target determines the size of the attention focus around the time of a saccade. These findings suggest that the attention focus selects the visual information used for a trans-saccadic comparison in order to generate the error signal.

Adaptation, Physiological↗

PubMed2000

A novel extracellular esterase from Bacillus subtilis and its conversion to a monoacylglycerol hydrolase.

A novel gene lipB, which encodes an extracellular lipolytic enzyme, was identified in the Bacillus subtilis genomic DNA sequence. We have cloned and overexpressed lipB in B. subtilis and Escherichia coli and have also purified the enzyme from a B. subtilis culture supernatant to electrophoretic homogeneity. Four different lipase assays were used to determine its catalytic activity: pH-stat, spectrophotometry, fluorimetry and the monomolecular film technique. LipB preferentially hydrolysed triacylglycerol-esters and p-nitrophenyl-esters of fatty acids with short chain lengths of </= 10 carbon atoms. Triolein, which is a typical substrate for true lipases, was not hydrolysed at all. These results led us to classify LipB as an esterase rather than a lipase. The catalytic triad of LipB consists of residues Ser78, Asp134, and His157 as demonstrated by amino-acid sequence alignments and site-directed mutagenesis. The nucleophile Ser78 is located in a lipase-specific consensus sequence, which is Ala-X-Ser-X-Gly for most Bacillus lipases. All other bacterial lipases contain a glycine residue instead of the alanine at position-2 with respect to the catalytic serine. We have investigated the role of this alanine residue by constructing LipB variant A76G, thereby restoring the lipase-specific consensus motif. When compared with LipB this variant showed a markedly reduced thermostability but an increased stability at pH 5-7. Determination of the specific activities of wild-type LipB and variant A76G using a monomolecular film of the substrate monoolein revealed an interesting result: the A76G substitution had converted the esterase LipB into a monoacylglycerol hydrolase.

Amino Acid Sequence↗

PubMed2000

Relation between the metrics of the presaccadic attention shift and of the saccade before and after saccadic adaptation.

A shift of the visual attention focus is known to precede saccades. However, how the metrics of both this presaccadic attention shift and the saccade are coupled is still unclear. We altered the saccade size by short-term saccadic adaptation to determine whether the attention focus would still be shifted to the location of the saccade target or to the modified postsaccadic eye position. The results showed that saccadic adaptation had no influence on the presaccadic attention shift. Thus either different processes determine the metrics of the attention shift and of the saccade or saccadic adaptation causes only modifications on a lower hierarchical level of saccade programming, thereby not influencing the metrics of the attention shift.

Adaptation, Physiological↗

PubMed1999

Orbital position dependency is different for the gain of externally and internally triggered saccades.

The gain of visually triggered saccades depends on orbital position. Centrifugal saccades have smaller gains and are slower than centripetal saccades elicited by the same target amplitude. We determined whether internally triggered saccades, e.g. scanning or memory saccades, exhibit the orbital position dependency evident in visually guided saccades. The search coil technique was used to record eye movements of healthy subjects while they performed horizontal 12.5 degree saccades under three paradigms (gap, scanning and memory saccades). Orbital position influenced externally triggered gap saccades but not the gain or peak eye velocity of scanning or memory saccades. These findings do not support the idea that position dependency caused by orbital mechanics is compensated for at the level of the common brainstem burst generator. Instead our results are consistent with the view that cortical output reflects the differences evident in the gain of visually triggered centrifugal and centripetal saccades.

Adult↗

PubMed1999

Intrasaccadic target steps during the deceleration of primary saccades affect the latency of corrective saccades.

This study investigates how visually guided saccades and subsequent corrective saccades are affected by a secondary target step occurring at different times during the primary saccade. Eye movements of human subjects were measured by means of a differential infrared light reflection technique while the subjects performed visually guided saccades to a laser spot in darkness. The target was stepped backward or onward during the targeting saccade. While the intrasaccadic target step did not influence gain, peak velocity or skewness of the primary saccade, it had a significant effect on the subsequent corrective saccade when the secondary target step occurred during the deceleration phase of the primary saccade: the latency of the corrective saccade was significantly increased compared with the one performed under the single-step control condition. This increase also occurred when single target steps were presented randomly intermixed with backward and onward double target steps and even between selected sub-samples of saccades with identical postsaccadic visual error. If the target step occurred early during the primary saccade, the latency of the corrective saccade was not changed. This indicates that visual information sampled during the deceleration phase of a saccade can lead to a cancellation of the normal trigger mode of corrective saccades.

Deceleration↗

PubMed1999

Internally and externally guided voluntary saccades in unmedicated and medicated schizophrenic patients. Part I. Saccadic velocity.

Saccadic eye movements were elicited in 30 schizophrenic patients before and in 17 of these 30 during antipsychotic treatment with neuroleptics, and compared with those of 12 age-matched controls under three different conditions: (a) the gap paradigm, which tests the visually triggered and visually guided saccades; (b) the anti-task paradigm, which tests the internally guided, visually triggered saccades; and (c) the memory paradigm, which tests the internally triggered and guided saccades. Eye movements were recorded by DC electro-oculography, and the peak eye velocities for the different saccades were calculated. We found that antipsychotic treatment with neuroleptics reduces the peak saccadic eye velocity. This effect is larger for internally guided saccades than for externally triggered and guided eye movements. The saccadic velocity of the unmedicated schizophrenic patients did not differ from that of the controls. Since patients with diseases of the basal ganglia primarily show abnormalities of the internally guided and triggered saccades, our findings indicate that neuroleptics influence the oculomotor loop through the basal ganglia and that this loop, by means of neuroleptic influence on the brainstem saccadic burst generator, also influences the peak velocity of the internally guided saccades. This contradicts the current idea of the role of the cortical input to the brainstem saccadic burst generator, which is thought to not be involved in the determination of saccadic velocity.

Adult↗

PubMed1999

Internally and externally guided voluntary saccades in unmedicated and medicated schizophrenic patients. Part II. Saccadic latency, gain, and fixation suppression errors.

Saccades were elicited in 30 schizophrenic patients before medication, in 17 of them during medication with neuroleptics, and in 12 healthy controls using six paradigms that tested different types of saccades: (a) the externally triggered and visually guided saccades; (b) the externally triggered and internally guided saccades (anti-saccades); and (c) the internally triggered and internally guided saccades (memory-guided saccades). Latency of the primary saccade, gain (eye amplitude to target amplitude), and percentage of unwanted saccades (fixation suppression errors) were calculated. The externally triggered and externally guided saccades were only slightly affected in the patients, indicating that the function of parieto-tectal pathways was preserved. In contrast, the internally guided and externally triggered saccades showed abnormally long latencies, slightly smaller gains, and an increased rate of suppression errors regardless of the medication status. These findings were even more pronounced in the internally triggered and internally guided saccades such as memory-guided saccades. According to animal experiments and studies on patients with disorders of the basal ganglia, the performance of these saccades is based on the function of the pre- and dorsolateral frontal cortex and its connections to the basal ganglia. The minimal improvement of some of the parameters after clinical improvement and during treatment with neuroleptics suggests that the eye-movement deficits are associated with abnormalities of schizophrenia, which do not basically change under medication with neuroleptics. The observed effects of neuroleptics also argue against a primary abnormality in the dopaminergic input to the frontal cortex - basal ganglia oculomotor loop and support the view that there is a primary disturbance of the cortical input to the oculomotor loop through the basal ganglia in schizophrenics.

Adult↗

PubMed1999

Saccade amplitude disconjugacy induced by aniseikonia: role of monocular depth cues.

The conjugacy of saccades is rapidly modified if the images are made unequal for the two eyes. Disconjugacy persists even in the absence of disparity which indicates learning. Binocular visual disparity is a major cue to depth and is believed to drive the disconjugacy of saccades to aniseikonic images. The goal of the present study was to test whether monocular depth cues can also influence the disconjugacy of saccades. Three experiments were performed in which subjects were exposed for 15-20 min to a 10% image size inequality. Three different images were used: a grid that contained a single monocular depth cue strongly indicating a frontoparallel plane; a random-dot pattern that contained a less prominent monocular depth cue (absence of texture gradient) which also indicates the frontoparallel plane; and a complex image with several overlapping geometric forms that contained a variety of monocular depth cues. Saccades became disconjugate in all three experiments. The disconjugacy was larger and more persistent for the experiment using the random-dot pattern that had the least prominent monocular depth cues. The complex image which had a large variety of monocular depth cues produced the most variable and less persistent disconjugacy. We conclude that the monocular depth cues modulate the disconjugacy of saccades stimulated by the disparity of aniseikonic images.

Aniseikonia↗

PubMed1998

Isolation of a Drosophila homolog of the vertebrate homeobox gene Rx and its possible role in brain and eye development.

Vertebrate and invertebrate eye development require the activity of several evolutionarily conserved genes. Among these the Pax-6 genes play a major role in the genetic control of eye development. Mutations in Pax-6 genes affect eye development in humans, mice, and Drosophila, and misexpression of Pax-6 genes in Drosophila can induce ectopic eyes. Here we report the identification of a paired-like homeobox gene, DRx, which is also conserved from flies to vertebrates. Highly conserved domains in the Drosophila protein are the octapeptide, the identical homeodomain, the carboxyl-terminal OAR domain, and a newly identified Rx domain. DRx is expressed in the embryo in the procephalic region and in the clypeolabrum from stage 8 on and later in the brain and the central nervous system. Compared with eyeless, the DRx expression in the embryo starts earlier, similar to the pattern in vertebrates, where Rx expression precedes Pax-6 expression. Because the vertebrate Rx genes have a function during brain and eye development, it was proposed that DRx has a similar function. The DRx expression pattern argues for a conserved function at least during brain development, but we could not detect any expression in the embryonic eye primordia or in the larval eye imaginal discs. Therefore DRx could be considered as a homolog of vertebrate Rx genes. The Rx genes might be involved in brain patterning processes and specify eye fields in different phyla.

Amino Acid Sequence↗

PubMed1998

Fixation errors and timing in sequences of memory-guided saccades.

We analyzed the relation between position and amplitude errors during the performance of sequences of saccades to previously memorized target positions in complete darkness. Although a complete compensation (on the average) for fixation errors was observed, groups of successive saccades could be identified which showed propagation of position errors. These groups are characterized by a long fixation time prior to the first saccade and short fixations prior to the remaining saccades. These findings indicate that sequences of eye movements can be performed in two different modes: (1) extraretinal information about the actual eye position is used to correct fixation errors; (2) pre-programmed groups of saccades with limited length can be performed with fixed amplitudes neglecting the actual eye position. These groups tended to occur predominantly at the end of a sequence.

Adult↗

PubMed1997

Unilateral cerebellar lesions affect initiation of ipsilateral smooth pursuit eye movements in humans.

To clarify the role of the cerebellum in pursuit initiation (first 100 msec), we used infrared oculography to examine the effect of unilateral cerebellar lesions on the initial (0-20 msec) and later (80-100 msec) periods as well as the steady-state response (200-300 msec) of horizontal smooth pursuit in 10 patients with unilateral cerebellar lesions. These results were compared with those of 17 age-matched healthy subjects. Smooth pursuit was elicited with a step-ramp target movement with randomized horizontal directions and velocities of 10 degrees/sec and 30 degrees/sec. In the first 20-msec pursuit, velocity was 22% lower toward the side of the cerebellar lesion than away from it and 16% lower in the period 80 to 100 msec (normal differences, 2% and 3%). Later (200-300 msec), the ipsiversive/contraversive difference was smaller, but pursuit velocity in both directions was significantly lower in patients than in normals. No lesion affected the floccular region and/or the nodulus/uvula. Five lesions extended so far medially that they could have affected the medial deep cerebellar nucleus (fastigial nucleus). The remaining five were in the lateral hemisphere in areas previously considered uninvolved in pursuit generation. Our findings prove that the cerebellum participates in human pursuit initiation and that lesions in the lateral cerebellum possibly affect smooth pursuit.

Adolescent↗

PubMed1997

Saccades in Gilles de la Tourette's syndrome.

Gilles de la Tourette's syndrome (GTS) is presumed to be an inherited disorder with an unclear pathophysiology. An involvement of the basal ganglia is suspected. Besides vocal tics, one of the main symptoms is the presence of motor tics. As eye movements are a specialized part of the motor system, we investigated whether they differed in some typical way in GTS patients. To study the control of saccades in GTS, different paradigms were used to elicit saccades, which were either externally triggered and visually guided or internally triggered and without visual target. GTS patients (n = 10) showed a significant increase of the latency of antisaccades, a highly impaired performance of sequences of memory-guided saccades, and an isolated reduction of the peak velocity in the antisaccades. Overall the results were similar to those found with similar paradigms in patients with Huntington's disease (HD). In analogy to the known pathology of HD, these findings can be attributed to ascending loops from the basal ganglia that inappropriately activate the frontal cortex, especially some of the eye movement-related areas there. This impairment seems to be a rather specific effect, since some oculomotor features that are preserved by structures of the frontal cortex, such as the effect of a fixation target on saccadic latency ("gap effect"), were normal.

Adolescent↗

PubMed1997

Impairment of the binocular coordination of saccades in strabismus.

To examine the link between binocular vision and binocular coordination of saccades we studied subjects with convergent strabismus since childhood with mild or no amblyopia: three subjects had small squint (< 10 prism D) and preserved peripheral binocular visual function with gross stereopsis; four subjects had larger squint (18-35 prism D) and no detectable stereopsis. A standard paradigm was used to elicit horizontal saccades; binocular recordings were made with the IRIS device. For subjects with small strabismus, saccades were disconjugate (unequal between the two eyes) typically by 1 deg. Subjects with larger strabismus exhibited even larger and more variable disconjugacy (typically 1.8 deg). Post-saccadic eye drift was consistently divergent in subjects with small strabismus and tended to reduce the convergent squint angle. In contrast, in subjects with large strabismus drift was convergent. The impairment of the binocular control of saccades is attributed to the deficiency of disconjugate oculomotor adaptive capabilities necessary to compensate for the natural asymmetries or changes in the two oculomotor plants; such deficiency would be more severe in subjects with large strabismus who have neither central nor peripheral binocular vision.

Adult↗