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Biomedical subjects

T Eggert

Publications and source records attributed to T Eggert.

43 records · Page 3Linked to original sources
PubMed1997

Deficiency of adaptive control of the binocular coordination of saccades in strabismus.

Disconjugate (different in the two eyes) oculomotor adaptation is driven by the need to maintain binocular vision. Since binocular vision is deficient in strabismus, we wondered whether oculomotor disconjugate adaptive capabilities are deficient in such subjects. We studied eight adult subjects with constant, long-standing convergent strabismus of variable angles (4-30 prism D). No subject had severe amblyopia. Binocular vision was evaluated with stereoacuity tests. Two subjects had peripheral binocular vision and gross stereopsis; two other subjects had abnormal retinal correspondence and abnormal or pseudo gross stereopsis. In the other subjects binocular vision and stereopsis were absent. To stimulate disconjugate changes of saccades, subjects viewed for 20 min an image that was magnified in one eye (aniseikonia). Subjects with residual peripheral binocular vision and even subjects with pseudo or abnormal binocular vision showed disconjugate changes of the binocular coordination of their saccades; these changes reduced the disparity resulting from the aniseikonia. In contrast, for subjects without binocular vision the changes were not correlated with the disparity induced by the aniseikonia. Rather, these changes served to improve fixation of one or the other eye individually.

Adaptation, Physiological↗
PubMed1997

Visually induced motion perception and visual control of postural sway in congenital nystagmus.

In congenital nystagmus (CN) the threshold for detecting motion of visual objects is increased. To determine whether this increase is due to a deterioration of visual motion signals or whether visual-vestibular interactions (which are necessary to judge object-motion in space) are also involved we examined how CN patients use visual motion signals to evaluate self-motion in perceptual and behavioral tasks. Using an optokinetic drum we measured the minimum optokinetic acceleration necessary to induced motion perception of the visual environment in CN patients. This threshold was significantly elevated in the CN patients compared with normals (20.1 deg/s2 to 3.25 deg/s2). We further addressed the question whether the elevation of this threshold is due to a deficiency in evaluating visual motion in general or to a specific modification affecting the percept of visual object-motion with respect to the inertial reference only. We thus measured the latency of visually induced self-motion perception, which was found to be very similar or even slightly smaller (1.7 +/- 0.7 s) compared with normals (2.2 +/- 1.7 s). Moreover, subjects with CN were found to use vision quite efficiently for the visual stabilization of posture (Romberg quotient 2.0 +/- 1.16), even if they did not reach the level of normals (Romberg quotient 3.7 +/- 1.1). The results indicate that CN affects the estimate of object-motion in a specific and much more severe way than the estimate of self-motion. The minimal effect of CN on self-motion perception can be explained by the low pass characteristics of the optokinetic input to self-motion perception. The specific deficiency in detecting object-motion indicates that adaptation to CN occurs on the level of visual--vestibular interactions for the perception of visual object-motion and not on the level of visual motion signals.

Adolescent↗
PubMed1996

Fast disconjugate adaptations of saccades in microstrabismic subjects.

In normal subjects, saccade amplitude inequality can be induced almost immediately when the image is made larger for one eye. This disconjugacy allows binocular fusion at the point of regard despite the image size inequality. It persists under subsequent monocular viewing which suggests a fast adaptive mechanism. This study tests whether such disconjugacy can be induced in subjects with microstrabismus who do not have foveal fusion. Three microstrabismic subjects viewed a random dot pattern the size of which was 10% larger in one eye. Within 40 sec horizontal saccades became larger in the eye viewing the larger image by 4-10%; the induced disconjugacy persisted under subsequent monocular viewing. Thus, fast disconjugate adaptation is possible in microstrabismus demonstrating that foveal fusion is not necessary to achieve it.

Adaptation, Ocular↗
PubMed1996

Disconjugate adaptation of the vertical oculomotor system.

Conjugate post-saccadic eye drift can be induced in normal humans if a visual pattern is made to drift after every saccade. This study examines the ability of normal humans to create disconjugate vertical post-saccadic drift. Identical fuseable patterns were presented dichoptically, one to each eye. At the end of each vertical saccade one pattern drifted up and the other down, by 5% of the saccade amplitude. Five subjects were trained for 2-3 hr. Eye movements were recorded with eye coils. Normal vertical saccades along the midline were remarkably conjugate and post-saccadic drift was minimal. Training produced only small disconjugate post-saccadic drift (0.14 deg) but substantial saccade amplitude disconjugacy (0.70 deg). For several subjects, the induced disconjugacies persisted even for saccades in the dark indicating that adaptive changes occurred in the binocular coordination of vertical saccades. Apparently vertical disparate post-saccadic retinal slip is not sufficient to stimulate significantly the saccade pulse-step matching mechanism which is believed to control post-saccadic eye drift. The changes we observed aimed to reduce position disparity and not retinal slip in each eye.

Adaptation, Ocular↗
PubMed1995

Position dependency of rapidly induced saccade disconjugacy.

We tested the ability of normal subjects to alter the conjugacy of their saccades in a position-specific manner. Five subjects dichoptically viewed a stereogram produced by two random-dot patterns. They immediately perceived a three-dimensional wedge with its apex closer to them. They were asked to saccade for 15 min back and forth between the apex and two lateral dots of the wedge. For fixation sequences between centre-right-centre, saccades immediately became larger in the right eye. For sequences between centre-left-centre, saccades immediately became larger in the left eye. For two subjects this non-monotonic position-specific disconjugacy compensated for the disparity of the stereogram almost perfectly. The disconjugacy persisted even under monocular viewing of one of the random-dot patterns. It diminished or disappeared immediately, however, when the random-dot pattern was shifted on the screen. We suggest the existence of a fast learning mechanism capable of producing position-specific disconjugacy by associating saccades with disparity. Such a mechanism would use a visual reference rather than the position of the eyes in the orbit.

Adult↗
PubMed1995

Immediate saccade amplitude disconjugacy induced by unequal images.

We tested the ability of normal subjects to make changes in the conjugacy of their saccades. Subjects dichoptically viewed a grid the size of which was 10% larger in one eye. The grids were centred onto a flat screen at 57 cm or 1 m from the subject. Horizontal saccades immediately became larger in the eye viewing the larger grid. For some subjects this disconjugacy persisted even under subsequent monocular viewing. Such persistent changes occurred mainly in the field where the required disconjugacy was divergent for centrifugal saccades, convergent for centripetal saccades. Vertical saccades also developed compensatory disconjugacy; its amplitude was smaller but less variable. To explain these results we propose a fast associative learning mechanism that pairs peripheral disparity with saccades and is capable of producing saccade disconjugacy even in the absence of disparity. For horizontal saccades a secondary conditioning of monocular depth cues by the disparity would also be involved.

Adult↗