Biomedical subjects

Ansgar Koene

Publications and source records attributed to Ansgar Koene.

5 recordsLinked to original sources

PubMed2006

Saccadic lateropulsion in Wallenberg syndrome: a window to access cerebellar control of saccades?

Saccadic lateropulsion is characterized by an undershoot of contralaterally directed saccades, an overshoot of ipsilaterally directed saccades and an ipsilateral deviation of vertical saccades. In Wallenberg syndrome, it is thought to result from altered signals in the olivo-cerebellar pathway to the oculomotor cerebellar network. In the current study we aimed to determine whether saccadic lateropulsion results from a cerebellar impairment of motor related signals or visuo-spatial related signals. We studied the trajectory, the accuracy, the direction and the amplitude of a variety of vertical and oblique saccades produced by five patients and nine control subjects. Some results are consistent with previous data suggesting altered motor related signals. Indeed, the horizontal error of contralesional saccades in patients increased with the desired horizontal saccade size. Furthermore, the initial directional error measured during the saccadic acceleration phase was smaller than the global directional error, suggesting that the eye trajectory curved progressively. However, some other results suggest that the processes that specify the horizontal spatial goal of the saccades might be impaired in the patients. Indeed, the horizontal error of ipsilesional saccades in patients did not change significantly with the desired horizontal saccade size. In addition, when comparing saccades with similar intended direction, it was found that the directional error was inversely related to the vertical saccade amplitude. Thus we conclude that the cerebellum might be involved both in controlling the motor execution of saccades and in determining the visuo-spatial information about their goal.

Aged↗

PubMed2006

Visual search for a target changing in synchrony with an auditory signal.

We examined whether the detection of audio-visual temporal synchrony is determined by a pre-attentive parallel process, or by an attentive serial process using a visual search paradigm. We found that detection of a visual target that changed in synchrony with an auditory stimulus was gradually impaired as the number of unsynchronized visual distractors increased (experiment 1), whereas synchrony discrimination of an attended target in a pre-cued location was unaffected by the presence of distractors (experiment 2). The effect of distractors cannot be ascribed to reduced target visibility nor can the increase in false alarm rates be predicted by a noisy parallel processing model. Reaction times for target detection increased linearly with number of distractors, with the slope being about twice as steep for target-absent trials as for target-present trials (experiment 3). Similar results were obtained regardless of whether the audio-visual stimulus consisted of visual flashes synchronized with amplitude-modulated pips, or of visual rotations synchronized with frequency-modulated up-down sweeps. All of the results indicate that audio-visual perceptual synchrony is judged by a serial process and are consistent with the suggestion that audio-visual temporal synchrony is detected by a 'mid-level' feature matching process.

Acoustic Stimulation↗

PubMed2004

Transfer of adaptation from visually guided saccades to averaging saccades elicited by double visual targets.

The adaptive mechanisms that control the amplitude of visually guided saccades (VGS) are only partially elucidated. In this study, we investigated, in six human subjects, the transfer of VGS adaptation to averaging saccades elicited by the simultaneous presentation of two visual targets. The generation of averaging saccades requires the transformation of two representations encoding the desired eye displacement toward each of the two targets into a single representation encoding the averaging saccade (averaging programming site). We aimed to evaluate whether VGS adaptation acts upstream (hypothesis 1) or at/below (hypothesis 2) the level of averaging saccades programming. Using the double-step target paradigm, we simultaneously induced a backward adaptation of 17.5 degrees horizontal VGS and a forward adaptation of 17.5 degrees oblique VGS performed along the +/- 40 degrees directions relative to the azimuth. We measured the effects of this dual adaptation protocol on averaging saccades triggered by two simultaneous targets located at 17.5 degrees along the +/- 40 degrees directions. To increase the yield of averaging saccades, we instructed the subjects to move their eyes as fast as possible to an intermediate position between the two targets. We found that the amplitude of averaging saccades was smaller after VGS adaptation than before and differed significantly from that predicted by hypothesis 1, but not by hypothesis 2, with an adaptation transfer of 50%. These findings indicate that VGS adaptation largely occurs at/below the averaging saccade programming site. Based on current knowledge of the neural substrate of averaging saccades, we suggest that VGS adaptation mainly acts at the level of the superior colliculus or downstream.

Adaptation, Physiological↗

PubMed2003

Attention-biased multi-stable surface perception in three-dimensional structure-from-motion.

Retinal velocity distributions can lead to a percept of three-dimensional (3D) structure (structure-from-motion [SFM]). SFM stimuli are intrinsically ambiguous with regard to depth ordering. A classic example is the orthographic projection of a revolving transparent cylinder, which can be perceived as a 3D cylinder that rotates clockwise and counterclockwise alternately. Prevailing models attribute such bistable percepts to inhibitory connections between neurons that are tuned to opposite motion directions at equal binocular disparities. Cylinder stimuli can yield not only two but as many as four different percepts. Besides the well-documented clockwise and counterclockwise spinning transparent cylinders, observers can also perceive two transparent half-cylinders, either convex or concave, one in front of the other. Observers are able to bias the time during which a percept is present by attending to one or the other percept. We examined this phenomenon quantitatively and found that in standard SFM stimuli, the percept of two convex transparent half-cylinders can occur just as often as the percept of (counter-) clockwise spinning cylinders. So far, however, all interpretations of experimental (neurophysiological) data and all proposed mechanisms for SFM perception have focused solely on the two classical cylinder percepts. Prevailing models cannot explain the existence of the other two percepts. We suggest an alternative model to explain attention-biased multi-stable perception.

Attention↗

PubMed2003

Mechanical interdependence of version and vergence eye movements.

In this paper, the authors investigate whether the idea of independent control of version and vergence eye movements is compatible with the mechanics of the eye plant. By computing the change in the axes of action of the eye muscles as a function of ocular vergence, they prove that, regardless of the muscle pulley locations, the required muscle activity for vertical version depends on the initial vergence angle. The binocular extension of Listing's law ('L2') describes how the torsional orientation of the eye depends on both gaze direction and ocular convergence. The authors show that for each vergence angle there is a range of possible muscle pulley locations that would cause independent control of version and vergence to result in L2. They also show that this mechanical explanation of L2 requires that the muscle pulleys move as a function of vergence.

Convergence, Ocular↗