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Biomedical subjects

K Hepp

Publications and source records attributed to K Hepp.

12 recordsLinked to original sources
PubMed1992

Calibration of three-dimensional eye position using search coil signals in the rhesus monkey.

A procedure is described to calibrate three-dimensional eye position with a dual-search coil implant in rhesus monkeys using a two-field magnetic system. The method allows one to determine the sensitivity of the search coils taking into account the presence of d.c. offset voltages. The orientation of the implant on the eye relative to a space-fixed reference frame is computed using fixations of targets arranged vertically. The critical steps of the procedure are discussed and documented by experimental data.

Algorithms↗
PubMed1991

Two- rather than three-dimensional representation of saccades in monkey superior colliculus.

Saccades are controlled by neurons in the brainstem reticular formation that receive input from the superior colliculus and cortex. Recently two quantitative models have been proposed for the role of the colliculus in the generation of three-dimensional eye movements. In order to test these models, three-dimensional eye movements were measured in the alert monkey to investigate whether the saccadic motor map of the superior colliculus is two-dimensional, representing retinal target vectors, or three-dimensional, representing three-dimensional motor error for the rotation of the eye. Electrical stimulation of the superior colliculus produced two-dimensional, not three-dimensional, eye movements. It is therefore concluded that the collicular motor map is two-dimensional.

Animals↗
PubMed1991

Listing's law for eye, head and arm movements and their synergistic control.

We have recorded eye, head, and upper arm rotations in five healthy human subjects using the three-dimensional search coil technique. Our measurements show that the coordination of eye and head movements during gaze shifts within +/- 25 deg relative to the forward direction is organized by restricting the rotatory trajectories of the two systems to almost parallel planes. These so-called "Listing planes" for eye-in-space and head-in-space rotations are workspace-oriented, not body-fixed. Eye and head trajectories in their respective planes are closely related in direction and amplitude. For pointing or grasping, the rotatory trajectories of the arm are also restricted to a workspace-oriented Listing plane. During visually guided movements, arm follows gaze, and the nine-dimensional rotatory configuration space for eye-head-arm-synergies (three degrees of freedom for each system) is reduced to a two-dimensional plane in the space of quaternion vectors.

Arm↗
PubMed1991

Smooth pursuit eye movements obey Listing's law in the monkey.

We have tested Listing's law in the monkey using the dual search coil technique to record 3-dimensional eye positions during smooth pursuit and during spontaneous eye movements in the light. Our results indicate that during smooth pursuit the eye positions obey Listing's law with a least the same accuracy as during spontaneous eye movements or during periods of fixations.

Animals↗
PubMed1991

Generation of vertical and torsional rapid eye movement in the rostral mesencephalon. Experimental data and clinical implications.

The riMLF is a nucleus in the rostral mesencephalon whose bilateral destruction leads to a palsy of vertical and torsional rapid eye movements. A unilateral lesion leads to a loss of torsional rapid eye movements in only one direction, but vertical rapid movements can still be generated with some reduction in their velocity. Single neuron studies in monkeys and anatomy support the concept that the riMLF together with the PPRF are the critical areas in the brainstem to generate rapid eye movements in 3 dimensions.

Animals↗
PubMed1989

On the generation of vertical and torsional rapid eye movements in the monkey.

The role of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in generating the vertical and torsional components of rapid eye movements was examined. The on-directions of burst neurons in the riMLF of alert Rhesus monkeys were obtained during quick phase nystagmus in three dimensions. The distinguishing feature of these burst neurons was the torsional component of their on-directions; neurons on the right side exhibited a clockwise component, from the point of view of the subject, while those on the left had a counterclockwise component. Vertical components could have up or down directions. This organization was verified by means of unilateral reversible inactivation of the riMLF using Muscimol. An injection in the right riMLF impaired the generation of quick phases with clockwise components while one on the left impaired counterclockwise components.

Action Potentials↗
PubMed1989

Rapid eye movement generation in the primate. Physiology, pathophysiology, and clinical implications.

The trajectories of rapid eye movements are usually described in a Cartesian coordinate frame with a horizontal, vertical and torsional component. The sensory to motor coordinate transformations for horizontal components of rapid eye movements can be localized to neurons of the paramedian pontine reticular formation (PPRF), where long-lead and short-lead burst neurons are found. The equivalent area for recoding of vertical and torsional movement components is situated in the rostral interstitial nucleus of the MLF (rostral iMLF). Pause cells in caudal midline structures of the PPRF help to coordinate the various movement components. Experimental inactivation of these different neuron population lead to palsies of rapid eye movement generation. A unilateral PPRF lesion leads to a loss of all horizontal rapid eye movements towards the ipsilateral side. A bilateral PPRF lesion involving caudal midline structures leads to a bilateral horizontal gaze palsy in addition to a severe disruption of vertical and torsional eye movements. A bilateral rostral iMLF lesion leads to a loss of all rapid eye movements with a vertical or torsional movement component. A unilateral iMLF lesion leads to a loss of all rapid eye movements with an ipsilateral torsional component.

Animals↗