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Biomedical subjects

K Hepp

Publications and source records attributed to K Hepp.

At least 19 recordsLinked to original sources
PubMed1999

Three-dimensional extraocular motoneuron innervation in the rhesus monkey. I: Muscle rotation axes and on-directions during fixation.

The rotation axis for each of the six extraocular muscles was determined in four eyes from three perfused rhesus monkeys. Measurements of the locations of muscle insertions and origins were made in the stereotaxic reference frame with the x-y plane horizontal and the x-z plane sagittal. The computed rotation axes of the horizontal recti were close to being in the x-z plane at an angle of about 15 degrees to the z axis. The rotation axes of the vertical recti and the obliques were close to being in the x-y plane at an angle of about 30 degrees to the y axis. In five alert rhesus monkeys, we simultaneously recorded extraocular motoneuron activity and eye position in three dimensions (3D). The activity of 51 motoneuron axons was obtained from the oculomotor (n=34), trochlear (n=11), and abducens nerve (n=6) during spontaneous eye movements. To extend the torsional range of eye position, the animals were also put in different static roll positions, which induced ocular counterroll without dynamic vestibular stimulation. Periods of 100 ms during fixation or slow eye movements (<10 degrees/s) were chosen for analysis. For each motoneuron, a multiple linear regression was performed between firing frequency and 3D eye position, expressed as a rotation vector, in both stereotaxic and Listing's reference frame. The direction with the highest correlation coefficient (average R=0.94+/-0.07 SD) was taken as the on-direction. Each unit's activity could be unequivocally attributed to one particular muscle. On-directions for each motoneuron were confined to a well-defined cone in 3D. Average on-directions of motoneurons differed significantly from the corresponding anatomically determined muscle rotation axes expressed in the stereotaxic reference frame (range of deviations: 11.9 degrees to 29.0 degrees). This difference was most pronounced for the vertical recti and oblique muscles. The muscle rotation axes of the vertical rectus pair and the oblique muscle pair form an angle of 58.3 degrees, whereas the corresponding angle for paired motoneuron on-directions was 105.6 degrees. On-directions of motoneurons were better aligned with the on-directions of semicircular canal afferents (range of deviation: 9.4-18.9 degrees) or with the anatomically determined sensitivity vectors of the semicircular canals (range of deviation: 3.9-15.9 degrees) than with the anatomically determined muscle rotation axes, but significant differences remain to be explained. The on-directions of motoneurons were arranged symmetrically to Listing's plane, in the sense that the torsional components for antagonistically paired muscles were almost equal, but of opposite sign. Thus, the torsional components of motoneuron on-directions cancel when eye movements are confined to Listing's plane. This arrangement simplifies the neuronal transformations for conjugate head-fixed voluntary eye movements, while the approximate alignment with the semicircular canal reference frame is optimal for generating compensatory eye movements.

Animals↗
PubMed1999

Feedback interactions between neuronal pointers and maps for attentional processing.

Neural networks combining local excitatory feedback with recurrent inhibition are valuable models of neocortical processing. However, incorporating the attentional modulation observed in cortical neurons is problematic. We propose a simple architecture for attentional processing. Our network consists of two reciprocally connected populations of excitatory neurons; a large population (the map) processes a feedforward sensory input, and a small population (the pointer) modulates location and intensity of this processing in an attentional manner dependent on a control input to the pointer. This pointer-map network has rich dynamics despite its simple architecture and explains general computational features related to attention/intention observed in neocortex, making it interesting both theoretically and experimentally.

Attention↗
PubMed1998

The collicular code of saccade direction depends on the roll orientation of the head relative to gravity.

This paper investigates the influence of static head tilt on the relation between activity in the motor layers of the superior colliculus (mSC) and saccadic oculomotor output. Based on single-unit recordings and electrical microstimulation in awake rhesus monkeys, we report that head roll changes the direction of the saccade vector generated by the mSC, with respect to a head-fixed coordinate system. Typically, the vector rotates in a direction that is opposite to the head roll direction. This rotation exceeds the amount of ocular counterroll that is observed as a result of difference in static head roll positions. We find that the mSC works in an oculocentric coordinate system that is biased in the direction of gravity. It is argued that the modification is a result of processes that occur downstream of or parallel to the mSC.

Animals↗
PubMed1996

Role of monkey nucleus reticularis tegmenti pontis in the stabilization of Listing's plane.

An important problem in motor control is how the nervous system deals with redundant degrees of freedom. It has been well documented that voluntary eye movements are constrained to a plane by Listing's law. Recent evidence has indicated that Listing's law is implemented downstream from the motor superior colliculus (SC), but controversy exists whether this synergy results from a neural control mechanism or from passive mechanical properties of the oculomotor plant. To address this problem, we have investigated the role of the caudal nucleus reticularis tegmenti pontis (cNRTP), which is functionally positioned inbetween the SC and cerebellar vermis, in the three-dimensional (3-D) control of saccades. In three rhesus monkeys, 3-D eye movements were measured while recording from single units in the cNRTP. In contrast to the SC, movement fields of cNRTP cells were best described by 3-D eye displacement vectors. We also performed electrical microstimulation with the eyes starting from a large range of initial eye positions. Evoked movements were always ipsilaterally directed but were often endowed with a fixed torsional component in either the positive or the negative direction. In two monkeys, small amounts of muscimol were unilaterally injected into the cNRTP. The results of these experiments strongly suggest that the cNRTP contributes to the stabilization of Listing's plane against torsional errors of the saccadic system. It is concluded, therefore, that the saccadic burst generator is 3-D, and that Listing's law is at least partially implemented by a neural control strategy.

Animals↗
PubMed1995

Deficits in torsional and vertical rapid eye movements and shift of Listing's plane after uni- and bilateral lesions of the rostral interstitial nucleus of the medial longitudinal fasciculus.

The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) contains burst neurons whose activity precedes rapid eye movements with a vertical and/or torsional component. To ascertain their causal role in the generation of conjugate eye movements, we placed uni- and bilateral kainic acid lesions in that region. Unilateral inactivation of the riMLF leads to a loss of all rapid eye movements with an ipsitorsional component (ipsitorsional is defined as movement of the upper pole of the ipsilateral eye in a temporal direction). Vertical eye movements are impaired in an asymmetric way, with downward movements slowed and upward movements little affected. Listing's plane is shifted in the contratorsional direction, i.e., we find a constant torsional offset for all eye positions. With bilateral lesions one observes a total loss of all vertical and torsional eye movements, while Listing's plane retains its shape and position. These results show that burst neurons in the riMLF play a decisive role in generating rapid eye movements with a vertical and torsional component.

Animals↗
PubMed1995

Theoretical explanations of Listing's law and their implication for binocular vision.

We shall discuss three theoretical explanations of Listing's law for conjugate eye movements with the head fixed: the original argument by Helmholtz, which is "sensorimotor" in its attempt to optimize vision by using internal feedback from the oculomotor system, and two comparatively simple recent explanations based on either visual or oculomotor performance. These geometrical demonstrations shed some light on recent generalizations of Listing's law to vergent eye movements.

Convergence, Ocular↗
PubMed1995

Influence of eye position on activity in monkey superior colliculus.

1. Most recording studies on the role of the monkey superior colliculus (SC) in eye movement generation have so far indicated that the code of the recruited population of cells is a fixed vector command representing the desired saccadic eye displacement vector, irrespective of the position of the eyes in the orbit. Experimental evidence from microstimulation, lesions, and neuroanatomy, however, suggests that the SC may have access to an eye position signal. 2. In this paper we have tested the hypothesis that SC activity is influenced by eye position, by recording from presaccadic burst neurons while monkeys made rapid eye movements in the light covering a large part of the oculomotor range. 3. In four alert rhesus monkeys, we obtained sufficient data from 57 SC single units. The activity of a substantial part of these cells (30/57) appeared to be significantly influenced by eye position. Although the tuning properties of these cells for saccade amplitude and direction remained invariant for changes in eye position, the peak firing rate of these units was systematically influenced by the position of the eyes in the head. 4. We have characterized this eye position dependence of a neuron's activity by a qualitative, model-independent, as well as by a quantitative model description (gain field), which takes into account both the tuning properties of the cell for eye displacement vectors and the dependence of eye position. 5. Although a majority of gain fields had their eye position sensitivity vector roughly aligned with the optimal saccade vector direction (colinear gain field, 17/30), a substantial part of the gain fields had their eye position sensitivity vectors in quite different directions, approximately homogeneously distributed with respect to the cell's ON direction. 6. We conclude that the SC has access to a signal related to the position of the eyes in the orbit. Several hypotheses on the possible functional role of this signal, in relation to the neural code of the motor map, are discussed.

Animals↗
PubMed1994

Oculomotor control: Listing's law and all that.

Previously, eye positions were characterized by the direction of sight, which has two degrees of freedom for conjugate movements and three degrees of freedom for vergent movements. Several groups have recently presented reliable data and unconventional models on binocular coordination, where all three degrees of freedom for one or both eyes have been taken into account. The results illustrate Bernstein's principle that in order to simplify control, the brain establishes unique relations between the target and motor space, in which non-Euclidean geometry of rotations is observed.

Animals↗
PubMed1993

Monkey superior colliculus represents rapid eye movements in a two-dimensional motor map.

1. Although the eye has three rotational degrees of freedom, eye positions, during fixations, saccades, and smooth pursuit, with the head stationary and upright, are constrained to a plane by Listing's law. We investigated whether Listing's law for rapid eye movements is implemented at the level of the deeper layers of the superior colliculus (SC). 2. In three alert rhesus monkeys we tested whether the saccadic motor map of the SC is two dimensional, representing oculocentric target vectors (the vector or V-model), or three dimensional, representing the coordinates of the rotation of the eye from initial to final position (the quaternion or Q-model). 3. Monkeys made spontaneous saccadic eye movements both in the light and in the dark. They were also rotated about various axes to evoke quick phases of vestibular nystagmus, which have three degrees of freedom. Eye positions were measured in three dimensions with the magnetic search coil technique. 4. While the monkey made spontaneous eye movements, we electrically stimulated the deeper layers of the SC and elicited saccades from a wide range of initial positions. According to the Q-model, the torsional component of eye position after stimulation should be uniquely related to saccade onset position. However, stimulation at 110 sites induced no eye torsion, in line with the prediction of the V-model. 5. Activity of saccade-related burst neurons in the deeper layers of the SC was analyzed during rapid eye movements in three dimensions. No systematic eye-position dependence of the movement fields, as predicted by the Q-model, could be detected for these cells. Instead, the data fitted closely the predictions made by the V-model. 6. In two monkeys, both SC were reversibly inactivated by symmetrical bilateral injections of muscimol. The frequency of spontaneous saccades in the light decreased dramatically. Although the remaining spontaneous saccades were slow, Listing's law was still obeyed, both during fixations and saccadic gaze shifts. In the dark, vestibularly elicited fast phases of nystagmus could still be generated in three dimensions. Although the fastest quick phases of horizontal and vertical nystagmus were slower by about a factor of 1.5, those of torsional quick phases were unaffected. 7. On the basis of the electrical stimulation data and the properties revealed by the movement field analysis, we conclude that the collicular motor map is two dimensional. The reversible inactivation results suggest that the SC is not the site where three-dimensional fast phases of vestibular nystagmus are generated.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
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↗