Retinal prosthesis: an encouraging first decade with major challenges ahead.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
R Eckmiller
Publications and source records attributed to R Eckmiller.
Learning retina implants with epiretinal contacts.
Retina implants are currently being developed by several interdisciplinary research consortia worldwide for blind humans with various retinal degenerative diseases. It is the aim of our retina implant project to develop a novel type of visual prosthesis to regain a moderate amount of vision such as perception of location and shape of large objects in the first stage and to approach reading quality in a subsequent stage. In our planned retina implant, a retina encoder (RE) outside the eye has to replace the information processing of the retina. A retina stimulator (RS), implanted adjacently to the retinal ganglion cell layer, has to contact a sufficient number of retinal ganglion cells/fibers for electrical elicitation of spikes. A wireless signal and energy transmission system has to provide the communication between the RE and RS. This paper outlines the retina implant project of our consortium of 14 expert groups and describes first results of the learning RE. The RE approximates the typical receptive field (RF) properties of primate retinal ganglion cells by means of individually tunable spatiotemporal RF filters. The RE as a cluster of RF filters maps visual patterns onto spike trains for a number of contacted ganglion cells. A concept is presented to train the individual RF filters in an unsupervised learning process, which employs neural networks in a dialog with the individual human subject. The desired aim of this dialog is an optimization of the visual perception by matching the various RF filter properties with those 'expected' by the central visual system for each contacted ganglion cell.
Animals↗
Recognition and tracking of impulse patterns with delay adaptation in biology-inspired pulse processing neural net (BPN) hardware.
The application of an electronic real time emulator for biology-inspired pulse processing neural networks (BPN) to recognition and temporal tracking of discrete impulse patterns via delay adaptation is demonstrated. The electronic emulation includes biologically plausible features, such as asynchronous impulses, membrane potentials and adaptive weights, as well as a mechanism to modify signal delays. The rule for the adaptation of impulse propagation delays is as follows: 'error neurons' detect temporal differences between single impulses of other neurons and adjust corresponding signal delay parameters. In the application presented BPN adapts its time delays in order to form a finely tuned match with a given sequence of three discrete impulses. After learning, BPN is capable not only of highly selective recognition of the learned impulse pattern but also of tracking a gradually changing impulse pattern. Tracking is achieved by continuously re-adjusting the delay profile. Delay adaptation (rather than weight adaptation) appears to be the more effective mechanism for such application.
Computers↗
Control of arm movements in a 2-dimensional pointing task.
The present study analyses in humans the control principles of sequential, unpracticed pointing movements in a 2-dimensional space. Our data reveal that variable pointing errors add up within such sequences. This finding supports the hypothesis that movement amplitude rather than position is the controlled variable of the investigated movements.
Attention↗
Video-oculographic measurement of 3-dimensional eye rotations.
An effective and relatively calibration-free method for the measurement of 3-dimensional eye rotations is described. The experimental method consists of recording with a video camera the positions of distinct globe markers provided by a tight-fitting, soft contact lens before and after an eye rotation. The mathematical analysis represents the eye rotation by a rotation vector (rotation axis and angle) and three, spherical Euler angles. The method is linear over the entire range of natural eye rotation, the resolution limited only by the video set-up used. Due to its easy applicability, the system is well suited for various eye movement measurements in clinical or research environments.
Eye Movements↗
A general numerical method evaluating three-dimensional eye rotations by scanning laser ophthalmoscopy.
A general computational method is described to specify completely the rotational state of the eye in three dimensions by scanning laser ophthalmoscopy (SLO). The method uses the simplex algorithm to fit the eye's rotational parameters to data given by n individually selected ocular fundus landmarks before and after the eye rotation. The rotational parameters are expressed as the rotation vector and three spherical Euler angles. The method, which was implemented in the C programming language, can be applied for various eye movement measurements in clinical and laboratory environments, including SLO.
Algorithms↗
Interaction of visual and non-visual signals in the initiation of smooth pursuit eye movements in primates.
The initiation of smooth pursuit eye movements (PEM) by visual and non-visual signals was analysed in humans and monkeys. While PEM latency ranged around 150 ms when a purely visual target was provided, it often dropped to about 0 ms, or even became negative, when target movement was coupled to the subject's arm; this suggests that signals about the intention to move the arm can be evaluated for PEM control. Eye movements always started in the visually correct direction, independent of the sign of coupling between arm and target; from this we conclude that intentional signals are not mere triggers, but also convey directional information. Short-latency PEM trials were intermixed with those characterized by normal latencies, which often resulted in bimodal latency distributions; this suggests that visual and intentional signals compete for the control of PEM.
Animals↗
Ocular torsion measured by TV- and scanning laser ophthalmoscopy during horizontal pursuit in humans and monkeys.
Ocular torsion during horizontal foveal pursuit and fixation was measured in five human subjects and two trained monkeys (Macaca fascicularis) by direct analysis of the ocular fundus rotation. In the monkeys the fundus images of either eye were generated with a TV-ophthalmoscope while the contralateral eye pursued an 8' visual target moving sinusoidally (0.3-0.9 Hz, +/- 5 degrees) in the horizontal plane. In the humans a scanning laser ophthalmoscope (SLO) generated the fundus image of the ipsilateral eye, which pursued the visual target (same parameters as in the monkeys) mixed electronically into the laser scan raster. The image sequences were stored on videotape and subjected to a frame-by-frame rotation analysis. In both the humans and the monkeys, torsion (fundus rotation about the visual axis) sinusoidally modulated (up to 8 degrees peak-to-peak) during foveal pursuit, approximately in phase with horizontal eye position. Intorsion (nasal movement of the upper eye pole) or extorsion was found during pursuit in the temporal or nasal direction. Torsion showed considerable intra-individual fluctuation and interindividual variability with regard to phase and modulation depth relative to the pursuit movements, and was interspersed with irregularly occurring rapid deflections. Torsion of the conjugate, nonpursing eye was similar to torsion of the pursuing eye. In contrast, torsion during fixation was only loosely correlated with horizontal eye position. Slow torsional drifts and large, rapid deflections (up to 6 degrees) occurred in either direction at a given fixation point in the horizontal plane. We conclude that ocular torsion during horizontal pursuit in primates is actively generated by a separate, neural oculomotor subsystem.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Dynamic adaptation of the blind pointing characteristic to stepwise lateral tilts of body, head, and trunk.
Blind pointing (i.e. pointing to visual targets without seeing the pointing arm) was investigated in normal subjects in response to stepwise lateral tilts (20 degrees) of the body, head, and trunk. Blind pointing positions of the right index finger on the outer surface of a hemispherical screen were measured relative to the positions of visual targets that were presented along a horizontal line (+/- 30 degrees in head coordinates) on the inner screen surface, thus yielding a blind pointing characteristic (BPC). (1) BPC is highly reproducible and can be subdivided into separate branches for the ipsi- and contralateral hemifields. These branches are rotated relative to the target line by individually different BPC angles pi i and pi c. (2) pi i exhibits characteristic time courses (measured within 10 min following a stepwise tilt) for each paradigm. (3) Body tilt (left ear down) causes a step-like increase in pi i of up to 14 degrees; body tilt (right ear down) causes a step-like decrease in pi i to about zero. (4) Trunk tilt (right shoulder down) produces a gradual decrease in pi i of up to 6 degrees (average time constant tau T = 5 min); trunk tilt (left shoulder down) produces a gradual increase in pi i of up to 4 degrees. (5) Head tilt (left ear down) causes an increase in pi i of up to 9 degrees followed by a gradual decrease (average time constant tau H = 6 min); head tilt (right ear down) causes a step-like decrease in pi i with unsignificant further changes. These findings are discussed in terms of a neural sensorimotor coordinate transformation process receiving separate, dynamic otolith and neck afferent influences.
Functional Laterality↗
A head-mounted device for measurement of pointing to visual targets without seeing the pointing arm.
A mobile, head-mounted device is introduced, which allows the presentation of visual targets and the measurement of pointing movements to these targets in visual open loop (i.e. blind pointing without seeing the pointing arm). This microcomputer-controlled, easy-to-operate device offers a variety of applications both in basic research and for clinical diagnosis in ophthalmology, neurology, and otolaryngology.
Arm↗
Computational model of the motor program generator for pursuit.
A parallel processing neural network model of a motor program generator (MPG) for pursuit eye movements (PEM) was developed. The MPG model consists of two neural networks (velocity maps), which represent velocity values theta R and theta L respectively, as eccentric locations on the map with zero in the center. Neurons are arranged in a circular layer and connected only to their immediate neighbors. The potential field P of all neurons is analogous to a flat circular membrane whose center can be pushed up or down. During PEM one of the two maps, which are connected in a push-pull fashion, always features an activity peak (AP) which travels with constant velocity vT from one neuron to the next. The shape of P defines whether AP travels in a circle (theta = constant), towards the periphery (theta increase) or towards the center (theta decrease). Such a model provides a novel approach for understanding neural generators of non-periodical motor programs.
Animals↗
Neural control of pursuit eye movements.
Explore the source record for details and available documents.
Animals↗
Goal-directed arm movements in absence of visual guidance: evidence for amplitude rather than position control.
The control of pointing arm movements in the absence of visual guidance was investigated in unpracticed human subjects. The right arm grasped a lever which restricted the movement of the right index fingertip to a horizontal arc, centered between the axes of eye rotation. A horizontal panel directly above the arm prevented visual feedback of the movement. Visual stimuli were presented in discrete positions just above panel and fingertip. A flag provided visual feedback on fingertip position before each pointing movement (Exp. A and B), or before a movement sequence (Exp. C). When subjects pointed from straight ahead to eccentric stimulus positions (Exp. A), systematic and variable pointing errors were observed; both kinds of errors increased with stimulus eccentricity. When subjects pointed from 30 deg left to stimuli located further right (Exp. B), errors increased with stimulus position to the right. Taken together, these findings suggest that pointing accuracy depends not primarily on stimulus position, but rather on required movement amplitude. When subjects performed sequences of unidirectional movements (Exp. C), systematic and variable errors increased within the sequence. A quantitative analysis revealed that this increase can be best described as an accumulation of successive pointing errors. We conclude that both findings, error increase with amplitude, and accumulation of successive errors, when considered together strongly support the hypothesis that amplitude, rather than final position, is the controlled variable of the investigated movements.
Arm↗
An on-line spike form discriminator for extracellular recordings based on an analog correlation technique.
The discrimination of single unit activity in extracellular recordings presents a serious problem when the signal-to-noise ratio is low or when the amplitudes of interspersed spikes are similar. By exploiting spike form, the system described here performs discrimination using on-line hardware template matching. Using analog delay lines, the combined deviation of 8 input signal values from 8 stored template values is calculated simultaneously. The 8 template values are selected by adjusting 8 cursors to the desired spike trace on a CRT; the spike form discriminator (SPIFODIS) then generates a deviation function which steeply drops to zero whenever form similarity occurs, allowing for easy triggering. The performance of SPIFODIS was compared quantitatively with that of a conventional amplitude trigger in two cases: when detecting a single unit with varied signal-to-noise ratios and when separating double units of equal amplitude. At signal-to-noise ratios between 2 and 1 the error rate for SPIFODIS was only 15-50% of that of an amplitude trigger. In double-unit recordings showing only form differences, spikes are discriminated with very low error rate, while an amplitude trigger fails completely.
Action Potentials↗
Smooth pursuit eye movements.
Explore the source record for details and available documents.
Adaptation, Physiological↗
Afferent innervation of extraocular muscles in the rat studied by retrograde and anterograde horseradish peroxidase transport.
After injection of horseradish peroxidase (HRP) into extraocular muscles of rat perikarya were labeled mainly along the medial edge of the ophthalmic subdivision of the trigeminal ganglion but not in the mesencephalic nucleus of the trigeminal nerve. Injections of HRP into the trigeminal ganglion labeled simple as well as branching and meandering free fiber endings in extraocular muscles. No evidence for muscle spindles was found, but the meandering endings may be considered as candidates for stretch receptors.
Animals↗
Neuroanatomical status of monkeys showing functional compensation following neonatal cerebellar lesions.
Neuroanatomical changes observed at the light microscopic level in various brain areas of four adult monkeys, who had various degrees of cerebellar ablation shortly after birth, are described in this study. Extensive neonatal hemilateral ablations of the cerebellar cortex (sparing the nuclei), which have previously been shown to leave the adult monkey with no discernible motor deficits, lead to substantial degeneration, mainly within the remaining cerebellum and the brain stem. In particular: 1) Ipsilateral to the lesion the intracerebellar nuclei and to some extent also the lateral vestibular nucleus are clearly reduced in size, whereas the contralateral cerebellum appears normal. 2) The principal olive and parts of the pontine nuclei show massive degeneration contralateral to the lesion. 3) Among the nuclei efferent to the cerebellum only the red nucleus contralateral to the lesion shows clear signs of degeneration. 4) Morphometric analysis of motor cortex and pyramidal tract reveals no systematic differences between the left and right sides, nor any other morphological indication of compensation. The morphological abnormality pattern in our monkeys is particularly similar to that described in cases of humans with olivo-pontocerebellar atrophies.
Animals↗
Compensation of oculomotor deficits in monkeys with neonatal cerebellar ablations.
The oculomotor performance of 11 monkeys, who had various degrees of cerebellar ablation shortly after birth, is described in this study. Detailed numerical results were obtained on three of these adult macaques after extensive training of specific pursuit eye movements and fixation. The vestibular nuclei were kept intact. In the presence of the intracerebellar nuclei, quite extensive neonatal ablations of the cerebellar cortex leave the adult monkey without any discernible oculomotor deficits. If ablation also includes the nuclei on one side, compensation is never complete even several years later: while vestibulo-ocular and saccadic responses seem normal, there are deficits in pursuit and gaze holding performance. The residual deficits vary with the extent of the ablation and are comparable with the pattern exhibited by acute hemicerebellar ablations. Monkeys who had their cerebellum including the nuclei essentially completely removed just after birth, could pursue, albeit with limited velocity, and hold gaze, albeit in a limited zone. Their vestibulo-ocular responses seemed unaffected.
Animals↗