Avi Caspi

Blind subjects implanted with the Argus II retinal prosthesis are able to improve performance in a spatial-motor task

Background/aims To determine to what extent subjects implanted with the Argus II retinal prosthesis can improve performance compared with residual native vision in a spatial-motor task. Methods High-contrast square stimuli (5.85 cm sides) were displayed in random locations on a 19″ (48.3 cm) touch screen monitor located 12″ (30.5 cm) in front of the subject. Subjects were instructed to locate and touch the square centre with the system on and then off (40 trials each). The coordinates of the square centre and location touched were recorded. Results Ninety-six percent (26/27) of subjects showed a significant improvement in accuracy and 93% (25/27) show a significant improvement in repeatability with the system on compared with off (p<0.05, Student t test). A group of five subjects that had both accuracy and repeatability values <250 pixels (7.4 cm) with the system off (ie, using only their residual vision) was significantly more accurate and repeatable than the remainder of the cohort (p<0.01). Of this group, four subjects showed a significant improvement in both accuracy and repeatability with the system on. Conclusion In a study on the largest cohort of visual prosthesis recipients to date, we found that artificial vision augments information from existing vision in a spatial-motor task. Clinical trials registry no NCT00407602.

Feasibility Study of a Retinal Prosthesis Spatial Vision With a 16-Electrode Implant

OBJECTIVE To demonstrate that an epiretinal prosthesis can produce patterned visual perception in patients blinded by photoreceptor degeneration who have no other treatment options. METHODS A totally blind subject with retinitis pigmentosa had a 16-electrode epiretinal prosthesis implanted. The implant is controlled wirelessly by an external computer or a head-mounted video camera. Spatial vision was assessed by measuring the subjects response to direct stimulation of patterns and by comparing the ability of the subject to identify the orientation of gratings with the system on and off. RESULTS In response to stimulation of 2 orthogonal rows of electrodes, the subject drew 2 lines with a mean (SEM) angle of 87.4 degrees (1.8 degrees) between them. With the system on, the subject identified the orientation of the grating target up to a spatial resolution that matches the spacing between the adjacent electrodes. In contrast, with the system off, the subject could not detect or identify the targets orientation. CONCLUSION Synchronized stimulation of different retinal locations with an epiretinal prosthesis implanted long-term can produce spatial vision with an acuity level determined by the distance between the electrodes. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT00279500.

Preliminary 6 month results from the argus tm ii epiretinal prosthesis feasibility study

The Argus™ II 60 channel epiretinal prosthesis has been developed in order to provide partial restoration of vision to subjects blinded from outer retinal degenerative disease. To date the device has been implanted in 21 subjects as part of a feasibility study. In 6 month post-implantation door finding and line tracking orientation and mobility testing, subjects have shown improvements of 86% and 73%, respectively, for system on vs. system off. In high-contrast Square Localization tests using a touch screen monitor 87% of tested subjects performed significantly better with the system on compared with off. These preliminary results show that the Argus II system provides some functional vision to blind subjects.

Scaling of horizontal and vertical fixational eye movements.

Eye movements during fixation of a stationary target prevent the adaptation of the visual system to continuous illumination and inhibit fading of the image. These random, involuntary, small movements are restricted at long time scales so as to keep the target at the center of the field of view. Here we use detrended fluctuation analysis in order to study the properties of fixational eye movements at different time scales. Results show different scaling behavior between horizontal and vertical movements. When the small ballistic movements, i.e., microsaccades, are removed, the scaling exponents in both planes become similar. Our findings suggest that microsaccades enhance the persistence at short time scales mostly in the horizontal component and much less in the vertical component. This difference may be due to the need for continuously moving the eyes in the horizontal plane, in order to match the stereoscopic image for different viewing distances.

A New Dimension in Retrograde Flow: Centripetal Movement of Engulfed Particles

Centripetal motion of surface-adherent particles is a classic experimental system for studying surface dynamics on a eukaryotic cell. To investigate bead migration over the entire cell surface, we have developed an experimental assay using multinuclear giant fibroblasts, which provide expanded length scales and an unambiguous frame of reference. Beads coated by adhesion ligands concanavalin A or fibronectin are placed in specific locations on the cell using optical tweezers, and their subsequent motion is tracked over time. The adhesion, as well as velocity and directionality of their movement, expose distinct regions of the cytoplasm and membrane. Beads placed on the peripheral lamella initiate centripetal motion, whereas beads placed on the central part of the cell attach to a stationary cortex and do not move. Careful examination by complementary three-dimensional methods shows that the motion of a bead placed on the cell periphery takes place after engulfment into the cytoplasm, whereas stationary beads, placed near the cell center, are not engulfed. These results demonstrate that centripetal motion of adhering particles may occur inside as well as outside the cell. Inhibition of actomyosin activity is used to explore requirements for engulfment and aspects of the bead movement. Centripetal movement of adherent particles seems to depend on mechanisms distinct from those driving overall cell contractility.

Phase-Synchronization Decay of Fixational Eye Movements

Abstract: In nonstationary noisy systems the traditional cross‐correlation method may not appropriately detect all cases of interdependencies between coupled systems. The phase‐synchronization method was previously found useful in detecting synchronization in several systems. We here applied the phase‐synchronization decay to study the synchronization between six combinations of binocular fixational eye movement components. We found that only two components were synchronized: the right and left horizontal with each other and the right and left vertical. Furthermore, the vertical‐vertical components were much more synchronized than the horizontal.

Multiple Saccadic Abnormalities in Spinocerebellar Ataxia Type 3 Can Be Linked to a Single Deficiency in Velocity Feedback

PURPOSE The purpose of the current research is to understand if the different eye movement abnormalities in patients with the same neurologic disease are related to varied disease processes or, alternately, do different patients adopt different strategies to overcome a singular brain deficiency. METHODS Using a magnetic search coil, we measured saccade dynamics, that is position and velocity waveforms, for patients diagnosed with spinocerebellar ataxia type 3 (SCA-3), also known as Machado-Joseph disease (MJD). RESULTS We observed that the saccadic waveform of the majority of the SCA-3 patients (7 of 10) exhibited dynamic overshoot, with the eye passing the desired endpoint and making a rapid correction before coming to rest. Patients with normal waveforms, that is with no dynamic overshoot, had saccades with relatively low peak velocity. CONCLUSIONS Velocity feedback in a closed loop control system is essential for providing a fast response without overshoot. Lack of a velocity feedback or an imbalance between position and velocity gains yields a tradeoff between response time and overshoot. While the goal of a saccade is to get to the desired position, models based on animal research suggest that the saccadic control also incorporates a velocity feedback. Results presented here indicated that all SCA-3 patients had deviations in the saccadic waveform, albeit of two types, either slow saccades or dynamic overshooting saccades. Using saccadic models based on animal research can explain how a single deficit, that is a mismatched velocity control of the motor error due to the disease, can yield these two different abnormalities in human patients.

Impaired vestibulo-ocular reflex (VOR) in spinocerebellar ataxia type 3 (SCA3): Bedside and search coil evaluation

Vestibulo-Ocular Reflex (VOR) abnormalities in cerebellar ataxias are a matter of renewed interest. We have previously reported vestibular areflexia in a group of Yemenite-Jews with Spinocerebellar Ataxia Type 3 (SCA3) who had clear bilateral pathological horizontal Head Impulse Test (HIT). The objective of this study was to evaluate the VOR of ten SCA3 patients who have variable bedside HIT responses by recording their eye movements using magnetic search coils and to correlate these results with their clinical and genetic data. Eight out of the ten patients have abnormal horizontal HIT detected by both clinical bedside examination and laboratory tests. Results of bedside HIT testing were significantly correlated with the VOR gain recorded using magnetic search coils. No significant correlations were found between VOR gain and other clinical or genetic data. Our study confirms the presence of defective VOR in SCA3 patients and corroborates the useful of the HIT as a reliable bedside test for diagnosis of VOR deficits.

The Saccadic system more readily co-processes orthogonal than co-linear saccades

Real-life visual tasks such as tracking jumping objects and scanning visual scenes often require a sequence of saccadic eye movements. The ability of the ocular motor system to parallel process saccades has been previously demonstrated. We recorded the monocular eye movements of five normal human subjects using the magnetic search coil technique in a double step paradigm. Initial target jumps were always purely horizontal or purely vertical. We were interested in the latency to onset of the second saccade as a function of direction in relation to the first saccade. When the inter stimulus interval (ISI) was 150 or 180 ms orthogonal second saccades were of significantly shorter latency than second co-linear saccades. When the ISI was 250 ms the latencies of orthogonal and co-linear second saccades were statistically indistinguishable. Based on these findings it is postulated that the ocular motor system can more readily co-process orthogonal than co-linear saccades.

Combined eye-head vs. head-only scanning in a blind patient implanted with the Argus II retinal prosthesis

The Argus II retinal prosthesis has a dissociation between the line of sight of the camera and that of the eye. The image-capturing camera is mounted on the glasses and therefore, eye movements do not influence the visual information sent to the implanted electrodes.