Luke E. Hallum

Retinal Neurostimulator for a Multifocal Vision Prosthesis

A neurostimulator application-specific integrated circuit (ASIC) with scalable circuitry that can stimulate 14 channels, has been developed for an epi-retinal vision prosthesis. This ASIC was designed to allow seven identical units to be connected to control up to 98 channels, with the ability to stimulate 14 electrodes simultaneously. The neurostimulator forms part of a vision prosthesis, designed to restore vision to patients who have lost their sight due to retinal diseases such as retinitis pigmentosa and macular degeneration. For charge balance, the neurostimulator was designed to stimulate with current sources and sinks operating together, and with the ability to drive a hexagonal mosaic of electrodes to reduce the electrical crosstalk that occurs when multiple bipolar stimulation sites are active simultaneously. A hexagonal mosaic of electrodes surrounds each stimulation site and has been shown to effectively isolate each site, increasing the ability to inject localized independent charge into multiple regions simultaneously.

Visual acuity measurement of prosthetic vision: a virtual-reality simulation study

A virtual-reality simulation tested prosthetic visual acuity for both rectangular and hexagonal phosphene grids. Thirteen normally sighted, untrained subjects were required to identify the Landolt C optotype over ten sessions. Overall performance, performance by filter setting (image analysis), and performance by size and orientation of the Landolt C optotype are reported. The results indicated that the hexagonal grid had a slight (4.1%) but statistically significant (p < 0.0001) performance advantage over the rectangular grid for correct identification of the testing symbol. It was also observed that both the phosphene grids had distinct performance profiles relating to their respective spatial orientation. Over the course of the ten sessions, learning afforded subjects an averaged improved performance of 10%.

Simulated prosthetic visual fixation, saccade, and smooth pursuit

A visual tracking task was administered to 20 subjects afforded simulated prosthetic vision (a phosphene array); a total of 3h data was taken from each subject over the course of 10 visits. The experiment assessed prosthetic visual fixation, saccade and smooth pursuit and the effect of practice. Further, we demonstrated an image analysis technique that assisted fixation and pursuit (but not saccade) accuracy, and required less vigorous movement of the phosphene array in pursuing the target. As measured by mean deviation from the target, fixation and pursuit accuracies were improved by 8.3 and 3.3 min of visual arc, respectively (35.8% and 6.8%), for inter-phosphene spacing of 1.9 degrees . The analysis technique, involving overlapping Gaussian kernels, was an heuristic design; this is the first step of an iterative, experimental approach to devising effective image analysis to be contained in an electronic vision prosthesis. The approach should ultimately afford implanted patients improved prosthetic visual function.

An efficient multiplexing method for addressing large numbers of electrodes in a visual neuroprosthesis

Recent clinical trials using modified cochlear implants employ a small number of electrodes to stimulate surviving retinal neurons in blind patients and indicate that spatially-mapped phosphenes may indeed be elicited through these means. The next obvious step forward in the path toward achieving a useful visual prosthesis for the blind will be to increase the quantity of stimulation sites such that shapes, characters and rudimentary images may be conveyed. An important objective that must be obtained in the pursuit of this task is the ability to configure and deliver the stimulation with sufficient speed so as to avoid delays that are perceived by the patient as flicker within the visual scene. As the quantity of electrodes within the prosthesis increases, so too does the complexity of achieving this objective. This paper describes a means through which large numbers of electrode sites may be efficiently addressed in a neurostimulation circuit so as to increase the rate at which said circuit may be configured for the delivery of stimulation.

Population representation of visual information in areas V1 and V2 of amblyopic macaques

Amblyopia is a developmental disorder resulting in poor vision in one eye. The mechanism by which input to the affected eye is prevented from reaching the level of awareness remains poorly understood. We recorded simultaneously from large populations of neurons in the supragranular layers of areas V1 and V2 in 6 macaques that were made amblyopic by rearing with artificial strabismus or anisometropia, and 1 normally reared control. In agreement with previous reports, we found that cortical neuronal signals driven through the amblyopic eyes were reduced, and that cortical neurons were on average more strongly driven by the non-amblyopic than by the amblyopic eyes. We analyzed multiunit recordings using standard population decoding methods, and found that visual signals from the amblyopic eye, while weakened, were not degraded enough to explain the behavioral deficits. Thus additional losses must arise in downstream processing. We tested the idea that under monocular viewing conditions, only signals from neurons dominated by - rather than driven by - the open eye might be used. This reduces the proportion of neuronal signals available from the amblyopic eye, and amplifies the interocular difference observed at the level of single neurons. We conclude that amblyopia might arise in part from degradation in the neuronal signals from the amblyopic eye, and in part from a reduction in the number of signals processed by downstream areas.

A quantitative analysis of head movement behaviour during visual acuity assessment under prosthetic vision simulation

In most current vision prosthesis designs, head movement is the sole director of visual gaze and scanning due to the head-mounted nature of the camera. Study of this unnatural behaviour may provide insight into improved prosthesis designs and rehabilitation procedures. In this paper, we conducted a psychophysical study to investigate the characteristics of head movements of normally sighted subjects undergoing a visual acuity task in simulated prosthetic vision (SPV). In 12 naïve, untrained subjects, we recorded spontaneous changes in the amount of head movements during SPV sessions compared to control (normal vision) sessions. The observed behaviour continued to be refined until five or six sessions of practice. Increased head movement velocity was shown to be correlated to improved visual acuity performance, up to 0.3 logMAR, an equivalent of detecting details at half the physical size compared to complete deprivation of head movements. We postulate that visual scanning can as much as double the spatial frequency information in prosthetic vision. Increased head movement velocity observed when subjects were attempting smaller test items and for low-pass filtering schemes with higher cut-off frequencies may be further evidence that higher frequency content may be available through visual scanning, unconsciously driving subjects to increase head movement velocity.

Learning prosthetic vision: a virtual-reality study

Acceptance of prosthetic vision will be heavily dependent on the ability of recipients to form useful information from such vision. Training strategies to accelerate learning and maximize visual comprehension would need to be designed in the light of the factors affecting human learning under prosthetic vision. Some of these potential factors were examined in a visual acuity study using the Landolt C optotype under virtual-reality simulation of prosthetic vision. Fifteen normally sighted subjects were tested for 10-20 sessions. Potential learning factors were tested at p<0.05 with regression models. Learning was most evident across-sessions, though 17% of sessions did express significant within-session trends. Learning was highly concentrated toward a critical range of optotype sizes, and subjects were less capable in identifying the closed optotype (a Landolt C with no gap, forming a closed annulus). Training for implant recipients should target these critical sizes and the closed optotype to extend the limit of visual comprehension. Although there was no evidence that image processing affected overall learning, subjects showed varying personal preferences.

Biological–Machine Systems Integration: Engineering the Neural Interface

The state of the art of biological-machine systems integration (BMSI) with an emphasis on neural interfacing is reported. The goal of BMSI from a medical viewpoint is to effectively replace or facilitate activation of, or recording from, neural elements in a part of the nervous system. BMSI will be firstly examined from a generic level whereby the current technologies for noninvasive and invasive neural recording and neural stimulation will be detailed. A case study in the area of visual neuroprosthesis will be presented to elucidate the current and future issues facing BMSI. This will include a discussion on biocompatibility, design of stimulating electrodes, the implanted microelectronic neurostimulator, and parallelization of stimulus encoding and delivery.

Psychophysics of Prosthetic Vision: I. Visual Scanning and Visual Acuity

Recipients of vision prosthesis prototypes have reported electrically elicited visual perceptions as discrete dots of light (phosphenes). Phosphenes construct the scenery in discontinuous small isolated patches, resulting in visual information deficit to a large portion of the visual field. Visual scanning therefore plays an important role in the utility of prosthetic vision. In a psychophysical study, normally sighted subjects undertook a visual acuity task in a simulation of prosthetic vision with scanning facilitated by head movements. Subjects who adopted the circular scanning technique (4/12) correctly identified >60% of the test items, compared to subjects with no particular scanning patterns (3/12) with <50%. Increased head movement velocity was correlated to increased performance; at optimal scanning velocities, we estimated a 50% increase in identification rate or a two-fold improvement in visual acuity threshold compared to otherwise complete lack of scanning movement. Improved performance likely resulted from positive interactions with the temporal processes of the human visual system, which may as much as double the spatial information of that originally afforded by the phosphene lattice

Contribution to the theory of prosthetic vision.

By way of extracellular, electrical stimulation of the visual pathway, the various approaches to vision prosthesis aim to provide crude, patterned vision to individuals with profound blindness. Common to all approaches is the implantable electrode array and the rendering of phosphenes—the actuated percepts occupying the visual field of the implantee. Thus prosthetic vision may be simulated, and underlying theories as to how to render it efficacious developed. We review the field of simulated prosthetic vision. Furthermore, with retinal prosthesis in mind, we suggest a revised approach—an approach with regard to sampling theory, the vertebrate central visual pathway, and eye movements. The parallel development of prosthetic vision theory, explored via simulation and bioengineering issues surrounding neurostimulator design and implantation has bearing on the success of clinical trials by numerous groups in coming years.