- •Visual Prosthetics
- •Preface
- •Acknowledgments
- •Contents
- •Contributors
- •1.1 The Visual System as an Engineering Compromise
- •1.2 An Overview of Human Visual System Architecture
- •1.2.1 Architecture and Basic Function of the Eye
- •1.2.2 Layout of the Retino-Cortical Pathway
- •1.2.3 Layout of the Subcortical Pathways
- •1.3 An Overview of Human Visual Function
- •1.3.1 Roles of Central (Foveal) Vision
- •1.3.2 Roles of Peripheral Vision
- •1.3.3 Roles of Dark-Adapted Vision
- •1.3.4 A Few Remarks Regarding Visual Development
- •1.4 Prospects for Prosthetic Vision Restoration
- •References
- •2.1 Introduction
- •2.2 Retina
- •2.2.1 Anatomy
- •2.2.2 Physiology and Receptive Fields
- •2.4.1 Anatomy
- •2.4.2 Physiology and Receptive Fields
- •2.6 The Role of Spatiotemporal Edges in Early Vision
- •2.7 The Role of Corners in Early Vision
- •2.7.1 Overview
- •2.8 Effects of Fixational Eye Movements in Early Visual Physiology and Perception
- •2.8.1 Overview
- •2.8.2 Neural Adaptation and Visual Fading
- •2.8.3 Microsaccades in Visual Physiology and Perception
- •References
- •3.1 Introduction
- •3.2 Background
- •3.3 Retinal Disease and Its Diversity
- •3.4 Retinal Remodeling
- •3.5 Retinal Circuitry
- •3.6 Retinal Circuitry Revision
- •3.7 Implications for Bionic Rescue
- •3.8 Implications for Biological Rescue
- •3.9 Final Remarks
- •References
- •4.1 Introduction
- •4.4 What Are the Limits to This Cortical Plasticity?
- •4.5 Possible Mechanisms Behind Brain Plasticity
- •4.6 Modulation of Brain Plasticity: Recent Developments
- •4.7 Neuroplasticity and Other Neuroprostheses Efforts
- •4.8 A Look at What Is Ahead
- •References
- •5.1 Introduction
- •5.2 Vision Changes Experienced by RP Patients
- •5.2.1 Overview
- •5.2.2 Visual Field Loss in RP
- •5.2.3 Changes in Color Vision and Glare Sensitivity in RP
- •5.2.4 Vision Fluctuations in RP
- •5.3 Visual Changes in Patients with Advanced Macular Degeneration
- •5.3.1 Changes Due to Wet AMD or Choroidal Neovascularization
- •5.3.2 Changes Due to Dry AMD or Geographic Atrophy
- •5.4 Charles Bonnet Syndrome
- •5.4.1 Overview
- •5.4.2 Complexity of Visual Hallucinations in CBS
- •5.4.3 Predictors and Alleviating Factors for CBS
- •5.5 Filling-In Phenomena (Perceptual Completion)
- •5.6 Remapping of Primary Visual Cortex in Patients with Central Scotomas from Macular Disease
- •5.7 The Preferred Retinal Locus for Fixation
- •5.8 Photopsias
- •5.8.1 Photopsias in RP
- •5.8.2 Photopsias in AMD and Other Ocular Diseases
- •5.9 Concluding Remarks
- •References
- •6.1 Introduction
- •6.2 Electrode–Electrolyte Interface
- •6.3 Electrode Material
- •6.3.1 Electrode Characterization
- •6.4 Overview of Electrode Materials for Neural Stimulation
- •6.5 Overview of Extracellular Stimulation
- •6.6 Safe Stimulation of Tissue
- •6.6.1 Mechanisms of Neural Injury
- •6.6.2 Parameters for Safe Stimulation
- •6.6.3 Stimulation Induced Injury in the Retina
- •References
- •7.1 Introduction
- •7.2 Power and Data Transmission
- •7.2.1 Wireline Connection
- •7.2.2 Inductive Coils
- •7.2.3 Serial Optical Telemetry
- •7.2.4 Photodiode Array-Based Prostheses
- •7.2.5 Thermal Safety Considerations
- •7.2.6 Conclusions: Comparing the Different Approaches
- •7.3 Tissue Response to a Subretinal Implant
- •7.3.1 Flat Implants
- •7.3.2 Chamber Implants
- •7.3.3 Pillar Arrays
- •7.4 Damage to Retinal Tissue from Electrical Stimulation
- •7.4.1 Effect of Pulse Duration
- •7.4.2 Electrode Size
- •7.5 Concluding Remarks
- •References
- •8.1 Introduction
- •8.2 Quasistatic Numerical Methods: The Admittance Method
- •8.2.1 Layered Retinal Model
- •8.2.2 Equivalent Electric Circuit
- •8.3 Three-Dimensional Activation Function Calculation
- •8.4 Safety of Implant
- •8.5 Conclusion
- •References
- •9.1 Pathophysiology of Retinal Degeneration
- •9.2.1 Outer Plexiform Layer
- •9.2.2 Inner Plexiform Layer
- •9.2.2.1 Bipolar Cell Excitation of Retinal Ganglion Cells
- •9.2.2.2 Amacrine Cell Modulation of Signal Processing
- •9.2.2.3 Inhibitory Transmitters
- •9.2.2.4 Acetylcholine and Dopamine
- •9.2.2.5 Neuropeptides
- •9.2.2.6 Putative neurotransmitters for retinal prosthesis
- •9.3 Neurophysiological Changes in Retinal Degeneration
- •9.4 Rationale for a Neurotransmitter-Based Retinal Prosthesis
- •9.4.1 Limitations of Electrical Stimulation
- •9.5 Technical Considerations and Design Approaches
- •9.5.1 Operating Principles for a Neurotransmitter-Based Retinal Prosthesis
- •9.5.2 Establishing a Retinal Prosthesis/Synaptic Interface
- •9.5.2.1 The Proximity Requirement
- •9.5.2.2 Convective Delivery of Neurotransmitters Via Microfluidics
- •9.5.2.3 Functionalized Surfaces for Neurotransmitter Stimulation
- •9.5.2.4 Synaptic Requirements for l-Glutamate Mediated Neuronal Stimulation
- •9.6 Summary
- •References
- •10.1 Introduction
- •10.2 Pioneering Experiments
- •10.2.1 Stimulation with No Chromophores
- •10.2.2 Azo Chromophores
- •10.3 Current Research
- •10.3.1 Caged Neurotransmitters
- •10.3.2 Pore Blocker and Photoisomerization
- •10.3.3 The Channelrhodopsins
- •10.3.4 Melanopsin
- •10.4 Synthetic Chromophores and Artificial Sight
- •References
- •11.1 Background
- •11.2 Physical Structure of Intracortical Electrodes
- •11.3 Charge Injection Using Intracortical Electrodes
- •11.3.1 The Intracortical Electrode as a Transducer
- •11.3.2 Charge Injection Limits
- •11.4 Intracortical Electrode Coatings
- •11.5 Characterization of Intracortical Electrodes
- •11.5.1 Cyclic Voltammetry
- •11.5.2 Electrode Stimulation Voltage Waveforms
- •11.5.3 Non-ideal Access Resistance Behavior
- •11.5.4 Non-linear Electrode Polarization
- •11.5.5 Determining Electrode Safety
- •11.6 Contrasts of In Vitro and In Vivo Behavior
- •11.7 Alternative Coatings for Improving Intracortical Electrodes
- •11.7.1 SIROF
- •11.7.2 PEDOT
- •11.7.3 Carbon Nanotube Coatings
- •11.8 Conclusion
- •References
- •12.1 Introduction
- •12.2 Responses of RGCs to Electrical Stimulation in Normal Retina
- •12.2.1 Epiretinal Stimulation
- •12.2.1.1 Target of Stimulation
- •12.2.1.2 The Site of Spike Initiation in RGCs
- •12.2.1.3 Threshold vs. Stimulating Electrode Diameter
- •12.2.1.4 Spatial Extent of Activation
- •12.2.1.5 Selective Activation
- •12.2.1.6 Temporal Response Properties
- •12.2.2 Subretinal Stimulation
- •12.2.2.1 Target of Stimulation
- •12.2.2.2 Threshold vs. Polarity of Stimulation Pulse
- •12.2.2.3 Spatial Extent of Activation
- •12.2.2.4 Temporal Response Properties
- •12.2.2.5 Dynamics of the Retinal Response
- •12.4 Responses of RGCs to Electrical Stimulation in Degenerate Retina
- •12.4.1 Epiretinal Stimulation
- •12.4.2 Subretinal Stimulation
- •12.4.2.1 Response Properties of RGCs
- •12.4.2.2 Activation Thresholds of RGCs
- •12.5 Cortical Responses to Retinal Stimulation
- •12.5.1 Spatial Properties Revealed by Cortical Measurements
- •12.5.2 Local Field Potentials
- •12.5.3 Elicited Responses Are Focal
- •12.5.4 Cortical Measurements Reveal Electrode Interactions
- •12.5.5 Temporal Responsiveness in Cortex
- •12.6 Suggestions for Future Studies
- •References
- •13.1 Introduction
- •13.2 General Considerations for Acute Retinal Stimulation Experiments
- •13.3 Surgical Technique
- •13.4 Threshold Measurements
- •13.5 Spatial Resolution and Pattern Perception
- •13.6 Temporal Resolution
- •13.7 Subretinal Versus Epiretinal Stimulation
- •13.8 Less Invasive Stimulation Procedures
- •13.9 Conclusions and Outlook
- •References
- •14.1 Introduction
- •14.2 Overview of Chronic Retinal Implant Technologies
- •14.2.1 The Retinal Implant AG Microphotodiode Prosthesis
- •14.2.2 The Intelligent Retinal Implant System
- •14.2.3 Second Sight Medical Products, Inc. A16 System
- •14.3 Thresholds on Individual Electrodes
- •14.3.1 Single Pulse Thresholds Using the SSMP System
- •14.3.2 Pulse Train Integration and Temporal Sensitivity
- •14.4 Suprathreshold Brightness
- •14.4.1 Brightness Using the Retinal Implant AG System
- •14.4.2 Brightness Using the Intelligent Medical Implant System
- •14.4.3 Brightness Using the SSMP A16 System
- •14.5 Spatial Vision
- •14.5.1 Spatial Vision with the Retinal Implant AG System
- •14.5.2 Spatial Vision with the Intelligent Medical Implant System
- •14.5.3 Spatial Vision with the SSMP A16 System
- •14.6 Models to Guide Electrical Stimulation Protocols
- •14.7 Conclusions
- •References
- •15.1 Background
- •15.2 Cortical Surface Stimulation
- •15.3 Intracortical Microstimulation
- •15.4 Optic Nerve Stimulation
- •15.5 What Is Known and What Needs to Be Done
- •15.6 Current Research Efforts
- •15.6.1 Optic Nerve Stimulation
- •15.6.2 Cortical Surface Stimulation
- •15.6.3 Intracortical Stimulation of Visual Cortex
- •15.6.4 CORTIVIS Program
- •15.6.5 Lateral Geniculate Stimulation
- •15.7 Microelectrode Arrays and Stimulation Hardware
- •15.7.1 Miniature Cameras
- •15.7.2 Animal Models
- •15.7.3 Image Processing and Phosphene Mapping
- •15.8 Conclusion
- •References
- •16.1 Introduction
- •16.2 Simulation Techniques and Basic Parameters
- •16.2.1 Gaze Tracking and Image Stabilization
- •16.2.2 Filter Engine Parameters
- •16.2.2.1 Raster Spatial Properties
- •16.2.2.2 Dot Spatial Properties
- •16.2.2.3 Temporal Properties
- •16.2.2.4 Dynamic Background Noise
- •16.2.2.5 Input Filtering/Windowing, Image Enhancement
- •16.3 Optotype Resolution and Reading
- •16.3.1 Visual Acuity
- •16.3.2 Reading
- •16.4 Face and Object Recognition
- •16.5 Visually Guided Behavior
- •16.5.1 Hand–Eye Coordination
- •16.5.2 Wayfinding
- •16.6 Visual Tracking
- •16.7 Computational Simulations
- •16.8 Conclusion
- •References
- •17.1 Introduction
- •17.2 Situating Image Analysis
- •17.3 The Experimental Framework
- •17.4 Tracking a Low-Resolution Target
- •17.5 Discussion
- •17.6 Conclusion
- •References
- •18.1 Introduction
- •18.2 Representation of Visual Space on the Visual Cortex
- •18.3 Cortical Stimulation Studies
- •18.4 Variability in Occipital Cortex
- •18.5 Phosphene Map Estimation
- •18.6 Psychophysical Studies with the Estimated Maps
- •References
- •19.1 Importance of Mapping
- •19.3 The Computer Era: Refining the Pointing Method of Phosphene Mapping
- •19.4 Verbal Mapping
- •19.5 Mapping Studies Using Subject Drawings
- •19.6 Recent Simulation Studies Using Phosphene Mapping
- •19.6.1 Tactile Simulations at Shanghai Jiao Tong University
- •19.6.2 Simulations in Our Laboratory
- •19.7 Concluding Remarks on Phosphene Mapping Techniques
- •References
- •20.1 Introduction
- •20.2 Principles for Assessment of Prosthetic Vision
- •20.2.1 Experimental Design
- •20.2.2 The Importance of Pre-operative Testing
- •20.2.3 Post-operative Assessment
- •20.2.4.1 Potential Approaches
- •20.2.4.2 Avoidance of Bias
- •20.2.4.3 Criteria for Sound Testing
- •20.2.4.4 Forced Choice Procedures
- •20.2.4.5 Response Time
- •20.2.4.6 Task (Perceptual) Learning
- •20.2.4.7 Establishing Criteria for Meaningful Change
- •20.2.4.8 Light Level
- •20.3 Vision Assessment in Prosthesis Recipients: Overview
- •20.3.1 Visual Function Assessment: Overview
- •20.3.2 Visual Performance Assessment: Overview
- •20.3.2.1 Measured Visual Performance
- •20.3.2.2 Self-Reported Visual Performance
- •20.4 Visual Function Assessment
- •20.4.1 Candidate Measures
- •20.4.1.1 Contrast Sensitivity (Contrast Detection)
- •20.4.1.2 Contrast Discrimination
- •20.4.1.3 Motion Perception
- •20.4.1.4 Depth Perception
- •20.4.2 Tests Used in Prosthesis Trials
- •20.4.3 Tests that Have Been Designed for Use with Prostheses
- •20.4.4 Vision Tests for Very Low Vision
- •20.5 Visual Performance Assessment
- •20.5.1 Measured Performance
- •20.5.2 Self-Reported Performance (Questionnaires)
- •20.6 Summary
- •References
- •21.1 Concepts of Functional Vision and Rehabilitation
- •21.1.1 Application to Orientation and Mobility
- •21.1.2 Application for Activities of Daily Living
- •21.1.3 Patient Lifestyle and Expectations
- •21.1.4 Congenital and Adventitious Vision Loss
- •21.2 Evaluation and Intervention with Prosthetic Vision
- •21.2.1 Evaluation
- •21.2.2 Intervention
- •21.3 Measuring Functional Outcomes
- •21.4 The Future
- •References
- •Author Index
- •Subject Index
15 Findings from Chronic Optic Nerve and Cortical Stimulation |
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15.6 Current Research Efforts
The research on a visual prosthesis using sites other than the retina can be divided into a number of sub categories. They are optic nerve stimulation, lateral geniculate, surface stimulation of visual cortex, intracortical stimulation of visual cortex, stimulation hardware, microelectrode arrays, miniature cameras and animal models. The ongoing research in each of these areas will be listed separately.
15.6.1 Optic Nerve Stimulation
Ren and co-workers at the Shanghai Jiao-Tong University in Shanghai, China has established a program called C-Sight to investigate implantation of penetrating microelectrodes in the optic nerve for a visual prosthesis [41]. They are investigating an image acquisition and processing system, a data telemetry system, a neural stimulator, and an implantable micro-camera system for an optic nerve visual prosthesis.
Another approach that is being actively studied in Germany is the use of regeneration microelectrode arrays. These electrodes consist of a wafer that has a number of holes into which nerve fibers can regenerate. The holes contain electrical contacts that enable single or a few nerve fibers to be stimulated. The optic nerve is cut and sutured to either side of the perforated microelectrode array. In rats, recovery of visual evoked potentials occurred in 2–8 weeks [30]. If regeneration through the perforated microelectrode arrays can be successful in primates and chronic stimulation of fibers can be shown to produce phosphenes then one could consider implanting this type of microelectrode in humans. One of the disadvantages of this type of microelectrode is that the optic nerve has to be cut and success of the implantation cannot be known for weeks or months. This might discourage some volunteers.
A group at Osaka University, Japan is investigating a different approach by stimulating the fibers in the optic nerve head inside the eye [26]. The advantages of this approach over the optic nerve cuff are that the exposure of fibers across the rim of the optic nerve head allows stimulation of small groups of fibers, and the intraocular surgical procedure is less invasive.
15.6.2 Cortical Surface Stimulation
After Dr. Dobelle died, his family donated the project, his patent and the technology to SUNY (State University of New York) at Stony Brook, in May 2006. Members from the staff at SUNY and Avery Biomedical Devices have teamed up to completely redesign the system used with the 16 patients implanted in Portugal.
310 |
E.M. Schmidt |
The redesign of the electrode array and electronics package will be completed before seeking FDA approval to implant patients in the USA.
Chowdhury and colleagues, in Australia, has been investigating a cat model for evaluating prototype cortical surface electrode arrays for a visual prosthesis [16, 17]. At present, this group does not have any plans for implanting human subjects.
15.6.3 Intracortical Stimulation of Visual Cortex
Because the National Institute of Health (NIH) is funded year to year by Congress, long-term patient care cannot be guaranteed. Thus the NIH administration decided not to continue the Visual Prosthesis Program for that reason. The scientists in the program were given the task of finding an appropriate University hospital that had access to the engineering expertise needed to carry out the Visual Prosthesis Program. Troyk and co-workers, at the Illinois Institute of Technology (IIT), formed a consortium consisting of IIT, University of Chicago (UC) and their Medical Center, EIC, and Huntington Medical Research Institute (HMRI). The NIH technology was transferred to IIT. The role of IIT is to develop implantable microelectrode arrays that contain RF powered and controlled stimulator packages and establish safe stimulation parameters for the microelectrodes [49]. EIC provides the electrochemistry expertise to properly develop iridium oxide stimulating electrodes [50]. The University of Chicago conducts the primate psychophysics experiments [6] and the Medical Center implants the primates in preparation for human implants. HMRI conducts safety experiments and histological evaluations of all implants. The Wilmer Eye Institute at Johns Hopkins University has been added to the consortium for evaluating human implants. A human implant is envisioned within 2 years
The University of Utah has conducted a number of studies aimed at implanting microelectrodes in the visual cortex. Normann and co-workers have developed a micro-machined electrode array consisting of 100 microelectrodes [37]. His group has conducted a number of studies that could lead to a human implant in the near future. They have looked at the histological effects of implanting these electrodes [35], the results of acute implantation in human neocortex [31] and the thermal impact of active arrays implanted in the brain [33]. They envision a intracortical visual prosthesis system employing 625 microelectrodes. The system receives video information from a micro-camera mounted in eyeglasses, processes the images with a computer and transmits the information over a telemetry system to stimulators on the electrode arrays.
15.6.4 CORTIVIS Program
A consortium of European Research Institutions has formed under the coordination of Dr. Fernandez in Alicante, Spain, called CORTIVIS [18]. The aim of the consortium is to develop a visual prosthesis based on intracortical stimulation of the visual