ARTIFICAL VISION. Regina Leung, Michelle Ngai

Transcription

1 ARTIFICAL VISION Regina Leung, Michelle Ngai

2 Overview 2 Anatomy and Physiology (A&P) Visual Pathway Blindness What makes vision prostheses possible? Engineering Design Translating between biology and electrical engineering Key/Common Components of vision prostheses Various implementations Three types: epiretinal, subretinal, optic nerve Future outlook

3 ANATOMY AND PHYSIOLOGY

4 Anatomy and Physiology Basic A & P of the Eye 4 3 inner layers of the eyeball Sclera Choroid Retina (most important in the visual pathway)

5 Anatomy and Physiology Basic A & P of the Eye 5 Path of light through retina Photoreceptors (convert light to electric signals): rods and cones graded potential Bipolar cell layer: bipolar cells, amacrine cells, and horizontal cells graded potential Ganglion cells action potentials Observe parallel pathways Bipolar and ganglion encode many properties of visible light (colour, intensity, etc)

6 Anatomy and Physiology Photoreceptors Physiology 6 Outer segment of photoreceptors contain photopigment Light causes photopigment to change form Result: cascade of chemical reactions: membrane less permeable to certain ions, alters photoreceptor membrane potential, send nerve impulse

7 Anatomy and Physiology Visual Pathway 7 1 st step: light rays focused, image formed on retina Image: retinal image inversion of visual field

8 Anatomy and Physiology Visual Pathway 8 Retina optic nerve lateral geniculate nucleus of the thalamus visual cortex

9 Anatomy and Physiology Visual Pathway - Retinotopy 9 Light falling on parts of retina will produce visual perception of image What if we want to reproduce that perception by stimulating optic nerve or visual cortex? Each target structure has a retinotopic organization/map

10 Anatomy and Physiology Blindness 10 Any component of visual pathway destroyed leads to blindness Examples: Retinal diseases (photoreceptors damaged) retinitis pigmentosa, age-related macula degeneration (AMD) Optic nerve fibers damaged: glaucoma

11 Anatomy and Physiology What makes vision prostheses possible? 11 3 KEY PHYSIOLOGICAL PRINCIPLES: 1) In healthy subjects, stimulation of retina, optic nerve, and cortex produced visual sensations (phosphenes specifically sensation of light). Same as visual sensation produced by that of visual pathway? No! Implication: electric current can be substituted for light to produce visual sensations 2) In diseases that cause blindness as mentioned above, remainder of neural structures of the visual pathway are still intact. Implication: visual prostheses that electrically stimulate the visual pathway can produce visual sensations in blind individuals HOW TO GENERATE VISUAL PERCEPTIONS IN A DAMAGED VISUAL SYSTEM WITHOUT LIGHT

12 What makes vision prostheses possible? 12 3 KEY PHSIOLOGICAL PRINCIPLES CONT D: 3. Retinotopic organization/map exists for each target structure (optic nerve, visual cortex). How to determine the pattern of electrical stimulation necessary at each structure to reproduce image (phosphene representation). 4. Semiconductor technology: each electrode small enough to stimulate one nerve improve acuity of image

13 ENGINEERING DESIGN

14 Engineering Design Biology Electrical Engineering 14 Components include: I. Convert light into electrical signals Camera II. Way of transforming visual space to the retinotopic organization of target structure Signal Processor III. Way to transmit power and signals to implanted electronics Power and Signal Transmitter IV. Way to interpret transmitted signals and stimulate corresponding electrodes to produce phosphenes Microelectronic chip V. Way to transmit signal to target structure electrodes VI. Biocompatibility

15 Engineering Design Power and Signal Transmission 15 Power and data both need to be transmitted to implant by external unit Balance between frequency differences Power (use low frequency for efficiency) vs. data (use high frequency for speed) Dual-band telemetry system Different frequencies to transmit data and power

16 Engineering Design Power and Signal Transmission Cont d 16 Battery Inductive * Infrared * Thermal Transmitter/ Energy Source N/A Coil LED, laser Body heat Receiver N/A Coil Photodiode, phototransistor Thermoelectric material Available Energy Low -restricted size/weight -good for devices with low consumption or occasional power drain (i.e. pacemaker) High Moderate Low Efficiency High Moderate Low Low Additional source: Biocells use glucose to provide energy *Transcutaneous links

17 Engineering Design Power and Signal Transmission Cont d 17 Transcutaneous links are commercially available Design considerations: Required data rate Location, size, and power consumption of implant Alignment of transmitter and receiver Current prostheses: inductive coupled coils with wireless radio frequency (RF) Infrared power Inductive Coupling Only for small distances magnetic field strength over coil axis decreases Air coupled Cannot use magnetic material to couple Adverse effects if exposed to external magnetic fields Can transmit unwanted electromagnetic interference from external devices

18 Engineering Design Microelectronic Chip 18 Requirements: Size, power consumption, and output capability (most important) Problem: electrode-retina interface has high impedance (so voltage needs to be high): High voltage breaks dielectrics in transistor May produce unacceptable amount of heat May be bad for the effectiveness of device Larger transistor required larger chip

19 Engineering Design Microelectronic Chip 19 Parts (of microstimulator) Controller Sequences output, sets gain, controls DAC, sets current/voltage references Stimulation sequencer Controls pulse width for each stimulus, interpulse interval, order of stimulation between different electrodes Stimulus generator Send current pulses to demultiplexer Demultiplexer routes stimulus to correct electrode Gain controller Adjusts gain of stimulus generator (maximum current) so different tissue areas can be stimulated with different scaled currents Charge cancellation circuitry (periodic discharge)

20 Engineering Design Electrode Arrays 20 Electrode number and spacing determine acuity of visual image Study: minimum 25 x 25 electrode array in 1cm^2, spacing 400 micrometres apart, for useful vision

21 Engineering Design Electrode Arrays Cont d 21 Electrode charge transfer efficiency: C/cm^2 Determines size of electrode and power requirements Higher the better 3 main materials: platinum iridium iridium oxide titanium nitride Platinum iridium used most often due to its resistance to corrosion Shape of array

22 Engineering Design Signal Processor 22 Purpose: Image pixelized image determine electrode stimulation pattern according to retinotopic organization of target structure

23 Engineering Design Signal Processor Cont d 23 Design criteria: flexibility (software can be changed) Signal parameters can lower threshold current in order to produce a nerve impulse (less power): as threshold pulse duration increases, threshold current decreases

24 Engineering Design V. Biocompatibility 24 Hermetic Packaging Not on par with other aspects of biomedical implants Barrier between circuits and body fluid Two major schemes developed: Hard shell/capsule Physical gap between chip and package (adds to overall size) Allows flexibility in fabricating individual components Thin film (as coating material) More technically difficult (compatible with chip process) Package potentially the same size as the chip Bottom line: package to protect device, but allow the system access to inputs/outputs (feedthroughs allow access to environment)

25 Engineering Design Biocompatibility 25 Humayun et al

26 IMPLEMENTATION

27 Implementation Stimulation 27 Three types we will cover today: I. Epiretinal II. III. Subretinal Optic Nerve

28 Implementation Retinal Prostheses 28 Epiretinal: bypasses photorceptors and biopolar cell layer and stimulates ganglion cells Subretinal: replaces photoreceptors and stimulates bipolar cell layer

29 Implementation I. Epiretinal Stimulation 29 Contains all basic components discussed before Camera, signal processor, wireless transmitter, microprocessor, electrode array

30 Implementation II. Subretinal Stimulation 30 Convert light to electrical impulse: microphotodiode No signal processing! Each microphotodiode has its corresponding metallic stimulation electric to pass nerve impulses to bipolar cells

31 Epiretinal Vs Subretinal 31 Epiretinal: better at disspating heat, signal processor is flexible, difficulty in chronic attachment to the retina Subretinal: uses more natural visual processing, lower current threshold, limitation of power implant generation

32 Implementation III. Optic Nerve Prostheses 32 Multielectrode spiral cuff electrodes placed around optic nerve Selective stimulation of axons in optic nerve Electrode implanted intracranially Electrode leads brought outside skull, through skin to external connector In past, direct connection required (now wireless) Neurostimulator comprised of four biphasic (bi-directional) sources for charge compensation (charge balance between active/excitable pulses and recuperating pulses No net electrical charge to tissues

33 Implementation Optic Nerve Prostheses Cont d 33 Note: the antenna would be if wireless transmission was used instead of direct connection (earlier method) Veraart et al

34 Implementation Optic Nerve Prostheses Cont d Examples of phosphene descriptions by patients Veraart et al Phosphenes generated by stimulation Large distribution in visual field Thresholds varied after implantation, but stabilized at low values Generally, seen as lumps/dots of various colours Background colour also can vary 34

35 Implementation Optic Nerve Prostheses Cont d Learning Curve Pattern Recognition Evaluated many times Scores increased Times decreased Pattern Orientation Used U s and L s Similar results Veraart et al 35

36 Future Outlook Remaining questions 36 Many questions also remain in many fields Need experiments on humans to answer more subtle questions about perception and electrical stimulation Biocompatibility Improve on biocompatibility testing: Current Nissel-type strains used to test biocompatibility (determine cellular density around implanted electrode) are not specific to a cell line (neurons, glia, astrocytes) and do not reveal status of cell (normal, degenerating, etc.) Effect of chronic electrical stimulation to tissues (paramount since device would be stimulated at high rates every day for long periods of time) Need to determine how to insulate device from biological medium for extended periods of time under electrical bias (parylene most promise) Improvement of hermetic packaging required for further progress

37 Future Outlook Remaining questions 37 Plasticity Changes in the organization of the visual system due to experience learning May cause problems? Unsure about how brain interprets patterns of stimulation from electrodes Reinvestigate the number of pixels required (for functional vision reading, recognizing unfamiliar spaces) with varying size of receptive fields and altering spatial relationships between receptive fields Right now, Cha et al claims 625 grayscale pixels Dissipation of power in living tissues Simulations for effect on eye in prostheses may be inaccurate since it did not use the final design of the implant How will temperature change near implant affect its biocompatibility?

38 QUESTIONS?

39 References 39 Humayun, Mark S. et al. A Variable Range Bi-Phasic Current Stimulus Driver Circuitry for an Implantable Retinal Prosthetic Device Humayun, Mark S. et al. Retinal Prosthesis Veraart, Claude et al. Pattern Recognition with the Optic Nerve Visual Prosthesis Veraart, Claude et al. Visual sensations produced by optic nerve stimulation using an implanted self-sizing spiral cuff electrode Maynard, Edwin M. Visual Prostheses