Abstract:
The invention is a method of automatically adjusting a retinal electrode array to the neural characteristics of an individual patient. By recording electrically evoked electroretinograms (eERG) to a predetermined input stimulus, one can alter that input stimulus to the needs of an individual patient. A minimum input stimulus is applied to a patient, followed by recording the eERG response to the input stimulus. By gradually increasing stimulus levels, one can determine the minimum input that creates a neural response, thereby identifying the threshold stimulation level. One can further determine a maximum level by increasing stimulus until a predetermined maximum neural response is obtained. However, eERG signals include a significant amount of noise. Applicants have developed novel techniques for artifact reduction and noise filtering to provide an accurate measure of neural activity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application Ser. No. 61/419,663, filed Dec. 3, 2010 for Fitting an Visual Prosthesis using Electrically Evoked Electroretinograms, the disclosure of which is incorporated herein by reference. This application is also related U.S. Pat. No. 7,483,751, Automatic Fitting for a Visual Prosthesis, the disclosure of which is incorporated by Reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to visual prostheses configured to provide neural stimulation for the creation of artificial vision. 
       BACKGROUND 
       [0003]    In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw “flames passing rapidly downwards.” Ever since, there has been a fascination with electrically elicited visual perception. The general concept of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthetic devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired. 
         [0004]    In the early 1930&#39;s, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital (visual) cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart. 
         [0005]    As intraocular surgical techniques have advanced, it has become possible to apply stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparatuses to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular visual prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide. 
         [0006]    Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on such a device. The passage of current causes changes in electrical potentials across visual neuronal membranes, which can initiate visual neuron action potentials, which are the means of information transfer in the nervous system. 
         [0007]    Based on this mechanism, it is possible to input information into the nervous system by coding the information as a sequence of electrical pulses which are relayed to the nervous system via the prosthetic device. In this way, it is possible to provide artificial sensations including vision. 
         [0008]    One typical application of neural tissue stimulation is in the rehabilitation of the blind. Some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface (epiretial). This placement must be mechanically stable, minimize the distance between the device electrodes and the visual neurons, and avoid undue compression of the visual neurons. 
         [0009]    In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrode assembly for surgical implantation on a nerve. The matrix was silicone with embedded iridium electrodes. The assembly fit around a nerve to stimulate it. 
         [0010]    Dawson and Radtke stimulated cat&#39;s retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses. These experiments were carried out with needle-shaped electrodes that penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson). 
         [0011]    The Michelson &#39;933 apparatus includes an array of photosensitive devices on its surface that are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a “bed of nails” having conductors which impinge directly on the retina to stimulate the retinal cells. U.S. Pat. Nos. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. Each spike pierces cortical tissue for better electrical contact. 
         [0012]    The art of implanting an intraocular prosthetic device to electrically stimulate the retina was advanced with the introduction of retinal tacks in retinal surgery. De Juan, et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan, et al., 99 Am. J. Ophthalmol 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina against the choroid and the posterior aspects of the globe. Retinal tacks are one way to attach a retinal array to the retina. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a visual prosthesis for use with the flat retinal array described in de Juan. 
         [0013]    A visual prosthesis must be adjusted to an individual patient. The most basic adjustment is mapping brightness levels to stimulation intensity, as this varies from patient to patient and from electrode to electrode for an individual patient. Hence, the visual prosthesis must be fit to the patient. As the number of active electrodes in a visual prosthesis increases, manually adjusting or fitting the visual prosthesis becomes tedious or impossible. Automatic fitting techniques are known such as those described in U.S. Pat. No. 7,483,751, Greenberg et al,  Automatic Fitting for a Visual Prosthesis . Greenberg describes automatic fitting using iris sphincter response, retinal recordings and cortical recordings. These techniques are effective but very expensive to implement. 
       SUMMARY 
       [0014]    The invention is a method of automatically adjusting a retinal electrode array to the neural characteristics of an individual patient. By recording electrically evoked electroretinogram (eERG) responses to a predetermined input stimulus, one can alter that input stimulus to the needs of an individual patient. A minimum input stimulus is applied to a patient, while simultaneously recording an eERG containing the response to the input stimulus. By repeating stimulation and recording at gradually increasing stimulus levels, one can determine the minimum input that creates a neural response, thereby identifying the threshold stimulation level. One can further determine a maximum level by increasing stimulus until a predetermined maximum neural response is obtained. However, eERG signals include a significant amount of noise. Applicants have developed novel techniques for artifact reduction and noise filtering to provide an accurate measure of neural activity. 
         [0015]    According to a first aspect of the invention, a method of fitting a visual prosthesis is proposed, including stimulating visual neurons with an electrical signal, detecting an indication of neural activity using an electroretinogram, comparing the indication of neural activity to a desired level of neural activity, and altering the electrical signal based upon a comparison of the indication of neural activity and the desired level of neural activity. 
         [0016]    According to a second aspect of the invention, the method according to aspect one, further includes applying a wavelet transform to the indication of neural activity 
         [0017]    According to a third aspect of the invention, the method further includes detecting an indication of neural activity in the fellow eye using an electroretinogram and subtracting the indication of neural activity in the fellow eye from the indication of neural activity in the implanted eye. 
         [0018]    According to a fourth aspect of the invention, the method according to aspect one, wherein the step of stimulating visual neurons includes increasing the electrical charge until an indication of neural activity is detected. 
         [0019]    Further embodiments are shown in the specification, drawings and claims of the present application. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  is a flow chart showing the method of processing eERG signals. 
           [0021]      FIG. 2  is a flow chart showing the process of auto fitting an electrode array. 
           [0022]      FIG. 3  depicts a block diagram of the visual prosthesis electronic control unit. 
           [0023]      FIG. 4  is a graph depicting a typical neural response to electrical input. 
           [0024]      FIGS. 5 and 6  show a retinal stimulation system adapted to be implanted into a subject. 
           [0025]      FIGS. 7 and 8  show a video capture/transmission apparatus or visor adapted to be used in combination with the retinal stimulation of  FIGS. 5 and 6 . 
           [0026]      FIG. 9  shows components of a fitting system according to the present disclosure, the system also comprising the visor shown in  FIGS. 4 and 5 . 
           [0027]      FIG. 10  shows the visual prosthesis apparatus in a stand-alone mode, i.e. comprising the visor connected to a video processing unit. 
           [0028]      FIGS. 11-12  show the video processing unit already briefly shown with reference to  FIG. 9 . 
           [0029]      FIG. 13   a  shows a LOSS OF SYNC mode. 
           [0030]      FIG. 13   b  shows an exemplary block diagram of the steps taken when VPU does not receive back telemetry from the Retinal stimulation system. 
           [0031]      FIG. 13   c  shows an exemplary block diagram of the steps taken when the subject is not wearing Glasses. 
           [0032]      FIGS. 14-1 ,  14 - 2 ,  14 - 3  and  14 - 4  show an exemplary embodiment of a video processing unit.  FIG. 14-1  should be viewed at the left of  FIG. 14-2 .  FIG. 14-3  should be viewed at the left of  FIG. 14-4 .  FIGS. 14-1  and  14 - 2  should be viewed on top of  FIGS. 14-3  and  14 - 4 . 
       
    
    
       [0033]    In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale. 
       DETAILED DESCRIPTION 
       [0034]    The present disclosure is concerned with a visual apparatus and a method for creation of artificial vision. In particular, the present disclosure provides a means of automatically fitting a visual prosthesis using an eERG. 
         [0035]    Subjects implanted with an Argus® II retinal prosthesis in the right eye (OD) participated in a study. Binocular eERGs were obtained, using Burian-Allen contact lens electrodes, by averaging up to 2750 epochs. Current levels ranged from below perceptual threshold to a maximum of ˜50 μA per electrode. Signal-to-noise ratios of raw eERGs were increased off-line using wavelet transformation (WT), for example the symlet 5 transform. The eERG was expected to be measurable only in OD. Eye movements and pupil responses may contribute to the eERG, but will also evoke a response in the contralateral or fellow eye (OS), since they are centrally controlled. Therefore, pupil and eye movements were recorded with an eye tracker, averaging up to 30 responses. 
         [0036]    eERG responses were recorded, which consisted of a negative peak (N 1 ) followed by a positive peak (P 1 ). In most subjects eERGs can be obtained bilaterally. We performed eye tracking and eERG recordings before and after eye dilation with tropicamide (1%) and phenylephrin (2.5%). At 30 μA, the pupil dilated in both eyes at 0.6 s, followed by a bilateral constriction after 1.2 s, with smaller amplitudes in the implanted than in the fellow eye (−0.1, +0.3 vs.-0.2, +0.5 mm). Pharmacologic dilation abolished these pupil responses. Eye movements were small (0.1 mm or less). Before dilation, the eERG N 1 -P 1  amplitude was 6 μV in both eyes. After dilation, the eERG amplitude was 2 μV in both eyes. To remove bilateral electrically evoked artifacts we subtracted the OS eERG from the OD eERG. No dilation drops were applied. Using this subtraction procedure we obtained reliable eERGs. At current levels between perceptual threshold and maximum comfort level, eERG amplitudes were 2-5 μV, N 1  latencies were 100-200 ms, and P 1  latencies 300-400 ins. eERG amplitudes correlated significantly (F-test, P&lt;0.05, r 2 &gt;0.9) with stimulus level. 
         [0037]    Bilateral artifacts, such as pupil responses, in the corneal eERG cannot be sufficiently reduced by using dilation drops. Even after dilation, a residual, but substantial electrical response persisted in the contralateral eye. This residual activity might reflect the neural component of the pupil reflex, while the myogenic component is blocked after dilation. Subtracting the contralateral eERG yields the best approximation of the eERG. 
         [0038]      FIG. 1  shows the filtering process applied to the eERG signal according to the present invention. Simultaneously, eERG signals are recorded in the stimulated eye and the fellow eye  114 . Visual neurons are stimulated using the visual prosthesis  116 . eERG recording stops  118 . Temporal alignment of the two eERG signals and the stimulus is important to remove the unwanted signal components. Next, a stationary wavelet transform, for example a symlet 5 transform, is applied to the eERG signal from both eyes to filter out any artifact caused by the implanted electronics  120 . Particularly, the telemetry coils of the visual prosthesis generate substantial noise. Next, the eERG signal from the fellow eye is subtracted from the eERG signal from the stimulated eye  122  to reduce biological artifacts, such as those associated with eye movement and pupil responses. Since, both eyes move together and both pupils respond together, subtracting the signal from the fellow eye removes the eye movement and pupil response artifacts. Next, eERG waveforms with a peak-to-peak amplitude exceeding a predetermined level in the first second are discarded  124 . This eliminates those responses with large artifacts that are not bilaterally symmetrical. The predetermined level is between 40 μV and 60 μV, but preferably 50 μV. Last, 100 to 200 epochs are averaged 126 to reduce random noise, such as background noise and small eye movements. Finally the output is returned and used the fitting process as described with respect to  FIG. 2   128 . 
         [0039]      FIG. 2  is a flow chart of the automatic fitting sequence. In the flow chart, the value N is the initial selected electrode, X is the neural activity recorded, and L is the level of stimulation. First L is set to 0, or some level known to be below the threshold of perception  140  and then incremented  142 . Electrode N is addressed  144 . The stimulation level is set to zero  146 , and then incremented  148 . The neural tissue is stimulated at the minimum level  150 . The stimulation is immediately followed by a recording of activity in the eERG  152 . One must be careful to distinguish between neural activity and electrical charge from the stimulating electrode. The neural response follows stimulation (see  FIG. 4 ). Simultaneous stimulation and recording requires that the recording phase be longer than the stimulation phase. If so, the stimulation and neural response can be separated digitally. If the recorded neural activity is less than a predetermined level  154 , the stimulation level is increased and steps  148 - 154  are repeated. While this is preferably a fully automatic process, it may be advantageous to first fit a subset of the electrodes using patient responses to properly calibrate the desired eERG signal levels. 
         [0040]    Once minimum neural activity is recorded, the stimulation level is saved in memory  156 . The level is then further increased  158  and stimulation is repeated  160 . Again stimulation is immediately followed by recording neural activity  162 . If a predetermined maximum level has not been reached, steps  158 - 164  are repeated until the predetermined maximum stimulation level is obtained. Once the predetermined maximum stimulation level is obtained, steps  142 - 164  are repeated for the next electrode. The process is continued until a minimum and maximum stimulation level is determined for each electrode  166 . 
         [0041]    The maximum stimulation level borders on discomfort for the patient. Because the automatic fitting process is automated, high levels of stimulation are only applied for a few microseconds. This significantly decreases the level of discomfort for the patient compared with stimulating long enough to elicit a response from the patient. 
         [0042]    The fitting process is described above as an incremental process. The fitting process may be expedited by more efficient patterns. For example changes may be made in large steps if it the detected response is significantly below the desired response, followed by increasingly small steps as the desired response draws near. The system can jump above and below the desired response dividing the change by half with each step. 
         [0043]    Often, neural response in a retina is based, in part, geographically. That is, neurons closer to the fovea require less stimulation than neurons farther from the fovea. Stimulation levels are also higher when the electrodes array does not contact the retina. Hence once a stimulation is level is set for an electrode, one can presume that the level will be similar for an adjacent electrode. The fitting process may be expedited by starting at a level near the level set for a previously fit adjacent electrode. 
         [0044]    Automating the fitting process has many advantages. It greatly expedites the process reducing the efforts of the patient and clinician. Further, the automated process is objective. Patient responses are subjective and may change over time due to fatigue. In some cases, a patient may not be able to provide the required responses due to age, disposition, and/or limited metal ability. 
         [0045]      FIG. 3  depicts a block diagram of the control unit. The block diagram is a functional diagram. Many of the functional units would be implemented in a microprocessor. A control unit  180  sets and increments a counter  182  to control the stimulation level of the stimulator  184 . The stimulation signal is multiplexed in MUX  186  to address individual electrodes  188 . After each stimulation step, the eERG returns a neural activity signal to a recorder  190 . The signal is compared to the stored minimum or maximum level (stored in a memory  192 ) in a comparator  194 . After programming, a signal from a video source  196 , or other neural stimulation source, is adjusted in a mapping unit  198 , in accordance with the minimum and maximum levels stored in the memory  192 . The adjusted signal is sent to the stimulator  184 , which in synchronization with MUX  186  applies the signal to the electrodes  188 . The electronics for the control unit could be external or within the implanted prosthesis. 
         [0046]      FIG. 4  is a graphical representation of the neural response to electrical stimulus. The vertical axis is current while the horizontal axis is time. Four curves  100 - 106  show the response at varying input current levels. An input pulse  108 , is followed by a brief delay  110 , and a neural response  112 . Hence, it is important to properly time the detecting function. It should also be noted that the delay period  110  becomes shorter with increased stimulation current. Hence, the system must separate the stimulation signals and neural response faster with increased current. The change in delay time may also be used as an additional indication of neural response. That is, the minimum and maximum may be determined by matching predetermined delay times rather than predetermined output levels. 
         [0047]    The exemplary retinal stimulation system shown in  FIGS. 5 and 6 , is an implantable electronic device containing an inductive coil  16  and an electrode array  10  that is electrically coupled by a cable  48  that pierces sclera of the subject&#39;s eye to an electronics package  14 , external to the sclera. The retinal stimulation system is designed, for example, to elicit visual percepts in blind subjects with retinitis pigmentosa. 
         [0048]    Human vision provides a field of view that is wider than it is high. This is partially due to fact that we have two eyes, but even a single eye provides a field of view that is approximately 90° high and 140° to 160° degrees wide. It is therefore, advantageous to provide a flexible circuit electrode array  10  that is wider than it is tall. This is equally applicable to a cortical visual array. In which case, the wider dimension is not horizontal on the visual cortex, but corresponds to horizontal in the visual scene. 
         [0049]      FIGS. 5 and 6  present the general structure of a visual prosthesis used in implementing the invention. 
         [0050]      FIG. 5  shows a perspective view of the implanted portion of the preferred retinal prosthesis. A flexible circuit  1  includes a flexible circuit electrode array  10  which is mounted by a retinal tack (not shown) or similar means to the epiretinal surface. The flexible circuit electrode array  10  is electrically coupled by a flexible circuit cable  12 , which pierces the sclera and is electrically coupled to an electronics package  14 , external to the sclera. 
         [0051]    The electronics package  14  is electrically coupled to a secondary inductive coil  16 . Preferably the secondary inductive coil  16  is made from wound wire. Alternatively, the secondary inductive coil  16  may be made from a flexible circuit polymer sandwich with wire traces deposited between layers of flexible circuit polymer. The secondary inductive coil receives power and data from a primary inductive coil  17 , which is external to the body. The electronics package  14  and secondary inductive coil  16  are held together by the molded body  18 . The molded body  18  holds the electronics package  14  and secondary inductive coil  16  end to end. The molded body  18  holds the secondary inductive coil  16  and electronics package  14  in the end to end orientation and minimizes the thickness or height above the sclera of the entire device. The molded body  18  may also include suture tabs  20 . The molded body  18  narrows to form a strap  22  which surrounds the sclera and holds the molded body  18 , secondary inductive coil  16 , and electronics package  14  in place. The molded body  18 , suture tabs  20  and strap  22  are preferably an integrated unit made of silicone elastomer. Silicone elastomer can be formed in a pre-curved shape to match the curvature of a typical sclera. However, silicone remains flexible enough to accommodate implantation and to adapt to variations in the curvature of an individual sclera. The secondary inductive coil  16  and molded body  18  are preferably oval shaped. A strap  22  can better support an oval shaped coil. It should be noted that the entire implant is attached to and supported by the sclera. An eye moves constantly. The eye moves to scan a scene and also has a jitter motion to improve acuity. Even though such motion is useless in the blind, it often continues long after a person has lost their sight. By placing the device under the rectus muscles with the electronics package in an area of fatty tissue between the rectus muscles, eye motion does not cause any flexing which might fatigue, and eventually damage, the device.\ 
         [0052]      FIG. 6  shows a side view of the implanted portion of the retinal prosthesis, in particular, emphasizing the fan tail  24 . When implanting the retinal prosthesis, it is necessary to pass the strap  22  under the eye muscles to surround the sclera. The secondary inductive coil  16  and molded body  18  must also follow the strap  22  under the lateral rectus muscle on the side of the sclera. The implanted portion of the retinal prosthesis is very delicate. It is easy to tear the molded body  18  or break wires in the secondary inductive coil  16 . In order to allow the molded body  18  to slide smoothly under the lateral rectus muscle, the molded body  18  is shaped in the form of a fan tail  24  on the end opposite the electronics package  14 . The strap  22  further includes a hook  28  the aids the surgeon in passing the strap under the rectus muscles. 
         [0053]    Referring to  FIGS. 7 and 8 , the glasses  5  may comprise, for example, a frame  11  holding a camera  13 , an external coil  17  and a mounting system  19  for the external coil  17 . The mounting system  19  may also enclose the RF circuitry. In this configuration, the video camera  13  captures live video. The video signal is sent to an external Video Processing Unit (VPU)  20  (shown in  FIGS. 9 ,  11  and  12  and discussed below), which processes the video signal and subsequently transforms the processed video signal into electrical stimulation patterns or data. The electrical stimulation data are then sent to the external coil  17  that sends both data and power via radio-frequency (RF) telemetry to the coil  16  of the retinal stimulation system, shown in  FIGS. 5 and 6 . The coil  16  receives the RF commands which control an application specific integrated circuit (ASIC) which in turn delivers stimulation to the retina of the subject via a thin film electrode array (TFEA). In one aspect of an embodiment, light amplitude is recorded by the camera  13 . The VPU  20  may use a logarithmic encoding scheme to convert the incoming light amplitudes into the electrical stimulation patterns or data. These electrical stimulation patterns or data may then be passed on to the Retinal Stimulation System, which results in the retinal cells being stimulated via the electrodes in the electrode array  10  (shown in  FIGS. 5 and 6 ). In one exemplary embodiment, the electrical stimulation patterns or data being transmitted by the external coil  17  is binary data. The external coil  17  may contain a receiver and transmitter antennae and a radio-frequency (RF) electronics card for communicating with the internal coil  16 . 
         [0054]    Referring to  FIG. 9 , a Fitting System (FS) may be used to configure and optimize the visual prosthesis apparatus. The Fitting System is fully described in the related application U.S. application Ser. No. 11/796,425, filed on Apr. 27, 2007, (Applicant&#39;s Docket No. S401-USA) which is incorporated herein by reference in its entirety. 
         [0055]    The Fitting System may comprise custom software with a graphical user interface running on a dedicated laptop computer  21 . Within the Fitting System are modules for performing diagnostic checks of the implant, loading and executing video configuration files, viewing electrode voltage waveforms, and aiding in conducting psychophysical experiments. A video module can be used to download a video configuration file to the Video Processing Unit (VPU)  20  discussed above and store it in non-volatile memory to control various aspects of video configuration, e.g. the spatial relationship between the video input and the electrodes. The software can also load a previously used video configuration file from the VPU  20  for adjustment. 
         [0056]    The Fitting System can be connected to the Psychophysical Test System (PTS), located for example on a dedicated laptop  30 , in order to run psychophysical experiments. In psychophysics mode, the Fitting System enables individual electrode control, permitting clinicians to construct test stimuli with control over current amplitude, pulse-width, and frequency of the stimulation. In addition, the psychophysics module allows the clinician to record subject responses. The PTS may include a collection of standard psychophysics experiments developed using for example MATLAB® (MathWorks®) software and other tools to allow the clinicians to develop customized psychophysics experiment scripts. 
         [0057]    Using the psychophysics module, important perceptual parameters such as perceptual threshold, maximum comfort level, and spatial location of percepts may be reliably measured. Based on these perceptual parameters, the fitting software enables custom configuration of the transformation between video image and spatio-temporal electrode stimulation parameters in an effort to optimize the effectiveness of the retinal prosthesis for each subject. 
         [0058]    The Fitting System laptop  21  of  FIG. 9  may be connected to the VPU  20  using an optically isolated serial connection adapter  40 . Because it is optically isolated, the serial connection adapter  40  assures that no electric leakage current can flow from the Fitting System laptop  10  in the event of a fault condition. 
         [0059]    As shown in  FIG. 9 , the following components may be used with the Fitting System according to the present disclosure. The Video Processing Unit (VPU)  20  for the subject being tested, a Charged Battery  25  for VPU  20 , the Glasses  5 , a Fitting System (FS) Laptop  10 , a Psychophysical Test System (PTS) Laptop  30 , a PTS CD (not shown), a Communication Adapter (CA)  40 , a USB Drive (Security) (not shown), a USB Drive (Transfer)  47 , a USB Drive (Video Settings) (not shown), a Patient Input Device (RF Tablet)  50 , a further Patient Input Device (Jog Dial)  55 , Glasses Cable  15 , CA-VPU Cable  70 , FS-CA Cable  45 , FS-PTS Cable  46 , Four (4) Port USB Hub  47 , Mouse  60 , Test Array system  80 , Archival USB Drive  49 , an Isolation Transformer (not shown), adapter cables (not shown), and an External Monitor (not shown). 
         [0060]    With continued reference to  FIG. 9 , the external components of the Fitting System may be configured as follows. The battery  25  is connected with the VPU  20 . The PTS Laptop  30  is connected to FS Laptop  10  using the FS-PTS Cable  46 . The PTS Laptop  30  and FS Laptop  10  are plugged into the Isolation Transformer (not shown) using the Adapter Cables (not shown). The Isolation Transformer is plugged into the wall outlet. The four (4) Port USB Hub  47  is connected to the FS laptop  10  at the USB port. The mouse  60  and the two Patient Input Devices  50  and  55  are connected to four (4) Port USB Hubs  47 . The FS laptop  10  is connected to the Communication Adapter (CA)  40  using the FS-CA Cable  45 . The CA  40  is connected to the VPU  20  using the CA-VPU Cable  70 . The Glasses  5  are connected to the VPU  20  using the Glasses Cable  15 . 
         [0061]    In one exemplary embodiment, the Fitting System shown in  FIG. 9  may be used to configure system stimulation parameters and video processing strategies for each subject outfitted with the visual prosthesis apparatus. The fitting application, operating system, laptops  21  and  30 , isolation unit and VPU  20  may be tested and configuration controlled as a system. The software provides modules for electrode control, allowing an interactive construction of test stimuli with control over amplitude, pulse width, and frequency of the stimulation waveform of each electrode in the Retinal stimulation system. These parameters are checked to ensure that maximum charge per phase limits, charge balance, and power limitations are met before the test stimuli are presented to the subject. Additionally, these parameters may be checked a second time by the VPU  20 &#39;s firmware. The Fitting System shown in  FIG. 9  may also provide a psychophysics module for administering a series of previously determined test stimuli to record subject&#39;s responses. These responses may be indicated by a keypad  50  and/or verbally. The psychophysics module may also be used to reliably measure perceptual parameters such as perceptual threshold, maximum comfort level, and spatial location of percepts. These perceptual parameters may be used to custom configure the transformation between the video image and spatio-temporal electrode stimulation parameters thereby optimizing the effectiveness of the visual prosthesis for each subject. The Fitting System is fully described in the related application U.S. application Ser. No. 11/796,425, filed on Apr. 27, 2007, (Applicant&#39;s Docket No. S401-USA) which is incorporated herein by reference in its entirety. 
         [0062]    The visual prosthesis apparatus may operate in two modes: i) stand-alone mode and ii) communication mode. 
       Stand-Alone Mode 
       [0063]    Referring to  FIG. 10 , in the stand-alone mode, the video camera  13 , on the glasses  5 , captures a video image that is sent to the VPU  20 . The VPU  20  processes the image from the camera  13  and transforms it into electrical stimulation patterns that are transmitted to the external coil  17 . The external coil  17  sends the electrical stimulation patterns and power via radio-frequency (RF) telemetry to the implanted retinal stimulation system. The internal coil  16  of the retinal stimulation system receives the RF commands from the external coil  17  and transmits them to the electronics package  14  that in turn delivers stimulation to the retina via the electrode array  10 . Additionally, the retinal stimulation system may communicate safety and operational status back to the VPU  20  by transmitting RF telemetry from the internal coil  16  to the external coil  17 . The visual prosthesis apparatus may be configured to electrically activate the retinal stimulation system only when it is powered by the VPU  20  through the external coil  17 . The stand-alone mode may be used for clinical testing and/or at-home use by the subject. 
       Communication Mode 
       [0064]    The communication mode may be used for diagnostic testing, psychophysical testing, patient fitting and downloading of stimulation settings to the VPU  20  before transmitting data from the VPU  20  to the retinal stimulation system as is done for example in the stand-alone mode described above. Referring to  FIG. 9 , in the communication mode, the VPU  20  is connected to the Fitting System laptop  21  using cables  70 ,  45  and the optically isolated serial connection adapter  40 . In this mode, laptop  21  generated stimuli may be presented to the subject and programming parameters may be adjusted and downloaded to the VPU  20 . The Psychophysical Test System (PTS) laptop  30  connected to the Fitting System laptop  21  may also be utilized to perform more sophisticated testing and analysis as fully described in the related application U.S. application Ser. No. 11/796,425, filed on Apr. 27, 2007, (Applicant&#39;s Docket No. S401-USA) which is incorporated herein by reference in its entirety. 
         [0065]    In one embodiment, the functionality of the retinal stimulation system can also be tested pre-operatively and intra-operatively (i.e. before operation and during operation) by using an external coil  17 , without the glasses  5 , placed in close proximity to the retinal stimulation system. The coil  17  may communicate the status of the retinal stimulation system to the VPU  20  that is connected to the Fitting System laptop  21  as shown in  FIG. 9 . 
         [0066]    As discussed above, the VPU  20  processes the image from the camera  13  and transforms the image into electrical stimulation patterns for the retinal stimulation system. Filters such as edge detection filters may be applied to the electrical stimulation patterns for example by the VPU  20  to generate, for example, a stimulation pattern based on filtered video data that the VPU  20  turns into stimulation data for the retinal stimulation system. The images may then be reduced in resolution using a downscaling filter. In one exemplary embodiment, the resolution of the image may be reduced to match the number of electrodes in the electrode array  10  of the retinal stimulation system. That is, if the electrode array has, for example, sixty electrodes, the image may be reduced to a sixty channel resolution. After the reduction in resolution, the image is mapped to stimulation intensity using for example a look-up table that has been derived from testing of individual subjects. Then, the VPU  20  transmits the stimulation parameters via forward telemetry to the retinal stimulation system in frames that may employ a cyclic redundancy check (CRC) error detection scheme. 
         [0067]    In one exemplary embodiment, the VPU  20  may be configured to allow the subject/patient i) to turn the visual prosthesis apparatus on and off, ii) to manually adjust settings, and iii) to provide power and data to the retinal stimulation system. Referring to  FIGS. 11 and 12 , the VPU  20  may comprise a case  800 , power button  805  for turning the VPU  20  on and off, setting button  810 , zoom buttons  820  for controlling the camera  13 , connector port  815  for connecting to the Glasses  5 , a connector port  816  for connecting to the laptop  21  through the connection adapter  40 , indicator lights  825  to give visual indication of operating status of the system, the rechargeable battery  25  for powering the VPU  20 , battery latch  830  for locking the battery  25  in the case  800 , digital circuit boards (not shown), and a speaker (not shown) to provide audible alerts to indicate various operational conditions of the system. Because the VPU  20  is used and operated by a person with minimal or no vision, the buttons on the VPU  20  may be differently shaped and/or have special markings as shown in  FIG. 12  to help the user identify the functionality of the button without having to look at it. As shown in  FIG. 12 , the power button  805  may be a circular shape while the settings button  820  may be square shape and the zoom buttons  820  may have special raised markings  830  to also identify each buttons&#39; functionality. One skilled in the art would appreciate that other shapes and markings can be used to identify the buttons without departing from the spirit and scope of the invention. For example, the markings can be recessed instead of raised. 
         [0068]    In one embodiment, the indicator lights  825  may indicate that the VPU  20  is going through system start-up diagnostic testing when the one or more indicator lights  825  are blinking fast (more then once per second) and are green in color. The indicator lights  825  may indicate that the VPU  20  is operating normally when the one or more indicator lights  825  are blinking once per second and are green in color. The indicator lights  825  may indicate that the retinal stimulation system has a problem that was detected by the VPU  20  at start-up diagnostic when the one or more indicator lights  825  are blinking for example once per five second and are green in color. The indicator lights  825  may indicate that the video signal from camera  13  is not being received by the VPU  20  when the one or more indicator lights  825  are always on and are amber color. The indicator lights  825  may indicate that there is a loss of communication between the retinal stimulation system and the external coil  17  due to the movement or removal of Glasses  5  while the system is operational or if the VPU  20  detects a problem with the retinal stimulation system and shuts off power to the retinal stimulation system when the one or more indicator lights  825  are always on and are orange color. One skilled in the art would appreciate that other colors and blinking patterns can be used to give visual indication of operating status of the system without departing from the spirit and scope of the invention. 
         [0069]    In one embodiment, a single short beep from the speaker (not shown) may be used to indicate that one of the buttons  825 ,  805  or  810  have been pressed. A single beep followed by two more beeps from the speaker (not shown) may be used to indicate that VPU  20  is turned off. Two beeps from the speaker (not shown) may be used to indicate that VPU  20  is starting up. Three beeps from the speaker (not shown) may be used to indicate that an error has occurred and the VPU  20  is about to shut down automatically. As would be clear to one skilled in the art, different periodic beeping may also be used to indicate a low battery voltage warning, that there is a problem with the video signal, and/or there is a loss of communication between the retinal stimulation system and the external coil  17 . One skilled in the art would appreciate that other sounds can be used to give audio indication of operating status of the system without departing from the spirit and scope of the invention. For example, the beeps may be replaced by an actual prerecorded voice indicating operating status of the system. 
         [0070]    In one exemplary embodiment, the VPU  20  is in constant communication with the retinal stimulation system through forward and backward telemetry. In this document, the forward telemetry refers to transmission from VPU  20  to the retinal stimulation system and the backward telemetry refers to transmissions from the Retinal stimulation system to the VPU  20 . During the initial setup, the VPU  20  may transmit null frames (containing no stimulation information) until the VPU  20  synchronizes with the Retinal stimulation system via the back telemetry. In one embodiment, an audio alarm may be used to indicate whenever the synchronization has been lost. 
         [0071]    In order to supply power and data to the Retinal stimulation system, the VPU  20  may drive the external coil  17 , for example, with a 3 MHz signal. To protect the subject, the retinal stimulation system may comprise a failure detection circuit to detect direct current leakage and to notify the VPU  20  through back telemetry so that the visual prosthesis apparatus can be shut down. 
         [0072]    The forward telemetry data (transmitted for example at 122.76 kHz) may be modulated onto the exemplary 3 MHz carrier using Amplitude Shift Keying (ASK), while the back telemetry data (transmitted for example at 3.8 kHz) may be modulated using Frequency Shift Keying (FSK) with, for example, 442 kHz and 457 kHz. The theoretical bit error rates can be calculated for both the ASK and FSK scheme assuming a ratio of signal to noise (SNR). The system disclosed in the present disclosure can be reasonably expected to see bit error rates of 10 −5  on forward telemetry and 10 −3  on back telemetry. These errors may be caught more than 99.998% of the time by both an ASIC hardware telemetry error detection algorithm and the VPU  20 &#39;s firmware. For the forward telemetry, this is due to the fact that a 16-bit cyclic redundancy check (CRC) is calculated for every 1024 bits sent to the ASIC within electronics package  14  of the Retinal Stimulation System. The ASIC of the Retinal Stimulation System verifies this CRC and handles corrupt data by entering a non-stimulating ‘safe’ state and reporting that a telemetry error was detected to the VPU  20  via back telemetry. During the ‘safe’ mode, the VPU  20  may attempt to return the implant to an operating state. This recovery may be on the order of milliseconds. The back telemetry words are checked for a 16-bit header and a single parity bit. For further protection against corrupt data being misread, the back telemetry is only checked for header and parity if it is recognized as properly encoded Bi-phase Mark Encoded (BPM) data. If the VPU  20  detects invalid back telemetry data, the VPU  20  immediately changes mode to a ‘safe’ mode where the Retinal Stimulation System is reset and the VPU  20  only sends non-stimulating data frames. Back telemetry errors cannot cause the VPU  20  to do anything that would be unsafe. 
         [0073]    The response to errors detected in data transmitted by VPU  20  may begin at the ASIC of the Retinal Stimulation System. The Retinal Stimulation System may be constantly checking the headers and CRCs of incoming data frames. If either the header or CRC check fails, the ASIC of the Retinal Stimulation System may enter a mode called LOSS OF SYNC  950 , shown in  FIG. 13   a . In LOSS OF SYNC mode  950 , the Retinal Stimulation System will no longer produce a stimulation output, even if commanded to do so by the VPU  20 . This cessation of stimulation occurs after the end of the stimulation frame in which the LOSS OF SYNC mode  950  is entered, thus avoiding the possibility of unbalanced pulses not completing stimulation. If the Retinal Stimulation System remains in a LOSS OF SYNC mode  950  for 1 second or more (for example, caused by successive errors in data transmitted by VPU  20 ), the ASIC of the Retinal Stimulation System disconnects the power lines to the stimulation pulse drivers. This eliminates the possibility of any leakage from the power supply in a prolonged LOSS OF SYNC mode  950 . From the LOSS OF SYNC mode  950 , the Retinal Stimulation System will not re-enter a stimulating mode until it has been properly initialized with valid data transmitted by the VPU  20 . 
         [0074]    In addition, the VPU  20  may also take action when notified of the LOSS OF SYNC mode  950 . As soon as the Retinal Stimulation System enters the LOSS OF SYNC mode  950 , the Retinal Stimulation System reports this fact to the VPU  20  through back telemetry. When the VPU  20  detects that the Retinal Stimulation System is in LOSS OF SYNC mode  950 , the VPU  20  may start to send ‘safe’ data frames to the Retinal Stimulation System. ‘Safe’ data is data in which no stimulation output is programmed and the power to the stimulation drivers is also programmed to be off. The VPU  20  will not send data frames to the Retinal Stimulation System with stimulation commands until the VPU  20  first receives back telemetry from the Retinal Stimulation System indicating that the Retinal Stimulation System has exited the LOSS OF SYNC mode  950 . After several unsuccessful retries by the VPU  20  to take the implant out of LOSS OF SYNC mode  950 , the VPU  20  will enter a Low Power Mode (described below) in which the implant is only powered for a very short time. In this time, the VPU  20  checks the status of the implant. If the implant continues to report a LOSS OF SYNC mode  950 , the VPU  20  turns power off to the Retinal Stimulation System and tries again later. Since there is no possibility of the implant electronics causing damage when it is not powered, this mode is considered very safe. 
         [0075]    Due to an unwanted electromagnetic interference (EMI) or electrostatic discharge (ESD) event the VPU  20  data, specifically the VPU firmware code, in RAM can potentially get corrupted and may cause the VPU  20  firmware to freeze. As a result, the VPU  20  firmware will stop resetting the hardware watchdog circuit, which may cause the system to reset. This will cause the watchdog timer to expire causing a system reset in, for example, less than 2.25 seconds. Upon recovering from the reset, the VPU  20  firmware logs the event and shuts itself down. VPU  20  will not allow system usage after this occurs once. This prevents the VPU  20  code from freezing for extended periods of time and hence reduces the probability of the VPU sending invalid data frames to the implant. 
         [0076]    Supplying power to the Retinal stimulation system can be a significant portion of the VPU  20 &#39;s total power consumption. When the Retinal stimulation system is not within receiving range to receive either power or data from the VPU  20 , the power used by the VPU  20  is wasted. 
         [0077]    Power delivered to the Retinal stimulation system may be dependent on the orientation of the coils  17  and  16 . The power delivered to the Retinal stimulation system may be controlled, for example, via the VPU  20  every 16.6 ms. The Retinal stimulation system may report how much power it receives and the VPU  20  may adjust the power supply voltage of the RF driver to maintain a required power level on the Retinal stimulation system. Two types of power loss may occur: 1) long term (&gt;˜1 second) and 2) short term (&lt;˜1 second). The long term power loss may be caused, for example, by a subject removing the Glasses  5 . 
         [0078]    In one exemplary embodiment, the Low Power Mode may be implemented to save power for VPU  20 . The Low Power Mode may be entered, for example, anytime the VPU  20  does not receive back telemetry from the Retinal stimulation system. Upon entry to the Low Power Mode, the VPU  20  turns off power to the Retinal stimulation system. After that, and periodically, the VPU  20  turns power back on to the Retinal stimulation system for an amount of time just long enough for the presence of the Retinal stimulation system to be recognized via its back telemetry. If the Retinal stimulation system is not immediately recognized, the controller again shuts off power to the Retinal stimulation system. In this way, the controller ‘polls’ for the passive Retinal stimulation system and a significant reduction in power used is seen when the Retinal stimulation system is too far away from its controller device.  FIG. 13   b  depicts an exemplary block diagram  900  of the steps taken when the VPU  20  does not receive back telemetry from the Retinal stimulation system. If the VPU  20  receives back telemetry from the Retinal stimulation system (output “YES” of step  901 ), the Retinal stimulation system may be provided with power and data (step  906 ). If the VPU  20  does not receive back telemetry from the Retinal stimulation system (output “NO” of step  901 ), the power to the Retinal stimulation system may be turned off. After some amount of time, power to the Retinal stimulation system may be turned on again for enough time to determine if the Retinal stimulation system is again transmitting back telemetry (step  903 ). If the Retinal stimulation system is again transmitting back telemetry (step  904 ), the Retinal stimulation system is provided with power and data (step  906 ). If the Retinal stimulation system is not transmitting back telemetry (step  904 ), the power to the Retinal stimulation system may again be turned off for a predetermined amount of time (step  905 ) and the process may be repeated until the Retinal stimulation system is again transmitting back telemetry. 
         [0079]    In another exemplary embodiment, the Low Power Mode may be entered whenever the subject is not wearing the Glasses  5 . In one example, the Glasses  5  may contain a capacitive touch sensor (not shown) to provide the VPU  20  digital information regarding whether or not the Glasses  5  are being worn by the subject. In this example, the Low Power Mode may be entered whenever the capacitive touch sensor detects that the subject is not wearing the Glasses  5 . That is, if the subject removes the Glasses  5 , the VPU  20  will shut off power to the external coil  17 . As soon as the Glasses  5  are put back on, the VPU  20  will resume powering the external coil  17 .  FIG. 13   c  depicts an exemplary block diagram  910  of the steps taken when the capacitive touch sensor detects that the subject is not wearing the Glasses  5 . If the subject is wearing Glasses  5  (step  911 ), the Retinal stimulation system is provided with power and data (step  913 ). If the subject is not wearing Glasses  5  (step  911 ), the power to the Retinal stimulation system is turned off (step  912 ) and the process is repeated until the subject is wearing Glasses  5 . 
         [0080]    One exemplary embodiment of the VPU  20  is shown in  FIG. 14 . The VPU  20  may comprise: a Power Supply, a Distribution and Monitoring Circuit (PSDM)  1005 , a Reset Circuit  1010 , a System Main Clock (SMC) source (not shown), a Video Preprocessor Clock (VPC) source (not shown), a Digital Signal Processor (DSP)  1020 , Video Preprocessor Data Interface  1025 , a Video Preprocessor  1075 , an I 2 C Protocol Controller  1030 , a Complex Programmable Logic device (CPLD) (not shown), a Forward Telemetry Controller (FTC)  1035 , a Back Telemetry Controller (BTC)  1040 , Input/Output Ports  1045 , Memory Devices like a Parallel Flash Memory (PFM)  1050  and a Serial Flash Memory (SFM)  1055 , a Real Time Clock  1060 , an RF Voltage and Current Monitoring Circuit (VIMC) (not shown), a speaker and/or a buzzer, an RF receiver  1065 , and an RF transmitter  1070 . 
         [0081]    The Power Supply, Distribution and Monitoring Circuit (PSDM)  1005  may regulate a variable battery voltage to several stable voltages that apply to components of the VPU  20 . The Power Supply, Distribution and Monitoring Circuit (PSDM)  1005  may also provide low battery monitoring and depleted battery system cutoff. The Reset Circuit  1010  may have reset inputs  1011  that are able to invoke system level rest. For example, the reset inputs  1011  may be from a manual push-button reset, a watchdog timer expiration, and/or firmware based shutdown. The System Main Clock (SMC) source is a clock source for DSP  1020  and CPLD. The Video Preprocessor Clock (VPC) source is a clock source for the Video Processor. The DSP  1020  may act as the central processing unit of the VPU  20 . The DSP  1020  may communicate with the rest of the components of the VPU  20  through parallel and serial interfaces. The Video Processor  1075  may convert the NTSC signal from the camera  13  into a down-scaled resolution digital image format. The Video Processor  1075  may comprise a video decoder (not shown) for converting the NTSC signal into high-resolution digitized image and a video scaler (not shown) for scaling down the high-resolution digitized image from the video decoder to an intermediate digitized image resolution. The video decoder may be composed of an Analog Input Processing, Chrominance and Luminance Processing and Brightness Contrast and Saturation (BSC) Control circuits. The video scaler may be composed of Acquisition control, Pre-scaler, BSC-control, Line Buffer and Output Interface. The I 2 C Protocol Controller  1030  may serve as a link between the DSP  1020  and the I 2 C bus. The I 2 C Protocol Controller  1030  may be able to convert the parallel bus interface of the DSP  1020  to the I 2 C protocol bus or vice versa. The I 2 C Protocol Controller  1030  may also be connected to the Video Processor  1075  and the Real Time Clock  1060 . The VPDI  1025  may contain a tri-state machine to shift video data from Video Preprocessor  1075  to the DSP  1020 . The Forward Telemetry Controller (FTC)  1035  packs  1024  bits of forward telemetry data into a forward telemetry frame. The FTC  1035  retrieves the forward telemetry data from the DSP  1020  and converts the data from logic level to biphase marked data. The Back Telemetry Controller (BTC)  1040  retrieves the biphase marked data from the RF receiver  1065 , decodes it, and generates the BFSR, BCLKR and BDR for the DSP  1020 . The Input/Output Ports  1045  provide expanded 10 functions to access the CPLD on-chip and off-chip devices. The Parallel Flash Memory (PFM)  1050  may be used to store executable code and the Serial Flash Memory (SFM)  1055  may provide Serial Port Interface (SPI) for data storage. The VIMC may be used to sample and monitor RF transmitter  1070  current and voltage in order to monitor the integrity status of the retinal stimulation system. 
         [0082]    Accordingly, what has been shown is an improved visual prosthesis and an improved method for limiting power consumption in a visual prosthesis. While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.