Abstract:
The present invention is an improved system for use of eye tracking including spatial mapping percepts in a visual prosthesis by presenting an electrically induced precept through a visual prosthesis, requesting a subject look to the direction of the percept and tracking their eye movement. Eye movement is both faster and more accurate than asking a visual prosthesis user to point to the location of a percept. This method can be beneficial in a retinal prosthesis, but is particularly useful in a cortical visual prosthesis where visual cortex does not match the retinotopic map. Methods are presented for calibrating an eye tracker. Eye tracking hardware may also be used for blanking video information base on the subject&#39;s natural blink reflex.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application incorporates by reference and claims priority to U.S. Provisional Application 62/298,390, for Spatial Mapping in a Visual Prosthesis by Tracking Eye Movement, filed Feb. 22, 2016. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to visual prostheses configured to provide neutral stimulation for the creation of artificial vision, and more specifically, new uses for eye tracking including an improved method of spatial mapping by tracking eye movement in a visual prosthesis and other applications of an eye tracker in a visual prosthesis. 
       BACKGROUND 
       [0003]    A visual prosthesis is an electronic neural stimulator that stimulates visual percepts with an array of electrodes, typically on the retina, LNG or visual cortex. Current technology for implantable neural stimulators and electrode arrays is quite limited, while high resolution video cameras are quite inexpensive. Cameras are typically mounted on the head, such as on a pair of glasses. This causes the user to scan with their head to observe a scene. It was suggested in 1996 (see Toward an Artificial Eye, IEEE Spectrum May 1996) that an eye tracker can be used to move the prosthesis field of view around a scene output by the camera to obtain more natural scanning by the visual prosthesis user. 
         [0004]    U.S. Pat. No. 7,574,263 teaches methods correcting spatial distortions in a visual prosthesis. While U.S. Pat. No. 7,574,263 teaches how to correct distortions, the method is manual and time consuming. 
         [0005]    U.S. Pat. No. 9,186,507 teaches that constant stimulation of neural tissue results in a gradual fading of percepts. It is advantageous to provide occasional breaks in neural stimulation to reset the neural pathway. 
       SUMMARY 
       [0006]    The present invention is an improved system for use of eye tracking including spatial mapping percepts in a visual prosthesis by presenting an electrically induced precept through a visual prosthesis, requesting a visual prosthesis user (subject) look to the direction of the percept and tracking their eye movement. Eye movement is both faster and more accurate than asking a subject to point to the location of a percept. This method can be beneficial in a retinal prosthesis, but is particularly useful in a cortical visual prosthesis where visual cortex does not match the retinotopic map. Methods are presented for calibrating an aligning an eye tracker. Eye tracking hardware may also be used for blanking video information base on the subject&#39;s natural blink reflex. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure. 
           [0008]      FIG. 1  shows a schematic view of an electrode array indicating a first stimulation test. 
           [0009]      FIG. 2  shows a schematic view of an electrode array indicating a second stimulation test. 
           [0010]      FIG. 3  shows the pupil&#39;s relative location between the start and end of the first stimulation test. 
           [0011]      FIG. 4  shows the pupil&#39;s relative location between the start and end of the second stimulation test. 
           [0012]      FIG. 5  shows the pupil&#39;s absolute location at the end of the first stimulation test. 
           [0013]      FIG. 6  shows the pupil&#39;s absolute location at the end of the second stimulation test. 
           [0014]      FIG. 7  shows the relationship of eye position to direction of gaze. 
           [0015]      FIG. 8A  shows recorded data without eye movement correction, shading matches the stimulation groups in the  FIG. 8B   
           [0016]      FIG. 8B  shows a schematic view of an electrode array include electrodes being stimulated. 
           [0017]      FIG. 8C  shows correction according to pupil location based on the simple model while coefficients were calculated based on linear regression. 
           [0018]      FIG. 8D  is a magnified version of  FIG. 8C . 
           [0019]      FIG. 8E  shows correction according to pupil location based on the simple model while coefficients were calculated based on a solver algorithm. 
           [0020]      FIG. 8F  is a magnified version of  FIG. 8E . 
           [0021]      FIG. 8G  shows correction according to pupil location based on the simple model while coefficients were calculated based on a solver algorithm. 
           [0022]      FIG. 8H  is a magnified version of  FIG. 8G . 
           [0023]      FIG. 9  show a video capture/transmission apparatus or visor adapted to be used in combination with the retinal stimulation of  FIGS. 16 and 17 . 
           [0024]      FIG. 10  shows components of a fitting system according to the present disclosure, the system also comprising the visor shown in  FIGS. 6 and 7 . 
           [0025]      FIG. 11  shows the external portion of the visual prosthesis apparatus in a stand-alone mode, i.e. comprising the visor connected to a video processing unit. 
           [0026]      FIGS. 12-1, 12-2, 12-3 and 12-4  show an exemplary embodiment of a video processing unit.  FIG. 12-1  should be viewed at the left of  FIG. 12-2 .  FIG. 12-3  should be viewed at the left of  FIG. 12-4 .  FIGS. 12-1 and 12-2  should be viewed on top of  FIGS. 12-3 and 12-4 . 
           [0027]      FIG. 13  is a perspective view of the implanted portion of the preferred visual prosthesis. 
           [0028]      FIG. 14  is a perspective view of the implanted portion of a cortical visual prosthesis. 
           [0029]      FIG. 15  is the perspective view of  FIG. 20  adding the location of the electrodes and the coil. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The present invention includes an improved spatial fitting and mapping system for a visual prosthesis. The system of the present invention maps projected locations of percepts, where a person perceives a percept from a visual prosthesis to the intended location of the percepts. The projected location may vary over time. This test results can be used to correct a visual prosthesis or spatially map the visual prosthesis. 
         [0031]      FIG. 1  shows a schematic view of an electrode array indicating a first stimulation test. Note that the image of the electrodes is tilted to match the observed tilt of the electrode array on a retina. Each test includes three stimulation patterns  102 ,  104  and  106 . 
         [0032]      FIG. 2  shows a schematic view of an electrode array indicating a second stimulation pattern. Again the array has the same tilt. The second test includes patterns  108 ,  110 , and  112 . 
         [0033]      FIG. 3  shows the pupil&#39;s relative location between the start and end of the trial with the first stimulation pattern. In each case the subject is asked to begin by look straight forward, and the look toward the location of a percept. Stimulation pattern  102  induces eye movement  114 ; stimulation pattern  104  includes eye movement  116 , and stimulation pattern  106  induces eye movement  118 . 
         [0034]      FIG. 4  shows the pupil&#39;s relative location between the start and end of the trial with the second stimulation pattern. In each case the subject is asked to begin by look straight forward, and the look toward the location of a percept. Stimulation pattern  108  induces eye movement  120 ; stimulation pattern  110  includes eye movement  122 , and stimulation pattern  112  induces eye movement  124 . 
         [0035]      FIG. 5  shows the pupil&#39;s absolute location at the end of the trial with the first stimulation pattern. Stimulation pattern  102  induces a percept at location  126 ; stimulation pattern  104  includes a percept at location  128 , and stimulation pattern  106  induces a percept at location  130 . 
         [0036]      FIG. 6  shows the pupil&#39;s absolute location at the end of the trial with the second stimulation pattern. Stimulation pattern  108  induces a percept at location  132 ; stimulation pattern  110  includes a percept at location  134 , and stimulation pattern  112  induces a percept at location  136 . 
         [0037]    The data shown in  FIGS. 1-6  is the result of experiments conducted by the applicants. The experimental setup consisted of two computers that were powered by the internal batteries. The first computer generated the stimulation patterns shown in  FIGS. 1 and 2 , and the second recorded video images of the pupil shown in  FIGS. 3 through 6 . Pupil recording and stimulation had synchronized timestamps. Stimulation patterns for each trial were created to stimulate four electrodes. A binary large object (BLOB) image was created on the computer using MATLAB and delivered to the Argus II system by connecting to the camera port using a VGA to NTSC adapter (Startech, VGA2VID). The image was such that there was white level over the four stimulated electrodes while the rest of the image was black. Pupil images were acquired at 30 frames per second using a USB camera (Microsoft, HD-6000) with an IR pass filter to block the room lighting in the visible spectrum. The pupil was illuminated by an IR LED (Osram, SFH 4050-Z). The pupil camera was mounted on a modified Pupil Lab frame (www.pupil-labs.com). The transmission coil of the Argus II (see  FIG. 1 ) was taped to the Pupil Lab frame. We didn&#39;t use the Argus II eye-wear since the stimulation patterns were created in the computer and there was no need for the camera. The stimulation pattern images were delivered to the recording computer using a VGA to USB adapter (Epiphan, DVI2USB3.0). The video streams from the pupil camera and the stimulation pattern were streamed and saved on the recording computer that had a Linux (Ubuntu 14.02) operating system. In addition to the video files, the application saved a META file with a timestamp for each frame. 
         [0038]    Stimulation waveforms on each electrode were set according to the programming that is used by the patient in daily activities with the Argus II. 
         [0039]    In each session, three patterns were interleaved and each pattern consisted of four neighboring electrodes. Stimulation duration was set to 0.6 s and the patient was instructed to move her eye to where she saw the light. The patient was remind to look straight at beginning of each trial. 
         [0040]    For each trial, we located the pupil frames at the time of the stimulation. The frames were presented on the computer (for example, see  FIGS. 3-6 ). We manually marked the pupil location at the beginning of the stimulation and in a resting position after the eye moved to the percept&#39;s location. It is worthwhile to note that data is presented in pixel coordinates of the pupil camera and not in degrees of gaze position. In order to convert the pupil location to pixel coordinates, the eye tracker had to be calibrated which is not trivial for blind patients (see  FIG. 7 ). 
         [0041]      FIGS. 1-6  shows examples of two sessions in which three different patterns where measured in each session.  FIGS. 1 and 2  show the array&#39;s layout with the electrodes that were stimulated in each session. The layout was rotated to account for the placement as measured by fundus imaging. The electrodes are pattern grouped, indicating the four electrodes that were stimulated in each trial.  FIGS. 3 and 4  show the relative spatial map, calculated as the difference in pupil position from the beginning of the trial to the end of the trial. In this case, the data were adjusted so that location of the pupil at the beginning of the trial was defined as the origin (0, 0).  FIGS. 5 and 6  show the absolute spatial map, as calculated by the absolute pupil location at the end of the trial. In this case, we ignored the pupil location at the beginning of the trial. For convenience sake, the pupil locations were adjusted so that the average across all patterns in all trials was at the origin. 
         [0042]    It can be seen that the spreads of the marked locations in the relative analysis ( FIGS. 3 and 4 ) method are narrower compared to the absolute case ( FIGS. 5 and 6 ). Comparing the standard error of the pupil location between the relative and absolute methods using a paired t-test showed a significance of p=0.03. 
         [0043]    In order to quantify the measured spatial map, we compared the relative orientation between the vectors  109  in  FIGS. 1 and 2 . For the first session the angle between the vectors, calculated from the layout of the array, is 101 deg. compared to 107 deg. that was calculated based on the measured spatial map. For the second session the angle between the vectors calculated from the layout of the array is 59 deg. compared to 50 deg. that was calculated from the measured spatial map. The relatively small discrepancy of 6 and 9 deg. in the measured orientation can be attributed to the fact that we estimated the orientation from pupil location and not from gaze orientation. The scaling on the horizontal and vertical dimensions from pupil location to gaze orientation probably is not the same. It is worthwhile to mention that the differences between theoretical and measured vector orientations are better than the average response error of a motion detection task of good performing implanted patients. Our experiment shows the feasibility in using eye movements as markers to measure the spatial mapping of a visual prosthesis. We observed that the relative pupil location is more confined relative to the absolute pupil location. This suggests that patients perceive the location of the phosphene relative to the instantaneous gaze position at the time of the stimulation. The relative location of the patterns we mapped matches the location on the array. Hence, the oculomotor system of a blind patient still functions and the patient can direct the gaze to the location of the phosphene. 
         [0044]    An eye tracker can be calibrated for blind patients. For example, we will analyze the glint of an array of LEDs in addition to pupil location. However, data presented here shows that we can spatially map the percept of a visual implant based on pupil location without gaze calibration.
       Referring to  FIG. 7 , in order to calibrate an eye tracker with a blind user of a visual prosthesis, i.e. finding the parameters that will convert pupil coordinates to world coordinates, we perform the following:   1. Have a system that will acquire pupil location and scene, front-facing camera with timestamps synchronized with stimulation of the visual prosthesis.   2. The world camera will be used to measure the location of the percept by locating the position of a hand-held marker in the space.   3. The system will stimulate a group of electrodes and ask the patient to place a hand-held marker that will mark the location of the perceived stimulation.   4. Repeat step #1 for several eye positions.   5. Based on the transformation model between pupil and gaze coordinates, find the parameters that will minimize the spread of pattern location for each group of electrodes.   The parameters that were found will be used to convert pupil to gaze in order to steer the line-of-sight of the prosthesis in real-time.
 
The location of the percept due to an electrical stimulation is a function of two factors:
 
The location of the stimulation on the retina and
 
The orientation of the eyeball (i.e. gaze).
       
 
         [0000]        X   world ( p,i )= X   implant   0   +X   pattern   0 ( p )+ X   gaze ( i ) 
         [0000]        Y   world ( p,i )= Y   implant   0   +Y   pattern   0 ( p )+ Y   gaze ( i ) 
       Where: 
       [0000]    
       
         
           
             X pattern   0 (p); Y pattern   0 (p) the location of a pattern p relative to the center of the implanted array 
             X implant   0 ; Y implant   0  implant the location of a pattern p relative to the center of the implanted array 
             X gaze (i); Y gaze (i) the location of a pattern p relative to the center of the implanted array 
             X precept (p,i); Y precept (p,i) the location of a pattern p relative to the center of the implanted array 
           
         
       
     
         [0056]    Simple Model: 
         [0000]        X   gaze ( i )= a   1   ·X   pupil ( i )+ a   0    
         [0000]        Y   gaze ( i )= b   1   ·Y   pupil ( i )+ b   0          Need to find four independent variables a 0 , a 1 , b 0 , b 1            
         [0058]    Advance Model: 
         [0000]        X   gaze ( i )= a   1   ·X   pupil ( i )+ a   2   ·Y   pupil ( i )+ a   0    
         [0000]        Y   gaze ( i )= b   1   ·Y   pupil ( i )+ b   2   ·X   pupil ( i )+ b   0          Need to find six independent variables a 0 , a 1 , a 2 , b 0 , b 1 , b 2            
         [0060]    We will get for the simple model: 
         [0000]        X   world ( p,i )= X   implant   0   +X   pattern   0 ( p )+ a   1   ·X   pupil ( i )+ a   0    
         [0000]        Y   world ( p,i )= Y   implant   0   +Y   pattern   0 ( p )+ b   1   ·Y   pupil ( i )+ b   0    
         [0061]    Or for the advanced model: 
         [0000]        X   world ( p,i )= Y   implant   0   +X   pattern   0 ( p )+ a   1   ·X   pupil ( i )+ a   2   ·Y   pupil   +a   0    
         [0000]        Y   world ( p,i )= Y   implant   0   +Y   pattern   0 ( p )+ b   1   ·Y   pupil ( i )+ b   2   ·X   pupil   +b   0    
         [0062]    X pattern   0 (p); Y pattern   0 (p)
       Is the theoretical of percept&#39;s location that pattern p will generate in coordinates relative to line-of-sight of the retina       
 
         [0064]    X world (p,i); Y world (p,i)
       Mark the theoretical location of the percept at trial i when pattern p is on Moving to real life       
 
         [0066]    X world (p,i); Y world (p,i)
       Is the actual percept&#39;s location that pattern p generated at trial i in coordinates array relative to line-of-sight of the retina       
 
         [0068]    X world   M (p,i); Y world   M (p,i)
       Mark the measured location of the percept at trial i when pattern p is on Preliminary results of patient testing is shown in  FIG. 8 :
 
 FIG. 8A  shows recorded data without eye movement correction, patterns match the stimulation groups in  FIG. 8B , note that the marked location for each group is not distinct.
 
 FIG. 8B  shows electrodes stimulated on the array.
 
 FIG. 8C  shows correction according to pupil location based on the simple model while coefficients where calculated based on linear regression.  FIG. 8D  is a magnified version of  FIG. 8C .
 
 FIG. 8E  shows correction according to pupil location based on the simple model while coefficients where calculated based on a solver algorithm.  FIG. 8F  is a magnified version of  FIG. 8E .
 
 FIG. 8G  shows correction according to pupil location based on the simple model while coefficients where calculated based on a solver algorithm.  FIG. 8H  is a magnified version of  FIG. 8G .
 
The layout of the array with the stimulated groups was rotated to account for the placement of the array on the retina.
       
 
         [0070]    For spatial fitting purposes, only the average value of the many trails is important. As can be seen in  FIGS. 8A through 8H , tests produce a range of values. The spread of these values can be used to guide down sampling. As noted above, the camera produced a much higher resolution image than the available electrode array. To get from the higher resolution camera to the lower resolution electrode array, down sampling is required. The range of the test samples is indicative of the area the subject perceives as the same place and indicative of the number of camera pixels that can be assigned to a single electrode. 
         [0071]    When eye tracking for spatial fitting or for normal use to alter video data according to the gaze of the eye, it is important record only when the eye is stationary. In addition to intentional eye movement to look at something the eye constantly moves in involuntary micro-saccades. An eye tracking camera does not measure eye movement, but samples eye location at regular intervals. Regardless of the sample rate, samples at the same location for more than 50 milliseconds indicate the end of the eye movement or saccade. 
         [0072]    As described in U.S. Pat. No. 9,186,507, stimulation of neural percepts fade with continuous stimulation. The 507 patent teaches multiple ways of interrupting stimulation to reset neural pathways. An eye tracking camera can also function as a blink detector. Interrupting stimulation each time the eye lid closes provides a natural reset of the neural pathways. It should be clear to one of skill in the art that other blink detectors are possible such as a light detector that measure the reduction of reflected light of the eye lid versus the eye or an electrical sensor that senses activation of the eye lid muscle. In additional to the reset of a natural blink, this is give visual prosthesis user an intuitive way to stop stimulation such as in response to a bright light or fatigue. A range of physiological changes may be detected and used to trigger an interruption of stimulation. A saccade can also be used as a signal to interrupt stimulation. Stimulation of an image mid-saccade provides little benefit and may be confusing to the user. It should also be noted that eye tracking sensors other than a camera can also be used. 
         [0073]    Referring to  FIG. 9 , the glasses  5  may comprise, for example, a frame  11  holding a camera  12 , an external coil  14  and a mounting system  16  for the external coil  14 . The mounting system  16  may also enclose the RF circuitry. In this configuration, the video camera  12  captures live video. The video signal is sent to an external Video Processing Unit (VPU)  20  (shown in  FIGS. 10, and 11  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  14  that sends both data and power via radio-frequency (RF) telemetry to the coil  2016  of the retinal stimulation system  1 , shown in  FIG. 13 . The coil  2016  receives the RF commands which control the 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  12 . 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  1 , which results in the retinal cells being stimulated via the electrodes in the electrode array  2010  (shown in  FIG. 13 ). In one exemplary embodiment, the electrical stimulation patterns or data being transmitted by the external coil  14  is binary data. The external coil  14  may contain a receiver and transmitter antennae and a radio-frequency (RF) electronics card for communicating with the internal coil  2016 . 
         [0074]    Referring to  FIG. 10 , a Fitting System (FS) may be used to configure and optimize the visual prosthesis apparatus shown in  FIG. 13 . 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. 
         [0075]    The Fitting System may comprise custom software with a graphical user interface running on a dedicated laptop computer  10 . 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. 
         [0076]    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. 
         [0077]    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 visual prosthesis for each subject. 
         [0078]    The Fitting System laptop  10  of  FIG. 10  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. 
         [0079]    As shown in  FIG. 10 , 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). 
         [0080]    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 . 
         [0081]    In one exemplary embodiment, the Fitting System shown in  FIG. 10  may be used to configure system stimulation parameters and video processing strategies for each subject outfitted with the visual prosthesis apparatus of  FIG. 11 . The fitting application, operating system, laptops  10  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  1 . 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. 10  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. 
         [0082]    The visual prosthesis apparatus may operate in two modes: i) stand-alone mode and ii) communication mode 
         [0083]    Stand-Alone Mode 
         [0084]    Referring to  FIG. 11 , in the stand-alone mode, the video camera  12 , on the glasses  5 , captures a video image that is sent to the VPU  20 . The VPU  20  processes the image from the camera  12  and transforms it into electrical stimulation patterns that are transmitted to the external coil  14 . The external coil  14  sends the electrical stimulation patterns and power via radio-frequency (RF) telemetry to the implanted retinal stimulation system. The internal coil  2016  of the retinal stimulation system  1  receives the RF commands from the external coil  14  and transmits them to the electronics package  2014  that in turn delivers stimulation to the retina via the electrode array  2010 . Additionally, the retinal stimulation system  1  may communicate safety and operational status back to the VPU  20  by transmitting RF telemetry from the internal coil  2016  to the external coil  14 . The visual prosthesis apparatus of  FIG. 11  may be configured to electrically activate the retinal stimulation system  1  only when it is powered by the VPU  20  through the external coil  14 . The stand-alone mode may be used for clinical testing and/or at-home use by the subject. 
         [0085]    Communication Mode 
         [0086]    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  1  as is done for example in the stand-alone mode described above. Referring to  FIG. 10 , in the communication mode, the VPU  20  is connected to the Fitting System laptop  10  using cables  70 ,  45  and the optically isolated serial connection adapter  40 . In this mode, laptop  10  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  10  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. 
         [0087]    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  14 , without the glasses  5 , placed in close proximity to the retinal stimulation system  1 . The coil  14  may communicate the status of the retinal stimulation system  1  to the VPU  20  that is connected to the Fitting System laptop  10  as shown in  FIG. 10 . 
         [0088]    As discussed above, the VPU  20  processes the image from the camera  12  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  2010  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. 
         [0089]    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  1  and the backward telemetry refers to transmissions from the Retinal stimulation system  1  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  1  via the back telemetry. In one embodiment, an audio alarm may be used to indicate whenever the synchronization has been lost. 
         [0090]    In order to supply power and data to the Retinal stimulation system  1 , the VPU  20  may drive the external coil  14 , for example, with a 3 MHz signal. To protect the subject, the retinal stimulation system  1  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. 
         [0091]    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  2014  of the Retinal Stimulation System  1 . The ASIC of the Retinal Stimulation System  1  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  1  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. 
         [0092]    One exemplary embodiment of the VPU  20  is shown in  FIG. 12 . 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 . 
         [0093]    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  12  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 vise 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 and BCLKR for the DSP  1020 . The Input/Output Ports  1045  provide expanded JO 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  1 . 
         [0094]      FIG. 13  shows a perspective view of the implanted portion of the preferred visual prosthesis. A flexible circuit  2001  includes a flexible circuit electrode array  2010  which is mounted by a retinal tack (not shown) or similar means to the epiretinal surface. The flexible circuit electrode array  2010  is electrically coupled by a flexible circuit cable  2012 , which pierces the sclera and is electrically coupled to an electronics package  2014 , external to the sclera. 
         [0095]    The electronics package  2014  is electrically coupled to a secondary inductive coil  2016 . Preferably the secondary inductive coil  2016  is made from wound wire. Alternatively, the secondary inductive coil  2016  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  14 , which is external to the body. The electronics package  2014  and secondary inductive coil  2016  are held together by the molded body  2018 . The molded body  18  holds the electronics package  2014  and secondary inductive coil  16  end to end. The secondary inductive coil  16  is placed around the electronics package  2014  in the molded body  2018 . The molded body  2018  holds the secondary inductive coil  2016  and electronics package  2014  in the end to end orientation and minimizes the thickness or height above the sclera of the entire device. The molded body  2018  may also include suture tabs  2020 . The molded body  2018  narrows to form a strap  2022  which surrounds the sclera and holds the molded body  2018 , secondary inductive coil  2016 , and electronics package  2014  in place. The molded body  2018 , suture tabs  2020  and strap  2022  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  2016  and molded body  2018  are preferably oval shaped. A strap  2022  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. 
         [0096]    While the description of the external portion of a visual prosthesis is described in terms of a retinal stimulator, the description is equally applicable to a cortical stimulator as shown in  FIG. 14 .  FIG. 14  shows a perspective view of an implantable portion of a cortical visual prosthesis.  FIG. 15  adds the locations of the electrodes and coil of the implantable portion. Note from this view the electrodes are show through the flexible circuit electrode array  2110 . That is the electrodes are on the other side. It is advantageous that the flexible circuit electrode array  2110  be made in a trapezoidal shape with the cable portion attached to the smallest side of the trapezoid. This shape better accommodates the target tissue on the medial surface of the visual cortex. The molded body  2119  holding the electronics package  2114  and the coil  2116  is arranged with the coil  2116  opposite the flexible circuit electrode array  2110 . The device is intended to be implanted with the flexible circuit electrode array  2110  attached on top of the package (toward the outside of the skull). This allows the electrodes to be on the same side of the flexible circuit electrode array  2110  as the bond pads connecting the flexible circuit electrode array  2110  to the electronics package  2114  and still face down toward the brain. The ceramic substrate portion of the electronics package  2114  to which the flexible circuit electrode array  2110  is attached is more delicate than the metal can portion. A mounting fixture  2115  covers and protects the electronics package  2114 , provides screw tabs for attaching the electronics package  2114  to the skull and further provides a heat sink to dissipate heat from the electronics package  2114 . The electronics package  2114 , coil  2116  and molded body  2118  are implanted within a hollowed out portion of the skull. Preferably the hollowed out portion does not go entirely through the skull. Only a small slot is needed to feed the flexible circuit electrode array  1210  through to its implanted location. This provides better protection to the brain than an implant where large portions of the skull are removed. The overall device is preferably about 9.7 cm in length. The electrode array portion  110  is preferably about 2.4 cm by 3.4 cm. The coil and electronics molded body is preferably 1.1 cm or less in width. Each electrode is preferably about 2 mm in diameter. 
         [0097]    It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. 
         [0098]    Accordingly, what has been shown is an improved visual prosthesis and an improved method for spatial fitting and image stabilization 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.