the following description is of the best mode presently contemplated for carrying out the invention . this description is not to be taken in a limiting sense , but is made merely for the purpose of describing the general principles of the invention . the scope of the invention should be determined with reference to the claims . fig1 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 . 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 electronics package 14 and secondary inductive coil 16 are held together by a molded body 18 . 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 . the preferred prosthesis includes an external portion ( not shown ) which includes a camera , video processing circuitry and an external coil for sending power and stimulation data to the implanted portion . fig2 a - d show typical perceptual responses collected from four patients . the perceptual responses differ in both the amplitude of the response curve and the shape of the response curve . all four patient perceptual responses , however , can be fitted by the function b = ai b where b is brightness , i is current amplitude , and a and b are parameters to be estimated from fitting the empirical data . three data points will adequately define the function . numerous statistical tools are available for automatically fitting the function to the three data points . in this example the x axis represent the amplitude of stimulation using a single pulse . the y axis represents the patient &# 39 ; s subjective rating of brightness where a stimulus rated as “ 10 ” is twice as bright as a stimulus rated as “ 5 ”. fig3 shows a flow chart of the fitting procedure . in this case , we are using patient &# 39 ; s ratings of subjective brightness but a measure of neural actively such a neural recording or pupil response ( described below ) could be used in an analogous fashion . first the fitting system must determine the perceptual brightness response to current relationship . this is accomplished by stimulating and measuring the subject reported brightness response rating at three points . it should be noted that the response is near linear in most cases . hence , two points can be used to approximate the response , but three points will yield a more accurate fit . first , a stimulus is presented 23 . if there is no response 24 , the stimulus is increase 25 and stimulation is presented again 23 . if there is a response to stimulus 24 and the response is pain 26 , the stimulation is reduced 27 and stimulation is presented again 23 . if there is a non - painful response it is recorded 28 in in non - volatile memory of the prosthesis device . recording the response may include subjective response , neural recording or other physiological response . this process is repeated to get the required number of recorded responses , usually 3 . if there are three recorded responses 29 , an equation or formula is derived to describe the relationship between current and brightness relationship 30 . the formula may be saved as an actual equation to be applied to the input value , or as a table of input and output values . it should also be noted that there must be a maximum charge limit sent in a visual prosthesis for safety reasons . hence , the current variations must be limited by the preprogrammed maximum change . in the preferred embodiment , current is mapped to brightness . it should be noted that other a factors which may affect brightness , such as voltage , pulse width or frequency , may be mapped by the same method . after the formula is established , input is received by the camera 31 ; the formula is applied to input data 32 ; and an output value is used to stimulate neural tissue 33 . fig4 is a flow chart of an automatic fitting sequence which may be employed to gain the three points needed for the method described in fig3 , or may be used as an alternative fitting procedure . in the flow chart , the value n is the selected electrode , x is the neural activity recorded , and l is the level of stimulation ( current amplitude . first n is set to 0 40 and then incremented 42 . the first electrode , electrode n , is addressed 44 . the stimulation level is set to zero 46 , and then incremented 48 . the neural tissue is stimulated at the minimum level 50 . the stimulation is immediately followed by a recording of activity in the neural tissue 52 . alternatively , recording can be done simultaneously by an adjacent electrode . if recording is done simultaneously , one must distinguish between neural activity and electrical charge from the stimulating electrode . the neural response follows stimulation ( see fig6 ). 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 54 , the stimulation level is increased and steps 48 - 54 are repeated . in most cases , the preset minimum level is any measurable neural activity . however , perception by the patient is the determining factor . if neural activity is detected and the patient reports no perception , the minimum level must be set higher . once minimum neural activity is recorded , the stimulation level is saved in memory 56 . the level is then further increased 58 and stimulation is repeated 60 . again stimulation is immediately followed by recording neural activity 62 . if a predetermined maximum level has not been reached , steps 58 - 64 are repeated until the predetermined maximum stimulation level is obtained . once the predetermined maximum stimulation level is obtained , steps 42 - 64 are repeated for the next electrode . the process is continued until a minimum and maximum stimulation level is determined for each electrode 66 . to obtain the subjective brightness or neural response for the necessary three points , one first finds the stimulus amplitude ( the intensity of the stimulus can also be varied along other dimensions ) which is barely detectable by the patient or provokes a minimally detectable neural response . one then presents the stimulus at that value ( e . g . the amplitude value v = 42 ) repeatedly until one has an accurate measurement of the subjective brightness or neural response at that stimulus intensity . one then finds the stimulus amplitude that is just under the safety limit or pain threshold , and measures the apparent brightness or neural response at that stimulus intensity . finally one finds apparent brightness or neural response for a stimulus whose amplitude is halfway ( or intermediate ) between those two points . if additional data points are desired , equal distant points such as 25 % and 75 % should be used . the range of intensities used for stimulation during operation of the device will fall within the range that is measured during the fitting procedure . very low or high intensity values may not be used in normal function /. 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 . 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 . often , neural response in a retina is based , in part , on geographical closeness . that is , neurons closer to the fovea require less stimulation than neurons farther from the fovea . 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 . automating the fitting process has many advantages . it greatly expedites the process reducing the efforts of the patient and clinician . further , the automated process based on measured neural responses is objective . patient perceptual responses are subjective and may change over time due to fatigue . in some cases , a patent may not be able to provide the required responses due to age , disposition , and / or limited metal ability . fig5 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 80 sets and increments a counter 82 to control the stimulation level of the stimulator 84 . the stimulation signal is multiplexed in mux 86 to address individual electrodes 88 . after each stimulation , the addressed electrode returns a neural activity signal to a recorder 90 . the signal is compared to the stored minimum or maximum level ( stored in a memory 92 ) in a comparator 94 . after programming , a signal from a video source 96 , or other neural stimulation source , is adjusted in a mapping unit 98 , in accordance with the minimum and maximum levels stored in the memory 92 . the adjusted signal is sent to the stimulator 84 , which in synchronization with mux 86 applies the signal to the electrodes 88 . the electronics for the control unit could be external or within the implanted prosthesis . fig6 is a graphical representation of the neural response to electrical stimulus . this figure is derived from actual recordings of a frog retina . response in a human retina will be similar and can be measured by a retinal electrode , implanted cortical electrode , or external cortical electrode commonly known as a visual evoked response or vep . 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 . either the stimulating electrode must be switched to a detecting electrode during the brief delay or detecting must occur on another electrode and continue long enough to record the neural response . it should also be noted that the delay period 110 becomes shorter with increased stimulation current . hence , the system must switch faster from stimulation mode to detecting mode 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 . as stimulation increases , it becomes more useful to employ an alternate recording means as described in the following alternate embodiments . in a first alternate embodiment , the recording electrode may be cortical electrode mounted on or near the visual cortex . temporary external electrodes placed on the scalp proximate to the visual cortex may record neural activity in the visual cortex . this allows the system to account for any variations in neural processing between the retina and the visual cortex . it , however , requires electrodes either implanted in the visual cortex or placed temporarily near the visual cortex . this alternate embodiment may be combined with the preferred embodiment by first using cortical electrodes to perform an initial fitting of the prosthesis in a clinic . thereafter , retinal recording may be used to readjust the prosthesis for any changes over time . fig7 shows the first alternate retinal prosthesis . a stimulating electrode array 150 is placed against the outer surface of a retina 152 ( epiretinally ). a cable 154 pierces a sclera 156 and attaches to an electronic control unit 158 . a return electrode 160 may be placed distant from the retina 152 . the stimulating electrode array 150 is a plurality of tiny electrodes . one or more recording electrodes 162 are placed in near the visual cortex . the recording electrodes may be temporary external electrodes , implanted electrodes under the scalp , or electrode implanted within the visual cortex . in a second alternate embodiment , the recording electrode may be either implanted in the iris , or placed externally near the iris . the iris contracts when increasing light levels enter the eye . electrical stimulation of the retina also causes the iris to contract , because the body perceives an increase in light entering the eye . conversely , the iris expands in response to a decrease in electrical stimulation . while the response of the iris is relatively slow , the neurological signals initiating a change in the iris respond quickly . measuring these signals may provide alternate feed back as to the body &# 39 ; s response to the electrical stimulus . alternatively , an optical device aimed at the eye may detect the diameter of the iris . fig8 shows the second alternate retinal prosthesis . a stimulating electrode array 210 is placed against the outer surface of a retina 212 ( epiretinally ). a cable 214 pierces a sclera 216 and attaches to an electronic control unit 218 . a return electrode 220 may be placed distant from the retina 212 . the stimulating electrode array 210 is a plurality of tiny electrodes . a recording electrode 224 is place in the periphery of the iris sensing electrical stimulus to the iris . in a third alternate device , electroluminescent pigments may be applied to the retina . electroluminescent pigments cause an individual cell to glow when it fires an action potential . a camera of the type used for retinal photos may detect neural response by detecting the electroluminescent glow of the applied pigment . accordingly , what has been shown is an improved method of stimulating neural tissue for improved response to brightness . 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 .
the invention is a method of automatically adjusting an electrode array to the neural characteristics of an individual patient . the perceptual response to electrical neural stimulation varies from patient to patient and the response to electrical neural stimulation varies from patient to patient and the relationship between current and perceived brightness is often non - linear . it is necessary to determine this relationship to fit the prosthesis settings for each patient . it is advantageous to map the perceptual responses to stimuli . the method of mapping of the present invention is to provide a plurality of stimuli that vary in current , voltage , pulse duration , frequency , or some other dimension ; measuring and recording the response to those stimuli ; deriving a formula or equation describing the map from the individual points ; storing the formula ; and using that formula to map future stimulation .