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 .