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 . a representative neural stimulation system 10 is shown in fig1 . such system typically comprises an implantable pulse generator ( ipg ) 12 , a lead extension 14 , an electrode lead 16 , and an electrode array 18 . the electrode array includes a plurality of electrode contacts 17 ( also referred to as “ electrodes ”). the electrodes 17 are arranged , for example , in an in - line array 18 near the distal end of the lead 16 . other electrode array configurations may also be used . the ipg 12 generates stimulation current pulses that are applied to selected ones of the electrodes 17 within the electrode array 18 . a proximal end of the lead extension 14 is removably connected to the ipg 12 , and a distal end of the lead extension 14 is removably connected to a proximal end of the electrode lead 16 , the electrode array 18 , is formed on a distal end of the electrode lead 16 . the in - series combination of the lead extension 14 and electrode lead 16 , carry the stimulation current from the ipg 12 to electrodes of the electrode array 18 . it should be noted that the lead extension 14 need not always be used with the neural stimulation system 10 . the lead extension 14 is only needed when the physical distance between the ipg 12 and the electrode array 18 requires its use . turning next to fig2 the neural stimulation system 10 is shown being used as a spinal cord stimulator ( scs ) system . in such configuration , the lead 16 , and more particularly the electrode array 18 , is implanted in the epidural space 20 of a patient so as to be in close proximity to the spinal cord 19 . due to the lack of space near the lead exit point 15 where the electrode lead 16 exits the spinal column , the ipg 12 is generally implanted in the abdomen or above the buttocks . the lead extension 14 facilitates locating the ipg 12 away from the lead exit point 15 . a more complete description of an scs system may be found in u . s . pat . no . 6 , 516 , 227 , which patent is incorporated herein by reference in its entirety . next , with respect to fig3 a , there is shown a functional block diagram of a system that automatically adjusts the amplitude of the stimulus current applied to neural tissue in accordance with the present invention . as seen in fig3 a , an electrode 17 is placed in close proximity to neural tissue 24 that is to be stimulated . the electrode 17 is electrically connected to a current pulse generator 13 which generates a stimulus pulse having a magnitude that is set by magnitude adjust circuitry 115 . the magnitude adjust circuitry 115 sets the magnitude of the stimulus pulse as specified by stimulation control circuitry 117 . the stimulation control circuitry 117 usually comprises some sort of processor , or state logic , that operates in accordance with a stored program or state diagram . it initially sets the magnitude of the stimulus pulse to a programmed or predetermined value . as the stimulus pulse is applied to the neural tissue 24 , an appropriate sensor s senses the coupling efficiency between the stimulus current and the neural tissue . that is , the sensor s provides a measure of how effective the applied stimulus is at stimulating the neural tissue 24 . the sensor s is connected to the magnitude adjust circuitry 115 so as to provide a feedback signal that indicates whether the magnitude of the stimulus needs to be adjusted up or down . for example , should the sensor s determine that very little of the energy is being coupled to the neural tissue 24 , then the feedback signal provided through the sensor s automatically causes the magnitude adjust circuitry 115 to increase the magnitude of the stimulus pulse so that the total energy coupled to the neural tissue 24 remains about the same . conversely , should the sensor s determine that more energy is being coupled to the neural tissue 24 , then the feedback signal provided through the sensor s automatically causes the magnitude adjust circuitry 115 to decrease the magnitude of the stimulus pulse so that the total energy coupled to the neural tissue 24 remains about the same . thus , it is seen that the magnitude adjust circuitry 115 automatically adjusts the magnitude , e . g ., amplitude , of the stimulus pulse so that the energy coupled to the neural tissue remains more or less the same . the sensor s may take various forms in accordance with the present invention . as described in more detail hereinafter , the sensor s may be , in one embodiment , a sensor that measures action potentials . in another embodiment , the sensor s may comprise optical circuitry that senses the optical properties of the body tissue surrounding or adjacent the target neural tissue that is stimulated . for example , the optical circuitry may comprise an optical transmitter and optical receiver that sense the reflective and transmissive properties of the body tissue surrounding the neural tissue . in accordance with this embodiment , optical energy of a prescribed frequency is emitted into the body tissue towards the target neural tissue 24 from an optical emitter included within the sensor s . preferably this optical energy is pulsed , but in some embodiments , it may be continuous . an optical receiver is placed in the sensor s near the optical emitter so as to receive any optical energy of the same frequency that is reflected from the body tissue and / or neural tissue . the amount of optical energy received by the optical receiver will vary as a function of the reflective and transmissive properties of the tissue through which the optical energy passes , and from which the optical energy reflects . these optical properties in turn vary as a function of the coupling efficiency between the stimulus current and the neural tissue . hence , a measure of these optical properties provides a measure of the coupling efficiency . in still further embodiments , the sensor s may sense electrical impedance , conductance , or other electrical properties indicative of coupling efficiency . in other embodiments , the sensor s may sense acoustical properties of the tissue surrounding the neural tissue 24 , e . g ., in order to determine the relative density , or amount of relative stress or pressure , within the tissue . in yet additional embodiments , the sensor s may sense the chemical properties , e . g ., ph , of the fluid in the epidural space . in one preferred embodiment of the present invention , as has been indicated , coupling efficiency is determined by measuring action potentials resulting from a stimulus pulse being applied to the neural tissue . thus , for purposes of such embodiment , the sensor s measures action potentials . [ 0037 ] fig3 b shows one method for generating and sensing action potentials in accordance with this embodiment of the invention . shown in fig3 b is a schematic representation of the distal end of the lead 16 , including four electrode contacts 17 a , 17 b , 17 c , and 17 d . ( it is to be understood that the lead 16 used with the neural stimulation system will have at least one electrode contact , and will usually have a plurality of electrode contacts , e . g ., 4 or 8 or 16 or more electrode contacts . the four electrode contacts shown in fig3 b is intended to be illustrative only , and not limiting . it is also to be understood that the physical and electrical connection with the electrode contacts 17 a , 17 b , 17 c and 17 d is typically made through wires 19 that pass through the body of the lead 16 . however , for simplicity of illustration in the schematic diagram of fig3 b , the electrical connection with the respective electrode contacts 17 a , 17 b , 17 c and 17 d is shown by wires external to the body of the lead 16 .) the electrode contacts 17 a , 17 b , 17 c and 17 d are positioned in the epidural space near the dorsal column fibers 24 within or near the spinal cord . in order to sense action potentials , stimulation pulses are applied from a pulse generator 30 , to a selected pair of the electrode contacts , e . g ., electrode contacts 17 a and 17 d . as connected in fig3 b , the polarity of the pulse generator 30 causes a current , represented by arrow 28 , to be emitted from electrode contact 17 d to the neural tissue 24 . the current 28 flows through the nerve tissue 24 and surrounding tissue and returns to the pulse generator 30 through electrode contact 17 a . the energy contained within the current 28 is coupled to the neural tissue 24 as a function of the coupling efficiency between electrode contacts 17 a , 17 d and the neural tissue 24 . this coupling efficiency may vary , for many reasons , such as postural changes , relative movement between the lead 16 and tissue 24 , or scare tissue maturation , to name just a few . as the tissue 24 is excited , or subjected to the stimulation current 28 , an action potential is created . such action potential is schematically represented in fig3 b by the wavy lines 32 . the action potential 32 may be sensed in various ways . one way is through electrode contacts 17 b , 17 c and a suitable pre - amplifier 40 . the output signal obtained from the pre - amplifier 40 thus becomes a measure of the sensed action potential . hence , by stimulating through one set of electrodes , 17 a , 17 d , and sensing through another set of electrodes , 17 b and 17 c , the action potential resulting from application of the current 28 to the nerve tissue 24 can be monitored . it is to be emphasized that the technique shown in fig3 b for sensing an action potential is only representative of various ways that could be used . for example , with appropriate gating circuitry , the action potential could also be sensed through the same electrode contact ( s ) that are used to apply the stimulation pulse . this is possible because the action potential associated with most excited nerve tissue typically follows application of the stimulus current by a short delay , e . g ., 0 . 1 to 3 milliseconds ( msec ). thus , through the use of appropriate gating circuitry that switches the action potential monitoring circuitry on only after the stimulus pulse has been applied , the same electrode contacts can be used for both purposes ( stimulating and monitoring ). alternatively , separate , dedicated electrode contacts may be used to monitor the action potential . such electrode contacts may be included on the lead 16 , or may be included on a separate lead . further , it should be noted that one electrode contact associated with stimulation and / or monitoring may be included as part of the case of the ipg 12 ( fig1 ), e . g ., as a ground or return electrode , in which instance both mono - polar stimulation and mono - polar sensing may be used . ( in contrast , the stimulating and sensing configuration illustrated in fig3 b represents bipolar stimulation and bipolar sensing .) in a similar manner , multi - polar stimulation and multi - polar sensing could also be employed . ( in this context , “ multi - polar ” refers to using multiple electrode contacts , i . e ., three or more , for stimulating and / or sensing .) finally , it should be pointed out that other types of sensors may be employed , in addition to electrode contacts connected to a sense amplifier , in order to sense the action potential or to sense other physiologic parameters that provide a measure of the coupling efficiency between the stimulating electrodes and the neural tissue . for example , a pressure transducer could sense changes in pressure that occur in the epidural space , or impedance measurements could be made between the stimulating electrodes , or chemical sensors ( e . g ., a ph sensor ) could measure changes in the chemistry of the fluids and tissue in the epidural space , or the like . turning next to fig4 there is shown a functional block diagram of a representative implantable pulse generator ( ipg ) 12 ( or , with respect to fig3 pulse generator 30 ) that may be used to practice the present invention . as seen in fig4 the ipg 12 is connected to a multiplicity of electrode contacts e 1 , e 2 , . . . en , where n represents an integer of at least 3 . the dotted - dashed line 102 in fig4 represents the boundary between the outside of the ipg case ( which is exposed to body tissues and fluids when the ipg is implanted ) and the inside of the ipg case ( which forms an hermetically sealed compartment wherein the electronic and other components are protected from the body tissues and fluids ). feed - through terminals 104 a , 104 b , . . . 104 n are thus used to provide an electrical path through the ipg case wall 102 . the feed - through terminals 104 a , 104 b , . . . are electrically connected to the electrodes e 1 , e 2 , . . . through wires within the lead 16 . thus , it is seen that each electrode contact e 1 , e 2 , . . . en is connected through a respective feed - through terminal 104 a , 104 b , . . . 104 n to a respective circuit node 106 a , 106 b , . . . 106 n within the hermetically sealed ipg case . this node , in turn is connected to a p - dac circuit 108 a and an n - dac circuit 110 a , as well as a sense amplifier 112 a . each of the other circuit nodes 106 b , . . . 106 n within the ipg similarly have a respective p - dac circuit , n - dac circuit , and sense amplifier connected thereto . a case electrode , case , may also be provided that effectively provides a common or return electrode that may be used with some stimulation and sensing configurations . in operation , in order to generate a stimulus current pulse that is applied between electrodes e 1 and e 2 , for example , the p - dac circuit 108 a , as controlled by control logic 120 over data bus 122 , causes a stimulation current having a specified amplitude to be emitted from the node 106 a , and hence to be emitted from the electrode contact el . at the same time , the n - dac circuit 110 b , similarly controlled by control logic 120 , causes a stimulation current of the same magnitude to be received through node 106 b , and hence through electrode contact e 2 . ( not shown in fig4 but assumed to be present , are coupling capacitors connecting the respective nodes 106 and feed - through terminals 104 .) in this way , a precisely controlled current is generated that flows from electrode contact e 1 to electrode contact e 2 through whatever body and nerve tissue resides between electrodes e 1 and e 2 . the duration of the current flow , i . e ., the width of the current pulse that is generated , is controlled by timer logic circuitry 124 . the operation of this output circuitry , including alternative embodiments of suitable output circuitry for performing the same function of generating current stimulus pulses of a prescribed amplitude and width , is described more fully in the above - referenced u . s . pat . no . 6 , 516 , 227 . each sense amplifier 112 a , 112 b , . . . 112 n connects its respective electrode node 106 a , 106 b , . . . 106 n to monitoring circuitry 126 . the monitoring circuitry 126 also monitors other signals 128 from various locations or components within the ipg , e . g ., battery voltage , charge current , etc . the control logic 120 , the timer logic 124 , and the monitoring circuit 126 are controlled or watched by a suitable micro - controller ( μc ) circuit 130 . the μc circuit 130 is coupled to the control logic 120 , the timer logic 124 , and the monitoring circuitry 126 over data buses 132 , 134 and 136 , respectively . suitable memory circuitry 140 is likewise coupled to the μc 130 , as is an oscillator and clock circuit 142 . the μc 130 , in combination with the memory circuit 140 and oscillator and clock circuit 142 , thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory 140 . alternatively , for some applications , the function provided by the microprocessor system may be carried out by a suitable state machine . power for the ipg is provided by way of a suitable power source 144 , such as a rechargeable battery . a power circuit 146 controls the charging or replenishment of the power source , as described more fully in the above - referenced u . s . pat . no . 6 , 516 , 227 . the power circuit 146 , the μc 130 and the monitoring circuitry 126 are also coupled to charging and telemetry circuitry 148 . an antenna coil 150 is likewise coupled to the telemetry circuitry 148 . it is through the antenna coil 150 that charging , forward telemetry and back telemetry signals may be received and sent to an external device , such as an external programmer or charging circuit , as described more fully in the above - referenced u . s . patent application , ser . no . 09 / 626 , 010 . ( in practice , separate coils may be used for charging , forward telemetry and back telemetry functions , as described more fully in the above - referenced u . s . pat . no . 6 , 516 , 227 , but for purposes of the present invention those distinctions are not relevant .) in fig4 the antenna coil ( s ) 150 is shown as being outside the hermetically sealed case of the ipg . in such configuration , feed - through terminals 103 are used to allow the coil ( s ) to be electrically connected to the charging and telemetry circuitry 148 , which are inside the hermetically sealed case . alternatively , if the case is made from a non - ferromagnetic material , such as titanium , or ceramic , the coil ( s ) 150 may be located inside of the hermetically sealed case . it is to be emphasized that that which is shown in fig4 is intended to be functional , and not limiting . those of skill in the art will be able to fashion appropriate circuitry , whether embodied in digital circuits , analog circuits , software and / or firmware , or combinations thereof , in order to accomplish the desired functions . [ 0055 ] fig5 is a timing waveform diagram that depicts various types of representative action potentials ( ap 1 , ap 2 , ap 3 ) that may be sensed as a function of time in response to a generated stimulus pulse ( sp 1 ). as seen in fig5 a stimulation pulse sp 1 is applied to selected electrodes at time t = 0 . shortly after the pulse sp 1 is applied , e . g ., at a time t 2 after t = 0 ( where t 2 may be , e . g ., from 0 . 1 to 5 milliseconds ( msec )), a monitoring window , having a time duration of w 1 msec , begins . the width of the monitoring time window w 1 may range , e . g ., from 1 - 10 msec . during the time window w 1 , the sense electrodes are monitored for the presence of an action potential . one representative action potential signal , ap 1 , shown in fig5 has an amplitude a 1 during a first monitoring window w 1 . at a known time t 1 thereafter , during a second monitoring window w 1 , the action potential ap 1 has changed to an amplitude of a 2 , where a 2 is markedly less than the amplitude a 1 . here , a marked change may be defined as one that has changed greater than about 10 - 20 % from a running average of the last 5 - 10 amplitude measurements . such marked change in amplitude of the action potential indicates a significant change has occurred in the coupling efficiency . such change could be caused , for example , by a postural change or relative movement between the electrode array and nerve tissue . in accordance with the present invention , and in response to sensing such a marked change between the action potential amplitudes a 1 and a 2 , correction circuitry programmed or wired into the μc 130 causes the amplitude of the stimulation current to increase , thereby maintaining the efficacy of the applied stimulus . had the amplitude a 2 been markedly greater than the amplitude a 1 , then the correction circuitry would cause the amplitude of the stimulation current to decrease . another representative action potential signal , ap 2 , shown in fig5 has an amplitude a 3 during the first monitoring window w 1 . at a known time t 1 thereafter , during a second monitoring window w 1 , the action potential ap 2 has changed to an amplitude of a 4 , where a 4 is only significantly less than the amplitude a 3 . as used here , the term “ slightly ” may be defined as a difference of less than about 5 % of the running average of the last 10 - 20 measurements as compared to the running average of 10 - 20 measurements taken several hours , e . g ., 12 - 36 hours , earlier . such slight change in amplitude of the action potential indicates a corresponding slight change has occurred in the coupling efficiency between the prior time and the present time . such change could be caused , for example , by a scare tissue maturation . in accordance with the present invention , and in response to sensing such a slight change between the action potential amplitudes a 3 and a 4 , correction circuitry programmed or wired into the μc 130 causes the amplitude of the stimulation current to increase a small amount , thereby maintaining the efficacy of the applied stimulus at a constant level . had the amplitude a 4 been slightly greater than the amplitude a 3 , then the amplitude of the stimulation current would be decreased a small amount . yet another representative action potential signal , ap 3 , shown in fig5 has an amplitude a 5 during the first monitoring window w 1 . at a known time t 1 thereafter , during a second monitoring window w 1 , the action potential ap 3 has changed to an amplitude of a 6 , where a 6 may or may not be much different than a 5 . however , the morphology of the ap 3 waveform ( where “ morphology ” means the waveform shape ) has changed markedly . such marked change in the morphology of the action potential waveform may likewise be used as an indicator that a change has occurred in the coupling efficiency . known techniques for determining the morphology of a waveform may be used to determine changes in the morphology over time . see , e . g ., u . s . pat . no . 5 , 685 , 315 , incorporated herein by reference . a simple technique , for example , integrates the action potential waveform , or other physiologic waveform , during the monitoring window w 1 so as to calculate the area under the morphology waveform curve . while such simple technique is not fool proof ( because it is possible for many different shaped curves to have the same area under the curve over a fixed time ), it is generally adequate for purposes of the present invention to detect a change in coupling efficiency . in accordance with the present invention , and in response to sensing such a marked change between the morphology of the action potential or other physiologic waveforms , correction circuitry programmed or wired into the μc 130 causes the magnitude of the stimulation current to increase , thereby maintaining the efficacy of the applied stimulus at a desired level . it is the energy content of the stimulus pulse that is adjusted in accordance with the invention when a change in the coupling efficiency has been detected . the energy content of the stimulus pulse is readily adjusted by adjusting the amplitude of the stimulus pulse . however , the energy content can also be adjusted by changing the width , or duration , of the stimulus pulse waveform , as well as the frequency with which the stimulus pulse is applied . thus , as used herein , the term “ amplitude ”, or “ magnitude ”, when used to describe the characteristics of the applied stimulus pulse , is meant to include pulse amplitude or pulse width or pulse frequency , or combinations of amplitude , width and frequency . [ 0064 ] fig6 is a flow chart that shows a method of practicing the invention in accordance with one embodiment thereof . although the flow chart is directed to the measurement of the action potential , or ap , it is to be understood that other physiologic parameter ( s ) other than the ap could also be measured , as has been previously indicated . thus , in the description of fig6 that follows , it should be understood that whenever “ ap ” is stated or used , the measurement of other physiologic parameters , such as optical properties , pressure , chemistry , or the like , of the tissue near the electrodes , may be made in lieu of , or as a supplement to , the measurement of the ap . as seen in fig6 a first step of the method involves programming the operating parameters ( block 202 ) into the ipg circuitry . such operating parameters include not only the operating regime for the neural stimulation system , e . g ., stimulation pulse amplitude , width , frequency , and electrodes , but also the parameters used by the invention to determine when a sufficient change in the sensed action potential has occurred so as to trigger the auto correction features of the invention . once all the operating parameters have been programmed , then a determination is made as to whether the auto correction feature of the invention has been programmed on ( block 204 ). if not , then the stimulator operates in accordance with its programmed operating regime without invoking the auto correction feature ( block 206 ). should new programming occur ( block 208 ), such that new operating parameters are loaded into the device memory ( which may include turning the auto correction feature on ), and should the device not be turned off ( block 210 ), then the process continues by returning to block 204 , where a new determination is made as to whether the auto correction feature is turned on . if auto correction is turned on ( block 204 ), then a stimulus pulse sp 1 is generated and applied to the specified electrodes ( block 212 ). next , the action potential ap , or other physiologic parameter , is measured during the prescribed time window ( block 214 ). once the ap or other parameter has been measured , then a determination is made as to whether the ap or other parameter has changed ( block 216 ). if not , then the process waits for the next sample time ( block 218 ), and if auto correction is not turned off ( block 220 ), then the process repeats ( blocks 212 , 214 , 216 ). if a determination is made that the action potential ap or other parameter has changed ( block 216 ), then a determination is made as to whether the energy coupling has increased or decreased , and how fast the change has occurred ( block 222 ). if the coupling has increased , then the magnitude of the stimulation pulse sp 1 is decreased an appropriate amount ( block 224 ). if the coupling has decreased , then the magnitude of the stimulation pulse sp 1 is increased an appropriate amount ( block 226 ). then , other parameters are revised or adjusted , as appropriate ( block 228 ) so that all the operating parameters are compatible with the changes made to sp 1 , and the process repeats , starting at block 204 . as described above , it is thus seen that the present invention provides a neural stimulation system wherein the output stimulus magnitude is automatically corrected or adjusted in order to compensate for coupling efficiency variations in the electrode - to - neural - tissue interface that cause more or less energy to reach the neural tissue from the electrode . variations in the coupling efficiency are determined , in one preferred embodiment , by sensing changes in a sensed action potential . in other preferred embodiments , variations in coupling efficiency are determined by sensing changes in the optical properties of the tissue surrounding the target neural tissue that is stimulated , by sensing changes in the pressure present within the tissue surrounding the target neural tissue , by sensing changes in the chemical properties of fluids within the epidural space , or the like . as further described above , it is seen that the invention provides a method of neural stimulation that includes measuring action potentials , or other suitable physiologic parameter ( s ), that are indicative of the coupling efficiency of the electrical stimulation current to the neural tissue , and automatically adjusting the magnitude of subsequent stimulating current pulses in order to compensate for variations in the measured coupling efficiency . while the invention herein disclosed has been described by means of specific embodiments and applications thereof , numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims .
a neural stimulation system automatically corrects or adjusts the stimulus magnitude in order to maintain a comfortable and effective stimulation therapy . auto correction of the stimulus magnitude is linked to the measurement of coupling efficiency . because the events that lead to the necessity of an output amplitude change are all associated with how much electrical energy is coupled to the neural tissue , and because there are several physiologic parameters that reflect in some measure how much energy is actually coupled to the tissue , the measurement of the one or more of such physiologic parameters is used as an indicator of the electrode &# 39 ; s effectiveness in providing therapeutic stimulation . the physiologic parameters that may be measured , and used by the invention as a measure of coupling efficiency , include : action potential ; the optical transmissive and / or reflective properties of the tissue and fluids surrounding or adjacent the target neural tissue ; the chemistry of the fluids and tissue near the electrodes , e . g ., in the epidural space ; or the changes in pressure that occur near the electrodes , e . g ., in the epidural space . the relative change in the measured physiologic parameter is used as an indicator of the obstruction to current flow between the electrodes and the neural tissue . such knowledge with respect to time thus permits the neural stimulation system to effectively auto - correct the output amplitude , thereby minimizing the occurrence of over - stimulation or under - stimulation .