a device 10 according to the present invention is shown in fig1 . the device 10 has a magnetic coil 12 , an amplifier 14 , and a waveform generator 16 . the waveform generator 16 ( e . g ., a general - purpose programmable computer or a purpose - built electric circuit ) provides an electrical pulse sequence to the amplifier 14 , which amplifies the electrical signals and provides them to the magnetic coil 12 . the magnetic coil 12 produces a magnetic field in response to electrical signals received from the amplifier 14 . over the region in which the subject &# 39 ; s brain is positioned , the magnetic field is a gradient magnetic field that is substantially uniform ( i . e ., the magnetic field strength varies substantially linearly in only one direction , e . g ., at about 5 g / cm , with the variation occurring from anterior to posterior across the subject &# 39 ; s head ) and unidirectional ( i . e ., the vectors representing the magnetic field all point in substantially the same direction , e . g ., along the long axis of the subject &# 39 ; s body ). ( alternatively , a magnetic coil can be used that generates a substantially uniform and unidirectional gradient magnetic field over only a region of interest of the brain , e . g ., the left prefrontal cortex .) the magnetic coil 12 is large enough to accommodate a subject &# 39 ; s head , with a diameter of , e . g ., about 35 cm ( 14 in .). when being treated with device 10 , the subject 18 lays down on a standard patient gurney 20 with a head support 22 , with his or her head positioned inside the coil 12 . other devices can also be used for administering the present treatment method . for instance , a conventional magnetic resonance imaging apparatus can be used . alternatively , instead of using a device such as device 10 that consists of separate components , the device can instead integrate one or more components , e . g ., to make the device easily portable . alternatively or additionally , the magnetic coil can be included in a hat - like structure , and the waveform generator , amplifier , and power source ( e . g ., a battery ) integrated into a control mechanism that the subject carries or wears , i . e ., on his or her subject &# 39 ; s belt . the subject can self - administer the treatment , and the treatment can be applied while the subject is lying down , standing , sitting , or in motion . alternatively or additionally , the control device can be pre - set to administer the treatment for specific periods at specific intervals or continuously . prior to receiving treatment using device 10 , a subject is selected as a candidate for enhancement of brain function . this selection is generally performed by medical professionals , e . g ., because the subject has been diagnosed as suffering a cognitive impairment . alternatively , a subject could self - select based on a perceived need or desire to enhance brain function . selection can be based on either subjective or objective criteria , including , e . g ., anxiety , moodiness , depression , lethargy , sleepiness , learning difficulties , and memory impairments . to administer the treatment , the subject &# 39 ; s head is positioned inside coil 12 , and subjected to a time - varying magnetic field . ( alternatively , the subject &# 39 ; s entire body could be positioned inside a full - body coil , and subjected to a magnetic field .) the magnetic pulse train used to generate the time - varying magnetic field is shown in fig2 . the pulse train comprises a sequence of pulses delivered at a high rate . as discussed in detail below , the magnetic field induces an electrical field in the subject &# 39 ; s brain . this electrical field can interact with neurons to cause cognitive effect . in light of this , the duration of each individual magnetic pulse is selected to be on the order of the refractory period of an axon , i . e ., on the order of several milliseconds ( e . g ., 1 to 10 milliseconds ), see , e . g ., e . r kandel et al ., principles of neural science , 1991 , which is incorporated by reference herein . thus , the pulse duration can be from on the order of 0 . 5 milliseconds to 10 milliseconds . for example , each pulse has a trapezoidal shape , with 128 microsecond ramp times ( from zero to plateau ) and 768 microsecond plateau times ( for a total duration of 1 . 024 milliseconds ). the pulses alternate in polarity , with a short gap between successive pulses . a single pulse train comprises 512 successive pulses , and so lasts for about a half - second . after a delay of about a second - and - a - half , the pulse train is repeated ( giving one pulse train every two seconds ), and the treatment concludes after about six hundred repetitions ( for a total treatment time of about 20 minutes ). alternatively , the second - and - a - half delay between successive pulse trains can be eliminated . at the plateau of each trapezoidal pulse , the maximum magnetic field strength is on the order of 5 - 10 g , with a magnetic field gradient of 0 . 33 g / cm . fig3 shows a three - dimensional plot of the resultant magnetic field . pulse sequences yielding maximum magnetic field strengths of up to about 50 g , and maximum magnetic field gradients of up to about 5 g / cm , can alternatively be used . these magnetic fields induce electric fields in the subject &# 39 ; s brain . the characteristics of these electric fields are defined by the magnetic field parameters according to maxwell &# 39 ; s equation : ∇× e ( x , y , z , t )=−∂ b ( x , y , z , t )/∂ t , where ∇× e is the curl of the electric field and ∂ b ∂ t is the rate of change of the magnetic field over time . in cartesian coordinates , this equation becomes : where the subscripts x , y , and z denote the component of the fields along those respective axes , see , e . g ., j . d . jackson , classical electrodynamics , 1975 , which is incorporated herein by reference . these equations describe fields in free space ( i . e ., fields produced in the absence of other material ). when conductive matter , such as brain tissue , is placed in the changing magnetic field , a charge distribution is also induced , resulting in an electric field . this electric field will affect the overall electric field in the head . this charge distribution can alter the free space electric field by up to about 50 %, see roth et al , electroencephalography and clinical neurophysiology , 81 : 47 , 1991 , which is incorporated herein by reference . the pattern of the effect of the charge distribution will depend on the shape and placement of the subject &# 39 ; s head . two local field distributions are of particular interest . in the first , the z - component ( superior - inferior component ) of the magnetic field has a uniform gradient in the y - direction ( anterior - posterior direction ), and the y - component has a uniform gradient in the z - direction : ( b x = 0 , b y = g ( t ) z , b z = g ( t ) y ), where g ( t ) is the value of the gradient . in this case , the electric field is given by : ( e x = e 0  ( t ) + 1 2  ( ∂ g  ( t ) / ∂ t ) · ( y 2 - z 2 ) , e y = 0 , e z = 0 ) , where e 0 ( t ) is a spatially constant field term that depends on the size of the coil and , consequently , the extent of the magnetic field . the preceding field description applies equally for the two other orientations , which is obtained by replacement of x with y , y with z , and z with x or by replacement of x with z , y with x and z with y , in both the vector components and coordinates . in addition , a given vector combination of these three field components , which forms an equivalent but rotated field , is also appropriate . thus , one approach to applying the new treatment techniques involves using a magnetic field that has a vector component with a gradient that is substantially uniform , e . g ., to within 10 %, in value or direction over a relevant volume of the subject &# 39 ; s brain , e . g ., a 8 cm 3 volume or the left prefrontal cortex . in another magnetic field distribution , the magnetic field is uniform over a local volume , which can be expressed as : ( b x = 0 , b y = 0 , b z = b ( t )). the corresponding local electric field is : ( e x = e 0 ( t )− a (∂ b ( t )/∂ t )· y , e y = e 0 ( t )−( 1 − a )(∂ b ( t )/∂ t )· y , e z = 0 ), where a is an arbitrary parameter determined by the details of coil winding . in both situations , if e 0 ( t ) is sufficiently large compared to ∂ g ( t )/∂ t · r 2 or ∂ b ( t )/∂ t · r , where r is an effective radius of the volume of interest , e . g ., the radius of a subject &# 39 ; s brain , then the local electric field is substantially uniform . the preceding field description applies equally for other orientations and rotations . fig4 shows the electric field waveform induced in the subject &# 39 ; s brain when subjected to the magnetic field waveform shown in fig2 . the electric field waveform is a sequence of alternating monophasic square pulses of alternating polarity . the width of each induced electric pulse corresponds to the ramping period for the magnetic field pulses , i . e ., 256 microseconds . for the 0 . 33 g / cm magnetic field pulse amplitude , the electric field amplitude is approximately 0 . 7 v / m . this electric field strength is approximately an order of magnitude less than the minimum peripheral nerve stimulation threshold of approximately 6 - 25 v / m , see , e . g ., j . p . reilly , medical and biological engineering and computing , 27 : 101 , 1989 , thus providing an appropriate margin of safety against causing pain or seizures in the patient . fig5 and 6 are contour and three - dimension plots of this electric field , respectively . these plots were made by modeling with biot - savart style integration , see , e . g ., j . d . jackson , classical electrodynamics , 1975 , which is incorporated herein by reference , for free space values using a magnetic field with a vector component having a gradient that is substantially uniform in value and direction . the plots in fig5 and 6 show that the electric field is substantially uniform in direction and changes slowly with distance . the direction of the induced electric field is determined by maxwell &# 39 ; s equations . for a magnetic field that has a gradient oriented from anterior to posterior across the subject &# 39 ; s head , the induced electric field is oriented in from right to left across the subject &# 39 ; s head . twenty - one people exhibiting symptoms of depression were selected by medical professionals and subjected to the present method . twelve subjects reported a post - treatment overall mood improvement of at least one point on the brief affect scale , which involves asking a subject to rate his mood after treatment compared to his mood at an earlier time , using a seven point scale : ( 1 ) very much improved , ( 2 ) much improved , ( 3 ) minimally improved , ( 4 ) no change , ( 5 ) minimally worse , ( 6 ) much worse , ( 7 ) very much worse . the results for forty - three visits by the twenty subjects are given in the table in fig7 . as reflected in fig7 the effects of the treatment on unmedicated subjects were most prevalent , with unmedicated subjects demonstrating mood improvement after seven of nine visits . this demonstrates that the new treatment is useful both as a substitute for and complement to drug therapy . the effects can be substantial ; four subjects reported a pronounced , sustained improvement that lasted over a week . there were three reports by patients of mood worsening . the treatments were administered using a general electric 1 . 5t signa mri scanner . after optional water suppression , slice selective excitation , and a spatial phase encoding pulse , the device applied a train of 512 trapezoidal alternating - polarity magnetic field pulses . these pulses were about one millisecond long , with ramp times of 128 microseconds and 768 microsecond plateau times . during the plateau of each pulse , the gradient was 0 . 33 g / cm , and the maximum magnetic field in the cortex was about 5 g . the entire train of 512 pulses was repeated every 2 seconds , six hundred times , for a total treatment time of 20 minutes . fig3 is a three - dimensional plot of this magnetic field , and fig2 is a diagram of the pulse train . the ‘ y ’ gradient coil in the magnetic resonance scanner , having an approximate diameter of about 90 cm ( 36 in . ), was used to apply this sequence , orienting the gradient in the anterior - posterior direction for the supine subjects . the gradient of the z - component of the magnetic field from this coil in the y - direction is uniform in both magnitude and direction over a subject &# 39 ; s brain to within about 5 %. the magnetic field induced an electric field in the brains of the subjects . this electric field was oriented from right to left , from the subject &# 39 ; s perspective , and had a magnitude of approximately 0 . 7 v / m . fig5 and 6 respectively show contour and three - dimension plots of this electric field modeled for free space values using a wire pattern for a coil similar to the ‘ y ’ gradient coil in the signa mri system and computed with biot - savart style integration . the induced electric field consisted of 256 microsecond monophasic square pulses , where each pulse has a single polarity and an amplitude of approximately 0 . 72 v / m . a diagram of this electric field waveform is shown in fig4 . to achieve the same electric field with a smaller coil , maxwell &# 39 ; s equations show that a higher magnetic field is needed . using a coil with a similar shape but smaller diameter , e . g ., a “ head - sized ” 35 cm ( 14 in .) coil instead of a 36 - inch “ whole - body ” gradient coil , to induce a similar same electric field magnitude would employ a magnetic field that reaches approximately 50 g in the head . the magnetic field used to induce such an electric field can have a vector component with a gradient that is slightly less uniform in value and direction , varying by about 10 % over the cranial volume . in addition , a higher magnetic field , e . g ., 100 g , can be used with a smaller coil that provides a vector component with a substantially uniform gradient over only a region , e . g . 8 cm 3 , of the brain . one example of an rtms technique uses a figure - 8 surface coil with loops that are 4 cm in diameter ( cadwell , kennewick , wash .). this coil is placed next to the scalp , and is usually positioned to direct the magnetic field at the prefrontal cortex of the brain , see , e . g ., george et al ., the journal of neuropsychiatry and clinical neurosciences , 8 : 373 , 1996 . an electric current is run through the magnetic coil to generate a magnetic field , specifically a sequence of single - cycle sinusoidal pulses where each pulse has a frequency of approximately 1800 hz ( or about 560 microseconds per pulse ). these pulses are delivered at a repetition rate of 1 hz ( i . e ., one single - cycle sinusoidal pulse every 1 second ), see , e . g ., george et al , biological psychiatry , 48 : 962 , 2000 ; eschweiler et al , psychiatry research : neuroimaging section , 99 : 161 , 2000 . this waveform is shown in fig8 . as the repetition period is much longer than the time span on the x - axis , only one single - cycle sinusoidal pulse appears in fig8 . the magnetic field generated by the fig8 waveform is shown in fig9 . the field reaches its maximum strength of approximately 10 , 000 g at the face of the coil . the strength of this magnetic field decreases rapidly as the distance from the coil increases , to about 0 g at about 6 cm to 8 cm , see , e . g ., cohen et al , electroencephalography and clinical neurophysiology , 75 : 350 , 1990 . fig1 shows the electric field waveform induced in the subject &# 39 ; s brain by the magnetic field shown in fig9 . this waveform consists of a series of 560 - microsecond single - cycle cosine pulses that repeat every 1 hz . fig1 shows the contour plot and fig1 shows the three - dimensional plot of the electric field induced in free space by the magnetic field shown in fig2 a . the electric field is approximately 120 v / m at the face of the coil , and falls to about 0 . 02 v / m on the side of the head opposite the coil . the contours of this rapidly diminishing electric field reflect the shape of the figure - 8 surface coil with 4 cm diameter loops , tilted at 45 °, and placed 6 . 7 cm vertically and horizontally from a position equivalent to the center of the head : the electric field forms roughly circular loops . a number of embodiments of the invention have been described . nevertheless , it will be understood that various modifications may be made without departing from the spirit and scope of the invention . accordingly , other embodiments are within the scope of the following claims .
the invention involves enhancing brain function by stimulating the brain using magnetic fields . applications of the new methods include improving the condition of individuals with cognitive disorders , such as depression , and studying the effects of neural stimulation using induced electric fields . these techniques can avoid deleterious effects of psychotropic pharmaceutical treatments , and provide a relatively safe , comfortable , inexpensive means of direct cranial stimulation .