fig1 is a schematic illustration of a neural stimulation system 20 , in accordance with an embodiment of the present invention . system 20 typically comprises an implantable neural stimulator 30 , an oral element 32 , and an external control unit 34 . stimulator 30 comprises an elongated support element 36 , one or more electrodes 38 fixed to the support element in a vicinity of a distal end thereof , and circuitry 40 coupled to the support element in a vicinity of a proximal end thereof . circuitry 40 typically comprises a wireless coupling element ( which typically comprises a coil ), and additional elements , such as one or more rectifiers , capacitors , amplifiers , or filters . one or more leads ( not shown in fig1 ), which pass along , through , or around support element 36 , couple electrodes 38 to circuitry 40 . alternatively , the leads function as the support element , i . e ., the support element does not comprise any structural elements in addition to the leads . further alternatively , the leads provide a substantial portion of the structural support of the support element , and the balance of the structural support is provided by other elements . for example , support element 36 may comprise the leads and a flexible sleeve surrounding the leads ; the leads supply most of the structural support of the support element , while the sleeve allows smooth passage of the leads through the greater palatine canal . circuitry 40 is shown schematically in fig1 ; the circuitry may employ one or more of the more detailed configurations described with reference to fig3 a - b , 4a - b , and 5a - d of u . s . patent application ser . no . 11 / 349 , 020 , filed feb . 7 , 2006 , entitled , “ spg stimulation via the greater palatine canal ,” which is assigned to the assignee of the present application and is incorporated herein by reference . stimulator 30 is adapted to be passed through a greater palatine foramen 42 of a hard palate 50 of an oral cavity 52 of a subject into a greater palatine canal 54 , such that electrodes 38 are brought into a vicinity of a sphenopalatine ganglion ( spg ) 56 . for some applications , the entire stimulator is contained within greater palatine canal 54 , while for other applications , at least a portion of the circuitry and / or the support element are positioned submucosally in the oral cavity . for clarity of illustration , the greater and lesser palatine nerves , and the greater and less palatine arteries are not shown in the figures . during an implantation procedure , stimulator 30 is typically passed through greater palatine foramen 42 posterior to the greater palatine nerve and artery , which are manipulated into an anterior position within the canal . for some applications , electrodes 38 apply a monophasic waveform to spg 56 , while for other applications , electrodes 38 apply a biphasic waveform . alternatively or additionally , waveforms and / or stimulation techniques may be used that are described in one or more of the patent applications incorporated by reference hereinbelow , or waveforms and / or stimulation techniques may be used that are known in the art of neural stimulation . fig2 is a schematic illustration of oral element 32 coupled to an oral appliance 92 , in accordance with an embodiment of the present invention . oral appliance 92 , which is typically shaped generally similarly to an orthodontic retainer , is configured to hold the oral element in a vicinity of or in contact with the roof of the oral cavity in a vicinity of implanted circuitry 40 of stimulator 30 . the use of oral appliance 92 , rather than mechanical coupling of oral element 32 to the roof of the oral cavity , generally reduces the likelihood of contamination . for some applications , oral appliance 92 is generally soft or semi - flexible , while for other applications , the oral appliance is generally rigid . for some applications , oral element 32 does not comprise power source 72 . instead , power is provided by a power source located outside of the oral cavity . for example , the oral appliance may be coupled by a cable to an external driver comprising a power source . for some applications , the driver is coupled to a headset or necklace worn by the subject . the driver or a separate external control unit , instead of oral element 32 , comprises all or a portion of circuitry 73 . for some applications , the driver is coupled to external control unit 34 , while for other applications , the driver comprises external control unit 34 . alternatively , oral element 32 is wirelessly coupled to external control unit 34 , which may or may not be coupled to the external driver . in an embodiment of the present invention , system 20 comprises a nasal element instead of or in addition to oral element 32 ( configuration not shown ). the nasal element is adapted to be inserted into a nostril of the subject , e . g ., into the nasal vestibule . the nasal element comprises at least one wireless coupling element 74 that is wirelessly coupled to transmit / receiver 40 of stimulator 30 , for transmitting / receiving power and / or data to / from the stimulator . in this embodiment , circuitry 40 of stimulator 30 is not necessarily positioned at the proximal end of the stimulator . for some applications , circuitry 40 of stimulator 30 comprises a wireless coupling element . wireless coupling element 74 of oral element 32 is adapted to wirelessly transmit energy and / or data to the wireless coupling element of circuitry 40 , and / or to wirelessly receive data form the wireless coupling element of circuitry 40 . for these applications , each of the wireless coupling elements typically comprises at least one coil . for some applications , the wireless coupling elements are wirelessly coupled to one another using induction , such as when the wireless coupling elements are positioned in close proximity to one another . alternatively , the wireless coupling elements are wirelessly coupled to one another using rf energy , such as when the wireless coupling elements are positioned at a greater distance from each other . further alternatively , the wireless coupling elements are wirelessly coupled to one another using another form of energy , such as ultrasound energy , in which case the wireless coupling elements comprises ultrasound transducers , e . g ., piezoelectric transducers . “ transducer element ,” as used in the present application including the claims , means an element adapted to wirelessly transmit and / or receive energy and / or data , including a coil , a piezoelectric transducer , and other wireless transducers known in the art . in an embodiment of the present invention , oral element 32 does not comprise wireless coupling element 74 . instead , power source 72 of the oral element is coupled to circuitry 40 using a wire that passes through mucosa 58 . the techniques of this embodiment are generally more energy - efficient than wireless energy / data transfer techniques . as a result , the battery of power source 72 of oral element 32 may need to be replaced or recharged less frequently , or not at all . for some applications , oral element 32 is adapted to be implanted in a tooth of the subject . for some applications , the implanted oral element comprises a wireless communication element for external wireless communication , such as of data . for some applications , power source 72 comprises a rechargeable or a replaceable battery . reference is made to fig3 , which is a schematic illustration of a neural stimulation system 120 , in accordance with an embodiment of the present invention . except as noted hereinbelow , elements of system 120 are the same as corresponding elements of system 20 having the same reference numerals . system 120 comprises implantable neural stimulator 30 and external control unit 34 . stimulator 30 comprises elongated support element 36 , one or more electrodes 38 fixed to the support element in the vicinity of the distal end thereof , and an implantable submucosal antenna 122 coupled to the support element in a vicinity of the proximal end thereof . submucosal antenna 122 is configured to be implanted in the roof of oral cavity 52 between oral mucosa 58 and a palate , e . g ., hard palate 50 and / or a soft palate 134 , and to generally conform to the shape of the palate . for some applications , the level of stimulation of the spg is determined by assessing an indirect physiological parameter of the subject related to the level of spg stimulation , such as cerebral blood flow ( cbf ) and / or bbb permeability . for some applications , assessment techniques described hereinbelow are used . for some applications , a healthcare worker enters the values of the indirect physiological parameter into system 20 , while for other applications , a device for measuring the indirect physiological parameters is coupled to system 20 , and communicates the parameters to the system . although electrodes 38 have been described as being applied to an spg of the subject , for some applications the electrodes are applied to another mts of the subject , as defined hereinabove . for some of these applications , electrodes 38 are passed through the greater palatine canal to the mts , while for other applications the electrodes are passed through only a portion of the greater palatine canal , or are advanced to the mts by another route . in an embodiment of the present invention , system 20 is used to reduce the risk of vascular dementia ( vad ). system 20 is configured to apply excitatory electrical stimulation to at least one mts , as defined hereinabove , such as spg 56 . the system applies the stimulation chronically , and configures the stimulation to augment cerebral blood perfusion to hypoperfused brain regions , by dilating cerebral vessels , thereby increasing cerebral blood flow ( cbf ) to brain tissue . alternatively or additionally , the stimulation causes the release of one or more neuroprotective substances , such as neuromodulators ( e . g ., nitric oxide ( no ) and / or vasoactive intestinal polypeptide ( vip )). such stimulation generally reduces the risk of development of vad , i . e ., delays or prevents the impending vad . typically , the vad includes vascular cognitive impairment ( vci ), multi - infarct dementia , strategic single - infarct dementia , post - stroke dementia , small vessel disease ( svd ) ( leukoaraiosis ), or a combination of more than one type of vad ( mixed vad ). in an embodiment of the present invention , a method for using system 20 comprises identifying that a subject is at risk of suffering from vad , and applying the stimulation responsively to the identifying . for some applications , the method comprises identifying that the subject already suffers from vascular cognitive impairment or minimal cognitive impairment . for some applications , identifying that the subject is at risk of suffering from vad comprises analyzing a radiological imaging status of the subject . recent large scale prospective studies ( for a review see schmidt r et al . ( 2007 ), cited in the background of the invention ) indicated that leukoaraiosis progresses over time and that its extent at baseline is an important predictor for the subsequent rate of lesion progression ( enzinger c et al ., schmidt r et al . ( 2003 )). for instance , individuals with early confluent and confluent leukoaraiotic changes tend to progress rapidly . moreover , the progression of leukoaraiosis closely relates to the rate of cognitive decline ( garde e et al ., schmidt r et al . ( 2005 )), and especially the decline in executive functions . for some applications , system 20 is used to apply stimulation to a subject that suffers from low cerebral blood perfusion , even without any clinical signs of minimal cognitive impairment , minimal cognitive impairment , or other vad . such subjects are often at greater risk for developing vad ( see , for example , meyer j s et al .). for example , low cerebral blood perfusion may be detected using the indicators of cbf described hereinbelow , using spect or ct / mri perfusion techniques known in the art , and / or using other techniques known in the art . for some applications , the method comprises identifying that an elderly subject displaying no symptoms of vad is at risk , and applying the stimulation in order to postpone the subject &# 39 ; s normally expected decline in cerebral blood flow ( cbf ). for some applications , the method comprises identifying that the subject suffers from reduced cardiac output , e . g ., caused by heart failure ( see , for example , pullicino et al . ( 2001 )). in an embodiment of the present invention , the vad includes post - stroke dementia , which develops in about one - third of stroke victims within three months after suffering from a stroke . in addition to identifying that the subject has suffered from the stroke , identifying that the subject is at risk of post - stroke dementia may comprise assessing an age of the subject at the time of stroke , a severity of the stroke , whether the subject suffers from hypertension , whether the stroke has recurred , and / or an indication of cognitive function of the subject during the acute state of the stroke . the stimulation is typically applied on a chronic , long - term basis , i . e ., for at least one week , such as at least two weeks , at least four weeks , at least three months , at least six months , or longer , such as for the remainder of the subject &# 39 ; s life . during this chronic treatment , stimulation is typically applied intermittently , such as during one session per day , or less frequently , such depending on the severity of assessed risk . for some applications , each session has a duration of between 1 minute and 6 hours , such as at least 5 minutes or at least 15 minutes , or between 2 and 4 hours , e . g ., about 3 hours or about 6 hours , or more than 6 hours . alternatively , the system is configured to apply such stimulation generally constantly , i . e ., 24 hours per day . further alternatively , the stimulation is applied less frequently than every day , such as once every other day ( e . g ., at least one minute during every 48 hours ), or more frequently than once per day , such as during two sessions per day . for some applications , the stimulation is applied during a plurality of stimulation periods which includes at least first and last stimulation periods . system 20 sets an inter - period interval between initiation of the first period and initiation of the last period to be at least 24 hours . for example , the first stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on a monday , and the last stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on a tuesday of the same week . optionally , stimulation is applied during at least one additional stimulation period between the first and last periods . for example , stimulation may be additionally applied from 1 : 00 a . m . to 4 : 00 a . m . on the tuesday . for some applications , the first period concludes simultaneously with the initiation of the last period , i . e ., the stimulation is applied constantly from the beginning of the first period until the conclusion of the last period . for example , the stimulation may be applied constantly from 1 : 00 p . m . on monday , january 1 to 4 : 00 p . m . on tuesday , january 2 , or constantly from 1 : 00 p . m . on monday , january 1 to 4 : 00 p . m . on monday , january 29 . alternatively , the initiation of the last stimulation period occurs after a conclusion of the first stimulation period , such that the stimulation is not applied during at least one non - stimulation period between the conclusion of the first stimulation period and the initiation of the last stimulation period . for some applications , the system sets the inter - period interval to be at least 48 hours , such as at least one week , at least two weeks , or at least four weeks . when using such greater inter - period intervals , the system typically , but not necessarily , applies stimulation during at least several additional stimulation periods between the first and last stimulation periods . for some applications , such additional stimulation periods may include a plurality of daily stimulation periods , applied on every day between the initiation of the first stimulation period and the initiation of the last stimulation period . for example , the first stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on monday , january 1 , the last stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on monday , january 8 , and the additional daily stimulation periods may occur from 1 : 00 p . m . to 4 : 00 p . m . on each day from tuesday , january 2 through sunday , january 7 , inclusive . for some applications , stimulation is applied for at least 30 minutes every day ( e . g ., at least 60 minutes every day ) between the initiation of the first stimulation period and the initiation of the last stimulation period . for some applications , stimulation is applied during a plurality of non - continuous stimulation periods during each 24 - hour period between the initiation of the first stimulation period and the initiation of the last stimulation period . for example , the first stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on monday , the last stimulation period may occur from 1 : 00 p . m . to 4 : 00 p . m . on wednesday , and stimulation may be applied during additional stimulation periods from ( a ) 1 : 00 a . m . to 4 : 00 a . m . on tuesday , ( b ) from 1 : 00 p . m . to 4 : 00 p . m . on tuesday , and ( c ) from 1 : 00 a . m . to 4 : 00 a . m . on wednesday , such that stimulation is applied during two stimulation periods during the 24 - hour period from 1 : 00 p . m . on monday to 1 : 00 p . m . on tuesday , and during two stimulation periods during the 24 - hour period from 1 : 00 p . m . on tuesday to 1 : 00 p . m . on wednesday . for some applications , the system is configured to set the inter - period interval to be no more than a maximum value , such as three , six , nine , or twelve months . for some applications , the system comprises a user interface , which enables a healthcare worker to enter a value for the inter - period interval . the system typically rejects values that are greater than the maximum value , such as by requiring the healthcare worker to enter another value , or by using the maximum value instead of the entered value . alternatively , the system notifies the healthcare worker if the entered value is greater than the maximum value ; optionally , the system allows the healthcare worker to override the notification . for some applications , the system is configured to store a maximum total time of stimulation per each time period having a given duration , and to apply the stimulation no more than the maximum total time per each time period having the given duration . for example , the given duration of each time period may be 24 hours . typical values for the maximum total time of stimulation per 24 - hour period include one hour , three hours , six hours , ten hours , and twelve hours . for some applications , the maximum total time of stimulation is predetermined , e . g ., by the manufacturer of the system , while for other applications , a healthcare worker enters the maximum total time of stimulation into the system . as used in the present application , including the claims , a “ stimulation period ” includes an entire period during which stimulation is applied , even though current is applied to the site only during a portion of the period , because of the duty cycle , on / off periods , and / or frequency of the current , for example . for some applications , the stimulation is applied bilaterally to both spgs , while for other applications , the stimulation is applied unilaterally to the mts ( e . g ., the spg ) that supplies the more affected hemisphere of the brain . for some applications in which the stimulation is applied bilaterally , techniques are used that are described in u . s . application ser . no . 11 / 573 , 993 , filed feb . 19 , 2007 , entitled , “ concurrent bilateral spg modulation ,” which is assigned to the assignee of the present application and is incorporated herein by reference . in some embodiments of the present invention , stimulation of an mts , such as the spg , is applied to treat a subject already suffering from vad . the stimulation generally stops or slows the progression of the dementia , and for some subjects improves cognitive ability . reference is made to fig4 , which is a graph 100 illustrating electrical stimulation strengths , in accordance with an embodiment of the present invention . excitatory stimulation of an mts ( e . g ., the spg ) induces changes in cbf , induces the release of one or more neuroprotective substances , such as neuromodulators ( e . g ., nitric oxide ( no ) and / or vasoactive intestinal polypeptide ( vip )), and / or modulates permeability of the blood - brain barrier ( bbb ). the inventors have found that excitatory stimulation of an mts at at least a minimum threshold strength increases cbf , and that the increase in cbf is related to the strength of the stimulation . the inventors have also found that at a sufficiently high strength , such stimulation modulates the permeability of the bbb , in addition to increasing cbf . “ strength ,” as used in the present application , including the claims , means a total charge applied to an mts in a given time period , e . g ., one minute , one hour , or one day . strength is increased or decreased by changing one or more parameters of the applied stimulation , such as the amplitude , number of cycles in a given time period , frequency , pulse width , or duty cycle ( e . g ., ratio of “ on ” to “ off ” time within a given cycle ), as described hereinbelow in greater detail . the y - axis of graph 100 indicates the strength of the stimulation of an mts . the strength of the stimulation is determined by the values of the parameters of the stimulation , such as voltage , current , frequency , cycles per time period , and duty cycle . stimulation at at least a minimum cbf - increasing strength 102 increases cbf . stimulation at such a strength also typically induces the release of one or more neuroprotective substances , such as no and / or vip . a maximum cbf - increasing strength 106 is the strength at which cbf is maximally increased , i . e ., further increases in strength do not further increase cbf . the bbb is opened , i . e ., the permeability of the bbb to larger molecules or substances that do not cross the intact bbb is significantly increased , by stimulation having a strength in a range 108 between a minimum bbb - opening strength 110 and maximum bbb - opening strength 112 ( beyond which increased strength does not result in additional opening of the bbb ). although minimum bbb - opening strength 110 is shown in graph 100 as being greater than maximum cbf - increasing strength 106 , this is not necessarily the case . in the present application , including the claims , stimulation of an mts is considered capable of inducing a “ significant ” increase in the permeability of the bbb if the stimulation is capable of inducing at least one of the following : ( a ) an increase in concentration of evans blue ( eb ) in brain tissue of a subject , such as a rat , of at least 100 % compared to a baseline concentration measured in a control rat . to determine the increase , permanent middle cerebral artery occlusion ( pmcao ) is induced in the rat , such as using techniques described hereinbelow with reference to fig6 . three hours after pmcao , stimulation is applied to the mts , and a bolus of eb 2 % at 1 ml per kg body weight of the rat is administered intravenously . the rat is sacrificed one hour after application of the stimulation and administration of the eb . to determine the baseline concentration , pmcao is induced in a control rat , three hours after pmcao an identical eb bolus is administered intravenously , but no stimulation is applied , and the control rat is sacrificed one hour after the administration of the eb ; and ( b ) a serum s100beta level of the subject ( indicative of clearance of the protein from the brain into the systemic circulation ), at a measurement time 45 minutes after initiation of mts stimulation , that is at least 30 % greater than a serum s100beta level of the subject measured at the beginning of the mts stimulation . although the above are indications of the “ significance ” of an increase in permeability of the bbb , use of the apparatus and performance of the methods described and claimed herein typically do not include measuring any of these indications . in particular , indication ( a ) is generally only possible to measure in an animal model ; if it were desired to conduct a human experiment , different techniques would likely be used , such as measuring the concentration in the brain of a radioactive isotope that is normally excluded by the bbb . for some applications , it is desirable to apply stimulation to an mts , and configure the stimulation to have a strength that induces an increase in permeability of the bbb that is even lower than a “ significant ” increase , as defined above . such a “ sub - significant ” increase in permeability of the bbb is considered to occur if the stimulation is capable of inducing at least one of the following : ( i ) an increase in concentration of eb , under the conditions defined in indication ( a ) above , of at least 20 %, such as at least 30 %, e . g ., at least 50 %; and ( ii ) a serum s100beta level , under the conditions defined in indication ( b ) above , that is at least 10 %, e . g ., at least 20 %, greater than the level of the subject measured at the beginning of the mts stimulation . for some applications , it is useful to define increased cbf as a percentage increase in cbf over a baseline level of cbf , which increase has at least a certain duration , e . g ., at least 5 minutes . typically , the baseline cbf level is either : ( a ) a normal baseline level for a subject , i . e ., prior to an adverse brain event , such as a cerebrovascular event , e . g ., a stroke , or ( b ) a post - event baseline level , prior to stimulation using the techniques described herein , and , optionally , prior to other treatment of the event . cbf is typically expressed as volume of blood flow per time per mass of the subject , e . g ., ml / min / 100 g . for some applications , increased cbf is expressed as an area under the curve ( auc ) of cbf with respect to baseline over a certain time interval . in an embodiment of the present invention , electrical stimulation system 20 is configured to apply excitatory electrical stimulation to at least one mts of a subject , and to configure the stimulation to increase cbf of the subject and / or induce the release of neuroprotective substances , without substantially opening the bbb of the subject . in other words , the system sets the strength of stimulation equal to less than minimum bbb - opening strength 110 , such as less than 90 % of minimum bbb - opening strength 110 , e . g ., less than 80 %, 70 %, or 60 % of minimum bbb - opening strength 110 . for some applications , the system is configured to increase cbf of the subject and / or induce the release of neuroprotective substances without increasing the permeability of the bbb to a level that produces a measurably - harmful clinical effect for the subject . the following table shows exemplary parameter ranges for some of the stimulation strengths and treatment protocols described hereinabove . as indicated in table 1 , for some applications system 20 provides stimulation by applying a plurality of cycles of stimulation , each cycle including an “ on ” period ( e . g ., between 2 and 90 seconds ) followed by an “ off ” period ( e . g ., between 8 and 90 seconds ). such cycles are applied a certain number of times per hour , typically spaced evenly throughout the hour . for example , if the cycles are applied four times per hour , the four cycles may be applied at the beginning of the hour , 15 minutes into the hour , 30 minutes into the hour , and 45 minutes into the hour , respectively . for some applications , each stimulation is applied in sets of two or more cycles . for example , if the stimulation is applied four times per hour , a set of two cycles may be applied at the beginning of the hour , 15 minutes into the hour , 30 minutes into the hour , and 45 minutes into the hour , respectively . in an embodiment of the present invention , during placement of electrodes 38 at an mts , as defined hereinabove , at least one physiological indicator of cerebral blood flow ( cbf ) is observed or measured concurrently with or after placement . for some applications , optimization of placement of electrodes 38 onto the appropriate neural structure is performed by activating the stimulator , and generally simultaneously monitoring cbf while manipulating the electrodes , and / or adjusting at least one parameter of the applied stimulation , so as to increase or decrease cbf , as appropriate . alternatively or additionally , this technique is used to verify the placement of electrodes 38 after implantation , and / or to select which combination of electrodes to use , such as by using the feedback algorithm described hereinabove . alternatively or additionally , a similar optimization process is performed , either during or after placement of electrodes 38 , to determine parameters of the applied current so as to achieve a desired effect , e . g ., on cbf or bbb permeability , as indicated by cbf . physiological indicators of cbf include , but are not limited to , the following : a measure of vasodilation of blood vessels of the eye , determined by unaided visual inspection or by using an instrument , e . g ., an instrument comprising machine vision functionality ; transcranial doppler ultrasonography measurements ; a measure of forehead perfusion , measured , for example , using laser doppler perfusion imaging ( ldi ) and / or using a temperature sensor ; and / or near infrared spectroscopy ( nirs ) measurements . carotid artery duplex ultrasound other appropriate measurements indicative of cbf for use with these embodiments of the present invention will be apparent to those skilled in the art , having read the disclosure of the present patent application . for some applications , one or more of the devices described hereinbelow with reference to fig5 - 8 are used for assessing a physiological indicator of cbf . fig5 is a schematic illustration of a vasodilation measurement instrument 230 , in accordance with an embodiment of the present invention . instrument 230 comprises an image sensor 234 ( e . g ., a ccd or cmos sensor , or another camera ) and processing circuitry 238 , in order to provide machine vision functionality . image sensor 234 is directed towards an eye 232 of the subject . the instrument measures the ratio of red to white in the sclera of eye 232 , or another indication of vasodilation . fig6 is a schematic illustration of a laser doppler perfusion ( ldi ) device 270 , in accordance with an embodiment of the present invention . ldi device 270 comprises a laser source 271 , a scanner 272 , and a computer 281 . scanner 272 is positioned near a forehead 241 of the subject for measuring forehead perfusion . fig7 is a schematic illustration of a thermometer 280 , in accordance with an embodiment of the present invention . thermometer 280 is positioned touching a forehead 241 of the subject for measuring forehead perfusion . fig8 is a schematic illustration of a transcranial doppler ultrasonography device 284 , in accordance with an embodiment of the present invention . transcranial doppler ultrasonography device 284 is positioned touching a head 288 of the subject for measuring cbf . for some applications , the measurement device , such as those described hereinabove with reference to fig5 - 8 , comprises an output unit 236 , such as a numeric display , tone generator , color display , or other output device , for outputting a signal indicative of the measured physiological parameter . alternatively or additionally , instrument 230 is coupled to an internal or external control unit of system 20 or 120 , and communicates the signal directly to the control unit . in an embodiment of the present invention , during placement of electrodes 38 at an mts , as defined hereinabove , penetration of a systemically administered dye into an eye of the subject is observed or measured concurrently with or after placement , as an indication of a level of increased permeability of the bbb . for example , the dye may include fluorescein dye . for some applications , optimization of placement of electrodes 38 onto the appropriate neural structure is performed by activating the stimulator , and generally simultaneously monitoring the penetration of the dye while manipulating the electrodes , and / or adjusting at least one parameter of the applied stimulation , so as to increase or decrease permeability of the bbb , as appropriate . alternatively or additionally , this technique is used to verify the placement of electrodes 38 after implantation , and / or to select which combination of electrodes to use , such as by using the feedback algorithm described hereinabove . alternatively or additionally , a similar optimization process is performed , either during or after placement of electrodes 38 , to determine parameters of the applied current so as to achieve a desired effect , e . g ., on cbf or bbb permeability , as indicated by bbb permeability . in an embodiment of the present invention , one or more of the above - described cbf - based assessment techniques are used by a healthcare worker after implantation to assess ( a ) whether electrodes 38 retain appropriate placement and contact with the mts , and / or ( b ) whether parameters of the applied current ( e . g ., magnitude , frequency , duration , scheduling ) continue to achieve the desired effect , e . g ., on cbf or bbb permeability . for example , such an assessment may be performed periodically during post - implantation follow - up care . in an embodiment of the present invention , the cbf - based assessment techniques described hereinabove are used to assist in determining the effective dosage and / or other parameters for presenting odorants to an air passage of the patient , as described in u . s . patent application ser . no . 10 / 512 , 780 , filed oct . 25 , 2004 , which is assigned to the assignee of the present application and is incorporated herein by reference . it is also to be appreciated that whereas some embodiments of the present invention are described with respect to implanting the electrical stimulator , for some applications the stimulator is temporarily or permanently inserted into the subject , and techniques described herein are used to optimize the temporary placement of the stimulator . as appropriate , placement of stimulator 30 may be facilitated by fluoroscopy , x - ray guidance , fine endoscopic surgery ( fes ) techniques or by any other effective guidance method known in the art , or by combinations of the aforementioned . typically , skin temperature and / or cerebral blood flow ( cbf ) is measured concurrently with insertion . cbf may be measured with , for example , a laser doppler unit positioned at the patient &# 39 ; s forehead or transcranial doppler measurements . verification of proper implantation of the electrodes onto the appropriate neural structure may be performed by activating the device , and generally simultaneously monitoring cbf . for some applications , stimulator 30 is implanted using techniques described in u . s . patent application ser . no . 10 / 535 , 024 , filed dec . 27 , 2005 , entitled , “ surgical tools and techniques for stimulation ,” which is assigned to the assignee of the present application and is incorporated herein by reference , and / or in the above - mentioned pct publication wo 04 / 043218 . for some applications , techniques described herein are performed in combination with apparatus and / or methods that are described in above - mentioned u . s . patent application ser . no . 11 / 349 , 020 . in an embodiment of the present invention , techniques described herein are performed in conjunction with techniques described in us patent application publication 2004 / 0220644 , which is assigned to the assignee of the present application and is incorporated herein by reference . in an embodiment of the present invention , a calibration procedure is performed , in which a test molecule is injected into the systemic blood circulation of the subject , and a threshold stimulation strength is determined by stimulating at least one mts , and gradually increasing the stimulation strength until the bbb is opened ( e . g ., as determined using a radioactive scanning technique ). system 20 applies therapeutic stimulation to an mts using a strength equal to a certain percentage of the threshold strength , typically less than 100 %. in an embodiment of the present invention , bipolar stimulation is applied , in which a first electrode is applied to a first mts , and a second electrode is applied to a second mts . in some embodiments of the present invention , techniques described herein are practiced in combination with techniques described in one or more of the references cited in the background of the invention section hereinabove and / or in combination with techniques described in one or more of the patent applications cited hereinabove . in some embodiments of the present invention , chemical stimulation of at least one mts is achieved by presenting chemicals , for example in a liquid or gaseous state , to an air passage of the subject , such as a nasal cavity or a throat , or in a vicinity thereof . the temporal profile and other quantitative characteristics of such chemical modulation are believed by the present inventors to have a mechanism of action that has a neuroanatomical basis overlapping with that of the electrical modulation of the mts . for some applications , chemical - presentation techniques described herein are practiced in combination with techniques described in pct patent application pct / il03 / 00338 , filed apr . 25 , 2003 and / or a us patent application filed sep . 27 , 2005 , entitled , “ stimulation for treating and diagnosing conditions ,” both of which are assigned to the assignee of the present patent application and are incorporated herein by reference . the scope of the present invention includes embodiments described in the following patent applications , which are assigned to the assignee of the present patent application and are incorporated herein by reference . in an embodiment of the present invention , techniques and apparatus described in one or more of the following applications are combined with techniques and apparatus described herein : u . s . provisional patent application 60 / 203 , 172 , filed may 8 , 2000 , entitled , “ method and apparatus for stimulating the sphenopalatine ganglion to modify properties of the bbb and cerebral blood flow ” u . s . patent application ser . no . 10 / 258 , 714 , filed oct . 25 , 2002 , entitled , “ method and apparatus for stimulating the sphenopalatine ganglion to modify properties of the bbb and cerebral blood flow ,” or the above - referenced pct publication wo 01 / 85094 u . s . provisional patent application 60 / 364 , 451 , filed mar . 15 , 2002 , entitled , “ applications of stimulating the sphenopalatine ganglion ( spg )” u . s . provisional patent application 60 / 368 , 657 , filed mar . 28 , 2002 , entitled , “ spg stimulation ” u . s . provisional patent application 60 / 376 , 048 , filed apr . 25 , 2002 , entitled , “ methods and apparatus for modifying properties of the bbb and cerebral circulation by using the neuroexcitatory and / or neuroinhibitory effects of odorants on nerves in the head ” u . s . provisional patent application 60 / 388 , 931 , filed jun . 14 , 2002 , entitled “ methods and systems for management of alzheimer &# 39 ; s disease ,” pct patent application pct / il03 / 000508 , filed jun . 13 , 2003 , claiming priority therefrom , and a us patent application filed dec . 14 , 2004 in the national stage thereof u . s . provisional patent application 60 / 400 , 167 , filed jul . 31 , 2002 , entitled , “ delivering compounds to the brain by modifying properties of the bbb and cerebral circulation ” u . s . provisional patent application 60 / 426 , 180 , filed nov . 14 , 2002 , entitled , “ surgical tools and techniques for sphenopalatine ganglion stimulation ,” pct patent application pct / il03 / 000966 , filed nov . 13 , 2003 , which claims priority therefrom , and a us patent application filed may 11 , 2005 in the national stage thereof u . s . provisional patent application 60 / 426 , 182 , filed nov . 14 , 2002 , and corresponding pct patent application pct / il03 / 000967 , which claims priority therefrom , filed nov . 13 , 2003 , entitled , “ stimulation circuitry and control of electronic medical device ,” and a us patent application filed may 11 , 2005 in the national stage thereof u . s . patent application ser . no . 10 / 294 , 310 , filed nov . 14 , 2002 , entitled , “ spg stimulation for treating eye pathologies ,” which published as us patent application publication 2003 / 0176898 , and pct patent application pct / il03 / 000965 , filed nov . 13 , 2003 , claiming priority therefrom pct patent application pct / il03 / 000631 , filed jul . 31 , 2003 , entitled , “ delivering compounds to the brain by modifying properties of the bbb and cerebral circulation ,” which published as pct publication wo 04 / 010923 , and u . s . patent application ser . no . 10 / 522 , 615 in the national stage thereof u . s . pat . no . 6 , 853 , 858 to shalev u . s . patent application ser . no . 10 / 783 , 113 , filed feb . 20 , 2004 , entitled , “ stimulation for acute conditions ,” which published as us patent application publication 2004 / 0220644 u . s . provisional patent application 60 / 426 , 181 , filed nov . 14 , 2002 , entitled , “ stimulation for treating ear pathologies ,” pct patent application pct / il03 / 000963 , filed nov . 13 , 2003 , which claims priority therefrom , and which published as pct publication wo 04 / 045242 , and u . s . patent application ser . no . 10 / 535 , 025 in the national stage thereof u . s . provisional patent application 60 / 448 , 807 , filed feb . 20 , 2003 , entitled , “ stimulation for treating autoimmune - related disorders of the cns ” u . s . provisional patent application 60 / 461 , 232 to gross et al ., filed apr . 8 , 2003 , entitled , “ treating abnormal conditions of the mind and body by modifying properties of the blood - brain barrier and cephalic blood flow ” pct patent application pct / il03 / 00338 to shalev , filed apr . 25 , 2003 , entitled , “ methods and apparatus for modifying properties of the bbb and cerebral circulation by using the neuroexcitatory and / or neuroinhibitory effects of odorants on nerves in the head ,” and u . s . patent application ser . no . 10 / 512 , 780 , filed oct . 25 , 2004 in the national stage thereof , which published as us patent application 2005 / 0266099 u . s . provisional patent application 60 / 506 , 165 , filed sep . 26 , 2003 , entitled , “ diagnostic applications of stimulation ” u . s . patent application ser . no . 10 / 678 , 730 , filed oct . 2 , 2003 , entitled , “ targeted release of nitric oxide in the brain circulation for opening the bbb ,” which published as us patent application 2005 / 0074506 , and pct patent application pct / il04 / 000911 , filed oct . 3 , 2004 , claiming priority therefrom pct patent application pct / il04 / 000897 , filed sep . 26 , 2004 , entitled , “ stimulation for treating and diagnosing conditions ,” which published as pct publication wo 05 / 030025 u . s . provisional patent application 60 / 604 , 037 , filed aug . 23 , 2004 , entitled , “ concurrent bilateral spg modulation ” pct patent application pct / il05 / 000912 , filed aug . 23 , 2005 , entitled , “ concurrent bilateral spg modulation ,” which published as pct publication wo 06 / 021957 u . s . patent application ser . no . 10 / 952 , 536 , filed sep . 27 , 2004 , entitled , “ stimulation for treating and diagnosing conditions ,” which published as us patent application publication 2005 / 0159790 u . s . patent application ser . no . 11 / 349 , 020 , filed feb . 7 , 2006 , entitled , “ spg stimulation via the greater palatine canal ” u . s . patent application ser . no . 11 / 465 , 381 , filed aug . 17 , 2006 , entitled , “ stimulation for treating brain events and other conditions ” u . s . patent application ser . no . 11 / 668 , 305 , filed jan . 19 , 2007 , entitled , “ stimulation of the otic ganglion for treating medical conditions ” in an embodiment of the present invention , electrical stimulation system 20 comprises circuitry described in one or more of the above - mentioned applications . in an embodiment of the present invention , an mts is stimulated using the magnetic stimulation apparatus and methods described in the above - mentioned u . s . patent application ser . no . 10 / 783 , 113 . as used in the present application and in the claims , the bbb comprises the tight junctions opposing the passage of most ions and large molecular weight compounds between the blood and brain tissue . it will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove . rather , the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove , as well as variations and modifications thereof that are not in the prior art , which would occur to persons skilled in the art upon reading the foregoing description . for example , elements which are shown in a figure to be housed within one integral unit may , for some applications , be disposed in a plurality of distinct units . similarly , apparatus for communication and power transmission which are shown to be coupled in a wireless fashion may , alternatively , be coupled in a wired fashion , and apparatus for communication and power transmission which are shown to be coupled in a wired fashion may , alternatively , be coupled in a wireless fashion .
a method is provided that includes identifying that a subject is at risk of suffering from vascular dementia . responsively to the identifying , a risk of development of the vad is reduced by applying electrical stimulation to a site of the subject selected from the group consisting of : a sphenopalatine ganglion , a greater palatine nerve , a lesser palatine nerve , a sphenopalatine nerve , a communicating branch between a maxillary nerve and an spg , an otic ganglion , an afferent fiber going into the otic ganglion , an efferent fiber going out of the otic ganglion , an infraorbital nerve , a vidian nerve , a greater superficial petrosal nerve , and a lesser deep petrosal nerve ; and configuring the stimulation to induce at least one neuroprotective occurrence selected from the group consisting of : an increase in cerebral blood flow of the subject , and a release of one or more neuroprotective substances . other embodiments are also described .