Patent Abstract:
the various embodiments described herein include methods , devices , and systems for enhancement of memory . in one aspect , a method includes : implanting intracranial electrodes in a patient , wherein the electrodes are placed selectively at a specific brain site ; determining a threshold for eliciting an after - discharge threshold of the patient , and stimulating the electrodes with current set below the after - discharge threshold while the patient is in a particular phase of sleep so as to enhance memory of the patient .

Detailed Description:
reference will now be made in detail to various embodiments of the present invention ( s ), examples of which are illustrated in the accompanying drawings and described below . while the invention ( s ) will be described in conjunction with exemplary embodiments , it will be understood that present description is not intended to limit the invention ( s ) to those exemplary embodiments . on the contrary , the invention ( s ) is / are intended to cover not only the exemplary embodiments , but also various alternatives , modifications , equivalents and other embodiments , which may be included within the spirit and scope of the invention as defined by the appended claims . the medial temporal structures , including the hippocampus and entorhinal cortex are critical for the ability to transform daily experience into lasting memories . for example , the hippocampus is among the first brain centers to be affected in alzheimer &# 39 ; s disease ( ad ) with resultant changes in memory performance . this study was undertaken to test the hypothesis that direct deep brain stimulation of the hippocampus or entorhinal cortex can alter memory performance . in an exemplary study , subjects included seven neurosurgical patients implanted with intracranial depth electrodes to identify seizure - onset zones for potential curative resective surgery . patients completed a spatial learning task where they learned various destinations within virtual environments . during half of the spatial learning trials focal electric stimulation was given below threshold required to elicit an after - discharge . while unilateral stimulation of the hippocampus had no effect on memory performance , there was a striking enhancement of memory following unilateral stimulation of the entorhinal region , the primary source of afferents into the hippocampus . entorhinal stimulation applied while subjects learned the location of landmarks resulted in better memory for these locations compared to those learned without stimulation . entorhinal stimulation resulted in resetting the phase of the theta rhythm recorded in the hippocampal electroencephelagram . deep brain stimulation of the entorhinal region enhanced memory for spatial information when applied at the stage of learning . these results raise the possibility that deep brain stimulation of the human entorhinal region that resets the hippocampal theta wave , could be an effective method to enhance memory and may prove beneficial in conditions , such as early ad or minimal cognitive impairment ( mci ), that affect learning and memory for new information . one will appreciate that stimulation applied during other stages may also enhance memory , for example , during the stage of retrieval of information , recall of information , and / or consolidation of information . loss of the ability to transform present experience to what can be later remembered is one of the most dreaded afflictions of the human condition . it is one of the first features of alzheimer &# 39 ; s disease ( ad ), which affects millions of people worldwide ( mount and downton , 2006 ; vas et al ., 2001 ) and is an unwelcome companion of the aging process becoming an increasing burden on individuals and society as life expectancy increases . decades of research and clinical observations have established that declarative memory , the ability to remember recently experienced facts and events , depends on the hippocampus and associated structures in the medial temporal lobe ( mtl ), including entorhinal , perirhinal and parahippocampal cortices ( for review see squire et al ., 2004 ). mtl structures are among the first affected in ad and other disorders affecting memory , with atrophy that is markedly disproportionate to the rest of the brain ( dickson , 2001 ). in ad the first affected mtl structure is the entorhinal region , the source of major input into the hippocampus ( braak & amp ; braak , 1991 ; hyman et al ., 1986 ; gomez - isla et al ., 1996 ), making declarative memory decline one of the earliest and most severe cognitive symptoms of the disease ( buckner , 2004 ; bélanger et al ., 1994 ). the mtl is also one of the earliest brain areas affected in normal aging with specific changes also affecting entorhinal - hippocampal connections within the perforant path ( burke and barnes , 2006 ; wilson et al ., 2006 ; yassa et al ., 2010 ). deep brain stimulation ( dbs ) has emerged in recent years as a powerful technique to treat a host of neurologic and neuropsychiatric disorders from parkinson &# 39 ; s disease and dystonia , to depression , obsessive compulsion disorder and others ( lang and lozano , 1998 ; davis et al ., 1997 ; mayberg et al ., 2005 ; vidailhet et al ., 2005 ). in some of these treatments the nature of the stimulation - induced modification of the neural circuit resulting in improvement is not completely understood and is often a matter of controversy . clearly , the ability to apply deep brain stimulation to modify neurological functions depends on the application of stimulation at distinct and specific sites in the complex neuronal circuitry underlying these functions . in rodents , electrical stimulation of the entorhinal perforant path has been shown to produce hippocampal long - term potentiation , acetylcholine release , and theta phase resetting , all of which are associated with improved memory function ( ehret et al ., 2001 ; feuerstein and seeger , 1997 ; pastalkova et al ., 2006 ; vertes , 2005 ; williams and givens , 2003 ). whether direct stimulation of this entorhinal pathway can enhance memory in humans is unknown . to the best of our knowledge , no animal studies using perforant path stimulation during learning have shown enhancement of hippocampal dependent memory . however , electrical stimulation of targets in the rodent lateral hypothalamus during learning resulted in improved performance on tests of subsequent memory ( redolar - ripoll et al ., 2002 ; soriano - mas et al ., 2005 ). memory enhancement occurred early and was specific to hippocampal - dependent memory tasks ( soriano - mas et al ., 2005 ). thus , stimulation seemed to act directly on specific neural circuits rather than general arousal or reward circuits . a few studies involving direct electrical stimulation of hippocampus in humans have generally shown a disruptive effect on memory . for example , halgren and colleagues ( halgren and wilson , 1985 ; halgren et al ., 1978 ; halgren et al ., 1985 ) showed behavioral deficits including memory impairments with stimulation of hippocampus above the threshold for eliciting an after - discharge in the electroencephalogram ( eeg ). in another study , bilateral stimulation of the hippocampus below the after - discharge threshold did produce deficits in visual recognition memory ( halgren et al ., 1985 ). more recently , it has been shown that stimulation of the hippocampus during encoding has negative effects on subsequent recognition memory for previously learned items ( coleshill et al ., 2004 ; lacruz et al ., 2010 ). however , a recent clinical study showed that stimulation of the hippocampal inputs ( the fornix / hypothalamic area ) in five alzheimer &# 39 ; s patients resulted in increased glucose metabolism of the temporal lobes ( laxton et al ., 2010 ), suggesting that stimulation of hippocampal afferents may have a beneficial effect . in the present study deep brain stimulation was applied to hippocampus and entorhinal region targets while neurosurgical patients learned various locations within a novel virtual environment in order to quickly deliver passengers to particular locations . subjects included seven patients with pharmacologically resistant epilepsy implanted with intracranial depth electrodes in order to identify seizure - onset zones for potential curative surgery . in applying deep brain stimulation to two sites in the mtl circuitry underlying declarative memory we set out to test the hypothesis that such site - specific stimulation applied at a particular phase of information processing will modify human memory . seven neurosurgical patients ( all right - handed , 3 female , 20 - 52 years old , mean age 35 . 4 ) with pharmacologically resistant epilepsy were implanted with intracranial depth electrodes for 7 - 10 days to determine the epileptogenic zone or area of seizure - onset for possible surgical resection . one will appreciate that the electrodes are immediately functional following the implant surgery . as such , one will appreciate that such electrodes may also be utilized to provide stimulation in chronic applications , including indefinite chronic stimulation . patients met clinical criteria for the procedure ( engel , 1993 ; fried et al ., 1993 ) and had not undergone prior neurosurgical resection . for patient demographics , including seizure foci and neuropsychological test scores see the following tables . table 1 lists the patient demographics , showing the 7 patients age , gender and handedness . table 2 lists clinical characteristics of patients . attention ( digit span ) was calculated using the wechsler adult intelligence scale ( wais iii ). verbal and non - verbal ( visual ) memory - related performance was assessed with the wechsler memory scale ( wms ), california verbal learning test ( cvlt ), and the rey - osterrieth complex figure test . executive function was measured using the trail making test ( trails b section ). neuropsychological test values are reported as percentages based on standardized scores . table 3 includes stimulated medial temporal lobe ( mtl ) regions , including right and left entorhinal region ( rec , lec ) and right and left hippocampal regions ( rah , lah ). also shown are the clinically determined seizure - onset zones for each patient . cap x indicates a region that fell with the epileptogenic zone for that patient . for each patient , mm abnormalities if any are also indicated . electrode placements may be determined based on clinical criteria . six of the seven patients had right and / or left entorhinal electrodes and all had at least one hippocampal electrode . in four of the seven patients who had both hippocampal and entorhinal electrodes implanted on the same side , eeg data was recorded and analyzed . all patients volunteered for the study by providing informed consent ; the study was approved by and conformed to the medical institutional review board at ucla . stimulation was current regulated , charge - balanced , with biphasic rectangular pulses set below the after - discharge threshold ( based on pretesting ; ranged from 1 . 0 ma - 2 . 0 ma ). patients were blind to stimulation condition and no patient reported noticing any effect of the stimulation . electrode contacts are stimulated through interface with a grass c - 12 stimulator , telefactor relay box , and stellate recording system . in this case stimulation was applied by external stimulator to demonstrate efficacy in this group of patients . one will appreciate that in chronic deep brain stimulation the stimulation may be applied by internal , specially implanted , stimulator ( s ). specifically , we used 5 sec on / off trains of stimulation at 50 hz with a 300 μsec pulse length using bipolar electrodes ( 0 . 059 cm 2 in surface area , 1 . 5 mm apart ). all current delivered ranged from 0 . 5 - 2 ma ( in some cases , 0 . 5 - 1 . 5 ma ) with stimulation ranging between 2 . 5 - 10 . 1 μc / cm 2 per phase ( 2 . 5 - 7 . 6 μc / cm 2 per phase ), well below the safe maximum used for chronic ( 30 μc / cm 2 per ph ) and acute ( 57 μc / cm 2 per ph ) stimulation ( agnew & amp ; mccreery , 1990 ; gordon et al ., 1990 ). the impedance measured was between 1 - 4 kω previous human studies using stimulation parameters of up to 3 . 0 v , 450 μs pulse width and 130 hz , have shown to be safe and well tolerated in patients with epilepsy with depth electrodes in the temporal lobe ( boon et al ., 2007 ), and similar stimulation levels may be used to control seizures in epilepsy . one will appreciate that various parameters may be used in accordance with the present invention . a neurologist was present during all sessions to monitor patient at bedside and view eeg data on - line . no seizures were elicited during stimulation in the current study ; similar stimulation levels have been used in clinical studies for seizure control in epilepsy ( velasco et al ., 2006 ; boon et al ., 2007 ; for review see jobst , 2009 ). patients completed a spatial learning task that consisted of navigation through a virtual environment and delivery of passengers to various stores ( fig1 a ). this “ yellow cab ” task has been used in several studies showing mtl neuronal recruitment during navigation ( ekstrom et al ., 2007 ; ekstrom et al ., 2003 ; ekstrom et al ., 2005 ; suthana et al ., 2009 ). one will appreciate that other tasks may be used including learning the names of persons , creating new associations such as face - name associations , and other types of learning and memory tasks . in order to familiarize patients with the tasks , alternate versions ( with new stimuli ) were given prior to testing . the experimental session consisted of alternating blocks of spatial learning and control conditions . during each spatial learning block , patients actively explored a virtual environment using a joystick by searching for passengers and delivering them to designated locations ( e . g . stores ). for all trials , passengers were positioned in the center of a rectangular grid - like city ( fig1 a ). patients learned to navigate to six stores in a virtual reality environment ; each store was repeated in each of the four blocks ( 24 total navigation trials ). starting point of each trial began from the previous trial &# 39 ; s store location and therefore varied on each trial . order of stores was counterbalanced within the task . for three of the stores ( white boxed stores , fig6 a ), stimulation occurred during the first 3 learning trials , and for the other 3 stores , no stimulation was given . learning the location of stores under stimulation and no stimulation alternated , and whether learning the first store location occurred under stimulation or not was counterbalanced across patients . each of the stores occurred equally often in the stimulation and non - stimulation condition across patients . during a “ stimulation ” store trial , stimulation was given throughout the entire trial in 5 sec on / off trains ; time of trials varied because the trial continued until the patient located the store ( average trial time = 14 . 76 ± 1 . 84 sec ). no stimulation was given during the fourth spatial block of trials ( block 4 ) in which memory performance was assessed and compared between those locations where stimulation had been previously applied , and those locations where no stimulation had been applied . two control tasks were interspersed within the spatial learning trials . these tasks were included to measure whether any effect of stimulation on the spatial learning task was due to improved motor or perceptual abilities . the first task was a guided navigation task in which , patients were instructed to follow the arrows on a screen using the joystick ( fig6 c ); each block lasted 1 min with stimulation during either the first or last 30 sec of the task ( counterbalanced across patients ). during the 30 sec stimulation periods , stimulation was given in 5 sec on / off trains . the second control task was a store - matching task where patients had to press the corresponding button in order to select one store that matched the target store presented above ( fig6 b ). block durations were also 1 min and each trial was present for 4 seconds with a 1 sec inter - trial interval . stimulation was given during every other store trial ; stimulated trials were counterbalanced across patients . all stimuli were presented using a macintosh laptop computer . pyepl ( python experiment program library ; http :// pyepl . sourceforge . net /) was used to present virtual reality stimuli and to record navigational routes and key press reaction times . spatial learning was quantified by first calculating the shortest path length ( ideal path ) from a passenger to the target store destination ( hartley et al ., 2003 ; newman et al ., 2007 ). next , the patients &# 39 ; actual path length was calculated . the key dependent variable in the study was the excess path length determined by subtracting the ideal path from their actual path to the store for each given trial ( see fig6 a ). the shorter the excess path length the better is the subject &# 39 ; s performance . reaction time performance on the control tasks was calculated using matlab ( mathworks , inc ). prior to implantation with depth electrodes , patients were scanned with a siemens trio head - only 3 tesla scanner . high in - plane resolution structural images with a matrix size of 512 × 512 ( spin echo , tr = 5200 ms , te = 105 ms , 19 slices , contiguous ; voxel size : 0 . 391 × 0 . 391 × 3 mm ) were acquired in the oblique coronal plane perpendicular to the long axis of the hippocampus . patients also received a 3 - t whole brain t1 weighted mp - rage gre scan ( tr = 1800 msec , te = 2 . 93 sec , voxel size = 0 . 9 × 0 . 9 × 0 . 8 mm ) as part of depth - placement planning . patients were then implanted with depth - electrodes by stereotactic methods for seizure monitoring as previously described ( fried et al ., 1999 ). following implantation with depth - electrodes , subjects received a spiral ct scan ( 1 sec rotation , high - quality ( hq ) mode , helical pitch 1 . 5 , 1 mm slice collimation , and a 0 . 5 mm reconstruction interval to localize electrodes ). cts were registered to the high - resolution mri and to the whole brain mri using a 3 - way registration in brainlab stereotactic and localization software ( www . brainlab . com ; gumprecht et al ., 1999 ; schlaier et al ., 2004 ). electrode contacts were registered and visualized on the high - resolution coronal mri ( fig2 ; fig7 ). mtl subregions were anatomically determined by boundaries that were demarcated based on atlases correlating mri visible landmarks with underlying cellular histology ( amaral and insausti , 1990 ; duvernoy , 1998 ). these methods have previously been used to localize microelectrodes and investigate structural and functional dissociations within human mtl subregions ( suthana et al ., 2009 ; ekstrom et al . 2008 , zeineh et al ., 2003 ). stimulation was given using the two most distal contacts for each electrode ( fig2 ). for the hippocampus , at least one contact fell within the ca1 region of the hippocampus , whereas for the entorhinal electrodes at least one contact fell within the alvear bundle / perforant path . for all patients &# 39 ; electrode localizations see the following table 4 . table 4 includes patient electrode localizations . region placements shown are for the two most distal macro - electrodes from the right and left anterior hippocampus ( rah and lah ), and right and left entorhinal cortex ( rec and lec ). medial temporal lobe ( mtl ) electrodes were also localized to specific gray or white matter regions within the entorhinal region and hippocampal subregions ca3 and dentate gyrus ( ca3dg ) or ca1 . for each patient , the threshold for eliciting an after - discharge was determined immediately prior to behavioral testing . the presence of after - discharges was determined by a clinical neurologist ( z . h or j . s ). the stimulation current level for the study was set 20 - 25 percent below the after - discharge threshold determined for each patient . we eliminated trials from our analysis where after - discharges occurred . for all patients , no seizures had occurred within 24 hours prior to testing . in the four patients with electrodes in the entorhinal and ipsilateral hippocampus , eeg data from the hippocampus were analyzed with matlab ( mathworks , inc ). each data record was filtered for the theta ( 3 - 8 hz ), alpha ( 9 - 14 hz ), beta ( 15 - 35 hz ), and gamma ( 35 - 100 hz ) frequencies and individually examined for artifacts and noise before being included in the eeg reset analysis . to determine if waveform resetting occurred in the hippocampus after stimulation of the entorhinal region , waveforms from the 5 seconds of each trial during and between each stimulation train were separately averaged . if phase resetting occurred during the stimulation trains , one should see greater alignment of waves and thus greater amplitude in the averaged waveform ( williams and givens , 2003 ). we then calculated the percent increase in theta resetting for 5 - sec post - stimulation onset compared to 5 - sec pre - stimulation onset ( fig5 ). also , we compared the power of the averaged waveforms for the stimulation and non - stimulation conditions for each of patient ( fig9 ). for non - stimulation trials , we obtained comparable averaged waveforms by averaging the waveforms for alternating 5 - second periods of the non - stimulation learning trials . to insure that resetting was not due to increases in the power of each trial &# 39 ; s rhythm , we also calculated the average power across all stimulation and non - stimulation navigation trials . we repeated the resetting analysis for each of the four frequency ranges ( theta , alpha , beta , and gamma ). in order to determine significant differences in performance between stimulation versus non - stimulation trials , we completed a 2 ( condition )× 3 ( block ) repeated measures anova for excess path length in each region ( entorhinal and hippocampus ). for block 4 , in which no stimulation was given , we did paired sample t - tests across trials ( p & lt ; 0 . 05 , bonferroni corrected , n = 18 trials ) to determine significant differences in excess path length for locations that had been learned under stimulation in blocks 1 - 3 compared to those that had been learned without stimulation . we also performed the paired sample t - tests across patients , which yielded the same results ( fig8 ; p & lt ; 0 . 05 , bonferroni corrected , n = 6 patients ). in addition , for each patient we calculated the percent reduction in excess path length on block 4 for locations that had been learned during stimulation compared to those learned without stimulation . for eeg analyses , we performed paired sample t - tests ( p & lt ; 0 . 05 ; bonferroni corrected ) to compare the power of the average theta waveforms during 5 - sec post - stimulation and 5 - sec pre - stimulation on navigation trials ( fig5 ). we did the identical comparison for the non - stimulation navigation trials ( fig5 ). we also compared the averaged theta power across all individual stimulation and non - stimulation navigation trials ( fig9 c ) using a paired sample t - test ( p & lt ; 0 . 05 ; bonferroni corrected ). lastly , the phase resetting analysis was repeated for each frequency range . fig3 shows the average of patients &# 39 ; behavioral performance during spatial learning with unilateral stimulation of the entorhinal region ( n = 18 trials ) or hippocampus ( n = 18 trials ). during the spatial learning task , patients picked up passengers and delivered them to stores within a virtual city . performance on each learning trial was measured by calculating the patients &# 39 ; excess path length ( deviation from the shortest path to the target location ). shorter excess path length equaled better performance . regardless of entorhinal region stimulation , there was a trend for excess path length to decrease across the first three blocks , ( anova stim x block 1 - 3 ; f ( 9 . 45 )= 3 . 145 ; p = 0 . 056 ). on the retention block ( block 4 ), when memory for the store location was tested without stimulation , there was a significant benefit ( shorter excess path length ) for those locations which had been previously learned during stimulation of the entorhinal region compared to locations which had been learned without stimulation ( n = 18 trials ; stim & gt ; non - stim , t =− 3 . 2807 ; p & lt ; 0 . 05 , corrected ). these results show that stimulation of the entorhinal region during learning results in enhancement of the performance on later recall . strikingly , enhancement occurred in every single one of the 6 patients tested with entorhinal stimulation ; average reduction in excess path length during retention with stimulation across all patients was 70 % ( fig4 ). reduced excess path indicates efficient navigation and better memory for the location of the store . maximum improvement is 100 % reduction , or a final excess path length of zero using the most efficient route . memory enhancement effects occurred despite the wide range of neuropsychological test scores ( see above table 3 ). for example , patient 2 , who scored in the impaired range on standardized tests of memory and executive function , showed an 86 . 9 % improvement for those locations learned during stimulation compared to those learned without stimulation . even patient 1 , who performed relatively well without stimulation , was nevertheless able to improve by 38 . 9 % to achieve optimal performance for those locations learned during stimulation . for patients 2 - 6 , navigation to all stores learned under stimulation was more efficient than navigation to any store location learned without stimulation , demonstrating the strong reliability of the effect . in contrast to the striking effects seen with stimulation of the entorhinal region , direct hippocampal stimulation resulted in no effect on the spatial learning task performance ( stim & gt ; non - stim , t = 0 . 0161 ; p = n . s . ; fig3 b ). in other words , memory performance was not enhanced , nor was it reduced , when patients navigated to locations , which had been previously learned while unilateral stimulation to the hippocampus was applied . neither entorhinal nor hippocampal stimulation significantly affected reaction time performance on the guided navigation control task ( fig8 a ) or the perceptual store - matching task ( fig8 b ). these results suggest that the benefits of stimulation in the spatial learning tasks were not due to general enhancement of perceptuo - motor processes involved in performing the spatial learning task . in four patients who had ipsilateral entorhinal and hippocampal electrode contacts , we investigated whether stimulation of the entorhinal region affected the hippocampal theta rhythm . supplementary fig4 a shows an example patient &# 39 ; s hippocampal theta resetting ( shown visually as an increase in power of the averaged waveform ) after stimulation of the entorhinal region . shown is the final average waveform of all stimulation and non - stimulation navigation trials during 5 sec pre - stimulus and during the 5 sec post - stimulation onset periods . hippocampal theta resetting after entorhinal stimulation showed a 44 . 3 ± 6 . 9 % increase during stimulation compared to before stimulation ( fig5 ; stimulation & gt ; pre - stimulation , t = 10 . 72 ; p & lt ; 0 . 05 ). there was no significant difference in the percentage change of theta resetting for comparable alternating 5 - second periods for non - stimulation trials ( non - stimulation & gt ; pre - non - stimulation , t =− 1 . 3548 ; p = n . s .). quantification of theta resetting across all 4 patients yielded a significant increase in power of the final average waveform during stimulation vs . non - stimulation trials ( fig9 b ; stimulation & gt ; non - stimulation , t = 5 . 39 ; p & lt ; 0 . 05 ). lastly , hippocampal theta resetting was not due to individual trial differences in theta power ( fig9 c ); average theta power of individual waveforms during stimulation vs . during non - stimulation trials was not significantly different . this suggests that stimulation did not merely evoke a larger theta rhythm , but rather phase - shifted an ongoing theta rhythm . we did not find significant changes during stimulation in either phase resetting ( power of the average waveforms ) or average power in all other frequency ranges ( alpha , beta , and gamma ). spatial navigation depends on spatial memory . most common tasks of daily living such as finding one &# 39 ; s care in a parking lot are dependent on the mtl . our results show that spatial learning can be enhanced by stimulation of the entorhinal region , a specific site within the mtl and the chief gateway into the hippocampus . indeed , stimulation of the entorhinal region while subjects were learning was associated with improved memory performance , as measured by speed and choice of route . one will appreciate that these circuits may be modulated during other types of declarative learning and under what circumstances the consequences of such modulation is augmentative . it is possible that stimulation will also facilitate verbal , autobiographical , and associative learning and memory , which have been shown previously to rely on the mtl region ( eichenbaum et al ., 2007 ; squire et al ., 2004 ; maguire et al ., 2001 ). thus , a complete characterization of stimulation &# 39 ; s effects on learning and memory will provide insights into whether electrical modulation of memory circuits could be used as a therapeutic strategy to enhance specific aspects of memory function in patients with memory disturbances . because of the mtl &# 39 ; s central role in the neuroanatomy of declarative memory , it is clear how the disruption of these structures fundamentally impairs declarative memory in ad patients . it may be possible to mitigate the functional burden of the disease for a time and slow the rate of progression in memory decline by stimulating and improving function within and between these brain regions . early results provide support for the idea of a safe , stereotactically guided surgical strategy for stimulating and enhancing neural function in early ad patients ( laxton et al ., 2010 ; tuszynski et al ., 2005 ). given that the entorhinal region is the first region affected in ad ( braak & amp ; braak , 1991 ; gomez - isla et al ., 1996 ), viable disease - modifying strategies would benefit from targeting the area of initial structural and functional compromise . certainly , given evidence that electrical stimulation can enhance neurogenesis in the hippocampus ( toda et al ., 2008 ), there is even the possibility of regeneration . the potential effectiveness of stimulation in enhancing memory has previously been demonstrated in one human neurosurgical patient who had deep brain stimulation ( dbs ) electrodes implanted in the hypothalamus in close association with the fornix in an attempt to treat morbid obesity ( hamani et al ., 2008 ). neuropsychological testing showed enhancement of verbal recall , measured 3 months after dbs , while performance on other neuropsychological tests was unchanged . stimulation of this fornix / hypothalamic region has also been shown to activate the medial temporal circuitry measured with eeg and positron emission tomography ( pet ) in 6 patients with early ad ( lacruz et al ., 2010 ). behavioral enhancement , was however , not clear in all patients perhaps due to the already increased rate of memory decline from the disease . our results presented here using a within - subjects design show that stimulation of the entorhinal region , the major source of cortical afferent input into the hippocampus can strongly enhance declarative memory . one question is whether stimulation can be effective during recall in addition to during learning . in the current study , stimulation during learning was sufficient to enhance subsequent memory when tested later without stimulation . one may appreciate that such stimulation might also enhance recall in addition to its benefits during learning . several animal studies have shown that stimulation of the entorhinal perforant pathway that elicits after - discharges and overt behavioral changes produces impairments in learning and memory ( lopes da silva et al . 1986 ; robinson et al . 1993 ; gilbert et al . 1996 ; hannesson and corcoran , 2000 ). in humans , stimulation of the hippocampus above the after - discharge threshold is sufficient to impair memory ( halgren and wilson , 1985 ; halgren et 1978 ; halgren et al ., 1985 ). other studies have shown that unilateral or bilateral stimulation below threshold for after - discharge elicitation also produced memory deficits ( lacruz et al ., 2010 ; halgren et al ., 1985 ; coleshill et al ., 2004 ). with the stimulation parameters used here , unilateral hippocampal stimulation below threshold for after - discharges did not affect subsequent spatial memory although it is possible that bilateral hippocampal stimulation would have been disruptive . the perforant path , consisting of major cortical afferent projections into the hippocampus ( insausti et al ., 1987 ) has been recently visualized in humans using high - resolution diffusion tensor imaging ( yassa et al ., 2010 ; augustinack et al ., 2010 ). although we did not acquire diffusion tensor images , localization of stimulating electrodes to subjects &# 39 ; high - resolution structural magnetic resonance images suggest placement within the human perforant pathway . one will appreciate that it may be beneficial to specifically target the perforant path to achieve the most therapeutic efficacy . the theta rhythm ( 3 - 8 hz ) is a large eeg potential recorded from the hippocampus in both rodents and humans ( buzaki , 2002 ; winson 1978 ; givens 1996 ; mormann et al ., 2005 ; ekstrom et al ., 2005 ; kahana et al ., 1999 ). theta phase resetting , which is the phase - locking of the theta rhythm to incoming sensory stimuli , ( adey , 1967 ; givens , 1996 ; vinogradova et al ., 1996 ; brankack et al ., 1998 ) has been suggested to enhance memory by allowing optimal encoding of novel stimuli . stimulation of the perforant pathway has previously been shown to result in theta resetting in the rodent hippocampus ( williams and givens 2003 ). in the current study , four patients who had ipsilateral hippocampal and entorhinal contacts exhibited hippocampal theta phase resetting during stimulation of the entorhinal region . in a study using functional magnetic resonance imaging ( fmri ) in human subjects , information learned during increased spontaneous activity in the entorhinal cortex was subsequently remembered better ( fernandez , et al ., 1999 ). these findings suggest that increased entorhinal input to the hippocampus can improve learning . although further studies are needed , these preliminary results provide evidence for a possible mechanism for stimulation - induced memory enhancement in humans . in summary , we present data here showing that stimulation of the entorhinal region during learning of a novel virtual city significantly enhanced memory performance and resulted in hippocampal theta resetting . these effects were regionally specific ; stimulation of the hippocampus yielded no changes in performance . this dissociation suggests that stimulating hippocampal afferents , rather than the hippocampus itself , results in more effective learning . furthermore , stimulation of the entorhinal region did not appear to enhance performance through improvement of general perceptuo - motor function , but rather through improved spatial learning . overall , these results raise possibilities of a future therapeutic strategy for ameliorating memory disturbances in diseases such as early alzheimer &# 39 ; s disease . the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teachings . the exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application , to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention , as well as various alternatives and modifications thereof . it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents . 1 ) amaral , d . g . & amp ; 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