consolidation 
 refers to   the idea that , following the experience of an event , the memory for 
 that event undergoes a process of stabilization that renders the memory more 
 resistant to brain injury or interference from similar experiences .
building on the 
 work of marr [ 1 , 2 ] , squire et al . 
   suggested that a memory representation or   memory trace was dependent upon both 
 medial temporal lobe ( in particular , the hippocampus proper ) and neocortical 
 structures , and that consolidation was the process by which cortical - cortical 
 connections within the trace were strengthened until eventually the memory 
 could be retrieved in the absence of the hippocampus .
the 
 question of whether a consolidated autobiographical memory , dependent primarily 
 on neocortex rather than hippocampus , is qualitatively unchanged from the 
 memory that was first encoded was not addressed explicitly in the squire et al . 
  proposal .
indeed , the consolidation 
 view inherently assumed that memories over time remained a faithful record of 
 the original event .
this assumption was 
 previously questioned by bartlett 
  , who demonstrated that memory retrieval was a constructive process rather 
 than a mere replay of the past . using 
 the now famous  war of the ghosts  story and what he called the method of 
 repeated reproduction , he showed that there was considerable variability in how 
 a story was recalled over time .
repeated 
 reproduction of the story typically led to a shortened , more stereotyped 
 version of it , with details either discarded , transformed , or added anew .
bartlett 's 
 study implied that changes in a memory after initial learning affected not only 
 the strength , but the content of that memory as well . in 
 recent years
one version 
 emphasizes that the role of the hippocampus in retrieval is time - limited and 
 that the informational components of memories are represented solely in 
 cortical regions  this has become known as the standard theory 
 of memory consolidation cf . [ 5 , 
 6 ] . 
 thus , the content of memories remains unchanged through the 
 consolidation process .
remote memories 
 retrieved solely from neocortex ( as the case in amnesic patients with 
 hippocampal damage ) should be as rich and detailed as remote memories retrieved 
 by comparable controls with an intact hippocampus . without further hippocampal involvement , the 
 content of consolidated memories should remain stable and consistent over time , 
 a faithful record of the original event .
the theory is supported by evidence of 
 temporally graded retrograde amnesia and a correlation between the severity of 
 retrograde amnesia and the severity of anterograde amnesia [ 712 ] .
nadel 
 and moscovitch   developed an alternative theory of memory consolidation , 
 known as the multiple trace theory ( mtt ) . 

similar to the standard theory of consolidation , mtt posits that the 
 establishment of long - term memories involves a lengthy interaction between the 
 hippocampal region of the medial temporal lobes ( mtls ) and neocortical regions 
 both adjacent to the mtl ( e.g. , perirhinal and parahippocampal cortices ) and at 
 a distance ( e.g. , prefrontal cortex ) . 

those memories that are reactivated , it is presumed , are made stronger 
 while others are forgotten . unlike 
 standard theory , mtt posits that the hippocampus remains an integral part of 
 the memory trace and is thus always involved in retrieval of long - term episodic 
 memories regardless of the age of the memory . 

evidence supporting this view comes from neuroimaging studies showing 
 that retrieval of detailed episodic memories activates the hippocampus no 
 matter how old these memories are [ 1418 ] and from studies showing that remote 
 episodic memories retrieved by amnesic patients lack the detail present in 
 remote episodic memories retrieved by an individual with an intact hippocampus 
 .
according 
 to mtt , each time an episode is retrieved and rehearsed , a new 
 hippocampally - dependent trace is created . 

retrieval , or reactivation , of a memory trace leads to reencoding , which 
 both strengthens and changes that trace making the details of the event more 
 accessible , either through an expansion of the original trace or creation of a 
 new , altered trace .
importantly , the 
 altered trace may incorporate additional components of the context of 
 retrieval , or even new information that is inadvertently ( or incorrectly ) 
 generated by the act of retrieval .
in 
 this regard , mtt provides a mechanism for bartlett s 
  notion that as memories age and consolidate , they are not just 
 strengthened , but also may be qualitatively altered .
the 
 present study examined the effects of the passage of time and repeated reactivation , 
 or retrieval , on remote autobiographical memories , and how medial temporal lobe 
 and neocortical structures change in response to these two variables using 
 functional magnetic resonance imaging ( fmri ) . 

twelve middle - aged participants recalled autobiographical memories for 
 events that occurred at least two years prior to the time of the study .
one group of 
 memories was retrieved during a single retrieval session on day 1 of the study 
 and not again until the day of the scan , which occurred 30 days later ( remote retrieval 
 condition ) .
another group of memories 
 was retrieved repeatedly during multiple retrieval sessions that occurred 
 weekly on days 1 , 7 , 14 , 21 , and 28 of the study and finally in the scanner ( multiple 
 retrieval condition ) .
the third group of 
 memories was retrieved during a single retrieval session on day 28 as well as 
 in the scanner ( recent retrieval condition ) . 
 on day 30
while the study focused primarily 
 on fmri patterns of activation within medial temporal lobe and other cortical 
 regions , the design also allowed us to examine the effect of the passage of 
 time and repeated retrieval on qualitative aspects of the retrieved 
 memories .
while 
 neither the standard theory nor mtt makes explicit claims about the qualitative 
 changes that occur to memories as they undergo repeated retrieval , considering 
 the assumptions of mtt outlined earlier , we hypothesized that multiple 
 retrievals would result in the memories becoming more accessible and more 
 detailed over time .
we further 
 hypothesized that , contrary to standard theory , activation within the medial 
 temporal lobe , including hippocampus proper , would be either maintained or 
 increased as a function of multiple retrievals in comparison to the mere 
 passage of time .
  twelve middle - aged participants ( ages 4063 ; 
 mean age 54.6 ; mean years of education , 16.2 ; range 1220 ) with no prior 
 history of head injury , neurological disorder , or psychiatric disorder participated 
 in this study . participants received 
 monetary compensation for their participation .   a list of typical life events , such as  your wedding day  or  a 
 birthday party ,  was used to generate memory prompting cues for the memory 
 retrieval sessions .
the list was an 
 extended version of the one developed by levine et al . .
participants were instructed to recall events 
 that occurred at least two years ago and extending as far back as they could 
 remember .
they were asked to provide the 
 approximate date of each memory to ensure that it occurred more than two years 
 ago .
they were also instructed to discuss 
 exclusively events that occurred in a specific place and time and that happened 
 only once .
each participant was 
 instructed to visualize the details of the event , mentally playing the event out as if it were a scene in a movie , while verbally describing all the details of 
 the event that they could remember , including what happened , who was there , 
 where they were , the physical details of the scene , and the time of day .
following recollection of each event , 
 participants were asked to rate the memory on several scales , including the 
 importance of the event both at the time it occurred and currently , the 
 emotionality of the event at the time it occurred and currently , how vividly 
 the memory was recalled , and their overall arousal or energy level at the time 
 of the event .
ratings were made on a 15 
 scale , respectively , representing not at all , somewhat , moderately , very , or 
 extremely .
participants were also asked 
 to rate how positive or negative the event was at the time that it occurred using 
 the following scale : very negative ( 3 ) , somewhat negative ( 1 ) , neutral ( 0 ) , somewhat positive ( + 1 ) , 
 and very positive ( + 3 ) . at 
 the   end of the interview session , participants were instructed not to ruminate 
 on any of the memories or relate the memories to friends or family until 
 completion of the experiment . 

the experimenter used the information derived 
 from the initial retrieval session to create specific cues for each memory for 
 use in subsequent retrieval sessions , for example , 
 in each of the retrieval 
 sessions that followed , participants were instructed to recall all the details 
 they could remember about the event , even if they had already mentioned them in 
 a previous retrieval session .
one 
 interviewer conducted all the initial interview sessions and another 
 interviewer conducted all the subsequent phone interview sessions .
the memory cues were presented in a new , 
 randomized 
 order at each retrieval session .
all 
 sessions were tape recorded and then transcribed afterwards .   in the initial 
 interview session ,
participants were provided with generic event cues until they 
 generated a list of 24 autobiographical memories as described in section 
 3.2 .
participants were asked to discuss memories 
 that were particularly memorable and rich in detail . if only a few aspects of a memory were 
 retrieved and no further information came to mind , the participant was asked to 
 move on to another cue .
the interviewer 
 kept track of the number of positively and negatively rated memories to ensure 
 that an approximately equal number of each was collected .
after the interview , the 24 memories were 
 divided into two lists of 12 , with each list including approximately the same 
 number of memories from each lifetime period ( childhood , adolescence , young 
 adulthood , and middle age ) , as well as roughly the same number of positive and 
 negative events .
one list was used in 
 the   remote retrieval condition and 
 the other list was used in the   multiple retrieval condition .
the remote retrieval items 
 were not retrieved again until the day of the scan ( day 30 ) and the multiple retrieval 
 items were retrieved during four additional weekly phone interviews scheduled 
 throughout the month , and then finally on the day of the scan ( days 7 , 14 , 21 , 
 28 , and 30 ) .   
they were provided with the 12 specific 
 memory cues from the multiple retrieval list derived from their memories 
 gathered on day 1 .   on day 28 , in addition 
 to retrieving items from the multiple retrieval condition as described above , 
 during the final phone session participants were interviewed exactly as they 
 were on day 1 for 12 additional autobiographical memories .
the memories met the same criteria as 
 memories in the other two conditions , having occurred over two years ago , and 
 including a similar number of positively and negatively valenced memories from 
 a similar distribution of life periods .
thus , 
 memories were obtained and retrieved under three conditions , as depicted in 
 figure 1 : remote retrieval  only retrieved 
 once , 30 days prior to the scan 
 session ; multiple retrieval  retrieved five times throughout the course of 
 the month leading up to the scanning session ; and recent retrieval  only retrieved once , 2 days prior to the scanning 
 session . 
  during fmri scanning , stimuli were 
 presented using dmdx presentation software   on high - resolution visuastim 
 digital goggles ( resonance technologies , inc .
participants 
 held a mouse in their right hand that was modified for use in the scanner .
participants were presented with all 36 
 memory cues described earlier in random order . 

participants were instructed to press the mouse 
 button as soon as they had read the memory cue and were aware of the memory 
 that the cue referred to .
they were 
 instructed to recall all of the details of the memory throughout the remainder 
 of the 12-second period , exactly as they had in each previous retrieval 
 session .
each memory cue was followed by 
 a 4-second  rest  period . during this 
 time
, participants were instructed to clear their minds and wait for the next 
 cue . following 
 scanning , participants were asked a series of follow - up questions regarding 
 their memories . for each memory , they 
 were asked whether or not they had been successful in the scanner in 
 remembering the memory that corresponded to the cue provided , and if so , if 
 they actively retrieved the details of the event for the full 12 seconds that 
 the cue was presented .  
images were collected on a general electric 
 3.0 tesla signa vh / i whole body echospeed scanner equipped with optimized acgd 
 gradients .
a sagittal localizer was 
 collected first for use in aligning t1-weighted anatomical images ( matrix = 256 
  256 , tr = 500 , te = 14 milliseconds , fov = 24  cm , sections = 31 , 4  mm , no 
 skip ) parallel to the anteroposterior commissural plane covering the whole brain .
following collection of the t1 images , 
 functional images were acquired in a single functional scan in the same 
 alignment as the t1 scans , using a single - shot spiral in / spiral out sequence 
  ( matrix = 64  64 , fov = 24  cm , tr = 2040 milliseconds , te = 30 milliseconds , 
 flip angle = 90 , 
 sections = 31 , thickness = 4  mm , no skip ) . 

a total of 400 volumes were collected , taking approximately 14 minutes 
 to complete .
finally , a high - resolution 
 spgr 3d anatomical volume was acquired ( 1.5  mm sections covering whole brain , 
 matrix = 256  256 , tr = 22 milliseconds , te = 4 milliseconds , flip angle = 30 , fov = 25  cm ) for coregistration 
 of images in mni coordinate space .
audio recordings of each of the five 
 retrieval sessions were transcribed for script analysis .
, three types of details were identified : internal , external , and 
 editorial .
internal details referred to 
 information that was central to the memory event itself , including the time , 
 place , date , and names of individuals , any specifics about the location or what 
 happened during the event .
these details 
 occurred or were present during the time frame of the event itself .
for example ,  this was during the summer 
 before i turned sixteen  provided the timing of the event 
external details reflected 
 general information not unique to the memory , or referred to events that occurred 
 outside of the time window of the memory event , or provided a judgment about 
 the present based on the past . for 
 example , 
i had gone on train rides in the past , to the grand 
  canyon and such  , provided context for the event 
taking your 
 first plane flight  but did not provide specific information about the event 
 itself .
editorial details included statements 
 made by the participant that reflected uncertainty , such as , 
now that i think about it , it had to have been   , providing no additional 
 information regarding the memory .
two independent 
 raters performed the script analysis on all memories , with inter - rater 
 reliabilities above 85% .
any 
 discrepancies were discussed and adjudicated by j. campbell . for 
 the purpose of analyses , internal and external details were added together and 
 are referred to as   total memory detail 
 count . for each memory
the total 
 number of words spoken by the participant was obtained using the word counting 
 function in microsoft word .
in addition , 
 three memories from each participant were selected at random for consistent analysis .
essentially , the phrases used to describe 
 each separable detail of each memory were analyzed for consistency across each 
 retrieval session . for retrieval 
 sessions on days 7 , 14 , 21 , and 28 ,
the number of details that were repeated 
 from the previous retrieval session was measured and expressed as a proportion 
 of the previous session details . for 
 example , if five details were described in the initial retrieval session on day 
 1 and four of those details were repeated during retrieval of the same memory 
 on day 7 , the consistency score would be 4/5 , or 0.80 .
single retrieval memories were retrieved for 
 a second time in the scanner on day 30 . 
 as a result , behavioral data from this session are not available for 
 analysis .  
analysis of functional neuroimages software 
 ( afni ; ) was used to examine images for motion or other artifact .
images were processed and analyzed using 
 statistical parametric mapping 2 ( spm2 , wellcome department of cognitive 
 neurology , university of glasgow , glasgow , scotland ) .
preprocessing included realignment , 
 normalization to a standard mni template ( http://www.mrc-cru.cam.ac.uk ) , and smoothing   using an 888  mm gaussian 
 filter .
the design was specified using a 
 hemodynamic response function ( hrf ) with partial derivatives for time and 
 dispersion .
the onset for each memory 
 trial was specified at 1 second prior to the response time for the memory cue 
 ( recall that participants pressed the mouse button when they recognized the cue 
 and began recalling the memory ) ; and duration was specified at the time from 
 the onset ( response time  1  second ) to the end of the 12-second stimulus 
 presentation period .
this localized the 
 time when the participants were actively recalling the memory and removed time 
 from the analysis when the participant was reading the cue .
other fmri studies have similarly modeled rt 
 into the fmri design by item matching , covariate analysis , or using rt 
 to temporally model onset of autobiographical memory elaboration [ 2628 ] .
contrast vectors were defined for each 
 participant , producing parameter estimates at each voxel for each contrast of 
 interest .
contrast images were then 
 submitted to a second - order random - effects group analysis 
 using the general linear model .
regions 
 of significant activation were identified using marsbar   by combining the 
 resulting group contrast images with either the specified anatomical masks from 
 the marsbar toolbox or masks drawn using marsbar based on clusters of 
 activation .
  the purpose of the behavioral analyses of 
 memories within the multiple retrieval condition was to determine whether or 
 not repeated recollection of the same event resulted in memories that were   less detailed , stereotyped , or 
 gist - like , as described by bartlett , or   more detailed and accessible , as predicted by mtt . for the multiple retrieval condition only , 
 item analysis for word count , total detail count , and editorial detail count
were 
 conducted within three separate repeated measures analyses of variance ( anova ) across 
 five retrieval sessions , days 1 , 7 , 14 , 21 , and 28 .
because of the large variability in the 
 length of individual memories , we conducted item analyses , with detail counts , 
 and so forth , for each memory included as a separate datum , rather than using 
 averages of memories across each participant . 
 it should be noted , however , that conducting the analyses using participant 
 averages for retrieval sessions did not change the overall pattern of results 
 although some differences across conditions no longer reached statistical 
 significance .
results 
 for word counts , total details , and editorial details are depicted in 
 figure 2 .
generally , the length of memories as 
 measured by both word count and number of details increased across the first 
 three retrieval sessions ( days 1 , 7 , 14 ) , and then remained stable across 
 subsequent retrievals ( days 21 , 28 ) . a 
 repeated measures anova confirmed that mean word count differed across 
 retrieval sessions ,   f ( 4,140 ) = 7.46 ,   p < .001 .
follow - up paired t - tests indicated that word 
 counts increased between retrieval sessions on day 1 and day 7 ,   t ( 1,143 ) = 2.403 ,    p < .05 , and again between day 7 and day 14 ,   t ( 1,143 ) = 3.215 ,   p < .005 . 

word count measures between day 14 and 
 day 21 and between day 21 and day 28 remained stable ( t s < 1 , nonsignificant ) . similarly , a repeated measures anova 
 confirmed that the total detail counts were significantly different across 
 retrieval session   f ( 4,140 ) = 6.549 ,   p < .001 , 
 with follow - up paired t - tests 
 indicating significant increases in total detail counts between day 1 and day 
 14 , t(1,143 ) = 2.09 ,  
p < .05 , and 
 day 7 and day 14    t(1,143 ) = 2.867 ,   p < .005 . the total detail counts between day 14 and 
 day 21 and between day 21 and day 28 were not significantly different ( t s < 
 1 , nonsignificant ) . while word count and total details increased across retrieval sessions , editorial 
 details decreased following the initial retrieval session ( see 
 figure 2 ) , 

although the overall number of editorial details was very small ( only 2 on 
 average per memory ) . a repeated measures 
 anova revealed that mean editorial detail measures across retrieval sessions 
 for the multiple metrieval condition on the item level were significantly 
 different   f ( 4,140 ) = 3.692 ,   p < .01 . follow - up paired t - tests indicated that 
 editorial details on day 1 differed from all other days , t's(1,143 ) > 2.98 ,   p < .01 , while days 728 did not differ 
 from one another , t s < 1.62 , nonsignificant . as 
 the amount of information in the memories increased over repeated retrievals , 
 so did the consistency of the specific details that were described .
the consistency measure for the subset of 36 
 memories that was evaluated increased across retrieval sessions , suggesting 
 that the story related by the participant was becoming more stereotyped or 
 scripted .
it also suggested that , while 
 new details were being added across the early sessions , details provided in 
 earlier sessions were maintained .
table 1 shows that phrase consistency increased significantly between day 7 and day 
 14 ,   t ( 1,35 ) = 2.22 , p < .05 , and between day 14 and day 28 , t ( 1,35 ) = 2.93 , p < .01 , with day 21 falling midway between days 14 and 28 .  
the overall increase 
 in word count and total memory details observed across retrieval sessions could 
 be attributable to multiple successive retrievals but could also be 
 attributable to the participant becoming increasingly comfortable with the 
 interviewer and the interview process . 

this may have resulted in an increased willingness to report more 
 details about their memories generally , regardless of how many times they were 
 retrieved previously . in order to 
 confirm that retrieval rather than personal comfort levels with the interview 
 process was driving the increase in details , we compared two sets of memories 
 retrieved on day 1 ( remote retrieval , multiple retrieval ) with two sets of memories 
 retrieved on day 28 ( recent retrieval , multiple retrieval ) .
we expected that the two sets of memories on 
 day 1 should not differ from one another in detail or word count , since they 
 were all retrieved for the first time in the same session . on day 28 , if repeated retrieval was 
 responsible for the change over time , then only details for memories in the multiple 
 retrieval condition should increase .
if 
 interview comfort was responsible for the change , then all memories retrieved 
 on day 28 , both within the multiple retrieval condition and the newly retrieved 
 memories in the recent retrieval condition , should increase .
a 
 two - factor repeated measures anova was conducted to examine the influence of 
 time ( day 1 versus day 28 ) and retrieval ( single versus multiple ) , and 
 indicated a significant interaction between time and retrieval for both word 
 count and total memory details , f ( 1,143 ) 
 = 6.43 , p < .01 and f ( 1,143 ) = 4.60 , p <    .05 , respectively . follow - up 
 t - tests revealed significant increases between day 1 and day 28 for the multiple 
 retrieval condition in both word count , t ( 1,143 ) 
 = 4.05 , p < .001 , and total details , t
( 1,143 ) = 2.64 , p < .01 . on day 28 , word 
 counts and details for memories in the multiple retrieval condition were 
 significantly higher than memories in the recent retrieval condition which were 
 retrieved only once , t(1,143 ) = 2.13 , p < .05 ; 
 and t ( 1,143 ) = 2.46 , p < .05 , respectively . in contrast , the differences in word count 
 and details between day 1 and day 28 for the two single retrieval conditions ( remote 
 retrieval versus recent retrieval ) did not approach significance , t s < 1 , nonsignificant .
the 
 results strengthen the conclusion that multiple retrieval sessions resulted in 
 memory recollections that were longer , more detailed , and more consistent , and 
 this increase was not due to a change in the reporting characteristics of the 
 participant during the course of the experiment . 
  while in the scanner , participants were 
 asked to respond by pressing the mouse button when they had completed reading 
 the memory cue and begun recalling the specific memory .
thus , reaction times may be taken as a 
 general indication of accessibility , or the effort required to retrieve the 
 memory .
reaction times for the three 
 memory conditions are presented in table 2 . 
 a repeated measures anova revealed that the mean reaction times differed 
 significantly between the three retrieval conditions f ( 2,128 ) 
 = 7.70 , p < .001 .
paired t - tests indicated that mean reaction 
 times were significantly longer for remote retrieval than multiple retrieval memories , t ( 1,129 ) = 3.71 , p < .001 , and shorter for the multiple retrieval compared to recent 
 retrieval memories , t ( 1,129 ) = 2.63 ,
p < .01 . the difference in reaction time between the remote 
 retrieval and recent retrieval conditions was not significant , t<1 .
note that the same pattern of differences was 
 observed when the analyses were conducted on the average reaction times per 
 participant , one - way anova , f ( 1,10 ) = 
 73.76 , p < .001 . for the subject - level analysis ,
the reaction time data suggest that memories 
 in the multiple retrieval session were the easiest to access , followed by recent 
 retrieval memories , and then memories in the remote retrieval condition .
this finding has implications for the imaging 
 results that follow .   in separate group
contrasts , each memory 
 condition was compared to rest at p < .005 
 uncorrected , in order to identify the general pattern of brain activation .
we expected to see considerable overlap 
 because in all three conditions participants are recalling well - established and 
 vivid memories .
figure 4 depicts the 
 distribution of brain activation observed in each condition compared to rest .
the results are consistent with previous 
 studies of autobiographical memory retrieval , indicating activation of 
 bilateral hippocampus , precuneus , lateral prefrontal cortex superior parietal 
 lobules , retrosplenial cortex , and left - lateralized superior temporal gyrus .
regions not commonly observed in studies of 
 memory retrieval , including bilateral caudate nucleus , thalamus , and orbital 
 frontal cortex , are also activated . 

hippocampal activation appears similar across the three conditions , with 
 bilateral activation in the middle region , extending to more 
 posterior regions in the left hemisphere .
mean 
 effect sizes were assessed using region of interest ( roi ) analyses . because of the significant overlap , a mask 
 was made of common active voxels across the three memory conditions .
the mask was then convolved with anatomical 
 masks from marsbar in order to identify those voxels that fell within major 
 anatomical regions showing activation , including left and right posterior 
 parahippocampal gyrus , left and right hippocampus proper , left and right 
 amygdala , and also bilateral caudate nucleus , superior temporal gyrus , 
 precuneus , and superior temporal gyrus . 

the mean effect sizes were obtained for each region from individual 
 datasets and were then compared directly across the three memory conditions in 
 spss with a repeated measures anova and follow - up paired t - tests .
table 3 shows the major regions of activation 
 across the three conditions , mean effect sizes , brodmann s areas , talaraich 
 coordinates , and contrast results for each of the regions .
the results show a general pattern of greater 
 activation for remote retrieval memories compared to recent retrieval , multiple 
 retrieval , or both memory types within the hippocampus , parahippocampal gyrus , 
 precuneus , and middle - frontal gyrus .
no 
 region showed greater activation for multiple retrieval compared to remote retrieval 
 memories .   
one problem with interpretation of these 
 results is that the three memory types differed in retrieval effort , as 
 measured by rt .
remote retrieval memories , 
 which were not recalled for over a month prior to scanning , took a significantly longer amount of time to retrieve 
 than either recent or multiple retrieval memories . this difference in rt
can influence the 
 amplitude of fmri signal , particularly since the data were modeled using 
 reaction time to define onset time , which then determined the duration of the 
 item as well . generally , longer item 
 durations will result in higher amplitude signal .
this 
 issue was approached in several ways . 
 first , a random - effects 
 group analysis directly comparing the multiple and recent retrieval conditions 
 was performed at p < .01 , uncorrected . 
 both conditions contained memories that had been retrieved only two days 
 prior to the scan , so memories were matched for recency of retrieval .
in addition , because the rts for the multiple 
 retrieval condition were shortest , any increased activations observed in this 
 condition can not be the result of increased retrieval time .
we hypothesized that multiple retrievals 
 would result in increased activation in brain regions associated with 
 recollection , compared to memories in the recent retrieval condition that were 
 recollected only once .
table 4 shows the results for this analysis , indicating that multiple retrievals resulted 
 in significantly greater activation in cortical , but not medial temporal , 
 regions .
increased activation was 
 observed in frontal , parietal , thalamic , temporal , and precuneus regions .
no medial temporal lobe region temporal lobe 
 region showed differential activation between the two retrieval 
 conditions .
in addition , no region 
 showed greater activation for recent retrieval memories compared to multiple retrieval 
 memories , despite the longer rts for recently retrieved memories . a 
 second analysis addressing this issue matched memories from each of the three 
 conditions on rts .
the previous analysis 
 suggested that multiple retrievals resulted in increased activation in 
 cortical , but not medial temporal lobe , regions .
the same increases should be evident 
 comparing multiple retrieved memories to both recently retrieved and remotely 
 retrieved memories , while controlling for rts .
one 
 method for dealing with differences in rts would be to add the rts as 
 covariates to the model , but this may be problematic given the relatively small 
 number of items in each memory condition and the assumption of a linear 
 relationship between rt and signal . 

instead , memories were matched across the three conditions based on rts 
 for each individual . using the criterion 
 of dropping fewer than 3 memories from each condition
, we were successful in 
 equating rts for 6 of the 12 participants , usually dropping either the shortest 
 rts in the multiple retrieval condition or the longest rts in the remote retrieval 
 condition .
the matched data sets were 
 compared directly in two separate random - effects group analyses comparing multiple retrieval with recent 
 retrieval , and multiple retrieval with remote retrieval .
a more liberal threshold    ( p <    .05 ) 
 was applied to the group contrasts in order to compensate for the loss of power 
 due to the smaller number of participants .
the mean number of memories included in each 
 condition was also well matched . in 
 addition , number of total details , editorial details , and word counts for the 
 selected memories were nearly identical to the detail and word counts for the 
 original memory sets from these participants , suggesting that our matching 
 procedure did not result in a biased subset of memories being included for 
 analysis .
the 
 random - effects 
 analysis provided results that were consistent with the previous direct 
 comparison of multiple retrieval and recent retrieval memories .
several brain regions showed greater 
 activation for multiple retrieval memories compared to both recent and remote retrieval 
 conditions , including left superior parietal lobule , right precuneus , bilateral 
 retrosplenial cortex , right superior temporal gyrus , and bilateral perirhinal 
 cortex . in the opposite contrasts , no 
 region showed greater activation for either recent or remote retrieval memories 
 compared to the multiple retrieval condition .
we 
 again performed roi analyses for medial temporal lobe regions as described 
 earlier , this time applied to the matched rt data .
the results listed in table 7 show no 
 significant differences in effect sizes for medial temporal lobe regions across 
 the three memory conditions .
the results 
 are consistent with the notion that the earlier differences in activation in 
 medial temporal lobe were driven by differentially longer item durations , 
 particularly for the remote retrieval memories .
the present study 
 examined the influence of repeated retrievals and the passage of time on the 
 subsequent retrieval of autobiographical memories .
results suggest that multiple retrievals , but 
 not the passage of time , have an impact on the representation of 
 autobiographical memories , reflected in both the quality of the memories during 
 subsequent retrieval and the pattern of regional brain activation as measured 
 by fmri .
we will first discuss the 
 behavioral data and then the fmri results and their implications for theories 
 of explicit memory consolidation .
multiple 
 retrievals of well - established memories resulted in three behavioral 
 changes : increased speed of access to 
 the memory , increased consistency in the manner in which memories were 
 described , and a gradual increase in recalled details across repeated retrieval 
 sessions , most prominently across the first three sessions .
the increase in speed of access is probably 
 due to the participant 's repeated exposure to the identical memory cues as well 
 as repeated rehearsal of the processes involved in search .
others have argued 
 that the access component of memory retrieval can be separated from the 
 reconstructive phase of recollection , where participants are actively 
 rebuilding the story of the memory , and these two components may have different 
 neural signatures . increased 
 consistency of recall may reflect scripting , or the development and refinement 
 of a narrative over multiple retrievals , that then accompanies a memory .
this 
 narrative becomes an integral part of the memory and may be an important 
 vehicle for the additions , deletions , and distortions that can occur in 
 autobiographical memories with time . 

this process is different than the changes described by bartlett 
   where stories 
 are condensed , schematized , and generally lose extraneous detail as they are 
 reproduced multiple times .
the 
 third behavioral change we observed , increased recall of details due to 
 retrieval practice , has been described by other researchers as well .
of particular relevance is the literature on 
 hypermnesia for episodic events , in which more details of an event are brought 
 to mind across several retrieval attempts even after the individual has 
 indicated that they can not recall any additional details .
although the typical hypermnesia paradigm 
 entails free recall of lists of words or pictures [ 30 , 31 ] , the phenomenon has 
 also been demonstrated using autobiographical memories [ 32 , 33 ] . repeated
recall of autobiographical memories 
 within a brief period of time ( an hour ) resulted in recollections that were 
 more consistent   and included more details of the original event ( e.g. , 
 details of the reading of the o. j. simpson verdict approximately eight months 
 after it was aired on television ) . 
 in the present study , we also found increased detailed recollection for 
 events over the first three retrieval sessions even though the retrieval 
 sessions were spaced by weeks , rather than minutes .
studies 
 of remote autobiographical memory rarely have the ability to clearly address 
 the issue of veracity ; that is , whether or not memory details produced by 
 participants actually occurred as they are reported .
the present study focuses on changes in 
 recollection over time in response to retrieval , rather than accuracy of the 
 recollections .
studies that address the 
 issue of accuracy most often rely on lists of words , pictures , or newly 
 acquired short stories , at the expense of the rich , emotional detail associated 
 with remote autobiographical memories that have been related many times and in 
 many different contexts , perhaps throughout a lifetime .
one notable exception to this is ulrich 
 neisser s analysis of the testimony of john dean .
neisser found that dean s exhaustive accounts 
 of the intensely emotional and important events surrounding the watergate 
 scandal occurring during the nixon administration were generally devoid of 
 correct details , despite the fact that dean was highly confident in the 
 accuracy of his recollections . 

nevertheless , neisser noted that the general information contained in 
 dean s memories  who knew what , who did what  was correct , even if the event itself had been 
 revised and reconstructed to a surprising degree , a phenomenon that he dubbed  repisodic 
 memory  .
the circumstances in which 
 multiple retrievals increase accuracy ( as in hypermnesia ) or result in 
 reconstructive and erroneous recollections ( as may be the case with 
 autobiographical memories ) have yet to be determined .
recently , marsh   distinguished between 
 the act of repeatedly retelling the story of a life event in social settings 
 with that of repeatedly recalling information in an environment such as a 
 psychology laboratory  the former deemphasizes accuracy and leads to 
 distortions , while the latter emphasizes accuracy and consistency . at this point
the 
 lability of memories during retrieval has been demonstrated elsewhere with very 
 different types of memory .
for example , 
 recent work with animals suggests that the act of retrieval or even partial 
 retrieval destabilizes the memory trace . 

nader et al .   have shown that following reactivation of a memory 
 trace , injection of a protein - synthesis inhibitor blocks reconsolidation 
 rendering the original memory trace inaccessible .
this result has been demonstrated with rats 
 in an amygdala - dependent fear conditioning paradigm   and also with appetitive , 
 food - rewarded spatial discrimination tasks mediated by both amygdala and 
 hippocampal regions [ 3739 ] .
consistent with 
 the animal work , robertson et al .   have demonstrated that retrieval or 
 practice of motor skills results in two independent outcomes that are quite 
 consistent with the formulations of mtt . 

first , the skill memory becomes fragile and susceptible to translation , 
 distortion , or the addition of new components . 

second , retrieval allows for reconsolidation of the original event , 
 which results in further strengthening and stabilization of the skill .
thus , a single long practice session of a 
 particular skill is less beneficial than several interleaved learning trials 
 which provided multiple   opportunities for reconsolidation , reminiscent of the 
 verbal learning paradigms of the 1960 's comparing the effects of spaced versus 
 massed retrieval .
robertson and cohen   make 
 the point that memories are not singular but include multiple components which 
 may be strengthened differentially by practice or retrieval , and may be 
 mediated by different brain mechanisms . 

for example , a rat learning a spatial maze learns the spatial layout of 
 the maze , and also 
 learns the response mapping to obtain the reward . in the present study , it is possible that 
 various behavioral changes observed , such as the speed of access , increased 
 consistency , and increased details , may be relatively independent of one 
 another and are influenced by different variables .
the 
 fmri results provide further evidence that episodic memory representations 
 change with repeated retrievals , but not with the passage of time .
not surprisingly , all memories showed a 
 similar distribution of activation that has been described in other studies of 
 autobiographical memory retrieval [ 15 , 42 ] . 

memories that were retrieved one month ago ( remote retrieval ) showed 
 greater activation across virtually all brain regions involved in memory 
 retrieval , including hippocampus , compared with both the recent and multiple memory 
 conditions .
interpretation of this 
 result , however , is complicated by the fact that memories that have not been 
 retrieved for a period of time ( in the present study , one month ) are more 
 difficult to access , as measured by response times . after equating rts across all retrieval 
 conditions , increased activation for memories in the remote condition was no 
 longer observed ; in fact , there were no measurable differences between the remote 
 and recent memory conditions , both sets of memories previously retrieved only on 
 a single occasion .
in 
 contrast , compared to the single retrieval conditions , memories that had been 
 retrieved multiple times elicited increased activation in a network of brain 
 regions , most notably in lateral prefrontal , parietal , cingulate , superior 
 temporal , and retrosplenial / precuneate regions , all regions that have been 
 previously observed during memory retrieval for emotional events [ 15 , 43 ] . in this case , increased activation was 
 associated with decreased reaction times , and hence can not be attributed to 
 differential effort in accessing the memories . 

increased cortical activation is predicted by both the standard theory 
 of consolidation and mtt , which suggest that cortical - cortical connections will 
 be strengthened as a memory is consolidated . 

however , mtt emphasizes the importance of repeated retrieval for 
 reconsolidation rather than the mere passage of time , while standard theory 
 does not directly address this issue .
we 
 assume that these cortical increases are related to the behavioral changes 
 described earlier , but further research is needed to clarify how the specific 
 behavioral changes are related to changes in fmri signal . in 
 contrast to cortical regions described above , with the exception of an anterior 
 bilateral region of perirhinal cortex ( ba area 28 ) ,
no differences in 
 activation were observed in hippocampus proper , entorhinal cortex , 
 parahippocampal cortex , or amygdala once memories were equated for 
 accessibility .
this does not appear to 
 be the result of decreased power due to smaller numbers of participants , 
 because significant activations for each condition compared to the rest control 
 condition were still observed in medial temporal lobe structures for all three 
 memory types , and clear differences were observed between conditions in other 
 brain regions , including perirhinal cortex . 

rather , medial temporal lobe activity was maintained across repeated 
 retrievals , neither increasing nor decreasing . 

it is important to note , however , that the present study emphasized 
 remote and emotionally salient memories , with nearly two thirds of the events occurring 
 in early childhood , adolescence , or early adulthood .
these remote memories may already have reached 
 an asymptotic level of hippocampal activation , and further increases in 
 activity may not be detectable using fmri . 
 the impact of multiple retrievals and the passage of time on newly 
 formed memories may show a very different pattern of results .
for example , there is ample evidence that 
 newly formed memories are reactivated during offline processes occurring 
 largely during sleep [ 4446 ] , which may play a larger role during the early 
 stages of the consolidation process . in 
 summary ,
the present results demonstrate two consequences of repeated retrieval 
 of remote , well - established autobiographical memories that are consistent with 
 the predictions of mtt .
first , repeated 
 retrieval of memories , but not the mere passage of time , resulted in memories that 
 were more accessible and more detailed , and ultimately lead to a consistent script 
 or narrative that was integrated with the memory .
second , repeated retrievals resulted in 
 increased activation within neocortical regions and maintenance of activation within 
 medial temporal lobe structures . despite 
 the remote nature of these memories , hippocampal activation was robust and did 
 not decrease across time or repetitions , findings that are contrary to the 
 predictions of the standard theory of consolidation .
whether or not hippocampal activation would 
 actually increase in newer , less well - established autobiographical memories as 
 a function of repeated retrieval and time remains to be seen .
clearly , involvement of hippocampus and 
 cortex in memory retrieval is complex , reflecting both the level of effort 
 required to retrieve old memories and the ongoing alterations of existing 
 representations as memories are retrieved and related .
further research will be needed to disentangle 
 the separate contributions to hippocampal and neocortical regions to the 
 distinct processes involved in memory retrieval .
 multiple trace theory ( mtt ) predicts that hippocampal memory traces expand and strengthen as a function of repeated memory retrievals . 
 we tested this hypothesis utilizing fmri , comparing the effect of memory retrieval versus the mere passage of time on hippocampal activation . while undergoing fmri scanning , participants retrieved remote autobiographical memories that had been previously retrieved either one month earlier , two days earlier , or multiple times during the preceding month . 
 behavioral analyses revealed that the number and consistency of memory details retrieved increased with multiple retrievals but not with the passage of time . while all three retrieval conditions activated a similar set of brain regions normally associated with autobiographical memory retrieval including medial temporal lobe structures , hippocampal activation did not change as a function of either multiple retrievals or the passage of time . 
 however , activation in other brain regions , including the precuneus , lateral prefrontal cortex , parietal cortex , lateral temporal lobe , and perirhinal cortex increased after multiple retrievals , but was not influenced by the passage of time . 
 these results have important implications for existing theories of long - term memory consolidation . 
1. INTRODUCTION
2. METHODS
3. RESULTS
4. DISCUSSION