Deprivation-related and use-dependent plasticity go hand in hand
After losing a hand , simple tasks such as tying your shoelaces become a daily challenge , resulting in serious implications for quality of life and employment ( Jang et al . , 2011 ) . How do different individuals adapt to such challenges , and what role does brain plasticity play in this adaptation ? By addressing these questions in individuals with congenital or acquired hand absence we hope to shed light on the scope for adaptive plasticity in the adult human brain , thereby providing important information for informing future neurorehabilitation strategies . It has been thought that adaptive plasticity is restricted in the adult primary sensory cortex following sensory deprivation , as shown recently in late blindness ( Baseler et al . , 2011 ) , suggesting limited scope for neurorehabilitation following sensory deprivation . Arm-amputation is a particularly powerful model for studying plasticity as it combines two major drivers for reorganisation – sensory deprivation and adaptive motor behaviour . Despite this , most accounts of plasticity following arm amputation focused on sensory deprivation , and in particular on apparently passive remapping of adjacent face or arm representation into the deprived cortex ( Lotze et al . , 1999; Ramachandran and Altschuler , 2009; Nava and Röder , 2011 ) . Although longer-range remapping has also been reported , such as intact hand representation in the deprived cortex ( Bogdanov et al . , 2012 ) , this is usually also explained as a passive result of inter-hemispheric dis-inhibition ( Werhahn et al . , 2002; Ramachandran and Altschuler , 2009; Simões et al . , 2012 ) . However , these accounts ignore substantial adaptations in motor behaviour that accompany absence or loss of a limb , which could be powerful drivers for plasticity ( Scholz et al . , 2009 ) . Here , we test whether altered limb–use patterns influence cortical reorganisation in individuals with unilateral hand absence , using functional magnetic resonance imaging ( fMRI ) and diffusion tensor imaging ( DTI ) . One aim of the current study was to assess upper limb use strategies in individuals with acquired and congenital hand absence . A further aim was to investigate whether functional cortical reorganisation underlies any observed variation in motor behaviour . Specifically , we predicted that the degree to which individuals used their residual-arm or intact-hand in daily life would reflect the degree of brain activity in the deprived cortex during movement of the corresponding limb .
We assessed day-to-day limb-use strategies in individuals with acquired ( n = 18 , 10 below elbow ) or congenital ( n = 11 , 10 below elbow ) hand absence ( see Table 1 for demographic details and prosthetic limb usage ) . Specifically , we quantified the extent to which individuals utilise their residual arms , relative to their intact hands ( Figure 1A ) , in daily activities . We collected data from acceleration monitors , worn by a sub-set of 20 participants ( 8 congenital ) while engaged in their normal routines . A laterality index , reflecting the relative number of movements performed by the intact hand vs the residual arm ( Figure 1B ) , showed that both groups moved the intact hand more than the residual arm ( t ( 7 ) = 3 . 48 , p=0 . 01; t ( 11 ) = 10 . 60 , p=0 . 001 , one-sample t-test compared to zero for the congenital and acquired groups , respectively ) . This could be due both to greater use of the intact hand and also to the different position of the accelerometer on the two sides of the body ( wrist vs arm ) , resulting in different acceleration profiles . Our main aim was to compare the degree of this laterality across the two 1-handed groups . We found that the acquired group exhibited stronger relative reliance on their intact hand , as demonstrated in a significantly greater lateralisation scores towards the intact hand , compared to the congenital group ( t ( 18 ) = −2 . 67 , p=0 . 016 , Figure 1B ) . 10 . 7554/eLife . 01273 . 003Table 1 . Demographic details of 1-handed individuals with acquired ( A ) and congenital ( C ) hand lossDOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 003AgeDeprivation age ( in years ) Amp . LevelSide/dominantCause of amputationCosmetic Pros . UsageFunctional Pros . UsageA0143384L/RTrauma20A0242224R/LNerve I*20A0321184R/LTrauma00A0446372L/RNerve I*10A0548201R/RTrauma15A0658112R/RTrauma15A073122L/RTrauma00A0854205L/LTrauma50A0947452L/LTumour13A1060342R/RTrauma05A1151354L/RInfection15A1247192L/RTrauma*05A1357484R/LInfection02A1456402L/RTrauma00A1522185L/RTrauma00A1643334L/RTrauma05A1750284L/RTrauma50A1852454L/RTrauma25C013104RDysmelia50C022404LDysmelia40C033504LDysmelia50C043105LDysmelia00C052504LDysmelia00C065404LDysmelia05C074905LDysmelia00C082204RDysmelia10C094904RDysmelia40C101804LDysmelia00C114602LDysmelia25Amputation levels: the level at which the residual arm ends . 1 = through shoulder , 2 = above elbow , 3 = through elbow , 4 = below elbow , 5 = through wrist; Side = side of amputation; dominant = hand dominance prior to hand loss ( based on self report ) : L = left , R = right; NI=nerve injury . *indicates potential partial spinal damage . Pros . Usage = Prosthetic limb usage ( frequency ) : 0 = never , 1 = rarely , 2 = occasionally , 3 = daily , 4 = more than 4 hr a day , 5 = more than 8 hr a day . 10 . 7554/eLife . 01273 . 004Figure 1 . Dissociative limb usage strategies in 1-handed individuals with congenital and acquired hand loss . ( A ) Schematic illustration of the residual and intact limbs . ( B ) Limb-use strategies , based on activity monitors for increased lateralised ( intact hand ) use . A laterality index , reflecting the relative number of movements performed by the intact hand vs the residual limb , was calculated using data from acceleration monitors ( mean ± s . e . m . ) , worn by 20 1-handed individuals ( 8 congenital ) while engaged in their normal routines . Positive values represent a tendency to use the intact hand more than the residual limb . Although both groups exhibit such a tendency ( t ( 7 ) = 3 . 48 , p=0 . 01; t ( 11 ) = 10 . 60 , p=0 . 001 , one-sample t-test compared to zero for the congenital and acquired groups , respectively ) , the acquired group shows a significantly greater preference for the intact hand compared to the congenital group ( t ( 18 ) = −2 . 67 , p=0 . 016 ) . ( C ) Questionnaire ratings ( mean ± s . e . m . ) for residual arm usage in daily activities were significantly greater for the congenital group ( t ( 1 , 27 ) = 3 . 65 , p=0 . 001 ) , suggesting they use their residual arm more frequently in daily activities , compared with the acquired group . ( D ) Questionnaire ratings for residual arm usage were negatively correlated with the laterality indices , measured based on acceleration monitoring ( r ( 20 ) = −0 . 53 , p=0 . 012 ) , validating the questionnaires as a measurement of habitual usage strategies between the residual and intact limbs . Scatter plot shows data for 1-handed individuals with congenital and acquired hand loss , frequency of prosthetic hand usage is indicated in the index to the right . Asterisks denote significance levels of *p<0 . 05; ***p<0 . 005 . DOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 004 These findings were further confirmed using questionnaire ratings from all 1-handed participants , quantifying the extent to which individuals incorporate their residual arm in daily tasks ( ‘Materials and methods’ ) . Ratings were significantly greater for the congenital group ( t ( 1 , 27 ) = 3 . 65 , p=0 . 001 ) , suggesting they use their residual arm more frequently in their daily routine , compared with the acquired group ( Figure 1C ) . This group difference remained significant after accounting for level of amputation and degree of functional and cosmetic prosthetic limb usage as covariates ( F ( 1 , 24 ) = 8 . 217 , p=0 . 009 ) . The questionnaire ratings for residual arm usage were negatively correlated with the accereometry-based laterality indices ( r ( 18 ) = −0 . 53 , p=0 . 012 , Figure 1D ) , validating the questionnaires as a measurement of habitual usage strategies between the residual and intact limbs . A similar correlation was also obtained after accounting for level of amputation and degree of functional and cosmetic prostheses usage ( r ( 15 ) = −0 . 451 , p=0 . 034 ) . Based on these dissociated adaptive motor strategies , we predicted distinct patterns of limb representation between the two 1-handed groups . All 1-handed participants and 22 intact ( 2-handed ) controls underwent fMRI , involving simple unilateral hand and arm movements ( fingers/elbow flexion and extension ) . Voxel-wise activation ( β ) patterns were compared between the three groups during hand and arm movements across the entire brain . Each of these two separate whole-brain contrasts resulted in a single cluster , centred on the hand knob of the central sulcus of the deprived hemisphere , spanning both the pre- and the post-central gyri . In line with their increased usage of the residual arm , congenital 1-handed participants showed increased activation during residual arm movements , compared with the other two groups ( Figure 2B ) . The resulting cluster showed two distinct local peaks of activation—in the anterior and posterior banks of the central sulcus ( t = 3 . 73; x = 38 , y = −20 , z = 54 for the anterior peak and t = 3 . 93; x = 48 , y = −20 , z = 54 for the posterior peak , coordinates are based on the MNI 152 brain template ) . Conversely , acquired 1-handed participants , who displayed increased use of their intact hand , correspondingly showed significantly increased activation during intact hand movements compared to the other groups ( Figure 2C ) . The resulting cluster had a single peak , centred on the posterior bank of the central sulcus ( t = 5 . 24; x = 42 , y = −18 , z = 48 in MNI 152 space ) . 10 . 7554/eLife . 01273 . 005Figure 2 . Limb-representation patterns in the deprived cortex reflect usage—whole brain contrasts . ( A ) Coloured lines delineate the boundaries of clusters activated during execution of movements using the feet ( green ) , arms ( blue ) , hands ( white ) , and lips ( pink ) in controls , projected on inflated hemispheres . ( B ) Whole-brain group comparisons for activation during residual/nondominant arm movements ( in 1-handed/control participants , respectively; deprived hemisphere is in front . Note that participants with above elbow deprivation were excluded ) . During movements of the residual arm , the congenital group showed increased activation compared with the acquired and control groups . This whole-brain contrast resulted in a single cluster , centred on the hand knob of the central sulcus of the deprived cortex , spanning the pre- and post-central gyri ( shown in orange ) . ( C ) Whole-brain group comparisons for activation during intact/dominant hand movements ( in 1-handed/control participants , respectively ) . During movements of the intact hand , the acquired group showed increased activation compared with the conjunction of the congenital and control groups . This whole-brain contrast resulted in a single cluster centred on the hand knob of the deprived cortex spanning the pre- and post-central gyri ( shown in orange ) . The square inserts in ( B and C ) show overlap between the clusters resulting from the whole brain group comparisons ( orange ) and the controls’ hand area ( white ) , as shown in ( A ) . No other significant clusters were identified here , or using the homologous contrasts between the other groups . DOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 005 Even though activations used to generate these two contrasts were measured during movements of different limbs ( hand vs arm ) , and on different sides of the body ( limbless side vs intact side ) , the differential activation was restricted in both cases to the hand area , as demonstrated by activation patterns in the control group ( Figure 2A and inserts in Figure 2B , C ) . No other significant clusters were identified in these comparisons , or using the homologous contrasts between the other groups . As demonstrated in Figure 2 , the two independent clusters , showing increased representation of the residual arm or the intact hand in the congenital and acquired groups , overlapped with the hand area of the control participants . This suggests that the area that would typically represent the missing hand has been recruited to support increased representation of the residual arm or intact hand . To investigate use-dependent plasticity specifically in the ‘deprived cortex’ , we constructed an independent region of interest ( ROI ) , based on the conjunction between phantom/nondominant hand movements in acquired amputees/2-handed controls . Amputees often experience vivid sensations of a phantom hand . Recently , this phenomenology has been supported by empirical evidence demonstrating that movements of a phantom limb elicit both central and peripheral motor signals that are different from those found during imagined movement ( Reilly et al . , 2006; Raffin et al . , 2012 ) . This phenomenon of maintained representation in the sensorimotor cortex , allowed us to reliably localise the representation of the missing hand ( Makin et al . , 2013 ) . Here , this pre-defined ROI was used to interrogate activity relating to other body parts , thus allowing us to test the usage-driven hypotheses independently . The borders of this ‘deprived cortex’ ROI , spanning primary somatosensory and motor cortices , are shown in Figure 3A . As the spatial resolution and co-registration methods used here are insufficient to reliably dissociate the somatosensory and motor primary cortices , cortical areas spanning the pre- and post-central gyri will be described here as ‘sensorimotor’ . 10 . 7554/eLife . 01273 . 006Figure 3 . Limb-representation patterns in the deprived cortex reflect usage—ROI analysis . ( A ) To define the deprived cortex , an independent ROI ( outline shown in black ) was derived from the conjunction between phantom/nondominant hand movements in acquired amputees/controls . For illustration purposes , the group contrast maps derived from the previous whole-brain analysis ( shown in Figure 2 ) are also overlaid ( in faded orange ) , highlighting the fact that whole-brain group differences are co-localised with the deprived cortex ROI . ( B and C ) In order to assess the degree to which the deprived cortex is used to represent other body parts , mean fMRI activation levels ( β ) for voxels within the deprived cortex ROI during residual arm ( B ) and intact hand ( C ) movements were calculated . Beta values were averaged ( ±s . e . m ) across the congenital ( white ) , acquired ( light gray ) and control ( black ) groups . Within-group comparisons confirmed greater representation of the relatively over-used limb in each group . Asterisks/Hashes denote significance levels of *p≤0 . 025; **p<0 . 005; #p<0 . 05 for corrected ( planned ) /and uncorrected ( exploratory ) comparisons , respectively . ( D ) Increased fMRI activation ( β ) in the deprived cortex during intact hand movements correlated negatively with questionnaire scores for residual arm usage ( associated with increased intact hand usage , Figure 1D ) , across the congenital and acquired groups ( r ( 28 ) = −0 . 43 , p=0 . 021 ) . Prostheses usage indices are shown in Figure 1 . DOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 006 We next extracted fMRI activation values ( β ) during movement conditions within the independent ROI . A mixed effect analysis of variance showed a significant interaction between groups and limbs ( F ( 2 , 47 ) = 3 . 67 , p=0 . 033 ) , reflecting dissociated recruitment of the deprived cortex by movements of the residual arm and intact hand in the two 1-handed groups ( Figure 3B–C ) . Post-hoc tests confirmed increased activation when congenital participants moved their ( relatively over-used ) residual limb , compared to acquired participants ( Mann Whitney U = 6 , n = 20 , p=0 . 001 ) and compared to controls ( t ( 30 ) = 2 . 37 , p=0 . 025 ) . Similarly , greater activation within the deprived cortex was observed when acquired participants moved their ( relatively over-used ) intact hand , compared to congenital participants ( t ( 26 ) = −3 . 68 , p=0 . 001 ) and controls ( t ( 37 ) = 2 . 96 , p=0 . 005 ) . These results were independent of participant A07 , who lost her hand at a young age ( t>2 . 81 , p<0 . 008 ) ( note that this participant was not included in the arm analysis above , due to her amputation level ) . The difference between the two 1-handed groups was also significant when accounting for level of amputation , and degree of functional and cosmetic prosthetic limb usage ( F ( 1 , 15 ) = 11 . 357 , p=0 . 004 and F ( 1 , 23 ) = 8 . 172 , p=0 . 009 for residual arm and intact hand movements , respectively ) . To complement the between-group findings reported above , we also performed within-group analyses to test for assymetrical patterns of limb representation . In particular , we tested whether the activity in the deprived cortex during movement of the over-used ( compensatory ) limb was greater than activity in the homologous cortex ( in the intact hemipshere ) during movements of the corresponding ( opposite ) limb ( Figure 4 ) . For the congenital group , activation in the deprived cortex during movements of the ( contralateral ) residual arm was greater than activation in the homolgous intact cortex during movements of the ( contralateral ) intact arm ( t ( 1 , 9 ) = 2 . 45 , p=0 . 037 ) . Similarly , for the acquired group , activation in the deprived cortex during movements of the ( ipsilateral ) intact hand was greater than activation in the homologous intact cortex during movements of the ( ipsilateral ) phantom hand ( Makin et al . , 2013 ) ( t ( 1 , 16 ) = 3 . 08 , p=0 . 007 ) . These results confirm that , within both groups , activity in the deprived cortex relating to the compensatory limb is greater than activity in the homologous cortex , relating to the corresponding opposite limb . 10 . 7554/eLife . 01273 . 007Figure 4 . Over-representation of the favoured limb for adaptive use results in intra-subject assymetry . Activation levels ( mean ± s . e . m ) during arm movements in the congenital group and during hand movements in the deprived cortex ROI ( white ) , and a homologous ROI contralateral to the intact hand ( gray ) . The congenital group showed increased contralateral activation ( in the deprived cortex ) during residual arm movements , compared to contralateral activation ( in the homolgous intact cortex ) during intact arm movements . The acquired group demonstrated significantly increased ipsilateral activation ( in the deprived cortex ) when moving their intact hand , compared with ipsilateral activation ( in the intact cortex ) during phantom hand movements . These results confirm adaptive changes in limb-representation in the deprived cortex in 1-handed individuals . Asterisks denote significance levels of *p<0 . 05; **p<0 . 01 . DOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 007 In addition to the hypothesised increase in activation for the limb favoured for adaptive usage , the ROI analysis in Figure 3B–C also revealed unexpected reduced activation in the 1-handed groups during movements of the limb which is not favoured for over-use , compared with controls . Specifically , the congenital group , who over-use the residual arm , showed reduced activation in the deprived cortex during movements of the intact hand ( t ( 31 ) = −2 . 61 , p=0 . 014 ) , whereas the acquired group , who over-use the intact hand , showed reduced activation in the deprived cortex during movements of the residual arm ( Mann Whitney U = 97 , n = 39 , p=0 . 025 ) . This result potentially reflects a competitive relationship between the representations of the two limbs in the deprived cortex in 1-handed individuals . To test whether functional reorganisation associated with adaptive movement strategies is present outside the primary sensorimotor cortex , we also explored patterns of limb representation in the cerebellar hand area ( lobule V of the anterior lobe ) of the acquired and congenital 1-handed groups . We defined an independent ROI for the deprived cerebellum , using the same criteria as used to construct the deprived cortex ROI . Note that the deprived cerebellum ROI is ipsilateral to the missing hand ( and opposite to the deprived cortex ROI ) . Similarly to the deprived cortex , the deprived cerebellum showed increased activation in the congenital group during residual arm movements ( t ( 26 ) = −3 . 33 , p=0 . 003 ) , and increased activation in the acquired group during intact hand movement ( t ( 18 ) = 2 . 75 , p=0 . 013 ) . This suggests that the patterns of altered representation identified in this study are not limited to reorganisation of body-part representations in the primary sensorimotor cortex . For the group results described above , we have compared the intact hand of 1-handed participants to the dominant hand of controls , and the residual arm of 1-handed participants to the non-dominant arm of controls . However , a third of the individuals with acquired deprivation lost their dominant hand ( Table 1 ) . To test whether group differences in hand dominance could influence the results , activation levels for non-dominant hand and dominant arm movements were extracted from a homologous ROI contralateral to the dominant hand , in an age-matched sub-group of the control participants . ROI-based comparisons between the acquired and control groups were repeated as above , while accounting for handedness using this sub-group , with similar results ( t ( 30 ) = 3 . 1 , p=0 . 004 for increased representation during intact hand movements and t ( 37 ) = −2 . 25 , p=0 . 032 for decreased residual arm representation during residual arm movements in acquired amputees vs controls ) . Phantom limb pain , prevalent in acquired amputees , may discourage these participants from using their residual arm . Indeed , within the acquired group a trend for a correlation between chronic phantom pain scores and usage was found ( r ( 16 ) = −0 . 40 , p=0 . 099 ) , such that individuals with worst chronic pain tend to use their residual arm less . Therefore , to test whether phantom pain could explain our observed imaging results , we repeated the comparisons between the two 1-handed groups described above ( using a univariate GLM ) with chronic phantom pain ratings as a covariate . Increased activation was maintained both in the congenital group ( relative to the acquired group ) during residual arm movements ( F ( 1 , 19 ) = 12 . 57 , p=0 . 002 ) and in the acquired group ( relative to the congenital group ) during intact hand movements ( F ( 1 , 27 ) = 6 . 10 , p=0 . 021 ) . These results suggest that the activation patterns presented above are not merely epiphenomenal to phantom pain . Although the congenital and acquired 1-handed groups differed in terms of usage strategies and activation patterns in the deprived cortex , individuals across these groups showed considerable overlap ( Figure 3D ) : acquired 1-handed individuals who nevertheless had high residual arm usage showed less of the intact hand activation that was typical of their group . Such individuals demonstrate that the differential reorganisational patterns observed in the two 1-handed groups may not be solely determined by deprivation age ( or previous afferent/efferent experience ) , but rather could relate to usage strategies . Indeed , when considering usage across both 1-handed groups we found that greater usage of the residual arm was associated with less activation during intact hand movements in the deprived cortex ( r ( 26 ) = −0 . 43 , p=0 . 021 ) , even when accounting for age of sensory deprivation ( r ( 25 ) = −0 . 36 , p=0 . 032 ) . This effect was strengthened when repeating the same partial correlation while only considering the prosthetic limb users ( r ( 19 ) = −0 . 61 , p=0 . 003 and r ( 18 ) = −0 . 56 , p=0 . 005 , respectively ) , suggesting that rehabilitation may strengthen the relationship between adaptive plasticity and usage , however further research will be required to identify the relationship between prosthesis usage and adaptive brain plasticity . Finally , the white matter connections supporting the functional changes associated with limb-usage were studied in voxel-wise comparisons ( Smith et al . , 2006 ) of fractional anisotropy ( FA , a measure of white matter microstructure ) , along the white matter skeleton . No significant differences in FA were found between the three groups , suggesting that deprivation in itself may be insufficient to induce significant white matter change ( although see Langer et al . ( 2012 ) for a study showing reduced FA in the corticospinal tract following several weeks of arm immobilisation ) . However , across both 1-handed groups , individuals showing greater activation in the deprived cortex during intact hand movements had higher FA values in the corticospinal tract and the inferior fronto-occipital fasciculus ( IFO ) in the deprived hemisphere ( Figure 5 ) . This voxel-wise non-parametric correlation was performed while accounting for participants’ age and deprivation age ( as nuisance regressors ) . No significant correlations were identified between FA and degree of activation in the deprived cortex during residual arm movements . 10 . 7554/eLife . 01273 . 008Figure 5 . Higher FA is associated with greater intact hand plasticity in 1-handed participants . Clusters in the corticospinal tract ( top ) and the inferior fronto-occipital fasciculus ( bottom ) ( red-yellow , p<0 . 05 , corrected ) show positive correlations between FA and intact hand fMRI activation within the deprived cortex ROI ( blue ) , while accounting for participants’ age and deprivation age . The bright/dark green lines denote the white matter skeleton/tracts ( based on the John Hopkins University atlas ) , respectively . Clusters have been dilated for presentation purposes . DOI: http://dx . doi . org/10 . 7554/eLife . 01273 . 008
We provide the first evidence that altered patterns of adaptive limb use , in individuals with unilateral hand absence , are reflected in distinct patterns of cortical reorganization . Using physiological measurements , we provide the first detailed description of patterns of daily limb usage in individuals with congenital and acquired limb-loss . Next , using fMRI we show that representation patterns in the deprived cortex of 1-handed individuals are contingent upon the limb-use strategy adopted by individuals , rather than a sensitive period in development: Individuals from either 1-handed group that rely more on their intact hands showed increased representation of the ( ipsilateral ) intact hand in the deprived cortex , irrespective of age at deprivation . These functional adaptations are further reflected in white matter structural integrity , as demonstrated using DTI . These findings shed new light on the extent of adult brain plasticity and have implications for rehabilitation . Loss of a limb can have devastating impact on quality of life in individuals who are typically young , employed and otherwise healthy . As a result of limb loss , 69% of sufferers have to change job or become unemployed ( Jang et al . , 2011 ) . How best to rehabilitate these individuals , and allow them to return to active and productive lifestyles , is unknown ( Nimhurchadha et al . , 2013 ) . While tremendous resources are dedicated to aiding 1-handed populations through the development of sophisticated prosthetic limbs ( Kuiken et al . , 2009 ) , a relatively large number of the individuals do not use them and prefer to carry out daily activities with their intact hand ( Jang et al . , 2011; Østlie et al . , 2012 ) . Clinicians are aware that different individuals adapt to limblessness in varying ways–with some relying on the intact limb ( Jones and Davidson , 1999 ) and others showing remarkable ability with their residual limb or a prosthetic limb ( Smurr et al . , 2008; Jang et al . , 2011 ) . Surprisingly , however , to our knowledge there has been no previous empirical study of the range of adaptive motor behaviours taken in 1-handed individuals in their natural environment ( Lindner et al . , 2010 ) . Based on subjective ( questionnaires ) and objective ( accelerometry ) usage monitoring , we provide empirical data on patterns of limb usage in these groups while engaged in their natural routine ( Figure 1 ) and then show that these daily usage strategies are a powerful driver of plasticity in the deprived cortex . Development of compensatory skills , whether through rehabilitation or through the natural course of adaptation , critically depends on learning and brain plasticity . Previous studies of maladaptive brain changes after amputation ( Flor et al . , 1995 ) have been instrumental in guiding new evidence-based rehabilitation interventions to treat phantom pain ( Chan et al . , 2007; Lotze et al . , 1999 ) . However , the study of brain changes after amputation has not previously been brought to bear on the clinical problem of adaptive motor behaviour . Understanding the scope for adult human brain plasticity is a key consideration for designing successful future neurorehabilitation approaches ( Krakauer , 2006; Koenig et al . , 2013 ) . We found that patterns of increased representation of displaced body parts in the deprived cortex closely relate to adaptive daily strategies of limb-usage in individuals with upper limb absence: people with congenital hand absence , who are better at incorporating their residual arms in daily tasks , activate their missing hand cortex during residual arm movements ( Figures 2B , 3B , 4 ) . This is consistent with previous reports of reorganisation associated with compensatory foot-use in individuals with bilateral congenital upper limb malformation ( Stoeckel et al . , 2009 ) . Conversely , the 1-handed individuals with acquired amputation , who are more dependent on their intact hands for daily activities , show strong activation in their deprived cortex when the intact hand is moving ( Figures 2C , 3C , 4 ) . However , this dissociative representation doesn’t exclusively rely on the cause of or age at deprivation: increased ipsilateral intact hand representation was associated with the usage strategy adopted by individuals ( Figure 3D ) , such that people who have learned to incorporate their residual arms in daily tasks tended to show reduced intact hand representation in the deprived cortex . Using both whole-brain and ROI analyses , our functional results demonstrate how plasticity within the deprived cortex may be shaped by adaptive experience . However , it is possible that adaptive strategies for compensatory limb-usage drive similar , yet subtler differences in other brain areas underlying limb usage . For example , using an ROI analysis , we have identified similar patterns of limb over-representation in the deprived cerebellum , which were not apparent in the ( corrected ) whole-brain comparison . We therefore cannot rule out the possibility that habitual usage strategies could produce further changes in brain representation , beyond the deprived cortex and even the sensorimotor system . Further research is therefore necessary in order to determine the full extent of adaptive plasticity following limb-loss . Finally , while no group differences in FA values were identified , the degree of intact hand representation in the deprived cortex correlated with FA values along the IFO and the corticospinal tracts of the deprived cortex ( Figure 5 ) . The IFO has been recently implicated in visuospatial representation , as well as visuomotor control ( Urbanski et al . , 2008; Migliaccio et al . , 2012 ) , which may be relevant for lateralised limb usage ( Makin et al . , 2010 ) . The structural differences in the corticospinal tract , associated with increased representation of the intact hand in the deprived cortex , may shed new light on the origin of bimanual plasticity in amputees , which has previously been regarded as a consequence of reduced inter-hemispheric inhibition ( Simões et al . , 2012 ) . Indeed , we find similar results in the deprived cerebellum , as identified in the sensorimotor cortex , despite fundamental differences in body-part topography ( Shambes et al . , 1978 ) in these two brain regions . One tentative interpretation of these findings is that longer-range plasticity ( where the deprived cortex is recruited by the intact hand ) elicits structural white matter change , whereas more local plasticity ( where the deprived cortex is recruited by the residual arm ) does not . However , the cross-sectional nature of the study does not allow us to differentiate between experience-dependent and pre-existing variations in white matter structure that might confer a bias towards intact hand representation in the deprived cortex . To conclude , we demonstrate how adaptive limb usage strategies may serve as powerful drivers of both functional and structural plasticity in adults . We show that the deprived cortex in people with either congenital or acquired hand absence is employed by whichever part of the upper limb individuals are relatively over-using ( compared to other 1-handed individuals ) to compensate for their disability . This occurs irrespective of whether this is a hand or an arm and the degree to which it is typically represented in the deprived brain area . By demonstrating that adaptive plasticity in amputees transcends the boundaries thought to restrict reorganisation after sensory deprivation in the adult human brain , our results may inspire future research , aimed at utilising neurorehabilitation to improve usage of both residual and intact arms , as well as artificial arms , in 1-handed individuals of all ages .
18 individuals with sustained unilateral upper limb amputation ( mean age ± s . e . m = 46 ± 3 , 6 with absent right hand ) , 11 individuals with a congenital unilateral upper limb deficit ( age = 35 ± 4 , 3 with absent right hand ) and 22 intact controls ( age = 41 ± 3 , 7 left hand dominant ) were recruited for the study ( see Table 1 for demographic details ) . Recruitment was carried through the Oxford Centre for Enablement and Opcare in accordance with NHS national research ethics service approval . Informed consent and consent to publish was obtained in accordance with ethical standards set out by the Declaration of Helsinki ( 1964 ) and with procedures approved by the NHS ( REC ref: 10/H0707/29 ) . Data from one individual with acquired deprivation and one control were discarded , due to excessive head movements ( >3 mm ) during fMRI and problems during DTI data acquisition ( respectively ) . In order to assess potential differences in rehabilitation strategies between the two amputee groups , use of residual arm was initially assessed using a revised version of the Motor Activity Log ( Uswatte et al . , 2006 ) . 1-handed participants were requested to rate how frequently ( never; sometimes; very often ) they incorporate their residual arm ( stump; either directly , or using a prosthesis ) in an inventory of daily activities , requiring varying degrees of motor control . The following items were used: taking money out of wallet; opening envelope; picking up/lifting glasses; picking up/holding up a phone; wiping off a kitchen counter or other surface; getting out of a car; stabilizing paper while writing; stabilizing dishes while carrying; carrying a cup or a can; carrying bags; getting up from a chair with arm rests; pulling chair away from table before sitting down; holding a book or a magazine/turning pages; typing on a keyboard/pressing mouse buttons; controlling a computer mouse; putting on your socks; putting on your shoes; tying shoe laces; inserting your ( intact ) arm through a sleeve; putting on makeup base , lotion , or shaving cream on face; washing hand or face; drying your hand or face; combing your hair; buttoning a shirt; zipping up a coat; peeling fruit skin; using a fork or spoon for eating . As previous motor activity logs were designed to assess mobility of a paralyzed hand , the questionnaire was modified while considering the unique confounds of unilateral amputees ( Lindner et al . , 2010 ) , and was aimed to accommodate participants with various levels of deprivation . In accordance with the original questionnaire , the items represented commonly encountered actions and covered a comprehensive range of activities , to accommodate various levels of skill of the residual arm . We focused on frequency , rather than quality of movement rating , as we were interested in both prosthetic limb and stump usage , which could not have been easily assessed . We used a scale of three frequency ratings , rather than five , since pilot testing showed that individuals tended to ignore the second and fourth options . Each item was scored ( between 0 and 2 ) , and the sum was divided by 54 , such that individuals were rated on a scale between 0 to 1 . To validate the usage questionnaires , and to study the relationship between increased reported residual arm usage and intact hand usage , limb acceleration data were collected from a subset of 21 limb-absent individuals ( 8 with congenital absence ) using the GeneActiv accelerometers ( sample rate: 100 Hz ) . Participants were given two strapped sensors , which they were asked to place on their wrist ( intact hand ) or the proximal aspect of the upper arm ( residual arm ) during 2 days with a typical daily routine ( mean number of hours per day ± s . e . m . = 15:08 ± 37 min ) . Data from one acquired participant was discarded , due to hardware malfunction . Data for each of the three movement axes was initially smoothed using a 500 ms kernel , to discard high frequency noise . To quantify the number of movements executed with each limb , the difference between maximal and minimal acceleration values were initially calculated within a sliding window of 400 ms for each movement axis separately . In each time window , movements were identified as difference in acceleration that was above a threshold of 0 . 2 m/s2 in at least one axis , provided that this increase was preceded and followed by periods of no movement ( a difference between maximal and minimal acceleration below 0 . 2 m/s2 ) in all axes . To account for whole body movements , as well as differences in number of hours of recordings , a ratio between the two limb movements was used , rather than an absolute number of movements . The movement laterality ratio was chosen [ ( intact−residual ) / ( intact+residual ) ] , as it portrays the extent of intact hand usage , given the contribution of residual arm movements . Task-based fMRI: participants were visually instructed to move their left/right hand ( finger movements ) , left/right arm ( elbow movements ) , feet ( bilateral toe movements ) or lips . The protocol comprised of alternating 12 s periods of movement and ‘rest’ . Each of the six conditions was repeated four times , in a counterbalanced order . Here we focus on results from intact/dominant hand and residual/nondominant elbow movements ( see Makin et al . , 2013 ) for information about phantom movements ) . Participants received extensive training on the degree and form of movements expected . Note that the movements were easy to execute and did not require any expertise . To confirm that appropriate movements were made at the instructed times , task performance was monitored visually both on- and off-line , using video recordings . The MRI measurements were obtained using a 3 Tesla Verio scanner ( Siemens , Erlangen , Germany ) with a 32-channel head coil . Anatomical data were acquired using a T1-weighted magnetization prepared rapid acquisition gradient echo sequence ( MPRAGE ) with the parameters: TR = 2040 ms; TE = 4 . 7 ms; flip angle = 8° , voxel size = 1 mm isotropic resolution . Functional data based on the blood oxygenation level-dependent ( BOLD ) signal were acquired using a multiple gradient echo-planar T2*-weighted pulse sequence , with the parameters: TR = 2000 ms; TE = 30 ms; flip angle = 90°; imaging matrix = 64 × 64; FOV = 192 mm axial slices . 46 slices with slice thickness of 3 mm and no gap were oriented in the oblique axial plane , covering the whole cortex , with partial coverage of the cerebellum . Two sets of whole brain diffusion weighted volumes were acquired using a generalized autocalibrating partially parallel acquisitions ( GRAPPA ) sequence , using the following parameters: 60 directions ( plus 8 vol without diffusion weighting ) ; b = 1000 s/mm; 65 axial slices; voxel size 2 × 2 × 2 mm; TR = 9600 ms; TE = 87 ms . All imaging data were processed using FSL 5 . 1 ( www . fmrib . ox . ac . uk/fsl ) . Data collected for individuals with absent right hands ( 6 acquired and 3 congenital participants ) , were mirror reversed across the mid-sagittal plane prior to all analyses so that the hemisphere corresponding to the missing hand was consistently aligned . Data collected for left-hand dominant controls ( n = 7 ) were also flipped , in order to account for potential biases stemming from this procedure . Functional data were analysed using FMRIB’s expert analysis tool ( FEAT , version 5 . 98 ) . The following pre-statistics processing was applied to each individual run: motion correction using FMRIB’s Linear Image Registration Tool ( MCFLIRT [Jenkinson et al . , 2002] ) ; brain-extraction using BET ( Smith , 2002 ) ; mean-based intensity normalization; high pass temporal filtering of 300 s; and spatial smoothing using a Gaussian kernel of FWHM ( full width at half maximum ) 5 mm . Time-series statistical analysis was carried out using FILM ( FMRIB’s Improved Linear Model ) with local autocorrelation correction . Functional data were aligned to structural images ( within-subject ) initially using linear registration ( FMRIB’s Linear Image Registration Tool , FLIRT ) , then optimized using Boundary-Based Registration ( Greve and Fischl , 2009 ) . Structural images were transformed to standard MNI space using a non-linear registration tool ( FNIRT ) , and the resulting warp fields applied to the functional statistical summary images . To compute task-based statistical parametric maps , we applied a voxel-based general linear model ( GLM ) , as implemented in FEAT . The block design paradigm was convolved with a gamma function ( Friston et al . , 1998 ) , and its temporal derivative was used to model the activation time-course at the individual level . For our main comparisons , contrasts between the intact/dominant hand and the residual/nondominant arm conditions were defined against the feet condition . Further contrasts were defined between intact/dominant arm ( in the congenital group ) and nondominant/missing hand ( in the acquired group ) vs feet movements . Group level analysis of spatial maps was carried out using FMRIB’s Local Analysis of Mixed Effects ( FLAME ) . Intact hand movements in the 1-handed groups was compared with dominant hand movements in controls , and residual arm movements were compared with non-dominant arm movements . Data from participants with above elbow deprivation ( 1 congenital and 7 acquired participants ) were excluded from the analysis of activity during arm movements . The cross-subject GLM included planned comparisons between each of the groups against the other two , while accounting for the unbalanced comparison . Z ( Gaussianised T/F ) statistic images were thresholded using clusters determined by Z>2 and a family-wise-error corrected cluster significance threshold of p<0 . 05 was applied to the suprathreshold clusters . To visualise the human homunculus , activation maps during feet , hands , arms and lip movements in the control group were thresholded at Z>3 , each map was mirror flipped and maps representing each of the four body parts were averaged , such that a symmetrical representation of each body part was achieved . For presentation purposes , statistical parametric activation maps were projected on the inflated surface of a representative participant’s cortex , using FreeSurfer . We followed the same ROI selection and thresholding procedures as previously reported in Makin et al . ( 2013 ) . Briefly , it has been demonstrated that movements of a phantom limb elicit both central and peripheral motor signals that are different from those found during imagined movement ( Reilly et al . , 2006; Raffin et al . , 2012 ) . Since the acquired , but not the congenital participants , displayed group activation in the sensorimotor hand knob during phantom hand movements ( Makin et al . , 2013 ) , the congenital group was excluded from the ROI definition . The ‘deprived cortex’ ROI was therefore defined using the conjunction of missing/non-dominant hand movements ( compared to feet movements ) in the acquired and control groups only . A pre-determined threshold of Z>7 was chosen , yielding a single cluster , centred on the hand knob of the central sulcus , contralateral to the missing hand . A second cluster in lobule V of the cerebellum , ipsilateral to the nondominant/phantom hand was defined separately . For within-participant comparisons , a homologous ROI for the intact hand was defined using the conjunction between controls and all 1-handed participants ( using a pre-determined threshold of Z>8 ) , yielding a comparable cluster contralateral to the intact hand . GLM parameter estimate values ( β ) of the low level statistical parametric maps of the contrasts described above were extracted from all voxels underlying the pre-determined ROIs and then averaged for each participant . Voxelwise statistical analysis of the FA data were carried out using Tract-Based Spatial Statistics ( TBSS [Smith et al . , 2006] ) . TBSS projects all subjects’ FA data onto a mean FA tract skeleton , before applying voxelwise cross-subject statistics . Diffusion data were initially corrected for eddy-currents and head motion using affine registration to the average of the non-diffusion-weighted volumes . A diffusion tensor model was fitted at every voxel to derive FA maps , which were non-linearly registered to group-specific templates created from 11 participants of each group ( FA thresholds > 0 . 2 ) . The resulting FA images were temporally concatenated into a single 4D file and averaged to create a mean ‘skeleton’ , representing the centre of all white matter tracts onto which participant-specific FA values were projected . Tract-based spatial statistics was performed voxel-wise , using a GLM and permutation-based non-parametric testing . The GLM included either planned comparisons between the three groups , or the beta values extracted from the deprived cortex ROI during intact hand movements for each of the individuals with hand absence . An age regressor of no interest was also included for the correlation analysis . Clusters were formed at t > 2 and tested for significance at p<0 . 05 , corrected for multiple comparisons across space . For presentation purposes , the FA clusters were dilated to fill the corresponding tracts . For analysis of limb-use data , independent-samples two-tailed t-tests were used to compare congenital and acquired groups . For ROI-based analysis of fMRI activation , intact hand movements in the 1-handed groups was compared with dominant hand movements in controls , and residual arm movements were compared with non-dominant arm movements . Data from participants with above elbow deprivation were excluded from the analysis of activity during arm movements . Between-group effects were initially statistically compared using a mixed-level ANOVA . For each limb , two comparisons were planned based on our behavioural results . Therefore , for follow-up unpaired t-tests of these planned comparisons , a p-value of p≤0 . 025 was used , to account for multiple comparisons . Other within/between-group effects were statistically compared using paired/independent-samples two-tailed t-tests ( α < 0 . 05 ) . Where significant departure from normality was found ( based on the Shapiro-Wilk test ) , nonparametric tests were used ( Man-Whitney or Wilcoxon , as appropriate ) . To test for correlations between questionnaire , acceleration data , and fMRI activation , two-tailed Pearson tests were used . To exclude the involvement of various confounds from the correlation analysis ( e . g . , level of amputation , age at deprivation ) , post-hoc one-tailed partial correlation analysis was performed . To exclude the involvement of various confounds ( e . g . , phantom pain ) from the group comparisons , univariate GLM comparisons were carried , with the confounding measurement as a covariate . To account for the potential contribution of phantom pain on the BOLD differences , chronic levels of phantom pain were assessed in each 1-handed individual . Chronic pain magnitude was calculated by dividing pain intensity ( 0: ‘no pain’—10: ‘worst pain imaginable’ ) by frequency ( 1—‘all the time’ , 2 —‘daily’ , 3—‘weekly’ , 4—‘several times per month’ and 5—‘once or less per month’ ) . This measure therefore reflects the chronic aspect of the pain as it combines both frequency and intensity , as used previously ( Draganski et al . , 2006; Makin et al . , 2013 ) . Comparison between correlations was assessed using a two-tailed Fisher r-to-z transformation . Statistical analysis was carried with SPSS version 18 .
Arm-amputation involves two powerful drivers for brain plasticity—sensory deprivation and altered use . However , research has largely focused on sensory deprivation and maladaptive change . Here we show that adaptive patterns of limb usage after amputation drive cortical plasticity . We report that individuals with congenital or acquired limb-absence vary in whether they preferentially use their intact hand or residual arm in daily activities . Using fMRI , we show that the deprived sensorimotor cortex is employed by whichever limb individuals are over-using . Individuals from either group that rely more on their intact hands ( and report less frequent residual arm usage ) showed increased intact hand representation in the deprived cortex , and increased white matter fractional anisotropy underlying the deprived cortex , irrespective of the age at which deprivation occurred . Our results demonstrate how experience-driven plasticity in the human brain can transcend boundaries that have been thought to limit reorganisation after sensory deprivation in adults .
The loss of a limb will have a profound impact on an individual’s daily life . Nevertheless , individuals can employ a variety of behavioural strategies to adapt to the loss of , say , a hand . Some become skilled at using the residual part of their arm , while others prefer to rely on their other hand . Their brain , too , will undergo major changes . Many studies have shown that the region of the brain that controlled a given limb can be “taken over” by another part of the body if that limb is lost . This process has been previously considered to be harmful , as it has been linked to experiences of pain arising from the missing limb . Now , Makin et al . have explored the links between changes in the behaviour of individuals missing a hand and changes in their brains . People who had been born without a hand or who had lost a hand in later life were asked to wear a device that recorded their movements as they went about their daily lives . The data revealed that people who had been born without a hand made relatively more use of their residual limb , while those who had lost their hand made relatively more use of their remaining hand . Moreover , these differences were reflected in patterns of brain activity . In the subjects born without a hand ( who were making relatively extensive use of their residual limb ) , the area of the brain that would otherwise control the ‘missing’ hand was activated when the subjects moved their residual limb . And in the subjects who had lost their hand , this brain region was activated when they moved their remaining hand . However , in individual subjects , the size of the effect depended on the usage preferences of the subject: for example , the minority of people who were born without a hand but nevertheless make extensive use of their intact hand showed a pattern of activation that resembled the average pattern seen in those who had lost a hand in later life . By providing new insights into the plasticity of brain and behaviour following the loss of a hand , the work of Makin et al . may aid the development of rehabilitation techniques to help patients to optimise the use of both their residual and their intact limbs .