in recent years , several neuroimaging techniques have been employed to investigate the 
 neural activity associated with motor imagery and the illusion of motion .
the human brain is 
 able to recognize changes in limb position and movement from both visual and somatosensory 
 stimuli , and the information conveyed to the brain by limb position and musculoskeletal 
 movement plays a vital role in forming a representational body image within the brain . 

collectively , these senses are referred to as kinesthesia . in this context , it has been 
 reported that the illusion of motion evoked by vibrating the tendon of a limb can be 
 effective in stimulating motor control and that it can aid motor learning1 .
the illusion of motion elicited by vibrating the tendon of a limb at the appropriate 
 frequency is mediated by firing of ia afferent fibers that occurs in response to muscle 
 spindle activity .
the muscle spindle receptors are particularly sensitive to the direction 
 and speed of limb movements ; thus , during tendon vibration , subjects experience an illusion 
 of motion , such as stretching of the affected muscles .
previous research has shown that the 
 tendon vibration frequency optimal for eliciting the illusion of motion lies between 70 and 
 100  hz2,3,4,5 .
one research group used magnetoencephalography ( meg ) to record 
 brain activity while subjects experienced the illusion of motion evoked by vibrating the 
 tendon of a limb6 and revealed that this 
 type of stimulation activated regions within the primary sensorimotor cortex , supplementary 
 motor area , and angular gyrus .
another research group used functional magnetic resonance 
 imaging ( fmri ) to measure neural activity during the same illusion of motion1 , 3 and 
 reported neural activation in the contralateral primary sensory motor cortex , dorsal 
 premotor cortex , supplementary motor area , cingulate motor cortex , and ipsilateral 
 cerebellum .
these motor areas are also known to be activated during both active limb 
 movements and motor imagery7 , 8 . with regard to clinical implications ,
gay et al . reported that patients with complex 
 regional pain syndrome ( crps ) type i experienced an improvement in both their pain symptoms 
 and range of motion following vibration of the tendon of the affected limb and the 
 associated illusion of motion9 .
goble et 
 al . reported an improvement in standing balance after vibration of the triceps tendon10 .
moreover , roll et al . reported that the 
 stimulation of illusory movements prevented the cortical disruption normally caused by 
 immobilization in healthy people and that the illusion of motion evoked by vibrating the 
 tendon of a limb could likely prove to be an effective prophylactic treatment following 
 surgery and cast immobilization with conservative therapy11 .
these basic and clinical research studies used a number of different stimulation devices , 
 and the optimal vibration frequency used to evoke the illusion of motion varied between 
 these studies .
while the neural activation patterns occurring during the illusion of motion 
 have been described previously , differences in brain activity associated with the use of 
 different vibration stimulation devices , or with varying vibration frequencies , have not yet 
 been investigated . in a clinical setting , it is important to determine the most effective 
 method of stimulation , as well the optimal stimulation frequency to evoke a beneficial 
 illusion of motion ; if the therapy can be delivered in a simple and cost - effective manner , 
 it can be made available to a larger number of patients .
therefore , in this study , we aimed 
 to compare the effects of a conventional vibration stimulation device with those of 2 handy 
 massagers , which are widely available at electronics stores .
we used functional 
 near - infrared spectroscopy ( fnirs ) to compare the patterns of neural activity occurring 
 during the illusion of motion evoked by vibrating a tendon with these 3 devices .
twelve right - handed college students ( 7 women and 5 men ; mean age , 21.6  years ; range , 
 2022  years ) with no previous history of neurological or other diseases participated in this 
 study . all subjects provided written informed consent after being fully informed of the 
 protocols and purpose of the study .
condition 1 was use of a vibration stimulation device ( sl-0105 lp ; asahi 
 seisakusho co. , ltd , saitama , japan ) set at 80  hz , which was determined to be the optimal 
 frequency in a previous study3 ; condition 
 2 was use of a handy massager ( ycm-20 ; yamazen corporation , osaka , japan ) at a frequency of 
 70  hz ; and condition 3 was use of a handy massager ( thrive md-01 ; thrive co. , ltd , osaka , 
 japan ) set at a frequency of 91.7  hz . the frequency of the 2 massage devices were limited by 
 their factory settings .
the order in which the devices were used on each subject was 
 randomized . during the trial
, subjects were instructed to relax in a sitting position with their eyes 
 closed .
it has been reported that the illusion of motion is unlikely to occur until the 
 muscle is relaxed1 .
subjects were also 
 instructed not to open their eyes until instructed to do so , as visual information can 
 disrupt the illusion of motion12 .
the 
 location selected for vibration stimulation was the common digital extensor tendon of the 
 right wrist joint .
the stimulation protocol consisted of 15  s of rest followed by 30  s of 
 stimulation and was applied three times for each condition .
the intensity of the illusion 
 evoked was assessed by each subject using a visual analog scale ( vas ) immediately after each 
 condition .
the illusion of motion was confirmed to reproduce movement of the wrist , and the 
 range of the illusion was measured using an electrical goniometer ( sg150 ; biometrics ltd . , 

we defined the  range of the illusion  as the range of tendon 
 vibration at which subjects experienced the illusion of motion . as a control ,
the skin 
 surrounding the extensor tendon of the wrist was stimulated using the same protocol .
this 
 allowed us to subtract the brain activity evoked by skin irritation and excitement of 
 meissner and pacinian receptors from the brain activity evoked by the illusory motion3 , 4 .
we measured neural activity in the brain by using an fnirs system ( foire-3000 ; shimadzu 
 corporation , kyoto , japan ) .
the fnirs system , which had continuous wave laser diodes with 
 wavelengths of 780 , 805 and 830  nm , was used to record cortical activity at a sampling rate 
 of 5  hz . in brief
, we employed a 49-channel system with 30 optodes ( 15 light sources and 15 
 detectors ) .
this system detects changes in cortical concentrations ( mm   mm ) of oxygenated 
 hemoglobin ( oxyhb ) , deoxygenated hemoglobin , and total hemoglobin , by applying the modified 
 beer - lambert law .
the optodes were positioned according to the international 10/20 system , 
 with cz located beneath the 7th light source and the other optodes located at intervals of 
 3.0  cm , centering around the 7th light source .
the fnirs topographic map covered the front 
 parietal area , which was divided into 8 different regions of interest ( rois , fig .
fifteen light sources ( red numbers ) and 15 detectors ( blue 
 numbers ) covered the frontoparietal area .
solid white numbers denote measuring 
 channels , which were divided into 8 regions of interest . ) , based on the functional anatomy of the parietal and prefrontal regions .
the left 
 sensorimotor cortex was covered by channels 17 , 18 , 21 , 22 , 26 , and 27 ; the right 
 sensorimotor cortex was covered by channels 14 , 15 , 19 , 20 , 23 , and 24 ; the left and right 
 premotor cortices were covered by channels 30 , 31 , 35 , and 36 , and by channels 28 , 29 , 32 , 
 and 33 , respectively ; the left and right prefrontal cortices were covered by channels 37 , 
 38 , 41 , 42 , 46 , and 47 , and channels 39 , 40 , 44 , 45 , 48 , and 49 , respectively ; and the left 
 and right parietal areas were covered by channels 3 , 4 , 8 , 9 , 12 , and 13 and channels 1 , 2 , 
 5 , 6 , 10 , and 1 , respectively ( fig .
it has 
 been reported that a delayed reaction is inherent in fnirs recordings ; therefore , data were 
 extracted beginning 5  s before the rest and task periods13 .  
fifteen light sources ( red numbers ) and 15 detectors ( blue 
 numbers ) covered the frontoparietal area .
solid white numbers denote measuring 
 channels , which were divided into 8 regions of interest . to compare the range of the illusion of motion and the intensity of the illusion under each 
 of the 3 conditions
we used spearman s rank 
 correlation coefficient to examine the relationship between the range of illusion of motion 
 and the reported intensity of the illusion . in the fnirs analysis , we selected oxyhb levels as markers of cortical activity because 
 oxyhb is the most sensitive indicator of locomotion - related changes in regional cerebral 
 blood ow14,15,16 .
moreover , there are 
 considerable individual differences in task - related changes in deoxyhb levels , probably due 
 to variable neurovascular coupling in the elderly16,17,18,19 .
changes in the oxyhb 
 levels were calculated during the task phases of rest and vibration , which were dened as 
 follows : the rest phase was the 15  s before the beginning of the 30  s task ( vibration ) 
 phase .
regional changes in the oxyhb level during the control and vibration phases were 
 obtained from each channel in each subject .
data for 3 repetitions were averaged for each 
 channel ; then the value for each region of interest was obtained by averaging data from 
 several channels ( fig .
1 ) in each subject . to evaluate the effect of cortical activation during the control and vibration periods , we 
 calculated the effect size ( es ) to adjust the inuence of differential path length factors 
 among subjects and cortical regions on oxyhb levels20 .
the es for the effect of the vibration task on activities was 
 calculated by the following formula : mean oxyhb value during vibration task - mean oxyhb value 
 during control task / standard deviation of oxyhb value during control task .
we calculated es for each channel16 , 21 and analyzed rois , including the right and 
 left primary sensory motor area , premotor area , and parietal association area .
moreover , we 
 employed roi analysis for the primary sensory motor area , premotor area , and parietal 
 association area to compare the responses between the left and right hemispheres under each 
 condition .
for this analysis , we used two - way anova and performed multiple comparisons using 
 scheffe s method .
all statistical analyses were performed using the statistical package for 
 the social sciences ( spss ) ver#17.0 ( spss , chicago , il , usa ) . the threshold for statistical 
 significance was set at p = 0.05 for all analyses .
under all 3 conditions , all 12 subjects experienced the illusion of motion . there was no 
 significant difference in the range of illusion or in the intensity of the illusion between 
 the 3 conditions ( p > 0.05 ) ( table 
 1table 1 .
range of illusion and illusion intensity under all conditionscondition 1condition 2condition 3range of illusion ( )42.8  16.838.3  13.238.8  14.0illusion intensity ( vas ; mm)75.2  21.970  20.470.9  19.9mean  sd .
the illusion intensity and range of illusion were 
 significantly correlated ( p < 0.05 ) ( table 
 2table 2 .
correlation between range of illusion and illusion intensityrange of illusioncondition 1condition 2condition 3condition 1condition 20.83condition 30.850.95*p<0.05 ; * * p<0.01illusion intensity ( vas)condition 1condition 2condition 3condition 1condition 20.83condition 30.870.92*p<0.05 ; * * p<0.01 .
vas : visual analog scale analysis of neural activity showed a significant increase in oxyhb level in the left 
 premotor cortex ( p < 0.05 ) , left and right sensory motor cortices , and parietal area ( p 
 < 0.01 ) under all 3 conditions ( table 
 3table 3 . comparison of effect size in regions of interest ( rois ) under the 3 
 conditionsroiillusioncontrolright parietal association areacondition 10.133  0.317 * * 0.350  0.472condition 20.152  0.306 * * 0.330  0.454condition 30.119  0.318 * * 0.208  0.469right primary sensorimotor areacondition 10.030  0.369 * * 0.323  0.433condition 20.101  0.342 * * 0.349  0.432condition 30.003  0.345 * * 0.384  0.456right premotor areacondition 10.006  0.358 0.314  0.687condition 20.020  0.357 0.153  0.688condition 30.057  0.378 0.102  0.704left parietal association areacondition 10.149  0.360 * * 0.325  0.403condition 20.046  0.359 * * 0.202  0.398condition 30.106  0.328 * * 0.240  0.420left primary sensorimotor areacondition 10.118  0.278 * * 0.192  0.256condition 20.114  0.278 * * 0.164  0.229condition 30.188  0.295 * * 0.256  0.243left premotor areacondition 10.147  0.415 * 0.149  0.382condition 20.168  0.397 * 0.107  0.382condition 30.106  0.416 * 0.198  0.399mean  sd .
multiple comparison analysis revealed no significant difference in brain 
 activity among the different vibration stimulation devices used .
in this study , we found no significant difference in the intensity of the evoked illusion 
 or in the range of the illusion of motion between the 3 conditions ; furthermore , the 
 illusion intensity and range of illusion of motion were significantly correlated under all 3 
 conditions .
this indicates that each one of the vibration stimulation devices used elicited 
 the same illusory effect .
we observed a significant increase in oxyhb concentrations in the 
 left premotor cortex , left and right sensory motor cortices , and parietal area under each of 
 the 3 conditions during the illusion of motion .
we also observed neural activation in the 
 same brain areas as previously reported1 , 3 , 6 .
previous studies investigating vibration stimulation of the tendon consistently reported 
 the optimal frequency for eliciting the illusion of motion as being between 70 and 
 100  hz2,3,4,5 however , these studies used different vibration stimulation devices . 

the stimulation frequencies of the 2 handy massagers used in this study were between 70 and 
 91.7  hz , which is within the range reported to be optimal for eliciting the illusion of 
 motion . in the current study , we demonstrated that these devices could reliably evoke not 
 only the same illusion intensity and range of illusion of motion but also the same patterns 
 of neural activity as that produced using more sophisticated vibration stimulation 
 devices .
previous studies have shown that the illusion of motion evoked by vibration stimulation of 
 the tendon activates the same brain areas as those activated during both active movement and 
 motor imagery7 , 8 . during active movement , efferent sensory information is input to 
 the brain as the muscle contracts , and information from the muscle spindles plays an 
 important role in the perception of movement and of changes in limb position .
the primary 
 motor cortex is crucial for kinesthetic information processing and perception of movement 
 and position1 , 22 . here , we observed increased activity in the primary motor cortex 
 during the illusion of motion .
this was most likely due to afferent input from muscle 
 spindles , as well as the illusion of motion caused by vibration stimulation of the wrist 
 tendon .
thus , we think that active movement and illusion of motion are associated with 
 similar patterns of brain activity . in addition
, previous studies showed similar brain activity patterns for motor imagery and 
 the illusion of motion evoked by vibration stimulation applied to a tendon8 .
notably , it has been reported that motor 
 imagery can be effective in promoting functional recovery of the upper limb in patients with 
 stroke28 .
further , the intensity of the 
 illusion can be decreased or increased by actively evoking motor imagery and applying 
 vibration stimulation to the appropriate tendon at the same time8 , 26 .
thus , by 
 employing a combination of the illusion of motion evoked by vibration stimulation and active 
 motor imagery , the potential beneficial effects in a clinical setting can be maximized for 
 both the techniques .
previous studies have also shown that illusory movements prevent the 
 cortical disruption normally caused by immobilization in healthy people11 and that the pain symptoms and range of motion of patients 
 with crps type i improved after they experienced the illusion of motion evoked by vibrating 
 the tendon of the affected limb9 . given the above findings
, we believe that the illusion of motion evoked by vibration 
 stimulation of a tendon could be effectively utilized in a clinical setting for prophylactic 
 and rehabilitative purposes . furthermore , our result showing that the illusion of motion can 
 be reliably evoked using a handy massager within the optimal stimulation frequency indicates 
 the possibility of making the technique easily accessible and affordable for a large number 
 of patients in a variety of clinical settings .
   [ purpose ] a number of different stimulation devices are used in basic and clinical 
 research studies , and their frequencies of use vary . however , 
 whether or not they are 
 equally effective has not been investigated . 
 the purpose of the present study was to 
 investigate neural activity in the brain during the illusion of motion evoked by 
 stimulating the tendons of the wrist extensor muscles using various vibration devices . 
 
 [ subjects ] twelve right - handed university students with no history of nervous system 
 disorder or orthopedic disease participated in the study . [ methods ] 
 the wrist extensor 
 tendon was stimulated using 3 different devices : 1 ) a vibration stimulation device 
 ( sl-0105 lp ; asahi seisakusho co. , ltd . , 
 saitama , japan ) , frequency 80  hz ; 2 ) a handy 
 massager ( ycm-20 ; yamazen corporation , osaka , japan ) , frequency 70  hz ; and 3 ) a handy 
 massager ( thrive md-01 ; thrive co. , ltd . 
 , osaka , japan ) , frequency 91.7  hz . 
 brain activity 
 was recorded during stimulation by using functional near - infrared spectroscopy . 
 [ results ] 
 increased neural activity was observed in both the premotor cortices and the parietal 
 region in both hemispheres in all 3 cases . 
 the level and localization of neural activity 
 was comparable for all 3 stimulation devices used . [ conclusion ] 
 this suggests that 
 subjects experience the illusion of motion while the tendon is being stimulated using any 
 vibration device . 