mirror neurons are cortical single - cells , first discovered in the macaque s motor 
 representation area ( f5 area : ventral premotor cortex)1,2,3 , that are activated when the monkey performs particular 
 object - directed actions , when it observes another individual performing the same action , or 
 when the monkey merely sees graspable objects4 , 5 . there may be a similar system in humans , 
 with increasing evidence that a putative human mirror neuron system exists2 , 5,6,7,8,9,10,11 , 
 with paradigms , like those used in monkeys , demonstrating cells which discharge both when a 
 movement is imagined and when the same movement is observed . a growing number of eeg studies have investigated changes in the sensorimotor cortex during 
 action observation or imagery12,13,14 . most previous 
 studies showed that the brain wave was changed , especially during observation , and suggested 
 that action observation would be more effective than action imagery for patients15 . however , there is a large body of evidence showing that the action system becomes activated 
 when viewing objects that afford actions16 , 17 . according to these studies , viewing 
 graspable tools activates motor regions . in other words , viewing a manipulable object linked 
 automatically into its motoric properties , which are associated with motor representations , 
 and therefore the mirror neuron system ( mns ) was activated . the mu rhythm is the eeg wave in the 813  hz frequency band seen over sensorimotor 
 cortex3 , 18 . it is most pronounced ( i.e. , incereased power ) when a person is at 
 rest , and becomes suppressed ( i.e. , decreased power ) both when observing actions of another 
 person and during action imagery16 . therefore , the mu rhythm is thought to reflect downstream modulation of mns activity . suppression has been successfully used to measure mirror neuron activity in human 
 adults13 , 16 , whereas increasing mu range reflects inhibition of the mns19 . recently , several action observation / imagery training studies have been conducted in 
 patients with limited physical activity20,21,22 , 
 with results demonstrating a clinical improvement in motor function20,21,22 . in addition , previous studies have investigated brain 
 wave changes such as relative spectral power15 , but there are few studies which analyzed the mu rhythm . moreover , 
 most studies compared the effect of action observation and action imagery , and little is 
 known about changes caused by subsequent action observation of the target objects . in patients with limited physical activity , visual stimuli from the environment are very 
 important in the recovery of motor learning . thus , we investigated the effects of action 
 observation and compared different visual objects on the mu rhythm . due to gender differences during action observation23 , only female subjects participated in this study ; 18 healthy adults 
 took part . all subjects were physically 
 healthy , had no neurological diseases , had not taken any medicines for the purpose of 
 treatment , had normal or corrected normal vision , and were right - handed based on the 
 edinburgh handedness inventory . all volunteers were informed of the sequence of the research 
 procedure , understood its details , and provided signed , informed consent prior to 
 participation . the study was approved by the ethics committee of kyungnam university , 
 republic of korea . scalp eeg was recorded using a multi - channel recorder ( neurofax eeg-1200 , nihon kohden , 
 tokyo , japan ) . the reference 
 electrodes were set to a1 and a2 ( ear lobes ) and recorded in the mono - polar mode . electrode 
 impedances was kept below 10 k. the lower filter was set at 0.53  hz and the upper filter at 
 35  hz , with a 256  hz sampling rate . all signals were a / d digitized and transferred onto a pc 
 for analysis with sampling frequency . fast fourier transform ( fft ) was used in order to 
 separate the mu band from the whole eeg trace . the experiment took place in a sound - isolated booth with a 22-inch monitor approximately 
 100  cm in front of the subject . they were asked to minimize any movements ( e.g. , eye blink , head and limb 
 movements ) while fixated on a computer monitor , in order to eliminate artifacts in the row 
 eeg trace during presentation of visual stimuli . stimuli consisted of different video clips of actions performed by hands and objects ( 3  s 
 duration ) that were recorded using a high - resolution digital camcorder and edited ( fig . 1fig . after 
 3  s , the action cue appeared for 1  s and then the hand action for 3 s. in the nvs 
 condition , the screen was then replaced with a blank black screen ; in the vs 
 condition , the target object remained on the screen for 3  s and thereafter was 
 replaced with a blank screen . in total , a trial length was 13  s , all subjects were 
 provided with 10 trials . ) . all subjects were divided into one of two groups :  visual stimuli ( vs ) and 
  non - visual stimuli ( nvs ) . each test condition lasted 130  s including breaks , comprising 
 randomized 13  s videos , and thus all subjects were provided with 10 trials of visual stimuli 
 in total . after 
 3  s , the action cue appeared for 1  s and then the hand action for 3 s. in the nvs 
 condition , the screen was then replaced with a blank black screen ; in the vs 
 condition , the target object remained on the screen for 3  s and thereafter was 
 replaced with a blank screen . in total , a trial length was 13  s , all subjects were 
 provided with 10 trials . eeg artifacts during visual stimuli were removed prior to analysis using a cosine window . eye blink , and eye and head movements were inspected visually to eliminate epochs with 
 artifacts . the mu amplitude at 813  hz was computed across trials for each condition using an fft that 
 was performed for each electrode . resulting power magnitudes ( v2 ) were averaged for each 
 time - locked block . for the statistical analysis of the action observation trials , mu 
 suppression was calculated by the difference in power ratios between the sub - blocks 
 ( e.g. , hand action observation ) and baseline blocks ( e.g. , black 
 screen)24 , 25 . a ratio was used to control for the variability in mu power as a result of individual 
 differences as opposed to mirror neuron activity . however , ratio data are inherently 
 non - normal , due to factors such as scalp thickness and electrode impedance , therefore a log 
 transformation was applied prior to statistical analyses although data were obtained from electrodes across the scalp , mu 
 rhythm is defined as oscillations measured over the sensorimotor cortex . activity of the 
 sensorimotor cortex during action observation is thought to reflect downstream motor 
 modulations1 , 26 , thus only data from c3 and c4 are presented24 , 27 . a one - sample t - test to examine suppression , as indicated by a log ratio significantly less 
 than 0 in left and right hemisphere electrodes during hand observation with objects , and an 
 independent - samples t - test of the difference between the two observation conditions , were 
 computed . a two - way anova was conducted with block ( pre / observation / post ) and condition 
 ( vs / nvs ) as factors . significant mu suppression was obtained in the action observation trials , in the c3 ( t(17)= 
 9.682 , p<0.001 , two tailed ) and c4 areas ( t(17)= 9.134 , p<0.001 , two tailed ) . there 
 was no significant difference in the amount of mu suppression in either region during action 
 observation blocks ( c3 , t(17)= 0.346 , p<0.734 , two tailed ; c4 , sphericity of the data was verified prior to performing the anova ( mauchly s 
 test , p>0.05 ) . block in both areas ( c3 , 
 f(2,34)=11.525 , p<0.001 ; c4 , f(2,34)=10.723 , p<0.001 ) . mu power was desynchronized during action observation 
 ( c3 : 1.011 v2  0.246 , p<0.001 ; c4 : 0.955 v2  0.249 , p<0.001 ) , and synchronized 
 post - observation ( c3 : 0.507 v2  0.169 , p<0.009 ; c4 : 0.485 v2  0.145 , p<0.004 ) . further , the mu power was lower post - observation than pre - observation ( c3 : 0.504 v2  
 0.209 , p<0.028 ; c4 : 0.470 v2  0.210 , p<0.040 ) ( table 1table 1.grand-averaged mu power amplitudes obtained across all trials pre - observationobservation*post - observationc3vs3.50  2.912.59  2.832.89  2.42nvs3.21  2.832.10  2.102.81  2.56c4vs3.46  3.012.59  2.832.92  2.41nvs3.13  2.802.09  2.132.73  2.43values shown are mean  sd . * p0.05 indicates significant difference between 
 pre - observation and observation ; p0.05 indicates significant difference 
 between observation and post - observation ; p0.05 indicates significant 
 difference between pre - observation and post - observation . eeg mu power is significantly 
 decreased by observing objects manipulated by a hand in both conditions . when the 
 action visual inputs are terminated , the mu power was significantly increased . 
 however , mu power was more increased in  nvs condition during the post - observation 
 period . ) . * p0.05 indicates significant difference between 
 pre - observation and observation ; p0.05 indicates significant difference 
 between observation and post - observation ; p0.05 indicates significant 
 difference between pre - observation and post - observation . eeg mu power is significantly 
 decreased by observing objects manipulated by a hand in both conditions . when the 
 action visual inputs are terminated , the mu power was significantly increased . 
 however , mu power was more increased in brain imaging studies have shown that the human mirror neuron systems are activated both 
 when performing and observing actions , with a number of eeg studies having investigated the 
 oscillatory activity of the cortex28,29,30 . 
 in this study , the 
 results show that the eeg mu rhythm is suppressed by the observation of actions on objects 
 the hands in both conditions , reflecting activation of the mns and consistent with previous 
 studies of the mu rhythm13 , 21 , 24 , 31 . these results suggest that mu suppression involves a 
 range of structures that modulate motor preparation activities and are sensitive to visual 
 input13 . in other studies comparing 
 brain waves in stroke patients and healthy subjects during action observation15 , both groups showed significant 
 differences in relative alpha power , suggesting action observation may be used as a 
 therapeutic method for cognitive intervention . in a clinical setting , based on these brain 
 wave changes , kawasaki et al.31 showed 
 that observational learning increased motor performance involving a ball rotation task , and 
 kim et al.32 suggested that action 
 observation can enhance the walking performance of patients with post - stroke 
 hemiparesis . when the action visual inputs were terminated , the mu power was significantly increased 
 during the post - observation period in both conditions , notwithstanding differences in visual 
 input . however , the applied analysis is imprecise in identifying between the conditions 
 during the post - observation period . focusing on comparing the  vs and nvs groups showed 
 that mu power was more greatly increased in the  nvs group ( table 1 ) . we suggest that this demonstrates the effects of visual 
 input between observed objects and disappeared objects , and is particularly relevant to mns 
 modulations . tucker and ellis33 showed 
 that the premotor cortex and related areas become activated when manipulable object appeared 
 and suggested that an active object representation is sufficient to generate affordance 
 compatibility effects based on execution . proverbio16 showed that the motor cortex was activated during visual perception 
 of tools with the actual possibility . these previous studies reflect the visuosomatomotor integration of the cerebral cortex and 
 that an observed object is automatically encoded30 , 
 33 . vision and action systems evolved 
 together to rapidly gather information from the environment and produce the appropriate 
 motor response17 , assuming that vision 
 flows fluently and automatically into action . therefore , we suggest arranging target objects 
 around patients with limited physical activity to help facilitate motor recovery . however some researchers have not found a difference in the magnitude of mirror neuron 
 activity between object and no object conditions27 . similarly , studies have observed that no suppression was found 
 while participants observed still objects3 . however , these studies differ with ours , as these were not performed 
 subsequent to action observation . we suggest that more visual input with action and object 
 observation may be more effective for cognitive rehabilitation purposes . we conclude that if visual stimuli ( i.e. , target objects ) are sufficiently provided in the 
 daily living environment during an observation training program , the program will be more 
 effective , especially in patients with cerebral damage of the nervous system .
[ purpose ] several action observation / imagery training studies have been conducted in 
 patients with limited physical activity showing improvements in motor function . however , 
 most studies compared effects of action observation and imagery , so little is known about 
 the changes caused by subsequent observation of target objects . moreover , few studies 
 analyzed brain wave changes in the eeg mu rhythm . 
 [ subjects and methods ] eighteen healthy 
 female adults participated in this study , and were divided into two groups :  visual 
 stimuli and  non - visual stimuli. eeg amplitude in the 813  hz frequency band over the 
 sensorimotor cortex was evaluated . 
 [ results ] significant mu suppression was obtained in 
 the action observation trials . 
 mu power showed a main effect of visual stimuli , with 
 decreased power during action observation , and increased power post - observation in both 
 conditions . comparing the 
 visual stimuli and  non - visual stimuli conditions during the 
 post - observation period , mu power demonstrated a greater increase in the  non - visual 
 stimuli condition . 
 furthermore , mu power was lower post - observation than pre - observation . 
 [ conclusion ] these results show the effects of visual input between maintaining target 
 objects and no visual input , and their relevance to modulations of the mirror neuron 
 system . 
 it also suggests that greater visual input may be more effective for cognitive 
 rehabilitation .