***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: OBJECTIVE To examine the effects of theta burst stimulation ( TBS ) and paired associative stimulation ( PAS ) on excitability in the human motor cortex . METHODS Sixteen healthy young participants received intermittent TBS ( iTBS ) or PAS to the primary motor cortex on two testing occasions , at least a week apart . Ten of the participants also received iTBS or PAS after conditioning with continuous TBS on two other occasions . Cortical excitability was assessed with single TMS pulses to the motor cortex . Motor evoked potentials ( MEPs ) were measured from the first dorsal interosseus ( FDI ) muscle before TBS or PAS stimulation , and every 10min for 60min after stimulation . Changes in excitability were compared against the potential for motor learning , assessed with the rotor pursuit task . RESULTS After the PAS protocol MEP amplitudes were significantly increased . This increase was greater than after intermittent TBS , which did not change MEPs significantly . Conditioning with continuous TBS showed no significant effect . Participants ' responses were not correlated across protocol s and were not correlated with rotor pursuit learning . CONCLUSIONS PAS was the only protocol which induced significant increases in MEP amplitude . SIGNIFICANCE PAS is robust in inducing excitatory cortical change . This makes it a suitable protocol for testing plasticity in healthy and patient groups Transcranial DC stimulation ( tDCS ) is a plasticity-inducing noninvasive brain stimulation tool with various potential therapeutic applications in neurological and psychiatric diseases . Currently , the duration of the aftereffects of stimulation is restricted . For future clinical applications , stimulation protocol s are required that produce aftereffects lasting for days or weeks . Options to prolong the effects of tDCS are further prolongation or repetition of tDCS . Nothing is known thus far about optimal protocol s in this behalf , although repetitive stimulation is already performed in clinical applications . Thus we explored the effects of different break duration s on cathodal tDCS-induced cortical excitability alterations . In 12 subjects , two identical periods of cathodal tDCS ( 9-min duration ; 1 mA ) with an interstimulation interval of 0 ( no break ) , 3 , or 20 min or 3 or 24 h were performed . The results indicate that doubling stimulation duration without a break prolongs the aftereffects from 60 to 90 min after tDCS . When the second stimulation was performed during the aftereffects of the first , a prolongation and enhancement of tDCS-induced effects for ≤ 120 min after stimulation was observed . In contrast , when the second stimulation followed the first one after 3 or 24 h , the aftereffects were initially attenuated , or abolished , but afterwards re-established for up to 120 min after tDCS in the 24-h condition . These results suggest that , for prolonging the aftereffects of cathodal tDCS , stimulation interval might be important The development of  stimulus selectivity in the primary sensory cortex of higher vertebrates is considered in a general mathematical framework . A synaptic evolution scheme of a new kind is proposed in which incoming patterns rather than converging afferents compete . The change in the efficacy of a given synapse depends not only on instantaneous pre- and postsynaptic activities but also on a slowly varying time-averaged value of the postsynaptic activity . Assuming an appropriate nonlinear form for this dependence , development of selectivity is obtained under quite general conditions on the sensory environment . One does not require nonlinearity of the neuron 's integrative power nor does one need to assume any particular form for intracortical circuitry . This is first illustrated in simple cases , e.g. , when the environment consists of only two different stimuli presented alternately in a r and om manner . The following formal statement then holds : the state of the system converges with probability 1 to points of maximum selectivity in the state space . We next consider the problem of early development of  orientation selectivity and binocular interaction in primary visual cortex . Giving the environment an appropriate form , we obtain orientation tuning curves and ocular dominance comparable to what is observed in normally reared adult cats or monkeys . Simulations with binocular input and various types of normal or altered environments show good agreement with the relevant experimental data . Experiments are suggested that could test our theory further Low-frequency ( 1 Hz ) repetitive transcranial magnetic stimulation ( rTMS ) can depress the excitability of the cortex locally and has been proposed for the treatment of disorders such as schizophrenia and epilepsy . Some have speculated that the depressant effect is related to long-term depression ( LTD ) of cortical synapses . Because in vitro LTD can be enhanced by pretreatment of synapses with higher-frequency stimulation , we hypothesized that if rTMS depression had mechanisms in common with LTD , higher-frequency priming would increase it also . In 25 healthy volunteers in two experiments , we measured motor-evoked potentials (  MEPs ) from TMS of the motor cortex to define the baseline response . Subthreshold rTMS ( 6 Hz , fixed rate or frequency modulated ) was used to prime the motor cortex , followed by suprathreshold 1 Hz stimulation for 10 min at just above the MEP threshold . Over the next 60 min , we recorded MEPs every 10 sec and found significant increases in the amount of cortical depression with both types of 6 Hz priming rTMS relative to sham . The MEP depression from 6 Hz-primed 1 Hz rTMS showed no evidence of decay after 60 min . Pretreatment with 6 Hz primes both 1 Hz rTMS depression and LTD . Although not conclusive evidence , this strengthens the case for overlapping mechanisms and suggests a potent new technique for enhancing low-frequency rTMS depression that may have experimental and clinical applications It has been 30 years since the discovery that repeated electrical stimulation of neural pathways can lead to long-term potentiation in hippocampal slices . With its relevance to processes such as learning and memory , the technique has produced a vast literature on mechanisms of synaptic plasticity in animal models . To date , the most promising method for transferring these methods to humans is repetitive transcranial magnetic stimulation ( rTMS ) , a noninvasive method of stimulating neural pathways in the brain of conscious subjects through the intact scalp . However , effects on synaptic plasticity reported are often weak , highly variable between individuals , and rarely last longer than 30 min . Here we describe a very rapid method of conditioning the human motor cortex using rTMS that produces a controllable , consistent , long-lasting , and powerful effect on motor cortex physiology and behavior after an application period of only 20 - 190 OBJECTIVE : To test the feasibility of creating a valid and reliable checklist with the following features : appropriate for assessing both r and omised and non-r and omised studies ; provision of both an overall score for study quality and a profile of scores not only for the quality of reporting , internal validity ( bias and confounding ) and power , but also for external validity . DESIGN : A pilot version was first developed , based on epidemiological principles , review s , and existing checklists for r and omised studies . Face and content validity were assessed by three experienced review ers and reliability was determined using two raters assessing 10 r and omised and 10 non-r and omised studies . Using different raters , the checklist was revised and tested for internal consistency ( Kuder-Richardson 20 ) , test-retest and inter-rater reliability ( Spearman correlation coefficient and sign rank test ; kappa statistics ) , criterion validity , and respondent burden . MAIN RESULTS : The performance of the checklist improved considerably after revision of a pilot version . The Quality Index had high internal consistency ( KR-20 : 0.89 ) as did the subscales apart from external validity ( KR-20 : 0.54 ) . Test-retest ( r 0.88 ) and inter-rater ( r 0.75 ) reliability of the Quality Index were good . Reliability of the subscales varied from good ( bias ) to poor ( external validity ) . The Quality Index correlated highly with an existing , established instrument for assessing r and omised studies ( r 0.90 ) . There was little difference between its performance with non-r and omised and with r and omised studies . Raters took about 20 minutes to assess each paper ( range 10 to 45 minutes ) . CONCLUSIONS : This study has shown that it is feasible to develop a checklist that can be used to assess the method ological quality not only of r and omised controlled trials but also non-r and omised studies . It has also shown that it is possible to produce a checklist that provides a profile of the paper , alerting review ers to its particular method ological strengths and weaknesses . Further work is required to improve the checklist and the training of raters in the assessment of external validity Long-term potentiation ( LTP ) and long-term depression ( LTD ) underlie most models of learning and memory , but neural activity would grow or shrink in an uncontrolled manner , if not guarded by stabilizing mechanisms . The Bienenstock-Cooper-Munro ( BCM ) rule proposes a sliding threshold for LTP/LTD induction : LTP induction becomes more difficult if neural activity was high previously . Here we tested if this form of homeostatic plasticity applies to the human motor cortex ( M1 ) in vivo by examining the interactions between two consecutive sessions of paired associative stimulation ( PAS ) . PAS consisted of repeated pairs of electrical stimulation of the right median nerve followed by transcranial magnetic stimulation of the left M1 . The first PAS session employed an interstimulus interval equalling the individual N20-latency of the median nerve somatosensory-evoked cortical potential plus 2 ms , N20-latency minus 5 ms , or a r and om alternation between these intervals , to induce an LTP-like increase in motor-evoked potential ( MEP ) amplitudes in the right abductor pollicis brevis muscle ( PAS(LTP ) ) , an LTD-like decrease ( PAS(LTD ) ) , or no change ( PAS(Control ) ) , respectively . The second PAS session 30 min later was always PAS(LTP ) . It induced an moderate LTP-like effect if conditioned by PAS(Control ) , which increased if conditioned by PAS(LTD ) , but decreased if conditioned by PAS(LTP ) . Effects on MEP amplitude induced by the second PAS session exhibited a negative linear correlation with those in the first PAS session . Because the two PAS sessions activate identical neuronal circuits , we conclude that ' homosynaptic-like ' homeostatic mechanisms in accord with the BCM rule contribute to regulating plasticity in human M1 
    Output:
    
The results showed that if the history of synaptic activity were high or low enough to displace the threshold , the expected effects of the test protocol would be the reverse .
This effect reversal is regulated by homeostatic mechanisms .
On the contrary , the effects of the test protocol would not be the reverse , and at most we experience a prolongation of the lasting effects if the aforementioned history is not enough to displace the threshold .
This effect prolongation is mediated by nonhomeostatic mechanisms .