Authors: Rogier B Mars Sven Bestmann John C Rothwell Patrick Haggard
Publish Date: 2007/07/19
Volume: 182, Issue: 1, Pages: 125-129
Abstract
The preparation of motor responses during the delay period of an instructed delay task is associated with sustained neural firing in the primate premotor cortex It remains unclear how and when such preparationrelated premotor activity influences the motor output system In this study we tested modulation of corticospinal excitability using singlepulse transcranial magnetic stimulation TMS during a delayedresponse task At the beginning of the delay interval participants were either provided with no information spatial attentional information concerning location but not identity of an upcoming imperative stimulus or information regarding the upcoming response Behavioral data indicate that participants used all information available to them Only when information concerning the upcoming response was provided did corticospinal excitability show differential modulation for the effector muscle compared to other taskunrelated muscles We conclude that modulation of corticospinal excitability reflects specific response preparation rather than nonspecific event preparationHumans and other primates are able to use prior information to prepare their motor system for a later response Rosenbaum 1980 This motor preparation is evident in a shorter reaction time and is reflected in preparatory neural activity An important question to ask when trying to understand the nature of preparatory neural processes is with what type of information a certain neural structure is dealing For instance in the domain of arbitrary or symbolic visuomotor associations Wise and Murray 2000 the dorsal premotor cortex PMd has consistently been shown to exhibit strong preparatory activity eg Kurata and Wise 1988 Toni et al 1999 However the type and locus of activity within PMd differs depending on the amount of information concerning the upcoming movement that is available to the participant Hoshi and Tanji 2000 Mars et al 2005 When preparatory information is given about only some attributes of the imperative stimulus activity can be found in the rostral part of PMd Conversely when the information about the imperative stimulus is sufficient to specify the response in advance preparatory activity is present in the most caudal parts of PMd which have access to the primary motor cortex and the spinal cord Mars et al 2005 Consistent with this finding modulation of activity of spinal neurons during movement preparation has been observed in monkeys Prut and Fetz 1999Studies probing human corticospinal excitability in delayedresponse paradigms using transcranial magnetic stimulation TMS however have yielded inconsistent results A number of studies have reported a decrease in corticospinal excitability during instructed delay periods but no responsespecific change in corticospinal excitability eg Touge et al 1998 Hasbroucq et al 1999 Conversely some recent studies reported increases in corticospinal excitability specific to the prepared response Van den Hurk et al 2007 Van Elswijk et al 2007 These conflicting results can be partly explained by differences across studies in the timing of the response and therefore in the degree of preparation or in the predictability of the TMS pulse Both of these factors have been shown to influence corticospinal excitability Van Elswijk et al 2007 Takei et al 2005 Interestingly Van den Hurk et al 2007 employed a range of delay periods varying between 1 and 9 s resulting in a low predictability of both response time and timing of the TMS pulse However these types of delays are quite unusual in preparation studies using TMS and may result in fluctuations in the level of motor preparation during the delay periodIn the present study we aim to investigate whether a specific increase in corticospinal excitability with selective response preparation can be found in a delayed response paradigm with delays in the range normally studied using TMS Furthermore we employ spatially compatible cues to instruct movement preparation Since previous studies eg Van den Hurk et al 2007 cued movement preparation using arbitrary cues cf Wise and Murray 2000 it remains to be seen whether these results generalize to other types of movement cueingParticipants were required to respond by pressing a button with either the left or right hand in response to a trigger cue The trigger cue was presented either to the left of the right side of fixation and consisted of a symbolic cue instructing one of the two possible responses At the beginning of each trial an instruction cue could give participants prior information on the location of the upcoming trigger cue or the movement it would instruct We probed corticospinal excitability during the delay period between the instruction and the trigger cues using singlepulse transcranial magnetic stimulation This allowed us to investigate whether the processing of advance motor information has distinct effects on corticospinal excitability over and above effects of prior information and general preparation Critically the timing of both response time and TMS pulse varied and was equally unpredictable between conditionsEleven healthy righthanded volunteers 4 women age range 18–31 years participated in the experiment All participants gave their informed consent prior to participation Experimental procedures were approved by the local ethics committee and performed according to the ethical standards laid down in the Declaration of Helsinki No participants reported any adverse effects of TMS The data of the last two out of four experimental blocks of one participant were discarded due to excessive head movementsSurface electromygraphic EMG was recorded from the right first dorsal interosseous FDI muscle with a bellytendon montage using Ag/AgCl surface electrodes Raw signals were amplified using Cambridge Electronic Design 1902 amplifiers with a bandpass filter of 1–2000 Hz Signals were stored on a personal computer for later analysis at a sampling rate of 5 kHz Participants were asked to maintain relaxation of the target muscle throughout the experimentTranscranial magnetic stimulation was applied over the hand area of the left motor cortex with a figureofeight coil 70 mm outer wing diameter using a Magstim 200 stimulator Magstim Co Whitland UK The coil was held tangentially over the left side of the scalp with its handle pointing backwards at an angle of approximately 45° from the midsagittal axis The site of stimulation was optimal to elicit motorevoked potentials MEPs in the right FDI muscle The coil position was marked on the participants head so that it could be maintained at that location during the experiment Stimulation intensity for the main experiment was set to evoke an MEP of 1 mV peaktopeak amplitude corresponding to a stimulator output of 30–60 for all participants
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