Online Maintenance of Sensory and Motor Representations: Effects on Corticospinal Excitability

Abstract

van den Hurk P, Mars RB, van Elswijk G, Hegeman J, Pasman JW, Bloem BR, Toni I. Online maintenance of sensory and motor representations: effects on corticospinal excitability. J Neurophysiol 97: [1642][1643][1644][1645][1646][1647][1648] 2007. First published November 29, 2006; doi:10.1152/jn.01005.2006. Flexible behavior requires the ability to delay a response until it is appropriate. This can be achieved by holding either a sensory or a motor representation online. Here we assess whether maintenance of sensory or motor material drives the motor system to different functional states, as indexed by alterations of corticospinal excitability. We used singlepulse TMS to measure corticospinal excitability evoked during the delay period of a novel paradigm in which task contingencies, rather than explicit verbal instructions, induced participants to use either sensory or motor codes to solve a delay-nonmatch-to-sample (DNMS) task. This approach allowed us to probe the state of the motor system while the participants were retaining either sensory or motor codes to cross the delay period, rather than the control of short-term storage driven by verbal instructions. When participants could prepare the movement in advance (preparation trials), the excitability of the motor cortex contralateral to the moving hand increased, whereas the excitability of the ipsilateral motor cortex decreased. The increase in excitability was confined to the prime mover, whereas the decrease in excitability extended to cortical territories controlling muscles unrelated to the response. Crucially, these changes in excitability were evoked only during preparation trials and not during trials in which subjects needed to maintain sensory items online (memory trials). We infer that short-term storage of sensory information and preparation of motor responses have differential and specific access to the output stage of the motor system. I N T R O D U C T I O NWe can cross time intervals interposed between stimuli and responses either by remembering events or by anticipating them (Rainer et al. 1999). Both strategies require the maintenance of task-relevant information in the brain, although the type of information maintained online might be different. When identification of a sensory stimulus and action selection are separated in time, a sensory representation of the stimuli needs to be held online (working memory; Goldman-Rakic 1987;Miyake and Shah 1999). Conversely, when action selection and movement execution are separated in time, subjects can prepare a specific response in advance (motor preparation;Tanji and Evarts 1976). Recent studies suggest that short-term storage of sensory information and movement preparation can be seen as conceptually and neuronally distinct phenomena (Curtis et al. 2004; Di Pellegrino and Wise 1996;Fuster 2000;Mars et al. 2005;Rowe et al. 2000). However, this issue remains controversial because other authors have argued that the same physiological mechanisms might support both motor preparation and working memory (C...

Effects of motor preparation and spatial attention on corticospinal excitability in a delayed-response paradigm

“…Although some studies have shown an increase in corticospinal excitability during movement preparation (e.g., Van den Hurk et al 2007;), the results have been mixed with some studies Wnding no eVect (Young et al 2000) or even a decrease in corticospinal excitability (Touge et al 1998;Hasbroucq et al 1999) during instructed delay periods. As discussed in the" Introduction", these conXicting results might be partly explained by diVerences across studies in the timing of the response, and therefore in the degree of preparation , or in the timing of the TMS pulse (Takei et al 2005).…”

“…Previous studies showing response-speciWc modulation of corticospinal excitability used arbitrary cues to instruct movement preparation (Van den Hurk et al 2007). However, movement information can reach the movement system via various neural pathways, depending on whether the instruction is provided using arbitrary or spatial cues (e.g., Toni et al 2001).…”

“…A number of studies have reported a decrease in corticospinal excitability during instructed delay periods, but no response-speciWc change in corticospinal excitability (e.g., Touge et al 1998;Hasbroucq et al 1999). Conversely, some recent studies reported increases in corticospinal excitability, speciWc to the prepared response (Van den Hurk et al 2007;). These conXicting results can be partly explained by diVerences 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 inXuence corticospinal excitability 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 Xuctuations in the level of motor preparation during the delay period.…”

“…Furthermore, we employ spatially compatible cues to instruct movement preparation. Since previous studies (e.g., 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 cueing.…”