Rhythmic synchronization of neurons in the beta or gamma band occurs almost ubiquitously, and this synchronization has been linked to numerous nervous system functions. Many respective studies make the implicit assumption that neuronal synchronization affects neuronal interactions. Indeed, when neurons synchronize, their output spikes reach postsynaptic neurons together, trigger coincidence detection mechanisms, and therefore have an enhanced impact. There is ample experimental evidence demonstrating this consequence of neuronal synchronization, but beyond this, beta/gamma-band synchronization within a group of neurons might also modulate the impact of synaptic input to that synchronized group. This would constitute a separate mechanism through which synchronization affects neuronal interactions, but direct in vivo evidence for this putative mechanism is lacking. Here, we demonstrate that synchronized beta-band activity of a neuronal group modulates the efficacy of synaptic input to that group in-phase with the beta rhythm. This response modulation was not an addition of rhythmic activity onto the average response but a rhythmic modulation of multiplicative input gain. Our results demonstrate that beta-rhythmic activity of a neuronal target group multiplexes input gain along the rhythm cycle. The actual gain of an input then depends on the precision and the phase of its rhythmic synchronization to this target, providing one mechanistic explanation for why synchronization modulates interactions.

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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...