P 1. Repetition suppression in sensorimotor system to adapt cortex to stimuli

Abstract

Repetition suppression (RS) (Grill-Spektor et al., 2006) of auditory evoked potentials (AEPs) is well known. As the sound stimulus is repeated, AEPs (N100 responses) decrease in amplitude and habituate (Fruhstorfer, 1971) . Recently, RS was demonstrated during mental imaging of movement (Hohlefeld et al., 2011) and repetition of hand gestures (Hamilton and Grafton, 2009) . These studies suggest that RS could have a role in motor control, while lacking evidence of causal relation between neuroimaging data and cortical motor output. This can be studied using transcranial magnetic stimulation (TMS). We studied whether the output of central motor network is controlled by RS with the hypothesis that RS could be a part of motor control mechanisms. Six right-handed healthy subjects (5 male) aged 22–58 years participated in the study. We measured RS of the cortical N100 responses with electroencephalography (EEG) (Näätänen and Picton, 1987) . We utilized a standard protocol in studying auditory habituation. Tones were delivered to the subject’s right ear at 60 dB above hearing threshold. The paradigm comprised of 160 tones in 40 trains, 4 tones within a train. The inter-train interval (ITI) was 20 s while the ISI between the tones within a train was 1s (Furubayashi et al., 2000) . Subsequently, the primary motor cortex area was comprehensively mapped using navigated TMS to locate the optimal cortical representation area of right first dorsal interosseous muscle. The resting motor threshold (rMT) was determined for each subject. Subsequently, we conducted a stimulation protocol with TMS at 120% of the rMT similar to that used in the auditory stimulation. During this, EEG and EMG responses were recorded. RS was evident in the cortical N100 response to auditory stimuli (31–37% decrease in amplitude) as well as to TMS (43–58% decrease in amplitude). More importantly, we also observed RS in the MEPs. The MEPs to the 2nd, 3rd and 4th stimulus in the train were 42–46% lower in amplitude than that to the 1st stimulus ( Fig. 1 ). The effect was similar in all subjects. In a control measurement with peripheral electric stimulation of ulnar nerve, we found no such effect. Our finding of RS in the central motor network is crucial for understanding the general control mechanisms of the motor cortex. However, the mechanism cannot be yet explained unequivocally. It appears that like more complex sensory networks, basic motor network in M1 is able to habituate to repeated stimuli. The recorded suppression of MEPs and TMS-EEG activity resembles the auditory habituation, which suggests that they all share same neuronal mechanisms of suppression. Hypothetically, the suprathreshold TMS may modulate the release of synaptic transmitters of the motor cortex and hence affect cortical excitability. One possible explanation for RS in a motor network is a servo-like mechanism by negative feedback from S1 after sensory feedback from movement (Catani et al., 2012) . We suggest that RS may be a general servomechanism of the cortex not limited to perception and control of the cortical input.