The subjective movement threshold (SMT)
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1
Universitätsmedizin-Berlin, Germany
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2
University of Potsdam, Germany
Self-awareness and action studies investigate how we are aware that an action was self-generated. Established in the framework of motor control, these studies argue that sensory predictions of future actions are a crucial component of self-awareness (Blakemore and Frith 2003). Previous studies investigated the temporal dissociation for the awareness of the intention to move and the actual onset of the movement. The temporal dissociation was found to be ca 73 ms, with the self-awareness of a movement preceding the actual movement (e.g. Libet 1999). The sense of agency is the feeling that our movements have a consequence. The comparison of predicted with the actual sensory consequence(s) is used to optimize motor control (Davidson and Wolpert 2003) and modulates the temporal dissociation; i.e. "temporal attraction", movement awareness and actual onset (Kalogeras 2002). Based on these results it is concluded that the sensory predictions of forward models play a pivotal role in self-awareness and action (Wolpert 2003). In contradistinction, control experiments have utilized non-volition induction of movements with transcranial magnetic stimulation (TMS). These experiments did not induce temporal attraction or dissociation. We investigate if TMS can induce undetected movements. The hypothesis is that the subjective movement threshold (SMT) is higher than the active motor threshold (AMT) and lower than the resting motor threshold (RMT). In five subjects (average age 23 +/- 5) we investigated with neuronavigated TMS in a randomized design the SMT, AMT and RMT. The stimulation target was defined by maximal muscle twitches (MEPs) over the first dorsal interosseus (FDI) muscle for minimal suprathreshold stimulation. RMT and AMT were defined following standard procedures. Not MEPs but movement awareness was used to define the SMT. All other parameters were kept identical. Positive muscle responses were larger than 50 µV. Movement awareness was defined by subjective report. All thresholds were quantified statistically within a 95% CI by an efficient maximum-likelihood algorithm (Awiszus & Feistner, 2007). We found significant results in all single subjects. The necessary stimulation strength was lowest for AMT, SMT and RMT, respectively. The control experiment identified the minimal stimulation strength necessary to induce a 1 mV response (1mVT). Here, we introduce the "subjective movement threshold" as a novel measure. The SMT thus provides a new window for investigations of self-awareness and action, especially as it possibly allows for the unconscious induction of movements.
Conference:
Bernstein Symposium 2008, Munich, Germany, 8 Oct - 10 Oct, 2008.
Presentation Type:
Poster Presentation
Topic:
All Abstracts
Citation:
Schmidt
S,
Mundt
M,
Ohl
S and
Brandt
SA
(2008). The subjective movement threshold (SMT).
Front. Comput. Neurosci.
Conference Abstract:
Bernstein Symposium 2008.
doi: 10.3389/conf.neuro.10.2008.01.112
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Received:
18 Nov 2008;
Published Online:
18 Nov 2008.
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Correspondence:
Sein Schmidt, Universitätsmedizin-Berlin, 82152 Planegg-Martinsried, Germany, sein.schmidt@charite.de