Abstract
BackgroundRegions of the dorsal visual stream are known to play an essential role during the process of mental rotation. The functional role of the primary motor cortex (M1) in mental rotation is however less clear. It has been suggested that the strategy used to mentally rotate objects determines M1 involvement. Based on the strategy hypothesis that distinguishes between an internal and an external strategy, our study was designed to specifically test the relation between strategy and M1 activity.MethodsTwenty-two subjects were asked to participate in a standard mental rotation task. We used specific picture stimuli that were supposed to trigger either the internal (e.g. pictures of hands or tools) or the external strategy (e.g. pictures of houses or abstract figures). The strategy hypothesis predicts an involvement of M1 only in case of stimuli triggering the internal strategy (imagine grasping and rotating the object by oneself). Single-pulse Transcranial Magnetic Stimulation (TMS) was employed to quantify M1 activity during task performance by measuring Motor Evoked Potentials (MEPs) at the right hand muscle.ResultsContrary to the strategy hypothesis, we found no interaction between stimulus category and corticospinal excitability. Instead, corticospinal excitability was generally increased compared with a resting baseline although subjects indicated more frequent use of the external strategy for all object categories.ConclusionThis finding suggests that M1 involvement is not exclusively linked with the use of the internal strategy but rather directly with the process of mental rotation. Alternatively, our results might support the hypothesis that M1 is active due to a 'spill-over' effect from adjacent brain regions.
Highlights
Regions of the dorsal visual stream are known to play an essential role during the process of mental rotation
Because subjects' responses to mental rotation were not recorded during the Transcranial Magnetic Stimulation (TMS) session, they were asked after the first part of the experiment whether they really used mental rotation and whether they needed and were able to repeat the operation during the five seconds of stimulus presentation
We included all 20 Motor Evoked Potentials (MEPs) amplitudes for analysis because the results were no different when we excluded the data of the four TMS pulses in each condition that were delivered at the moment of the switch between trials
Summary
Regions of the dorsal visual stream are known to play an essential role during the process of mental rotation. The external strategy, on the other hand, describes the imagination in front of the "inner eye" of a self-rotating object driven by an external force In this case, M1 is not necessary for task performance and should not be activated [3]. The same holds for objects that are usually grasped and manipulated in the real world, like tools More abstract stimuli such as Shepard & Metzler figures or objects that cannot be manually rotated in the real world (e.g. houses or 2-D figures) are thought to trigger the external strategy because their rotation by external forces appears easier to imagine [6,9]. We assessed the choice of a mental rotation strategy in dependence on the different categories of stimuli using a self-constructed post-test questionnaire
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