Visuomotor mental rotation is impaired by transcranial magnetic stimulation of parietal cortex

NemoImage

13, Number

6, 2001,

Part 2 of 2 Parts 10

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SENSORIMOTOR

Visuomotor mental rotation is impaired by transcranial magnetic stimulation of parietal cortex Sven Bestmann, Kai Thilo, Dieter Sauner, John Rothwell MRC Human Movement and Balance Unit, Institute of Neurology, London, UK Introduction: Brain imaging studies have demonstrated the involvement of parietal cortex in mental rotation processes. Recently, Jordan et al. [l] showed that, regardless of stimulus properties, the strongest activations occur in the inferior and superior parietal lobes bilaterally. Furthermore, it has been suggested that these activations increase with higher cognitive demand [2]. However, other lesion and imaging studies demonstrate unilateral as well as bilateral involvement in mental transformation processes. In addition, the importance of these areas to mental transformation processes remains unclear. By using transcranial magnetic stimulation (TMS), we aimed to investigate whether parietal areas are involved in a mental rotation task with an additional motor component. Method: With local ethics committee approval ten subjects performed a center-out pointing task with delayed visual feedback. On appearance of a directional arrow, subjects were instructed to perform a hand movement in the direction of the arrow or at 35,70, 105 or 140” clockwise to it, depending on prior instruction. Instruction was given to move as fast and accurate as possible. Twenty trials per condition were collected. Reaction time (RT) to movement onset and the directional accuracy over the initial 100 ms of movement were analyzed. Short trains of TMS (4 pulses at 20 Hz, 120 % of resting motor threshold, figure-of-eight coil, 90 mm diameter) were applied on every trial over parietal sites P3, P4 (international 10-20 system) or the leg area under the vertex (Cz) starting 100 ms after stimulus onset.

In line with previous studies [3], the RT increased linearly with increasing angle of rotation in all conditions ( 464 2 44 ms (SD.) for diit pointing or 0’ of rotation up to 803 ? 70 ms for 140” rotation, mean 0.92). A two-way ANOVA revealed a significant interaction between the amount of rotation and the site of TMS stimulation. (F(8,72) = 2 * 5, P < 0 * 05). Post hoc analysis showed that compared with vertex stimulation, stimulation over P3 and P4 increased the reaction time for large rotation angles (by -65 ms for 105” and by -85 ms for 140’ rotation angles, P < 0 * 05). However, no significant effect of parietal stimulation on the accuracy or variance of the movement could be observed when compared to vertex (Cz) stimulation. Diacuasion: The present study demonstrates the involvement of parietal cortex in visuomotor mental transformation processes. However, the finding that only large rotation angles were affected by stimulation over parietal cortex suggests that parietal involvement in mental rotation processes increase with increasing processing load. Furthermore, it is notable that TMS did affect the speed of task processing but did not bias the accuracy of the final movement output significantly. Finally, the effects were similar for right (P4) and let? parietal (P3) stimulation which, in line with recent imaging studies on mental rotation [l], do not indicate a lateralisation of the effect. References 1. Jordan K. et al. NeuroImage 2001; 13:143-152 2. Tagaris G.A. et al. NeuroReport 1996; 7:773-776 3. Georgopoulos A.P. et al. Exp. Brain Res. 1987; 65: 361-370

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