Although the existence of directional motor deficits (DMD) associated with movement planning and/or execution seems to be widely recognized, neglect and single cell studies examining their neuroanatomical foundation have produced contradictory and inconclusive findings. The present study assessed the occurrence of DMD following the application of repetitive transcranial magnetic stimulation (rTMS) over two regions, as commonly reported in the neglect literature, namely the right middle frontal gyrus (rMFG) and the right angular gyrus (rAG). Fourteen healthy subjects underwent rTMS while performing an auditory choice task, involving pointing toward two laterally located targets, under internally (i.e., pointing side freely selected) and externally guided conditions (i.e., pointing side guided by spatial auditory cues). In order to examine whether subjects compensated for induced deficits with the help of vision, visual feedback was occluded at movement onset in half of the trials. rTMS applied to the rAG significantly increased reaction times (RTs) for leftward internally-guided movements. In contrast, rTMS applied to the rMFG reduced the likelihood to complete leftward internally-guided movements under blindfolded conditions. These effects suggest that DMD might involve cognitive processes contributing to the different stages of motor control, such as movement selection and goal maintenance.
Keywords: Directional motor deficits; Repetitive TMS; Right angular gyrus; Right middle frontal gyrus; Unilateral neglect.
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