Crossing the hands disrupts tactile spatial attention but not motor attention: evidence from event-related potentials.

During covert shifts of tactile spatial attention both somatotopic and external reference frames are employed to encode hand location. When participants cross their hands these frames of references produce conflicting spatial codes which disrupt tactile attentional selectivity. Because attentional shifts are triggered not only in Attention tasks but also during covert movement preparation, the present study aimed at investigating the reference frame employed during such 'motor shifts of attention'. Event related brain potentials (ERPs) were recorded during a Motor task where a visual cue (S1) indicated the relevant hand for a manual movement prior to a tactile Go/Nogo stimulus (S2). For comparison, we ran a tactile Attention task where the same cue (S1) now indicated the relevant hand for a tactile discrimination (S2). Both tasks were performed under uncrossed and crossed hands conditions. In both Attention and Motor tasks similar lateralized components were observed following S1 presentation. Anterior and posterior ERP components indicative of covert attention shifts were exclusively guided by an external reference frame, while a later central negativity operated according to a somatotopic reference frame in both tasks. In the Motor task, this negativity reflected selective activation of the motor cortex in preparation for movement execution. In the Attention task, this component might reflect activity in the somatosensory cortex in preparation for the subsequent tactile discrimination. The presence of multiple and conflicting spatial codes resulted in disruption of tactile attentional selection in the Attention task where attentional modulations of tactile processing were delayed and attenuated with crossed hands as indicated by the analysis of ERPs elicited by S2. In contrast, attentional modulations of S2 processing in the Motor task were largely unaffected by the hand posture manipulation, suggesting that motor attention employs primarily one spatial coordinate system.