ERP investigation of the reference frames that guide shifts of attention triggered during covert tactile-spatial orienting and manual movement preparation

Both somatotopic and external reference frames encode the hand location during covert shifts of tactile spatial attention. When participants cross their hands, these spatial codes are in conflict and attentional selectivity is disrupted. Because shifts of attention towards the hand are triggered also when participants covertly prepare a manual movement, we investigated which reference frames guide such motor-induced attention shifts. Event Related Potentials (ERPs) were recorded during a motor task where a visual cue indicated the relevant hand for a manual movement prior to a tactile Go/Nogo stimulus. For comparison, we run a tactile attention task where the same cue now indicated the relevant hand for a tactile discrimination. Both tasks were performed under uncrossed and crossed hands conditions. Lateralized ERP components indicative of covert attention shifts (ADAN, LDAP) were present and unaffected by the hand posture manipulation in both tasks. Towards the end of the preparation interval, the LRP in the motor task and a negativity contralateral to the cued hand in the attention task reversed polarity in the crossed hands condition. As expected, in the attention task enhanced N140 for tactile stimuli presented to the cued hand were found with uncrossed but not with crossed hands. In contrast, in the motor task no difference emerged between N140 modulations with uncrossed and crossed hands. This pattern of results suggests that initial external and later somatotopic codes guide shifts of attention in motor and attention tasks. However, a conflict arises only for tactile selection when hands are crossed, while hand selection for motor execution seems to be based mainly on one (possibly external) spatial code.