Competing representations of reaching directions in the frontoparietal network

• ¹ University of Birmingham, United Kingdom
• ² University Hospital Birmingham, United Kingdom

We report two EEG experiments addressing frontoparietal function in the planning of directional hand movements. The experiments were conceived within the framework of the affordance competition model of action selection (Cisek 2007). We aimed to evaluate whether competitive interactions between neuron populations coding for different potential movements yield measurable effects in scalp-recorded EEG. Experiment 1 used a movement precuing paradigm implemented as a centre-out task, modeled after a single unit recording study (Bastian et al. 2003). The trial structure comprised the presentation of a cue followed by a response signal at SOA 1.2s (see Figure). The cue could indicate one, two, or three possible movement directions. The stimuli were presented on a PC monitor, while responses were made by movements of the left or right index finger over a smooth horizontal surface in front of the participant. The finger movements were recorded using a motion tracking device. Reaction time analyses demonstrated faster response times when cues provided full information compared to partial information conditions. Moreover, responses were faster when the precue indicated two compared to three possible movement directions. Consistent with this scaling in RTs, EEG analyses revealed a scaling in the amplitude of the ADAN component as well as the fore period LRP. In addition to these anticipated effects, which are consistent with the predictions of the affordance competition model, we found a biphasic time course of the ADAN, with initial parietal activation followed by frontal activity. Experiment 2 sought to confirm that the scaling of ADAN and LRP was due to competition between representations of different movement directions instead of determined by the number of response alternatives. Here, cues indicated one direction, two directions separated by a small angle (45º) or two directions separated by a wide angle (90º). Responses were made by joystick movements, accompanied by cursor movements on the visual display. The results showed a predicted RT advantage when two possible movements were separated by a wide compared to a small angle. However, the scaling of ADAN and LRP amplitude was no longer present. Instead, we observed effects on the posterior N2 component. Thus, with movements relying on proprioceptive information (Exp. 1) effects consistent with competition were seen over the frontal cortex. With movements partly relying on visual guidance (Exp. 2), related effects were localized over the parietal cortex. Together, the results suggest – in accordance with the affordance competition model - that task and response characteristics determine where in the frontoparietal network the competition between representations for different reaching directions is played out.