The Parallel Encoding of Actions Within a Sequential Grasping Task: Strategic, Motor, or Perceptual Interference Effects
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Every day we are constantly performing consecutive grasping actions to complete our desired goals. Although we have a good scientific understanding about how the brain programs one grasping action towards a single object, we still have a very limited knowledge about how the brain programs a grasping action within the context of a sequential task. The aim of this dissertation was to investigate whether the characteristics of the second object in a sequence can strategically influence or interfere with how the first object is grasped. Specifically, we explored whether the size and/or the orientation feature of the second object would have any effect on how the grip aperture and grip orientation is selected when performing the first action towards a cylindrical object that varied between two sizes. The task requirement associated with the second object varied across several experiments: participants were either grasping the second object, performing a perceptual judgment about it, or grasping it but with vision occluded. Although the first object was a cylinder that could afford to be grasped with a wide range of grip orientations, the results never revealed that the orientation of the second object had a direct influence on the grip orientation selected to grasp the first object across all experimental manipulations. This also remained true when only strictly manipulating orientation (no size manipulation). However, when the second object was smaller than the first object, we found a consistent pattern of results where participants reached out with a smaller peak grip aperture relative to when the second object was the same size or larger. Based on these results, it is suggested that the second object within a sequential task will only interfere with the grasping kinematics towards the first object when both action plans have overlapping features, and most importantly this is not dependent on providing a strategic benefit to the overall movement. It is speculated that this specific pattern of results is due to a motor interference effect driven by the parallel encoding of grasping actions within the anterior intraparietal area (i.e., the grasping circuit).