Exploring Upper Limb Sequence Behaviours in Egocentric and Allocentric Reference Frames

Voluntary behaviours, such as reaching, are essential for manipulating and exploring our environment. The current body of literature, however, has predominantly investigated reach behaviours through tasks such as peg-moving, tapping, dotting, and circle drawing. The objective of this study was to investigate the order and direction of reaching behaviours in a sequential tapping task in both an egocentric and an allocentric reference frame. Gaze behaviours were observed to explore intent to reach in the sequential task. Implementing reference frames in an upper limb motor control task might be of clinical importance when exploring rehabilitation techniques post-traumatic brain injury. It was hypothesized that when one’s resting hand was in view (egocentric reference frame), the initial reach would be towards the resting hand; otherwise the long axis would be the location of the first reach (allocentric reference frame). Participants were expected to move in the clockwise direction with their right hand, and counter-clockwise direction with their left hand, similar to circle-drawing tasks. Gaze behaviours were expected to precede hand movements towards each target. Right-handed participants were asked to perform a sequential dot tapping task on a touchscreen in either an egocentric (i.e. resting hand is beside the touchscreen) or allocentric (i.e. resting hand is on their thigh) reference frame, using either their preferred or non-preferred hand. Eye movements were tracked to help identify if gaze was coupled to reaching movements. Results indicated that participants initial movement was towards the target that was closest to their midline; moving to the target near the midline might be the most efficient route towards the initial target to reduce trajectory errors and energy expenditure. Participants were equally as likely to move in the clockwise direction with the left hand as they were with the right, perhaps due to the right-hand-left-hemisphere system undergoing the decision-making process, as the right hand always performed the task first. Similar to previous gaze research in sequential tasks, eye movements guided the hand to their initial target. A saccade was made towards the first target to be tapped, whereby gaze shifts occurred to each consecutive target in the sequence prior to tapping. These results demonstrate that in a sequence task, young adults choose the most efficient means to direct their reaches by creating an egocentric reference frame within the peripersonal space; and eye movements help in initiating where and how these movements are made. This study can be elaborated upon by future research in special populations, as well as for rehabilitation purposes in the retraining of an affected limb in post-stroke and traumatic brain injury patients