Abstract
On-line control of movement is essential for fast and efficient interactions with our environment. Previous studies, utilizing abstract patterns such as simple geometric shapes, have demonstrated that on-line corrections in response to target perturbations are made without any sudden directional changes in hand/arm movement. In this study we used a perturbation paradigm to assess the effects of visual attention and object affordance on the efficiency of arm movements to both geometrical patterns and images of everyday objects. Participants were required to touch (using a hand-held stylus) the left or right edge of a centrally presented stimulus, randomly selected from a range of geometrical and object patterns (n=30). The stimuli were deliberately chosen to induce a degree of attentional bias and/or affordance bias about the vertical axis. Perturbations, coincident with movement onset, occurred on 20% of trials and were elicited by the appearance of a small red circular target at the far left or right edge of the stimulus. On perturbation trials, participants were required to touch the red dot. The distance from the starting position of the hand to the stimulus was 40 cm. The efficiency (path smoothness) of on-line adjustments to target perturbations was reduced when the direction of the perturbation was incongruent with the affordance and/or attentional bias of the stimulus. These factors significantly delayed the movement transition to the perturbed target. Moreover, unlike previous research, the on-line corrections were generally of sudden onset, giving a non-smooth character to the path trace. We conclude that the efficiency of on-line corrections to reach and touch a target is critically dependent on the attentional bias of the target and/or any action afforded by it. These results have significant practical implications for the design of interactive display units where fast, accurate observer responses are required.