Examining the influence of marginally modified constraints on motor behaviour

Abstract

The thesis examines the influence of visual misperceptions of the primary environmental constraints related to motor behaviour, and explores whether motor adaptation can be caused implicitly by introducing systematic, undetectable changes in key constraints in the performance environment. The first two experiments (Chapter 2) were conducted to determine if a simple manipulation of the height of the uprights can cause participants to perceive the dimensions of the rugby goalposts differently. The findings support the existence of a rugby goalpost illusion. Misperceptions of the dimensions of the goalposts may influence decisions about where to place the ball when converting a try. In Chapter 3, a series of experiments tested this hypothesis both in a laboratory setting and in real life games. Experiment 3 showed that narrow goalposts caused kickers to place the ball further from the try-line than wide goalposts. Furthermore, misperceptions of the uprights width induced by the rugby posts illusion caused kickers to place the ball differently when making kicks (Experiment 4). By introducing pitch-markings in Experiment 5, we tested whether the effect of the illusion is moderated by use of familiar cues in the environment. An observational study (Experiment 6) suggested that the influence of the rugby posts illusion, intimated in our experimental work, is not evident in real life. Both Chapters 4 and 5 were designed to examine whether very subtle changes in constraints in the environment can be used specifically to induce implicit motor learning. In Chapter 4, the smallest detectable difference between two levels of sensory stimulus was determined for simple line drawings representative of rugby goalpost uprights (Experiment 7) or crossbars (Experiment 8). The findings from Chapter 5 suggested that (implicit) motor adaptations might be caused by gradual incremental changes in environmental constraints.