The effect of skill level, external frame of reference and environmental changes on one-handed catching

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Abstract
Two groups of subjects designated ‘good’ (n= 12) and ‘poor’ (n= 10) catchers on the basis of a task-specific criterion were required to catch, one-handed, balls projected from a ball-projection machine under eight different conditions. Four of these comprised the ‘environment illuminated’ conditions (full light; hand obscured by an opaque screen; a see-through screen and an external vertical reference frame in the field of view) and four others the ‘environment dark’ conditions (catching hand wearing a glove painted with luminescent paint and u. v. illuminated ball in an otherwise dark room; ball illuminated by u. v. light; luminescent hand and external luminescent vertical reference plus u. v. illuminated ball; u. v.-illuminated ball, plus luminescent vertical reference). While clear and significant differences - as was to be expected from the selection criteria imposed - were shown between the groups of ‘good’ and ‘poor’ catchers for all three dependent variables-number of catches, spatial errors and temporal errors-it was demonstrated that, for both groups, degrading the environment, by reducing the information available, resulted in a significant performance decrement on the dependent variables ‘number of catches’ and ‘temporal errors’. ‘Poor’ catchers were also shown to make significantly more spatial errors than ‘good’ catchers. Whereas the latter finding confirms earlier findings reported in the literature, the significant effect on performance of degrading the environment -particularly with respect to temporal errors - is new. The finding is discussed in the framework of the use of ‘tau’ (time to contact) in making judgements about when to initiate the grasp response.