Discriminating the role of binocular information in the timing of a one-handed catch

Previous work using prediction-motion tasks has indicated that time-to-contact estimates are based exclusively on binocular information when the approaching object is small. The aim of this study was to examine the influence of object size on the use of binocular information sources in specifying time-to-contact in one-handed catching. Subjects (n=10) were required to time their grasp to catch different-sized balls (60, 80, and 100 mm in diameter) approaching with a constant spatial trajectory and constant velocity of 2.0 m/s. Binocular information was manipulated with a telestereoscope that increased the individuals' effective interocular separation. Subjects performed six trials with each of the different-sized balls in normal and telestereoscopic viewing conditions. It was found that subjects closed the hand earlier when catching the small and medium balls under telestereoscopic viewing compared with normal viewing; furthermore, that subjects closed their hand earlier for the small and medium balls compared with the large ball under telestereoscopic viewing. With regard to hand aperture, there was an effect of ball size, with the hand being opened to a wider aperture for each increase in ball size. Subjects also opened the hand to a wider aperture when catching the small and medium balls under telestereoscopic viewing. These findings are consistent with the notion of an increased reliance on the use of binocular information when the approaching object is small. Furthermore, they indicate that binocular and monocular sources of information are not used in isolation, but rather in combination to support the timing of one-handed catching.