P. van Hof

The relation of unimanual and bimanual reaching to crossing the midline.

The present study assessed the development of reaching for objects positioned in front of the contralateral shoulder. In particular, it examined how the development of crossing the midline is related to the development of bimanual reaching. Twenty infants were observed longitudinally at 12, 18, and 26 weeks of age while reaching for two balls (3 cm and 8 cm in diameter) located at three positions (ispsilateral, midline, and contralateral). The reaches were analyzed from video recordings. With age, the infants increasingly adapted the number of hands used to the size of the object. The number of reaches crossing the body midline increased with age. Furthermore, the majority of the midline crossings were part of two-handed reaches for the large ball and occurred at or after onset of bimanual reaching. Together, these findings strongly suggest that the development of crossing the body midline emerges in the context of bimanual reaching. It was concluded that the need to grasp a large ball positioned contralaterally with two hands induces midline crossing. Hence, the development of midline crossings is not exclusively dependent on organismic constraints (e.g., the maturation of hemispheric connections), but rather on their interaction with environmental constraints (e.g., object size).

Visual constraints in the development of action

The chapters aim is to understand the role of visual information in the control of avoidance and interception behaviors in infancy from the ecological psychology approach to perception and action. We show that during infancy developmental change in action is associated with the use of different information sources and that this process of attunement promotes the perceived action possibilities (affordances). In the final section, we position these findings within Milner and Goodales two-visual system model, which holds that perception and action are mediated by two functionally and neuron-anatomically separate visual (sub-)systems.