Looking with the head and eyes: a developmental study
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From a very early age, infants use their heads, eyes and hands to explore the world of objects around them. The infant therefore has to develop a hierarchy of stabilized systems: trunk, head and eyes must work in coordination to allow effective control of the arms and hands. In particular gaze has to be stable. Previous research into the stabilization of gaze has mainly concentrated on how eye movements compensate for head movements. There is little information on the role of the head in gaze stabilization, either for adults or for infants. The head and eye coordination of a group of adults was tested under two situations; when tracking a moving target and when compensating for body movement while gaze was fixed on a stationary target. Movement of the target or subject could be either predictable or unpredictable. It was found that the head played an important role, whether the target or subject was moving. Head control was equally good under both conditions, but was superior when movement was predictable. A group of infant subjects were tested longitudinally on the same tasks in order to chart the development of the role of the head in looking. Testing was at three week intervals between the ages of 10 and 28 weeks. As with the adults, the head was found to play an important role, control improved over the tested period, showing a surge around 16-20 weeks. Unlike adults, the performance of the infants was much better when they rather than the target were moving. Deficiencies in the development of gaze stabilization would have serious implications for perceptuo-motor development. A brain-damaged infant was tested under similar conditions in an exploratory longitudinal study between the ages of 21-28 weeks. He was shown to be principally deficient in head rather than eye control, particularly in the visual tracking task.