My research is concerned with perceptual-motor processes in infants and adults. One of the human infant's earliest motor accomplishments is reaching for objects. This fundamental skill opens up a new realm of exploration in which the infant is no longer a passive observer but an active participant. When an infant reaches for an object, we should marvel at many aspects of this seemingly simple behavior. Speed and grace characterize the action of the six-month-old infant's reach, although only six to eight weeks earlier the same infant was at the beginning stages of learning to reach. In order to reach successfully the infant must localize the object in three-dimensional space, plan a trajectory for the hand, open the hand in preparation for grasping, and finally execute the motor plan that results in a successful grasp. Some situations may increase the demands. For example, the object's size or orientation may require that the hand be preshaped or rotated in a certain way, or the object may be moving so that the infant must take the changing location into account when planning the trajectory. Or, vision of the hand and background can be varied so that the infant can see the object but not the hand, or neither object nor hand. We believe that the careful study of reaching behavior will reveal the growth and interplay of both motor and cognitive development during infancy.
After more than a decade of research, we now know that infant reaching is characterized by enormous flexibility and rapid learning, a process that combines achievement of motor control with acquisition of new cognitive abilities. We have conducted a series of studies in which infants reached for objects under conditions varying in visual input from full light to total darkness.
Rachel Keen
Development of Sensorimotor Processes and Cognition in Human Infants
The purpose has been twofold: 1) to determine the extent to which infants can control motor action without visual guidance of their limbs, and 2) to investigate infants' ability to represent objects and events that they are unable to see. From a number of studies we conclude that infants can reach for and grasp objects easily and accurately without visual guidance. Around six months of age they reach readily for sounding objects (such as a toy rattle) in total darkness, when presented from a variety of locations. They pre-orient their hands appropriately for horizontal and vertical rods painted with glow-in-the-dark paint, just as they do when the same rods are presented in the light. When a moving, glowing object swings past them in the dark, they are able to catch it, just as they can do in the light. Our data include behavioral scoring of the infants' reaching movements from videotape and kinematic data obtained from a motion analysis system (Optotrak). Both types of data reveal the remarkable similarity of reaches made in the light and in the dark. Parallel studies with adults have shown that their reaches under similar light/dark circumstances are greatly affected when sight of the hand is lost. Average velocity, peak velocity and the timing of the peak, duration of the reach, and the aperture between thumb and index finger are all affected by asking subjects to reach for objects in the dark. Although infant reaching is very impres- sive, it differs from adult reaching in a number of ways. We are interested in tracing the developmental changes that take place as the child gains more experience and skill in reaching.
A second line of research concerns an auditory phenomenon called the "precedence effect," which refers to our ability to suppress echoes. Sound produced in an enclosed space inevitably produces reflections off surrounding surfaces such as walls, ceilings, floors, and nearby objects. These reflections or echoes color the original sound and enhance its loudness, but they are not identified as separate sounds from new sources unless the delay between the original sound and the echo is quite long. This phenomenon has been called the precedence effect to emphasize the greater weight given to the directional information in the first sound wave to strike the ear, compared to the weight given to later-arriving sound waves. The strength of the first sound wave (or leading sound) is surprisingly strong; even when the echo is produced at the same intensity as the leading sound, the listener will still localize the sound at the leading location. Many of our studies have measured the echo threshold, defined as the shortest delay between leading and lagging sounds at which the echo (or lagging sound) is perceived as a separate sound, coming from a different location. To study the precedence effect, we present sounds to adult listeners in an anechoic chamber, a special room that has foam-covered walls, ceiling, and floor to absorb all echoes. We present sound through an array of loudspeakers that simulate one original sound followed by one or more delayed sounds or echoes. The listener's task is to judge where the echo is coming from by making the appropriate response on a button box.
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