Rachel K. Clifton

Visual information and object size in the control of reaching

The role of vision in the control of reaching and grasping was investigated by varying the available visual information. Adults (N = 7) reached in conditions that had full visual information, visual information about the target object but not the hand or surrounding environment, and no visual information. Four different object diameters were used. The results indicated that as visual information and object size decreased, subjects used longer movement times, had slower speeds, and more asymmetrical hand-speed profiles. Subjects matched grasp aperture to object diameter, but overcompensated with larger grasp apertures when visual information was reduced. Subjects also qualitatively differed in reach kinematics when challenged with reduced visual information or smaller object size. These results emphasize the importance of vision of the target in reaching and show that subjects do not simply scale a command template with task difficulty.

Problem Solving in Infancy: The Emergence of an Action Plan

Young childrens strategies were evaluated as they grasped and used objects. Spoons containing food and toys mounted on handles were presented to 9-, 14-, and 19-month-old children with the handle alternately oriented to the left and right. The alternating orientations revealed strategies that the children used for grasping items. Younger children usually reached with their preferred hand, disregarding the items orientation. In the case of the spoon, this strategy produced awkward grasps that had to be corrected later. Older children anticipated the problem, alternated the hand used, and achieved an efficient radial grip (i.e., handle grasped with base of thumb toward food or toy end) for both orientations. A model of the development of action-selection strategies is proposed to illustrate planning in children younger than 2 years.

Object Representation Guides Infants' Reaching in the Dark

Infants were presented with two sounding objects of different sizes in light and dark, in which sound cued the objects identity. Reaching behavior was assessed to determine if object size influenced preparation for grasping the object. In both light and dark, infants aligned their hands when contacting the large object compared with the small object, which resulted in a reach with both hands extended for the large object and reach with one hand more extended for the small object. Infants contacted the large object more frequently on the bottom and sides rather than the top, where the sound source was located. Reaching in the dark by 6 1/2-month-olds is not merely directed toward a sound source but rather shows preparation in relation to the objects size. These findings were interpreted as evidence that mental representation of previously seen objects can guide subsequent motor action by 6 1/2-month-old infants.

The Beginnings of Tool Use by Infants and Toddlers

Children (aged 9, 14, 19, and 24 months) were encouraged to use tools to achieve a demonstrated goal. Each tool was most efficiently applied when held by the handle with the thumb toward the head of the tool in a radial grip. The tools were presented at midline and oriented to the left and right on alternating trials, so the children who managed to grasp a tool in both orientations with the radial grip demonstrated planning of actions in advance. The tools included a spoon, hairbrush, toy hammer, and magnet; the goals were to feed ones self, feed another, brush ones hair, brush anothers hair, hit pegs, and retrieve metal objects. Children were found to use more radial grips with the self-directed tools (i.e., hairbrush-to-self and spoon-to-self), indicating that they could plan their actions better when directed toward the self than toward an external goal.

Where's the ball? Two- and three-year-olds reason about unseen events.

Children 2, 2 1/2, and 3 years of age engaged in a search task in which they opened 1 of 4 doors in an occluder to retrieve a ball that had been rolled behind the occluder. The correct door was determined by a partially visible wall placed behind the occluder that stopped the motion of the unseen ball. Only the oldest group of children was able to reliably choose the correct door. All children were able to retrieve a toy that had been hidden in the same apparatus if the toy was hidden from the front by opening a door. Analysis of the younger childrens errors indicated that they did not search randomly but instead used a variety of strategies. The results are consistent with the Piagetian view that the ability to use representations to guide action develops slowly over the first years of life.

Proximodistal structure of early reaching in human infants.

Abstract Nine infants were tested, at the age of onset of reaching, seated on their parent’s lap and reaching for a small plastic toy. Kinematic analysis revealed that infants largely used shoulder and torso rotation to move their hands to the toy. Many changes in hand direction were observed during reaching, with later direction changes correcting for earlier directional errors. Approximately half of the infants started many reaches by bringing their hands backward or upward to a starting location that was similar across reaches. Individual infants often achieved highly similar peak speeds across their reaches. These results support the hypothesis that infants reduce the complexity of movement by using a limited number of degrees-of-freedom, which could simplify and accelerate the learning process. The proximodistal direction of maturation of the neural and muscular systems appears to restrict arm and hand movement in a way that simplifies learning to reach.

Breakdown of echo suppression in the precedence effect

A new phenomenon is reported in which a change in spatial location of the leading sound source disrupts the normal echo suppression of the precedence effect. Click trains were presented through two loudspeakers, one leading the other by a few milliseconds. When leading and lagging signals were switched, listeners heard clicks momentarily for as long as several seconds from both loudspeakers before echo suppression was reestablished.

How Infants Use Vision for Grasping Objects

The role of vision was examined as infants prepared to grasp horizontally and vertically oriented rods. Hand orientation was measured prior to contact to determine if infants differentially oriented their hands relative to the objects orientation. Infants reached for rods under different lighting conditions. Three experiments are reported in which (1) sight of the hand was removed (N = 12), (2) sight of the object was removed near the end of the reach (N = 40, including 10 adults), and (3) sight of the object was removed prior to reach onset (N = 9). Infants differentially oriented their hand to a similar extent regardless of lighting condition and similar to control conditions in which they could see the rod and hand throughout the reach. In preparation for reaching, infants may use the current sight of the objects orientation, or the memory of it, to orient the hand for grasping; sight of the hand had no effect on hand orientation.

Infant pitch perception: Evidence for responding to pitch categories and the missing fundamental

While numerous studies on infant perception have demonstrated the infants ability to discriminate sounds having different frequencies, little research has evaluated more sophisticated pitch perception abilities such as perceptual constancy and perception of the missing fundamental. In the present study 7-8-month-old infants demonstrated the ability to discriminate harmonic complexes from two pitch categories that differed in pitch by approximately 20% (e.g., 160 vs 200 Hz). Using a visually reinforced conditioned head-turning paradigm, a number of spectrally different tonal complexes that contained varying harmonic components but signaled the same two pitch categories were presented. After learning the basic pitch discrimination, the same infants learned to categorize spectrally different tonal complexes according to the pitches signaled by their fundamental frequencies. That is, the infants showed evidence of perceptual constancy for the pitch of harmonic complexes. Finally, infants heard tonal complexes that signaled the same pitch categories but for which the fundamental frequency was removed. Infants were still able to categorize the harmonic complexes according to their pitch categories. These results suggest that by 7 months of age infants show fairly sophisticated pitch perception abilities similar to those demonstrated by adults.

When they were very young: Almost-threes remember two years ago☆

Abstract Five children who had participated 15–19 times between 6–40 weeks of age in a study of the perception of auditory space returned to the laboratory two years later. The original experimental sequence, which entailed trials in both light and darkness, was carried out along with several additional memory probes. Five control subjects, age, and sex mates who had not previously participated, also experienced the test procedure. Behavior was videotaped and recorded by two observers, and the control and experimental groups were compared on a series of measures. Several measures provide indications that the 2-year-olds remembered what they did in the laboratory two years before; specifically, they retained memory for early action sequences. The findings are of particular interest because a transition from preverbal to verbal functioning marks this 2-year period of development.