Touch the table before the target: contact with an underlying surface may assist the development of precise visually controlled reach and grasp movements in human infants.

Multiple motor channel theory posits that skilled hand movements arise from the coordinated activation of separable neural circuits in parietofrontal cortex, each of which produces a distinct movement and responds to different sensory inputs. Prehension, the act of reaching to grasp an object, consists of at least two movements: a reach movement that transports the hand to a target location and a grasp movement that shapes and closes the hand for target acquisition. During early development, discrete pre-reach and pre-grasp movements are refined based on proprioceptive and tactile feedback, but are gradually coordinated together into a singular hand preshaping movement under feedforward visual control. The neural and behavioural factors that enable this transition are currently unknown. In an attempt to identify such factors, the present descriptive study used frame-by-frame video analysis to examine 9-, 12-, and 15-month-old infants, along with sighted and unsighted adults, as they reached to grasp small ring-shaped pieces of cereal (Cheerios) resting on a table. Compared to sighted adults, infants and unsighted adults were more likely to make initial contact with the underlying table before they contacted the target. The way in which they did so was also similar in that they generally contacted the table with the tip of the thumb and/or pinky finger, a relatively open hand, and poor reach accuracy. Despite this, infants were similar to sighted adults in that they tended to use a pincer digit, defined as the tip of the thumb or index finger, to subsequently contact the target. Only in infants was this ability related to their having made prior contact with the underlying table. The results are discussed in relation to the idea that initial contact with an underlying table or surface may assist infants in learning to use feedforward visual control to direct their digits towards a precise visual target.

Two types of memory-based (pantomime) reaches distinguished by gaze anchoring in reach-to-grasp tasks.

Comparisons of target-based reaching vs memory-based (pantomime) reaching have been used to obtain insight into the visuomotor control of reaching. The present study examined the contribution of gaze anchoring, reaching to a target that is under continuous gaze, to both target-based and memory-based reaching. Participants made target-based reaches for discs located on a table or food items located on a pedestal or they replaced the objects. They then made memory-based reaches in which they pantomimed their target-based reaches. Participants were fitted with hand sensors for kinematic tracking and an eye tracker to monitor gaze. When making target-based reaches, participants directed gaze to the target location from reach onset to offset without interrupting saccades. Similar gaze anchoring was present for memory-based reaches when the surface upon which the target had been placed remained. When the target and its surface were both removed there was no systematic relationship between gaze and the reach. Gaze anchoring was also present when participants replaced a target on a surface, a movement featuring a reach but little grasp. That memory-based reaches can be either gaze anchor-associated or gaze anchor-independent is discussed in relation to contemporary views of the neural control of reaching.

Increasing task precision demands reveals that the reach and grasp remain subject to different perception-action constraints in 12-month-old human infants.

The reach and grasp follow different developmental trajectories, but are often considered to have achieved nearly adult-like precision and integration by 12 months of age. This study used frame-by-frame video analysis to investigate whether increasing precision demands, by placing small reaching targets on a narrow pedestal rather than on a flat table, would influence the reach and grasp movements of 12-month-old infants in a complementary or differential fashion. The results reveal that placing the target atop a pedestal impaired the infants's ability to direct an appropriate digit towards the small target, but did not produce a corresponding decrease in the frequency with which they used an index-thumb pincer grip to grasp the target. This was due to the fact that, although infants were more likely to contact the target with a suboptimal part of the hand in the pedestal condition, a greater proportion of these suboptimal contacts ultimately transitioned to a successful index-thumb pincer grip. Thus, increasing task precision demands impaired reach accuracy, but facilitated index-thumb grip formation, in 12-month-old infants. The differential response of the reach and grasp to the increased precision demands of the pedestal condition suggests that the two movements are not fully integrated and, when precision demands are great, remain sensitive to different perception-action constraints in 12-month-old infants.