[John Anderson 950227.2200 EST]
I was looking through some back issues of Science that just arrived,
and I came across this article by ALH van der Meer, FR van der Weel,
DN Lee (1995) "The functional significance of arm movements in
neonates" Science (3 Feb 1995) 267:693-695, which some CSG-L readers
may find interesting. This is from the same group, the Perception in
Action Laboratories of the University of Edinburgh, that published the
Nature article "Where we look when we steer", which I posted about
last summer (940708.2140). Here is the abstract:
"Arm movements made by newborn babies are usually dismissed as
unintentional, purposeless, or reflexive. Spontaneous arm-waving
movements were recorded while newborns lay supine facing to one side.
They were allowed to see only the arm they were facing, only the
opposite arm on a video monitor, or neither arm. Small forces pulled
on their wrists in the direction of the toes. The babies opposed the
perturbing force so as to keep an arm up and moving normally, but only
when they could see the arm, either directly or on the video monitor.
The findings indicate that newborns can purposely control their arm
movements in the face of external forces and that development of
visual control of arm movement is underway soon after birth."
The small forces that pulled on their wrists were exerted by weights
-- 0%, 10%, or 25% of the estimated weight of the baby's arm -- on the
ends of pulleys, with the force applied roughly parallel to the body
axis of the infant and toward the toes. When the infant could see its
arm, either directly or in the video monitor, it moved its hand up and
down in the same place (ie in its field of view) even when the weights
were applied, but when it was unable to see the arm, the weights
pulled the arm down towards the toes. This result seems fairly solid,
judging from Fig 2. They also say that the amplitude of movement
(estimated by the standard deviation of the position coordinate) is
greater -- ie the infants move their arms more -- when they can see
them. This result seems shakier to me, because the greatest
difference is only 1 SD between seen (4 SD) and unseen (3 SD).
However, the difference is significant as indicated by mixed measures
ANOVA, F(2,12) = 11.93, P < 0.002; I don't really know what these
numbers mean.
The authors conclude that these kinds of movements help "in the
construction of a bodily frame of reference for action", which the
infant needs "to successfully direct behavior in the environment.
Because actions are guided by perceptual information, building a frame
of reference for action requires establishing information flow between
perceptual input and motor output."
John