A DEVELOPMENTAL TRANSITION IN PREHENSION MODELED AS A CUSP CATASTROPHE

The purpose of the study was to show that the change from reaching wit hout grasping to reaching with grasping during the first 6 months of l ife carried the characteristics of a discontinuous phase transition (c atastrophe). A cross-sectional study was carried out with 58 infants b etween 60 and 408 days old. The infants were seated in a specially des igned seat, and presented with nine detachable balls on a black curved board within reaching distance al shoulder height. The number of reac hes without and with grasping were scored from video. A cusp catastrop he model was fitted to the data. A Likelihood-Ratio test indicated tha t rite likelihood of the cusp model was significantly higher, p < .001 , than a linear regression model. The cusp model was also compared wit h a logistic model. Akaike's Information criterion for the cusp catast rophe exceeded the logistic model, thus indicating a general better-fi t. Based on prior research, the following potential control parameters were chosen: crown-heel length, total body weight, arm length, arm ci rcumference, ponderal index, arm volume, arm weight, and body position relative to the horizontal. The cusp model predicted that arm weight and arm circumference significantly contributed to the control paramet ers. It was found that these two variables had their largest contribut ion to the asymmetry control parameters. (C) 1998 John Wiley & Sons, I nc.