Mobile rotation and epochs of mobile movement.

For any episode of infant movement, the starting position of the mobile is arbitrary. Furthermore, while the built-in Matlab function atan2.m can be used to convert position to angle, each time the mobile passes the point on its orbit which corresponds to 0 radians, atan2 inserts an artificial jump of 2π radians to signify the beginning of the next cycle. Mobile X and Y positions were transformed using trigonometric properties to calculate rotational angle of the mobile, ranging from 0 to π radians. Mobile angle reset to 0 each time the mobile stopped moving. However, the pseudoactivity from the reset did not contribute to the present data analysis and was removed for more accurate visualization. Dividing the cumulative sum of the absolute change in angle by two across an interval provides the number of rotations in that time period. (Dividing by two is necessary as 2π radians = 1 rotation).

A filter was constructed to investigate whether the length of time that the infants kept the mobile in motion changed across the tethered phase. The filter identified bouts of mobile motion when mobile velocity exceeded 30 mm/s for at least 150 ms. Mobile epochs were defined as bouts of mobile motion that resulted in at least a quarter of a full rotation. Epoch start and stop times were then retained for further analysis.