Kelso (1982) showed that two body limbs, swaying in anti-phase coordination (simultaneously left and right), shifted to in-phase (swaying inwards and outwards simultaneously) at a certain critical frequency. So far, such a critical phase shift has mainly been explained in terms of mathematical models based on the stability of the attractors.
However, we think that such a shift should be better explained by information inavailability and control difficulty at high frequencies. We hypothethized that at high frequencies, information necessary to maintain the anti-phase relationship may be inadequate, (i.e., the movement is so rapid that whether the hands are actually in-phase or not cannot be perceived clearly), and it may be easier to control homologous muscles together as in the in-phase motion (Cohen, 1971). Thus, a phase shift would occur at a higher frequency.
To test our hypothesis that information and control are important in the critical phase shift, we asked 11 participants to trace circles with both hands using ink-depleted ball-point pens over two marked circles (10 cm in diameter) on a horizontal surface for 30 seconds in each trial. In the beginning of each trial, both hands started from the extreme left position and cycled in a clockwise direction. The frequency of drawing was timed with a metronome. Eight different frequencies were used (60, 80, 104, 126, 138, 152, 168, and 184 cycles per min). Analysis was performed to check at what frequency the initial anti-phase circle drawing could not be maintained. To increase the haptic information about the phase of the two hands, plastic sticks (8 cm long, 1 mm thick, and 3 mm wide) were fixed on each of the marked circles radiating from the centers, either only at the top, or at both the top and the bottom of the two circles (see figure). We hypothesized that the critical phase shift would be delayed to a higher frequency when the stick provides increased information. With two sticks, the critical phase shift would be advanced as interference increases with two sticks in each circle. Results show both shifts indicating that information and control are important in producing the critical phase shift.