In the current research, we manipulated two stimulus dimensions, frequency and amplitude, that are useful for defining the visual stimulus in optical flow. [TABLE 1] Temporal frequency or the period of the stimulus was manipulated by oscillating the room at different frequencies, such as once every two sec. In our research, the wall oscillated at two frequencies: 0.3 and 0.6 Hz. Amplitude corresponds to the distance that the walls moved before reversing direction. In our research, the walls moved 9 and 18 cm. It is useful to note that the covariation between these two dimensions specifies the velocity of the visual stimulus. As can be seen, the mean velocity ranged between 2.7 and 10.8 cm.
Table 1: Stimulus velocities.
The sample included 40 infants who were divided evenly among four age groups: 5, 7, 9, and 13 months of age. Ages were selected to straddle the period during which infants are learning to sit so that experience with sitting could also be evaluated.
Infants were tested while sitting in the moving room. Each trial followed the same sequence of events. First the toy in the window on the front wall was activated to attract the infant's attention, and then the walls began to oscillate back and forth for approximately 10 sec. Following each trial, infants were given a brief opportunity to stabilize their posture before the next trial began. Infants were tested twice in each of the four frequency x amplitude conditions, and they were also tested twice in a variable frequency condition. These trials were presented in random order and were preceded as well as followed by a baseline trial in which the walls did not move.
Postural responses were measured while infants sat in a standard infant bicycle seat mounted to a force plate. [FIGURE 2] This figure is a schematic representation of the force plate. Center of pressure (COP) was the dependent measure and was measured for the two orthogonal axes by calculating the difference between the moment arms and normalizing by the weight of the child. The equations are shown on the figure. A change in the posture of the infant will shift the COP along the axes of the plate.
Figure 2: Schematic representation of the force plate.
In this study, COP was time sampled every 20 msec (50 Hz) and so was the wall movement. Postural sway was operationalized as the change in COP along the anteroposterior axis from one sample to the next. Wall movement was operationalized as the change in anteroposterior position of the room from one sample to the next. For each trial, we calculated the time series for wall movement and postural sway by extracting the first 512 samples. [FIGURE 3] Examples of the time series for both sway and wall movements are shown in the left panels of this figure. Position is plotted on the vertical axis and time is plotted on the horizontal axis. As can be seen, the COP and the wall oscillates back and forth over time.
Figure: An example pair of time series. (A) Time series for the wall movements. (B) Time series for the sway. (C) Fourier power spectrum of the wall movements. (D) Fourier power spectrum of the sway.