The modulation of intermanual interactions during the specification of the directions of bimanual movements

The different effects of measurement noise on the intermanual correlations of terminal and initial directions result from the different ranges of coordinates and can easily be illustrated by way of simulation. We generated two sets of correlated normally distributed directions (μ=30, σ²=5 ρ=0.8). For each direction we determined the x- and y-coordinates for vector lengths of 20 and 150 mm. These coordinates were rounded to units of 0.5 mm, corresponding to the coordinates used in the determination of initial and terminal directions in the experiment. For each length the directions were re-computed from the rounded coordinates. For a typical sample of 1000 cases the means were 29.98° and 30.02° for the left and right hand, respectively, the standard deviations were 2.22 and 2.25°, and the intermanual correlation was 0.8048. Means (and standard deviations) for the re-computed terminal directions were 29.98° (2.22°) and 30.02° (2.25°) with a correlation of 0.8050; means (and standard deviations) for the re-computed initial directions were 29.96° (2.25°) and 30.03° (2.30°) with a correlation of 0.7751. The differences in standard deviations and correlations are consistent across samples, but not the difference in the means.