Effects of lags on human operator transfer functions with head-coupled systems.

The effects of operator learning and target velocity on head tracking performance with and without lags has been studied. Five lags (0, 40, 80, 120, 160 ms) between head movement and target image movement, and three target velocities (2, 3.5, 5 degrees.s-1 r.m.s.) were investigated with eight male subjects and band-limited random target motions. Head tracking transfer functions, tracking error spectra, mean radial tracking error, and subjective difficulty ratings were obtained. Head tracking performance was significantly degraded by lags greater than, or equal to, 40 ms (in addition to a system lag of 40 ms). Both the input-correlated tracking error and the uncorrelated tracking error increased with increasing lag. No significant improvement in head tracking performance was found through practice with an 80-ms lag. As the lag increased, operators increased their gains at frequencies above about 0.5 Hz and reduced their phase lags at low frequencies (about 0.1 Hz) but failed to fully compensate for the increased display lag. The increased human operator gain was associated with increased operator phase lag at higher frequencies (above 0.5 Hz). To improve head tracking performance in the presence of lags, a lag compensation technique is needed to prevent undesirable changes in tracking strategy.