Previous studies have shown that humans use both optic flow and the target visual direction in active control of self-motion. Here we develop a methodology that allows a more sensitive measurement of the observer's separate reliance on these cues to steer toward a target. Three observers were asked to use a joystick to steer toward a target with three types of displays, an empty screen with only a target visible, a textured ground plane, and a textured ground with reference posts. To tease apart the observer's use of optic flow and target visual direction cues, we perturbed both heading (Yh) and the simulated gaze direction (Yg) in the display using independent sums of seven harmonically unrelated sinusoids (0.1-2.18 Hz and 0.11-2.21 Hz). The former shifted heading from the target while the latter kept heading intact but shifted the target visual direction on the screen. Observers had control of their heading but not their simulated gaze direction (i.e., Yh is a closed-loop task while Yg is an open-loop task). Ninety-second time series of heading error, gaze direction, and joystick displacement were Fourier analyzed and averaged across six trials. For all three observers, as displays contained more optic flow information, the heading RMS error decreased (mean error: 5.99?°, 4.50?°, and 4.33?° for the empty, the textured ground, and the textured ground with posts displays respectively), and observers increasingly controlled heading compared to gaze disturbance (mean ratio of control power correlation: 0.82, 1.08, and 1.40, respectively). Furthermore, Bode plots (frequency response plots) revealed a significant decrease of sensitivity to gaze disturbance (mean control gain: 5.53, -1.03, and -3.76 dB respectively). These findings show that with enriched optic flow displays observers rely more on heading and less on visual direction to steer toward a target.
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