Submitted on March 2, 2004
Accepted on December 18, 2004
Effect of Contrast on the Active Control of a Moving Line
Li Li1*, Barbara T. Sweet1, and Leland S. Stone1
1 Human Factors and Technology, NASA Ames Research Center, Moffett Field, CA, USA
* To whom correspondence should be addressed. E-mail: lli{at}mail.arc.nasa.gov.
In many passive visual tasks, human perceptual judgments are contrast-dependent. To explore whether these contrast-dependencies of visual perception also affect closed-loop manual control tasks, we examined visuomotor performance as humans actively controlled a moving luminance-defined line over a range of contrasts. Four subjects were asked to use a joystick to keep a horizontal line centered on a display as its vertical position was perturbed by a sum of sinusoids under two control regimes. The total RMS position error decreased quasi-linearly with increasing log contrast across the tested range (mean slope across subjects: -8.0 and -7.7% per log2 contrast, for the two control regimes, respectively). Frequency response (Bode) plots showed a systematic increase in open-loop gain (mean slope: 1.44 and 1.30 dB per log2 contrast, respectively), and decrease in phase lag with increasing contrast, which can be accounted for by a decrease in response time delay (mean slope: 32 and 40 ms per log2 contrast, respectively). The performance data are well fit by a Crossover Model (McRuer and Krendel 1974), which allowed us to identify both visual position and motion cues driving performance. This analysis revealed that the position and motion cues used to support manual control under both control regimes appear equally sensitive to changes in stimulus contrast. In conclusion, our data show that active control of a moving visual stimulus is as dependent on contrast as passive perception and suggest that this effect is due to a shared contrast sensitivity early in the visual pathway, prior to any specialization for motion processing.
