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Li Li Human, Barbara T Sweet, Leland S Stone; Effect of contrast on the active control of a moving line. Journal of Vision 2003;3(9):541. doi: 10.1167/3.9.541.
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© ARVO (1962-2015); The Authors (2016-present)
In passive speed-matching tasks, perceived speed is contrast-dependent with little evidence of saturation at high contrast (e.g. Stone & Thompson, 1992), while simple 1D direction discrimination saturates at near threshold contrasts (e.g. Watson & Robson, 1981). To explore the use of speed information in a manual control task, we examined observers' performance as they actively controlled a moving luminance-defined line for a range of contrasts (2, 3, 4, 8, 16% around 22 cd/m2) and for two different controller dynamics. The stimulus consisted of a Gaussian-blurred (SD = 1 ) horizontal line. Four observers (three naïve) were asked to use a joystick to keep a horizontal line centered on a 34 × 26 display as its vertical position was perturbed by the sum of 10 harmonically-unrelated (0.02 to 2.18 Hz) sinusoids. Two control paradigms were tested in blocked conditions: joystick displacement generated a command proportional either to the rate of change of line position or to the rate of change of line velocity. Four-minute time series of line position and joystick displacement were Fourier analyzed and averaged across 6 sessions. For all four observers, the RMS error decreased quasi-linearly with increasing log contrast across the tested range (mean slope: −8.0 & −7.7% per log2 contrast unit, averaged across observers, for the two control paradigms, respectively). Bode plots (frequency response plots) showed a systematic increase in sensitivity (mean slope: 1.61 & 1.52 db per log2 contrast unit, respectively) and decrease in phase lag at high frequencies, which can be accounted for by a decrease in response delay (mean slope: −31 & −26 ms per log2 contrast unit, respectively). The finding that performance continues to improve even for relatively high contrasts suggests that speed information is used in the manual control of a moving line under both control paradigms.
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