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Abstract:
DeValois and DeValois (Vis Research, 31, 1619-1626) have shown that a moving carrier behind a stationary window can cause a perceptual misplacement of this envelope in direction of motion. The authors also found that the bias increased with increasing carrier speed and eccentrcity. Yamagishi et al. (2001, Proceedings of the Royal Society, 268, 973-977) showed that this effect can also be found in visuo-motor tasks. To see whether variables such as eccentricity and grating speed increase the motion-induced perceptual shift of a motion field also in an action task, a motor-control experiment was created in which these variables were manipulated (eccentricity values: 0 deg, 8.4 deg and 16.8; speed values: 1.78 deg/sec, 4.45 deg/sec and 7.1 deg/sec). Participants had to keep a downward-sliding path aligned with a motion field (stationary Gaussian and horizontally moving carrier) by manipulating the path with a joystick. The perceptual bias can be measured by comparing the average difference between correct and actual path position. Both speed and eccentricty had a significant impact on the bias size. Similarly to the recognition task, the bias size increased with increasing carrier speed. Contrary to DeValois and DeValois’ finding, here the perceptual shift decreased with increasing eccentricity. There was no interaction of the variables. If we assume an ecological reason for the existence of a motion-induced bias, it might be plausible to see why the bias is smaller in an unnatural task such as actively manipulating an object that is in an eccentric position in the visual field (hence the decrease of bias magnitude in the periphery). Contrary to this, recognition tasks carried out in the periphery of the visual field are far more common and therefore might “benefit” from the existence of a motion-induced localization bias. As expected, task difficulty increased with increasing speed and eccentricity. It seems interesting to further compare action and perception tasks in terms of factors influencing the localization bias in these different task types.
