A comparison of localization judgments and pointing precision.

We compared the precision of perceptual localization and manual pointing. A Gaussian blob was presented 6 degrees to the right or left of a central fixation spot on a CRT screen. Above and below the blob, vertical lines were displayed. On each trial, the blob was slightly offset to the right or left with respect to the lines. The subjects had to judge whether the blob appeared to the right or to the left of the vertical lines. At the same time, they had to point to the center of the blob with their index finger. Precision for perceived position was significantly better than precision for pointing. Performance in these two tasks correlated highly between the subjects. Overall, subjects pointed more leftward on trials where they judged the blob to be to the left of the lines. There was also a significant correlation for each subject between the pointing error and the perceived location error, calculated by partialling out the effect of the physical offset. The results are in agreement with the idea that the signals determining the perceived location of an object are used to guide the motor system in pointing toward it.