Abstract
Models of line bisection implicitly consider distance to be the metric by which spatial extent is processed. For example, if a 20 cm line is presented visually, the brain infers or computes its length from the visual angle subtended. An alternate hypothesis would suggest that length (D) is determined from the product of velocity (V) over time (T). We refer to this as the DVT model, which reflects an ‘indirect’ computation of spatial extent because it does not rely on a direct measurement of distance (D). To investigate the DVT model in a healthy population, we conducted a series of experiments which measured pointing and ocular judgments of spatial extent using the line bisection task. We manipulated line length, position, and the direction of ocular scanning prior to bisection. Scanning led to different biases in bisection than did free viewing suggesting that the mechanism involved in scanning introduced additional perceptual biases of spatial extent. Pointing behavior showed a robust influence from scan direction (i.e., left-to-right scanning created a bias leftward to that of right-to-left scanning), whereas the speed of scanning was inversely related to ocular fixation biases (i.e., slower speeds induced exaggerated biases). We were unable to show a strong effect of timing on bisection behavior perhaps because of the probe(s) used. Rather, to our surprise, we found that ocular behavior, presumably operating in a gaze-centered reference frame, and pointing behavior, operating in a hand-centered reference frame, produced distinct patterns of bisection. In general, pointing behavior generated systematic errors that were impervious to manipulations such as length, line position, or speed of scanning, whereas ocular behavior was far more variable and more susceptible to these manipulations. This suggests that judgments of spatial extent can be made independently for the hand and eye.