Line copying: distinct "where" and "aiming" spatial bias in healthy adults

Spatial bias in natural, implicit tasks such as reaching and grasping may manifest differently from that in arbitrary laboratory-experiment line bisection tasks. Because spatial processing in everyday activities is difficult to quantify, it is important to study spatial behavior in an implicit laboratory task. Drawing tasks of copying lines or objects integrate spatial perceptual-attentional ("where") input and motor-intentional ("aiming") output, and may be more implicit than line bisection because participants are unaware that the placement of their drawings will be assessed.

We examined whether it is possible to distinguish "where" and "aiming" spatial biases in a line-copying task. We examined changes in "where" and "aiming" biases in response to bottom-up versus top-down cues (hemispace presentation and drawing direction).

In 13 healthy adults, we collected copied-line displacements and lengths in both the natural (left-right congruency) and reversed (left-right incongruency) viewing conditions, to distinguish "where" and "aiming" biases.

Participants displaced lines leftward (P=0.01) as they copied, displaying primarily a "where" bias. They displaced lines in the drawing direction irrespective of viewing condition, a finding consistent with induced "aiming" effects (P=0.291). Presenting lines on participants' right or left side did not affect the "where" spatial bias. Cues did not affect copied-line lengths.

We showed that an implicit laboratory-experiment task of copying lines can discern complex stages of spatial processing in healthy adults. Further evaluation of this task will greatly contribute to the understanding of mechanisms of human spatial cognition.