Idiosyncratic profiles of collinearity error using segments and dot pairs

It is well known that judgments of oblique line segments are more variable and less accurate than are judgments of horizontal or vertical segments, i.e., the "oblique effect." A prior study from our laboratories confirmed these differentials for a task in which the collinearity of segments at various angular positions was judged. Further, that study found that each observer manifests a distinct, idiosyncratic profile of errors across the 360 degrees range. These error tendencies are conspicuous in models derived by harmonic analysis, and we describe significant excursions of a given model as "delta errors." The present experiments found complex profiles of delta error with various stimulus and test conditions. A given subject manifested similar models of delta error when judging collinearity of dot pairs versus line segments. In the prior work the segments to be judged were asymmetrically positioned upon the test sheet. However, the asymmetric positioning is not responsible for the errors, as the present work found errors excursions with a round display field. Similar profiles of delta error were found when subjects were allowed to mark an open space, versus being required to respond at a specific distance (indicated by a target circle). This addresses questions of whether the error should be measured as an angle. In the two final experiments, we present evidence that the source of delta error within the nervous system is at the point of binocular synthesis of the information from the two eyes, or beyond, and the effects are not due to errors of reaching. Potential neural substrates for these complex, idiosyncratic error tendencies are discussed.