On knowing which way to trace: direction errors during visual curve tracing.

McCormick and Jolicoeur's (1991; 1994) zoom lens model of visual curve tracing proposes that curve tracing involves tracking a curve with a variable size local operator. Unspecified in their model is how the executive function guiding the processing field of this operator initially knows which direction to trace. An experiment was conducted to determine whether observers occasionally curve trace in the wrong direction. A typical curve tracing task involving the determination of whether two dots occur on the same line in a display consisting of two dots and two intertwining lines was used. Lengthening the curve segment at the beginning of a to-be-traced curve resulted in a slowing of response times only to dot locations close to the end of the to-be-traced line. Apparently, observers calculate curve tracing direction based on the spatial location of the second (i.e., target) dot, which can result in erroneously tracing in the wrong direction at dot locations farther along the line.