Academic journal article Canadian Journal of Experimental Psychology

On Knowing Which Way to Trace: Direction Errors during Visual Curve Tracing

Academic journal article Canadian Journal of Experimental Psychology

On Knowing Which Way to Trace: Direction Errors during Visual Curve Tracing

Article excerpt

Abstract 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.

The time that it takes to determine that two markers appear on the same curved line increases as the curve distance between the two markers increases. This finding suggests that subjects must visually trace the intervening line segment (Jolicoeur, Ullman, & MacKay, 1986). McCormick and Jolicoeur (1992) demonstrated that curve tracing likely involves shifting visual attention, and proposed a computational model of this operation (McCormick & Jolicoeur, 1991; 1994). This model is based loosely on the zoom lens model of shifting visual attention proposed and explored extensively by Eriksen and his colleagues (e.g., Eriksen & St. James, 1986) in which visual attention is thought to be analogous to a variable-sized spotlight. Within McCormick and Jolicoeur's framework, curve tracing is thought to proceed by serially applying a local operator having a variable-sized processing field. The model has proven to be a better predictor of the pattern of response times found in curve tracing tasks than alternative models such as Jolicoeur, Ullman, and MacKay's (1991) bipartite local operator (McCormick & Jolicoeur, 1991; 1994), and converging evidence has continued to support the notion that this variable-sized operator is the same mechanism as Eriksen's variable-sized focus of attention (McCormick, 1994).

In addition to tracking the curve by a local operator, a number of other processes are necessary to perform the typical curve tracing task. In one version of this task, subjects are presented with two intertwining but nonintersecting lines and two dots. The stimuli are constructed such that one dot is always positioned at the same spatial location as the initial fixation stimulus and another occurs at one of eight possible locations on one or the other of the curves. The subject's task is to decide as quickly and as accurately as possible whether the two dots are on the same curved line. In addition to curve tracing, mechanisms are necessary for performing functions such as the initial encoding of the stimulus, including processes to locate the general position of the second dot. Also necessary are some executive functions such as selecting the direction to trace, starting and stopping the tracing process, and the complex decision processes that are involved in integrating the information necessary for selecting the correct response. The present report is concerned with how the curve tracing executive chooses the direction to trace.

In previous descriptions of the zoom lens model of curve tracing, it was assumed that tracing proceded from the centrally located dot toward the end of the line. On a 'same' trial the operator reached a second dot and initiated an appropriate response; on a 'different' trial the operator reached the end of the curve and then, along with other accumulated information, decided that the two dots were not on the same line. As can be seen in Figure 1, the centrally located dot was never positioned at the very end of a curve, but was usually a few degrees of visual angle from the end of a curve. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.