Academic journal article Perception and Psychophysics

Attentional and Automatic Processes in Line Tracing: Is Tracing Obligatory?

Academic journal article Perception and Psychophysics

Attentional and Automatic Processes in Line Tracing: Is Tracing Obligatory?

Article excerpt

When comparing two target elements placed on the same convoluted curve, response times are dependent on the distance between the targets along the curve, despite being separated by a constant Euclidean distance. The present study assessed whether such line tracing is obligatory across the whole of the line even when the task demands do not require it, or whether it is an optional strategy that can be disregarded when the circumstances favor a different method of attentional deployment. Three experiments were conducted to assess whether attention can select only a portion of a curve to trace when it is strategically sensible to do so. The results suggest that attention can indeed jump over portions of a line that are irrelevant to task performance before tracing has begun. However, the final experiment suggests that line tracing may continue beyond the task-relevant portion of the line. We conclude that line tracing is a strategy whose initial deployment can be influenced by top-down factors, rather than an obligatory response triggered by the stimuli-although, once engaged, line tracing may be hard to stop.

When we are presented with a top-down plan of a maze, it might be assumed that we will traverse the pathways with our eyes, in a fashion similar to how we might explore the same maze on foot. Indeed, this appears to be the case. Crowe, Averbeck, Chafee, Anderson, and Georgopoulos (2000) asked participants to judge which exit would correctly lead out of a series of mazes. They found that response times (RTs) increased with the length of the path from the start point to the exit and were also related to the number of turns within the main path. Eye movements, consisting of a number of saccades and fixations, closely followed the main path, although the convolution of the path in between subsequent fixations was such that it led Crowe et al. to conclude that the main path is "mentally traversed" away from the point of fixation. Once covert attention reaches a certain point, a saccade is initiated to bring the focus of the eye in line with the focus of covert attention. At this point, covert attention moves away once again, traversing the maze until it needs to call another saccade.

These results argue for a covert, serial search of the maze. Similar conclusions were drawn by Jolicceur and colleagues (Jolicceur & Ingleton, 1991; Jolicceur, Ullman, & Mackay, 1986, 1991; McCormick & Jolicur, 1991, 1992, 1994). Their basic paradigm required participants to view two convoluted lines or curves, one of which would pass through the point of fixation. (We refer to all stimuli as lines, though it should be noted that most stimuli used in these experiments involve curved lines rather than straight lines.) The task was to say whether two targets were on the same line or on different lines, without moving the eyes. The first target was always at the point of fixation, and the second target could appear at one of eight locations arranged in a circle around the fixation point, so that all potential locations were the same distance in Euclidean space from the center. Four of the potential target locations were on the same line as the first target, and four were on the other line. When targets appeared on the same line-although the absolute distance between all combinations of targets was equalthe second target could increase in distance from the first target in regard to its relative position along the convoluted line. Jolicceur(Jolicoeuretal., 1986,1991) noted that RTs to targets on the same line varied as a function of the relative distance between the first and second target along the line. It appeared that when judging whether the two targets were on the same line, this required participants to move covert attention along the line. The further along the Une that attention had to travel, the longer it took for participants to make the correct decision. These results were found even when the displays only appeared for a fraction of a second, thus ruling out eye movements as the main cause of the effect. …

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