Visual search is a common scanning procedure that people use to locate objects in real life. During a typical experiment with the visual search task, a participant is re-quired to search for a specific target in a visual scene; the number of targets and nontargets (distractors) or feature dimensions (e.g., colors) may be manipulated to evaluate reaction times and performance on the visual search task. Visual search is often affected by neurological deficits and, therefore, requires clinical attention for problem identifi-cation and further management of symptoms. However, most lab instruments for visual search studies may be in-appropriate for clinical evaluation. Therefore, we modi-fied a popular assessment instrument, the cancellation task, from a paper-and-pencil test to a computer-based test (Wang, Huang, & Huang, 2006) for education and clinical administration. To better express the spatiotempo-ral process during the assessment, we introduced a pattern identification mechanism for visualized representation of visual search patterns.
Previous studies have indicated that the features shared by the target and the distractors, as well as the number of search items, will affect reaction times and the perfor-mance of visual search (Hogeboom & van Leeuwen, 1997; Müller & Found, 1996; Scharroo, Stalmeier, & Boselie, 1994; Treisman, 1991; Wolfe, 2001). Treisman and Gelade (1980) proposed the feature integration theory, in which two types of visual search tasks are distinguished: feature search and conjunction search. Feature search, in which a single feature, such as size, color, orientation, motion, cur-vature, or depth, solely defines the target, can be executed
easily. Feature search is performed in parallel in the pre-attentive stage, and reaction time will not be affected by the number of distractors. In contrast, conjunction search is a more complex task requiring the examination of tar-gets and nontartar-gets that share the same features. As the number of distractors increases, participants require more time to complete the conjunction search task.
Visual search tasks have been used to study the deploy-ment of visual attention in a visual scene (Bundesen, 1990; Bundesen, Habekost, & Kyllingsbaek, 2005; Johnson & Proctor, 2004). An individual’s intention controls the al-location of attention and guides the attentional focus and the processing of selected information consecutively. Pos-ner and Rafal (1987) reported that visual-scanning deficits can disrupt visual attention. Shifts in visual attention (i.e., the orienting process) are usually accompanied by motor behavior (overt movements of the head and eyes to align the fovea with the spatial locus of the target) and can be ob-served by means of reaction time for motor output. There-fore, visual attention can be observed through people’s motor behavior when they perform a visuospatial task.
In the studies of visuospatial attention, an individual has to search for a specific target in a search scene, which involves the spatial processing with the temporal compo-nent of the task (Robinson & Kertzman, 1990; Snowden, Willey, & Muir, 2001). When an individual searches for a target scattered in a visual scene but fails to point out the target and its location, it is likely that the person has deficits in visuospatial attention. The cancellation test is one of the most popular tools for assessing visuospatial
383 Copyright 2008 Psychonomic Society, Inc.
Visualized representation of visual search
patterns for a visuospatial attention test
HO-CHUAN HUANG
National Kaohsiung University of Applied Sciences, Taiwan
AND TSUI-YING WANG
National Cheng Kung University, Taiwan
Cancellation tests have been widely used in clinical practice and in research to evaluate visuospatial attention, visual scanning patterns, and neglect problems. The aim of the present work is to present a visualized interface for the visuospatial attentional assessment system that can be employed to monitor and analyze attention per-formance and the search strategies used during visuospatial processing of target cancellation. We introduce a pattern identification mechanism for visual search patterns and report our findings from examining the visual search performance and patterns. We also present a comparison of results across various cancellation tests and age groups. The present study demonstrates that our system can obtain more processing data about spatiotem-poral features of visual search than can conventional tests.
Behavior Research Methods 2008, 40 (2), 383-390
doi: 10.3758/BRM.40.2.383
384 HUANG AND WANG
but also the cancellation process. However, the system did not support a visualized interface for analyzing the visual search path and attentional center, which could increase our understanding of attentional deployment.
In the present article, we present a new visuospatial at-tentional assessment system for assessing and obtaining more detailed information of attentional processes and performance. We use the cancellation test to assess atten-tion performance, the efficiency of attenatten-tional shifts, and the strategies of visuospatial search in a visual scene. A visualized tool is introduced that presents the geographi-cal location of the attentional center for visuospatial atten-tional processes. In addition, a pattern identification algo-rithm is proposed for automatically identifying the type of visual search pattern. With the assistance of computerized tools, we can understand individual discrepancies in the process and strategy of visuospatial search better.
Visual Search Pattern and Attentional Center During a visual search task with more than one target, all the stimuli marked by the participant can be linked quentially to form a visual scanpath. A scanpath is the se-rial deployment or shifts of the attentional spotlight across displayed visual information mapped in iconic memory (Posner & Peterson, 1990). We define a scanpath as “the total of the cancellation sequences (i.e., links of marked stimuli) based on the temporal order of stimuli marked during the visual search process.” In other words, a scan-path shows the attentional shifts for stimuli marked from one location to the next across the visual scene. With the assistance of the CACTS scanpath analysis, we were able to monitor the attentional shifts of participants in order to understand their deployment of visuospatial attention.
A visual scanpath can be as simple as a straight line or as complex as a visual search graph. We classified a visual search graph into three search pattern types: a hori-zontal, a vertical, and a mixed search pattern. To avoid trivial processes in identifying search patterns by human-made detection, we established a pattern identification algorithm with an evaluation measure, the search pattern index, for automatically identifying the visual search tern (Figure 1). Two expressions relevant to the search pat-tern index are
line(i j )
Xj Xi2 Yj Yi
2 (1) and
slope(ij) | Sy/Sx|, (2)
where Sx (Xj Xi) and Sy (Yj Yi); (Xi, Yi) and
(Xj, Yj) are the coordinates of the stimuli i and j,
respec-tively; line(ij) is the distance between two marked stim-uli i and j; slope(ij) is the slope of the straight line (i.e., line(ij)) and is used to identify the type of the straight line (horizontal or vertical). Two variables, H-sum and V-sum, are used to calculate the total length of paths for horizontal and vertical lines, respectively. If the value of slope(ij) is less than 1 or the value of Sy is zero, the line will be
horizontal, and the length of line segment line(ij) will be accumulated into the H-sum variable. On the other hand, if the value of slope(ij) is larger than 1 or the value of Sx
attention (Byrd, Touradji, Tang, & Manly, 2004; Lowery, Ragland, Gur, Gur, & Moberg, 2004). Traditional paper-and-pencil cancellation tests are used for clinical settings and research to evaluate visuospatial attention, visual-scanning patterns, and neglect problems (Halligan, Burn, Marshall, & Wade, 1992; Weintraub & Mesulam, 1985). The formats of cancellation tests vary widely in terms of complexity, with respect to stimulus shapes, set sizes, and array layouts (Friedman, 1992; Halligan et al., 1992; Wil-son, Cockburn, & Halligan, 1987).
Despite the extensive application of cancellation tasks in research, there are relatively few reports on the spatial aspects of cancellation performance in healthy popula-tions. Some studies have examined error distributions for the left and right halves of cancellation forms, find-ing no significant difference for small samples of healthy adults (Gauthier, Dehaut, & Joanette, 1989; Weintraub & Mesulam, 1987, 1988). Geldmacher and colleagues stud-ied verbal and nonverbal cancellation performance in a large sample of healthy adults and reported conflicting findings for visuospatial attention bias. They suggested that the reading habit may be an interacting factor to be considered for the directional bias (Geldmacher & Alhaj, 1999; Geldmacher, Doty, & Heilman, 1994).
In other experiments, cancellation tests have been used as a tool for understanding visual exploratory performance during the target-searching process. It has been assumed that the extent of the disorganization of cancellations may relate to visual inattention. Healthy participants usually cancel targets in organized patterns, generally horizontal (e.g., left to right) or vertical (e.g., top down) (Donnelly et al., 1999; Gauthier et al., 1989; Weintraub & Mesulam, 1988). How-ever, Mark, Woods, Ball, Roth, and Mennemeier (2004) reported no significant correlations between omissions and search organization variables for cancellation performance. They used a small camera to record the test performances of 18 stroke patients and conducted a frame-by-frame analysis on the searching path, with marked stimuli identified by Cartesian (x, y) coordinates. Difficulties arise when one at-tempts to replicate the study with a large sample through such an all-by-hand method. We suggest that further exami-nation of the topography of target searching is necessary to rule out visual attention problems and that an automated procedure is required.
Computerized technologies have been widely used in different assessment tools. Both Donnelly et al. (1999) and Potter et al. (2000) used computers to record and analyze spatiotemporal performance, such as premovement time (initial movement), movement time (time between two successively marked stimuli), drawing time (completing a cancellation), and pause time (the preparatory process). These computer- assisted assessment tools more sensitively measured visuospatial attention performance than did tra-ditional assessment tools. Wang et al. (2006) constructed the computer- assisted cancellation test system (CACTS) to investigate the performance of visuospatial attention in a population of schoolchildren. We used CACTS to analyze visual attention performance in terms of visual search pat-tern, moving time, pause time, and performance, which allowed us to observe not only the correct/error responses,
VISUOSPATIAL ATTENTION REPRESENTATION 389 1999) and intrinsic motor behavior (Schwartz, Adair, Na, Williamson, & Heilman, 1997) may have an interaction effect on the left-advantage phenomenon. Further inves-tigations that consist of samples with a wider educational background, various neurological conditions, and a com-bination of other assessments with CACTS would provide more evidence for understanding the visuospatial perfor-mance of an individual.
CONCLUSION
Many traditional assessment tools are being replaced by computer-assisted assessments because they are time-saving, less biased, more precise, and more efficient to process and because they have more storage capacity. In the present study, we found that a computerized assess-ment system obtained more detailed information than a traditional paper-and-pencil assessment did about an indi-vidual’s attentional processes and performance. A visual-ized tool was presented to locate the attentional center and monitor the visual scanpath in order to understand shifts in visual attention.
We found that the CACTS offered preliminary data about visuospatial attention and a visual scanpath for the target search task, which may have implications for the understanding of visual reading patterns on a screen. We also found that the visuospatial attention performance of the participants was as good as was expected.
AUTHOR NOTE
This work was supported in part by Grant NSC93-2614-S-151-001 from the National Science Council (NSC), Taiwan. We thank the NSC for their financial support and all the research members involved in this project. We also thank the reviewers for the valuable comments and suggestions. Correspondence concerning this article should be addressed to T.-Y. Wang, Department of Occupational Therapy, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan (e-mail: michwang@mail.ncku.edu.tw).
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