Sensitivity and validity of visual search models for multiple targets

Efficiency improvement by navigated safety inspection involving visual clutter based on the random search model

Navigated inspection seeks to improve hazard identification (HI) accuracy. With a tight inspection schedule, HI also requires efficiency. However, lacking quantification of HI efficiency, navigated inspection strategies cannot be comprehensively assessed. This work aims to determine inspection efficiency in navigated safety inspection, controlling for HI accuracy. Based on a cognitive method of the random search model (RSM), an experiment was conducted to observe the HI efficiency in navigation, for a variety of visual clutter (VC) scenarios, while using eye-tracking devices to record the search process and analyze the search performance. The results show that the RSM is an appropriate instrument, and VC serves as a hazard classifier for navigation inspection in improving inspection efficiency. This suggests a new and effective solution for addressing the low accuracy and efficiency of manual inspection through navigated inspection involving VC and the RSM. It also provides insights into the inspectors' safety inspection ability.

Evaluating eye fixation and color contrast effects in the measurement of visual lobe sizes: comment on Tsang, et al. (2013)

The visual lobe refers to the useful field of view, functional visual field, or conspicuity area in single eye fixation, and the visual lobe size is determined by the conspicuity of target presented amongst background characters. This comment discusses a study by Tsang, Chan, and Yu (2013) that investigated the effect of color contrast on the visual lobe. First, this comment suggests a way to improve the precision of their method for measuring a visual lobe. Measuring positions of the participants' eyes during visual search with an eye tracker would allow checking the eyes' fixation on the center of the search field, improving the precision of visual lobe measurement. This comment also suggests a more optimal experimental design to investigate the effect of color contrast on visual lobe size. Tsang, et al. used colors for target and background that had similar contrast (i.e., differences in chromaticity) for the experiments. If the color-contrast ratio in each experiment condition had differed enough, the color contrast effect on the visual lobe could have been investigated more effectively.

Effect of color contrast on visual lobe shape characteristics

This study examined the effects of the color contrast of target and background objects on visual lobe area and shape characteristics. For visual lobe mapping the participants had to locate a single colour target amongst a homogeneous background of non-targets during a brief presentation. Targets of six different colors (red, yellow, green, cyan, blue, purple) were tested, the non-targets were black and the background was metallic grey. The results showed that color contrast did not affect visual lobe parameters under the constant luminance contrast condition of 6:1 tested in the experiment. Subjective preference evaluations showed that the blue and purple targets resulted in the highest visual comfort and overall preference, respectively, while the yellow target was the most negatively rated. The color preferences here were somewhat different from the previous color combination studies. It might be due in part to the constant luminance contrast and display polarity used here.

Effect of display polarity and luminance contrast on visual lobe shape characteristics

The effect of display polarity and luminance contrast on visual lobe (effective visual field) shape characteristics was studied using three levels of luminance contrast with combinations of positive and negative polarities. The binocular effective visual field for a detection task, with a peripherally presented target (V) embedded in a homogeneous competing background (Xs), was mapped on 24 imaginary axes passing through the fixation point. The results showed that visual lobes mapped using positive polarity were statistically larger in area, rounder and more regular in shape than those for negative polarity. The medium contrast condition lobes were more symmetric and regular than low contrast condition lobes, and lobe area and perimeter increased with increasing luminance contrast ratio. Under the interaction of positive polarity and high luminance contrast, visual lobes were found to be larger, smoother and rounder. The high level of luminance and contrast however resulted in a higher degree of visual discomfort. The results indicated that positive polarity and contrast of medium (26:1) to high (41:1) levels are possible display settings for better visual lobe characteristics and better anticipated search performance. Practitioner Summary: The effect of display polarity and luminance contrast on visual lobe shape characteristics was examined with uniform stimulus materials in this study. The results help to identify the optimum display settings for luminance contrast and display polarity to enhance lobe shape characteristics and hence search performance in industrial inspection tasks.

Validating the random search model for two targets of different difficulty

A random visual search model was fitted to 1,788 search times obtained from a nonidentical double-target search task. 30 Hong Kong Chinese (13 men, 17 women) ages 18 to 33 years (M = 23, SD = 6.8) took part in the experiment voluntarily. The overall adequacy and prediction accuracy of the model for various search time parameters (mean and median search times and response times) for both individual and pooled data show that search strategy may reasonably be inferred from search time distributions. The results also suggested the general applicability of the random search model for describing the search behavior of a large number of participants performing the type of search used here, as well as the practical feasibility of its application for determination of stopping policy for optimization of an inspection system design. Although the data generally conformed to the model the search for the more difficult target was faster than expected. The more difficult target was usually detected after the easier target and it is suggested that some degree of memory-guided searching may have been used for the second target. Some abnormally long search times were observed and it is possible that these might have been due to the characteristics of visual lobes, nonoptimum interfixation distances and inappropriate overlapping of lobes, as has been previously reported.

Target difficulty, priority assignment of attentional resources, foveal task load, and order of testing of foveal loading on visual lobe shape characteristics

Effects of the difficulty of a peripheral target, the priority assignment of attentional resources for simultaneous peripheral and foveal tasks, and the foveal task load and order of testing of cognitive foveal loadings on visual lobe shape characteristics were investigated. Analysis showed that lobe shape characteristics were affected by target difficulty but not the low foveal load. For the tasks used here, attentional resources were sufficient for participants to perform both peripheral and foveal tasks concurrently; therefore, priority assignment of attentional resources had no effect on lobe shape. With regard to order of testing of foveal loading, lobe roundness, boundary smoothness, and vertical symmetry improved with a positive practice effect for the groups tested in the high level-low level order. The implication is that providing training or practice to participants on a task with a higher level foveal load could optimize lobe roundness, boundary smoothness, and symmetry. Performance on the foveal task was better with easy peripheral targets than difficult targets and better for the foveal-primary than for the peripheral-primary conditions, presumably because of the larger proportion of attentional resources allocated to the foveal task for these two groups.

Display polarity, stimulus exposure duration, and visual lobe shape

This study investigated the effects of visual display polarity and stimulus exposure duration on visual lobe shape. Analysis showed that regardless of display polarity and exposure duration combinations tested here, visual lobe contours were slightly irregular and asymmetric, of medium roundness, with a moderately rough boundary, and horizontally elongated with a mean length-width ratio of 1.53. Visual lobes mapped with negative display polarity were significantly rounder, slightly more regular and more symmetric along the vertical axis, compared with those mapped with positive polarity. Under the different polarity conditions, there were no significant differences in visual lobe area, perimeter, boundary smoothness, and elongation. When stimulus exposure duration increased from 200 to 400 msec. and from 200 to 300 msec., there were significant increases in the visual lobe area, perimeter, roundness, boundary smoothness, and regularity. No such changes were found when duration increased from 300 to 400 msec. Exposure durations did not have a significant effect on the shape categories of elongation and horizontal symmetry for the different stimuli. There were no statistically significant interactions between polarity and stimulus exposure duration for any of the lobe shape indexes used here.

Visual lobe shape and search performance for targets of different difficulty

In quantitative models of visual search it has usually been assumed that visual lobe area shape was sufficiently regular to be approximated by a circle or ellipse. However, the irregularities in visual lobe shapes that have been found in studies involving extensive lobe mapping have suggested that lobe shape may have important implications for visual search performance and for the accuracy of mathematical models used for performance prediction. However, no systematic research on the relationship between the shape aspect of visual lobes and search performance seems to have been carried out and no comparisons of visual lobe shape characteristics under the effect of target difficulty have been reported. The current study was conducted to achieve two major objectives in two experiments. Experiment 1 used two different targets (letter 'O' and letter 'Y') to map the visual lobes of subjects in order to provide a systematic and quantitative comparison of lobe shape characteristics and experiment 2 was to investigate the correlation of visual lobe shape characteristics with visual search time under the effect of target difficulty. The visual lobes of 28 subjects were mapped on 24 imaginary and regularly spaced meridians originating from the centre of the visual field to resemble the full field mapping situation. Five categories of shape indices, viz. roundness, boundary smoothness, symmetry, elongation and shape regularity were investigated. The results of this study demonstrated that the visual lobe shapes of subjects elongate horizontally with medium level of roundness, high levels of boundary smoothness, symmetry and regularity for an easy target (O) against a homogeneous background of 'X's. When a difficult target (Y) was used, the visual lobes of the subjects were still elongated horizontally but to a smaller extent and with a low level of roundness, medium level of boundary smoothness and regularity and a similar high level of symmetry to the easy target. Moreover, significant correlations between shape indices and visual search time were found, suggesting mathematical models for predicting search time should not merely rely on area but also should consider visual lobe shape indices. Finally, a universal mathematical model containing several visual lobe shape indices was developed, which was applicable in the prediction of visual search time for a range of similar search tasks.

Quantifying visual field shape for improving accuracy of search performance prediction

The current study calculated a set of 16 shape indices of six subjects (4 men and 2 women, 21-28 years old) for easy and precise quantification of visual lobe shapes using Visual Lobe Measurement System (VILOMS) software. The indices can be used in mathematical models of visual search for better understanding of search behaviour and higher accuracy of search performance prediction. General patterns in the shape indices showed that visual lobes of the subjects were slightly irregular, with low sphericity, a moderately rough boundary, horizontally elongated and slightly asymmetric along both axes. Given the complex relationships among visual lobe area, lobe shape, and search strategy, it seems that a more detailed analysis than hitherto attempted will be necessary. Quantifying lobe shape with the 16 shape indexes is a useful step to performing such analysis.