E L S E V I E R International Journal of Industrial Ergonomics 18 (1996) I 13-119

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Industrial Ergonomics

Foveal acuity, peripheral acuity and search performance: A review

Alan H.S. Chan a,,, Alan J. Courtney b a Department of Manufacturing Engineering, City University ofHong Kong, Tat Chee Auenue, Kowloon Tong, Hong Kong

h Department of Industrial and Manufacturing Systems Engineering, Universi O" of Hong Kong, Hong Kong, Hong Kong

Received 20 August 1994; revised 10 June 1995

Abstract

Some of the more important results from 30 years of research on foveal and peripheral acuity, visual search performance and the relationships between them are summarised in this paper. The visual lobe area is a useful concept for representing peripheral acuity and has proven to be important in predicting performance for visual search tasks. Different approaches to measuring lobe area and experimental results relating peripheral acuity and visual search performance are summarised. Methods of measurement include full field mapping, eight-axis visual field mapping, four-, two- and one-axis lobe length measurement, and a card-sorting task. A brief review is also made of the various measures of static and dynamic foveal acuities using different types of tasks, their applications and factors affecting acuity values.

Relevance to industry

Visual lobe area related to visual search performance is a useful concept in designing various displays and controls. Its measures and applications summarized in this paper will serve as basic guidelines for industrial designers.

Keywords: Foveal acuity; Peripheral acuity; Visual lobe; Visual search

1. Introduction

Foveal acuity and peripheral acuity are two im- portant subject factors related to visual search perfor- mance (Megaw, 1979). A variety of measures and techniques have been used for measuring these two visual abilities in order to improve understanding of the visual search process. Research work over the past 30 years has been ongoing in the areas of

measuring static and dynamic foveal acuity, measur- ing sizes and shapes of the effective visual fields under different conditions, simplifying the processes of measurement, and investigating the relationships between foveal acuity, peripheral acuity and the visual search process. This paper summarises some of these research findings.

2. Foveal acuity

Corresponding author. Tel.: (852) 2788 8439, Fax: (852) 2788 8423, E-maih meachan@cityu.edu.hk

Foveal acuity is the abili ty to discriminate fine detail and is a basic visual function important for

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many industrial tasks, e.g. identification of potential targets in an inspection process. It measures the resolution capability of the visual system in terms of the smallest high-contrast detail perceived at a given distance (Olzak and Thomas, 1986). Several methods of measuring foveal acuity are available and all involve directly or indirectly a description of the visual angle occupied by the target.

2.1. Types of tasks measuring foveal acuiO'

A variety of tasks are used for measuring the two main categories of foveal acuity; static and dynamic.

2. I. I. Static L, isual acui O' Static visual acuity has been widely employed for

examining underlying visual mechanisms subserving spatial vision and as a screening device in perfor- mance settings. One of these static tasks is an identi- fication task where subjects are asked to recognise a target similar to that in the Snellen eye chart. It has practical significance because it does measure letter identification which is a need in real life. Another type is a resolution task which requires subjects to discriminate a separation between the parts of a target, usually a grating pattern. The acuity is usually measured in terms of the reciprocal of the visual angle in minutes of arc subtended at the eye by the smallest detail that can be discriminated. The stan- dard score is one. Landholt rings, cyclical gratings and checkboards are commonly used for this type of task. A detection task requires a subject to judge whether a target is present or absent and it measures the visual angle of the target that can be detected. A L, ernier acuity task requires subjects to tell whether an upper vertical line is displaced to the right or the left of the lower line and it measures the visual angle of the smallest displacement that can be detected. Whenever we need to match two lines, such as unlocking a combination lock or aligning a dial on a scale in precision equipment, we are using vernier acuity. A stereoscopic acuity task measures the ability to differentiate the different images received by the retinas of the two eyes of a single object that has depth and is measured by the difference between the parallactic images of two similar targets that are at just noticeably different distances from each other.

2.1.2. Dynamic visual acuity tasks Dynamic visual acuity is the ability to resolve

detail when there is relative motion between the observer and the object. It is usually poorer than static visual acuity (Morgan et al., 1983), sometimes, rather obviously, because the eyes cannot move as fast as the test pattern, so the critical part of the pattern may not be consistently registered on the fovea (Murphy, 1978). Dynamic acuity was found to deteriorate rapidly as the rate of motion exceeded about 60 deg/sec (Burg, 1966).

2.2. Factors qff'ecting foveal acuity

Two main categories of factors, viz. subject fac- tors and stimulus factors affect the acuity values measured. Subject factors include pupil size, age and training while stimulus factors include luminance, glare, contrast and exposure time.

3. Peripheral acuity

When we fixate a low-contrast target point in a uniform background we have maximum visual acuity along the line of sight and acuity decreases approxi- mately linearly into the periphery, falling off more rapidly in the far periphery. The limit of peripheral sensitivity for particular target and background char- acteristics has been represented in terms of probabil- ity of target-acquisition as a function of eccentricity from fixation (Davies, 1968). The terms functional visual-field, visual lobe, and conspicuity area have been employed to describe the effective area within which a target would be seen with a given probabil- ity, e.g,, 50% frequency of seeing. Visual conspicuity has been defined by Engel (1971) as "that combina- tion of properties of a visible object in its back- ground by which it attracts attention via the visual system, and is seen as a consequence". Cole and Jenkins (1980) provide a discussion on the alterna- tive measures of conspicuity area. Although the con- spicuity area is derived from the primary measure of probability of seeing, it has greater generality than the primary measure because it is not specific to a given eccentricity, it is easy to understand, and it provides a scale with clearly defined endpoints. An- other measure of peripheral acuity is visual lobe size

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which is a function of the target and background under consideration; targets of different conspicuity yielding different visual lobe areas. There are large individual differences in lobe area even for people with similar foveal acuity (Bellamy and Courtney, 1981).

3.1. Testing around border area

In measuring the conspicuity area, Engel (1971) tested only the border region using a limited number of exposures and determined the boundary in which the target object was seen. His test required periph- eral form detection of an object against a back- ground, and he found irregularities in the shape of monocular areas indicating a local decrease in sensi- tivity which, he suggested, may be due to retinal blood vessels or peculiarities of neural organisation. Unfortunately, Engel did not report the criterion used to define the lobe boundary. In a study of foveal load, Ikeda and Takeuchi (1975) found irregularly shaped monocular functional visual fields, but the technique they used was to examine the border area only to the extent thought necessary by the experi- menter during the course of the experiment. Never- theless, the great variations in functional visual field between and within subjects prompted them to com- ment that physiological characteristics were unlikely to cause such variability. They suggested that the attitude of the subjects towards the task, i.e., conser- vative or risk taking may be an important factor.

3.2. Full field mapping

The functional binocular visual field has been found to be ovaloid with the longer axis horizontal, the area increasing as a function of stimulus-pre- sentation time, and for the majority of subjects a significant larger proportion of the area was above the fixation point than below it (Chaikin et al., 1962). Using a peripherally presented target in a regular background with more than 200 target loca- tions, Courtney and Chan (1985b) found that the binocular functional visual-field boundaries were very irregular, with regions of insensitivity within the field areas, and there were large differences between the subjects tested. Irregular boundaries were also reported by Chaikin et al. (1962) using 81 target

locations and larger target and background characters than used by Courtney and Chan (1985b) but the contours found by Courtney and Chan (1985b) were more irregular than might have been expected. A possible explanation for the areas of insensitivity is that selective interference from background objects occurs to a more marked extent in those areas. Such an effect would be important for visual search as Townsend et al. (1971) found interference persisted even with prolonged viewing time. In an 18-month study, Courtney (1989) confirmed, for whatever rea- sons, the presence of boundary irregularities and areas of insensitivity. Since a large number of data are required to map the functional visual-field fully (Chaikin et al., 1962, used 38,880 observations ex- tending over six weeks), many researchers have used some estimates of field size (or some measures proportional to field size) rather than shape for the purpose of relating the functional visual-field area to search performance (Johnston, 1965; Bellamy and Courtney, 1981; Gallwey, 1982).

3.3. Eight-axis uisual field mapping

In studies relating visual field to search perfor- mance and in models of search performance involv- ing consideration of visual-field size (Overington, 1979; Brown, 1979), it is usually assumed that the functional field is sufficiently regular to be approxi- mated by relatively few exposures of targets on a limited number of meridians (usually four or fewer). A knowledge of the shape of the visual field may lead to the evaluation of a more accurate and reliable quantitative measure of the visual field which can then be used for evaluation of the role of field-size and shape in visual search performance. Such knowl- edge can also justify the assumption of circular or elliptical field shape and the measurement of four or fewer meridians in determining area. The binocular functional visual field for a detection task, with a peripherally presented target embedded in a homoge- neous competing background, was mapped on 8 axes passing through the fixation point (Courtney and Chan, 1985c). In this experiment, the method used to determine the eight-axis lobe boundary was derived from a simple method originally designed to provide a gross measure of lobe area relevant to visual search. The method yielded boundary dimensions for

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each meridian that represented the number of loca- tions at which targets were detected. Therefore, loss of sensitivity anywhere along a meridian affected the location of the boundary. Such a boundary usefully shows the relative extent of detection capability along each meridian and is highly significantly related to search performance; however, it does not show the exact location of areas of low sensitivity within the lobe. The results showed that there was no signifi- cant difference between the top and bottom or be- tween the left and fight meridians. Thus the lobes would appear to be symmetrical if only the horizon- tal and vertical axes are considered. Taking vertical length as the average of top and bottom field lengths and horizontal length as the average left and right field lengths, the horizontal length was significantly greater than vertical length. No significant correla- tion was found between vertical and horizontal length. The 8-axis area correlated highly with area based on only the horizontal and vertical axes but the latter area gave no indication of even gross irregular- ities in shape of the visual field.

3.4. Four-axis lobe length measurement

meridians to obtain monocular 75% liminal bound- aries; subjects were required to indicate the direction of the gap in the broken ring. Temporal, nasal, superior and inferior meridians were tested in a fixed order for each subject. Leachtenauer (1978) used a similar procedure, but he combined two monocular fields for analysis, the order of meridian testing was randomly assigned, and he did not report the expo- sure time. The visual lobe measured by Bellamy and Courtney (1981) involved peripheral form detection using one target against a competing background. They used four meridians: right, left, top, bottom, and binocular viewing to establish an estimate of lobe area based on the errors made by subjects in reporting the target location. This is a more difficult search task than that used by Courtney and Chan (1985a) and there was no significant difference be- tween horizontal and vertical lobe dimensions. A recent study (Sanders and Bruck, 1991) of investigat- ing the effect of presentation time on the size of the visual lobe was performed with targets on the hori- zontal and vertical axes and the results showed a significant increase in lobe size as presentation time increased.

Courtney and Chart (1985a) plotted four-axis functional visual fields for subjects tested with tar- gets on eight axes passing through the fixation point. The data were extracted from two sets of orthogonal axes and the results also showed that most subjects had irregularly shaped binocular fields. In a study of isoresponse time regions proposed for the evaluation of visual search performance in ergonomic interface models (Kee et al., 1992), a stimulus of background character Xs with target on four axes was used. The results showed that the seven main effects investi- gated - expectancy, field heterogeneity, target uncer- tainty, density, size contrast, peripheral position, and meridian - were significant in affecting the average search time.

3.6. One-axis lobe length measurement

Gallwey (1982) used 'about' 330-ms exposure and two meridians: left and right. The lobe size 'score' was the weighted mean probability of detect- ing a target. Chan and Courtney (1993) investigated the effects of cognitive foveal load on a primary peripheral single-target detection task with the use of horizontal two-meridian stimulus lines. Mild evi- dence of tunnel vision was obtained. With same sets of stimulus lines, Chan and Courtney (1994) studied the effects of priority assignment of attentional re- sources on tunnel vision. The results indicated that tunnel vision was most prominent when the foveal task was primary.

3.5. Two-axis lobe length measurement 3.7. Card-sorting task

Some researchers made measures only on the vertical and horizontal axes (Johnston, 1965; Leacht- enauer, 1978; Bellamy and Courtney, 1981). John- ston (1965) used white broken tings on a plain background, exposed at various eccentricities on four

Other than the various types of tachistoscopic methods of measurement of visual-lobe size men- tioned above, Courtney and Chan (1985d) showed that a card-sorting task provided a rapid and simple means of estimating relative visual-lobe size. They

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reported that a 1 min card-sorting test using only one target against a homogeneous background was shown to be sensitive to subject differences in visual lobe area. With the type of sorting-test material proposed, the whole testing procedure would, at the most, require a 1 min practice period followed by a 1 min test trial.

years and with only one subject had near acuity less than 20/25, Courtney and Chan (1985b) showed that near and far acuity did not correlate significantly with search performance. It might be argued that foveal acuity and search times are unrelated because of the possibility of very small scatters in foveal acuity measurements.

4. Foveal acuity, peripheral acuity and search performance

4.1. Foveal acuity and search performance

Erickson (1964a) showed that foveal acuity had no significant effect on time to locate a target in a static structured display. Similarly, Harris and Chaney (1969) found that foveal acuity was not a good predictor of inspection performance. While Johnston (1965) has obtained a low correlation be- tween foveal and peripheral acuity, search speed and foveal acuity measures were unrelated. However, foveal acuity is a better predictor of detection capa- bility than peripheral acuity for more difficult tasks (Erickson, 1964b); with increasing difficulty of dis- crimination of a target from its background, there is an increasing dependency on foveal rather than pe- ripheral vision. Bellamy and Courtney (1981) re- ported that subjects' search performance measured in card-sorting time was not sensitive to acuity nor to age for subjects under 30 with near acuity of at least 20/25 (Snellen notation), but was sensitive to acuity and age when subjects of all ages and those with acuity of less than 20/25 were included. Courtney (1984) found that foveal acuity correlated with search times for subjects with age ranging from 15 to 52 years. But no significant correlation was found when the old (over 30 years) subjects were excluded from analysis. Courtney and Chan (1985d) reported that when subjects of less than 20/25 acuity and over 30 years of age were eliminated from analysis there were still significant correlations between acuity and card-sorting data. Their results also suggested that near foveal acuity seemed to play a more important role in search as the subjects gained experience and good acuity was still advantageous to jobs which are not demanding. Using a somewhat homogeneous group of 54 subjects of ages ranged from 18 to 24

4.2. Foveal acuity and peripheral acuity

Johnston (1965) found a low correlation between foveal and peripheral acuity. As mentioned earlier, there are large individual differences in lobe area even for people with similar foveal acuity (Bellamy and Courtney, 1981). Courtney and Chan (1985d) showed that for subjects below 30 years of age with near acuity of at least 20/25 (Snellen notation), foveal acuity correlated significantly (p < 0.05) with horizontal lobe dimensions and with lobe area based on 'correct' response data.

4.3. Peripheral acuity and search performance

Erickson (1964a) found that peripheral acuity was negatively correlated with search time for laboratory tasks. Bellamy and Courtney (1981) made use of the relationship between search time and peripheral acu- ity established for laboratory tasks by Erickson (1964a) and Johnston (1965), and for real targets with trained observers in photointerpretation by Leachtenauer (1978). The apparent importance of peripheral acuity for search speed indicated that pe- ripheral acuity may prove to be a useful selection and diagnostic test for people performing visual search tasks. Bloomfield (1975) summarised the equations, relating peripheral acuity and search time, developed by Bloomfield and Howarth (1969) and Bloomfield et al. (1974), as t e t l / D 2, or toLI/Q 2, where t is the mean search time, D is the discrim- inability of the target or fault from the rest of the display and Q is the extent into the periphery that discrimination can be made. Bellamy and Courtney (1981) used this relationship between search and peripheral acuity through the concept of the visual lobe. Engel (1971) defined the conspicuity area as the retinal field in which an object is capable of being noticed during a single eye pause, when the observer has no fore-knowledge of the location of

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the object. Visual search proceeds in a series of fixation pauses of 200-300 ms average duration. The amount of information available in a single fixation will depend upon many factors (see, for example, Bellamy and Courtney, 1981) but, clearly, the peripheral acuity of the searcher and the physical characteristics of the target and background are of great importance.

4.4. Visual-field shape and search performance

The marked irregularities in visual-lobe reported by Courtney and Chan (1985b), if found typical, may have important implications for search and mathe- matical models of search. The validity of the as- sumptions about probability with single fixation of discovering a target based on regular, homogeneous functional visual-field areas become questionable. Models of search performance that assume a regular homogeneous functional visual-field area also may not be valid. The results indicated that the shape of the binocular functional visual-field was irregular and contained areas of insensitivity such that it may be necessary to consider shape as well as area to understand the effects of functional visual-area on search. It was suggested that the 'looking without seeing phenomenon' noted by Megaw (1978) and the difficulty experienced by some subjects in finding a target, even after repeated scanning, may be partially explained by irregularities in the functional field. A subject may not be aware of loss of sensitivity or of extensions to field area (Courtney and Chart, 1985a), therefore, even with a good knowledge of target characteristics it would be difficult for the subject to select an appropriate search strategy. Evidence that some subjects do not have a clear idea of their limitations in detecting targets in the periphery comes from a study by Courtney (1984) and reported by Bellamy and Courtney (1981). When asked to use a scale from 0 to 100 to rate their confidence in the detection of peripheral targets, subjects frequently gave very high ratings (i.e., near 100) to their esti- mate of target location when in fact they were totally wrong, and vice versa: they gave very low ratings (i.e., near 0) to their estimate of target location when they were right. It would be difficult to explain such extreme ratings in terms of idiosyncratic use of the rating scale.

5. Summary

In summary, the relationship between foveal acu- ity and search performance is likely to be significant in groups of subjects with wide variation in foveal acuity and age, searching for targets of low conspicu- ity. Subjects having good foveal acuities may not be good in performing peripheral acuity tests. The vi- sual lobe area is a useful concept and measure for representing peripheral acuity and has proved to be significant in predicting performance in visual search tasks. But a comprehensive understanding of rela- tionship between search performance and peripheral acuity relies on the study of shape as well as area of visual lobe.

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