NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

National Research Council (US) Committee on Vision. Emergent Techniques for Assessment of Visual Performance. Washington (DC): National Academies Press (US); 1985.

Cover of Emergent Techniques for Assessment of Visual Performance

Emergent Techniques for Assessment of Visual Performance.

Show details

TWO MODES OF VISUAL PROCESSING

Background

Vision plays a role not only in the perception of objects but also in spatial orientation (maintenance of body posture, perception of self-motion, and locomotion). The function of vision in spatial orientation as well as its controlling parameters is different in many fundamental ways from its contribution to the resolution of fine detail. In particular, fine detail is unnecessary for many visually controlled tasks (Leibowitz et al., 1980). The concept of two visual systems or two modes of processing visual information (Held, 1968, 1970; Ingle, 1967; Schneider, 1967; Trevarthen, 1968; Leibowitz and Post, 1982) is helpful to clarify these differences. The two modes of visual processing are focal and ambient.

The focal mode in general answers the question of “what” about objects perceived. Most studies of vision, particularly in relation to performance evaluation, have been concerned with focal vision. The ambient mode is concerned with “where” objects are located relative to the observer and where the observer is located in space. Focal and ambient vision differ in a number of ways.

(1)

The focal mode is almost exclusively visual, while the ambient mode acts in concert with the vestibular, somatosensory, and auditory senses to subserve spatial orientation, posture, and gaze stability.

(2)

Object recognition by the focal mode can operate over the full range of spatial frequencies. The ambient mode is adequately activated by low spatial frequencies typically stimulating large areas of the visual field.

(3)

Adequate luminance and lack of refractive error are critical for some aspects of focal vision (visual acuity, for example) but play a much less important role in ambient vision. The low spatial frequencies subserving ambient vision are less sensitive to the degradation of retinal image quality by refractive error or by reduction of illumination.

(4)

Focal vision is less efficient in the peripheral visual field. Although ambient functions are less efficient if restricted to a small area of the periphery compared with central vision, unlike focal vision, ambient functions improve when larger areas of the visual field are stimulated.

(5)

Focal vision typically involves attention, while ambient visual functions are more reflexive in nature. Reading while walking illustrates the fact that although attention is dominated by the focal-mediated reading task, spatial orientation is adequately maintained by the ambient mode with little or no conscious effort.

Implications

The idea of two modes of visual processing has important implications in several areas of vision. This section indicates specific directions for future research in areas for which there is a need to increase our understanding of the role of the different modes of processing.

Spatial Disorientation and Motion Sickness

In recent years, the importance of sensory mismatch within the ambient mode has come to be recognized as a cause of spatial disorientation and motion sickness. Whenever there is disagreement, based on previous experience, between the sensory input provided to the gaze stability and the spatial orientation systems, for example, a person can experience disorientation and/or nausea (Reason and Brand, 1975).

Vehicle Guidance and Night Driving

The two modes of processing can be functionally dissociated. Spatial orientation is adequate in the absence of the ability to recognize objects due to refractive error or reduction of luminance level. This selective degradation may be a factor in nighttime driving accidents. Vehicle guidance is a dual task: steering relies on ambient vision while recognition of signs and hazards is mediated by focal vision. At night, ambient vision functions as well as in daylight. Since the driver's self-confidence derives from the ability to steer the vehicle, and since he or she is not aware of the reduction in the ability to recognize hazards with the degraded focal system, nighttime driving speeds are often too fast to permit a timely response to infrequent and unexpected hazards on the roadway (Leibowitz and Owens, 1977).

Visual Narrowing Under Stress and Cortical Brain Damage

The two modes can be dissociated in other situations as well. Under various kinds of stressors, reaction time to objects imaged in the peripheral visual field may be increased, or the objects may not be detected at all. This phenomenon is referred to as tunnel vision or narrowing of the visual field (Leibowitz et al., 1982). Even more dramatically, studies of patients with cortical brain damage have demonstrated that spatial orientation can be carried out completely without awareness when the stimuli are imaged on areas of the visual field that are scotomatous as tested by conventional perimetry (Weiskrantz et al., 1974). Thus, focal and ambient vision can be dissociated either by brain damage or by the nature of the attentive demands in certain tasks such as occur when driving a vehicle. An implication of this functional dissociation is that the phenomenon of visual narrowing could result from the concentration of focal vision due to shifts of attention. Ambient vision, which does not require attention, is probably unaffected by attentional narrowing. A critical factor is that traditional static perimetry makes use of a focal task requiring attention that can be redirected by the observer. Ambient vision seems largely to be reflexive and therefore may not be as susceptible to modification by attention shifts. Whether selective degradation of focal vision while ambient function remains intact is also characteristic of visual narrowing resulting from stressors, such as hypoxia or excessive gravitational forces, has not yet been determined.

Because both focal and ambient vision are critical in human performance, it is important that visual tests be employed that are sensitive to both functions. Most tests of vision in current use evaluate only focal vision and are therefore of limited usefulness in predicting performance in many situations, particularly those involving spatial orientation.

Aircraft Instrumentation

Because ambient visual functions are reflexive, they present potential advantages with regard to the display of orientation information in aircraft over symbolic displays that involve learning and interpretation (Leibowitz and Dichgans, 1980). As pointed out by Head (1918), processes that require higher levels of information processing are more vulnerable to loss during stress than reflexive functions. This concept is incorporated in the Malcolm Peripheral Vision Horizon Display, which provides a wide-angle artificial horizon in order to more adequately stimulate the ambient system (Malcolm et al., 1975).

Gaze Stability

Most tests of visual resolution involve a stationary observer viewing a static target that requires gaze stability but places minimum demands on these systems. In many real-life situations in which the observer and/or the target is moving, smooth eye movements are subserved by both a reflexive and a voluntary system. The reflexive system is activated either by moving visual contours typically stimulating large areas of the visual field (optokinetic nystagmus) or by acceleration of the head (vestibulo-ocular reflex). Analogous to ambient function, this system is reflexive and does not involve awareness. Its function is to maintain a stable retinal image during head movement. Voluntary fixation in foveated animals is subserved by the phylogenetically newer pursuit system. Since the principal function of this voluntary system is to facilitate object recognition by maintaining images on the fovea, it subserves focal vision (see the section on dynamic visual acuity).

Interaction Between Focal and Ambient Vision

Although the ambient system can function adequately in the absence of focal vision, focal vision is not independent of disturbances of the ambient system. Disruption of gaze stability mechanisms, either vestibular or optokinetic, when the head is in motion results in retinal image motion. Such inappropriate image movement lowers contrast and reduces spatial resolution. Another consequence of ambient dysfunction is disorientation and/or motion sickness. Gastric symptoms associated with intersensory mismatch within the ambient system demand attention and interfere with object recognition and visually mediated judgments. Illusory object or self-motion frequently occurs when, in order to compensate for ambient dysfunction, the pursuit system is activated to preserve gaze stability (Leibowitz and Post, in press). Such illusory motion is difficult if not impossible to distinguish from true object or self-motion.

Recommendations

The working group recommends that research be conducted on how to integrate tests of ambient vision with tests of focal vision. For example, the contrast sensitivity function represents a significant improvement in the evaluation of focal vision. Some tests of ambient vision are available, but they are not as well developed. Although there exist excellent techniques for assessing vestibular sensitivity, the integrated function of the components of the ambient system has not been extensively investigated. Quantitative evaluation of body sway has shown considerable promise in clinical diagnosis and represents a potentially powerful methodology for assessing task performance (Dichgans and Brandt, 1978). Individual differences in illusory self-motion (vection) and induced tilt are marked, but their origin and significance are unknown. Sensitive measures of optokinetic nystagmus are in extensive clinical use, but the visual parameters have not been studied in detail. Questions such as the relative contribution of various areas of the visual field and the role of spatial frequency, contour extent, and contrast remain to be answered. In many respects, then, the ambient system and in particular its visual component represents an uncharted frontier with important implications for psychophysics, medicine, and human engineering. In evaluating the role of vision in adjustment, tests of focal function are indispensable but incomplete. Tests of spatial orientation are wanted and wanting.

Copyright © National Academy of Sciences.
Bookshelf ID: NBK219039

Views

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...