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OCR for page 25
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please TWO MODES OF VISUAL PROCESSING 25 Regan, 1975, 1980) and physiological data (Regan and Cynader, 1982) suggest the existence of specialized visual mechanisms sensitive to object movement in depth. Perimetric analysis of sensitivity to motion in depth (Beverly and Regan, 1983) reveals large individual differences and considerable heterogeneity. Some individuals are unable to discriminate an expanding retinal image fromSome individuals are unable to discriminate an expanding retinal image froma a lateral motion, while others are highly sensitive to the difference. Because of these individual differences and because this sensitivity to depth may be highly correlated with a pilot's flying ability (see below), the working group concludes that this area should be more extensively investigated. In one study, pilots were measured on their sensitivity to motion in depth (Kruk et al., 1981). The pilots viewed a square on a display screen that randomly expanded or contracted in size, simulating motion in depth. The pilots were instructed to keep the size of the square as constant as possible using a control that counteracted the size change. Accuracy in performing this task was taken as an index of depth sensitivity. The pilots were also tested on an Air Force flight simulator on flying tasks such as landing in fog, formation flying, and low-level bombing accuracy under counterattack. Pilots who performed well on the depth-tracking task also performed well on the flying tasks. In another study, sensitivity to depth motion was found to correlate well with flying performance in an actual airplane that was tracked by means of telemetry (Kruk and Began, 1983). Conclusions and Recommendations The working group concludes that the depth motion tracking task has potential for screening pilots and others involved in precise visual-motor tasks and that its potential should be further developed and explored. Like measures of dynamic visual acuity, dynamic depth tracking involves both the visual system and the oculomotor system. Superior performance on both of these tasks probably involves the integration of these two systems. The working group believes that the existence of a rather simple dynamic tracking task, which is rather easy to administer and which predicts the ability to perform complex flight tasks, would be a significant development in the assessment of vision. The working group recommends that further studies of this technique be undertaken. These studies should focus on finding the optimal test conditions giving the highest predictability of actual flight performance as well as other visual-motor tasks such as vehicular driving. TWO MODES OF VISUAL PROCESSING use the print version of this publication as the authoritative version for attribution. 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
OCR for page 25
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please TWO MODES OF VISUAL PROCESSING 26 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. use the print version of this publication as the authoritative version for attribution.
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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please TWO MODES OF VISUAL PROCESSING 27 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. use the print version of this publication as the authoritative version for attribution.
OCR for page 25
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please TWO MODES OF VISUAL PROCESSING 28 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 use the print version of this publication as the authoritative version for attribution.