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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
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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
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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.
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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.
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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
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