Visual Direction

Visual direction

 A Brief History 

The laws of visual direction describe a method by which the visual system estimates the visual directions of binocular targets. Alhazen, Wells, and Hering originally formulated this method. Ono revised the Laws in 1991 and Howard and Rogers again revised them in 1995. This method is known as “Hering’s Laws of Visual Direction” (1942). One of Hering’s laws, “The Law of Common Binocular Direction”, states that the directions derived from the two eyes’ images will be perceived as if the observer is viewing the scene from a single vantage point between the two eyes. This point is called the “Cyclopean Eye”. More recently, Map and Ono (1999) have gone as far as to assert that the cyclopean eye is both a logical and functional necessity for judging the direction of objects.

In 2000, C.J. Casper and Raymond van Ee showed from their experimental findings that the cyclopean concept could also be explained by angular information without the need for cyclopean eye. They suggested that binocular perception is incompatible with vision from a single vantage point and the concept of the cyclopean eye is “sometimes inappropriate and always irrelevant” as far as vision is concerned.

When we understand SSSRD and the cortex eye and how the two eyes coordinate, we can clearly observe that the comments made by all the previously mentioned people are not well founded, mainly due to those individuals not fully understanding the function of the cortex eye. It plays a very important function in vision science by creating the mathematical structure that allows depth perception to be judged. Previously illustrated, the vantage angle of each eye is retained from a single vantage point of the cortex eye. The purpose of the cortex eye is not for visual direction and it is certainly not ‘irrelevant’ as far as vision is concerned. As we have observed, the function of the cortex eye works in complete coordination with all the other functions in the visual process, a notion pervasively illustrated and explained throughout this book.

Figure 6.1 is an example of what Hering correctly, but unknowingly, observed in the context of SSSRD and the cortex eye. When we observe these targets in the context of SSSRD and the cortex eye, the black spot on the pane of glass is ‘F’ and it is aligned with the distant tree ‘D’ with only the left eye. ‘C’ is the house and it is aligned with ‘F’, the black spot, with only the right eye. ‘F’, ‘C’, and ‘D’ are each viewed as if aligned in a straight-ahead direction, when ‘F’ is the point of focus. Hering interpreted this observation as if the house and tree were viewed from a single location. The observation was correct but could not be logically explained. The only explanation for this observation can be derived by first gaining a complete understanding of SSSRD in conjunction with the cortex eye.

Figure 6.1: Hering’s observation, in the context of SSSRD and the cortex eye.

The house ‘C’ is located on the visual axis of the right eye, VR, farther than the fixation point. The tree ‘D’ is located on the visual axis of the left eye, VL1, farther than the fixation point, which is signified as ‘F’ in the diagram. Both VR and VL1 are the sections of the visual axis after they intersect and pass through the fixation point ‘F’. This is how the black spot, the house, and the tree, were all observed aligned by Hering. In SSSRD combined with the cyclopean eye, the rotation of the visual axes to a central position results in these three targets being viewed in a straight-ahead direction in the cortex eye. This occurs because the house and tree are selectively located on the visual axis of each eye.

We have observed and proved this process by natural observation, SSSRD, occlusion, and the function of the cortex eye. When we examine Hering’s observations in the context of SSSRD and the cortex eye, it will become clear that they are correct but misconstrued, as previously illustrated in the cortex eye as an observation farther than the fixation point. The cyclopean eye, as described by Hering, cannot be a law of common binocular visual direction.

‘F’ is the black spot on the pane of glass, ‘C’ is the tree, and ‘D’ represents the house. If we were driving from point ‘F’ to point ‘D’, we would not be driving in the same visual direction as point ‘C’. Similarly, if we were driving in the direction of point ‘C’, we would not be driving in the same direction as point ‘D’. In the cortex eye, these three points appear to be in the same straight-ahead direction; but they are not. The tree and the house are selectively positioned farther than the fixation point, on the visual axes of the right and left eye. When the two visual axes in the cortex eye rotate and become a single axis on pivot ‘F’, the black spot, the fixation point, ‘C’ and ‘D’, appear to be in a straight-ahead direction. However, these targets could be tens of metres apart in opposite directions and visually we never use a target farther than the fixation point to judge visual direction. 

As previously shown in the mathematical structure of depth perception, the purpose of the function of the cortex eye is principally for computing depth perception, not to direct visual direction. Targets in space are viewed out of position in the cortex eye and the reason for this is to create a mathematical structure by which depth and distance judgement is computed by the brain and not for the judgement of visual direction. The rotation of the visual axes in the cortex eye is tiny in the context of the entire visual field, in particular nearer than the fixation point. Farther than the fixation point, it can be very large, as already illustrated in figure 5.12. This proven fact rules out any possibility of the cortex eye playing any part in visual direction.

The object of our visual direction is always the fixation point, but targets farther than the fixation point are never the objects of our visual direction, even though targets are viewed out of position in the cortex eye. In isolation, they are still viewed at the exact vantage angle of each eye relative to the fixation point from a central location between the two eyes. The cortex eye is the eye the brain creates in the visual cortex and its location is the centre relative to the two eyes. It is vision from a single vantage point but retains the vantage angle direction of all targets viewed by each individual eye yet plays virtually no role in visual direction.

The rotation of the axes in the cortex eye, to form this single eye, does not interfere with visual direction or with the vantage-viewing angle of each eye being maintained, particularly in the largest retinal-section of the dominant eye, nearer than the fixation point. This is achieved by the way the overlap in the cortex eye occurs. All targets in the area of section A, which overlaps section B, are still viewed in the visual direction of the dominant eye. Consequently, the dominant retinal-section is always the largest retinal-section, nearer than the fixation point. It is always constant and it is from this largest retinal dominant sectional area that our absolute visual direction is judged. Without the dominant eye and the way it is structured as an integral part of the visual system, we would be unable to perform tasks that require higher levels of visual accuracy.

The dominant eye

The coordination of SSSRD and the cortex eye logically explains another of the most profound problems that has been debated for many decades, visual direction. Similar to our approach to stereopsis, we followed a misguided path in trying to understand stereo-vision by observing random dot stereograms in the same way we followed a misguided path by trying to understand visual direction as a cyclopean phenomenon.

In 1903, Rosenbach discovered that everyone has a dominant eye, even though both eyes could have equal vision. This was a very significant discovery, the importance of which has never been fully understood or elaborated in the scientific sphere. Rosenbach used a simple sighting test, with both eyes open, the subjects were requested to aim at a distant object using their index finger. He discovered that most people preferred the image of one eye to that of the other. The dominant eye was identified by alternate occlusion, when viewing with the dominant eye, the index finger and the target are aligned, but when viewing with the other eye, the finger appeared offset.

Almost three decades later, Hillemanns confirmed the findings of Rosenbach in 1927. His study proved that approximately 40% of non-strabismus people were right-eye dominant, while approximately 20% were left-eye dominant, with the remaining 40% uncertain. Since then, many other analysts have substantially confirmed these results, including Coren & Kaplan (1993), Crider (1994) and Porac & Cohan (1996).

Using different methods, some of the most notable tests are the “the Freiburg Ocular Prevalence Test” and the “Haase Stereo-balance Test”. In 1994, Lang proposed that prevalence of one eye is due to partial suppression of the other eye, which renders double images on the border of Panum’s areas unremarkable. What is very surprising about these results is that the positive aspects of having a dominant eye are largely ignored. In fact, Haase’s assertion was that ocular prevalence can be and should be eliminated by phoria-correcting prisms, in patients who suffer from eyestrain. Lang suggested that ocular prevalence may be due to partial suppression of one eye and this helps to disregard double images at stereo-disparities close to the limits of Panum’s area.

As important as the dominant eye is, nobody has ever defined its extent and boundaries, or explained the principal reason as to why it is dominant. Through SSSRD, we can completely understand how and why the dominant eye is dominant. The dominant eye controls the largest retinal-section nearer than the fixation point. For a right-eye dominant person, this retinal-section is section A1 and it extends from the horizontal boundary G to the retinal-section boundary VL (the visual axis of the left eye nearer than the fixation point). I only became aware of the importance of the dominant eye during my research in separate sectional suppression and retinal dominance, in particular when examining its role in visual direction. The cortex eye plays a limited role in angular visual direction in the sense that it creates different disparities at different angular views, but it plays no role in absolute visual direction: the dominant eye plays the lead role in absolute visual direction.

As illustrated in SSSRD and the cortex eye, in three-dimensional space the vantage angle views of each eye’s direction are retained. Targets aligned with the left eye in section B with targets in section B1, retain their vantage angle and alignment relative to ‘F’ in the cortex eye. Likewise, targets aligned with the right eye in section A with targets in section A1, retain their vantage angle and alignment relative to ‘F’ in the cortex eye. However, the dominant eye controls the largest retinal-section, section A. Not only is it the largest retinal-section, but it controls the vital area which allows dominance to exist, and that area is the area that lies between the visual axes of the right and left eye, nearer than the fixation point. The left eye only controls this area when the person is left-eye dominant and as a result, direct and accurate visual direction in the cortex eye is only judged by one eye, and that eye is the dominant eye. All targets viewed in section A on both sides of the visual axis VR1, nearer than the fixation point of the dominant right eye, are aligned in the visual direction of that eye.

Targets are consistently viewed in the same visual direction in this area. For accurate alignment when aiming at a target (the fixation point), the object of aiming should be close to or on the visual axis of the dominant eye that is focused on the target. This instinctively occurs with left and right-eye dominant players of sports where intricate aiming is required, for example, in snooker, darts, and clay pigeon shooting, among others. Without a dominant eye, this would not be possible and we would have no visual accuracy or consistency in our visual direction.

As illustrated and reiterated throughout this book, the retinal-section of the dominant eye is the largest retinal-section nearer than the fixation point. The visual axis of the right eye nearer than the fixation point is located in this area and all targets on the left and right side of this axis are binocularly aligned with the target or fixation point. However, the dominant eye executes the alignment of these targets and, consequently, it dominates visual direction of practically all targets located nearer than the fixation point.

The target point in visual direction is always the fixation point. If we walk, run drive, or aim at any target in any sport, the fixation point is always the target point in visual direction. Similarly, most people’s actions are orientated from the side of the dominant eye. For example, most right-eye dominant people have a preference to use their right hand for writing and their right foot for kicking a football. Likewise, most left-eye dominant people have a preference to use their left hand and left foot in the same situations. Without a dominant eye, visual direction would be seriously impaired.

The dominant eye is not a condition that requires treatment, neither is it a mere byproduct of evolution, it is part of the actual evolution of the eye. The dominant eye, like the entire structure of the visual system, evolved over millions of years and as a result, it is integral with the fundamental mechanisms of SSSRD and the cortex eye. It is therefore a critical component of this structure and it is crucial that we understand its workings. In the cortex eye, part of the dominant retinal-section A overlaps section B, but the visual direction of all targets is retained in the overlap, so the function of the cortex eye in no way interferes with the visual direction of any target in the retinal-section of the dominant eye. Again, this is another example of the amazing coordination that is evident throughout the entire structure of the visual system.

To fully understand the importance of the dominant eye and the confines of its retinal-section, we must also understand how this retinal-section is incorporated in the function of the cortex eye. Figure 6.2A illustrates the retinal-section A, of a right-eye dominant person. It is the largest retinal-section nearer than the fixation point. It incorporates VR1, the visual axis of the dominant right eye, and it extends from the parallel focal plane at ‘G’ to the visual axis of the left eye, VL, nearer than the fixation point. It is the only retinal-section nearer than the fixation point, where targets can be aligned left and right of the fixation point with consistently accurate visual direction.

Figure 6.2: All targets in section A are in the in the visual direction of the right dominant eye

It is the only retinal-section nearer than the fixation point where targets can be aligned with the fixation point on the visual axis of the dominant right eye, without being viewed in double. The reason why this is so is that the visual axis of the dominant eye is not a retinal-section boundary, unlike the visual axis of the weaker eye, which is a retinal-section boundary.

When the function of the cortex eye occurs, the areas that lie between these two visual axes overlap, as evident in figure 6.2B. Consequently, this particular area of section A is overlapping with an area in section B when the two axes rotate to form a single axis. The overlap does not effect the dominance of the eye as the dominant right eye is still dominant in this overlap area, and the original vantage angle direction of all targets in figure 6.2A (in that entire retinal-section area) are accurately retained in figure 6.2B.

Visual direction is essential in order to allow us to correctly align two targets in space; that is to drive, to run, to walk, to aim at a target in space, or to point out a target or object in space for identification. In order to perform any of these tasks, we must have a true visual direction from point A to point B. Point A is the location of the observer and point B is the fixation point and the target of direction. For any animal to freely navigate with visual accuracy in space, to hunt and protect itself from other prey, they need a dominant eye, and without one, none of these feats would be possible. Targets farther than the fixation point play little or no part in visual direction, simply because the fixation point is always the target of our visual direction.

For a right-eye dominant person, section A (the retinal-section of the right eye) is the largest retinal-section nearer than the fixation point. Section B (the retinal-section of the weaker left eye) is the smallest retinal-section nearer than the fixation point. For this reason, the so-called ‘weaker eye’ cannot be the dominant eye. However, it is always dominant in it’s own retinal-section, in the same way the right and left eye are dominant in sections A1 and B1, farther than the fixation point. It is however, the dominance of the right eye in section A, which controls visual direction for a right-eye dominant person. We also have to remember that each eye is dominant to a lesser degree in as far as that the dominance is not noticed in all the other retinal-sections. The following illustrations help to explain the reasons for why this is so.   

Figure 6.3: Target in the visual direction of the left and right eyes.

In figure 6.3A, target 1 and target 2 are viewed ‘D1’ distance left, and right of ‘F’. In the cortex eye, in figure 6.3B, the targets are still viewed the same ‘D1’ distance left and right of the fixation point ‘F’. The left and right eyes view the two targets in the same visual direction relative to the fixation point ‘F’, before and after the function of the cortex eye. However, the two targets are viewed in different directions, not in a common direction.

The visual direction for a right-eye dominant person is always a target in section A of the dominant right eye. The reason for this is that we can only have one visual direction. The visual direction of the weaker left eye is very limited because its small retinal-section only extends to the visual axis of the left eye, VL. Targets are accurately aligned with the fixation point on or close to the visual axis VR1 of the dominant eye and this cannot occur on VL, because it is a retinal-section boundary, where targets are viewed in double or offset with the fixation point. 

Figure 6.4: Targets in the visual direction of the left eye, in section B/B1; targets in the visual direction of the right eye, in section A/A1.

Target 1 and target 4 are aligned with the left eye D1, ‘D2’ distance left, and right of ‘F’, in sections B and B1. Target 2 and target 3 are aligned with the right eye D1, ‘D2’ distance right, and left of ‘F’, in sections A and A1, in figure 6.4A. In the cortex eye, in figure 6.4B, the targets are still viewed the same ‘D1’ and ‘D2’ distance left and right of the fixation point ‘F’ with each eye. Target 1 and target 4 are aligned with the left eye D1, ‘D2’ distance left, and right of ‘F’, in sections B and B1 and are in the visual direction of the left eye. Target 2 and target 3 aligned with the right eye D1, ‘D2’ distance right, and left of ‘F’, in sections A and A1, and they are in the visual direction of the right eye. The left or right eye controls visual direction in all the retinal-section divisions, including the two monocular areas. In SSSRD and the cortex eye, targets in every retinal-section have the visual direction of the eye that they are viewed with, but as already stated, targets farther than the fixation point only play passive roles in visual direction. The dominant eye judges absolute visual direction.

A limited role is played by visual direction in section B of the left eye. On rare occasions, when it does play a role in visual direction (observed later in this chapter), it often causes confusion. The weaker eye as a result is the eye most prone to attack from a flying missile, be it a fly or piece of mud, and in particular if the missile comes from the dominant side. The dominant eye is the largest retinal-section and controls left and right of its visual axes and it always has more time to react. If both eyes controlled visual direction, then we would have constant confusion and visual inaccuracy: an unsolvable problem, similar to the theory of retinal correspondence.

Figure 6.5: Targets aligned straight in front of the eyes, not in the same visual direction.

The purpose of this figure is to dismiss those comments made over the years, including those made by Hering, that inferred that visual direction is in the so called ‘cyclopean eye’ and that it is the average of the two monocular visual directions of the two eyes. Target 1 is viewed ‘D1’ distance right of ‘F’ and ‘D1’ distance left of ‘F’ with both the left and the right eye. Therefore, it is in the average straight-ahead direction of the two monocular views, when with the focus on ‘F’ in figure 6.5A. However, target 1 is not viewed in a straight-ahead direction when ‘F’ is binocularly focused upon.

In the cortex eye in figure 6.5B, it is evident that it is viewed to be in a direction ‘D1’ distance left of ‘F’ with the dominant right eye. Target 1 is still viewed in the same visual direction with the dominant right eye only. This illustration also proves that visual direction of a target is not the average of the two eyes. Target 1 is in a straight-ahead visual direction with ‘F’, using the average of the two eyes, but it is not in a straight-ahead direction in the cortex eye. It is viewed only in the visual direction of the dominant right eye and the object of visual direction is always the fixation point. In figure 6.5, target 1 is a perfect example of why visual direction is not the average monocular location of objects in space. Target 1 is not on a visual axis; it is located between the two axes. Yet, its visual direction is in the view of the dominant right eye with the fixation point. This holds for all targets that lie between the visual axes of the left and right eyes, nearer than the fixation point. The three principle reasons why section A is the dominant area of the dominant eye are:

·    It is the largest retinal-section nearer than the fixation point.

·    It is the only retinal-section nearer than the fixation point in which the visual axis of the dominant eye is not a retinal boundary.

·    It is the only retinal-section nearer than the fixation point in which targets on the visual axis, or on either side of the visual axis, can only be viewed in the same visual direction in line with the fixation point.

Hering described in the ‘Law of Identical Visual Direction’ that all objects on the path of the chief rays to the fovea of the two eyes, appear to be in the same primary visual direction. He maintained that this relationship holds over the entire visual field for all points that occupy the central portion of the retinas. In other words, Hering is saying that all objects on the visual axes of both the left and right eyes appear to be in the same visual direction. It is true that targets appear to be in the same visual direction when they are located on the visual axes of both eyes, nearer or farther than the fixation point, but this is not the entire visual field. In fact, the visual axes are only a tiny part of the visual field. The cortex eye can generally not judge visual direction without causing total confusion. In the context of SSSRD and the cortex eye, there are numerous other constraints, as previously illustrated in this book. For example, no target can be correctly aligned with the fixation point on the visual axis of the left eye, nearer than the fixation point, because this is a retinal-section boundary.

No target can be correctly aligned with the fixation point on the visual axis of the right eye, when it is located farther than the fixation point, because this axis is also a retinal-section boundary. As already illustrated in describing SSSRD and the function of the cortex eye, the vantage angle view of each eye before and after the function of the cortex eye, is retained in the single image. The object of visual direction is always the fixation point or a target left or right of the fixation point. The dominant eye can only judge the visual direction to such a target because it can accurately judge the visual direction of objects moving left or right of the fixation point. As such, no point can be aligned with the fixation point with the average direction of both eyes. Any point aligned with the fixation point nearer than the fixation point is viewed only by the visual direction of the dominant eye.

Observers:

All visual direction tests consisted of simulated models of each individual illustration and the same five right-eye dominant observers participated in each test. Great care was taken to ensure that all tests were performed in optimal light conditions, in three-dimensional space.

Results:

All five observers corroborated that for a right eye dominant person, section A is the largest retinal-section in the visual field, and it includes the visual axis of the right eye and extends to the visual axis of they left eye. Consequently, this sectional area controls visual direction. They also confirmed that each eye is also dominant in its own particular sectional area. The significance of this observation is that practically all our visual direction is judged by the dominant retinal-section A of the right eye. A very significant observation confirmed by all observers, was the change in the focal plane that occurs due to change in the visual angle of the eyes. This change also causes a perceived change in the distances of existing targets, even when the head is in a static position. Also confirmed, this perceived change was born from a perceived depth judgement to the fixation point and focal plane, and is mathematically accounted for by using the same formula.