'Lucy' and The Arc of Visual Perception

Visual Arc of Perception of A.Afarensis

In his book subsequent to the discovery of the Lucy fossils Donald Johanson shows a profile of what an upright A. Afarensis would look like and life-size models of this are exhibited standing upright in museums, amongst others the American Natural History Museum.

Comparison of the arc of visual perception of this model, or reconstruction of A. Afarensis with that of modern humans shows a significant difference.

If we consider that the facial plane is delineated between the surface of the skull at the centre of the forehead and the surface of the front teeth, (ignoring the projection of the modern human nose). Then modern man has eyes set in the head which at their normal, comfortable inclination are focused at about 90° to the plane of the face. In other words if a modern man is standing upright looking forward in a comfortable stance, his facial plane is vertical to the ground and his eyes focus into the distance in a direction which is parallel to the ground. His vertical ‘arc of visual perception’ includes the ground from about a half metre ahead of his feet. (See diagram below).

The Vertical Arc of Visual Perception of Modern Man

In the A. Afarensis model (see diagram below) the eyes are similarly positioned with a focus forward and horizontal to the ground, however facial plane itself is inclined at about 30° to the vertical. In this model the eyes are therefore presented as being focussed at an angle of about 60° to the facial plane. Whilst in this reconstruction the horizontal visual arc of perception would appear to be similar to that of modern man, the total vertical arc appears to be less than 115°.

If this attitude of the head were maintained, the visual arc downwards would be severely restricted due to the width and protrusion of the jaw. Accordingly, if we project a line from the eye downwards past the surface of the nose and jaws. We can calculate that, depending on the height of the creature, the downward vision immediately forward would be restricted so that the ground up to one and a half metres immediately ahead would be out of the vertical visual arc of perception.

Alternatively for A. Afarensis to have it’s visual arc covering the ground from about one pace in front of it, it would need to hold it’s head so that the attitude of it’s facial plane would be as near vertical as possible. It would accordingly have its chin resting on its chest. If the normal stance of A. Afarensis were as depicted then such an abnormal position would have been very uncomfortable to maintain indefinitely. After a few minutes, the strain on the muscles of the neck would be severe, as we can easily test by personal experiment on ourselves. We can therefore only assume that, if A. Afarensis was upright, then the attitude of the head in motion was as depicted and that accordingly it’s normal arc of vision did not cover the ground ahead for nearly three full walking paces. (See diagram below)

The Vertical Arc of Visual Perception of A.Afarensis

Note: In these diagrams the height of A. Afarensis is taken as the suggested 4’ 8” or 1.48 metres for an adult male and that of modern man to be 5’ 9” or 1.8 metres. The length of pace of A. Afarensis has been scaled down to 45cm compared to that of a man of 1.8 metres who would have a walking pace of about
50 - 55cm.

Inclining your head backwards so that your facial plane is about 30° to the vertical and walking over a rough and uneven ground surface would give some idea of the effect of this visual restriction. It is clear that such a visual restriction would have a serious effect on the bipedal co-ordination of A.Afarensis in its suggested posture. To examine to what effect this would have had in practice it is necessary to look at the co-ordination of man today for a comparison.

The Senses, Memory and Co-ordination

Of the five senses vision is the main one involved in the co-ordination of movement, and while touch, smell and hearing can also influence movement, taste is not generally involved.

If a sense is recognised by the fact that it receives external signals, then there is in fact a sixth sense, the sense of balance, which is influenced by the force of gravity. This of course is a vital factor controlling movement.

There are however some additional ‘internal senses’ which have a major role in co-ordination. One is the mental perception of the position of the limbs relative to the rest of the body, the ‘proprioceptive’ sense.

A practical example of the use of this would be a gymnast on a single bar attempting a forward somersault. In this exercise without such a positional sense, accurate placement of the feet on the bar would not be possible.

Memory, or experience, also has a direct effect on co-ordination and a simple example of this would be as follows.

When out running in open grassland we observe that the colour of the grass ahead of us changes to a deeper green. Experience tells us that this normally means that the ground there contains more moisture than that at our present position. This visual signal and its stimulation of memory may trigger an automatic reaction of altering course to avoid this potentially difficult area.

This whole process of stimulus, assessment and analysis of both the visual signals and the triggered memory and the subsequent physical reaction may well be done completely unconsciously, while the conscious mind is focusing on other more important matters.

The memory that influences our actions can be from an experience that occurred at any time in our past up to the very short term. This includes memory or experience such as an awareness of our present physical condition and our physical and co-ordinational capabilities.

Co-ordination of Physical Actions and Reactions

When on the move all the external and internal senses referred to plus memory can simultaneously be involved in co-ordination. For instance immediate information from the visual and tactile senses can be combined and referred to balance and experience/memory and result in a physical reaction. These mental processes are therefore highly complex and involved using and connecting different parts of the brain and, as we will see later, are extremely fast. How complex and fast can be demonstrated by looking at a situation faced by an Orienteer running in natural terrain.

Orienteering courses vary in length but these can be 8 to 10 Kilometres or more. Courses are marked out on a specially produced, large scale and detailed contour map. A number of ‘controls’ or markers are set out on the ground at specific physical features as shown on the map. The object is to navigate to each of these controls and return to the start as quickly as possible. As the fastest wins, for the top competitors, this means travelling as fast as the local conditions allow. This normally means running, apart from where there are exceptionally difficult conditions.

Therefore we will consider the circumstances faced by a good orienteer over a short distance in a competition and then analyse some of these actions and reactions in detail. The following is a description of just a few paces of the progression of a good orienteer around a course.

The Orienteer is running in forest and the ground is from immediate prior experience, firm and even and the pace and footfall adjusted accordingly to allow for a hard landing, or impact, of the feet and for good purchase for propulsion. The visual focus is on overall long-range navigation and is aimed well ahead on the features in the way and on significant features to either side.

The Orienteer at this moment has, as expected, found a good firm base with his right foot, confirmed by the tactile sense of the sole of the foot on meeting the ground. Some of the muscles of the leg are accordingly contracting, propelling the body upwards and forwards. The left leg having completed its propulsion action is in the air moving forward. The position of the next placement of the left foot has been mentally programmed previously and as it descends at speed to the ground a firm sensory reaction at a specific point in the descent of the foot is expected.

The ground however is not firm at this point and in fact is very soft. As the foot descends into this soft ground to just 15 or 20 mm below the surface at least two warning ‘messages’ are sent to the brain. One through the ‘positional’ sense in effect says, “The foot has gone past the point where the ground should be.” The second, from the tactile sense of the sole of the foot through the shoe says, “The ground is not firm.”
By this time the right foot has already left the ground and is in the air moving forward for the planned next pace.

Of the numerous mental and physical actions and reactions that occur at this instant the main ones are as follows.

The muscles of the left foot and leg are instructed to relax, to cancel the planned positional contractions for propulsion and to allow the foot to feel for a sound base under the soft surface.

The right leg is simultaneously instructed to shorten its pace from the planned long one and to stamp down as quickly as possible. This is in order to provide a more immediate support for the body and to be able to relieve the potential stress on left leg and foot if the solid base under the soft surface is found to be awkward or unsound.

At the same time the head and the eyes snap down from their focus further ahead to focus on the surface immediately in front. Firstly, if possible, to view the new placement position of the right foot and then to reprogram the subsequent placement of both feet.

It transpires that the soft ground under the left foot conceals a hard rigid object. As the foot descends a further 25mm into the soft ground it contacts the rigid object, perhaps a root or a stone, with the outer left front part of the sole. The right foot meanwhile has not yet descended to the ground to provide alternative support for the body. The left foot is, as stated, feeling for a base and the muscles are relaxed. The foot, ankle and leg cannot however sustain the stress involved in such an offset support position. So again the almost instantaneous reaction is for the left ankle and foot muscles to flex and allow the foot to cant to one side. The angle through which the ankle can rotate laterally is fairly limited and this limit is reached quickly in the continuing descent of the left foot. Once this angle is reached and stress on the ankle becomes severe, as indicated by pain signals to the brain, this initiates a reaction of moving the knee to the right to allow the lower leg itself to cant to that side.

Further if this action is insufficient to relieve the stress both on the ankle and the leg, it will need to be followed by the transfer of weight by, and an inclination of, the torso to the right. This transfer is better described as ‘throwing’ the weight of the body to one side as it is an extremely fast instinctive movement involving the muscles of the thighs and the spine and lower torso.

By this time the instructions to the right leg will have been to stamp down more urgently, but at the same time to be prepared for an unsure footing as this placement position has not been programmed. However if this is not possible to do this in time to relieve the left limb, and/or the stimulus is severe enough, the result may well be to continue with ‘throwing’ the torso to the right and consequently allowing a ‘planned’ fall in this direction. This fall would be a case of accepting the lesser of the two evils, in terms of potential injury.

All these co-ordinated actions and reactions are designed to prevent the left foot and leg, in these circumstances, from being subjected to stress of a magnitude sufficient to cause injury. Of course any planned fall would also be co-ordinated, involving the visual senses, as far as possible to avoid injury. Injuries may still be the result if the circumstances are unfavourable and/or the fall cannot be adequately controlled, but they may well be less serious than a broken ankle or leg.

It is clear that all of these sensory, programming and muscular actions and reactions take place simultaneously and at an incredible speed and that these are just a small part of the innumerable mental–physical signalling processes taking place concurrently.

All these various signals are being sensed, transmitted to the brain, received by it, processed by it, decisions made transmitted to the muscles, received by them and acted on, within minute fractions of a second. And this is not all, these processes are carried out on top of and concurrently with, innumerable other decisions as to foot placement two or three or more paces ahead, route choice, etc. etc. etc. This indicates that the eyes have to be able to register numerous factors simultaneously and means that the focus clearly cannot be on all these at one time. The brain is therefore controlling movement on the basis of images from all parts of the total cone of visual perception and doing so with astounding speed and organisation. This is a highly developed capability, requiring fast transmission of a huge amount information from the sensory areas to the brain, processing it and referring to stored experience, and signalling back to the appropriate muscles. The muscles in use when running in these circumstances include the majority of the 600 main body muscles.

This is an extremely complex task in combining well-developed and fast mental programming and acute senses with excellent balance and fast, precise muscular reactions.

The speed of human physical reaction to stimulus in these circumstances can be roughly calculated by relating the velocity of the runner over the ground to the distance of the foot movement referred to earlier. A reaction instigated by 30mm of movement by the foot at a modest velocity over the ground of 4 metres a second would give a physical reaction time measured from stimulus to muscular contraction of about 0.007 seconds.

It is facile to compare the enormous power of the human brain to a computer. However to attempt to put this in some sort of perspective, no computer, if given just a fraction of the co-ordinational problem described above would, I suggest, be capable of solving it and issuing instructions within the necessary time constraints.

I suggest that the overall mental capacity necessary for such advanced bipedal co-ordination is not generally acknowledged and is well underestimated. This may well be due to the fact that in modern man this highly developed mental capability is generally not utilised.

Hominid Bipedal Motion

It is generally assumed that we evolved either from a quadrupedal or knuckle walking, forest floor dwelling ape such as the gorilla or from a tree dwelling ape such as the gibbon. The differences in the shape, dimension and the alignment of the skeletal bones between ape and man are significant. The progression therefore to a permanently upright posture would be governed by, amongst other things, the need for a substantial alteration in skeletal structure. It is clear that the process of natural selection leading to these changes would take a considerable length of time.

Before examining this progression however it is important to differentiate between an occasional bipedal capability and a permanently and fully erect bipedal locomotion.

Here we can look at our cousin the chimpanzee for clarification. The chimp is capable of standing almost upright and walking this way and it is clear that their sense of balance in this position is good. This locomotion however is ungainly for anatomical reasons and is not normally used and then only for short distances.

When in quadrupedal motion the facial plane is vertical and the placement of the rear feet is often positioned to either side of the front knuckle position.

It is quite evident that the reason for choosing this posture, apart from the anatomical, is for good visual coverage of the ground surface for both front and rear foot placements.

Accordingly therefore, while the chimp is capable of balanced upright motion, it normally chooses a quadrupedal locomotion.

As the example of the Chimpanzee shows, any hominid in the initial stages of the progression to fully upright would be capable of moving in a near erect posture. This however would not normally be used except perhaps at a slow walk in a well known environment, when the tactile sense of the soles of the feet, more than the visual sense, would be used to test the ground surface for obstacles and secure placement.
However when the necessity occurred for a perambulation of any distance, in unfamiliar terrain or at any velocity, for good vision it would be imperative to angle the facial plane at or about vertical.

When in quadrupedal motion the attitude of the upper spine of a Chimpanzee is approximately horizontal to the ground. The attitude of the head on the spine is such that the facial plane is about vertical to the ground, or at an angle of 90° to the spine. The attitude of the facial plane of modern man is also about vertical to the ground as is now the upper spine. Thus, in the evolution of quadrupedal ape to fully bipedal man today, the spine has moved through an arc of about 90° while the facial plane has remained vertical.

Therefore if we evolved from a similar creature then this progression would involve a gradual change in the attitude of the top section of the spine, in relation to the ground surface, through an arc of about 90° to vertical. It would also involve a gradual change in the angle that the facial plane subtends with the spine from about 90° to parallel with it.

Next >

Home >

Copyright Romun Press 2006. All rights reserved. Website by A-line Graphics.