'Lucy'
and The Arc of Visual Perception
Introduction
It is now generally believed that our primate ancestors living over
three million years ago walked upright. The main factors that have
influenced this belief are the discovery of the ‘Lucy’ fossils at Afar
in Ethiopia, together with a set of footprints in hardened volcanic
ash in Laetoli in Tanzania. Both of these have been dated as coming
from a similar period of between 3 and 4 million years ago. Studies
of these and other fossils have been interpreted as showing that a
fully upright, bipedal locomotion was practised by primates at this
time, leading further to the suggestion that these primates lived by
foraging, hunting and scavenging on the open Savannah. After the discovery
at Afar these fossil remains were identified as being a separate species
and named Australopithecus Afarensis and were suggested as being from
our ancestral line.
There is however one fundamentally simple, but vitally important,
characteristic that I believe has not been considered in presenting
this creature as having a permanent upright posture. This is the visual
‘arc of perception’.
Of all the senses vision is by far the most important, commanding
70% of all the sensory receptors of the human body. It is the main
source of information from the external environment and it is suggested
that 90% of the information stored in the brain is of visual origin.
It is therefore the main influence in our relationship with the natural
world in controlling our actions in it and our reactions to it.
As with all our senses and capabilities this has developed
for good reason. Nature, through the process of Natural
Selection, does not waste resources creating an ability that is not
utilised to the full at some stage. The development of this sense
therefore has had a significant influence on the adaptation of man
to the natural environment and consequently on the evolutionary progression
to modern man.
The Arc of Visual Perception
In humans the clear focus of the eyes
is a cone of a few degrees in width. However the total
‘cone’ of visual perception is, in most people, very wide.
This cone of perception, or rather the combined cones of
each eye, covers a total arc of 180° - 200° in a horizontal
plane and up to about 145° in the vertical plane (see Diagrams below).
This ‘cone’ of visual perception is the total external area from which
rays of light are received, through the cornea and the lens, by the
retina of the eye.
 
To demonstrate the wide horizontal arc of perception, consider a person
standing in the middle of the halfway line of an empty
football field focusing his eyes straight ahead at the goal posts.
If a second person on the sideline to his right or left, say 50 metres
away, walks from a position behind him and moves into a position on
the halfway line, at about 90° to his line of vision. The eye of the
first person will then detect this movement. In other words, while
the focus of the eyes is at right angles to this image, the brain has
received a visual signal through the eyes and is aware of the incursion
into this space.
The natural instinct would then be to turn and focus on
this perceived movement to establish what this object is
and whether it is a potential danger or otherwise worthy of interest.
The total vertical arc of vision is not as large as the horizontal
arc, the upper periphery being restricted by the eyelids and lashes
and the eyebrows, and the lower to a lesser extent by the cheekbones.
This normal vertical arc of visual perception, with the eyes at horizontal
focus, includes the surface of the ground from about half a metre ahead,
or about one walking pace.
As with all our abilities this visual ability, or sensitivity, has
evolved to this extent for a reason. For example if out walking in
easy, flat natural terrain, we become aware of an obstruction 20 or
so metres ahead, right in our chosen path, such as a small rock. Our
eyes focus on this object momentarily, and the fact that this is in
our path and is a potential danger is mentally registered. This is
normally done quite unconsciously. Once this potential danger is noted,
we no longer need to keep our eyes focused on this object alone as
our brain has worked out roughly how many steps it will take us to
reach it. Our brain via our visual senses maintains an indirect ‘watch’
on this object and reminds us when nearing it, to refocus on it, if
necessary. Then, again if it is necessary, it reprograms the co-ordinates
and adjusts our pace or direction to avoid stepping on it.
In other words through the visual cortex the brain can track any object
within the wide arc of perception of the eyes, without having the eyes
focus directly on it. If it has been noted to be of interest in some
way then a mental ‘watch’ is kept on this object and, when time or
circumstance permits, the focus may be returned to check up on it.
While such an object may not be in focus, it is however still visible
right up to the periphery of the cone of visual perception, or up to
the point where no light from it reaches the eye.
An obstacle such as the rock on the ground in our path
is sensed by the eye and the image of it monitored by the
brain, without it being in the main area of focus of the eyes, right
up to the point where we would actually step on it.
Vision and Co-ordination
When we are in motion the sense of vision
is by far the most influential source of external information
for the co-ordinational processes leading to any actual movement.
Co-ordination, with particular reference to human locomotion,
I will here define as the process of the movement of the
limbs and the body initiated by the contraction or relaxation of
various muscles, responding to signals sent by the brain, itself
reacting to external stimuli received by the senses and also, as
discussed later, some internal stimuli. In other words the sequence
of sensory stimulation, to mental analysis and reaction, on to physical
reactions, or movement.
When in motion we unconsciously make numerous, simultaneous co-ordinational
decisions, and often change them as quickly due to circumstance, amongst
many other things as to actual placement of the feet. These actions
may result from the innumerable visual and other sensory stimuli reaching
the brain from all parts of the body and the brain is continually assessing
this information and either immediately acting on it or storing it
or ignoring it.
With respect to the visual senses, continuous light images stimulate
the retina of the eye, which refines these and signals on to the visual
part of the sensory cortex. Computation and assessment by the brain
of any relevant sensory information then takes place. When moving,
decisions made are then passed on as instructions to the motor cortex
and thence via the spinal cord to the motor neurons activating the
relevant muscles of the body involved in the programmed movement.
Most physically fit and well co-ordinated people are able to avoid
an obstacle, such as the rock mentioned earlier, completely unconsciously
and automatically. Those people with poorly trained co-ordination or
subnormal eyesight may need to refocus on such a simple, small object
or feature, perhaps more than once, and in particular when it is very
close. On the other hand those with well trained co-ordination skills
such as Cross Country Runners or Orienteers who are used to running
in these circumstances are able to automatically adjust pace and/or
direction to avoid such an obstacle without at any time refocusing
on it.
Vision and Foot Placement Programming
When in motion a bipedal human
is continually viewing the ground surface ahead and unconsciously
programming the placement of feet some distance ahead. Of course in
the modern urban environment most surfaces are manufactured to be as
secure, even and flat as possible so that it is possible to walk in
many situations without taking much notice of where the feet are to
be placed. The visual concentration can accordingly be mainly on other
aspects of the local environment through which we move.
When on the move in natural terrain however the concentration and
visual focus, for this aspect of locomotion, is dependent on the surface
conditions. For example when walking along the coastline there can
be beaches of firmly packed flat sand on which it is possible to walk
as on a city pavement. Where the foreshore is comprised of, say of
a jumble of loose, smooth surfaced stones or pebbles of varying shapes
and dimensions of up to 200mm or so, some of which are not firmly positioned
and may move when stepped upon, it is a completely different matter.
In this situation it is necessary to visually concentrate on the position
of each foot placement and also to feel our way, testing with our feet
some rocks which our eyes tell us may be insecurely based. Some poorly
co-ordinated, unfit, sedentary people may be fearful of attempting
such a traverse while on the other hand some athletes such as Orienteers
may be able to run in these conditions.
The maximum potential speed of motion would clearly depend on the
conditions and the fitness and the co-ordinational abilities of the
individual.
In the first case of firm flat sand it would be possible
to run at our maximum individual speed, in the second,
for most people, it would be dangerous to proceed other than at a
slow walk. The reason for this is the need to concentrate on foot
placement immediately ahead.
For those with poor co-ordination such concentration and
focus would be on each individual foot placement one by
one, coupled with ‘feeling’ for the stability of each stone with
the tactile and positional senses of the foot.
For those with well-trained co-ordination, whether walking or running
and dependent on the degree of difficulty, the focus would be on foot
placement perhaps four to five paces ahead. The brain via the visual
cortex programs or co-ordinates such foot placements and for these
four or five paces the actual, subsequent positioning of each foot
on each stone is not normally visually verified or checked with the
full focus of the eyes. These placements are, in these circumstances,
of necessity irregular. In other words the locomotion may be by means
of a sequence of short steps, leaps, skips etc., all possibly involving
deviations to the overall course or direction.
This means that this co-ordinated movement of the limbs is planned
and pre-programmed into short-term memory. The visual senses utilising
the peripheral out of focus vision of the eyes, are monitoring these
immediate foot placement positions and adjusting them where necessary.
To examine how this works in a practical sense we can use a simple
example of crossing a road. You are on a pavement and decide to cross
the road to the other side. Your gaze sweeps across the scene and you
assess a route and decide on it. In doing so you absorb a vast amount
of visual information, much of which you have not directly focused
on, are unconscious of and could not later describe even if you were
asked to. Some of this information includes, say, the positions of
a lamppost, a parking meter, the kerb, a stormwater drain, the condition
of the pavement and the road, parked vehicles, people etc. etc.
Having planned your route, your brain then gives the signals or instructions
to activate numerous muscles that result in your walking to the kerb,
perhaps changing direction to avoid the meter and lamppost. Then to
stepping down onto the road, avoiding the drain and walking over the
road in a direction to avoid cars parked on the other side.
Generally, once you have swept the scene visually, you do not need
to focus on the obstacles as you come to them. For example you have,
unconsciously, noted the position of the kerb in relation to your own
position and you have judged its height above the road surface. So
in negotiating this particular obstacle you will not normally lower
your gaze and focus on it to double-check on these factors. This is
partly due to the initial assessment and judgement and partly because
they continue to be monitored by your brain via the visual cortex.
This can be demonstrated by the following. While you step towards
the kerb and stand on it, your focus is horizontal looking for traffic.
In the gutter there is an object that was obscured in your initial
visual sweep. While you are stepping onto the edge of the kerb, the
object moves. Your attention is immediately drawn to this movement
and your instinct makes you look down and focus on this object.
It is clear therefore that, without your being aware of it, your brain
is monitoring this area of your total arc of vision.
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