Why Do Humans Have two Front-Facing Eyes? An Analysis and Some New Ideas.

"Eyes in the front, the animal hunts. Eyes on the side, the animal hides."


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Humans are primates and all primates have two front-facing eyes.  Why?  

Currently, the explanation goes like this: The binocular vision provides stereoscopic or three-dimensional (3D) view that helps to locate and pin-point objects more precisely.  This is good if you are hunter/predator; and it helps in arboreal living - gives ability to swing and jump more accurately between tree branches - good if you live in trees. 

Early primates were indeed tree dwellers; and besides finding insects to eat, they ate plant leaves and fruits.  None of the primates were predators in the usual sense of the term. Chimps are the closest relatives of humans; and among the great apes, only humans and chipms eat meat but only infrequently.  Traditional human societies appear to have relied more heavily on plant-based diets. 

So one might think that more likely front-facing eyes evolved to help in arboreal living - living in trees helped early primates to stay safe from predators and also allowed easy access to tree leaves and fruits.  The problem with these theories is that the earliest primates were actually nocturnal and relied on smell more than vision.  But see also.

For many millions of years, great apes have lived mostly on ground and have not been hunters.  I would think that input from the front facing eyes serves a more fundamental purpose and is not necessarily wholly related to predation or arboreal living. I base this suggestion on the fact that in the human brain, neurons devoted to visual processing number in hundreds of millions and take up about 30% of the cortex - as compared with 8% for touch and just 3% for hearing.  Each of the two optic nerves, which carry signals from retina to the brain, consists of a million fibres; each auditory nerve carries a mere 30,000.  It would be unusual for evolution to invest so much energy in visual processing if it only helped humans to carry out relatively minor activities of hunting and tree dwelling. 

So, why do primates have front facing eyes?  In this publication, I want to examine this question in more detail - (1) by looking at the evolution tree of primates, (2) I shall discuss the pros/cons of having front-facing eyes against eyes on the sides of the head. Lastly, (3) I shall argue that the complex society/environment, that humans have been living for the past million years or so, requires the most elaborate vision system and a good part of the 'new brain' was earmarked for processing the 'superior' visual signals possible by having two front-facing eyes.  

(1) Primates: Let us look at the evolution of primates that goes back to some 60 million years.  The next two slides summarise the evolution tree of primates:
(please click on a slide to see full page image; press Escape to return to main text)

Let us start with Prosimians - the first primates. Prosimians are nocturnal, have large eyes with a tapetal (retro-reflecting) layer behind the retina to help night vision, but their eyes are not as well positioned for 3D vision as are the eyes of other primates. They have well-developed sense of smell and hearing; a larger proportion of prosimian's brain is devoted to the sense of smell than the sense of vision.  Prosimians are insectivorous, also eating fruits.  It is plausible that two front-facing eyes helped the prosimians to live in trees and search for insects during the night. However, it is only about 20 million years ago that in the apes, eyes developed to have the full bony sockets, full colour vision etc. 
For completeness, I list some common traits that primates (apes) share:

Among the apes, humans are unique in having a much larger brain relative to the body mass - human brain is 1.3 kg that is almost 3 times the size of a chimp brain even though they have similar body mass.  This divergence is due to the rapid development of the preforntal cortex in humans starting some 2 million years ago.   

(2) Vision with side-facing and front-facing eyes: Most animals have either front-facing or side-facing eyes.  Conventional wisdom is that hunters/predators have front-facing eyes as the binocular vision provides greater accuracy in determining distance and location of the prey; animals, who are preyed on, have side-facing eyes as the monocular vision provides almost a 360 degrees view and helps in detecting an approaching predator.
I have drawn the following slide to explain the difference:

 
Depth Perception:  In binocular vision, we perceive depth/distance of objects by receiving information from two different angles.  If we close one eye - we have monocular vision as many two-eyes-on-the-sides animals also have. However, with one eye only, our depth perception does not seem to be enormously different; the question arises how can/do animals perceive distances of objects with monocular vision?  

We can do this because the brain uses a variety of depth cues.  The brain has a big memory bank and large processing power; and it seems to do a pretty good job of interpreting depth cues. Binocular vision just makes the depth perception so much better. For animals, it is a trade off between limited 'good' coverage of about 180 degrees or nearly full 360 degrees 'good-enough'/functional coverage.  One chooses what gives the best chance of survival.
Wiki has a detailed article on depth cues; here, I shall discuss only some of the most important cues: 

Relative Size Cue:  If two objects (e.g., two trees) are known to be the same size and if one subtends a larger visual angle on the retina than the other, the object that subtends the larger angle appears closer.

By observing the angle projected by the object on the retina, the brain can determine the absolute distance of the object using the previous knowledge of its size.

Similarly, the brain uses perspective (parallel lines converging in the distance) to reconstruct relative distances of two parts of some large object (a building or landscape features)
     
Motion Parallax:  
Transverse Motion:  Parallax is the apparent change in position of an object relative to distant background objects resulting from a change in position of the observer.  
The relative motion of an object against the background objects gives hints about its distance; for example, when travelling in a train or a car, nearby objects pass quickly while far off objects appear stationary. The use of motion parallax for depth perception is widespread throughout the animal kingdom.  Birds bob their heads to achieve motion parallax, squirrels move in lines perpendicular to an object of interest to locate its position etc.

Radial Motion:  If an object is moving towards you, its image on the retina increases in size - the changing image size enables the observer to not only see the object as moving but also to perceive the distance and the speed of the moving object.  

Accommodation:  Mammals, birds and reptiles vary the optical power of the eye by changing the shape of the elastic lens.  Fish and amphibians vary the power by changing the distance between a rigid lens and the retina. 

The far point may usually be taken as a distant point at infinite distance and focusing on an object at finite distance allows the brain to perceive its distance.

The muscles inside the eye (ciliary muscles) bring about the mechanical changes in the lens.  The sensation of contracting or relaxing of the ciliary muscles in focusing on a nearby object is sent to the visual cortex where it is used to interpret distance/depth of the object.  Accommodation is most effective in judging distances less than 2 metres. In humans, the accommodation amplitude can be up to 15 Dioptres.

Recent Work:  Some fascinating work has been done in quantifying the superiority of binocular vision over monocular vision.  An interesting conclusion is that for large distances of greater than about 25 metres, the two visions have similar precision; but for closer objects, binocular vision precision, depending on the availability of depth cues from the surrounding, can be up to 40 times better than for monocular vision (see 1, 2). 

Visually Guided Behaviour:  Studies with visually guided behaviour like walking over and around obstacles have observed that walkers were quicker by about 10% when using binocular vision and also  judged the height of obstacles with greater precision.  There was higher uncertainty in monocular vision, leading to greater reliance on feedback (from depth cues) in the control of movements. 

Seeing Objects behind Obstacles 

It is not only in depth perception that binocular vision is superior, but significantly, results from recent research point to advantages of binocular vision in carrying out many aspects of daily activities. 

Structure of the Eye:  Actually, nobody tells you about the scrappy and incomplete information about the surrounding environment that the eye sends to the brain - the brain fills in the voids, and this can be dodgy.  To understand 'the trip-wire act' that vision processing is, we need to learn how the eye collects visual data about the surroundings.  Essentially, one thinks of the retina as an extension of the brain and it is at the retina that external light is received to generate the electro-chemical signals which are then processed in different parts of the brain (see Section 2). The interpretation of the results again requires the brain to fill in lot of details using guesswork and can also be dodgy.  It is surprising that, most of the time, the brain appears to make a decent job out of the whole process.
   
First, I describe the anatomy of the eye and the retina in the following slides. 

In humans, the retina is a 0.5 mm thick layer of cells and covers 72% of the spherical eyeball of 22 mm diameter. The retina covers about 150 degrees vision area in front of the eye. 

The optical disc  where the bundle of 1 million nerve-fibres leave the retina is an area of about 3 mm square. It contains no light-sensitive detectors creating a blind spot in the vision.

Light detectors are called rods and cones.  There are 100 million rods (sensitive to the brightness of light but not its colour), while the 7 million cones in the retina are sensitive to colour but not very sensitive to brightness.

Cones are primarily concentrated in the central retina (see slide) as  hexagonal mosaic in the fovea and its surrounding macula (diameter = 5.5 mm). Fovea is the region of sharpest visual acuity, is very small in size and contains no rods.  The pit in the macula (parafovea) is 1.5 mm diameter. The area surrounding the fovea has the largest concentration of rods and has the most sensitivity to light.  

  
The fovea of the eye sees the spatial details and full colour of the objects with the greatest sharpness - but that is a tiny angular range - of the order of a few degrees.  The reason, that we can see the bigger picture spanning almost 130 degrees range without moving the head, is  that our eyes constantly dart about, fixating for a fraction of a second and then moving on.  The jerky movements are called saccades.  We make about three saccades per second, each lasting between 20 and 200 millionth of a second; we have no concious control on saccades - the brain manages it.  While saccades are happening, we are effectively blind. The brain does not use information picked up during a saccade but uses guesswork to fill in the details.

How is Visual Information Processed by the Brain:  

{This section may be missed out without loss of continuity}

The information from the eye is carried by the axons of the retinal ganglion cells to the midbrain.  In the brain,  visual processing is akin to an orchestra, where clusters of cells in different parts of the brain co-operate to process different components of visual information such as vertical or horizontal orientation, colour, size, shape, movement etc. 

The following two slides show a schematic of how visual information from the eye flows to the brain. I refer you to the original lectures for more information on this topic.

The visual information received is then analysed by various parts of the brain; the brain collects the results and constructs a picture of the external view.  Using memory and previously held information, the brain updates/fills-in any missing information to form a full picture. All of this is done in the blink of an eye!  Generally, it does a good job too.  
But a word of caution here:  It is not too difficult to  fool/manipulate the brain.  It uses past experience to construct from somewhat incomplete information that the eye sends and guesses what the missing information might be.  It is not too difficult for the brain to get the whole thing wrong.  The subject of optical illusions (see an infographic with many examples here) and hallucinations provide fascinating case studies where brain gets the results totally wrong. Eyes play a central role in meta-communications and bizarre effects like Uncanny Valley are observed directly as a result of brain's processing of visual signals. Dreaming is another example when teh brain creates visual perception when no external input is present.

And, it is not only in processing visual information, the brain is equally fallible in interpreting hearing, smell, taste, emotions and in many other decisions it makes.    

(3) Humans and Vision:  The above discussion leaves unanswered the question - 'why we have two front-facing eyes?' To some extent this is of academic interest only.  What we really want to understand is - why evolution has invested so much, almost 30-50% of brain resources - in processing vision related information. This is unique to humans and we shall try to speculate why such a large proportion of the increase in human brain size over the last 2 million years might have gone to vision related processing.

We learn about the world we live in through our senses - there are 21 accepted senses including 4 belonging to vision (brightness and 3 colours - red, green and blue). Vision, sound, smell and touch are the only senses that provide us information about the external environment.  Smell and touch are relevant only for short distances; sound may be good for medium distances of a few km or less but is of very poor spatial resolution. Vision is the only sense that properly connects us to the outside world and is our main way to interact with our surroundings. Humans rely heavily on vision to guide our behaviour and perceive the world.

Unlike all other animals who are mainly concerned with the search for food and safety from predators, humans have learnt to manipulate the environment for their benefit.  Many factors like learning to use tools, controlling fire, agriculture, living in societies with mutually accepted rule and regulations etc. have elevated humans to become the most powerful species that has ever lived.  This has been made possible by the visual input processed by the brain.  Imagine the amount of visual information that would be fed to the brain from the ever increasing activities that humans are involved with - that will require a massive supercomputer to handle .  It is no wonder that the human brain consumes greater than 20% of basal metabolic energy even though it is only 2% of the body mass.  The brain also consumes energy at a more or less constant rate of 20 Watts whether you are solving maths problems or sleeping or sitting quietly in the sofa - it has to organise itself to be ready for efficiently analysing the next input; (For comparison Titan supercomputer uses 4,000,000 Watts of electricity!)  
It seems the vision perception department is always looking for more resources and this might be the reason why the size of the human brain has increased so rapidly over the past million years or so.
Humans have retained the two front-facing eyes because the binocular vision provides a far superior depth perception than a monocular vision; and helps the brain to more accurately perceive reality.

The brain's perception of reality involves a lot of filling-in of missing information from guesswork, and the reliability of the resulting perception is a matter of discussion.  There are many examples when our perception is wrong by a long margin - but that is all we have just now.     


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