Please read the post “The Perfection & Deception of Perception” before you read this post for maximum impact and benefit.
When we watch a film at the cinema, no matter where we look on the screen, the image is detailed, crisp and clear. It is uniform in quality and even at the corners and edges the details are sharp and in focus. It doesn’t matter where we choose to look the picture is good.
The implications of the large cinema screen are that when we look away from one area of the screen to view part of the picture on another part of the screen, the detail is still there on the area of the screen we are no longer looking at. The detail doesn’t stop being there just because we aren’t looking at it anymore.
When we look out onto the world it appears like we are looking out on a large cinema screen. What we see is a seamless, uninterrupted image that is clear and in focus just like the cinema screen. When we move our eye from one area of the image to another, we see a beautiful panning action worthy of the best cinematographer with no jumping juddering or blurring and with absolutely consistent detail and focus across the image. Unlike the large cinema screen though, this is an illusion, the very clever end result of a number of different complex biological systems interacting with one another to create this impression.
To visualize what our eye actually sees when we look at a scene, imagine the cinema screen again but instead of a uniformly clear and focused image, imagine the whole screen is out of focus and blurry with all the colour washed out except for a small circular area on the cinema screen about the size of a basketball. This small circle is crisp, clear and in focus with well-defined colours.
To understand why our eye sees an image like this requires a basic understanding of the structure and function of the eye.
Light passes through our pupil and is focused by the lens on our retina. The light sensitive cells on the retina then send data via the optic nerve to our visual cortex in the brain to be processed to produce an image we can understand.
The retina is not uniform in structure. There is a tiny pitted area on the retina called the fovea on which the lens centres the focused image. The fovea though tiny accounts for 50% of the data sent from the eye to the visual cortex. The other 50% of the data comes from the rest of the retina. So 50% of the data used to create the image in our brain comes from an area that is approximately 1/10,000th of our total visual field. The other 50% comes from the other 99.9999% of the retina.
The fovea differs from the rest of the retina in other ways too. It is densely packed with light sensitive cells called cones which see colour. We have three different types of cone, each of which contains a pigment that responds to a different wavelength of light – green, red or blue and release differing amounts of different neurotransmitters depending on the wavelength and intensity of that light. Depending on the intensity of each wavelength, each receptor will release varying levels of neurotransmittor on through the optic nerve, and in the case of some colors, no neurotransmitter. Just like mixing paints, we see different colours by combining the information from the primary colours detected by 3 different cones. Due to the fact that we need input from 3 cones to see a colour, their response time is comparatively slow so they aren’t good at detecting very quick changes or movement. They are also very poor at seeing in low levels of light.
The part of an image we see with our fovea is called foveal vision. What we see with the rest of our retina is called peripheral vision.
The rest of the retina has a high density of light sensitive cells called rods. Rods are the simpler of the two cell types, as they really only interprets “dim light”. Since Rods are light intensity specific cells, they respond very fast, and to this day rival the quickest response time of the fastest computer. Rods control the amount of neurotransmitter released, which is basically the amount of light that is stimulating the rod at that precise moment. One simple experiment is to go out at night and look at the stars (preferably the Orion constellation) using peripheral vision (side view). Pick out a faint star from the periphery of your eye and then look at it directly. The star should disappear, turn and look at it from the periphery again, it will pop back into view.
In summary and roughly speaking, foveal vision is a very small area at the centre of our visual field that sees things in sharp crisp colourful detail but is bad at spotting movement or change where as peripheral vision, the majority of our visual field sees very low detail, is poorly focused and the colour is very washed out but is excellent at noticing movement and change.
Here is an example of what a line of text looks like to our eye when reading which demonstrates the effect of foveal vision.
If 99.9999% of our visual field is actually a black and white blur, how then does it appear to be a clear focused colourful image?
This is where evolution and survival have played a large part in the design and function of our visual system. Being able to focus 50% of our attention on 0.0001% of an image allows us to concentrate intensely on a specific object when we need to. In primitive cultures, this may have been prey we were hunting or a tool we were making. If we received an equal amount of information from the whole of our retina, our brain would have to perform a massive amount of processing to filter out all the stuff we didn’t want to be distracted by whilest attempting to perform the focused task, most of which would be out of focus anyway because of the nature and function of lenses.
So the eye naturally performs the filtering for us. While we focus our attention on what we are interested in everything in the periphery is seen in much lower detail to prevent our visual cortex from being overwhelmed with data. However, the peripheral vision is one of the fastest reacting light sensitive systems known to man and is fantastic at spotting sudden movement. If while we are intensely focused on our prey or our tool making, a predator should start to hunt us, our peripheral vision is the perfect tool to spot its stealthy movement towards us out of the corner of our eye and alert us to the danger. Our basic survival program would make us then look directly at the source of movement and register the detail of the movement using foveal vision.
So this explains why we have 2 types of vision and the different types of information they are providing to our visual cortex but does not explain why it appears to us that we can see everything clearly. The reason is that our visual cortex holds an image of what is within our visual field in our ‘minds eye’. It creates this image by taking a series of ‘still’ pictures by moving our focus of attention (fovea) around the image to collect the details of the image. The static image is then retained in the brain in apparent detail and focus. The brain again through evolution has developed a program that tells it what is most important or most likely to change in an image and automatically instructs our eyes to keep looking back to those areas with the fovea to check for change in detail.
Here is an animated example showing how a human subjects foveal vision scanned a still photograph of a house, gathering the detail of the image for the mental snapshot. You will need to press the “Next” button on the webpage sveral times until you see the picture of the house, then press the “start” button.
One possible explanation as to why the brain maintains an out of date image in our minds eye is that when we move our eye to a different area of an image, the eye moves so fast that our visual system cannot refresh fast enough to see the detail and the image should just be a blur. To prevent us seeing blurred vision when our eye moves, the visual cortex ignores the blurred data and the static image in the mind is used to hold the image steady.
Millions of years of testing have shown that the system works perfectly as designed but it does introduce the anomalies demonstrated in the previous post. Most people report being aware of something moving at the edge of their visual field but are amazed when it turns out to be a gorilla that they didn’t see. The reason the gorilla is not seen by the majority of people is because I set you a focused task that required you to override your urge to investigate the movement of the gorilla detected by your peripheral vision. For the gorilla to be recognised as anything other than a movement you needed to move the image of the gorilla onto your fovea, which in turn would have made you lose count of the passes made by the white team. Fail a task – no never – I’d rather ignore a gorilla 
So simply put, we only see what we look at directly and what we look at directly depends on what our brain thinks is most likely to change or be of importance to us. If we don’t look directly, we can’t see! On a more philosophical note, our beliefs affect what we expect to see or not see, so they affect what we look for in life. If we look for something we will see it, (search and you shall find) if we don’t there is a chance we may not see it even if it’s right in front of our noses just like the gorilla. In modern life we are just so busy, we barely ever have time to stop, sit back and look around and see what’s really out there. In this fully occupied state it becomes easy for others to manipulate this design flaw in our visual system to their own advantage and influence our behaviour in ways we cannot even begin to imagine.
There is a famous story of the Conquistadors arrival in South America in their ships. It is said that the native South Americans brains having never seen a man made structure like a European battle ship before, simply ignored the ships on the horizon. Their brains weren’t programmed to look for hazards like that and so like the Gorilla, they simply didn’t see the arrival of the vehice of their own demise.
So I invite you to listen to Madonna’s famous track Frozen with new eyes and ponder on what you might not be able to see because you haven’t taken time to look yet or simply because you think it’s not worth looking because you don’t believe it’s there. When you have done that you might like to watch some other fun experiments which demonstrate the brains innability to see what it doesn’t expect to happen here and here.
No comments:
Post a Comment