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The Blind Spot

Dr. Joanne Paul- MBBS (UWI), FRCPCH (UK), FRCP (Edin)

In medical school, a few good many years ago, we learnt that the retina has light sensitive cells, cones, and rods, that transmit messages to the brain about what is seen. Cones transfer colour and are good for daylight vision and perceive fine detail. Rods pick up black and white and are very light sensitive,  perceiving size, shape, and brightness. Rods are also the ones we use for night vision. In fact, babies only see black and white and shades of grey when they are born. Although their rods are mature at birth, their cones are not developed and thus it takes a while for them to see colour. They only start perceiving colour at 3 to 4 months old.

A blind spot generally is an area where a person’s view is obstructed. The blind spot in the eye is an area on the retina (otherwise called the optic disk) where the optic nerve fiber and blood vessels exit the back of the eye. There are no light sensitive cells at that spot and thus that part of the retina cannot visualize and is called the blind spot. So essentially every person has a tiny gap in their vision where they are totally blind. We typically do not notice this visual gap in our day to day lives. Some researchers suggest that with both eyes open, the visual fields overlap, and can fill in the missing information for the opposite eye. The brain itself takes this information from the opposite eye, adds the visual cues in the environment and the calculation of different images from eye movements to make up an image to accommodate for the visual gap. So essentially the brain presumes and calculates what should be seen in that visual gap and then passes it off as what is actually seen.

The occipital cortex and other areas in the brain deal with processing of visual stimuli. Maybe about 50% or more of the brain is involved indirectly or directly with vision and vision processing. Some parts process the visual stimuli. Then another part processes what the visuals mean to us in terms of navigating our environment, recognition, and spatial awareness. Other parts help with memory connection so we attach meaning to what we see so when we see it again, there would be context and enhanced decision making. Another part ensures our mental focus is on the correct object out of all the other things in our field of vision at that point.

For such a small area of the body, vision is a very important part of brain function. Most people think of human vision like a camera. The direct photographic image we get from the eyes is actually upside down. Vision, they have found, is more similar to speech than like a photograph. The brain machinery actually spends time to construct what we see. The brain combines all the images, reviews shapes, size, occlusion, close objects, objects far away, environmental cues, and non-visual cues like sound and self-motion, to construct a 3-Dimensional image that denotes what we see. This processing can take time. To allow this to happen on a practical level, our vision can be up to 15 seconds behind real time. Instead of analyzing every snapshot, what we ‘see’ or perceive in a given moment is an average of the images of the past 15 seconds. It is like the brain refresh time for vision is every 15 seconds, giving the brain time to process and interpret.

Going even further, not only does the brain construct what we see, it can reconstruct what we see. This means that aside from the brain taking time to compose the vision, it also edits vision. Examples of this can be seen with the café wall visual illusion where the horizontal lines are straight and parallel but because the vertical lines are irregular and squiggly, the brain interprets the horizontal ones as bent and not in parallel. The Ebbinghous illusion or Titchener circles are of similar concept where the inner circles are of the same size but because the larger and smaller circles around the inner circle change the context, the brain interprets the circles as different sizes. There is also a similar version with the checker board shadow illusion where the adjacent colour and shadow affects what we see because the brain edits based on context. The brain takes context, edits and reconstructs the images to suit. Supposedly the brain also reconstructs the image to protect the individual and enhance our sense of normalcy and security.

Going even further, (how far does this writer want to go man), there is something called perspective taking vision. If the brain is constructing and reconstructing to suit context and mental health protection, it makes sense that the most accurate vision is a multiperspective one, even including a perspective that is diametrically opposed to yours. If one could see something from multiple viewpoints and perspectives then and only then, could you see the object or thing or issue in its entirety, or as much entirety as you can. If we automatically on some level, see what we want to see then we are blind to our own blindness. Seeing and understanding from various other persons’ points of view may be the real true vision, without the blind spot.

 

Dr Joanne F Paul is a Lecturer, a Paediatric Emergency Specialist, and a member of TEL institute

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