Vision occurs when light enters the cornea (the outer covering of the eye) and travels through the pupil (the opening) to focus on the retina, which translates the light into nerve impulses that are sent via the optic nerve to the visual cortex in the brain, creating images.
But how we “see” the world is dependent on a lot of things. For example, our species determines which wave lengths we can see in the electromagnetic (EM) spectrum. As you can see in the chart of the EM spectrum, only a narrow portion of it is detectable to the human eye. This is called “visible light”.
What’s pretty amazing is that many species of birds, insects, and fish can see into the ultraviolet range of the spectrum (take a look at the picture below that shows how humans and bees see flowers). We can’t see ultraviolet because we are missing the protein in our retinal receptors that would allow us to be sensitive to that wavelength. If we could see ultraviolet, we would see the world very differently.
Damage to any part of the visual system, from cornea to cortex, changes how our vision works. It’s not terribly surprising that damaging the eye impacts our ability to see the world. A cloudy lens, made so by cataracts, turns our world blurry and indistinct. What is surprising is that damage to the brain can permanently alter how we perceive visual information. For example, lesions–perhaps brought about by a stroke or head trauma–in the occipital cortex can cause partial vision loss or visual hallucinations and illusions. Several forms of agnosia result from damage to this part of the brain. Agnosia is an inability to identify something despite a lack of damage to the memory centers of the brain or to the sensory input system (eyes, optic nerve, etc.). Despite the object being clearly visible and familiar to the viewer, they struggle to determine what it is. However, given a chance to touch the object, identification typically soon follows. Prosophenosia (also called face blindness) is a form of agnosia that results from damage to the occipital cortex. Individuals with this disorder suffer from an inability to recognize faces. Even people as familiar as their parents or children become unrecognisable. Damage to the occipital cortex can lead to a variety of visual deficits, including:
• Defects in vision (Visual Field Cuts).
• Difficulty with locating objects in environment.
• Difficulty with identifying colors (Color Agnosia).
• Production of hallucinations
• Visual illusions – inaccurately seeing objects.
• Word blindness – inability to recognize words.
• Difficulty in recognizing drawn objects.
• Inability to recognize the movement of an object (Movement Agnosia).
• Difficulties with reading and writing.
As we age, on average, our ability to process visual information begins to decline. This is due to either physical changes to the eye or degradation at the level of the cortex or both. In terms of cortical function, this slow decline usually begins as early as 25 years of age. The good news is, there are things we can do to improve how we extract information from our visual world. This will be the topic of my next post.
So, knowing all of this, we come to realize that what we see when we look at the world is a function of what information the receptors in our eyes are sensitive to and how our brain interprets that information. Check out some of these cool visual illusions, for example. Here we see a group of images that, when viewed by a perfectly healthy visual system, induce various misperceptions or illusions. We end up seeing things that really just aren’t there (click here to access a site that has more optical illusions but also provides more detailed information regarding how they occur).
The way we see the world isn’t the only way to see it.