What is Vision?

It may seem a little strange at first, but if you relax we will solve the puzzle of vision together.

There are essentially four steps to vision.

• First we have to gather light into our eye.

• The light has to be channelled to the back of the eye.

• Transduction occurs.

• The information goes to our brain where we interpret it.

Let’s break down the 4 steps in a little more detail.

Step One: Gathering Light

For the AP it is important to understand that we only see a small fraction of the light spectrum.  There are all kinds of light waves out there from long ones (infrared, microwaves or radio waves) to short ones (ultraviolet waves X-rays or even gamma waves (like what made the Hulk)).

The light we can see is in what we call the visible light spectrum, and from shortest to longest goes violet, indigo, blue, green, yellow, orange and red.  The height of the light wave determines it’s intensity or brightness.  While the length of the wave determines it’s hue (color).  So when we look at am object, these light waves enter our eye.

Step Two: The Light Channeled within the Eye

Once the light hits the eye it goes through a variety of structures.  Take a look at the diagram below.

The white part of our eye is called the cornea and basically protects and helps reflect light.  The light goes through a hole in our eye called a pupil.  The pupil is like the shutter on a camera, it opens or closes to let light in.  The colored part of your eye is called the iris.  The iris is a muscle that sole job is to open (dilate) or close (constrict) the pupil.  So when the light goes through the pupil it first hits the lens.  The lens is almost like a magnifying glass that reflects the light toward the back of our eye.

The lens is constantly changing shape depending on whether we are looking at objects close to us or far away.  The bending of the lens is called accommodation.  When adults become old, their lens becomes rigid and they cannot reflect light properly to the back of their eye and need reading glasses.  A really cool thing about the lens is that it reflects the light upside down and inverted toward the back of the eye (retina).  Our brain must switch the image back right side up or we would have serious perception issues.

The structure of the eye in Hebrew and Japanese (I think), in case this clears some things up for you.

Step Three: Transduction Psychology Definition

Ok, so how does our eye turn the light into neural impulses so that our brain can understand.  Most of the process occurs on the back of the eye called the retina.  The retina is the most important part of our eye (it is often referred to as the brain of the eye).  First, it is important to know that the retina is made up of several layers of cells and the light must pass through all of them to experience transduction (kind of like a water filtration process).

The first layer of cells to be activated by light are called the rods and cones.  Rods see only black and white and are spread throughout the outside of the retina.  Cones see color and are located in the center of the retina known as the fovea.  Rods out number cones by about 20 to 1.  Since the cones are located in the fovea (in the center of the retina) we see color objects better if they are directly in front of us.   Since rods are located on the periphery of the retina- we see black and white better in out peripheral vision.

Ok- so the lens reflects the light back to the retina and it hits the rods and cones.  If the rods and cones fire- they send the information to a second layer of cells called bipolar cells (you have to no nothing about these bipolar cells except they pass on the light to the next set of cells).  So the bipolar cells give the information to a layer of cells called the ganglion cells.

The axons of the ganglion cells make up our optic nerve which sends the information to the thalamus in our brain (where the optic nerve hits the retina is sometimes called our blind spot– I will show you how to find it in class).  In case they ask (which I doubt they will- but if you are going for that 5 on the AP- the specific part on the thalamus that attaches to the optic nerve is called the lateral geniculate nucleus (LGN) and the area that the optic nerves crosses/intersects in our head (remember our cerebral cortex is contralateralized) is called the optic chiasm.  This is a very simplified version of transduction psychology definition in the eye- but I think it suites our purposes (suites our purposes- what an adult and nerdy thing to say).

Step Four: Vision in the Brain

We should already know from the brain chapter that the thalamus sends the visual information to the occipital lobe in the cerebral cortex.  We interpret the image in the visual cortex in the occipital lobe.  When you watch TV- you see all kinds of things at the same time. Let’s say you are watching 24 and Jack Bauer is about to torture some random terrorist to save 145,000 people.

You see Jack’s shape, his motion, his colors and all kinds of other cool stuff about Jack at the same time.  Scientists say that in our visual cortex we have specific cells that all have specific jobs.  Some of these cells may just see shape.  While other cells have the sole job to see motion.  We call these types of cells feature detectors.

OK- there is ONE last aspect left to cover about vision.  That is how do we see color.

There are two theories of color vision.

• Trichromatic theory: this theory is actually quite simple (so I like it more).  It says that we have three types of cones in our retina.  We have cones that detect red, blue and green and from a combination of those three colors we can see almost everything.  Now you artists out there are now saying, dude- those are not primary colors!! The problem is that you are thinking in terms of paint, not light.  Go check out a Projector TV and tell me what color the three bulbs are.  The problem with this simple theory is that is does not do a good job explaining color blindness or afterimages.  Ok – what is an afterimage.   Stare at the red dot in the green square and count to forty.  Then stare at the white square and tell me what you see (actually you really can’t tell me, so tell yourself- or your mom).

You should see a greenish/blue dot in a reddish/purple background.  That is an afterimage.

• Opponent-Process Theory:  this theory states we have three types of receptor cones and they each handle a pair of colors (red/green, yellow/blue, and black/white).  If one sensor/color is firing, it slows the other from firing.  The theory does a good job at explaining afterimages.  Your cones, after firing red for awhile, will rest and fire the opposite green, when not being stimulated.  It also explains color blindness well.  Most people that have trouble seeing colors usually cannot see either tints of red/green or blue/yellow.