Lesson 7: The Visual Process

The Beginning of the Visual Process

Let's start with another video and worksheet Links to an external site. Download worksheet Links to an external site.

: Vision: Crash Course A&P #18 Links to an external site.

 

Types of cells in the eye

Think of how gossip spreads through a corporate environment. The information may start out as simple, but as the message goes from person to person, it gets bigger, more complicated, and (depending on your point of view), better and better! That is kind of how the eye works. Millions of little people (cells) are trying to communicate a message to the next one in line, and each cell trying to improve upon that message, often by highlighting the 'juiciest' bits and downplaying the less important information.

The eye has 3 layers of cells. The first layer actually lines the back of the eye, these cells are called photoreceptors. They are the first ones to receive the message (the first ones to "see the light"). The middle layer, coming toward the front of the eye, has 2 main types of cells called horizontal and bipolar cells. These cells try to make the message more dramatic and clear before they pass it along. They send the message along to the ganglion cells, which are responsible for passing it along to the brain. So, it all starts with the photoreceptors ... 

Photoreceptors: Millions of Tiny Light-Detectors

Photoreceptors (sometimes simply called receptor cells and are specifically divided into rods and cones) detect light. Basically, they produce electricity depending on how much light they absorb. The amount of electricity produced is proportional to the amount of light that hits the cell. Photoreceptors do this by producing a neurotransmitter called glutamate. Just to make this fun for students, they do this in a confusing way!!! The more light absorbed into the cell = the less glutamate produced by the cell = the more electricity produced. Get it? Think about it this way:

Say a 100 watt lightbulb "attacks" a photoreceptor with it's powerful light ray. The poor photoreceptor would get scared and stop producing glutamate at once as if he is holding his breath (exactly 100 watts worth of it) and produce a 100 watt shock of electricity with his scream! Well, it isn't a perfect analogy, but I hope you get the idea.

There are 2 types of photoreceptors you may have heard of:

Rods:

Respond best to dim light, they do not detect color and best detect peripheral motion and large objects. There are about 100 million rods in your eye. This is good if you want to see whether your child has gotten up from the table out of the corner of your eye (the eyes in the back of mother's head may well be rods!)

Cones:

Respond best to bright light, they are the ones that detect color and are best for visual acuity. 5 million cones are in your eyes right now. I know this because you wouldn't be able to read this print if you did not have them!

 

Horizontal Cells: Ignoring the boring photoreceptors.

Horizontal cells modify the strength of the signals sent by the photoreceptors. Horizontal cells generally attenuate the strength of the message sent by the photoreceptors, but they do it disproportionately depending on the strength of the initial signal. Specifically, a very strong signal (bright light) is attenuated just a little and a very weak signal (dim light) is attenuated a great deal. Horizontal cells are responsible for lateral inhibition which is responsible for enhancing the edges along the canvas as well as for the Mach Band illusion below.  

Think of horizontal cells as the supervisors at a company. They do not want to appear to encourage gossip and will always try to curb the chit-chat at the water cooler. However, if the gossip is "good" i.e. if it is credible enough and juicy enough, you know they will listen and pass it along at their next opportunity!

Bipolar Cells: Sending the good stuff along.

Like horizontal cells, bipolar cells make direct contact with the photoreceptors. Bipolar cells respond positively to a reduction in glutamate from the photoreceptors (meaning more light is detected). The job of bipolar cells is to recombine information received by receptor and horizontal cells and pass this on to the ganglion cells.

Bipolar cells try to discern the "truth" from the message; picking out what is a salient fact versus what is mere gossip before they try to impress their bosses by passing the message along.

Ganglion Cells: Lonely at the top!

Ganglion cells receive information from bipolar cells and pass that information along to the brain. Actually, the axons of ganglion cells go directly to the brain. There are far fewer ganglion cells than there are photoreceptors, about 100 million photoreceptors to 1.25 million ganglion cells.

The ganglion cells are the last to process the message before it goes to the big boss (the brain).

Visual Sensitivity vs. Resolution

You just learned that there are way more photoreceptors than there are ganglion cells. 

Convergence is the ratio of photoreceptors to ganglion cells. There are 100 million photoreceptors to 1.25 million ganglion cells. Clearly, there must often be several photoreceptors to only 1 ganglion cell. If there is high convergence, there are many photoreceptors per ganglion cell. If there is low convergence, there are few receptors per ganglion cell.

Sensitivity is the ability to detect stimuli, whereas resolution is the ability to discern the precise spatial properties. For example, have you ever ducked because you sensed an object being hurled at your head, only to peek from your cowered position to see a little, itty bitty fly? You have demonstrated great visual sensitivity, you were able to detect an object coming right for you (good job). However, you did not have the time to look at the object and get high resolution. Thus, you couldn't tell a fly from a bowling ball!

Convergence is the enemy of resolution!

As such, the more photoreceptors to a ganglion cell, the less likely you will be able to discern the object accurately. However, the MORE likely you are to detect the object's presence. This is the trade-off between resolution and sensitivity.

In the fovea (the middle of the macula) convergence can is about 1:1, or one photoreceptor to one ganglion cell, which is exceptionally good resolution. As we extend outward into the periphery of the retina, the ratio can get as high as several hundred photoreceptors to one ganglion cell. This underlies 2 specific types of visual perception:

	Foveal vision	Peripheral vision
Type of receptor	Cones	Rods
Number of receptors to ganglion cell	Few Receptors	Many Receptors
Best conditions	Bright Light	Faint Lights
Resolution	Good Detail / Acuity	Poor Detail
Color vision	Good Color Vision	Poor Color Vision

The duplex solution simply states that our eyes are equipped with both kinds of visual input. The scientific terms are 

Scotopic Vision

Vision under dimly lit condition. Therefore, it is the vision associated primarily with the rods in the periphery of the retina. For example, detecting something from the corner of your eye. 

Photopic Vision

Vision that requires a lot of light. It is the vision associated with the cones in the center of the retina. For example, reading this sentence. 

Anatomical Structures and Functions of the Human Eye

Lets get an idea about the anatomy of the eye:

 

Sclera:

The white part made of tough, hard material covering all the outer eye except the cornea. The sclera protects the eyeball.

Cornea:

The transparent portion of the outermost fibrous coat of the eyeball that covers the iris and the pupil and is continuous with the sclera. The cornea protects the eye while allowing light to penetrate.

Iris

The colorful part (iris comes from the word for rainbow). The outer layer of the iris consists of pigment and the inner layer consists of blood vessels.

Pupil

The black part in the middle of your eye. The pupil expands and contracts to let just the right amount of light in - in the dark it expands, and in the light it contracts. The pupil also responds to your autonomic nervous system, for example, if you are scared your pupils will expand to let in more light.

Lens

The transparent structure inside the eye that focuses light rays onto the retina; the lens is right behind the iris and allows us to focus. The lens must be transparent. Cataracts, or opaque film covering the lens, severely impairs vision.

Structure, Location and Function of the Retina

Finally, we get to the back of the eye to a structure called the retina. We have mentioned this important area in the back of the eye before, but now we will look at it's structure. Again, the retina lines the back wall of the eye where light gets transuded into neural signals by the the photoreceptors (rods and cones). The retina is where the magic happens! 

Three major components of the retina are:

Macula: the center of the retina where visual acuity is sharpest.

Fovea: a depression within the macula of the retina of the eye that contains a single layer of cones with no overlapping blood vessels;  this is the region of greatest visual acuity.

Optic disk: a blind spot where there are no photoreceptors because the axons of the cells in the eye are heading back to the brain.

Photoreceptors and the distribution of rods and cones across the retina. 

The center of the retina has a higher concentration of cones, responsible for the increase in visual acuity. In fact, in the very center of the fovea, only cones are found. As we move out toward the periphery of the back of the eye we find a higher concentration of rods.

Perception - Making Sense of that Pattern of Energy 

Perceiving is Believing - Crash Course Psychology #7 Links to an external site.

 

Take a minute to take some notes: Look over the Magritte image (above) - where do you tend to look first? What can you see in your peripheral vs. foveal vision? How do you move your eyes over the piece to 'take it all in'?