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PSY 214 Lecture # (09/14/2011) – (Introduction to Vision) Dr. Achtman

Written by: Briana Ramirez, [email protected] of 8

Corrections: • A correction needs to be made to NTCO3 on page 3 under excitatory transmitters. It is possible

to excite a neuron without sending information to another neuron. For example, in figure 2.12 p. 31, the neuron was excited, but it was not excited to the level to have an action potential (the dotted line). Due to this, the information is not sent to other neurons unless the action potential is reached.

• A correction also needs to be made on NTC03 on page 3 under relationship between excitatory and inhibitory transmitters on the fifth bullet point. When all the transmitters are inhibitory this also stops the spontaneous firing while the stimulus is applied. This is shown by no action potentials in the stimulated portion of part E in figure 2.13. Announcements:

• Dr. Achtman has put the CD that coincides with the textbook in her office mailbox.

Lecture Notes: Review

• Last class we looked a lot at electricity and the receptive field. • Today we will go back and look at Stimulus and Transduction in terms of sight. Transduction

occurs on the level of the receptor. It involves changing one type of energy into another.

Figure 1.1 p. 6

PSY 214 Lecture 4

Topic: Introduction to Vision Chapter 3, pages 44-54



The Stimulus of Vision

Figure 3.1 p.44

• The stimulus of vision (in humans) is the visible light portion of the electromagnetic spectrum. In figure 3.1 the wavelength goes from shorter on the left to longer on the right. Wavelength is the distance from the peak of one wave to the peak of the next.

• The electromagnetic spectrum is a continuum of waves. • Visible light ranges from 400 - 700 nanometers. A nanometer is 10-9 meters. White light

includes all wavelengths of visible light. • In the visible spectrum of light blue has a shorter wavelength than red. Therefore, different

color has different wavelengths. • Light can be measured as either a wave or a photon. Most of the light we see is reflected light. Structure of the Eye

Figure 3.2 p.45

• The first lens of the eye is the cornea. • The iris is the color of the eye. In the center of the iris is the pupil, which appears black because it is

a whole in the eye. • The red portion in figure 3.2 is filled with liquids called Vitreous and Aqueous Humor. • The fovea is the center point of focus that contains only cones. The periphery is the images of

everything that is not the central focus point, and contains both rods and cones. • The light must pass through all these layers to get to the receptors in the retina. All layers of cells in

the back of the eye are called the retina. The receptors for light in the retina are called the Rods (purple) and cones (red).

• Transduction occurs in the rods and cones!



• The attended stimulus is not always the object being focused on the fovea. You can make a conscious effort to make the attended stimulus something in the periphery. In class we used the example of trying to look at an attractive person without blatantly staring at them.

The Lens’ of the Eye • The cornea is a fixed lens and does not change shape. It does 80% of the focusing. • The only way to change the shape of the cornea is LASIK eye surgery. • The lens is adjustable because the ciliary muscles change its shape. The lens does the remaining

20% of the focusing. • Astigmatisms are due to the cornea. People with astigmatisms are born with them. • Glaucoma is a condition that clouds the optics because the eye does not receive light input.

Figure 3.3 p.45

• Near Point of Vision: is how near to the eye the object can be and still be in focus

• The near point moves further away with age. This is called presbyopia. • The reason for the near point is the lens. The reason it gets further away with age is because the

ciliary muscles weaken and the lens hardens making it harder to accommodate.

Figure 3.4 p.46 Rods and Transduction

• There are 120 million rods, which are named for their rod - shape.



• The outer segment of the rod is called the visual pigment which reacts to photons. This is where transduction occurs. (As well as in the cones, named for their cone - shape.)

Figure 3.6 p.48

• Opsin and retinal change their shape when a photon interacts with it. This is called isomerization.

Figure 3.7 p. 48 • This change in shape leads to an enzyme cascade and changes the voltage of the cell. The enzymes

help to break down proteins and facilitate the sight process.

Retinal Processing and the Distribution of Rods and Cones Differences in Rods and Cones 1) Shape

• Rods are shaped like long cylinders and Cones are small and tapered

2) Number • There are 120 million Rods and 6 Million Cones.

3) Distribution • Only Cones are found in the fovea. There are approximately 50,000 cones in this location.

• Rods and the remainder of the Cones can be found disbursed along the periphery. There are more rods then cones in the periphery.



Figure 3.12 p. 51 • The Blind Spot is where there are NO Rods or Cones. The reason for the blind spot is because it is

where the Optic Nerve exits your eyeball. • Therefore, if an object is in your blind spot you are not able to see it. The blind spot is different in

people because it depends on the angle at which the nerve leaves the eye. • The reason you are often unaware of your blind spot is for several reasons:

1) You are constantly moving your eyes 2) Your brain will fill in the empty space based off your surroundings

3) the blind spot is in different locations in each eye

Figure 3.14 p. 52 Retinal Diseases

• Macular Degeneration - people cannot see what they direct their eyes to - the macula is the fovea plus a little bit of the surrounding area

- It is age related and there are some medications to slow it down - This is represented by part (a) in Figure 3.13

• Retinitis Pigmentosa – extreme tunnel vision - there is a genetic component and it gets worse as it progresses

- This is represented by part (b) in Figure 3.13



Figure 3.13 p. 53

Dark Adaptation • The dark adaptation experiment starts with the lights turned on. The experiment officially begins

when the lights are then turned off. The method of adjustment is used so that the observer can adjust the light so they can just barely see it is there. The experiment takes 25 - 30 minutes to complete.

Figure 3.18 p. 53

• We are now going to see how rods, cones, and rods and cones together adjust to the dark. - In figure 3.19 the bottom of the scale reads High, which means dimmer light and high sensitivity.

The top of the scale reads Low, referring to low sensitivity and bright light. • The green curve represents only cones and this is done by shining the light directly in front of the

observer so that it lands on their fovea. This shows that cones adjust rapidly to the dark in the first six minutes, however after this point they level off and do not become and more sensitive to the dark as time goes on.



• The purple line represents only rods. This shows that they adapt more slowly to the dark, but after the first 10 minutes they continue to adapt to the darkness and allow for our night vision more than cones after this ten minute mark. They way that they study only rods is to use patients who are monochromats which means they were born without cones and can only see in black, white, and shades of grey.

• The red line is a mixture of both rods and cones. We can see that this line is a mixture of the two individual curves. It mimics the cones curve for the first 10 minutes because that is what controls our vision. However, after those ten minutes are up it mimics the rods because they take over the night vision.

Figure 3.19 p. 54

For more information: • http://www.youtube.com/watch?v=0HzWmldLDHI – This youtube video describes the structure

of the eye and how light is focused on the retina and relayed to the brain. • http://www.youtube.com/watch?v=hMHKJF31lBg&feature=related – This youtube video is good

for seeing how the lens changes shape depending on how close or far away an abject is. (Accommodation)

• http://www.youtube.com/watch?v=f-YkzgfgN2k – This youtube video is interesting if you would like to know how the LASIK eye surgery procedure is completed.

• http://www.accessexcellence.org/AE/AEC/CC/vision_background.php - This website goes into detail on how our ideas of how we see have changed since the time of Aristotle. It also goes over the biological structure of the eye and explains opsin and retinal again if you need clarification.

Real-life example: • Every day light reflects off of objects and then enters our eye and is focused through our lenses

and displayed on our retina. For example, as college students nearly every day we go to class and see the teacher either writing on the blackboard, or we look at slides they have prepared.



Depending on how close or far away you sit from the blackboard depends on how your eye compensates for the reflected light.

• This also can be seen in everyday life by people who wear glasses. This shows that their lens have trouble adapting in order to present a clear image on the retina. Therefore, an additional lens is added by the persons wearing of glasses.

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