psy 214 - nazareth collegerachtma8/psy214/files/ntc05-2.pdf · • the purkinje scale (1825) is the...

PSY 214 Lecture #5 (09/19/2010) – (Introduction to Vision) Dr. Achtman

Written by: {Dan Waldbillig}, {[email protected]} of 12

Corrections: • A correction needs to be made to NCT04-1 on page 2; however, this is more of another note,

rather than a correction. In the center of the iris is the pupil, which is in fact a hole in the eye, but it is black because the tissues in the eye absorb much of the color, causing the pupil to appear black. (See figure 3.2 on pg. 45.)

• A correction, or another note that should be added to NCT04-1 is on page 6, regarding dark adaption. The experiment does take 25-30 minutes to complete, but it should be noted that if you continue to test the subject, their adjustment to the light would not become any greater. It will reach its maximum potential of focus at 25-30 minutes, and stay at that same rate. (See Figure 3.18 on pg. 53.)

Announcements: • There will be the first Sensation and Perception Test on Monday, October 3rd, which

is a Monday. The test will cover the first four chapters of the textbook. Refer to the syllabus.

Lecture Notes: Review

• Last class, the main topics we discussed were:

o Visual Stimulus, which is the visible part of the electromagnetic spectrum. o Parts of the eye and structure including the optic nerve and retina.

o SOME of the differences between rods and cones. o Dark adaption and how it occurs.

o Also discussed, was certain type of retinal diseases.

PSY 214 Lecture 5

Topic: Introduction to Vision (Part 2) Chapter 3, pages 56-68



Figure 3.2 on PG 45. (Showing parts of the eyes)

Spectral Sensitivity

Figure 3.21 A on PG 56

• The X-axis represents how long the wavelength is. The Y-axis represents relative threshold on the left, and relative sensitivity on the right.

• These graphs answer the question, “Is there a light we are more sensitive to?” They also showing how much of that color needs to be represented in order for you to see it.

o The answer to the question is yes that we are more sensitive to yellow, since it has the lowest amount present for us to recognize it. The color we are least sensitive to is blue.

• The equation converting threshold to sensitivity is….. Sensitivity =1/threshold (THh) • The threshold is how bright the color has to be for us to detect the given color.

• This sensitivity is dependent upon a combination of rods and cones.



Spectral sensitivity of Rods and Cones

Figure 3.21 B on PG 56

• This chart shows the difference between your cone sensitivity and your rod sensitivity. • The chart shows that your cones are most sensitive to bright lights, such as the colors green

and yellow. • Your rods are more sensitive to darker colors used in night time/dusk, including the colors

blue and green. • This is why everything looks so bright and vibrant in the daytime and so dark and gloomy in

the night. • The Purkinje Scale (1825) is the shift from cone vision to rod vision. This causes a better

perception of short wavelengths during dark adaption. • This shift shows what you experience on an everyday basis.

Absorption Spectrum

Figure 3.24 on PG 57



• The difference between rod and cone spectral sensitivity is caused by differences in absorption of the spectra.

• An absorption spectrum is a plot of the amount of light absorbed by a substance against its wavelength of the light.

• This graph shows the absorption spectra of the rod and cone pigments. • The cones have three absorption spectra and because there are three different pigments.

Each one of the three spectra’s has it’s own receptor. • The spectral sensitivity curve is much determined by the medium and long wavelength

pigments. • The rod and cone sensitivity is determined by the properties of the rod and cone visual

pigments.

Differences between Rods and Cones 1. The Shape 2. Distribution of Retina

3. The number of rods and cones (many more rods in the eye than cones) 4. Temporal response to light (dark adaption)

5. Spectral sensitivity

The Retina

• This is a diagram of a segment of the retina, which is the tissue in the back of the eye. • The receptor cells include rods and cones. These are the farthest layer from the optic nerve.

• The receptor cells are responsible for transduction. • The middle layer is made up of bipolar cells; these will synapse with the rods and cones and

transfer the signal.



• The ganglion cells, and layer closest to the optic nerve fibers, transmit the signal out of the eye. Ganglion cells are the only cells that fire action potentials. This is also the strongest signal since the ganglion cells have to send the signal such a far distance to the brain.

• Numbers for a perspective (per eye)

1. 120 million rods 2. 6 million cones

3. 1 million ganglion cells Types of cells in the retina

1. Receptor Cells (rods/cones) 2. Bipolar cells

3. Ganglion cells 4. Horizontal cells-Send signals sideways between ganglion cells and the bipolar cells.

5. Amacrine cells-sends signals sideways between the bipolar cells and the rods and cones.

Stimulation of Rods and Cones


• This figure shows rods on the left diagram and cones on the right diagram.

• All of these rods and cones in both diagrams are being stimulated by a light source. • The numbers represent the number of the response units generated by the rods and cones in

response to an intensity of 2. • The red figure shows the ganglion output, while the blue shows the rod/cone output.

• The ganglion cell on the left side is shown to need a level of ten to fire an action potential. • Rods have convergence, which allows all sources feed to a single ganglion cell, and it is

able to fire an action potential. (Left diagram)



• The convergence in the rods make it so there is low light that can be sensed since the cells work together. This increases sensitivity and you obtain a lot of information by this.

• Cones in the fovea DO NOT have convergence. There is a 1:1 ratio of cones to ganglion cells, which creates no response and no action potential.

• The advantage of the 1:1 ratio in the fovea is that this is how you obtain details.


• Figure 3.28 shows the 1:1 ratio that cones are able to have. It shows that two lights are being shown and two lights are being transmitted.

• Rods show that two lights are being shown. But since they work together and communicate to only one ganglion cell, the rods only show one light being transmitted.

• Rods show you that you have stimulation, while cones are able to show you the detail to them through this 1:1 connection.

Visual Acuity in eccentric locations • This is shown by the Jaguar example in class. There were two slides, in one, everything was

in focus, and in the other picture, just the outer part of the picture was out of focus. • The differences were not visible at first to each student because their focus was on the

jaguar. • This being because your rods can’t be focused on the outside of the picture, since our focus

was on the jaguar in the center, so the details were not detected.



What the Horseshoe Crab teaches us about Inhibition



• The horseshoe crab has lateral inhibition, which means each lens in the eye is located directly over a receptor creating a 1:1 ratio.

• The horseshoe crab’s eye is made up of hundreds of ommatidia (small lenses located over the receptor).

• In figure 3.31, this idea is shown, by recording from an electrode over “A”.

• A fires a lot, when only A is stimulated. When both A and B are stimulated, the cell fires less. When the intensity is increased with A and B, then cell fires even less. The more cells that fire, the less intense the cell fires because they inhibit each other.

• The brighter the light, the less it wants to send the signal. (Think of the “shh” cells from class)



• This shows us that when more than one place is stimulated, it is more inhibited by the other locations where it is sensing light because of the lateral plexus, and these “shh” responses from cell to cell.

Examples of Lateral Inhibition


• This example shows us the lateral inhibition in humans.

• As you look at the picture, your eyes notice grey ghosts at the intersection between 4 squares. As you try and focus on the grey ghosts, you notice they disappear.

• This example shows the inhibition that is transmitted across the retina.



How Hermann Grid Works


Why does receptor A look Grey?

• It has twice as much inhibition, since it’s at the intersection of all four black squares. • A is getting input from all 4 around it.

• As for B, it is not seen as a ghost because; L and R are not being stimulated in this example. • Your periphery sees the grey ghosts, since it’s not your focus; there is no 1:1 connection.

• When you try to look right at it, you cannot see it because it then has a 1:1 connection and is pooling from other cells, which makes it just white, and the ghost disappears.

Mach Bands

Figure 3.35 A on PG 65

• This is another type of lateral inhibition. • There are seven shades of grey in which you perceive edges as darker than the middle, even

though they are solid colors.




• Figure 3.35, part B represents a graph that is what the color actually should look like. • Figure 3.35, part C, shows how we perceive the color difference.

• We perceive B and C darker than A and D because they are near the edge and we have are limited to lateral inhibition

Lateral Inhibition in Vision

• This diagram shows how inhibition works from a stimulation standpoint. • Cell A, B and C are all getting intense stimulation, while D is getting moderate stimulation.

• Cells A and B are being inhibited from both sides, thus not passing on the signal hardly at all.

• Cell C is only being inhibited from one side since side D is not inhibiting it. This is because it is not receiving intense stimulation, but rather moderate stimulation.



Simultaneous Contrast


• The two center squares reflect the same amount of light into your eyes, because they are the same object and color. They look different because of simultaneous contrast.

• Simultaneous contrast occurs when our perception of the brightness or color of one area is affected by the presence of a surrounded area.

• This is why the inner square on the left looks darker than the inner square on the right, when in fact their the same color.

How lateral inhibition is used to explain simultaneous contrast effect


• This is an example of simultaneous contrast based on lateral inhibition. • Both squares receive the same amount of illumination.

• The size of the arrows indicates the amount of lateral inhibition. • Since the square on the left receives more inhibition, it appears darker, even though both

squares are the same color and shape.



For more information: http://www.youtube.com/watch?v=ePSz6oQ2cuk In this video, the receptive field is explained thoroughly, along with how rods and cones communicate exclusively with ganglion cells. The parts and structure of the eye are also discussed in this lecture. http://reference.findtarget.com/search/spectral%20sensitivity/ I found this website very helpful in clarifying the spectral sensitivity of the rods and cones. If you are still confused about spectral sensitivity, this website may be helpful in clarifying it for you. http://www.indiana.edu/~p1013447/dictionary/lat_i.htm This website is very helpful in clarifying the idea of lateral inhibition. If all the diagrams and grids confuse you to how this works, this website is helpful. It explains in the simplest form possible and can be helpful to hearing the material presented in a different way for better understanding. Real-life example: • A real life example that everyone can connect with in this chapter is the idea of rod and cone

vision and relative sensitivity. Referring to figure 3.22 on pg 56, one can see the colors that rod and cones are sensitive to. My real life example is, imagine your walking outside around 3 pm when the sun is shining and there are no clouds in the sky. The flowers seem bright colors such as bright yellows and reds and the grass and trees seem vibrantly green. This is your cone vision working since it is so sensitive to green and yellow (bright colors). Then imagine you are walking outside around 8 pm. Now, the grass looks like shades of grey, trees seem dark grey and everything looks dull. This is because your rod vision is picking up on the colors it is most sensitive to, those colors being dark greens and dark blues. This real life example shows the difference between rod vision and cone vision on a daily basis.

• Everyday we have notebooks and paper in class to record. Next time you’re in class, lift your paper or notebook about 5 inches from the desk. You will see the shadow reflected represents mach bands, exactly like the example from the textbook. You will recognize the light and dark bands just as mach bands represents.

psy 214 - nazareth collegerachtma8/psy214/files/ntc05-2.pdf · • the purkinje scale (1825) is the...

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