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Title: Vision: the mismatch between external reality and the filtered and distorted perceptions that are provided by sensory processing of information

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Assignment Goals
Source Material
Student Instructions
Guiding Questions
Writing Prompt
Calibrations and Answer Keys

Assignment Goals

The goal for this assignment is to build an appreciation of the marvelous tailoring of evolution that makes it possible for us to extract complex information from sensory input. You will apply general principles that are true for all the senses to explore in detail the mismatch between external reality and the filtered and distorted perceptions that are provided by sensory processing of information.

Sensory input, the information flow into the brain, is alike for all the sensory systems including taste, touch, vision, hearing, smell, pain, equilibrium, blood pressure, chemoreception, and proprioception. In this activity, you will investigate in detail one example showing how the sensory nervous system both dissects and integrates information before sending that information to the brain. By dividing a sensory field into small areas that can be monitored individually, sensory neurons extract detailed information that is then integrated by combining features within and between receptive fields to determine the relationships between stimuli.

One of the general principles that you will apply is the concept that nerve cell depolarization opens calcium channels, and then the influx of calcium triggers exocytosis of synaptic vesicles and neurotransmitter release. Nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release.

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Source Material

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Source Material Resources:
How the retina works - Article in pdf format written by Helen Kolb in 2003. Kolb (2003) American Scientist 91 (1): 28-35. Used by permission of American Scientist, magazine of Sigma Xi, The Scientific Research Society. Great color diagrams and photomicrographs. For best results, go to the article and select the link to email it to yourself.
URL: http://www.americanscientist.org/template/AssetDetail/assetid/16218
Signal transduction in the photoreceptors - assignment resource
URL: http://www.hhmi.org/senses/a/a140.htm
The retina - A simple description.
URL: http://faculty.washington.edu/chudler/retina.html
Sensing change -
URL: http://www.hhmi.org/senses/a120.html
Hermann grid optical illusion -
URL: http://www.brainconnection.com/teasers/?main=illusion/hermann
Direct and indirect pathways or How the Retina Works -
URL: http://www.americanscientist.org/template/AssetDetail/assetid/16218/page/4#17761
Perception of edges - The video on this web site requires real player.
URL: http://www.hhmi.org/lectures/webcast/ondemand/97webcast4/receptive.html

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Student Instructions

Before you begin, use your textbook and glossary to become familiar with these basic structures and functions:

Then look at a white wall in bright light. Look at the wall through a dark opaque tube with one eye, and compare that brightness to the brightness of the wall as seen through the other eye without the tube. To understand this distortion (since we know that the wall has the same brightness with and without the tube) it is necessary to understand how information is processed before it is sent by the ganglion cells through the optic nerve to the brain.

Your textbook may not have enough detailed information to answer all your questions as you work to understand this process, so fill in the details by referring to the recommended web-based source materials.

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Guiding Questions

  1. How does the white wall look different when viewed through an opaque tube?
  2. Describe the organization of the different kinds of neuron cells in the retina.
  3. Describe how a photorecepter cell responds to a photon of light. List the intracellular signals and describe how the photorecepter cell changes in response to light.
  4. Tell what signal is sent by photorecepter cells and compare that signal sent in response to light with the signal sent in the absence of light.
  5. What is the role of the horizontal and/or amacrine cells?
  6. What do ganglion cells do?
  7. For full credit, use at least 10 of the 14 terms listed under source materials.

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Writing Prompt

Trace the signal transduction pathways and the steps from reception of the external light signal in the eye to transmission of the information about the brightness of the light. Your job is to explain why the brightness of a wall looks different when viewed through an opaque tube compare to what you see without the tube by describing what happens to the signal from reception to transmission through the ganglion cells and then to the optic nerve.

Write in paragraph form and use the html flags at the beginning and at the end of each paragraph. Any time you do not write in your own words, use quotation marks and include a bibliography.

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Calibrations and Answer Keys
High Quality Calibration

The wall looks brighter when viewed through a narrow opaque tube compared to the wall viewed without the tube. To begin with, the lens of the eye functions to focus the light on the retina where phototransduction occurs. The light first passes through the retinal layers with ganglion cells, bipolar cells, amacrine cells, and horizontal cells before it is detected by rhodopsin molecules in the rod cell membranes at the back of the retina. Rhodopsin has two components: opsin is an integral membrane protein and retinal is a vitamin A derived molecule that changes shape in response to absorption of a photon of light.

In the darkness when rhodopsin is not active, intracellular cyclic GMP (cGMP) levels are high and both sodium channels and potassium channels are open in photorecepter cell membranes. The sodium/potassium ATPase continuously pumps sodium out of the cell and potassium into the cell, so the ion leak through the open channels allows sodium to flow back into the cell and potassium to flow out. The sodium influx and potassium efflux establishes a membrane potential that is partially depolarized to -40 mV rather than the -70 mV that is typical of most nerve cells. At the depolarized rod cell membrane potential, glutamate neurotransmitter is constantly released to stimulate the neighboring bipolar neuron.

When light activates rhodopsin, a signal is sent through a G protein called transducin that responds by reducing the rod cell intracellular cGMP levels and this signal closes the ion channels that permit sodium influx. As a result, the cell becomes hyperpolarized and the glutamate neurotransmitter that normally stimulates the neighboring bipolar neuron is no longer released.

Most bipolar cells are stimulated by neurotransmitters coming from multiple photorecepter rod cells. Depending on the type of glutamate receptor, glutamate excites some bipolar neurons but inhibits other bipolar neurons. Bipolar neurons then synapse with ganglion cells and these connections can also be excitatory or inhibitory. Each ganglion, through connections to bipolar cells, also responds to a particular circular area of retina known as the visual field. There are two types of visual fields that are established by lateral connections from the horizontal and amacrine cells. Lateral connections that are inhibitory can cancel out part of the excitatory information. The neuron associated with the "on-center/off-surround" visual field responds strongly when the light is brightest in the center of the visual field. In contrast, the neuron associated with an "off-center/on-surround" visual field will be inhibited with bright light in the center. These functions of lateral inhibition allow detection of edges or lines, and this information is then transmitted by the ganglion cells through the optic nerve to the brain.

Since the opaque tube through which the wall is viewed provides a dark edge to the field of vision, there is consequently less lateral inhibition of the light signal and we interpret the color of the wall as being brighter than what we see without the dark surrounding tube in the visual field.



1. Are there spelling errors? Hint: Copy the text into a word processing program and use the spell check function.

Yes
No

Answer: No
Feedback : none
2. Is the writing in the students' own words or are quotation marks used to indicate any sections that are not written in the student's own words and is a reference list included for those quotations?

Yes
No

Answer: Yes
Feedback : Most of this information is paraphrased from the Silverthorn text.
3. Does the paragraph address the first guiding question, in other words, does it provide an explanation for the fact that the white wall looks brighter when viewed through an opaque tube?

Yes
No

Answer: Yes
Feedback : The wall looks brighter when viewed through a narrow opaque tube compared to the wall viewed without the tube. To begin with, the lens of the eye functions to focus the light on the retina where phototransduction occurs. The light first passes through the retinal layers with ganglion cells, bipolar cells, amacrine cells, and horizontal cells before it is detected by rhodopsin molecules in the rod cell membranes at the back of the retina. Rhodopsin has two components: opsin is an integral membrane protein and retinal is a vitamin A derived molecule that changes shape in response to absorption of a photon of light.
4. Photorecepters send signals to bipolar cells and these transmit the signals to ganglion cells. Is the relationship betweeen these three cell types in the retina explained?

Yes
No

Answer: Yes
Feedback :

The wall looks brighter when viewed through a narrow opaque tube compared to the wall viewed without the tube. To begin with, the lens of the eye functions to focus the light on the retina where phototransduction occurs. The light first passes through the retinal layers with ganglion cells, bipolar cells, amacrine cells, and horizontal cells before it is detected by rhodopsin molecules in the rod cell membranes at the back of the retina. Rhodopsin has two components: opsin is an integral membrane protein and retinal is a vitamin A derived molecule that changes shape in response to absorption of a photon of light.

and

Most bipolar cells are stimulated by neurotransmitters coming from multiple photorecepter rod cells. Depending on the type of glutamate receptor, glutamate excites some bipolar neurons but inhibits other bipolar neurons. Bipolar neurons then synapse with ganglion cells and these connections can also be excitatory or inhibitory. Each ganglion, through connections to bipolar cells, also responds to a particular circular area of retina known as the visual field. There are two types of visual fields that are established by lateral connections from the horizontal and amacrine cells. Lateral connections that are inhibitory can cancel out part of the excitatory information. The neuron associated with the "on-center/off-surround" visual field responds strongly when the light is brightest in the center of the visual field. In contrast, the neuron associated with an "off-center/on-surround" visual field will be inhibited with bright light in the center. These functions of lateral inhibition allow detection of edges or lines, and this information is then transmitted by the ganglion cells through the optic nerve to the brain.
5. Rhodopsin is the protein pigment in photoreceptor cells that responds to a photon of light. Is it clear from the writing that this student knows this function for rhodopsin?

Yes
No

Answer: Yes
Feedback :

The wall looks brighter when viewed through a narrow opaque tube compared to the wall viewed without the tube. To begin with, the lens of the eye functions to focus the light on the retina where phototransduction occurs. The light first passes through the retinal layers with ganglion cells, bipolar cells, amacrine cells, and horizontal cells before it is detected by rhodopsin molecules in the rod cell membranes at the back of the retina. Rhodopsin has two components: opsin is an integral membrane protein and retinal is a vitamin A derived molecule that changes shape in response to absorption of a photon of light.

In the darkness when rhodopsin is not active, intracellular cyclic GMP (cGMP) levels are high and both sodium channels and potassium channels are open in photorecepter cell membranes.


6. In photorecepter cells high levels of the signal transduction molecule, cGMP, cause sodium influx and cell depolarization. Reduction of cGMP closes sodium channels and hyperpolarizes the cell. Does the student demonstrate understanding of this relationship between cGMP levels and sodium influx?

Yes
No

Answer: Yes
Feedback : In the darkness when rhodopsin is not active, intracellular cyclic GMP (cGMP) levels are high and both sodium channels and potassium channels are open in photorecepter cell membranes. The sodium/potassium ATPase continuously pumps sodium out of the cell and potassium into the cell, so the ion leak through the open channels allows sodium to flow back into the cell and potassium to flow out. The sodium influx and potassium efflux establishes a membrane potential that is partially depolarized to -40 mV rather than the -70 mV that is typical of most nerve cells. At the depolarized rod cell membrane potential, glutamate neurotransmitter is constantly released to stimulate the neighboring bipolar neuron.
7. Photorecepter cells release neurotransmitter in the absence of light. They reduce neurotransmitter release in proportion to the amount of light exposure. Does the student show that they understand this opposite relationship between the amount of neurotransmitter released and the amount of light stimulation?

Yes
No

Answer: Yes
Feedback : When light activates rhodopsin, a signal is sent through a G protein called transducin that responds by reducing the rod cell intracellular cGMP levels and this signal closes the ion channels that permit sodium influx. As a result, the cell becomes hyperpolarized and the glutamate neurotransmitter that normally stimulates the neighboring bipolar neuron is no longer released.
8. Lateral connections between neurons in the retina can be inhibitory. Does this student explain lateral inhibition?

Yes
No

Answer: Yes
Feedback : Most bipolar cells are stimulated by neurotransmitters coming from multiple photorecepter rod cells. Depending on the type of glutamate receptor, glutamate excites some bipolar neurons but inhibits other bipolar neurons. Bipolar neurons then synapse with ganglion cells and these connections can also be excitatory or inhibitory. Each ganglion, through connections to bipolar cells, also responds to a particular circular area of retina known as the visual field. There are two types of visual fields that are established by lateral connections from the horizontal and amacrine cells. Lateral connections that are inhibitory can cancel out part of the excitatory information.
9. Ganglion cells respond to an area of the retina called the visual field and they send this information through the optic nerve to the brain. Does the student identify the relationship between the visual field and the message sent by a ganglion cell to the brain?

Yes
No

Answer: Yes
Feedback : Most bipolar cells are stimulated by neurotransmitters coming from multiple photorecepter rod cells. Depending on the type of glutamate receptor, glutamate excites some bipolar neurons but inhibits other bipolar neurons. Bipolar neurons then synapse with ganglion cells and these connections can also be excitatory or inhibitory. Each ganglion, through connections to bipolar cells, also responds to a particular circular area of retina known as the visual field. There are two types of visual fields that are established by lateral connections from the horizontal and amacrine cells. Lateral connections that are inhibitory can cancel out part of the excitatory information. The neuron associated with the "on-center/off-surround" visual field responds strongly when the light is brightest in the center of the visual field. In contrast, the neuron associated with an "off-center/on-surround" visual field will be inhibited with bright light in the center. These functions of lateral inhibition allow detection of edges or lines, and this information is then transmitted by the ganglion cells through the optic nerve to the brain.
10. Lateral inhibition makes the wall look dimmer when viewed without the tube. The dark tube reduces lateral inhibiton, and that is why the wall looks brighter when viewed through the tube. Does the student relate lateral inhibition to the percieved brightness of the wall?

Yes
No

Answer: Yes
Feedback : Since the opaque tube through which the wall is viewed provides a dark edge to the field of vision, there is consequently less lateral inhibition of the light signal and we interpret the color of the wall as being brighter than what we see without the dark surrounding tube in the visual field.
11. How would you rate this text?
10 Highest
9
8
7
6
5
4
3
2
1 Lowest
Rating: 9
Feedback : none

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Mid Quality Calibration

The perception of grey when looking at a wall without the opaque tube "is a result of local gain control in the retina. The 'gain' of a signal is the average amount of power transmitted in that signal; for an image, setting the signal gain means setting the brightness level. Cells in your retina adjust the brightness of an image in the same way that the brightness knob on your computer monitor does, by adjusting the intensity of the light signal." Your eyes adjust the light in small sections called the visual field.

The size of these small sections is determined by the size of neural receptive fields in the retina. Lateral connections between cells help the retina to determine the edges of shapes. Lateral connections in the retina are inhibitory, meaning that they decrease the gain of a signal; in other words, lateral connections serve to turn down brightness. Lateral connections are formed by horizontal and amacrine cells within the retina.

When light shines on the photoreceptor cells, the pigment rhodopsin responds to the photon by reducing the cGMP levels and this closes the sodium channels and hyperpolarizes the cell. As a result, nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release.

The response is complex because photoreceptor nerotransmiter release can result in both excitatory postsynaptic potential or inhibitory postsynaptic potential, depending on the type of neurotransmitter receptor in the bipolr cell that receives the signal from multiple photoreceptor cells. At any rate, bipolar cells and ganglion cells, that receive signals from the bipolr cells, can both recieve not only the message from the photorecptor that there is bright light (absence of neurotransmitter) but they can also receive lateral inhibition signals from neighboring neurons that tell whether the neighboring spots in the visual field are exposed to light. "Lateral connections from the horizontal and amacrine cells give the bipolar cells a receptive field shaped like two concentric discs. Each bipolar cell in the second layer is the manager of a group of photoreceptors, so these discs span the receptive fields of the photoreceptors underneath. As a manager, the bipolar cell can compare the reports of photoreceptors under its discs to detect spatial relationships between regions of light and dark. An on-center bipolar cell is strongly activated by a spot of light in the center disc of its receptive field surrounded by darkness in the outer disc. An off-center bipolar cell is strongly activated by light in the outer disc of its receptive field, with darkness in the center. These concentrically shaped receptive fields enable bipolar cells to detect edges, or transitions between regions of light and dark."

In looking at the white wall through a dark, opaque tube, the dark regions are not stimulated with light and so they have no inhibitory lateral signals to turn down the "gain" for that signal. This leaves the light signal bright. With the other eye, with no peripheral dark sections, most of the receptive field is flooded with white light and there is strong lateral inhibition; the result is that the gain here is turned down, and that area appears grey.

  1. BrainConnection.com, Illusions, Scientific Learning Corporation, 1995 University Ave., Suite 400, Berkeley, CA 94704, © 1997-2000 Scientific Learning Corporation, http://www.brainconnection.com/BC/BC_GetPage.pl?pid=Fh_Illusions_Dir/Fh_Illusions06 Accessed April 1, 2000.
  2. BrainConnection.com, NeuroSeries: Vision, Scientific Learning Corporation, 1995 University Ave., Suite 400, Berkeley, CA 94704, © 1997-2000 Scientific Learning Corporation, http://www.brainconnection.com/BC/BC_GetPage.pl?pid=Fh_Anatomy_Dir/Fh_Anatwk4 Accessed April 1, 2000.


1. Are there spelling errors? Hint: Copy the text into a word processing program and use the spell check function.

Yes
No

Answer: Yes
Feedback :

The perception of grey when looking at a wall without the opaque tube "is a result of local gain control in the retina. The 'gain' of a signal is the average amount of power transmitted in that signal; for an image, setting the signal gain means setting the brightness level. Cells in your retina adjust the brightness of an image in the same way that the brightness knob on your computer monitor does, by adjusting the intensity of the light signal." Your eyes adjust the light in small sections called the visual field.

The size of these small sections is determined by the size of neural receptive fields in the retina. Lateral connections between cells help the retina to determine the edges of shapes. Lateral connections in the retina are inhibitory, meaning that they decrease the gain of a signal; in other words, lateral connections serve to turn down brightness. Lateral connections are formed by horizontal and amacrine cells within the retina.

When light shines on the photorecepter cells, the pigment rhodopsin responds to the photon by reducing the cGMP levels and this closes the sodium channels and hyperpolarizes the cell. As a result, nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release. The response is complex because photorecepter nerotransmiter release can result in both excitatory postsynaptic potential or inhibitory postsynaptic potential, depending on the type of neurotransmitter receptor in the bipolr cell that receives the signal from multiple photorecepter cells. At any rate, bipolar cells and ganglion cells, that receive signals from the bipolr cells, can both recieve not only the message from the photorecptor that there is bright light (absence of neurotransmitter) but they can also receive lateral inhibition signals from neighboring neurons that tell whether the neighboring spots in the visual field are exposed to light. "Lateral connections from the horizontal and amacrine cells give the bipolar cells a receptive field shaped like two concentric discs. Each bipolar cell in the second layer is the manager of a group of photoreceptors, so these discs span the receptive fields of the photoreceptors underneath. As a manager, the bipolar cell can compare the reports of photoreceptors under its discs to detect spatial relationships between regions of light and dark. An on-center bipolar cell is strongly activated by a spot of light in the center disc of its receptive field surrounded by darkness in the outer disc. An off-center bipolar cell is strongly activated by light in the outer disc of its receptive field, with darkness in the center. These concentrically shaped receptive fields enable bipolar cells to detect edges, or transitions between regions of light and dark."

In looking at the white wall through a dark, opaque tube, the dark regions are not stimulated with light and so they have no inhibitory lateral signals to turn down the "gain" for that signal. This leaves the light signal bright. With the other eye, with no peripheral dark sections, most of the receptive field is flooded with white light and there is strong lateral inhibition; the result is that the gain here is turned down, and that area appears grey.
2. Is the writing in the students' own words or are quotation marks used to indicate any sections that are not written in the student's own words and is a reference list included for those quotations?

Yes
No

Answer: Yes
Feedback : none
3. Does the paragraph address the first guiding question, in other words, does it provide an explanation for the fact that the white wall looks brighter when viewed through an opaque tube?

Yes
No

Answer: Yes
Feedback : The perception of grey when looking at a wall without the opaque tube "is a result of local gain control in the retina...
4. Photorecepters send signals to bipolar cells and these transmit the signals to ganglion cells. Is the relationship betweeen these three cell types in the retina explained?

Yes
No

Answer: Yes
Feedback : The response is complex because photorecepter nerotransmiter release can result in both excitatory postsynaptic potential or inhibitory postsynaptic potential, depending on the type of neurotransmitter receptor in the bipolr cell that receives the signal from multiple photorecepter cells. At any rate, bipolar cells and ganglion cells, that receive signals from the bipolr cells, can both recieve not only the message from the photorecptor that there is bright light (absence of neurotransmitter) but they can also receive lateral inhibition signals from neighboring neurons that tell whether the neighboring spots in the visual field are exposed to light. "Lateral connections from the horizontal and amacrine cells give the bipolar cells a receptive field shaped like two concentric discs. Each bipolar cell in the second layer is the manager of a group of photoreceptors, so these discs span the receptive fields of the photoreceptors underneath. As a manager, the bipolar cell can compare the reports of photoreceptors under its discs to detect spatial relationships between regions of light and dark.
5. Rhodopsin is the protein pigment in photoreceptor cells that responds to a photon of light. Is it clear from the writing that this student knows this function for rhodopsin?

Yes
No

Answer: Yes
Feedback : When light shines on the photorecepter cells, the pigment rhodopsin responds to the photon by reducing the cGMP levels and this closes the sodium channels and hyperpolarizes the cell. As a result, nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release.
6. In photorecepter cells high levels of the signal transduction molecule, cGMP, cause sodium influx and cell depolarization. Reduction of cGMP closes sodium channels and hyperpolarizes the cell. Does the student demonstrate understanding of this relationship between cGMP levels and sodium influx?

Yes
No

Answer: Yes
Feedback : When light shines on the photorecepter cells, the pigment rhodopsin responds to the photon by reducing the cGMP levels and this closes the sodium channels and hyperpolarizes the cell. As a result, nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release.
7. Photorecepter cells release neurotransmitter in the absence of light. They reduce neurotransmitter release in proportion to the amount of light exposure. Does the student show that they understand this opposite relationship between the amount of neurotransmitter released and the amount of light stimulation?

Yes
No

Answer: Yes
Feedback : When light shines on the photorecepter cells, the pigment rhodopsin responds to the photon by reducing the cGMP levels and this closes the sodium channels and hyperpolarizes the cell. As a result, nerve cell hyperpolarization makes it less likely that calcium channels will open so that there will be less of a chance for calcium to signal neurotransmitter release.
8. Lateral connections between neurons in the retina can be inhibitory. Does this student explain lateral inhibition?

Yes
No

Answer: Yes
Feedback : The response is complex because photorecepter nerotransmiter release can result in both excitatory postsynaptic potential or inhibitory postsynaptic potential, depending on the type of neurotransmitter receptor in the bipolr cell that receives the signal from multiple photorecepter cells. At any rate, bipolar cells and ganglion cells, that receive signals from the bipolr cells, can both recieve not only the message from the photorecptor that there is bright light (absence of neurotransmitter) but they can also receive lateral inhibition signals from neighboring neurons that tell whether the neighboring spots in the visual field are exposed to light. "Lateral connections from the horizontal and amacrine cells give the bipolar cells a receptive field shaped like two concentric discs. Each bipolar cell in the second layer is the manager of a group of photoreceptors, so these discs span the receptive fields of the photoreceptors underneath.
9. Ganglion cells respond to an area of the retina called the visual field and they send this information through the optic nerve to the brain. Does the student identify the relationship between the visual field and the message sent by a ganglion cell to the brain?

Yes
No

Answer: No
Feedback : As a manager, the bipolar cell can compare the reports of photoreceptors under its discs to detect spatial relationships between regions of light and dark. An on-center bipolar cell is strongly activated by a spot of light in the center disc of its receptive field surrounded by darkness in the outer disc. An off-center bipolar cell is strongly activated by light in the outer disc of its receptive field, with darkness in the center. These concentrically shaped receptive fields enable bipolar cells to detect edges, or transitions between regions of light and dark."
10. Lateral inhibition makes the wall look dimmer when viewed without the tube. The dark tube reduces lateral inhibiton, and that is why the wall looks brighter when viewed through the tube. Does the student relate lateral inhibition to the percieved brightness of the wall?

Yes
No

Answer: Yes
Feedback : In looking at the white wall through a dark, opaque tube, the dark regions are not stimulated with light and so they have no inhibitory lateral signals to turn down the "gain" for that signal. This leaves the light signal bright. With the other eye, with no peripheral dark sections, most of the receptive field is flooded with white light and there is strong lateral inhibition; the result is that the gain here is turned down, and that area appears grey.
11. How would you rate this text?
10 Highest
9
8
7
6
5
4
3
2
1 Lowest
Rating: 7
Feedback : none

Top

Low Quality Calibration
Conversion of a photon of light that is detected by a photoreceptor cell is called transduction. Transduction is surprisingly intricate—so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein—rhodopsin—that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells—and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.

1. Are there spelling errors? Hint: Copy the text into a word processing program and use the spell check function.

Yes
No

Answer: No
Feedback : none
2. Is the writing in the students' own words or are quotation marks used to indicate any sections that are not written in the student's own words and is a reference list included for those quotations?

Yes
No

Answer: No
Feedback : Much of this text is borrowed without recognition of the author of the Signal transduction in the photorecepters web page.
3. Does the paragraph address the first guiding question, in other words, does it provide an explanation for the fact that the white wall looks brighter when viewed through an opaque tube?

Yes
No

Answer: No
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
4. Photorecepters send signals to bipolar cells and these transmit the signals to ganglion cells. Is the relationship betweeen these three cell types in the retina explained?

Yes
No

Answer: No
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived. ONLY TWO TYPES OF CELLS ARE LISTED.
5. Rhodopsin is the protein pigment in photoreceptor cells that responds to a photon of light. Is it clear from the writing that this student knows this function for rhodopsin?

Yes
No

Answer: Yes
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
6. In photorecepter cells high levels of the signal transduction molecule, cGMP, cause sodium influx and cell depolarization. Reduction of cGMP closes sodium channels and hyperpolarizes the cell. Does the student demonstrate understanding of this relationship between cGMP levels and sodium influx?

Yes
No

Answer: Yes
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close.Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
7. Photorecepter cells release neurotransmitter in the absence of light. They reduce neurotransmitter release in proportion to the amount of light exposure. Does the student show that they understand this opposite relationship between the amount of neurotransmitter released and the amount of light stimulation?

Yes
No

Answer: Yes
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
8. Lateral connections between neurons in the retina can be inhibitory. Does this student explain lateral inhibition?

Yes
No

Answer: No
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
9. Ganglion cells respond to an area of the retina called the visual field and they send this information through the optic nerve to the brain. Does the student identify the relationship between the visual field and the message sent by a ganglion cell to the brain?

Yes
No

Answer: No
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
10. Lateral inhibition makes the wall look dimmer when viewed without the tube. The dark tube reduces lateral inhibiton, and that is why the wall looks brighter when viewed through the tube. Does the student relate lateral inhibition to the percieved brightness of the wall?

Yes
No

Answer: No
Feedback : Conversion of a photon of light that is detected by a photorecepter cell is called transduction. Transduction is surprisingly intricate-so intricate that the process is not yet fully understood for most of the senses. In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein-rhodopsin-that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade. The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential. In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell. But in the light, the channels close. Then the electrical potential inside the cell becomes more hyperpolarized. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells-and thus alerts bipolar cells in the next layer of retinal cells that a photon of light has arrived.
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Rating: 4
Feedback : This text does not deal with the problem posed - the perceived brightness of a white wall - and the concept of lateral inhibition is not addressed. Furthermore, copying text from a web page without recognition of the source is plagiarism.

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