Please read chapter 2 (if you don't have a book yet, please let me know). After reading chapter 2, please respond to the following questions:
What were two things from the chapter that you found interesting? Why were they interesting to you? Which two things did you find the least interesting? Why? What did you read in the chapter that you think will be most useful to in understanding the visual system? Finally indicate two topics or concepts that you would like me to cover in more depth in class.
Note: Keep in mind that you will be allowed to bring in the blog posts to class with you when you take exams. Be sure to use the terms and terminology in your posts.
The first item that I came across that was interesting to me was the "photic sneeze reflex". I am notorious in my family and among my friends for doing this. Everytime I get out of the car or walk out of a store into bright light I almost always immediately sneeze. I found it interesting that they have no concrete evidence as to why this happens and it is possibly due to crossed wires in the brain. The second thing that I found interesting was the discussion of a blind spot via the fundus. The fundus is the back layer of the retina which does not contain photoreceptors. However, we do not normally notice the blind spot because our visual fields fill in the information surrounding it.
The couple of items I found the least interesting was the discussion of the five major classes of neurons: photoreceptors (light-sensitive), horizontal cells (contact photoreceptor and bipolar cells), bipolar cells (synapse with rods or cones & with horizontal cells & then onto ganglion cells), amacrine cells (contacts with bipolar & ganglion cells & one another), and ganglion cells (receive info from photoreceptors via bipolar & amacrine cells and transmit info to brain & midbrain). I found this part of the text very difficult to read because of the complex language being discussed. Each cell contacts different cells and they are easily confused with one another. There are also different types of bipolar and ganglion cells that could easily get confused. Bipolar cells can be diffuse (receive input form multiple cones), midget (receive input from a single cone), ON (respond to increase in light), and OFF (respond to decrease in light). Ganglion cells can be P (receive excitatory input from single midget bipolar cells & feed the parvocellualr layer of the lateral geniculate nucleus whatever that is) and M (look like umbrellas & receive excitatory input from diffuse bipolar cells & feed the magnocellular layer fo the lateral geniculate nucleus).
I think the most useful information to understand the visual system is how the eyes work to adjust and take in light. Light is absorbed, scattered, reflected, transmitted, or refracted and only when a photoreceptor in the retina absorbs light can it become a sensation. In the anatomy of the eye the pupil controls the amount of light that reaches the retina by using the pupillary light reflex. The light then travels to the retina where the process of "seeing" begins. At the retina the light is transduced (conversion from light to neural energy) through the use of photoreceptors that sense the light. There are two main photoreceptors in the duplex retina: rods (dim light) and cones (daylight, acuity & color). Through a series of events which the photoreceptors start by contacting horizontal and bipolar cells we are able to have a visual sensation.
The first topic I would like to go more in depth in class are the different cells and the processes by which they help create the visual sensation. Up to that point I was able to follow the text well however it gets a bit more complicated when they discuss the different cells. A second topic that I would like to go more in depth about is the idea of center-surround receptive fields (region on the retina where visual stimuli influence a neuron's firing rate).
I found the sneeze reflex to be really interesting as well. I am just relieved to know that I am not allergic to light! haha
My favorite was the sneeze effect! I was thinking all along that something was wrong with me and then the first week of class I realize there's not! It's so funny to think about! And there's much more to it than I thought!
The most interesting things from chapter two were explaining actually how seeing stuff works; meaning how does the information from the eye is sending to
the brain that we can actually anlayze the color, gaudiness, and shap. Also, interesting is how can we see in dark and how we can adapt the light.
In the book, there is a wide explanation about how light on its way become a sensation that can be absorbed, reflected, and transmitted. The most important part in the eye is retina that is responsible for contais roads and cons and then it receive an image from the lens and it sends to the brain through the nerves.
The author talks about some diseases that damage your eye and make difficulties in vision. One of the diseases that the author introduces in the book is
retinitis pigmentosa (RP). It is a family of hereditary diseases that progresively degeneratea the retina that afftects night vision and peripherial vision.
The least interesting thingsd in chapter two were talking about the physics stuff and electromagnetic radiation. There are two ways to conceptualize light,
as a wave and as a stream of photons. Another thing that I was not interested in was a description of ganglion cells. There are different types of retinal P
and M ganglion cells. Ganglion cells receive their input from bipolar and amacrine cells. Why there were not interesting things to me? Because it is hard to
read about very technical, "dry" stuff.
The most useful to in understanding the visual system will be, in my opinion, the part called :Eyes that see the light" It explains briefly how through the nevrous system we can get the information from what we see to the brain and then analyze it.
I would be interested to cover in class in more depth topic about optical diseases and the thing that we can see in the dark.
One concept that I found interesting was how our eyes communicate with our brain. I found this interesting because I didn’t know that for our eyes to communication with our brain the information from the eye is passed on to over 1 million ganglion cells which process the information and send it to the brain through axons located on the back of eye. Another concept that I found interesting was how our eyes adapt to low light and bright light. I found this interesting because I was unaware that pupil dilation is only responsible for a small portion of our visuals systems ability to adapt to light and dark conditions. The main contributors are the rods and cones, with the rods providing sensitivity in low lights, with the cones taking over in bright lights. One thing I did not find very interesting was the section about different eye conditions such as emmetropia or myopia, I didn’t find this very interesting because I felt like it was out of place and didn’t contribute to the understanding of the visual system. Another topic I didn’t find very interesting was on Retinitis pigmentosa, I didn’t find this very interesting because I felt like it didn’t contribute to the overall understanding of the vision system. I think that the make up of the eye with be key in understanding the visual system, as well as understanding retinal informational processing. The two topics I would like to be covered more in class would information on retinal information processing and center surround receptive fields.
I agree that it is amazing that our eyes can in a way converse with our brain and that the light has to pass through so much information just to allow us to see the image. It's amazing what our bodies can do. If we didn't have classes we may never know what something as simple as an eye is capable of!
Another concept that I found interesting was how our eyes adapt to low light and bright light. I found this interesting because I was unaware that pupil dilation is only responsible for a small portion of our visuals systems ability to adapt to light and dark conditions........this just sparked a thought, what role does the eyes make-up play in drug effects on the eye.
I also found this topic very interesting. I always thought that pupil dilation was the main, if not only, way our eyes adjusted to day and night. Before reading this chapter, I never would have guessed that there were two other things occurring that contributed to the adjustment from day vision to night vision. Having an understanding of on and off center cells defiantly helped me to understand the other ways in which our eyes adjust to different levels of light. Along with pupil dilation, our visual system uses different types of photoreceptor in different situations and by effectively throwing away photons we don't need. It also responds to the contrast between adjacent retinal regions, and the ganglion cells do their best to ignore whatever variation in overall light level is left over.
I also think it would be interesting to learn about what role the make-up of the eye would have one drug effects. I wonder if it the drugs would inhibit the eyes from regulating the amount of light that is allowed into the eye...this would cause everything to appear brighter and possibly cause problems adjusting to dark lighting..just a thought. Maybe this is something we could discuss in class in the future.
Although parts of the chapter were very definition heavy, I found the chapter as a whole to be very eye opening (bad pun anyone?). I was pleasantly surprised about the amount of detail the authors went into while explaining how the eyes work. One of the most interesting parts for me was when the authors explained how things go wrong with the optics of the eye. The authors explained that in order to focus on a distant object there must be no refractive error so the refractive power of the eye is matched to the length of the eyeball. The author explains that the refractive error of Myopia (nearsightedness) occurs when the eyeball is too large for the optics needed and the image will be in front of the retina (blurry image). Hyperopia (farsightedness) is the opposite and the image will be behind the retina. Luckily the eyes can accommodate (lenses get fatter when looking at closer objects) in order to correct for this problem in milder cases. Another interesting thing about Hyperopia is that most babies are born with this condition because the optical parts of the eye develop faster than the actual size of the eyeball (making the eyeball smaller than the optics). I studied optics in another class and I remember working out problems to figure out where the image would be located but I really feel this section of the book helped me understand why we used such techniques. I think it would be very interesting to delve deeper into things that can go wrong with vision in class. I feel that if I understood why things can go wrong it would help me further understand the intricacies of the structure and function of the eye.
I also found the difference between the photoreceptors cones (day light vision, fine visual acuity and color) and rods (night vision) to be interesting. It is amazing how specialized something like the eye is in that they absorb different colors due to different pigment molecules in their inner segment. Simply the type of protein (opsin) in this visual pigment molecule determines the wavelengths that are absorbed. Rhodospin is the pigment found in rods and cones have one of the other three types. It amazes me how such minor differences in chemical makeup can make such a huge difference in physiology. I would definitely like to learn more in class about the research about the third type of photoreceptor (in our ganglion) that research suggests adjusts our biological rhythms. Does this photoreceptor have anything to do with things like Seasonal Affective Disorder?
The least interesting subject for me was probably the types of cells in the lateral (horizontal and amarcrine) and vertical pathways ( biolar, photoreceptors, and ganglion cells). I found this to be less interesting because this area of the text was very definition heavy and I found myself needing to read the margins more. Although less interesting it might be helpful to go back over this material in class in order to clearly identify the differences between all the different types of cells discussed. Perhaps this section would be more interesting if I learn the definitions and the read the section again in order to get the big picture.
This text is unique in the way it gives little tasks for you to check your vision etc. and I think that these little activities are actually helpful to cement the ideas in your mind. Those activities along with clinical examples such as “The Man Who Could Not See Stars” (Retinis pigmentosa: death of photoreceptors and breakdown of pigment epithelium) are probably the most helpful in my learning of the visual system. P.S. Did anyone actually try the pen light example? I actually had a pen light and it worked but I wish the author would have warned that my eyes would hurt so bad I couldn’t read for about a half hour!
After reading chapter 2 I feel that the most interesting thing about the ocular system is it's shear complexity. We all learned the basics in high school biology (rods, cones, optic nerve, etc.), but I had no idea that so many different systems of neurons had to interact with each other to produce the signal that is sent to the occipital lobe. The complexity of this system must have taken eons of evolution.
The second part of the chapter that interests me is it's brief explanation of the amount of research it took to discover all these different mechanisms. The book only breifly mentions the reserach done by Dr. Kuffler using cats, frogs, and horseshoe crabs, so I would like to find out exactly how he tapped electronic measurement equipment into the very tiny and sensitive ganglion cell of a cat.
Personally, I didn't find any part of the chapter to be particularly disinteresting. I recognize that all of the different aspects of the chapter are useful in giving students the big picture concept of what makes us able to percieve an image.
I feel that the most useful information in this chapter is the different occular defects that can occur. Most of us need corrective lenses now and certainly all of us will need them at some point in out lives so understanding what can go wrong and how to fix it is very useful.
The topics that I would like to be covered further in class are the wide variety of differnt kinds of cells and their specific function. The book does a great job of explaining each type of cell, but I would just like a more layman-friendly breakdown of the process so that I can better grasp the communication between them. I am having particular trouble with how these cells work into the large and small geniculate nucleous.
I also found the "photic sneeze reflex" to be the most interesting. People would think I was crazy when I kept sneezing because of the sun. Now I have proof! What's even more intriguing is that scientists still don't have a definite answer for this situation. Francis Bacon, however, proved Aristotle wrong in saying that this is not a reaction from the heat of the sun. Instead, Bacon guessed that the sunlight makes the eyes water which in turn makes moisture seep into the nose. This was proved wrong as well, however. Now current scientists are thinking that this sneeze reflex is because of crossed wires in the brain. This was all very interesting to me because it has been a part of my life and I have been affected by it.
Another topic that really intrigued me was how many parts there are to the eye. It takes a lot more science and pieces to the eye than I thought to make a picture. The first tissue the light touches is the cornea, which is transparent. The cornea interests me as well. This layer has many nerve endings which is what forces the eye to close and create tears if it is scratched. Next is the aqueous humor, a fluid, which supplies oxygen to the cornea and the lens, which is also transparent. What really interested me about the eye that I had never learned was that the pupil is simply a hole in the iris. After the light passes through the lens, it goes into the vitreous humor, which fills up most of the eye. The light is then refracted by this. The final stage is that the refracted light comes into focus by the retina. I had no idea that the function of the eye was so complex, and it's really interesting to me how scientists can figure that type of thing out.
I think the least interesting topic in this chapter to me was the topic about all of the cells because I couldn't really grasp the big picture all that well. So I think that's one topic that would be helpful for you to explain in class. I think the most useful information from this chapter was the section "Eyes that see the light" which explained all about the different parts of the eye. It may be helpful to cover that in class as well more in depth.
I actually know someone who was sneezing so much becasue of the sun. It's funny how much we dont know but it is actually scientifically proved.
Eyes are our way of seeing light and outside stimuli. Our eyes forms images of the outside world, this enables us to recognize and understand objects. The cornea of our eye is the first tissue which visional cues reach. There is a rich supply of transparent sensory nerve endings within the cornea, the visional cues are brought into focus at the retina. Accomodation is the process by which the eye changes to see different things both far and near, our eyes get fater and skinner based on the distant of the object. The ability for our eyes preform accommodation lessens as we get older, this is called presbyopia and bifocuals were invented to help with this problem. They aid in seeing both distant and close objects.
Light is reflected when it bents or changes as it moves through something or our eyeball to our retina. If there is an error within our eyeball eyeglasses can bend light for it to be properly focused on the retina, or correct myopia(nearsightedness) and hyperopia(farsightedness). In the retina, light and visual stimuli is transduced into neural energy that can be understood and experienced by the brain. This process is powered by photoreceptor cells.
Photorecpetor cells are composed of two types rods and cones. Rods function well under dim illumination and is why animals with all-rod retinas are nocturnal where cones need brighter illumination to work.
Bipolar cells receive information with the help of 50 photoreceptors and passes it on to ganglion cells which then send messages off to the brain in the optic nerve. Each ganglion cell has a small window in the world known as it receptive field. Some ganglion cells are more sensitive and tell different things like what color the outside world and its light is.
I think that my favorite part of this chapter was learning about how the eyes works, the different cells and photoreceptors that go into vision, and understanding why a lot of people need glasses to understand and recognizes outside stimuli. The chapter was a little dry for me.
One of the topics I found very interesting in chapter 2 was the blind spot in each of our eyes due. The blind spot is caused by the optic disc where the optic nerve, arteries and veins enter and leave the eye. Figure 2.6 demonstrates this concept. This is interesting to me because every time we look at something we aren't seeing a part of it. fortunately for humans we have two eyes with different blind spots so we do not notice the blind spots.
Another interesting topic in chapter two was the story of the man who couldn't see the stars. He had a condition called retinitis pigmentosa, which causes deterioration of photoreceptors most commonly effecting peripheral vision and night vision. This was interesting to me because it shows how important the rods are. most of the rod shaped photoceptors are on the sides of the eye which is why you can see dim starts better using peripheral vision.
One concept I learned from chapter two to help be better understand the visual system is the concept of light. Light has to be defined as a wave and a photon in order to under stand the visual system. Light gets to our eyes as "waves" but is absorbed as photons. The other part of the chapter that is most useful in understanding the visual system is the different parts of the eye and how they work. The content in this chapter is pretty hard to understand and visualize how it works. I would like to hear more about the individual parts in class and maybe a video or something to help visualize the physical process of seeing.
The blind spot like in your vehicle.
A couple things I found interesting about chapter 2 is myopia and hyperopia. I have known for awhile what nearsightedness and farsightedness are but I didn’t know exactly what they mean so I find it interesting that myopia is nearsightedness and hyperopia is farsightedness. Another aspect of these terms that I find interesting is that if you have one of these conditions, your eyeball is either too large or too small, which I didn’t know was a problem someone could have. Astigmatism is also interesting to me because I know several people who have the condition but I never fully understood that it means that the cornea is unequally curved.
The concepts that I found least interesting were all the cells that make up the retina: horizontal cells, amacrine cells, bipolar cells, and ganglion cells. Although each of these types of cells is important in the make-up of our retina and therefore extremely important in learning about vision, I find the bigger picture more interesting.
Although there are several little things that work together to create vision, I think that the fact that eyes can absorb light and form an image because of the brain is most useful. If vision was solely up to the eye, vision wouldn’t be very useful at all. It’s the power of the brain that really affects what we see and how we see things.
I like the way to put the last few sentences "If vision was solely up to the eye, vision wouldn’t be very useful at all. It’s the power of the brain that really affects what we see and how we see things." It is a good way of stating how the brain helps the visual system work especially how the brain uses adaptation to reverse the image the correct way, etc.
I think most of us seem to agree that the different cells and complexity of the retinal structures were not necessarily the least interesting but the most difficult to comprehend. However, I think after going through the topic in class today I am getting a better understanding of it.
There were several things I found interesting in the chapter that somewhat all correlated together. I’m not the best at retaining technical information and what “x leads to y and z and so forth. One thing I did find interesting was that of communicating to the brain via ganglion cells. Ganglion cells receive their input from bipolar and amacrine cells, which process this input further and sends messages off to the brain through their axons, which gather in the back of the eyeball and emerge together as the optic nerve. There are two types of ganglion cells which include P and M. P ganglion cells are small ganglion cells that receive excitatory input from single midget bipolar cells in the central retina and feed the parvocellular layer of the lateral geniculate nucleus. On the other hand, M ganglion cells feed the magnocellular “large cell” layer of the lateral geniculate nucleus and has an umbrella-like appearance. Furthermore, there is a receptive field “the region on the retina in which visual stimuli influence a neuron’s firing rate.” Kuffler found that the spatial layout of the ganglion cell’s receptive field is essentially concentric; that is, a small circular area in the center responds to an increase in illumination and a surrounding annulus responds to a decrease in illumination. In understanding the ganglion cell there is what is called an On-center and Off-center cell. The On-center is where the cell depolarizes in response to an increase in light intensity, whereas the Off-center cell decreases in light intensity. In addition, retinal ganglion cells act as a filter by editing the information they send on to the brain. The ganglion cells are most sensitive to differences in the intensity of the light in the center and in the surround, and they are relatively unaffected by the average intensity of the light. The amount of light has to do with contrast which is the difference in luminance between an object and the background or between lighter and darker parts of the same object. Overall, ganglion cells, together with the bipolar, amacrine, and horizontal cells act as an image filter, transforming the raw image into a new representation.
A second thing I found interesting was that of retinal information processing. The retina contains five major classes of neurons which include: photoreceptors, horizontal cells, bipolar cells, amacrine cells and ganglion cells. The retina contains around 100 million photoreceptors. There are two types of photoreceptors which involve rods and cones, basically photoreceptors are light-sensitive receptors in the retina. In addition, both types of photoreceptors consist of an outer and inner segment and synaptic terminal. Visual pigment molecules of a protein, the structure of which determines which wavelengths of light they absorb and a chromphore, which captures light photons. Each photoreceptor has only one of the four types of visual pigments found in the human retina. Each cone has one of the other three pigments which respond to long, medium, and short wavelengths. Short wavelengths are sensitive cones which consist of about 5-10% of all the cones and are missing from the center of the fovea. The foveal center is dichromatic meaning it has only two color sensitive cones. There are more long wavelength sensitive cones than medium wavelength sensitive cones.
One thing that I found least interesting was the section on eyes that see light just because it seems pretty redundant in what I have learned in previous years. An eye forms an image of the outside world allowing us to use light to recognize objects, not just determine whether light is present and what direction it is coming from. The first tissue that light will encounter is the cornea which is the window into the eyeball since it is transparent. It is transparent because it contains no blood vessels or blood, which would absorb light. It does however have a rich supply of transparent sensory nerve endings which are there to force the eyes to close and produce tears if the cornea is scratched, preserving its transparency. The watery fluid in the anterior chamber of the eye is called the aqueous humor and the lens inside the eye that enables changing focus is the crystalline lens. To get to the lens, the light must pass through the pupil, which is the hole in our muscular structure also known as the iris. The pupil controls the amount of light that reaches the retina. After light passes through the lens it enters the vitreous chamber (the space between the lens and the retina, where it is refracted for the final time by the vitreous humor which is the transparent fluid that fills the vitreous chamber in the posterior part of the eye. Overall, only some of the light will actually reach the retina which is a light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.
A second thing that I found least interesting was the four optical components of the eye. If you have emmetropia you have the perfect match which there is no refractive errors because the refractive power of the eye is perfectly matched to the length of they eyeball. If you have myopia also known as nearsightedness which I have it means the eyeball is too long for the optics, but can be corrected with negative lenses. If one has hyperopia also known as farsightedness the eyeball is too short for optics, therefore needs to be corrected with positive lenses. One can have a visual defect caused by unequal curving of one or more the refractive surfaces of the eye, usually the cornea which is called astigmatism which I have also in both eyes.
What I think will be most useful in understanding the visual system is that photons are absorbed by encounters with dust, vaporized water and many more and some of the light is scattered by these kinds of particles. Light is reflected when it hits a surface giving it color. In fact, most of the light bounces off the surface accounts for that surface’s light appearance. Also, most light striking a dark surface is absorbed. Light that is neither reflected nor absorbed by the surface is transmitted through the surface. Light is refracted, or bent as it is transmitted. Refraction also occurs when light passes from air to water or into the eyeball. Furthermore, visible light waves have wavelengths between 400 and 700 nanometers. The light waves themselves are not colored; it is only after our visual system interprets an incoming wave that we perceive the light as violet or red. Also, the two ways to conceptualize light is through a stream of photons and as a wave. Photons are tiny particles that each consist of one quantum of energy.
Some topic or concepts that I would like covered in more depth are: Lateral inhibition, amacrine cells, and bipolar cells vs midget bipolar cells.
Key Terms: Hyperopia, Emmetropia, Myopia, Ganglion cells, P and M ganglion cells, retina, fovea, cornea, iris, pupil, refraction, transparent, aqueous humor, crystalline lens, vitreous humor, absorbed, scattered, reflected, transmitted, image, wave, photon, photoreceptors, astigmatism, cones, rods, contrast, filter, amacrine cells, On and Off-center cells.
When the cornea is not spherical, the result is astigmatism. With astigmatism, vertical lines might be focused slightly in front of the retina, while horizontal lines are focused slightly behind it or vice versa. If a person has a reasonable degree of uncorrected astigmatism, one or more of the lines might appear to be out of focus, while other lines appear sharp. Lenses that have two focal points (that is, lenses that provide different amounts of focusing power in the horizontal and vertical planes) can correct astigmatism.
Moreover, Vision begins in the retina, when light is absorbed by rods or cones. The retina is like a minicomputer that transduces light energy into neural energy. Retinal ganglion cells have center-surround receptive fields and are concerned with changes in contrast (the difference in intensity between adjacent bits of the scene). The retina sends information to the brain via the ganglion cells, neurons whose axons make up the optic nerves. The visual system deals with large variations in overall light intensity by a) regulating the amount of light entering the eyeball, b) using different types of photoreceptors in different situations, and c) effectively throwing away photon we don't need.
Lateral inhibition is antagonistic neural interaction between adjacent regions of the retina. When it comes to the illusion called Mach Bands, there is a relation with lateral inhibition. The illusion known as Mach bands is due to the face that there is an illusory bright stripe and an illusory dark stripe. These dark and bright stipes are illusions formed by the visual system. Mach suggested that the illusory stripes were produced by lateral interactions in the retina. Moreover, if a luminance ramp edge is filtered by a center surround (or lateral inhibition) weighing function, the output will have an undershoot and an overshoot that seem to correspond to Mach bands.
Center Surround has 4 things. 1) light hyperpolizes the cone (or rod). It turns on the cone and thereby excites the bipolar cell directly underneath. That bipolar cell then excites its ganglion cell. 2) The neighbor cones also excite horizontal cells. The horizontal cells send processing laterally and inhibit the center bipolar cell. Cells are randomly firing. 3) A small spot of light excites the bipolar cell but not its neighbors. There is no inhibition, so it is free to get really excited and excite the ganglion cell, which fires like crazy. 4)A ring of light excites only the neighbors. Now, the bipolar cell is strongly inhibited, with no excitation. In response to this strong silencing of the bipolar cell, the ganglion cell shuts down as well. it will not turn on again until the light is turned off, at which time you will see a rebound "off-response."
I disagree that this was one of the interesting topics, but I found center surround cells to be difficult to understand and quite complicated. There are On-center cells, which are cells that depolarize in response to an increase in light intensity in its receptive field center. This type of cell increases its firing when a light is turned on in the center of the receptive field. It decreases when the light is turned on the field surrounding the center. Some of these ganglion cells do the opposite, however, and are called Off-center cells. These ganglion cells (in the retina) depolarize when they are reacting to a decrease in light intensity in the receptive field center. These ganglion cells act as a filter that allows the passage of some frequencies or digital elements and blocks the passage of others. They actually edit the information they send to the brain. Also, ganglion cells are most sensitive to the differences in light intensity in the center/surround, and aren't really affected by the average intensity of the light. This helps our vision because the average of intensities of light falling on the retina can vary a lot depending on different situations. The contrast, or difference in luminance between an object and the background (or lighter and dark parts) will be somewhat the same in many lighting conditions. This topic also correlates with the video we saw about the cat and the neurons firing as well as mach bands and illusory contour.
I really find the aspect of our eye called accommodation interesting. Its amazing that the lens in our eye can be manipulated to focus on objects at such a large range of distance. If you think of the lens in a camera, one lens has only one distance. The zooming action on a camera comes from the interaction of 2 lenses and even that process is so inferior to the focusing ability of the human eye. The ciliary muscles around the lens contract and bend the lens into a very precise shape to focus the light entering through the cornea. It would take literally dozens of different sized and shaped lenses to be able to reproduce the clarity and focus that the eye produces with one lens. That is amazing. Our natural lens is so successful because of the fact that it is not a rigid piece of glass but a flexible piece of tissue. It has no direct blood flow so that it can remain transparent and therefore relies on the aqueous humor to deliver oxygen and nutrients to its cells.
The pigment portion of our eyes, the pigment epithelium, is pretty cool. I've always wondered whether eye color has an effect on our vision, perhaps it modifies individual color perception. I mean the pigment is there to capture photons of light, but certain colors only capture certain colors of light and reflect everything else. So someone with brown eyes is reflecting more brown light than someone with blue eyes. Does this mean that the person with brown eyes is receiving less "brown stimulation" to their brain? It is also interesting that rods and cones seem to be the only neural receptors that do not follow the action potential process. Rather they operate in what are called graded potentials. Basically the rod, or cone, is stimulated by light in the area called the chromophore. Positively charged calcium ions leave the cell causing the cell to become more negatively charged enabling an electrical signal to be sent to the next cell. But rather than an "all or nothing" fire these cells operate with a neurotransmitter called glutamate. The amount of glutamate in the synapse between the receptor and the bipolar cell is inversely proportional to the amount of light being absorbed. This allows the signal to be graded as very strong or not so strong depending on the amount of light stimulating the receptor. The information is sent to the bipolar cell which is receiving information from as many as 50 other receptors (or in the case of cones as little as one). It formulates a sort of piece of an image that is combined with the other thousand pieces in the brain. The book discusses the trade off between sensitivity and acuity. The pooling of receptors into one bipolar cell allows that cell to scan much more information to determine if any of the receptors were stimulated. But if some where and some weren't and the pattern by which they were stimulated kind of gets lost when you consolidate all the signals into one cell's processing. In the fovea cones have a 1:1 ratio to their bipolar cells, which explains why we have such fine acuity in the center of our focus, and why peripheral vision is so unclear.
I would like to hear more about off center and on center ganglion cells. Ganglions in general are pretty cool its like they are miniature brains deciding what information is relevant enough to pass on and largely leaves out constants like average light level and whatnot. It looks for changes not constants and when there is no contrast to report the cell just chooses not to increase its firing rate to send any information. We talked about this in class, it's adaptation. Almost every aspect of our bodies operate in a way to expend the least amount of energy possible. If information is unnecessary or constant, down to the very level of our cells, we ignore it.
Two things that I found interesting from ch. 2 was the structure of the eye and all parts work together and the photic sneeze reflex. You never really think about everything that goes into you ability to see. I think it's interesting how the retina is so complex and has so many cells in that one area. I never thought about sneezing when you are suddenly exposed to sunlight. I usually do sneeze when coming out of a movie theather but i thought it was just something in the air. I think it's intersting that that is a response to sunlight.
Two things that i found least interesting were specifics of how the cells work. I thought the textbook could have done a better job of how the bipolar cells work. I like when text books give examples and pictures instead of just listing a bunch of words. It makes it a lot easier to understand what they are talking about. The other thing I found least interesting is lateral inhibition. Again I found this least interesting because of how the book presented the information.
For understanding the visual system I found the beginning of the chapter to be most useful. It highlighted what everything was and its functions. I liked how the book used the visual images to explain how the eye works. I think that it is a lot easier to understand the material with the visual aids.
Two things I would like to hear more about in class are about how the cells like the bipolar cells, ganglion cells and the amacrine cells in the eye work in the visual system. I would also like to hear a little more about mach bands.
One of the topics from chapter 2 that I found interesting was the topics of myopia, hyperopia, and emmetropia. Emmetropia is what occurs when the refractive power of the four optical components of the eye perfectly match the length of the eyeball. When emmetropia does not occur it is due to the eyeball either being too long or too short. When the eyeball is too long, the image is focused in front of the retina and the image will appear blury and is called myopia or nearsightedness. When the eyeball is too short, the image is focused behind the retina and is called hyperopia or farsightedness. Both myopia and hyperopia can be corrected with lenses. This topic interests me because I have been wearing glasses since I was in second grade and I think it is interesting to learn what causes someone to be either nearsighted or farsighted.
Another topic from chapter 2 that I found interesting was the fundus and the blind spot. According to the text, the fundus is the back surface of the eyes and is what doctors look at to view the retina and the optic verve. when looking at the fundus the doctor would see a white circle which is known as the optic disc. this is where the arteries and veins that feed the retina enter the eye and where the axons of ganglion cells leave the eye via the optic nerve. also, it contains no photoreceptors (which are light sensitive receptors in the retina, that when they sense light they can stimulate neurons in the intermediate layers, including bipolar cells, horizontal cells and amacrine cells) and is therefor blind. The book explains two different ways to see your own blind spot. Normally you do not see your blind spot because the brain just fills in what is missing. I have heard about the blind spot before in my bio-psychology class but never really knew what caused it. Now that I know about the fundus and the optic disc, I know what cases the blind spot and why.
The two things in the chapter that I found the least interesting were the concepts of the outer segment and the inner segment of a photoreceptor and the synaptic terminal. The outer segment is part of a photoreceptor which contains photo-pigment molecules and the inner segment is the part of the photoreceptor that lies between the outer segment and the cell nucleus. The synaptic terminal is the location where axons terminate at the synapse for transmission of information by release of a chemical transmitter. The reason why I found these two terms and concepts least interesting is because it seemed like the authors of the text were throwing a lot of terms at the readers without going into great detail or explanation about them. I don't think that throwing in these terms really contributed much to my knowledge of how vision works and how the eyeball is constructed.
I think that what I read in chapter 2 about the five major classes of neurons in the retina will be most useful in understanding the visual system. Those 5 classes are photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganclion cells. There are 2 main types of photoreceptors and they are rods and cones. rods are photoreceptors that are specialized fro night vision and cones are photoreceptors that are specialized for daylight vision, fine visual acutity and color. Since human retinas have both rods and cones they are considered to be duplex retinas. Rods and cones operate best under different lighting conditions. Rods work better when there is dim lighting, whereas cones need bright light to work at their full potential. Another important difference between rods and cones is that rods cannot signal differences in color. Cones, on the other hand, have one of three different photopigments that differ in the wavelengths at which they absorb light most efficiently.
The next major type of neuron is the horizontal cells. they are specialized retinal cells that contact both photoreceptor and bipolar cells. They are a big part in lateral inhibition which is antagonistic neural interaction between adjacent regions of the retina.
Amacrine cells are retinal cells that are found in the inner synaptic layer which make synaptic contacts with bipolar cells, ganglion cells and one another. They are a part of the lateral pathways and run perpendicular to the photoreceptors. According to the text, the precise function of these cells is still unknown.
There are a few different kinds of bipolar cells and the type determines the function. Diffuse bipolar cells recieve input from as many as 50 photoreceptors, pools this informations, and passes it on to a ganglion cell. Midget bipolar cells receive input from single cones and pass this information on to single ganglion cells.
Ganglion cells receive their input from bipolar and amacrine cells, process this input further and send messages off to the brain through their axons, which gather in the back of the eyeball and emerge together as the optic nerve. There are also numerous types of ganglion cells. midget ganglion cells are small ganglion cells that receive excitatory input form single midget bipolar cells in the central retina. parasol ganglion cells are an unistratistied ganglion cell that looks like and umbrella and receives input form diffuse bipolar cells.
Two topics that I would like to cover more in depth in class would be photoactivation and hyperolarization. According to the text photoactivation is activation of the photoreceptors by light and hyperpolarization is and increase in membrane potential where the inner membrane surface becomes more negative than the outer membrane surface.
I just wanted to elaborate more on your comment "Cones, on the other hand, have one of three different photopigments that differ in the wavelengths at which they absorb light most efficiently." Each photoreceptor includes one of four types of visual pigments found in the retina. These pigments alllow the receptor to absorb certain wavelengths of light. Rods have the pigment called rhodopsin. Since all rods contain the same pigment they cannot signal differentes in color. Cones on the other had have one of three pigments that respond to short (S-cones=blue), medium (M-cones=green), and long (L-cones=red) wavelengths. Because cones have different pigments they are able to signal differences in color and provide the basis for our color vision.
I found this chapter extremely difficult to read. I think I'm going to have to go through it a couple more times before fully understanding the inner workings of the eye. However, my first time through I was able to pick up some information for the post.
I found the most interesting part of the chapter to be the information on photic sneeze reflex. I thought it was funny that Aristotle believed it happened because of heat on the nose. However, it's interesting to me that it's still unknown as to why this happens. This happens to me all the time and I often use the idea to make myself sneeze when I'm unable to.
Another section of the chapter that I enjoyed reading was the "Man Who Could Not See Stars". This section was about a disease known as retinitis pigmentosa. The text states that the disease is characterized by the degeneration of the pigment epithelium and the death of photoreceptors. It also states that rods are affected before the cones therefore you first start to notice impairment in the peripheral vision and under low light conditions.
The parts of the chapter that I disliked the least was the basically the explanations of how the eye works. I found the way it was described extremely confusing and therefore can't really say much on the information right now. I plan on going back and reading again and possibly looking up some information online (how the eye works for dummies possibly) to make myself better understand it. Hopefully then I can come back and write more about the topic.
I think one thing that will help me in class is how we actually see. The books states that often times people think that our eyes work like video cameras, which I know isn't true but it's much easier to think of it this way. The text describes the visual system as a series of filters. It talks about how each stage in our visual system has it's own responsibility for extracting a particular aspect of the visual world and then passing it on the the next stage.
One thing I would like to learn more about in class is the center-surround receptive fields. I've heard about Kuffler's experiments before but I still find it extremely interesting for some reason. The study resulted in two important pieces of information. One our ganglion cells respond best to spots of a particular size and two the cells are most sensitive to differences in the intensity of light and are basically unaffected by the average intensity.
The two things that I liked the most from chapter 2 are rods and cone, and filter. First, I thought rods and cones were interesting because after reading the chapter I actually got a better understanding of rods and cones. It is quite interesting to me that rods and cones differ in that one works better in the light and one works better in the dark. Rods, which work better in the dark, are also bigger than cones. Rods however, cannot tell you anything about color. On the other hand, cones work better with more light. Cones are smaller but can distinguish different colors. This was interesting to me. The second thing that I found very interesting was the filter. I found this interesting because that our filter lets some stuff in but keeps some stuff out. I thought it was pretty cool that our brain doesn’t actually interpret everything that we see because of the filter.
The two things that I found least interesting were communicating to the brain via ganglion cells and light. First, I honestly found communicating to the brain via ganglion cells because I did not understand it. As I was reading that section I was lost. I tried to pull it together to get a better understanding but it just wasn’t happening. I did understand the receptive field and ON-center field stuff but I just couldn’t tie it together to get a better understanding of the bigger picture. Second, I found light to also be a boring part in the chapter because it is something I have always known about growing up. I have previously learned about light waves, so that stuff seemed a bit repetitive. I am just more interested about learning new stuff, which the rest of the chapter definitely provided!
I think the most helpful thing that I learned about in the chapter that will help me understand the visual system is the overall layout of the eye. The cornea is the first tissue of the eye. It is transparent. The pupil is a hole in the iris. The iris lets more or less light into the eye, it also tells us colors. The retina, detects light and tell the brain how the light relates to objects in the world, this is where seeing really begins. Knowing this basic understanding of the eye really helps me put the bigger picture into focus.
Finally, the two topics that I would like for you to cover more in depth in class are, communication to the brain via ganglion cells, and Retinitis pigmentosa (RP). First, as I stated earlier I don’t have a very good understanding of the communication to the brain by way of ganglion cells. That section of the chapter went completely over my head. I know it is important to better understand that. Second, Retinitis pigmentosa (RP) sounds like something I might have. My eyes are terrible, the always have been. While reading this, it was kind of scary. I can really see myself having this as I grow older. Everyone in my family has poor eye sight, so I would love to learn more about RP.
I also found the difference between rods and cones interesting! Rods and cones are important for our adaptation to light and dark. Rods are sensitive to dim light but become overwhelmed in bright light. Cones are much less sensitive the rods and they take over in bright light. In bright light rods do not function well and cones are able to recover their sensitivity quickly in bright light to adapt. Rods are not as quick to recover and in dim light take up to 30mins to adapt. The rod and cone adaptation allows us to not be bothered by variations in light levels. Ganglion cells take it a step further by ignoring whatever variation in light level is left over
Ch. 2
I went back and reread chapter two because I feel like there was a lot more information in there that I should have pulled. As I read, I focused on the retina, color vision, lateral inhibition, center surround cells, and astigmatism. First, I reread the retina section in great detail. I knew the retina is an important player in the visual system, but I didn’t know how important it really is. This is where seeing begins! The retina is in the back of the eye, it consists of rods and cones. The purpose of the retina is to take and image and from the lens and send it to the brain. The retina sends the image to the brain by the optic nerve. The retina also brings things into focus. The retina detects light, but not all light makes it in. I thought this was interesting. There are also photoreceptors and neurons in the retina. Rods and cones are also found in the retina. Rods and cones always fascinate me because they serve completely different purposes. The purpose of the rod are to see at night; cones to see during the day. Next, I read in more detail about color vision. After seeing the PowerPoint in class I had to go back and read because I didn’t get a very good understanding the first time. It is very interesting to me that we really only see red, blue, and green. Then how is it that we see all the other colors that we see? All the other colors that we see are combinations of the red, green, and blue that we see. Seeing color takes place in the retina by changes in pigments. So why are some people color blind? This is due to an absence of cones. I really feel I learned a great deal on the second go around on this. Third, I learned more about lateral inhibition. I remember seeing the slides in class that were about mach bands. Lateral inhibition are fascinating to me. These are those cool things that trick our brain. I think the most straight forward way for me to remember lateral inhibition is that what the eye sees is not exactly what it reports to the brain. Therefore, some of the things are made up by the brain to fill in for what is not reported. I found the “lateral inhibition network” to be an interesting player in this. This is where information from neurons is lost. Both the retina and pupil are also important in lateral inhibition. Lateral inhibition explains optical illusions. I remember you explaining mach bands in class and I didn’t really know what you were talking about, but after going back and reading that section again, I have a much better understanding. This (lateral inhibition filtering out neurons) explains why the two boxes side to side that are different shades appear to have a light and dark line near the center. Next, I went back and reread the part about center surround cells. When you were going over this in class I caught a good chunk of it but I feel I learned even more by going back to read it. This all takes place in the receptive field. It interest me how many things are firing off in my eye without me even knowing what is going on. Light is what turns on the rods and cones that excite the bipolar cell (which I still don’t completely understand). The bipolar cell excites the ganglion cell and those fire like crazy! The example of this that jumps out at me is the Herman grid illusion that you showed in class. I remember seeing all these little dots that were not really there. Finally, the last thing from this chapter I went back to read was astigmatism because I know this is very important, especially because I personally have horrible eyes. My cornea has to be wrecked, there is no way it can be perfectly round. As a child, I think I remember my optometrist telling me that I had astigmatism but I never knew what it was. The best way the book explains it is “vertical lines may be focused slightly inforont of the retina, while horizontal lines are focused slightly behind it (or vise versa).” So this is what my eye doctor was saying back then… If lenses with two focal points are the way to fix it, I wonder if my glasses have two focal points?
Sorry about this one being a lot longer, but after seeing the powerpoint, a lot more things started clicking
One of the topics from chapter 2 that I found interesting was the topic of Cataracts. Cataracts are opacities of the lens and they are caused by irregularity of the crystallins. Crystallins are a class of proteins that make up the lens. The lens is normally transparent because of the crystallins. Anything that interferes with the regularity of the crystallins will result in loss of transparency. Cataracts impact the people of the world in so many ways. How will they make a living for their families once they are so bad they can't work? How will they afford the surgery to remove them? Your eyes can make or brake you.
Another topic from chapter 2 that I found interesting was the topic of the seven basic types of retinal cells and how they connect to one another. Rods and cones, which rest against a layer of pigment epithelium cells, are the only types of cells in the retina that respond directly to light rays, while ganglion cells are the only type of cells in the retina that communicate directly with the brain (via the optic nerve). In between these two layers of cells is a third layer, composed of horizontal, bipolar, and amacrine cells. These intermediate neurons transform the raw light information communicated by the receptors into a form that is more usable by the brain. Way Cool!
One thing that I found least interesting was the illustration of the electromagnetic spectrum from gamma rays to radio and television waves. I did find it interesting how they say that the light waves themselves are not colored; it is only after our visual system interprets an incoming wave that we perceive the light as "violet" or "red".
Another least interesting was the P and M ganglion cells. M ganglion cells are cells that resemble little umbrellas that receive excitatory input from diffuse bipolar cells and feed the magnocellular layer of the lateral geniculate nucleus. The P ganglion cells are small ganglion cells that receive excitatory input from single midget bipolar cells in the central retina and feed the parvocellular layer of the lateral geniculate nucleus.
The most useful information would have to be the structure of the eye. It is better to know all the part names before trying to understand their functions. Pigment Epithelium is a thin layer of cells is sandwiched between the retina and sclera. It supplies nutrients to retinal receptor cells and holds reserve pigment molecules that the receptors use to detect light rays. Ciliary muscles are responsible for changing the shape of the lens to bring close objects into focus. They are connected to the lens by tiny fibers called the “zonules of Zinn.” The iris expands and contracts to regulate the amount of light entering the eye through the pupil (the hole in the middle of the iris). When you are outside in the bright sunlight the iris expands (causing the pupil to shrink) to block out most of the light that would enter your eyeball, so that the image on your retina is not “overexposed.” When lighting conditions are dim, the iris contracts (causing the pupil to enlarge) to allow as much light as possible to enter, giving you the greatest chance possible to see what is there in the dim light. The retina is the computer of the eye—the place where light information is transduced into neural firing. The retina, as well as the pigment epithelium and sclera, are not represented to scale here. In actuality, the retina is the same thickness as a piece of paper!
Cones and rods in the retina detect light energy and transduce it into neural energy. Neural signals from the photoreceptors eventually pass through retinal ganglion cells and exit the retina via the optic nerve. Note that the retina thins into a dented region directly behind the pupil. This area is called the fovea (a term derived from a Greek word meaning “pit,” describing its shape), and is the portion of the retina that “sees” the world most clearly. The fovea is packed with color-sensitive cones and processes images with extremely high resolution. The interior of the eye is filled with the vitreous humor—a jelly-like substance that helps the eye maintain its ball shape and refracts light rays. The vitreous is generally clear (otherwise, light would never make it to the retina!), but contains some small opaque particles that can occasionally be seen as “floaters.” Four structures—the cornea, aqueous humor, lens, and vitreous humor—all refract light rays and help to focus images on the retina. The lens, while not the most powerful refractor (the cornea actually does more of the focusing), is the only one of these structures that can change its refractive power to bring objects at different distances into focus. The lens becomes fatter to focus on near objects and returns to its normal, thinner, shape when viewing far objects, a process called accommodation. All humans experience a gradual stiffening of their lenses with age. This decreases the ability to focus on near objects, a condition known as presbyopia. At about 40–50 years old, most adults will need glasses to read. The glasses compensate for the focusing power that the lens has lost. Lenses can also form cataracts and become opaque, blocking light from entering the eye. Treatment of cataracts usually involves removing the natural lens and replacing it with an artificial one. The space between the cornea and the lens/iris is filled with aqueous humor, a transparent substance derived from blood that has had proteins and other suspended particles removed. The aqueous humor supplies oxygen and other nutrients to the cornea and lens, removes waste products from these and other structures, and transmits and helps to focus light rays. The axons from ganglion cells in the retina gather together in the optic nerve for their journey into the brain (this is why these two structures blend into each other in the figure). The small area of the retina where the optic nerve emerges contains no receptor cells, forming a “blind spot” in your visual field. Your brain automatically fills in the blind spot with information from surrounding areas. You can observe the filling-in process using the figure below. Close your left eye, focus on the black circle with your right eye, and move your head closer or farther from the monitor (you will probably have to be about 6 inches away) until the lines connect to each other across the white circle. The cornea, which is continuous with the sclera but clear, is the first eye structure that light encounters as it is transmitted into the eye. The cornea serves as a barrier to the outside world and does much of the focusing and refracting of light rays as they pass through it. A damaged cornea can be very painful, due to the large numbers of free nerve endings (pain detectors) in them. Minor scratches usually heal on their own. Permanent damage can often be corrected by corneal transplant surgery. The sclera is the tough outer covering of the eye that holds the rest of the structures in. It is continuous with the cornea in front, and forms a sheath around the optic nerve in back. The sclerae are the “whites of your eyes.” The tiny red lines that you see in the sclerae when you look in the mirror (especially if you are tired) are blood vessels. The indentation on the side of the sclera towards your nose is a tear duct (in the frontal view of the eye, it is on the left). Know it will be a lot easier to understand what is going on in that eye of yours.
Finally, the two topic that I would like for you to cover more in depth in class is retinitis pigmentosa. A progressive degeneration of the retina that affects night vision and peripheral vision. RP commonly runs in families and can be caused by defects in a number of different genes that have recently been identified.
After going back and reading the chapter again I was able to figure out a little more about the eye. I still find the information boring but I was able to figure out more of what I read. The inner workings of the eye is an extremely complex topic. The first part of the eye that light goes through is the cornea. This is a transparent tissue that contains no blood vessels or blood. The cornea has multiple nerve endings which are what forces the eyes to close and produce tears if necessary. The cornea is generally spherical in shape. When it is not spherical is is called astigmatism. An astigmatism can cause blurred vision, because the irregular shape of the cornea prevents the light from being focused to a single point. My eye doctor always told me when you have an astigmatism you’re eye is shaped more like a football than a baseball. Behind the cornea is the aqueous humor which is a fluid derived from blood. This supplies oxygen and nutrients to the cornea as well as removes waste. The pupil is located is a hole in the iris (the part the gives our eyes color). The pupil controls the amount of light that is allowed into the eye. The iris expands and contracts according to how bright the light is which allows more or less light to reach the retina. After being allowed in the light would reach the vitreous humor which is a gel=like substance and is generally transparent. The vitreous chamber comprises about 80% of the volume of the eye. After going through the vitreous chamber light reaches the retina which detects light and then transfers information about the aspects of light that are related to objects. The retina converts light energy into neural energy which can then be understood by our brains. The retina is comprised of several layers and is an extremely complex structure. The retina contains about 100 million photoreceptors, 90 million rods and 4-5 million cones, which capture light and initiate seeing by producing chemical signals. Rods function in dim lighting conditions and can only provide information in black/white. Cones function in brighter conditions and have three photopigments which allow us to see color. One part of the retina contains no photoreceptors and is usually referred to as our blind spot, however, it’s name is the optic disc. The optic disc is where arteries and veings enter the eye and where axons leave the eye via the optic nerve.