Reading Activity Week #3 (Due Monday)

One thing from the chapter I found interesting is the development of spatial vision. I find it amazing that an infants’ peak contrast sensitivity can reach adult levels by 9 weeks old but that the rest of their vision still has to develop for many years. I also found the condition of amblyopia to be very interesting. Amblyopia is caused by strabismus, which is when one eye is turned so that both eyes don't see the same things. Having this condition and not fixing it soon enough can cause damage forever because the cortex can't process the information a new eye sends out. Luckily, doctors can spot these conditions early enough that they can be fixed.

Two things I found very uninteresting is the cycles per degree and layers of the LGN. I found these things uninteresting mostly because I didn't understand them. The cycles per degree seem so miniscule that it's hard to grasp how they can even be that important. Although I think the LGN is interesting on the big scale of things, I don't think each separate part is.

This whole chapter was pretty confusing to me so I'd like to have more clarification on pretty much everything.

Has anyone else noticed that the time stamps for the blogs are off? I posted the above comment at 6pm on Monday. ;)

One topic I found interesting was how eye doctors measure our eyesight. Although this wasn't 100% clear to me, I learned that acuity is the smallest spacial detail that can be resolved in our eyesight. Eye doctors measure this in terms like 20/20. However, visual scientists measure our eyesight by talking about the smallest visual angle of a cycle of the grating that we can see. The cycle is a pair that consists of one dark bar and one bright bar. Using letters to decide whether one had perfect vision or needed glasses was invented by Herman Snellen. He designed the set of block letters so that the letter was five times the size of the stokes that made up the letter. He used the gratings, which are parallel bars or lines in these letters to decide this as well. According to Snellen, visual acuity is the distance at which a person can just barely identify letters/ the distance at which someone with normal eyesight can identify the letters. It is called 20/20 because the people were placed at 20 feet away and the size of the letters were adjusted instead of the viewers. Also, a 20/20 letter is at 5 arc minutes or .083 degree. Therefore, if you can read a 20/20 letter, you can see the detail that subtends 1 minute of arc. Therefore, if a person has to be 20 feet away from a letter that someone who has normal vision can read at 40 feet, that person would have 20/40 vision. I found this very interesting, since I never knew really any of that!
I found spacial frequencies to be very interesting to me. In the picture in our book of Abraham Lincoln, we were able to tell who the picture was by simply squinting our eyes. The picture then looked almost as though it was the picture we always see, instead of the blocks. Also, there were pictures of a boy in low frequency and high frequency as well. It was stated that different spacial frequencies enhance different types of information. The low frequency photo showed mainly the outline of the face while the high frequency photo showed information in fine detail of the face.
I found the types of cells described by Hubel and Weisel somewhat uninteresting. They stated that cortical neurons come in many varieties such as simple cells and complex cells. Simple cells have defined regions and include edge detectors and stripe detectors. The edge detector would see light on on side and darkness on the other side of its receptive field. The stripe detector sees a line of light at a particular width and is usually surrounded by darkness on both sides. In complex cells, however, instead of just responding when a stripe is presented in the center of the receptive field, the complex cell will respond when the stripe is present anywhere in the receptive field. The other parts of this were a little bit confusing to me, so I'd like to learn more about that.
One other topic that I found was uninteresting, at least from the textbook perspective, would be the topics about the lateral geniculate nuclei, which is a structure in the midbrain, or thalamus, that receives information from the ganglion cells in the retina and leads to the visual cortex. This also included the topic about striate cortex which receives inputs from the lateral geniculate nucleus. These things did not keep my attention and was hard to grasp. Therefore, I'd like to learn about these things as well. Any other information from the chapter would be helpful because it was hard to stay focused throughout.

Overall I found it very interesting how specialized our visual systems are. The lateral geniculatenuclei (LGN)(located in the thalamus with input and output conncections) act as a relay statin and is a structure with six layers. Interestingly, the top two and bottom four layers have very different functions. The top two layers (magnocellular layers) get their input from M ganglion cells and its pathway responds to fast movements . The bottom four layers (parvocellular layers) get their input from P-ganglion cells and are used in focusing on details of non-moving objects. The specialized nature extends to the research findings of Hubel and Wiesel on the striate cortex. I also it interesting that neurons respond at varying degrees depending on the orientation of the line in the receptive field (orientation tuning). However, I did find it odd that our visual acuity and contrast sensitivity are better for horizontal and vertical objects (than oblique targets). It was also very interesting to learn about spatial-frequency channels (pattern analyzer ran by cortical neurons) that detect specific ranges of frequencies and orientations. The low frequencies deal are in charge of analyzing the broad outlines of something where the higher frequencies are in charge of the fine details. Although I could already tell that the hidden person activity was Abraham Lincoln, it was pretty cool that the mask somewhat disappeared when I squinted my eyes. This chapter was fairly hard to follow at times but it is becoming increasingly apparent how complex the visual system actually is. I only wish that the authors used more examples throughout this chapter in order to organize the concepts more easily in my mind. I feel that I learn best with examples ( I think that is why I enjoyed things like the high frequency and low frequency pictures) and it would help my retention of the material to go over more examples in class as well as what happens when certain areas of the visual system are not working properly. The section of the chapter that I enjoyed least was the part on sin wave gratings mostly because I found it difficult to see the big picture. The authors attempted to explain real life applications of this concept in the section “Why Sine Wave Gratings” but I wish they would of elaborated more.

This chapter looks at how our eyes can pick up images on objects in our world and how it communicates that information to the brain. Can we ever see the object how it really is? the details and the size of the object? Seeing 20/20 does not mean one sees objects perfectly how they are, it means that a normal, average person can see and identities the object at 20ft away. Thus being able to see is all on a comparative. The book states that Acuity is the smallest detail that cane been seen, and as our book states, our fovea is the area of the area that picks up the detail. We have the most cones within the fovea. Thus we can see things more clearly when they are perceived directly on our fovea, and our head naturally turns so that images reflect on to our fovea if we need to pick up detail.

All Information from the photoreceptors makes its way through the retina to the ganglion cells and from there is sent along the optic nerve to the brain. The neurons in our visual cortex make up what is known as the striate cortex. This area evaluates the information as strips, and edges and such. Thus from what I understand, ganglion cells respond to spots or “stars” of light as the book describes them, where on the other hand our striate cortex interprets the information to be stripes lines, or edges.

Our ganglion cells sent information to the visual cortex. Our visual cortex is responsible for orderly mapping our world and determining information about our visual field, the width, color, and other characteristics of the stripes of the objects we see. The primary visual cortex or the striate cortex is the first of the cortex to receive information, this part of our brain orientating our world, by filtering some stimulates while responding closely to others. Our lateral geniculate nucleus is the link between the retina and the visual cortex. This area is where the thalamus receives information from the ganglion cells and communicates to the visual cortex. There are two layers in the LGNs. The larger and lower of the two layer is the magnocellular level. This level receives information from the M ganglion cells; cells that respond to large, fast-moving objects. The other level, the parvocellular level, receives input from P ganglion cells which responds to stationary objects.

Scientists are nterested in how our eyes see these contrasts, like stripes or grating, and use tools such as the Sine Wave Gratings. If the white and the dark part of the grating fall on separate cones then one can make out stripes. On the other hand when the spatial frequency is too high, both the dark and bright stripes fall within the receptive center, making it more one tone. This part is the most confusing for me, and I am sure that I don’t have it exactly right, I would like to talk about this more in class.

What I find the most interesting learning about the different part of the eye and brain, and how they work together and separately. What if we did not have a properly functioning visual cortex. Would we not see edges or contrast between objects or angles just see blobs of light in every direction. What if we had a working fovea which intakes details, without a properly functioning visual cortex. What would we see.

One of the things that I found interesting in Chapter 3 was the description of the 20/20 vision. It was interesting to learn that they do not describe acuity in terms of angles (the angle subtended by an object at the retina) and cycles (for a grating a pair consisting of one dark bar and one bright bar). They instead describe visual acuity by making you read numbers and letters of varying sizes. Since I do wear glasses I have taken this test quite often, but it is nice to know exactly what the purpose is and how they determine my visual acuity from it. Also, the history of how the modern visual test came about was very interesting. I never would have guessed that it started with a simple set of block letters. Today, 20/20 vision is considered to be perfect, and I have always wondered where the 20/20 came from. After reading the text I know that the first 20 is from being positioned 20 feet away from what you are reading. And that the second number depends on how far away from it you are when you can read it. It was also interesting that young adults should actually have 20/15 vision.

The second thing that I found interesting in chapter 3 was the contrast sensitivity function (CSF). The CSF is "a function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus". In other words, the CSF is a upside down u-shaped graph that shows what is visible to us. On the y-axis of the graph, is the spatial frequency (cycles/degrees) and on the x-axis contrast sensitivity. The book provides a figure that allows you to test your own CSF. I thought this was a really neat example and helped me understand the topic better.

The topics that I had a hard time concentrating on or understanding are also the topics that I found the least interesting. These topics are Cortical magnification and the striate cortex (AKA primary visual cortex and V1) Cortical magnification is defined in the book as the amount of cortical area (usually specified in millimeters) devotedto a specific region, ex the visual field. The Striate cortex is defined as the area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus as well as feedback from other brain areas and is responsible for processing visual information. I would like to go over these topics more in depth so that I can gain a better understanding of them.

I think that an understanding of the lateral geniculate nuclei (LGN's) will be the most useful to understanding the visual system. According to the text, LGN's act as relay stations on the way from the retina to the cortex. There are six layers to the LGN's. The bottom two layers are larger than the top four layers and are called the magnocelllular layers. The top and smaller layers are refered to as the parvocullular layers. The magnocullular layers receive input from parasol ganglion cells in the retinas, whereas the parvocullular layers receive input form midget ganglion cells. Also, another difference is that the magnocellular pathway responds to large, fast-moving objects and the parvocellular pathway s responsible for processing details of stationary targets.

Another important aspect to understanding the visual system is topographical mapping. This is the orderly mapping of the world in the lateral geniculate nucleus and the visual cortex. It provides us with a neural basis for knowing where things are in space. the begin the left LGN receives projections from the left sides of the retinas in both eyes and the right LGN receives projections from the right sides of the retinas. Then each layer of the LGN receives input from one or the other eye. Layers 1, 4 and 6 of the right LGN listen to the left eye (also refered to as the contralateral (referring to the opposite side of the body or brain)) while layers 2,3 and 5 receive input from the right (ipsilateral-- referring to the same side of the body or brain). Each of these layers has its own organized map of a complete half of the visual field (topographical map).

One of the things I would like to go over in class are sin wave gratings. The text defines them as "a grating with a sinussoidal luminance profile". The figure in the text explains that depending on the width and distance between black and white bars with pink dots, the image that appears will be different. I would just like some more clarification on this topic to be sure that I am understanding it more clearly. A few other things I would like to go over in class would be columns and hypercolumns, simple cells and complex cells and end stopping. Some of these things were difficult to understand in the text, but I do believe they are important things to understand when it comes to the functioning of the visual system.

The first thing I found interesting was the concept of selective adaptation which I am considering doing my topical blog over. Anyways, adaptation is a reduction in response caused by prior or continuing stimulation. The human visual system contains individual neurons which I’m referring to the tilt aftereffect which is the perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation. When it come to adaptation to high contrast it results in a loss of sensitivity for spatial frequencies close to the adapting frequency, but not for spatial frequencies that are much higher or lower than the adapting frequency. Selective adaptation causes the neurons most sensitive to the adapting stimulus to become fatigued. Higher contrast is needed after adaptation for a test grating to be able to stimulate neurons. Neurons responsive to too much higher or too much lower spatial frequencies are not fatigued by the adaptation procedure, so contrast sensitivity for these spatial frequencies is not affected. Moreover, the transfer of adaptation effects from one eye to the other thus implies that selective adaptation occurs in cortical neurons.

The second topic I found intriguing was that of the striate cortex and its receptive fields and so forth. This is also called the primary visual cortex (V1) which defined is the area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus, as well as feedback from other brain areas and is responsible for processing visual information. The striate cortex contains on the order of 200 million cells. When it comes to cortical magnification objects imaged on or near the fovea are processed by neurons in a large part of the striate cortex, but objects imaged in the far right or left periphery are allocated only a tiny portion of the striate cortex. Furthermore, there are receptive fields in the striate cortex one of which is orientation selectivity which the cell is tuned to detect lines in a specific orientation in the same way that a piano key in tuned to produce a specific note. An individual neuron responds best when the line or edge is at just the right orientation and hardly at all when the line is tilted more than 30 degrees away from the optimal orientation. There are narrow tuning functions that each striate cortex neuron functions as a filter which in this context is an acoustic, electrical, electronic or optical device, instrument, computer program or neuron that allows the passage of some frequencies or digital elements and blocks the passage of others. Hubel and Wiesel discovered that cortical cells responds well to moving lines, bars, edges, and gratings, Moreover, many neurons respond greatly when a line moves in one direction (left to right) but not all when the same line moves from right to left. Lastly, The property of the receptive fields of striate cortex neurons by which demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when presented in another is known as ocular dominance.

One thing that I found the least interesting was visual acuity. Acuity is the smallest spatial detail that can be resolved. In addition, a cycle is one repetition of a black and white stripe (a pair consisting of one dark and one light bar). Visual acuity has a lot 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. The angle that would be formed by lines going from top and bottom or left and right depending on the orientation of the stripes of a cycle on the page, through the center of the lens and on to the retina. In addition, the visual system samples for grating discretely, through the array of receptors at the back of the retina. If the entire cycle falls on a single cone we see nothing but a gray field or may experience a phenomenon known as aliasing which are misperceived cycles which are longer than they actually are.

A second thing that wasn’t very fascinating to be was the retinal ganglion cells and stripes probably because it came a lot easier to me then other subjects in this chapter. Cells respond vigorously to spots of light. Each ganglion cells also responds well to certain types of stripes or gratings. When the spatial frequency of the grating is too low, the ganglion cell responds weakly because part of the fat, bright bar of the grating lands in the inhibitory surround, damping the cells’ response. Similarly, when the spatial frequencies are too high, the ganglion cell responds weakly because both dark and bright stripes fall within the receptive field center, washing out the response. Responses depend on the phase which is relative position of a grating. If the grating phase is shifted by 90 degrees, half the receptive field center will be filled by a light bar and half by a dark bar and similarly for the surround. There will be no net difference between the light intensity in the receptive field’s center and its surround. A second 90 degree shift puts the dark bar in the center and the light bars in the surround, producing a negative response. A third shift returns to the situation after the first shift, with the overall intensities in the center and surround equivalent and the cell therefore blind to the grating.

I think the retinal ganglion cells and stripes along with spatial frequencies would be concepts most useful to understanding the visual system, except the book doesn’t give a long description of spatial frequencies. The spatial frequency channel is a pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies. I also think simple and complex cells would be helpful. Simple cell are cortical neurons that define excitatory and inhibitory regions. An edge detector prefers to see light on one side of its receptive field and darkness on the other side. A stripe detector responds best to a line of light that has a particular width, surrounded on both sides of darkness. On the other hand, complex cells cannot be simply predicted from the responses to stationary bars of light. The simple cell might respond only if a stripe is presented in the center of its receptive field, a complex cell will respond regardless of where the stripe is presented as long as it is somewhere within the cell’s receptive field. Simple cells are considered phase sensitive, whereas complex cells are considered phase-insensitive.

I actually had a few topics that I would like to be covered more in depth in class which are the lateral geniculate nucleus, sine waves, columns and hypercolumns and contrast sensitivity function (CSF). Spatial frequency refers to the number of times a pattern, such as a sine wave grating repeats in a given unit of space. The human contrast sensitivity function (CSF) is shaped like an upside-down U. The LGNs act as a relay station on the way to the retina to the cortex. It has 6 layers and the bottom two of which are magnocellular layer and the four on the top are parvocellular layers. The manocellular receive input from M ganglion cells in the retina, and the parvocllular receive input from P ganglion cells. Columns are a vertical arrangement of neurons. Hypercolums are 1mm block of striate cortex containing 2 sets of columns, each covering every possible orientation (0-180 degrees), which one set preferring input from the left eye and one set preferring input from the right eye.

The two things in chapter 3 that I found most interesting were orientation tuning and ocular dominance. I found orientation tuning interesting because of how precise and complex the system of sight is. This is all new information for me, and while it might not be a big deal that neurons in striate cortex tend to respond more optimally to some orientations than to others, as well as the big difference in the response depending on the orientation is just a bit mind boggling for me. That is to say that selectivity, a concept I usually save solely for people in making decisions, that my neurons have preferences for sight makes me think of sight as more than just how I get about on a daily basis. Similarly, with ocular dominance it is again the preferences of the striate cortex neurons that intrigue me, to know that my body is making decisions without me, that one eye is more responsive than the other, and all without me having a say is just a bit unnerving. I have always been under the false impression that I was making all the decisions.

The two things I found least interesting in chapter 3 were spatial frequency and cycles per degree, because even while I was reading it I felt like it was going over my head. I think if they were explained more fully in class, I might have a better chance of grasping the concepts. It is probably less the topics and more the fact that I am often overwhelmed in science classes and this all seems incredibly technical. It all makes some semblance of sense to me in pieces, but the whole is a bit fuzzy, perhaps lecture will straighten things out.

The first thing I found interesting was the overall theme of the chapter. It amazes me what goes into making us see what we see. All of these things are things that we are never consciously aware of. We never think about what takes place from the moment our eye sees something to when the brain process what we see. I am sitting here typing this paper and cannot even fathom how fast this process works. The second thing I found interesting was figure 3.14 the inverse cortical magnification factor. I found this interesting because if very clearly laid out how images are reversed from out eyeball to our striate cortex. The third thing would be the tilt effect. It is interesting because of the selective adaptation. The two things that I found least interesting were columns. I do understand that each the right and left eye have separate columns but I don’t understand how each column plays into the big picture. The thing that I read in the chapter that will be most helpful in the chapter is how the cerebral cortex translates what our brain sees. Also, the visual cortext and how it translates everything such as colors and width. The two topics I would like you to cover more in class are spatial frequency and retinal ganglion cells.

Chapter 3 elaborates on the center-surround nature of ganglion cells, and in conjunction with the lecture from this week it has helped me to get a much firmer understanding of how ganglions operate as well as how visual fields are used to organize the system of receptors on our retinas. Ganglion cells receive information from bipolar cells and amacrine cells. Bipolar cells collect information from many receptors, (diffuse bipolar cells receive information from as many as 50 rods), or a single receptor. (midget bipolars receive input from 1 cone) Amacrine cells are horizantal and act to relay information from photoreceptors to bipolar cells and ganglion cells. This horizontal communication allows receptors to be parts of visual fields for multiple ganglion cells. The overlap of visual fields is necessary. If one visual field is firing at a high rate and sending the message that it's center is being stimulated, an overlapping visual field could be being inhibited because the stimulation is falling in the surround not the center. As the book said our visual system is concerned with contrast, not with constant. The overlap of the visual fields allows us to further distinguish the contrasting stimulus with the constant surrounding. So when we see a star we see a dot in the sky. The dot does not disappear when it leaves the center of one visual field and goes into the surround because another visual field that is overlapping is able to account for the incoming information. We see a precise dot in the sky not a smudge with abstract edges. We know our brain exaggerates edges but the process begins on the retina. The center surround layout allows ganglions to pinpoint stimulation and ignore what it wants. (either the center or the surround depending on whether it is center ON or center OFF) Chapter 2 refers to convergence and divergence. This is what the horizontal and amacrine cells do, they converge information from receptors and pass it on but they also diverge and send the information from the receptors to several ganglions, creating the overlap effect of the visual fields. The book illustrates the use of gratings, or series of black and white lines, to get a peak into the organization of the visual fields that are used by our ganglion cells. The term spatial frequency refers to the number of times a pattern is present in a given amount of space. Basically this refers to the interaction of the amount of space between each line in a grating and the angle at which the grating strikes the eye. If the distance between the receptive fields and the retina increases the angle at which the stimulation strikes the receptive fields changes. If you get further the receptive field takes in more, possibly more than one set of white and black stripes. Phase refers to the position of the target on the receptive field and it's effect on the amount of stimulation being received by the center and surround. As we saw in class if the center and the surround are both equally stimulated there is no change in firing rate. Because the receptive fields do not increase their firing rate no lines are distinguished and the grating appears as a gray constant. This grating stuff seems like it is pretty far from applicable to reality but in truth we experience gratings every time we open our text books. Printers put ink on a page in horizontal lines. The space between the lines is so small that our receptive fields take in many lines and cannot distinguish the space between. The result is our perception of an image or word on the page.

The lateral geniculate nuclei are the first stop in the brain for visual information. This area is not cortex, or the higher processing portion of the brain, rather it is just a collection of neurons where lower order processing happens before the information is sent to the cortex. There are two geniculates, one on each side of the brain. The left incorporates information from the right visual field and the right information from the left visual field. **Note I tried to say receptive fields earlier, referring to the field made of input from several receptors and interpreted by a single ganglion cell. Visual field here refers to the world that we see, our window of sight. I don't want there to be any confusion but these terms are so similar sounding that maybe that is inevitable. There is a left half and a right half of this field. As with everything else our brains are organized in such a way that our left brain controls the right half of our body and the right brain the left half. The way images are focused onto the back of the retina is pretty crazy. It is kind of like a mirror, when the light strikes our retina the image is backward in the same way a mirror is backward. The left portions of both retinas receive light from the right part of our visual field. The right portions of the retinas receive the light from the left visual field. The left geniculate does not receive information only from the right eye, rather the right visual field, which is experienced by the left part of the retinas of both eyes. Both geniculate nuclei are made of 6 layers. The top four consist of what are called parvocellular layers and receive input from midget ganglion cells, which receive from midget bipolar cells which receive from single cones. The bottom two layers are called magnocellular layers because the neurons are physically larger in these layers. These neurons receive from parasol ganglions which receive from diffuse bipolar receptors which receive from many rods. It is amazing how much more is dedicated to the cone receptive fields. I mean we've discussed the thought that there are two visual systems and the chief difference between them is that one uses cones and obtains a clear focused image and one uses rods and goes for the bigger picture kind of information. Our fine detail vision must be very important in evolutionary terms because so much more energy and literally more biological material is dedicated to cone vision. (i claim the term cone vision) The book discusses the terms contralateral and ipsalateral meaning almost literally different side and same side, respectively.

The book discusses Hubel and Wiesel's famous study in which they discovered neurons in the occipital lobes of cats that specifically increased their rate of fire when a line was presented to the visual field. More research led to the discovery that different neurons are programmed to recognize lines at different angles and orientations. This response feature is called oriental tuning.