Reading Activity Week #7 (Due Monday)

For some reason, I didn't find this chapter as interesting as the chapters that we have been reading lately. I had a hard time concentrating or gaining interest in any of the topics that were covered. I know that motion detection is an important aspect of the visual system, but it just wasn't presented in an interesting way. Maybe it was just me...but I struggled with this chapter and would really like to cover a lot of it in class.

The first thing that I think is important in this chapter is the two different types of motion that our visual system recognizes. The first type that is discussed in the text is known as first-order motion. This is the motion of an object that is defined by changes in luminance. First-order motion focused on luminance-defined objects. There are objects that is defined by changes in luminance. The second kind of motion detection that is discussed in the textbook is known as second-order motion. Second-order motion is the motion of an object that is defined by changes in contrast or texture, but not by luminance. Second-order motion looks at texture or contrast-defined objects that change position over time Contrast-defined objects are objects that are defined by changes in contrast or texture, but not by luminance.

Apparent Motion, or the illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession, is not without its faults. The book describes the most common version for how the visual system is believed to detect motion. According to the text, there are many different steps in recognizing motion. Cell M (motion detection cell) receives input from cell A (location one) and cell B (location two of moving object). However, this could mean that one large item is exciting both cells instead of one object moving from point a to point b. Thus cell D, receives input form A and delays transmission of this imput for a short period of time. Cell D also has a fast adaptation rate (meaning it fires when cell A initially detects light, but quickly stops firing if the light remains shining on A receptive field. Cells B and D are then connected to neuron X, a multiplication Cell. By delaying receptor As response mechanism that is sensitive to motion. This explanation was developed by Wener Reichardt to explain how flies detect motion. There have been several elaborations to this model. One is by Barlow and Levick which uses what eletrical enginners would can an "AND gate" Cell X fires and only if both its inputs (B and D) are firing simultaneously, and it passes this message on to the motion detection cell M.

Mentioned previously was the fact that there are some problems with apparent motion and the models for how our visual system perceives motion. These are known as the correspondence and aperture problems. The correspondence problem or motion is the problem faces by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame one. The aperture problem is the fact that when a moving object is viewed through an aperture (or receptive field), the direction of motion of a local feature of part of the object may be ambiguous.

Now that we know a little more about first-order motion, we should move on to second-order motion. The main difference between the two is that in first-order motion, things actually move from point a to point b, and in second-order motion nothing moves. Instead, our visual system perceives movement due to contrast colors or textures. Second-order motion proves that matching discrete objects across movie frames is not necessary for motion perception. Also, there is some research that shows that our visual systems may have specialized mechanisms for second-order motion. Studies have found patients who lack first-order motion have defects in different areas of the brain than those who lack second-order motion. There is an evolutionary need for second-order motion. When trying to detect motion in an object that is effectively camouflaged, very little first-order motion will be noticed by our retinas, but our second-order motion vision signals will be generated.

Another topic that was somewhat interesting in this chapter was the section covering eye movements. The Superior Colliculus is a structure in the mid brain that is important tin initiating and guiding eye movements. Our eyes experience several different types of eye movements everyday. It will experience involuntary eye movements and voluntary eye movements. There are three different types of voluntary eye movements. Smooth Pursuit, is the first type of eye movement in which the eyes move smoothly to follow a moving object. Another type is vergence. Vergence is a type of eye movement in which the two eyes move in opposite directions (both eyes turn toward the nose [convergence] or both turn away from the nose [divergence]). The last form of voluntary eye movements is saccade. Saccade is a rapid movement of the eyes that changes fixation from one object or location to another. These are also known as fast jumps of the eye. You can decide to make a sccade deliberately, but whether you are thinking about it or not, ou will make three or four saccades every second of every minute of every hour of the day. Our eyes are more liekly to make a saccade in response to contours than to broad featureless area of an image. we also make eye movements that are based on the content of a scene and on our specific interest in that scene.

One of the things that I found least interesting in this chapter was the section that discussed the problem of discriminating motion across the retina that is due to eye movements versus object movements. One of the answers to this problem is saccadic suppression. this is the reduction of visual sensitivity that occurs when one makes a saccadic eye movement. Saccadic suppression eliminate sthe smear from retinal image motion during an eye movement. The other answer to this problem is that the brain sends out two copies of each order to move the eyes, the motor system solves the problem of why an object in motion may appear stationary. one copy goes to the eye muscles another goes to an area of the visual system that has been names the comparator. the comparator is an area of the visual system that receives one copy of the order issued by the motor system when the eyes move. the comparator can compensate for the image changes caused by the eye movement. which inhibites any attempts by other parts of the visual system to interpret the changes as object motion.

Another topic that I didn't find very interesting was that of optic array. Optic array was coined by J.J. Bibson to describe the collection of light rays that interact with objects in the world in front of a viewer. Some of these rays strike our retinas, allowing us to see. Apparently when we move through our environment, we experience patterns of optic flow that are used by our visual systems to determine where we are going. optic flow is described as the changing angular positions of points in perspective images that you experience as you move through the world. The focus of expansion is the point in the center of the horizon from which, when you are in motion, all point in the perspective image seem to emanate. the focus of expansion is one aspect of optic flow.

The topic that I think is most useful in learning more about the history of psychology is the section that discusses the experiments that were done on monkeys to learn more about motion detection in the visual system. Newsome and Pare trained monkeys to respond to a set of moving dots. There was one set where all of the dots moved in the same direction, another set in which only half the dots moved in the same direction and all the others moved in random directions, and a third set that had only 20% of the dots moving in the same direction. Once the monkeys were fully trained, they could recognize the correlated motion direction when only 2% of the dots were moving in the same direction. Then, the researchers lesioned the monkeys MT areas (Middle temporal lobe, an area of the brain thought to be important in the perception of motion).The results showed that the monkeys needed about ten times as many correlated dots in order to correctly identify the direction of motion. Despite this, the monkey's ability to discriminate the orientation of stationary patterns was generally unimpaired. Also, the monkeys performance in the correlated dot motion task improved markedly during the weeks following the lesion, presumable because they learned to use other brain ares to discriminate motion.

In class, I would like to learn more about Akinetopsia. Akinetopsia is a rare neuropsychological disorder in which the affected individual has no perception of motion. I would also like to discuss the topic of motion aftereffect (MAE). Motion aftereffect is defines as the illusion of motion of a stationary object that occurs after prolonged exposure to a moving object.

Terms: first-order motion, luminance-defined object, second-order motion, texture-defined/contrast-defined object, apparent motion, Werner Feichardt's model, Superior colliculus, Barlow and Levick's AND gate, correspondence problem, aperture problem, smooth pursuit, vergence, saccade, saccadi suppression, omparator, optic array, optic flow, focus of expansion, akinetopsia, motion aftereffect, middle-temporal lobe.

In chapter 7 i found motion to be a very important part of the visual system. This includes motion aftereffect which I also thought was interesting. This is the illusion of motion of a stationary object that happens after a long exposure to a moving object. I relate this to playing guitar hero. I would concentrate on guitar hero so hard and for so long that after I was done, everything was moving upward because that's how my eyes were used to looking. The computation of visual motion was hard to follow but was kind of interesting as well. Motion involves a change in position over time. There are two adjacent receptors, neurons A and B, which are separated by a fixed distance. A spot of light (bug) moving left to right would past through A then through B's receptive field. Motion detection, M, would fire in response to the moving light or bug, and would also respond to the large stationary bug that spreads across both receptive fields. Since this becomes a problem, we have a cell, D, that receives input from A and fires when A first detects light, but stops firing if the light keeps shining on A's receptive field. Also, B and D are connected to a multiplication cell, X. When B and D are both active, X will fire. So delaying A's response (D), and then multiplying it by B's response (X), it creates a sensitivity to motion. This topic interests me as well as the next topic I discuss, so I would like to learn more about these.

Apparent motion is another aspect of this, and it is the illusory or impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. This was developed my Sigmund Exner back in 1875. He set up a device that would create sparks that were separated from each other by space and time. Even though there were two different sparks, people who saw this could swear that they saw a single spark moving from one place to another. We see apparent motion today in tv and computers. We see one fluid motion even though we know in cartoons that the cartoon is just a series of frames put together and shown to us at a fast speed. Aperture is also a part of this and is described as an opening that allows only a partial view of the object. The correspondence problem, however, is the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame one. We have motion detections for all directions, so that makes this difficult. Aperture problem, however, is when a moving object is viewed through an aperture, the direction of motion of a certain part of the object may be indefinite.

All of the terms that went along with the motion aftereffects revisited topic were kind of uninteresting to me. For example, interocular transfer is the transfer of an effect such as adaptation from one eye to the other. First order motion, however, is the motion of an object that's defined by changes in luminance whereas second order motion is the motion of an object that is defined by changes in contrast or texture, but not by luminance. Luminance-defined object is an object that is defined by changes in reflected light. Texture-defined object, or contrast defined object, is an object that is defined by changes in contrast or texture but not by luminance. These change positions over time like luminance-defined objects.

The last topic that wasn't quite as interesting to me is was the topic called using motion information to navigate. The optic array is the collection of light rays that interact with objects in front of a viewer. Some of these rays hit our retina which enables us to see. Optic flow, however, are patterns in our image that allow us to determine where we're going. Another term used was focus of expansion. This is the point in the center of the horizon (when we are in motion) where all points in that image seem to stem from. This is an aspect of optic flow. This topic goes further, but it was hard for me to be attracted to this topic for some reason. It's hard for me to understand why this certain topic will apply to my life so I think that it may be the least helpful to us.

The middle temporal lobe is the area of the brain that is important in the perception of motion. Interestingly, certain neurons are specific for motion in one particular direction. Second order motion is very interesting and happens when the motion of an object is defined by changes in texture or contrasts and not luminance. Research suggests that a separate mechanism is used for second order motion than first order motion (motion defined by changes in luminance). I also found akinetopsia interesting. Akinetopsia is a disorder where the person cannot perceive motion. As the authors explain the object change where they are located but the individual cannot see the movement. The person can tell the objects changed positions but cannot perceive them moving. Although it is rare, I would still like to learn more about akinetopsia because I find it very interesting. I would also like to learn more about the physiology of eye movements. I think the key to enjoying and understanding motion perception is to remember how important it is for us to have. Without perception of motion it would be impossible to avoid collisions with moving objects in our environment. Reminding myself of the importance of motion perception helped me to enjoy parts of this chapter that would otherwise be boring. I liked the development of motion perception the least because essentially all the authors told us is that reflexive eye movements to moving targets are present when the baby is born and global motion doesn’t fully develop until three or four years. I wish the authors would have went into more detail about the development process.
Terms: middle temporal lobe. Second order motion. First order motion. Akinetopsia.

One subject that I found interesting was the topic on where in our
brain is responsible for us detecting motion. One area that is thought
to be important is the middle temporal lobe. The vast majority of
neurons in this area of the brain are selective to motion in one
direction. These neurons show little selectivity to form or color.
Several researchers have set out to prove this by looking at monkeys
and their finding suggests that the MT is the site of global motion
detection neurons in our visual system. Another topic that I found
interesting was motion aftereffects. These aftereffects can often be a
response to fatigue motion detectors. An example is when you look at
waterfall for a long period of time our detectors sensitive to
downward motion become Fatigued and when we look away our detectors
for upward motion fire faster and objects appear to be moving up. When
talking about these effects we are talking about first order motion or
motion of an object that is defined by changes in luminance. The opposite of this is second order motion or motion of an object that is defined by changes in contrast or texture and not by luminance. One topic that I did not find very interesting was saccadic suppression and the comparator. I didn?t find this very interesting because I thought it was difficult to read and there wasn?t much information. Another topic that I did not find very interesting was the man who couldn?t see motion. I didn?t find this very interesting because I felt like this topic was simply tacked on to the end of the chapter. One topic I would like to go over more in class is eye movements. I would also like to go over the eye movements that are associated with reading.
Terms
Middle Temporal Lobe
First order motion
Second Order Motion

The first thing I found interesting in Chapter 7 is the optical illusion on page 168. It is so powerful and noticeable that I had to cover it while I read the adjacent page. The apparent rotation of the 'rollers' was too hard to tune out. The chapter is focused on motion perception. It begins by explaining MOTION AFTEREFFECT by an anecdote with a man and a waterfall. This is the illusion of motion of a stationary object that occurs after prolonged exposure to a moving object. The actual movement of an object is explained by constructing a neural circuit in which the receptive cells fire into fewer and fewer ganglion that interpret the motion and its direction. Another important type of motion detection isn't actually motion in reality. APPARENT MOTION can be demonstrated by a flip-book of pictures that only change subtly between each frame. The differences are perceived as movement.
2 problems of motion detection are the CORRESPONDANCE PROBLEM in which the motion detection system cannot determine which feature in frame one corresponds to a a feature in frame 2. (better illustrated by the drawings) The APERTURE PROBLEM arises when a moving object viewed through a receptive field a specific moving part on that object may be ambiguous. The most important visual system component in the detection of motion is the MIDDLE TEMPORAL LOBE.
Page 117 lists some of the very important aspects of objects that can signal motion to our visual system and the way it interprets it: INTEROCULAR TRANSFER is the transfer of an effect (like adaption) from one eye to the other; first-order motion is the motion of an object that is defined by changes in luminance; luminance-defined object is delineated by changes in reflected light; second-order motion is an object that is defined by changes in contrast or texture, but not by luminance; texture-defined objects change in contrast or texture but not in luminance.
We are able to see because of our optic array, this is the collection of light rays that has hit objects in front of us and bent into our visual field, when this field moves we are experiencing optic flow(J.J. Gibson 1957)
We are very good as a species at detecting BIOLOGICAL MOTION, in which the points of interest are the motion of an animal's anatomy, and estimating the TIME TO COLLISION of an object in a trajectory towards us.
The chapter also explains the movement of the eye itself. An important aspect of this is SMOOTH PURSUIT, this is the movement of the eye at a constant rate as an object moves, rather than in a jerky motion. 6 muscles control the movement of the eye. VERGENCE separates the focus of the eyes, SACCADE is a rapid movement of the eyes that changes fixation from one object or location to another, and REFELXIVE EYE MOVEMENT is an automatic and involuntary movement. A very interesting point is made when the author describes how our eyes saccade as we read. The bias is towards the right and we can only perceive words when we stop so we are really reading in 50 millisecond bursts.
I would like a further explanation of the middle temporal lobe in class.

The two things in chapter seven i found interesting are aperture and correspondence problems. Aperture problem is the fact that when a moving object is viewed through a receptive field, the direction of motion of a local feature or part of the object may be uncertain. This is really cool and reminds me of the spinning tops that light up. I think we all probably had one but when you look at the lights on top you didn’t know which direction the top was spinning. That is a good example of what the aperture problem is.

The correspondence problem has something to do with motion detection. Is the top spinning left or is the top spinning right. Who knows? Well this correspondence “problem” is similar to the one we faced when studying binocular vision. There, the issue was which points on the left retina corresponded with which points on the right retina. As noted in the chapter on binocular vision, correspondence is only a problem for the researchers studying motion perception; as we will see, the visual system is remarkably adept at “solving” the problem. Like the D cell when it adapts quickly.

Even though we have to choose the not so interesting it is all somewhat interesting. So I chose the Interocular transfer. Everything goes together so you can’t mention one thing about motion perception and not mention the other. Interocular transfer is the transfer of an effect from one eye to the other. We know that motion detection neurons must be located at some point at the beginning of the visual system, after information from the two eyes has been combined. Motion-sensitive cells are found as early as the striate cortex. This is where information from the two eyes is only partially integrated—hence the fact that the interocular MAE is not as strong as the MAE you experience if you view the stationary objects with the same eye that saw the moving objects. Information from striate cortex neurons is sent on to other areas of the brain, most notably the medial temporal (MT) lobe, for further processing.

And to continue on we will talk about how certain neurons in the brain are specialized for motion detection. Moreover, each motion-sensitive neuron is most responsive to a single direction of movement. Researchers believe that all of these direction-specific neurons are constantly firing in a sort of dynamic equilibrium. When you are viewing a stationary scene, the various neurons’ responses cancel each other out, so that no motion is perceived. If elements in the scene start moving, for example, to the right, neurons sensitive to rightward motion will increase their firing rate, leading to the perception of rightward motion. That is why MAE occurs.

In order for us to understand where Sigmund Exner and the other entire psychologist who are studying motion perception get all there information we would have to know all the small details about the visual system. Knowing these small details will help us in understanding the history of their psychology.

In class, I would like to learn more about the Computation of the visual motion. It talks about how to build a motion detector. It gives the definition of the word motion as something that change position overtime. Well apparent motion gives you the impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. This has something to do with the computation of visual motion when they talk about the motion detection cells. They say M cannot simply add up excitatory inputs from A and B. So they have to add one named D. I know very confusing but D has a fast adaptation rate and works together with A and B. When working together the three connect with neuron X. M though is responding between A and B to make up lag. I need a detailed, visual view of what is going on here, I am a bit bewildered. It would be really cool to hear more about the different types of eye movements because you could probably tell us some great stories or show us some visual examples. Most of the eye movements we have all heard of like vergence which is a type of eye movement in which the two eyes move in opposite directions. When, for example, both eyes turn toward the nose or away from the nose. That is called convergence and divergence. When the eye is having rapid movements that change fixation from one object or location to another that is called saccade. Some movement can be automatic or involuntary which then is called reflexive eye movement. The eye is amazing and the more in-depth we get the better.

Terms: Computation of the visual motion, motion detector, apparent motion, correspondence problem, Aperture problem, MAE, Interocular transfer, Sigmund Exner, saccade, vergence, convergence, divergence, reflexive eye movement

One of the things I found interesting in this chapter was the motion after effect (MAE). MAE is very similar to the color after effect. When we view something in constant motion in one direction(for at least 15 seconds) and then look at a stationary object we see movement in the opposite direction. The text explains that when we view a stationary object our neurons that are tuned to different directions of movement are balanced. When we look at an object in motion for an extended period of time the neurons that are sensitive to the movement (upward, downward) become fatigued. When we move from the moving object to a stationary object the fatigued neurons fire slower than the others therefore we see the object moving in the opposite direction.

Another section that I found interesting was on apparent motion. Apparent motion was first demonstrated by Sigmund Exner in 1875. We still experience apparent motion in our everyday lives through tvs and computers. The text uses the example of an animated cartoon. Cartoons are just a series of still pictures that are shown at high speeds giving the appearance of motion. When I read this section I was reminded of the flip books that I made as a child. I would draw a series of pictures that were a little different each time with one picture on each page. When they are all complete and in book form if you flip through the pages quickly the subject of the picture looks like it's moving.

One other thing that I found really interesting was biological motion which was discovered by Gunnar Johansson in 1975. Biological motion is just the "pattern of movement of living beings". I would be interested in finding out more about this topic.

One topic that I found uninteresting was tau. Tau is "information in the optic flow that could signal time to collision (TTC) without the necessity of estimating either absolute distance or rates". The text uses an example of a cricket ball flying at your head. In this case you wouldn't have the ability to measure absolute distance or speed of the object so instead tau is used to determine TTC. Tau works like this. As the ball approaches the image of it on your retina gets larger. Tau then is the ratio of the retinal image size at any given moment to the rate the image is expanding.

Another part that I found uninteresting to read about what types of eye movements. Our eyes are constantly in motion. If we look at an object and try to keep our eyes stationary then they involuntarily move to keep the image fresh. If somehow your eyes become temporarily paralyzed then the visual world will gradually fade away (ok, so that part is kind of interesting). There are also voluntary eye movements. The types are vergence and saccade. Vergence is an eye movement where both eyes move in opposite directions (convergence and divergence). Another type is saccades. This is "rapid movement of the eyes that changes fixation from one object or location to another".
As I mentioned earlier I would like to talk more about biological motion (this is probably what I'll do my topical blog on though) and akinetopsia. Akinetopsia is a disorder where the person has no perception of emotion. For some reason I tend to find disorders one of the most interesting parts of each chapter.

TERMS: motion after effect, color after effect, neurons, apparent motion, biological motion, tau
time to collision, absolute distance, retina, vergence, saccade, akingetopsia

Motion aftereffect (MAE) is the illusion of motion of a stationary object that occurs after prolonged exposure to a moving object. After viewing motion in a constant direction for a sustained period of time, we are able to see a stationary object that we view subsequently such as rocks around a waterfall, as moving in the opposite direction.

The first thing I found interesting was eye movements. A type of eye movement in which the eyes move smoothly to follow a moving object is known as smooth pursuit. When it comes to the physiology and types of eye movements, one way to get some inkling of the role of these brain structures is to stimulate them with small electrical signals and observe the movements of the eyes. For example, if a cell in the midbrain (superior colliculus) of a monkey is stimulated, the monkey’s eyes will move by a specific amount in a specific direction. Every time that cell is stimulated, the same eye movement will result. Stimulating a neighboring cell will produce a different eye movement. Superior colliculus is a structure in the midbrain that is important in initiating and guiding eye movements. In addition, there are three types of voluntary eye movements. Most obvious, perhaps are the smooth pursuit movements that we make when tracking a moving object. Vergence eye movements occur when we rotate our eyes inward (converging the eyes) or outward ( diverging the eyes) to focus on a near or far object. The third type of voluntary movements are saccades: fast jumps up to 1000 point from one spot to another. The eyes are more likely to make saccades in response to contours than to broad featureless areas of an image. We make eye movements that are based on the content of a scene and on our specific interests in that scene. We make about 172,800 saccades per day. There are also reflexive eye movements which is a movement of the eye that is automatic and involuntary. For example, when the eyes move to compensate for head and body movement while maintaining fixation on a particular target. Optokinetic nystagmus (OKN) is another reflexive eye movement in which the eyes will involuntarily track a continuously moving object. OKN is characterized by the eyes moving smoothly to say the right in pursuit of an object moving to the right, and then snapping back. Furthermore when is comes to eye movements and reading which is really interesting to me we English readers don’t get as much out of reading as the Hebrews. Reading English involves fixating for roughly a quarter of a second, and then making a saccade of about 7-9 letter spaces. Readers of English are able to gain information from up to 15 characters to the right of fixation, but only 3-4 characters to the left. Thus the perceptual span is asymmetrical. Readers of Hebrew which read from left to right have the reverse asymmetry. Readers of both Hebrew and English can switch this asymmetry depending on which language they are readings, so the asymmetry is attentional, not a product of limitations imposed by the visual system.

The second concept I found interesting was computation of visual motion. One possible objection to the neural circuit is that it doesn’t require continuous motion in order to fire. An image of a bug that appears in A’s receptive field, then disappears and they reappears in B’s receptive field a short time later will drive M to respond just as strongly as if the image had moved smoothly across the two receptive fields. Apparent motion is the illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. This illusion was first demonstrated by Sigmund Exner. Exner set up a contraption that would generate electrical sparks separated from each other by a very short distance in space and a very short period of time. The two different perceptual objects—observers swore they saw a single spark moving from one position to the other. In addition there is a concept known as aperature which is an opening that allows only a partial view of an object. For example, beneath the mask, the square moves exactly as before, but if you view the movie, you will clearly perceive up and won, not diagonal motion. Furthermore, correspondence problem (motion is the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame 1. because we have motion detection in all directions, one detector will sense the diagonal motion implied by matching the circle labeled A with the circle labeled C. but another detector will sense the vertical motion implied by matching circle A with circle B. These detectors compete to determine our overall perception. Aperture problem is the fact that when a moving object is viewed through an aperature or a receptive field, the direction of motion of a local feature or part of the object may be ambiguous. Without the mask, the diagonal motion detector clearly determines which direction we perceive the square to be moving. Moreover, when we view a stationary object, the responses of neurons tuned to different directions of motion are normally balanced. That is, neurons sensitive to upward motion fire at the same rate as neurons sensitive to downward motion, so the signals cancel out and no motion is perceived. Interocular transfer is the transfer of an effect such as adaptation from one eye to the other. Evidence suggests that MAE in humans is caused by the same brain region shown to be responsible for global motion detection in monkeys’ cortical area MT. Furthermore, first order motion is the motion of an object that is defined by changes in luminance. Luminance defined object is an object that is delineated by changes in reflected light. Second order motion is the motion of an object that is defined by changes in contrast or texture, but not luminance and texture defined (contrast defined) object is an object that is defined by changes in contrast or texture, but not by luminance. The only thing that changes in our second order motion movie is the strips of dots are inverted from one frame to the other.

The first thing I found least interesting was using motion information to navigate. The optic array is the collection of light rays that interact with objects in the world in front of a viewer. Some of these rays strike our retinas, enabling us to see. When we move through our environment, we experience patterns of optic flow that our visual systems use to determine where we are going. Optic flow is the changing angular positions of points in a perspective image that we experience as we move through the world. The focus of expansion is the one place in the visual field that will be stationary, so the visual system can locate this stationary point, it can determining the heading. The mere presence of optic flow indicated locomotion’s, and a lack of flow is a signal that you are stationary. If you have ever left your car in neutral with the brake off at the top of a hill, you many have experienced a situation in which optic flow alone tells you that you are in (unintentional) motion. Outflow indicated that you are approaching a particular destination; inflow indicates retreat assuming that your head is facing forward you will also experience outflow when you look over your shoulder as you walk forward. Furthermore, the motion of animals and people known as biological motion that helps us identify both the moving object and its actions. We can use biological motion to identify whether a set of moving lights is attached to a male or female walker. The mechanisms that analyze biological motion obey different rules for integrating motion over space and time than do mechanisms for other forms of complex motion. Biological motion appears to play an important role in how we understand human actions. Moreover, when it comes to avoiding imminent collision there is a concept known as time to collision (TTC) which is the time required for a moving object to hit a stationary object, such as a cricket ball hitting a baseball player’s hat. Tau is the information in the optic flow that could signal TTC without the necessity of estimating either absolute distances or rates. The ration of the retinal image size at any moment to the rate at which the image is expanding is Tau and TTC is proportional to Tau. The ratio of the retinal image size at any moment to the rate at which the image is expanding is tau, and TTC is proportional to tau. The great advantage of using tau is to estimate TTC is that it relies solely on information available directly from the retinal image.

The second concept I found least interesting was saccadic suppression and the comparator. When we make a saccade, the visual system pretty much shuts down for the duration of the eye movement (visual activity is suspended in a similar way when we blink). Saccadic suppression acts mainly to suppress information carried by the magnocellular pathway. Although saccadic suppression eliminates the smear of the moving world during a saccade, it seems as if we should still be disturbed by the sudden displacement of the objects in front of us. By sending out two copies of each order to move the eyes, the motor system solves the problem of why an object in motion may appear stationary. One copy goes to the eye muscles another goes to an area of the visual system that has be called the comparator. The comparator can then compensate for the image changes caused by the eye movement, inhibiting any attempts by other parts of the visual system to interpret the changes as object motion.

The concept I thought would be most useful to understanding the visual system is definitely the middle temporal lobe (MT) which is an area of the brain thought to be important in the perception of motion. The vast majority of neurons in MT are selective for motion in one particular direction, but they show little selectivity for form or color. MT is the site of global-motion detection neurons in the visual system. A study was done on Monkeys which shows that when neurons were stimulated in a specific direction, the dots they were seeing were actually moving in the opposite direction. Furthermore, some aspects of motion perception are already evident at birth. For example, reflexive eye movements to moving targets are present in newborns. On the other hand, sensitivity to global motion, which is thought to reflect processing in MT, appears to develop more slowly, reaching maturity at about three to four years of age, while sensitivity to motion defined form and biological motion takes even longer.

The two topics I would like covered more in depth are akinestopia which is a rare neuropsychological disorder in which the affected individual has no perception of motion. The other concept is saccadic suppression which is the reduction of visual sensitivity that occurs when we make saccadic eye movements. Saccadic suppression eliminates the smear from retinal image motions during an eye movement.

Key Terms: Motion aftereffect, apparent motions, aperture, correspondence problem, aperture problem middle temporal lobe, interocular transfer, first order motion, luminance defined object, second order motion, texture defined object, optic array, optic flow, focus of expansion, time to collision, tau, biological motion, smooth pursuit, superior colliculus, vergence, saccade, reflexive eye movement, saccadic suppression, comparator

This chapter was slow but got good at the end. We perceive motion in two ways. First order motion refers to the perception of luminance based objects with well defined edges. Second order motion refers to the perception of motion in texture based objects. Motion is detected by stimulation of multiple receptive fields in a particular order. If adjacent receptive fields are stimulated it is likely that the source of the stimulation moved across the visual array. The messages from two adjacent receptive fields are sent to a motion detector cell. The message from the first receptive field is delayed by another cell. The delayer cell is also a quick adapting cell. Meaning it will inhibit the signal from the first receptive field if the pattern of stimulation to this field doesn't change. The motion detector cell receives input from both receptive fields at the same time. If both fields register stimulation the motion detector cell sends a "yes i detect motion" signal. The adaption rate of the delayer cell allows the motion detector cell to determine in the stimulation in the first receptive field has changed. If not there is no signal from the first receptive field and this particular motion detector cell does not send a yes message. Motion perception is created by stringing thousands of these 2 to 1 receptive field detector cell patterns together to cover the entire visual array. The yes and no messages from the detector cells are sent to yet another layer of detector cells. 2 receptive fields to 1 detector cell, 2 detector cells to 1 higher detector cell. Now signals from motion detector cells are compared, and the signal won't pass on unless both cells register motion. The diagram on page 157 was a favorite of mine. It is somewhat vague but it sparked a lot of thought on the process.

There is a certain threshold that stimulation must cross to be classified as either moving or not moving. The stimulation on a receptive field must change, and stimulation on an adjacent receptive field must occur shortly after. Because our receptive fields are limited by size and by how many signals they can send in a period of time, the illusion of motion is fairly easy to create. Apparent motion occurs when we perceive motion but no actual motion of the object occurred. Either the object simply shifted position in the visual array, or the original object disappeared and another object that has similarly defined edges is stimulating receptive fields that are adjacent to the ones stimulated by the original object. This is the case with flipbooks, or cartoons. What we see is a collection of thousands of stationary images, but they are shown so rapidly that it appears to the the same image moving across our visual array.
Areas in the middle temporal lobe contain neurons that are sensitive to motion in particular directions, just like the orientation sensitive neurons in the LGN and the visual cortex.

One thing I found most interesting was how our brain discriminates between changes of position caused by actual movement (or at least stimulation that can be perceived as movement) and the movement of our eyes. Our eyes are constantly moving, for the most part unconsciously. Research done on monkey brains showed that were specific neurons in the superior colliculous that were responsible for detecting movement in a corresponding area of the visual field, and then sending a message to direct the eye to that area of the visual field. Large movements of the eye are called saccades. One reason that we are able to keep track of our visual world even with the massive movements our eyes sometimes make is called saccadic suppression. This refers to the visual systems tendency to sort of shut down for the duration of the dramatic eye movement. The cognitive structure responsible for reconciling eye movements with the steady visual perception we are used to is called the comparator. The comparator seems to be a theoretical concept, but it receives a copy of the message sent from the superior colliculous to the ciliary muscles. It the reconciles our perception by eliminating contrast that is caused by eye movement.

First order motion, second order motion, motion detector cell, delayer cell, middle temporal lobe, saccade, saccadic suppression, comparator, superior colliculous