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Lecture 11

PSC 131 Lecture 11: Motion

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PSC 131
Geng Joy

I. Lecture 11 – Motion (May 15, 2017) A. Class Summary 1. Ways to Perceive Motion a. Real, illusory, motion aftereffect inferred…what does this tell us about how motion is computed in the brain? 2. How is Motion Computed? a. Motion detectors b. The aperture problem 3. Area MT 4. Eye-Movements: Perceiving Motion vs Producing Motion B. Ways to Perceive Motion 1. First Order Motion a. Local changes in contrast b. Illusory motion from first order principles c. The image is like throbbing; it doesn’t look completely still d. This image is actually completely static 2. Apparent Motion – the (illusory) impression of smooth motion resulting from the rapid alternation of objects appearing in different locations in rapid succession a. Ex: television, movies, and cartoons; marquee lights; flip books 3. Motion Aftereffects a. Spiral Demo: Staring at the spiral, the spiral was spinning away b. What did you see after the spiral stopped? i. When it stopped, the wallpaper looked like it was spiraling towards you c. Motion Aftereffect (MAE) – the illusion of motion of a stationary object that occurs after prolonged exposure to a moving object d. Gives clues to how motion neurons result after being fatigued C. How is Motion Computed? 1. How would you build a motion detector? a. Motion is a change in position over time b. You need something that can integrate time 2. How would you build a motion detector based on changes in the position of an object over time? a. Start with two adjacent receptors i. Each responds to the presence of an object in a particular position b. Need to communicate the difference in timing of each response i. Incorporate a delay from one location ii. This delay accounts for change in time iii. Allowing the cell to detect the coincidence of movement c. This cell will only be able to detect motion going in a specific direction 3. Motion Detection a. Constructing a neural circuit for the detection of rightward motion i. Need two cells at least in adjacent and slightly overlapping receptive fields ii. There needs to have a delay on signal from A in order to know it will be at B iii. It will not misfire if there’s something large in the receptors, and it triggers both A and B independently b. The key to motion is that one signal will be delayed and that allows a second layer neuron to see that there is motion when both signals go through c. Two receptive fields will release signals simultaneously to the next layer and it will appear to show motion 4. How would this network treat apparent motion? a. There are lots of neurons that will be sending info to the next layer b. Each has a delay; the next layer will have sensitivity to change over time c. The same neuron will send 2 signals: one will be delayed, one will not i. It can talk to different neurons to send more info d. There are probably displays for different speeds as well e. Apparent motion: the network would interpret instant snapshots that move across the plane as motion the same way it would interpret actual motion 5. The “Aperture” Problem a. Aperture – an opening that allows only a partial view of an object b. Aperture Problem – the fact that when a moving object is viewed through an aperture (or a receptive field), the direction of motion of a local feature or part of an object may be ambiguous i. Even for the little motion detectors, they have small windows to the world and what they interpret can be part of a different global motion type stimuli c. Previous model can explain motion detection, but it cannot explain how we avoid the aperture problem 6. Aperture Problem – Observation of small portion of larger stimulus leads to misleading information about direction of movement (demo in slow motion) a. Activity of a single V1 cell does not provide accurate information about direction of movement b. You can’t tell if it’s moving up and down or diagonally i. You need a global detector to see diagonally; local detector sees horizontally and vertically ii. It’s ambiguous what direction the stimuli is moving 7. Building a Global-Motion Detector a. Similar to population coding where you need to introduce a new layer to your motion detection b. V1 local-motion detector  global-motion detector (looks for motion in a particular direction) c. Adding another layer to the motion detecting network opens the aperture more D. Visual Area MT 1. Visual Area MT 2. Locating Human MT a. Scanning people’s brains to see when they look at moving dots b. See that back V1, MT are lit up (activated) c. MT is more active
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