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Lecture 24

01:830:301 Lecture 24: Visual Guidance of locomotion and Sound

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Lecture 24 – Visual Guidance of locomotion and Sound Visual Guidance of Locomotion -motion informs us about: (Two components of motion is speed and direction) -heading direction (how can I represent motion in each point in a big screen? What symbols can I use?  you can use an Arrow) -Vector field: fancy way of saying that at each x-y location of the image, you’re going to have an arrow -length of arrow  speed -direction of arrow  direction of motion -this is what the motion content is going to look like.  that kind of motion is useful, because it tells you which direction you’re headed within the scene  and also, as a result, we can tell if we’re in a collision course with another object. -Imagine you’re walking to times square and you’re changing direction  and there are many people walking and you can tell someone is going to cross your path at some point – (when do we adjust our speed or direction?) how soon? – doesn’t always have to be moving it could just be a tree or a table – you still have to worry about the collision course)  This is what we call an Optic Flow. -how soon a “collision” is likely to occur Optic Flow: refers to the flow pattern that you get on the retinas as the result of the relative motions of different objects and different surfaces in the world (Vector Field – at each x-y location, you have an arrow that tells you how fast the object is moving and in what direction) OPTIC FLOW Forward Motion  it would look like and expansion (outward flow) in the image look like Star Wars beginning scene when the stars expand and it looks like you’re going through space and the stars are going past you. -you’re going to get a smear in the outward direction -if you do this with a camera with a long exposure time  you would produce a smear and it would look like you are going forward Backward Motion  contraction (inward flow)  Image – This is what optic flow looks like (dot tells you the x-y location that we’re currently talking about and the length of the line that is attached to the dot explains which direction and speed of the dot is moving) -at each dot (the current x-y location) in the image, you have a direction and speed of motion -At which points in space are moving the fastest  the points that are in the front that is closest to you in the bottom Focus of expansion -which points in space are not moving at all (zero speed)  the horizon and the ones closest to the middle There is a point in the middle that every point is pointing toward that point (and that point is not moving- 0 speed) -The middle point is called Focus of Expansion (all of the other arrows are expanding outwards starting from that point)  this gives us the HEADING DIRECTION (it will make it look like you’re going forward) COLLISION COURSE  Time of Collision: How soon a “collision” will occur? -if you know the heading direction, you know if you’re going to collide with other objects Time to collision: how much time you have before collision  if you want to adjust your direction and speed, you need to know when you’re colliding Animals: The seagull dive into water when they hunt fish  they know exactly when to fold their wings before landing in the water (they’re good at computing time of collision) -How can we estimate time of collision? Important Factor: If I know how fast I am moving in the world and how far (distance) the water is  then I can compute time to collision One possibility: (natural possibility) -estimate the depth, distance of object (D) -estimate our own speed of motion (or how the fast water surface is coming towards you) (S) -Time to Collision  Time = Distance / Speed (Recall: Speed = Distance/Time) -this would be a natural possibility, but there are some difficulties because this approach acquires youtoknowthe actualspeedin theworld(andit’sdifficulttomeasuretheexactspeed) -difficult to estimate or measure your exact/precise speed and the absolute depth (not relative) of the object  mathematical point of view it is very natural, but it’s difficult to measure D and S in a precise estimate of the absolute speed and depth. -This is NOT exactly how the brain computes the time to collision  (both of components, (D and S) instead the strategy that brain uses is to use quantities that are much easier to measure directly on the image The visual system uses: 1. Current size of object on the retina  (not the true size of object in the world) it is the size of the image that is projected on the retina (you don’t have to make any complications to get that) 2. The rate at which the projected image on the object is expanding/ growing in the image (on the retina) -D and S were both properties of the 3D scene, whereas these are properties that can be measured directly on the image you can simply measure these two properties in the image on retina How does this help us (knowing the 1 and 2)? When would collision have occurred? At what point? -when the image of the object is entirely filled on the retina  you would know you collided  within how much time did the image fill the retina -by combining the ratio of 1 and 2, we can see within how much time the image is going to fill the entire visual field. TAU(Time to collision) = ratio (1) / (2)  time to collision is given by Tau Key difference with this strategy with the previous strategy: the two quantities 1 and 2 simply requires measuring certain variables within the image itself, it doesn’t require to estimate any other quantities in the world (like the distance or speed in 3D Space) -Using Tau doesn’t require the actual/true depth or the true speed (the speed in 3D space) of the object (our speed relative to the object) -even infants show the “looming affect” (looming stimulus: object is getting larger) in the TV  they think it’s going to collide even if it’s a flat screen. (End of Motion chapter) HEARING – Auditory Perception To understand auditory perception, we must study: 1. Acoustic energy/sound 2. Structure and function of the ear (the sense organs) 3. Processing of acoustic information in the ear and the brain Difference between acoustic energy and sound: Acoustic energy: physical variable Sound: purely perceptual quantity What is Sound? -a vibrating surface generates COMPRESSIONS and RAREFACTIONS within the medium (e.g. air) -ex: tuning fork  moving back and forth really quickly – creates compressions (air molecules compressed together) and rarefactions(air molecules spread apart) in the air -the density of molecules (compressed or separated) determine air pressures -Compression: increase inAir Pressure -Rarefaction: decrease in Air Pressure -so that means, in order to plot an acoustic wave, I can plot air pressure on the y-axis AcousticEnergy: wearereferringtowavesofcompressionandrarefactionthroughthemedium Sound: purely a perceptual experience that is based on physical energy that is present in the world -Tree Philosophy question: if a tree fell in the middle of the forest, would there be sound?  no (because sound is a perceptual experience and if there’s no one there to have this perceptual experience, there is not sound) but it would have acoustic energy (because there is a measure) -In order to have C and R (increases and decreases in airpressure) you need a medium because you need molecules that you can compress or separate  if you don’t have molecules, you’re going to have a vacuum  you CANNOT hear through a vacuum -Electromagnetic waves CAN travel through vacuum (they have different properties than sound waves  light has dual nature  can be particles an
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