Motion Perception Lecture Notes PDF

Perceiving motion Jason Bell, Wednesday Sept 18 2024 Page 1 of 36  Today’s lecture Part 1 What is motion: space and time How do we detect both things in a circuit? Apparent motion The aperture problem Motion in the brain: MT This is th...

Perceiving motion Jason Bell, Wednesday Sept 18 2024 Page 1 of 36  Today’s lecture Part 1 What is motion: space and time How do we detect both things in a circuit? Apparent motion The aperture problem Motion in the brain: MT This is the ‘where’ pathway Part 2 Using motion information Optic flow & navigating Biological motion Time to collision The role of eye movements Saccades and shutting the world off! Page 2 of 36 1 Lets start with a demo Motion aftereffect (MAE): The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object. https://neurosciencenews.com/waterfall-illusion-brain-14138/ Existence of MAE implies an opponent process system, like that of colour vision. So motion is processed as a visual attribute AND as part of a set of interconnected processes. But… unlike colour, motion isn’t encoded at discrete points in space (contrasting cone responses) Motion involves changes in space, across time Page 3 of 36 1 Coding motion How would you build a motion detector? Motion is just a change in position over time. Start with two adjacent receptors. Page 4 of 36 1  Coding motion To distinguish motion, the circuit needs additional neurons Register the change in position Incorporate a delay. Accounts for change in time This type of circuit is called a Reichardt detector Page 5 of 36 1 And this principle extends further The motion detector circuit in the previous slide works, but it doesn’t cover a very big area. Is there a way string several motion detector circuits together and cover a larger area? Yes Page 6 of 36 1 Using the rules: Apparent motion and how TVs, displays and cartoons work Apparent motion: The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. First demonstrated by Sigmund Exner in 1875. Demonstrates the motion detector circuit does not need real motion in order to fire. But not everything looks natural Page 7 of 36 1 A word on frame rates and smooth motion Think about drawing pictures in the corner of your book and then flipping through the pages. The rate that you show these images is called a frame rate Higher frames per second (FPS) will appear smooth. Lower FPS will appear jerky https://youtu.be/pfiHFqnPLZ4?si=R6XyX-O4XJlyxRjj Your monitor also has a framerate. We describe these in frequency, or Hertz (Hz) Page 8 of 36 1 A word on frame rates and smooth motion Of course this is an issue for filming/recording the real world too… Real objects move but recording a video involves a series of discrete static images- this is FPS So when we watch authentic motion we are still experiencing apparent motion. See below about FPS in relation to shooting movies 24FPS and shutter speed https://www.premiumbeat.com/blog/advanced-look-into-frame-rates/ Page 9 of 36 1 The aperture problem Correspondence problem (motion): The problem faced by the motion detection system of knowing which feature in frame 2 corresponds to which feature in frame 1 – We are wired to detect all motion directions Aperture: An opening that allows only a partial view of an object: This is how a neuron works: receptive fields – 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 Page 10 of 36 1 The aperture problem 9m 45s in: https://www.youtube.com/watch?v=i2ETv2N1z5E Page 11 of 36 1 The aperture problem 11:08 in https://www.youtube.com/watch?v=i2ETv2N1z5E Page 12 of 36 1 Solving the aperture problem Given that motion within any single aperture (or receptive field) is ambiguous, how might the visual system correctly perceive the overall motion of objects? Motion information from several local apertures (or receptive fields) can be combined to determine the global motion of the object Page 13 of 36 1 Solving the aperture problem Whichever possible motion direction is the same in all apertures is the true global motion direction of the object Page 14 of 36 1 Motion in the brain Motion processing must occur at multiple stages in order to solve the aperture problem. This implies distinct populations sensitive to local motion at small scales, and to global motion, at larger scales We can say something about where global-motion detectors are: Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects. So its an M, or “where” pathway visual signal Middle temporal lobe (MT) also plays an important role in motion perception The vast majority of neurons in MT are selective for motion in a particular direction Page 15 of 36 1 Motion in the brain Page 16 of 36 1 Demonstrating the role of MT Newsome and Pare (1988) conducted a study on motion perception in monkeys. They trained monkeys to respond to correlated dot motion displays. A threshold task for global motion perception (absolute limit for detecting coherent motion) Not different colours in the real experiment Page 17 of 36 1 Demonstrating the role of MT Newsome and Pare (1988) conducted a study on motion perception in monkeys. They trained monkeys to respond to correlated dot motion displays. The MT area of the monkeys was lesioned. Result: Monkeys needed about ten times as many dots to correctly identify direction of motion. And in humans Akinetopsia: A rare neurophysiological disorder in which the affected individual has no perception of motion. Caused by disruptions to cortical area MT Page 18 of 36 1 Lesion studies A quick note on lesion studies… Invasive (sometimes terminal) Lesions may be incomplete or may influence other structures. E.g. unplugging a TV impairs TV perception… An alternative is electrical stimulation of MT neurons? E.g. TMS, tDCS. A newer approach Can be done on humans. Avoids problems of lesion studies Biases motion detection thresholds or appearance in a temporary manner Page 19 of 36 1 Pt 2. Using motion information Part 2- using motion information to make sense of the world Optic flow for navigation Biological motion for recognising Time to collision estimates Eye movements Distinguishing between retinal motion and object motion Saccadic suppression Page 20 of 36 1 Optic flow and navigating How do we use motion information to navigate? Optic array: The collection of light rays that interact with objects in the world in front of a viewer. Term coined by J. J. Gibson. Optic flow: The changing angular position of points in a perspective image that we experience as we move through the world. Optic flow demonstration https://youtu.be/ysGM3CfBVpU Page 21 of 36 1 Optic flow and navigating Focus of expansion (FOE): The point in the centre of the horizon from which, when we are in motion, all points in the perspective image seem to emanate. This is one aspect of optic flow. The focus of expansion tells the observer which way they are heading. Page 22 of 36 1 Biological Motion: determining structure from patterns of motion http://www.biomotionlab.ca/Demos/BMLwalker.html This is how the Xbox kinect works. Also how they test bowling actions in cricket (done at UWA) Page 23 of 36 1 Biological motion in the brain Pattern of movement gives rise to the recognition of the object Including: gender, animal species, mood Processing appears to involve the superior temporal sulcus (STS) Page 24 of 36 1 Time to collision (TTC) Avoiding imminent collision: How do we estimate the time to contact (TTC) of an approaching object? Tau (): Information in the optic flow that could signal TTC without the necessity of estimating either absolute distances or rates 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 Example: You are driving and then suddenly realize that the car in front of you is getting really big, really fast. You slam on the brakes because you realize that a collision is imminent if you don’t. Of course, this is not the only cue- as I’ll describe in week 10, monocular and binocular depth cues aid distance perception too… Page 25 of 36 1 Eye movements and motion The problem the visual system has to distinguish between motion on the retina caused by eye movements versus motion on the retina caused by moving objects Lets consider a simple example A) stare at dot, pencil moves on retina B) stare at pencil, everything else moves on retina Page 26 of 36 1 Eye movements and motion Why do we perceive the pencil to be in motion in the first case, but perceive the dot to be stationary in the second case? After all, both items moved across our retinas Because in the second case there was an eye movement This table explains why Object Retinal Movement? Eye Movement? Movement? No No No Yes Yes No Yes Yes No No Yes Yes Page 27 of 36 1 Eye movements Types of eye movements: Smooth pursuit: The eyes smoothly follow a moving target Ou experienced this in example B in the previous slide Saccade: A rapid movement of the eyes that changes fixation from one object or location to another Let’s try a demo in class to show the difference between the 2 eye movements 2 fingers held in front. Track one finger moving vs switch which finger you look at Vergence: The two eyes move in opposite directions, as when both eyes turn towards the nose. We’ll talk more about this in depth perception (wk 10) Reflexive: Automatic and involuntary eye movements Page 28 of 36 1 During saccades, we basically don’t see Saccadic suppression: The reduction of visual sensitivity that occurs when we make saccadic eye movements Saccadic suppression eliminates the smear from retinal image motion during an eye movement You can observe saccadic suppression yourself in a mirror (or not…) Let’s try this demo in class with a mirror Look at your left eye, then your right. Did you see your eyes move? To me this is one of the coolest parts of vision. We don’t notice the world shut down every time we move our eyes!! i.e. 2 to 3 times per second We make saccades multiple times per second. Up to 172,000 per day (and then there is sleep- e.g. REM sleep) Page 29 of 36 1 But the visual system plans for saccades How else do we compensate for eye movements to preserve the stability of the visual world? Dynamic remapping of receptive fields A saccade is planned but not yet executed. Some neurons in parietal cortex remap their receptive fields relative to upcoming fixation location Saccade is executed. Receptive fields are already processing information from new location before eye lands there. Receptive fields of neurons in the frontal eye fields also transiently shift inward towards the new point of fixation Page 30 of 36 1 But the visual system plans for saccades Some neurons in parietal cortex remap their receptive fields relative to upcoming fixation location Receptive fields are already processing information from new location before eye lands there. Receptive fields of neurons in the frontal eye fields also transiently shift inward towards the new point of fixation So the visual system plans for an eye movement before we “choose” to make it… Page 31 of 36 1 And eye movements are not random Saccades are not random but rather, tend to focus on ‘interesting’ points in an object/scene: e.g. Page 32 of 36 1 So how do eyes move? Six muscles are attached to each eye and are arranged in three pairs: Controlled by an extensive network of structures in the brain Superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements When this structure is electrically stimulated, eye movements result Page 33 of 36 1 Today in summary Part 1 what is motion , how is it encoded and where? The Reichardt detector. Real motion is not required in this circuit. Apparent motion shows us that The aperture problem: local ambiguity is solved into global motion perception Global motion is processed in area MT Page 34 of 36 1 Today in summary Part 2 We use motion information For navigating. i.e. Optic flow and the Focus of Expansion (FoE) To reveal structure: Biological motion movement reveals structure and identity To avoid or interact with objects: time to collision (TTC): Tau, and disparity help us Eye movements Two main types. Saccadic and smooth pursuit Comparison of the two identifies object movement Saccadic suppression reduces our percept of smear/blur Eye movements planned and we process the location we will jump to! Superior Colliculus important in control/guidance of eye movements Page 35 of 36 1 Next time Don’t forget the essay is due next Tuesday the 24th 11:59pm Next Wednesday we talk about depth perception Page 36 of 36 1