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Lecture 6

PSYB51 lecture 6 .docx

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Matthias Neimier

Lecture 6- PSYB51 What is the problem of univariance in colour vision? Please explain. What does it mean for vision based on a single type of colour- sensitive cone? How can the problem of univariance be solved?  The problem of univariance can be solved by having tricolor rods  Firing rate varies—universally based on: wave lengths, and intensity of the light. >cones will response differently depending on wavelength. 3 cones: Large = red Medium= green Short= blue  More cones= more richer colour vision Eye Movements
 – 6 muscles controlled by cranial nerves – Oculomotor control
 – Different eye movements
 – How do we achieve spatial constancy? Why talk about eye movements in a course on perception? - It could affect how we see the world - It is driven by attention - Motion -> quickly look at something ugly, dangerous, interesting, etc. - Move your eyes to points of interest so that you have a higher resolution - Eye movements are important to enhance our vision  Eye movements: six muscles are attached to each eye and are arranged in three pairs: – Inferior/superior/lateral/medial rectus – Inferior/superior oblique  Rectus muscle runs straight  Inferior rectus= bottom  Superior rectus= top  Boney hook= trochlea > As soon as you rotate your head more than 5degrees, your eyes counter rotate (you don’t really notice that- vision stays stable). > Passed 5 degrees, eyes make a quick motion, so a little disruption in vision (gaze is not stable) > Third dimension is rotation along sight > For each dimension of rotation, you would need 2 muscles > Muscles don’t do anything but pull Controlled by 3 cranial nerves  Extensive network
 – Frontal eye fields & parietal cortex => LOOK AT DIAGRAM – Superior colliculus: Structure in midbrain that plays important role in initiating and guiding eye movements – When stimulated with electrical signals, eye movements can be observed – Frontal eye field is the most important eye field in the cerebral cortex – Parietal cortex: somewhere around that area, there’s a region called lateral interpiratal area  Eye movements: – Smooth pursuit: Eyes move smoothly to follow 
 moving object – Saccade: Rapid movement of eyes that change fixation from one object or location to another -- Vergence eye movements (convergence and diversion movements): Type of eye movement in which two eyes move in opposite directions, done deliberately – Fixational eye movements, microsaccades (tiny versions of the saccades; it refreshes the eye) -> the eye never stays stable, it slides a bit  Function of smooth pursuit eye movements: keep object of interest stable and on the fovea.  Demonstration: Look at the background with the pencil moving across in the front you see that the eraser is clearer than the background  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? 
 – Because in one case there is an eye movement  Similar effects can be observed with saccadic eye movements  Function of saccades: move (rotate) fovea to object of interest, move as quickly as possible to reduce travel time during which vision is blurred.  Yarbus (1967): scanpaths reveal intentions and interests.  3-4 saccades/sec  False motion & retinal smear during saccade Why don’t we notice that?  Spatial constancy: Tricky problem of discriminating motion across the retina that is due to eye movements vs. object movements – Demonstrate this – Why does this happen? – Saccadic suppression (of vision, incl. motion): Reduction of visual sensitivity that occurs when one makes a saccadic eye movement; eliminates smear from retinal image motion during an eye movement  the vision starts shaking if you close one eye  When you push the eye, it moves – Remaining questions: > How does the brain know when to suppress? > Insufficient: displacements across saccades should result in apparent motion illusions (but doesn’t) > Botox was injected to the eye muscles to make it unusable. It appeared to him as if things were jumping around Compensation theory: Perceptual system receives information about the eye movement and discounts changes in retinal image that result from it (proposition) – Motor system sends motor command to eye muscles – A copy of that command (―efference copy‖, or colorily discharge) goes to an area of visual system that has been dubbed ―comparator‖ -> efference is anything that comes out of the brain -- Comparator compensates for image changes caused by the eye movement, inhibiting any attempts by other parts of the visual system to interpret changes as object motion [LOOK AT DIAGRAM] Space Perception and Binocular Vision  Euclidian geometry: Parallel lines remain parallel as they are extended in space – Objects maintain the same size and shape as they move around in space  they don’t shrink – Objects get bigger as they approach our eyes – Which sense is governed by Euclidian geometry?  Problem for vision: recover 3D info from 2D projections – Most depth cues can be derived from geometrical consequences of the projection  The two retinal images of a three-dimensional world are not the same! -> Parallax  Binocular disparity: The differences between the two retinal images of the same scene. It is the basis of stereopsis; a vivid perception of the three- dimensionality of the world that is not available with monocular vision.  Monocular depth cues vs. Binocular depth cues: One eye vs. two eyes  Binocular depth cues (from overlapping visual fields) provide: – Convergence – Stereopsis: is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision – Ability of two eyes to see more of an object than one eye Monocular Cues to Three-Dimensional Space – Occlusion – Relative size
 – Position cues
 – Familiar size
 – Aerial perspective – Linear perspective – Motion cues  Occlusion: A cue to relative depth order when, for example, one object obstructs the view of part of another object – Nonmetrical depth cue: provides information about depth order but not magnitude. -> ex. we know there’s a square behind a circle; we just don’t know how far it is -- (Metrical depth cues: Provide quantitative information about distance)  Size and position cues: – Relative Size: A comparison of size between items without knowing the absolute size of either one (things UP on a picture means it’s closer to the horizon, and DOWN means closer to us) Size and position cues cont’d: – Texture Gradient: A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away – Relative Height: Objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image  The more remote parts in planes situated below the eye, appear higher (the projection EF of BC appears higher than the projection DE of AB). • Natural scene statistics. > There are two pictures of a smokey landscape. Th
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