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Lecture 6


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Matthias Niemeier

PSYB51 Lec 6 SAQ: what is the problem of univariance and 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? - Can’t tell different colours - Just see different shades and luminance - Named univariance because only have 1 type of colour receptor (ex. s cones – only sensitive to lower, shorter wavelengths - Makes us confuse all colours - Photoreceptors not specific  sensitive to many wavelengths Eye movements - Micro saccades – jerky, little eye movements that refresh the retina - Most acute vision is in the fovea - Need eye moevments to move our fovea around - Specifically designed for vision o No other movements are as specialized - Optimizes vision - Simplest kind of movement Eye movements: based on 6 muscles – 6 muscles are attached to each eye and are arranged in 3 pairs - Counter rotations in the head – when spinning eye one way, eyes go the other way - 3 dimensions of rotation: o Left/right o 2 x 3 = 6 muscles - 2 types of eye muscles: o RECTUS  On the TOP and BOTTOM and on the TEMPORAL and NASAL sides  Inferior for bottom/superior/ lateral – away from eye/ medial o Oblique  Wrap around the the eye  Inferior – underneath eye ; for torsions and vertical  Superior oblique Controlled by 3 cranial nerves: - 3, 4, 6 - CN 3 – Oculomotor innervates almost all eye muscles except 2 - CN 4 – trochlear nerve - superior oblique muscle (runs through trochlear) o Projects to the opposite eye - CN 6 abducens nerve – Extensive network - *know: eye movements are controlled by cerebral cortex (done consciously) - 2 main centers of eye control – frontal eye fields, parietal cortex o Project onto superior colliculus  Superior colliculus: midbrain structure – guides eye movements - Signals split up into HORIZONTAL and VERTICAL components Eye movements: - Smooth pursuit : a car is driving down the street and you follow with your eyes - Saccades: jerky eye movements, very quick; meant to move fovea to objects of interest - Vergence eye movements o Convergent eye movements: following an object getting closer to our eyes o Divergence: following an object moving away from the eye Smooth pursuit - Demo with pencil - Meant to keep stuff stable on our fovea - We compensate automatically for our eye movements - Eye movements are meant to optimize vision – to stabilize and mess around with it Function: keep object of interest stable and on the fovea - vision interpreted differently - same effects seen with saccadic eye movements - world stays stable but eyes move around Function of saccades: move fovea as quickly as possible to reduce travel time which blurs vision - have false motion – things seem like theyre moving but its just our eyes - ex. picture of saccades on a face o knots mean fixation is not perfect – not completely stable Spatial Constancy - problem of discriminating motion across the retina due to eye movements or movements in the world - how do we solve? Problem of spatial constancy - demo that works  fixate on one area of a picture to the other and see if it seems like it is moving around Saccadic Suppression - during saccadic movement, we don’t see the world moving  motion perception doesn’t work that well; gets suppressed - reduction of visual sensitivity – sensitivity to motion – when making an eye movement - how does the brain know when to suppress something? - Displacement across saccades results in apparent motion illusion - Just vision alone is not sufficient Resolve through: Compensation theory: perceptual system receive information about eye movement and based on this info, discounts changes in retinal image that results from it th - Like a 7 sense - Motor systems send motor command to eye muscles  efference o Efference can be copied “efference copy” goes back to the visual system called a “comparator” - Comparator compensates for image changes caused by the eye movement inhibiting any other parts of the visual system interpreting changes - The brain is not aware that the eye is moving – retina sends information - Efference copies demo: eye tapping o Also: tickling your own hand does not actually make it tickle but if someone else does it, it feels more ticklish o Ticklish is the signal to the brain that something might be happening to your hand o When doing it to yourself, brain ignores it because it knows what you are trying to do o Part of comparator is associated with cerebellum - Corollary discharge signal = efference copy - If information about retinal movement and efference copy get to comparator, brain knows it is an eye movement and not movement from the environment Depth perception Intro to space perception - Euclidian geometry o Parallel lines stay parallel as they stretch in space o An object of a certain size will be the same size no matter the distance from it  Seem like they get smaller as we walk away from them  Take up less space on the retina o We don’t sense geometry directly, we see a projection of Euclidian geometry on our 2D vision o Does not work with touch or haptics - Problem with vision: o To perceive depth and space, need to recover 3D info from 2D porjections o Not able to derive the true depth o Most depth cues are about recovering 3D info from 2D projections – has to do with results of projections  Torsions that happen when looking at projections Ex. parallax - One geometrical fact is one eye is left of other, and therefore are horizontally apart - Vantage point is different for each eye - Horizontal/retinal disparity: gives rise to depth Binocular disparity: diff between two retinal images of the same scene, basis of stereopsis – vivid perception of 3D of the world not available with monocular vision Monocular vs binocular depth cues - Binocular depth cues come from overlapping visual fields provide: o Convergence  Amount of rotation of the 2 eye
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