PSY280 Last Lecture Study Sheet

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Matthias Niemeier

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Lecture 10: Eye Movements, Depth, and Attention Eye Muscles: - 4 rectus muscles: inferior/superior/lateral/medial - 2 oblique muscles: inferior/superior Cranial Nerves controlling the eye: - Oculomotor (III) – Listens to all other eye muscles - Trochlear (IV) – Listens to superior oblique of contralateral - Abducens (VI) - to draw away (from the nose) – Listens to lateral rectus of ipsilateral - Muscle information is sent to frontal eye fields and the parietal cortex Superior Colliculus: - Structure in the midbrain important for initiating and guiding eye movements - Gets input from retinal ganglion cells, which help plan eye movements 4 Types of Eye Movements 1. Smooth pursuit: - Voluntary eye movement in which the eyes move smoothly t o follow a moving object 2. Vergence: - Voluntary eye movement where the eyes move in opposite directions (either converging towards the nose, or diverging away from the nose) 3. Saccade: - Voluntary/involuntary eye movement where eyes rapidly change fixat ion point (e.g. reading) - Spatial constancy: Problem of discriminating motion across the retina due to conflict (eye movement vs. image movement) – - Saccadic suppression: Reduction of visual sensitivity that occurs during saccadic eye movements – eliminates the smear from retinal image motion during an eye movement. It suppresses information carried by the magnocellular pathway (in between saccadic movements) - Compensation theory: Perceptual system receives “efference copy”/corollary discharge (aw ay from brain) regarding eye movement and discounts changes in retinal image due to these eye movements. Examples: eye tapping, tickling Take into account eye movements that cause differences in retinal image and we can choose to ignore these using efferent copy/corollary discard - the brain is informed that eye movement is happening. Efference = signal from brain to flow out. Goes to a comparator with other image signals towards visual cortex - Comparator: Area in visual system; receives motor system comm and (along with eye muscles); compares motion signal with image movement signal to compensate for image changes caused by eye movements – sends information to the visual cortex 4. Fixational/Microsaccade: - Involuntary, small jerk-like movements - Important for fine spatial judgments (e.g. sewing) Reflexive Eye Movement: Automatic eye movement to compensate for head and body movement while maintaining fixation on a particular target (e.g. walking). Also known as vestibular eye movements and operate via the VOR (vestibular -ocular reflex) Binocular Summation: Combination of signals from each eye to increase performance on tasks compared to one eye alone Binocular Disparity: Differences between two retinal images of the same scene (e.g. recall putting top on pen); the basis for stereopsis (vivid perception of the 3D world not available with monocular vision; impression of three-dimensionality) - Receptive fields found in left eye with phase -shifted inputs from receptive field in right eye – phase shift corresponds to binocular disparity and could be used to derive depth from two retinal images Stereopsis: Ability to use binocular disparity as a cue to depth - visual system exploiting the regularities of projective geometry to recover the 3D world from its projections on a pair of 2D surfaces Projective geometry: Geometry describing the transformations that occur when the 2D world is projected onto a 2D surface (e.g. the retinal image , shadows) Monocular Depth Cues - Occlusion: Cue to relative depth order; most reliable (wrong only during “accidental viewpoints”); does not give information about depth magnitude (non-metrical depth cue ) – only relative depth or the depth order - Aerial Perspective /Haze : Depth cue based on understanding of light scattered in the atmosphere (e.g. distant objects are fainter, closer objects mo re opaque) Geometric Distortions of 3D -to-2D projections - Size and Position: Distance of an object can reflect on its size; relative size is viewed (comparison of size between items) – relative metrical depth cue - Relative Height: Objects at different distances from viewed on ground plane will form images of different heights in the retinal image – relative metrical depth cue - Texture (Relative Size + Relative Height): Depth cue based on fact that items of same size form smaller images when they are far away (and the size change occurs smoothly across the image – this is important in giving the right sense of depth; recall rabbit images) - Familiar Size: Knowing what size something ought to be (typical size) – absolute metrical depth cue - Lin ear Perspective: Depth cue based on fact that lines that are parallel in the 3D world will appear to converge in a 2D image. The vanishing point is where parallel lines receding in depth converge Foreshortening: Visual effect of object appearing shorter because it is angled toward the projection screen/retina/picture plane (e.g. hand pointed away from viewer, door image opening outward to viewer) - Motion: Depth cue based on head movement; geometric information obtained from different positions relative to the eye at the same time (example: sitting inside a train, objects closer shift position more than objects farther away); geometric information obtained from an eye in two different positions at two different times is similar to stereopsis (two eyes, dif ferent position at same time) – relative metrical depth cue - Intraocular and eye muscles Binocular Depth Cues - Vergence : Eyes converging (far to near) and diverging (near to far) ; extent of vergence gives us information on the depth of the object - absolute metrical depth - Binocular disparity used for stereovisi on: Differences in retinal image of the same scene Intraocular/Extraocular Muscle Depth Cues Accommodation: Process by which the eyes change its focus by adjusting the lens (e.g. making it fatter as gaze is directed to closer objects) Vergence: Ability of two eyes to turn inward; reduces the disparity of a feature to (near) zero Stereopsis Implementation: o Input from two eyes converge to cell (striate cortex – where most neurons are binocular) o Binocular neuron: two receptive fields; same orientation, spatial -frequency tuning, preferred speed and direction of motion o Binocular neurons respond when retinal ima ges on corresponding points of retinas (neural basis for horopter) o Other binocular neurons tuned to particular binocular disparity o Can be used as both a metrical (interests dorsa/”where”) & non-metrical (interests temporal/”what”) depth cue Correspondin g retinal points: Geometric concept where points on the retina of each eye where the monocular retinal images of a single object are formed are at the same distance from the fovea in each eye (two foveas are also corresponding points) – recall: crayon example – red crayon is as far from fovea in both eyes Horopter: Location of objects in space whose images lie on correspo nding points in the two retinas (same point on left & right retina); surface of zero disparity; position in the world depends on the current state of the convergence of the eyes; objects significantly closer/far from horopter form images on noncorresponding points in the two eyes (double-vision) Panum’s fusion area: Region of space in front and behind horopter where binocular single vision is possible Diplopia: Double vision when stimuli falls outside of Panum’s fusional area Cross Disparity: Occurs when object is in front of horopter (near); object is displaced to the left of right eye and to the right of left eye (crossed) Uncrossed Disparity: Occurs when object is behind the horopter (far); object is displayed to the left in the left eye and to the right of the right eye (uncrossed) Absolute Disparity: Difference in the retinal coordinates in left and right eyes of the object (e.g. positions on the retina) Relative Disparity: Difference in absolute disparities of two objects (e.g. two objects – relative position on the retina) Combining Depth Cues Bayesian approach: Statistical model based on Bayes’ insight that prior knowledge could influence estimates of the probability of a current event or stimulus – (e.g. what is and what is not likely to occur) o Explores how the visual system decides what we’re actually seeing based on the more likely interpretation o Probability that the world is in State A given observations O can be calculated through P(A) x P(O|A)/ P(O) o Ideal observer analysis: Theoretical observer with best information able to combine different sources optimally (compare human performance to ideal observer) o Optimal inference: Perception choosing the solution that is “most likely” – or “optimally possible” Specific distance tendency: Objects in a dark environment (e.g. a lighted orb in a dark room) are judged to be at a distance of 2 -4m Equidistance tendency: Objects near each other in the retinal image (neighbouring objects) appear to be the same distance to the observer Natural Scenes: The distance of objects relatively near the observer tends to be overestimated, whereas the distance of objects that are fur ther away tends to be underestimated. Finally, the apparent distance of objects on the ground varies with the angle of declination of the line of sight: objects on the ground that are at least several meters away appear closer than they really are, and with increasing distance are judged to be progressively more elevated than warranted by their physical position - Perceived distance predicted via probability distribution of physical distances Grating Scales: Leftward bias associated with greater negativity over the right brain Illusions and Construction of Space o Since visual perception of the world is a best guess about the causes of visual input, illusions may occur when guesses are wrong o 2D illusions: A plausible guess about the 3D world as it is being depicted in a 2D picture
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