Lesson 4 Visual Perception

LESSON 4: VISUAL PERCEPTION Our Visual System - - Electromagnetic light energy is converted into neural electrochemical impulses 1. Light reflected from a surface enters the eyes via the transparent cornea, bending to pass through the pupil at the center of the colored iris. 2. Behind the iris, the thickness and shape of the lens adjust to focus light on the retina, where the image appears upside down and backward. Vision is clearest at the fovea. 3. Light-sensitive receptor cells in the retinal surface, excited or inhibited by spots of light, influence the specialized neurons that signal the brain’s visual centers through their bundled axons, which make up the optic nerve. The optic nerve creates the blind spot. Light bounces off an object in the world Projected onto the retina The retina transduces the light (turns it into neurons firing) THREE MAIN LAYERS OF RETINA: - Ganglion cells - Amacrine cells (horizontal cells, bipolar cells) - Photoreceptors (rods and cones) VISUAL PATHWAYS IN THE BRAIN: - “what” – Temporal lobe lesions in monkeys (can indicate where but not what) “where” – Parietal lobe lesions in monkeys (can indicate what but not where) From eye to back of the brain (Occipital lobe) where visual info is processed - Contralateral (based on visual field) - The wiring from the retina to the occipital lobe of the brain is quite orderly - Electromagnetic radiation (light waves) hit the eye. Neural firing (action potentials) go from eye to brain. VISUAL PERCEPTION - Perception: the process of recognizing, organizing and interpreting information - What we see (visual perception) is not exactly what’s out there in the world 1. DISTAL OBJECT - (distal – distance, i.e. out there) 2. 3. - INFORMATIONAL MEDIUM Light reflected off his face (atmosphere, time of day) PROXIMAL STATUS (in proximity) Light absorbed by retina in the eye (projected onto 2D surface) 4. PERCEPTUAL OBJECT - What we actually experience seeing - Day vs. Clay = context and past experience/knowledge matters, i.e. “top-down effects on perception”  Sometimes we fail to perceive what does exist  Sometimes we perceive things that do not exist  What we perceive is not identical to the distal object out there  What we perceive is not the proximal stimulus. Our brain uses cues and context to interpret the proximal stimulus that hits the retina PERCEPTUAL BASICS: - Sensory adaptation – occurs when sensory receptors change their sensitivity to the stimulus Our senses respond to change – Ganzfeld effect 1. Perceptual Constancy - Object remains the same even though our sensation of the object changes. - Size constancy vs shape constancy - Each person's experience creates perceptual constancy - constancies of size, colour, shape, and brightness 1. Size Constancy - the tendency to perceive an object as being on one size no matter how far away the object is 2. Color Constancy - the tendency to perceive objects as keeping their color even though different light might change the appearance of their color sense 3. Shape Constancy – the tendency to perceive the same shape in an object no matter what angle you view it from 4. Brightness Constancy – the tendency to perceive an object as being equally bright even when the intensity of the light around it changes THEORIES OF PERCEPTION: - Bottom-up vs top-down theories Bottom-Up processing theories  Direct perception  Template theories  Feature-matching theories  Recognition-by-components theory 1. GIBSON’S THEORY OF DIRECT PERCEPTION - The information in our sensory receptors is all we need - No complex through process (ex: use texture gradients as cues for depth and distance, mirror neurons start firing 30-100ms after a visual stimulus) 2. TEMPLATE THEORIES - Compare to templates ion memory until a match is found - Problem of imperfect matches (cartoony versions of animals) 3. FEATURE-MATCHING THEORIES - Recognize objects by feature - Detect elements and assemble them into more complex forms. Brain cells respond to specific features (lines and angles)  Hubel & Wiesel (1979) – each neuron only responded to a very specific situation in a very specific place in the visual field  Simple cells: bars/edges  Complex cells: respond to moving edges, bars/edges detect bars of particular orientation  Hypercomplex cells: moving edges of a particular orientation and length, particular colors (simple and complex cells), moving in a particular direction - Pandemonium Model (Four kinds of demons)  Image, feature, cognitive, decision demons 4. RECOGNITION BY COMPONENTS THEORY - Breaks objects down into “geons” (for geometric ions) Sensation does not equal perception TOP-DOWN INFLUENCES: - When reading sentences, “bottom-up” would be seeing the lines/curves that make up each letter, forming each letter into a word, forming words into sentences, and so on Bottom-up theories have trouble accounting for the fact that we can understand jumble of letters (order of letters inside of a word) Top-down influences on perception The Likelihood Principle: we perceive the world in the way that is “most likely” based on our past experiences Perception is not automatic from raw stimuli (making inferences, guessing from experience)  Word Superiority Effect: better recognition of letters presented within words compared to isolated letters or letters in non-words  Configural Superiority Effect: objects presented in context are easier to recognize than objects presented alone GESTALT’S VIEW OF PERCEPTION - The whole is more than a sum of its parts Law of Prägnanz – individuals organize their experience in as simple, concise, symmetrical, and complete manner as possible These principles allow us to make sense of what we see 1. Closure: the tendency to perceive a complete or whole figure even when there is something missing; there are gaps in what our senses tell us 2. Figure-Ground perception: the perception of figures against a background; what you see depends on which you are looking at and it can change as your eyes move from one area to another 3. Proximity: the perceptual tendency to group together visual and auditory events that are near each other; we group things together because they are close to each other 4. Similarity: the perceptual tendency to group things together because they are alike 5. Continuity: the perceptual tendency to group stimuli into continuous patterns (following the smoothest path) 6. Common Fate: the tendency to perceive objects that are moving together as belonging together 7. Law of Good Form: the tendency to organize forms in simplest way possible 8. Relative size: objects farther away appear to be smaller while objects that are larger appear to be closer 9. Height in plain: distant objects in a picture appear higher while closer objects are lower in the visual field MOVEMENT: 1. Phi Phenomenon: mistaking apparent movement for the real thing 2. Stroboscopic Motion: the illusion of movement produced by the rapid progression of images or objects that are not moving at all; example – movies DEPTH PERCEPTION - This allows us to develop the ability to judge depth or distance away of objects. We perceive this distance by using monocular and binocular cues There is evidence that depth perception begins to develop early in life. The Visual Cliff experiment showed this, the very young infants seemed to be unafraid when placed on the Cliff at the edge of the apparent drop-off. By nine months, infants responded with the fear to the drop off. Other studies showed that they wouldn't even cross the “cliff” when called by their mothers. Crawling apparently signaled the need to sense this danger. The only animal that would regularly cross the “cliff” was a rat. This was because they don't use their vision to sense danger. Their whiskers told them that the surface was still solid, so they moved across it. MONOCULAR CUES - There are several of these which allows us to judge how close or far away objects are 1. Linear perspective: the tendency to see parallel lines as coming closer together, or converging, as they move away from us 2. Interposition (overlap, relative position): the tendency to perceive an object as being closer if it blocks another object 3. Shadows and highlights: objects with highlights on their surface and shadows beneath appear to be closer 4. Texture Gradient: the more texture or detail we see in an object, the closer we perceive it to be 5. Motion Parallax: The tendency of objects to seem to move forward or backward depending on how far away they are from the viewer; An example is looking at nearby things out the car window versus objects farther away. 6. Atmospheric Perspective: The tendency to perceive hazy or smoky objects as farther away. BINOCULAR CUES: - Both eyes are needed to perceive these cues. 1. Retinal Disparity: Binocular cue for perceiving depth based on the difference between 2 images of an object that the retina receives. As the object moves closer; As the object gets closer, the disparity gets greater. 2. Convergence: Movement of eye muscles that cause pupils to come closer together as an object is brought closer. OPTICAL ILLUSIONS: - This is a false visual perception of a stimulus. These are not hallucinations where there is no external stimulus. They exist; we just see them inaccurately. FUSIFORM GYRUS IN TEMPORAL LOBE: - Pattern recognition, facial recognition, high expertise in any item (birds, cars) recognition Expert individuation hypothesis EVIDENCE FOR SEPARATE SYSTEMS: 1. Prosopagnosia - Inability to recognize faces after brain damage - Ability to recognize objects is intact 2. Associative Agnosia - Difficulty with recognizing objects - Can recognize faces DEFICITS IN PERCEPTION - Disruption of the “how” pathway – OPTIC ATAXIA – cannot use vision to guide movement, unable to reach for items Disruption of the “what” pathway – AGNOSIA – inability to recognize and identify objects or people  Simultagnosic – normal visual fields, yet act blind; perceives only one stimulus at a time – single word or object  Prosopagnosia – inability to recognize faces, including one’s own