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Lecture 4

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University of Toronto St. George
Connie Boudens

Lecture 4 - 01-28-12 From last week.... The Visual System PR Rods lower sensitivity lower acuity colour blind periphery of retina Cones higher sensitivity/acuity colour sensitive in fovea There is a blind spot. Our brain fills in the blank perceptually. A series of neurons communicates information from the retina to the cortex in the eye: PR BC Ganglion cells and optic nerve (goes from eyeball, where the blind spot is, and carries information to different parts of the brain) in thalamus: LGN in cortex V1, primary visual cortex There's some neural activity in the individual system in that visual system diagram. One's a bright stimulus and one is dull. It can excite or not excite. If we think of two images coming in, cell B has information coming in from both cell A and C where cell C is only getting information from cell B, cell D would not have information coming in. This is what elicits one of our optical illusions i.e. Mach bands, lateral inhibition between neighbouring cells and the retina. Because the two pieces of information are coming into your visual stream, there's some inhibition going on in between your cells, it is trying to find the edges to make the perception of the white band look darker than it actually is. The other example, the checkerboard with the cylinder, square B is the same level of darkness as A. There's also a shadow coming from the cylinder that compensates for the darkness as well. Single-cell recording the frequency of the action potential are recorded as different stimuli presented to a subject. Researchers can manipulate what's being shown and record how they fire. What makes them fire? There are studies involving animals, there are certain cells firing in the brain. There's this receptive field. The main thing is that in the RF, there's a center and surround. When something is stimulated in the centre, the neural firing frequency increases from normal. When something is in the surround, it actually inhibits/decreases from normal firing. How is this all related? The receptive field of the V1, they are related to line orientation. These kind of cells are edge detectors. Flat, no stimulus, when it's an actual angle, there's more activity, when it's upright, there's lot of activity. Different neurons in V1 are specialized, resulting in parallel processing, not serial processing. Parallel processing in the visual pathway: P (parvocellular) Cells M (magnocellular) cells The what system: Identification of objects Occipital-temporal pathway Visual agnosia The where system: locations of objects and guiding our responses Occipital-parietal pathway Problems with reaching for seen objects Binding problem is how do we recognize this as one single unit when you have so much information coming in from many areas. While things are firing about one particular object, it'll also be firing about the movement. Our brain interprets that synchrony as coming from one object, this is the neural synchrony. The systems themselves, the spatial position, the visual areas that are processing these features - like shape and colour - know that spatial position of the object. You can think of the coordinates on your GPS. Attention is also critical for the binding of visual features. When attention is overloaded, people will make conjunction errors. E.g. recognizing the colour and letter while concentrating on other tasks, you may mix up the colours, that's the idea of overloading the visual stream (attention is overloaded), this is the idea of conjunction errors. *** Lecture 4 Lecture outline: Form perception object recognition word recognition feature nets different objects, different recognition systems top down influences on object recognition Typoglycemia The words are spelled wrong but we are able to recognize what the words are. Recognizing objects why is this important? If we couldn't recognize objects, we are incapable of moving on in our daily lives, recognize the toothbrush, keys, etc. Sometimes it's hard to imagine what object recognition is because it comes natural to us. But if we think about when we first saw something that was brand new or something that was outside your domain of expertise; i.e. going to Lowe's/Home Depot, you have no clue what it does; if you used the tool, that would look to you just like a table or whatever, you got used to seeing those objects. It's a valuable part of our daily lives and is crucial for learning. This is especially important for recognizing people; friend or foe, family. Beyond the information given Form perception form perception: shape and size object recognition: ID The whole is greater than its parts: Knowledge about something gives us more information about an object more than the simple visual features that we see. E.g. Necker Cube If we look at the cube at the top, you can either see cube A/B. Only one can be seen at a time, and you can pop back and forth E.g. the vase/faces You either see two faces looking at each other or a white vase. Your knowledge helps interpret which you see. E.g. staircase What you see is going beyond the information because it is only lines. E.g. skull/lady in mirror Form Perception People resolve ambiguity in everyday situations. In the fruit basket image, there's an apple behind a banana. Your ability to interpret scenes that are ambiguous are governed by a few principles 1) similarity there's red dots/greyish pink dots, we tend to view this as dots in a column because the colours are similar. 2) proximity rather than 15 dots on a screen, people will say that there are three groups of dots. We connect the dots closest in proximity as being in one group 3) good continuation if something seems like it's being interrupted by another object, we think of simple terms of continuation. 4) closure we look at the triangle with big gap in the left and right side. We can look at it and it feels like a triangle to you, it's one image instead of two. 5) simplicity We tend to think about it as two pieces of wood placed on top of the other, we take the simplest explanation and we perceive it without any effort or thought The Mona Lisa example We ID the head as being something different than the outer object. Alternatively, the actual pattern is influencing us in other ways. The circle/loop example there's actually 5 separate circles. Because of the shape and pattern, we're taking it as one whole spiral and it takes effort to see the separate circles The X/V example we take the simplest explanation possible. We avoid interpretations like coincidences or bringing in extra theories. We tend to see it as an X or two lines crossing. It's a coincidence that there is precision for it at the junctions to be two Vs The LIFT/black background example People usually see the BG as the foreground, they don't see the white necessarily, as soon as you ID as the foreground, you see that there are letters there. Until you had the basic information feed in, you have some knowledge and it changes the way you see what's on the page there. The OPTICAL ILLUSION/field example You can't go back once you seen it; as you begin to apply knowledge to it, top down processing, it's always available to you. The next time you see it, you're not interpreting from bottom up anymore, it becomes automatic. PERCEPTION word with shadows It doesn't really have information to inform you of the letter structure. We interpret the dark lines around it as being a shadow which we perceive as an edge. We consider them as one object. It gives us the sense of a closure as well. Brain areas for basic visual feature brain areas for large scale form Interactive Object recognition form perception, the process through which the basic shape and size of an object are seen object recognition, the process through which the object is identified E.g. The "Dolphin" example top down information is influencing what we see. We've probably been exposed to situations where a loving couple embraces from arts, real life, school, TV., media, etc. If you show this to children, they see nothing except dolphins. The idea is that this is a good visual information to be shown that once you have a certain external information, it's hard not to see that. E.g. THE CAT (example from the textbook) H and the A are the exact same shape and we say that in a context of what's there, we don't have an issue of seeing the first on
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