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Chapter 10

PSYCH 1XX3 Chapter 10: Module 10- Form Perception 2

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Joe Kim

Module 10 Form Perception 2 Feature Detectors - Magno and parvo cells in the retina transduce the light stimulus into a neural pulse - Magno cells are found mainly in the periphery of the retina and are used for detecting changes in brightness as well as motion and depth - Parvo cells are found throughout the retina and are important for detecting color, pattern and form - Ganglion cells and their small receptive fields are crucial to object recognition - From the retina, the axons of these cells exit the eye via the optic nerve, travel to the LGN and end up in the occipital lobe - Cells are particular about what makes them fire, called feature detectors - In 1952, Hodgkin and Huxley recorded the electrical activity in an individual neuron of the squid, and this paved the way for other researchers to use this technology to see how individual neurons respond to specific stimuli - In 1959, Lettvin et al discovered the optic nerve of a frog that responded only to moving black dots called bug receptors - Hubel and Wiesel spent years extending the work in the cortex of cats and monkeys, and won a Nobel prize in 1981 o Began their exploration of the visual cortex by trying to learn what types of stimuli the individual cortical cells respond to o They did this by putting microelectrodes in the cortex of a cat to record activity when shown different stimuli o Realized that neurons must respond to stimuli that are more complex than flashes of light (which were previously being used) o Found that each neuron is very specific about what will make it fire the most o These cells fire maximally to stimuli of a certain shape, size, position and movement and this defines the receptive field for that cell - Simple cell: respond maximally to a bar of a certain length and orientation in a particular region - Complex cell: responds maximally to a bar of a certain length and orientation, regardless of where the bar is located within the receptive field - Hyper-complex cells: responds maximally to a bar of a particular orientation that ends at specific points within the receptive field - The layout of the visual scene is preserved in the visual cortex - Neighboring objects in the field are processed by neighboring areas of the brain - The largest amount of the cortex is devoted to processing information from the central part of the visual field, which projects onto the fovea - Each region of the cortex receives some input from a small piece of the visual field and within each region there are specific features that get analyzed - For a particular part of the field, there are neurons that are maximally fired, responding to specific parts like color or orientation - Parallel firing by multiple areas of the brain increases speed Ventral Stream - The processing of the visual input in the primary visual cortex involves specific cells responding to relatively specific features from a small portion of the visual field - Combined to give a new meaning - The combination begins in the extrastriate cortex, aka the visual association cortex. It has multiple regions that each receive a different type of information from the primary visual cortex about the scene (one for color, one for movement, another for orientation) - In the extrastriate cortex where the information begins to segregate into two streams according to they type of information it is o Dorsal (where), processes where objects are located in the visual field and how they are moving o Dorsal takes information from the primary visual cortex to the parietal cortex which processes spatial information o Ventral (what), processes information about what the object is, like color and form o The ventral stream takes information from the primary visual cortex and sends it to the temporal cortex - The temporal cortex is arranged in vertical columns that are oriented perpendicular to the surface of the cortex - Neurons in the temporal lobe respond to very specific stimuli that are more complex, like hands and faces, feet, apples and chairs (among many others) - Each layer respon
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