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Lecture 10

PSYCH 1XX3 Lecture 10: Module 10: Form Perception II (2017)

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Joe Kim

Module 10: Form Perception II Feature Detectors Magno and Parvo Cells • Found in retina • Transduce light into neural impulse • Magno mainly in periphery of retina – detect changes in brightness, depth, motion • Parvo found throughout retina – detect colour, pattern, form • From these ganglion cells – their axons exit retina via optic nerve to LGN • LGN to primary cortex in occipital lobe • Feature Detectors – what makes the cells in primary visual cortex fire Hodgkin and Huxley • Pioneered method for recording activity in neurons • Now able to see how individual neurons respond to specific stimuli • Lettvin – neurons in frogs on responding to moving black dots “Bug Detectors” Hubel and Wiesel • Accidently discovered feature detector cells • Used microelectrodes on cats to record neural activity when different visual stimuli presented • Neurons didn’t fire until cracked slide was presented – cats I followed crack, neurons fired intensely • Realized – neurons respond to stimuli more complex than diffuse flashes of light • Each neuron specific about what makes it fire most • Cells fire maximally to certain shape, size, position, and movement Simple Cell • Respond maximally to a bar of a certain orientation in a particular region of the retina • Example o Following simple cell responds most to horizontal bar o Is bar changes orientation/moves out of region – cell is inhibited (fires less than baseline) o Receptive field of simple cell = opponent fashion Complex Cell • Responds maximally to a bar of a certain orientation and direction of movement, regardless of where the bar is located with the receptive field • Example o Some cells don’t care where bar is located a will continue to fire o Some do care, get highest response at certain location, direction, angle Hypercomplex Cells • Responds maximally to a bar of a certain orientation and direction of movement, ending at specific points within receptive field • Have a region of inhibition at the end – making them sensitive to length of bar • Example o Most response when bar of light is in one region o Weak response when touching off Topographical Organization • Preserved in visual cortex Ventral Stream • Visual info must be combined to make sense of a visual scene • Begins in extrastriate cortex (visual association cortex) – surrounds primary visual cortex • Has many sub regions – each receives different type of information from primary cortex (colour, movement, orientation, etc) • Here info separates into dorsal or ventral stream depending on the type of information • Dorsal Stream – “where” – where objects are located and how they’re moving, goes to parietal lobe (processes spatial information) • Ventral Stream – “what” – what object is, form and colour – goes to temporal cortex (feature information comes together) Columns in Temporal Cortex • Temporal cortex arranged in columns oriented perpendicular to surface of cortex • Cells here respond to ver specific and more complex stimuli (hands, faces, chairs, apples, etc) • Each column = 5 layers of neurons responding to complex stimuli from same category • Each layer within column responds to slightly different features o One column of neurons may respond best to apples – layer one responds to red apples, layer 2= green apples, layer 3= yellow apples, layer 4= small apples, layer 5 = big apples • Not every object coded by specific neuron • Rather, objects presented by activity patterns across many cells in different parts of brain Development of Pattern/Object/Face Recognition Infant Pattern Recognition • Infants don’t perceive patterns, objects, and faces like adults • Preferential looking method used to study patterns
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