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Exam 1 week from today • in class • assortment of question types including written answers Read this article! QuickTime™ and a decompressor are needed to see this picture. Final Project • Research grant proposal • start thinking about a hypothesis and research question • start thinking about the techniques you would use to answer the question How does the visual system represent visual information? • Brainstorm this: what are the different ways the visual system might encode a feature? How does the visual system represent visual information? • Brainstorm this: what are the different ways the visual system might encode a feature? How does the visual system represent visual information? • Brainstorm this: what are the different ways the visual system might encode a feature? – “labeled lines” • many different subnetworks of neurons - activity in a network indicates presence/nature of a feature – spike timing • absolute rate or # of spikes per second might indicate presence/nature of a feature • “multiplexed” – Hybrid of these two Visual Pathways • Image is focused on the retina • Fovea is the centre of visual field – highest acuity • Peripheral retina receives periphery of visual field – lower acuity – sensitive under low light Visual Pathways • Retina has distinct layers Visual Pathways • Retina has distinct layers • Photoreceptors – Rods and cones respond to different wavelengths Visual Pathways • Retina has distinct layers • Amacrine and bipolar cells perform “early” processing – converging / diverging input from receptors – lateral inhibition leads to centre/surround receptive fields - first step in shaping “tuning properties” of higherlevel neurons Visual Pathways • Retina has distinct layers – signals converge onto ganglion cells which send action potentials to the Lateral Geniculate Nucleus (LGN) – two kinds of ganglion cells: Magnocellular and Parvocellular • visual information is already being shunted through functionally distinct pathways as it is sent by ganglion cells Visual Pathways • visual hemifields project contralaterally – exception: bilaterally representation of fovea! • Optic nerve splits at optic chiasm • about 90 % of fibers project to cortex via LGN • about 10 % project through supperior colliculus and pulvinar – but that’s still a lot of fibers! Note: this will be important when we talk about visuospatial attention Visual Pathways • Lateral Geniculate Nucleus maintains segregation: – of M and P cells – of left and right eyes P cells project to layers 3 - 6 M cells project to layers 1 and 2 Visual Pathways • Primary visual cortex receives input from LGN – also known as “striate” because it appears striped on some micrographs – also known as V1 – also known as Brodmann Area 17 Visual Pathways • Primary cortex maintains distinct pathways • M and P pathways synapse in different layers W. W. Norton How does the visual system represent visual information? How does the visual system represent features of scenes? • Vision is analytical - the system breaks down the scene into distinct kinds of features and represents them in functionally segregated pathways • but… • the spike timing matters too! Visual Neuron Responses • Unit recordings in LGN reveal a centre/surround receptive field • many arrangements exist, but the typical RF has an excitatory centre and an inhibitory surround • these receptive fields tend to be circular - they are not orientation specific How could the outputs of such cells be transformed into a cell with orientation specificity? Visual Neuron Responses • LGN cells converge on “simple” cells in V1 imparting orientation specificity Visual Neuron Responses • V1 maintains a map of orientations across the retina because each small area on the retina has a corresponding cortical module that contains cells with the entire range of orientation tunings