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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
• 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 higher-level 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
W. W. Norton
• Primary cortex maintains distinct pathways
• M and P pathways synapse in different layers
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
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