Exam 1 week from today in class assortment of question types including written answers

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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

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 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

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