Exam 1 week from today

• in class

• assortment of question types including written answers

Read this article!

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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 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

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