Exam in 12 days

• in class

• assortment of question types including written answers

Read this article!

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

• Themes to notice:

– Contralateral nature of visual system

– Information is organized:• According to spatial location• According to features and kinds of information

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: bilateral

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