Lecture 3: Visual Pathways

Components of the visual pathways

optic nerve, optic chiasm, lateral genicualte nucleus, superios colluculus, visual cortex

The pathways

Rods and cones → (intermediate layers of cells that create excitatory/inhibitory connections) → ganglion cells on the retina → optic nerve → optic chiasm → LGN → V1

Cortical Magnification

the visual field map in V1 in which the fovea takes up a disproportionately large region of V1

Lateral Inhibition

Produced by adjacent receptors that are connected by inhibitory synapses - active neuron inhibits the activity of its neighboring neurons

Center-surround receptive fields

• act like “contrast detectors”, responding most to visual stimuli with high contrast

• Happens in ganglion cells due to the lateral inhibition

Where do ganglion cells go?

• most end at the LGN

• a bit at the superior colliculus (a region involved in controlling eye movement)

Optic Chiasm

Where signals from each half of the retina meet

Binocular region of the visual field

The are where the view of the left eye and right eye merge

LGN

• Organizes info from retina based on: which eye it came from and which type of receptor it came from (rods or cones)

• Receives feedback (top-down) signals from cortex

• Regulate the signal from retina, sending fewer impulses to cortex

LGN → V1

LGN sends most of its neuron signals to the primary visual cortex (V1), which then move to V2, V3, etc.

V1 physiology

Hubel and Wiesel discoved that V1 neurons respond best to oriented edges.

• 3 types of neurons in V1: simples cells, complex cells, hypercomplex cells

Simple cells

excitatory and inhibitory areas arranged side by side, sensitive to orientation and position

Complex cells

respond best to orientation, motion, direction

Hypercomplex cells

respond best to orientation, motion, direction, length

Receptive field properties in V1

V1 neurons show a graded response to their preferred stimulus

• a simple cell tuned to a vertical edge will also respond to other orientations that deviate slightly from vertical

Orientation tuning curve

measures the relationship between orientation and firing

Retinotopic map

Map of the retina on the cortex

• that two points that are close together on an object and on the retina will activate neurons that are close together in the brain

edge enhancement

an increase in perceived contrast at borders between regions of the visual field due to the center-surround receptive field (ex. mach bands)