PSYCH N110 Chapter 10 Terms

Retinofugal projection

The pathway carrying visual information from the retina out to the brain

Order of the retinofugal projection

Optic nerve, optic chiasm, optic tract, LGN

Optic chiasm

Where the optic nerves meet; axons from the nasal half of each retina cross to the other side

Partial decussation

Only nasal retina axons cross at the chiasm; temporal retina axons stay on the same side

Left visual field goes to which hemisphere?

The right hemisphere (and right visual field goes to the left)

Visual hemifield

Half of the visual field (left or right of the fixation point)

Optic tract

Axons after the chiasm, carrying information about the opposite visual hemifield

Main target of the optic tract

The lateral geniculate nucleus (LGN) of the thalamus

Retinal projection to the hypothalamus

Synchronizes circadian rhythms (sleep-wake cycle) with daylight (suprachiasmatic nucleus)

Retinal projection to the pretectum

Controls pupil size (pupillary light reflex)

Retinal projection to the superior colliculus

Orients the eyes and head toward visual stimuli (eye movements)

LGN

Thalamic relay for vision; receives retinal input and sends it to primary visual cortex

How many main layers does the LGN have?

Six

Magnocellular LGN layers

Layers 1-2 (ventral); receive input from M-type ganglion cells

Parvocellular LGN layers

Layers 3-6 (dorsal); receive input from P-type ganglion cells

Koniocellular LGN layers

Thin layers between the main layers; receive input from non-M non-P ganglion cells

Does one LGN layer get input from both eyes?

No; each layer gets input from only one eye, keeping the eyes separate

Which LGN layers get contralateral vs. ipsilateral eye input?

Contralateral: 1, 4, 6. Ipsilateral: 2, 3, 5

Where does most input to the LGN come from?

Primary visual cortex (feedback), not the retina

Optic radiation

Axons from the LGN to primary visual cortex

Primary visual cortex names

V1, striate cortex, Brodmann area 17; in the occipital lobe

Why is V1 called striate cortex?

A visible stripe in layer IV made by the dense LGN input

Which V1 layer receives LGN input?

Layer IVC

Layer IVC alpha vs. IVC beta

IVC alpha gets magnocellular input; IVC beta gets parvocellular input

Ocular dominance columns

Bands of layer IV that each receive input from only one eye, alternating left and right

Where do the two eyes' inputs first combine?

In V1, outside layer IVC, creating binocular receptive fields

Ocular dominance plasticity

If one eye is deprived during a critical period early in life, its ocular dominance columns shrink and the open eye takes over

Retinotopy in V1

V1 holds an orderly map of the visual field

Why is the fovea over-represented in V1?

Many more ganglion cells serve the fovea, so it takes up a disproportionately large area of cortex

Orientation selectivity

Many V1 neurons fire best to a bar or edge at a particular angle

Direction selectivity

Some V1 neurons fire best when a stimulus moves in a particular direction

Simple vs. complex cells

Simple cells have separate ON and OFF regions; complex cells respond to oriented edges anywhere in their receptive field

Who discovered orientation-selective cells in V1?

Hubel and Wiesel

Blobs

Cytochrome-oxidase-rich patches in V1 that receive koniocellular and parvo input; involved in color processing

Dorsal stream

V1 to MT (V5) to parietal cortex; analyzes motion and location ('where' / 'how')

Area MT (V5)

Dorsal-stream area specialized for motion

Ventral stream

V1 to V4 to inferotemporal (IT) cortex; analyzes form, color and object recognition ('what')

Area V4

Ventral-stream area important for shape and color

IT (inferotemporal) cortex

End of the ventral stream; complex receptive fields such as face-selective cells

Which pathways feed the dorsal vs. ventral streams?

Dorsal mostly magnocellular (M); ventral mostly parvocellular (P) and blob

Optic radiation vs. acoustic radiation

Optic radiation: LGN to V1. Acoustic radiation: MGN to A1. Same thalamus-to-cortex relay logic