Chapter 4: The Visual Brain

Week 5

Optic chiasm

in the optic tract

where optic nerve in each eye splits in half

nasal retinae cross over + temporal rinae stay on the same side

optic tract

optic nerve starting at the chiasm and into the brain

Contralateral representation of visual space

left visual world goes to the right side of brain

vise versa

Ipsilateral organisation

same-side organisation

temporal retina projects to the same side of the brain

contralateral organisation

opposite side organisation

nasal retina projects to the opposite side of the brain

lateral geniculate nucleus

1 in each hemisphere

in thalamus

relays info from optic nerve to visual cortex

magnocellular layers

layers of lateral geniculate nucleus with big cells

receive input from M ganglion cells

parvocellular layers

layers of lateral geniculate nucleus with small cell

receive input from P ganglion

koniocellular layers

layers of the lateral geniculate with extra tiny cells

receive input from K ganglion cells 

M cells (parasol retinal)

project to the magnocellular layer

10% of ganglion cells

highly sensitive to light 

P cells (midget retinal)

project to parvocellular layer

80% of ganglion cells

low sensitivity to light

sensitive to wavelength

input from a single cone

K cells (bistratified retinal)

koniocellular layer

10% of ganglion cells

low sensitivity to light

sensitive to wavelength 

P pathway (parvocellular)

midget retinal ganglion cells

k pathway

starts with bistratified retinal ganglion cells

projects to koniocellular layers of LGN

M pathway

starts with parasol retinal ganglion cells

projects to the magnocellular layers of the LGN

superior colliculus

top of brain step

beneath thalamus

controls eye movements

smooth pursuit eye movements

voluntary tracking eye movements

saccades

most common and fastest eye movements

used to look at one object to the next

primary visual cortex V1

cerebral cortex

gets input from LGN (occipital lobe)

early visual processing

retinotopic map

point by point relation between retina and V1

cortical magnification

some sensory receptors get more space in the cortex than others

ex. fovea has more cortical area than the periphery

simple cells

V1 neurons respond to stimuli with specific orientations to objects in the receptive field

ex. simple cells like stimulus that produce the strongest response

orienting tuning curve

graph shows the typical response of a simple cell to stimuli or diff orientations

complex cells

neurons in V1 that respond to a variety of stimuli in diff locations

preference to moving stimuli

end stopped neurons

respond to stimuli that end within the receptive field

ocular dominance column

in v1

made of neurons that get input only from left or right eye

orientation column

in v1

neurons with similar responses to the orientation of a shape presented to them

hypercolumn

1mm block of v1

has ocular dominance AND orientation columns 

blobs

groups of neurons in the v1 sensitive to colour

interblobs

groups of neurons sensitive to orientation in vision

extrastriate cortex (secondary)

visual areas in the occipital lobe other than v1

v2

starts with visual associations rather than processing the input

ventral pathway

starts with midget and bistratified reinal ganglion cells

continues into inferotemporal cortex into temporal 

“what” pathway → object identification and colour vision

dorsal pathway

starts with parasol retinal ganglion cells

continues into parietal lobe

“where” pathway → location of objects and their movement

object agnosia

vision is fine but you can’t identify and recognise an object

inferotemporal cortex

in the temporal lobe

gets input from the ventral visual pathway

object identification

V5 (mt)

area in occipital lobe in dorsal pathway

motion detection and perception

blindsight

presence of visual abilities even though a person experiences blindness because of damage to V1

scotoma

area of partially or totally destroyed cells

causes a blindspot in a region of the visual field

conjugate gaze palsies

neurological disorder that affects eyes coordinating movements

internuclear ophthalmoplegia

conjugate palsy resulting from damage to the brain stem region (medial longitudinal fasciculus)