visual pathways (ch 4) - exam 2 (1/3)

optic nerve

an unmyelinated collection of axons from ganglion cells

• these axons make up the blind spot in the eye

optic chiasm

the left and right optic nerves meet here to cross information

• information goes from each eye to both hemispheres of the brain, and each hemisphere receives input from the contraleteral visual field

  • ipsilateral information and contralateral information

ipsilateral

information from the visual world that does not cross the optic chiasm and remains on the same side of the brain

• goes to layers 2, 3, and 5 of each LGN

• example: L temporal (out to the left) retina receives information from the R visual field and stays on the same side through the LGN

  • L nasal retina receives information of the R visual world as well, but this information will cross over to the L LGN

contralateral

information from the visual world that crosses the optic chiasm to go from 1 side to the other

• layers 1, 4, an 6 of the LGN

• example: L nasal retina receives information of the R visual world - this information crosses over to the L LGN

  • L temporal retina receives information from the R visual field but stays on the same side through the LGN

optic tract

contains crossed (contralateral) fibers from 1 eye and uncrossed (ipsilateral) fibers from the other eye

• optic nerve from each eye splits in half here after meeting in the optic chiasm

• left visual world is processed in the R hemisphere of the brain, right visual world is processed in the L hemisphere of the brain

• number of fibers depends on the placement of the eyes (humans v

contralateral representation of visual space

left visual world is processed in the R hemisphere of the brain, right visual world is processed in the L hemisphere of the brain

LGN and superior colliculus

2 main pathways from the optic tract (where does visual info go after the optic tract?)

1. optic nerve

2. optic chiasm

3. optic tract

4. LGN or superior colliculus

   1. LGN to visual cortex

list the path of the primary visual pathway

superior colliculus

the “where” system (in the primary visual pathway)

superior colliculus

located at the top of the brainstem beneath the thalamus - controls automatic saccades (rapid eye movements from 1 focal point to another) and smooth eye movements (tracking)

• purposeful eye movements - positions eyes in the right place when you want to look at something

  • orienting head = “where”

• activates multisensory cells when a visual stimulus has another (e.g., auditory) stimulus attached with it - enhancement

• receives approximately 10% of information from ganglion cells/optic tract

saccades

controlled by the superior colliculus - rapid eye movement from 1 object to another

smooth eye movements

controlled by the superior colliculus - requires fixation within the foveal area to track something as it moves

• example: watching a bird fly across the sky

  • you can’t move your eyes like this just by scanning the room or reading; eyes consistently move from 1 object to another unless focusing on something actively moving, resulting in this

LGN

the “what” system in V1

LGN

located near the thalamus - responsible for organizing and regulating information from the retina before it is sent to the cortex, and receives it back in a feedback information loop

• bilateral structure with 6 layers of cells and a retinotopic map of the retina

  • ipsilateral information: 2, 3, 5

  • contralateral information: 1, 4, 6

• 2 types of pathways: parvo and magno

  • parvo: layers 3, 4, 5, and 6

  • magno: layers 1 and 2

• functions

  • relay station: info flows in from different places in the brain and is sent out to different places in the brain

  • edge detection: light/dark, edges/boundaries

  • reticular activating system: sleep/wake cycles

  • organizes info: which eye info comes from, color vs noncolor, movement, form information

  • receives feedback from visual cortex

magno (magnocellular)

pathway of the LGN

• large cells that fire in bursts when triggered

  • rods AND cones

• responsible for movement and flickers/light detection

  • larger receptive fields that are sensitive to light but not color

movement, light detection

what the magno pathway of the LGN is responsible for

parvo (parvocellular)

pathway of the LGN

• small cells (midget retinal ganglion cells) with a sustained firing pattern when triggered

  • cones - fovea

• responsible for color, fine texture pattern, depth - visual acuity

• layers 3, 4, 5, and 6 of the LGN

color, fine texture pattern, depth, visual acuity

what the parvo pathway of the LGN is responsible for

3, 4, 5, 6

layers (#) of the LGN dedicated to the parvocellular pathway

1 and 2

layers (#) of LGN dedicated to magnocellular pathway

parvo and magno (parvocellular and magnocellular)

2 pathways of the LGN

ipsilateral: 2, 3, 5

contralateral: 1, 4, 6

LGN cell layer organization

retinotopic organization

point by point topography of the retina

• preserves information about where in the world infomation comes from and which eye each image comes from

• keeps spatial structure - cells next to each other process things that are next to each other in areas next to each other

  • LGN and V1

LGN and V1

the 2 structures in the visual pathway with retinotopic organization

bilateral

a structure that is this has 1 in both hemispheres of the brain

• LGN and V1

LGN and V1

the 2 bilateral structures in the visual pathway

striate cortex, V1

names for the primary visual cortex

V1

primary visual cortext in the occipital lobe - info comes in from the LGN to here

• essential for all higher perception and cognition, contianing approximately 250 million cells

• bilateral structure

  • left hemisphere receives visual info from the R visual world, and the right hemisphere receives visual info from the L visual world

  • meets in center to represent the fovea

• organization

  • retinotopic organization/representation of the retina

  • cortical magnification

• 6 layers - LGN (parvo/magno) connections with some layers of the cortex

• simple cells, complex cells, hypercomplex cells

retinotopic, cortical magnification

organization of V1

cortical magnification

in V1 - in which the cortex increases processing for a specific area/information

• more space in the cortex is dedicated to some sensory receptors than others

• something in the fovea is not fully magnified by the cortex, but receives more cortical area than the periphery

convergence to divergence

in which visual information condenses from rods and cones (~100 million cells) to ganglion cells (~1 million cells) and disperses amongst the striate cortex (~250 million cells)

Hubel and Weisel

researchers responsible for discovering visual cortex and the preferences of different cells in V1

• simple, complex, hypercomplex

simple, complex, hypercomplex

the 3 types of neurons in the striate cortex that respond best to/prefer specific types of edge information shown to the eye

simple cells

in V1 - type of neuron with excitatory and inhibatory areas arranged side-by-side that respond best to edges/lines of a certain orientation

• static

  • position selectivity: specific area of a visual field

  • orientation selectivity: certain orientation

• color selectivity - dark against light OR light against dark

complex cells

in V1 - type of neuron that likes moving edges, lines, and bars in any area of the visual field

• can like dark against light AND light against dark

• direction selectivity

hypercomplex cells

in V1 - type of neuron that likes moving curved lines/bars

size selectivity

in reference to V1 neurons - responds best to things of a certain size

• similar to ganglions with smaller/larger receptive fields

orientation selectivity

in reference to V1 neurons - simple cells respond best to lines/bars of a certain fixed orientation

position selectivity

V1 neurons - simple cells prefer stimuli to be in a specific area within the visual field

• physical placement matters for peak response

• not an issue in complex cells

color selectivity

in reference to V1 neurons - simple cells prefer different color combos (either/or)

• complex cells will respond to both

direction selectivity

in V1 neurons - complex cells like when information is moving in a specific direction

size, orientation, position, color

selectivity of simple cells in V1

direction

selectivity of complex cells in V1

dorsal and ventral

2 streams for processing visual info in the extrastriate cortex (beyond V1)

• lesion studies

dorsal

pathway in extrastriate cortex that processes “where” information

• in parietal lobe (responsible for attention and somatosensory system)

• magno layers of the LGN - movement

parietal - attention and somatosensory system

what lobe is the dorsal pathway in? what is it responsible for?

ventral

pathway in extrastriate cortex that processes “what” information

• in temporal lobe (memory, language, and auditory system)

• parvo layers of the LGN - visual acuity, color, texture, to perceive and recognize objects

temporal - memory, language, auditory system

what lobe is the ventral pathway in? what is it responsible for?

parvo

layers (pathway) of the LGN that the ventral pathway in extrastriate cortex is connected to

magno

layers (pathway) of the LGN that the dorsal pathway in extrastriate cortex is connected to

monkey lesion studies

study that led to the discovery of the dorsal/ventral pathways, in which severing each one resulted in an inability to either find an object (dorsal) or remember what an object is (ventral)

functional modularity

different parts of the brain function with different responsibilities in the cortex

visual agnosia

in which a person cannot recognize a visual stimulus (2D or 3D) just by looking at it

• they don’t know what it is due to brain damage, not within the eye itself

prosopagnosia

inability to recognize faces

object agnosia

type of visual agnosia in which a person cannot recognize physical (3D) objects, not just visual stimulus

achromatopsia

inability to recognize color (not the same as colorblindness, which happens in the cones)

• seeing things in B/W/G

visual agnosia, prosopagnosia, object agnosia, achromatopsia

types of issues that arise when there is damage to the different parts of the visual cortex

relay station, edge detection, reticular activation system, organizes info, receives feedback from visual cortex

LGN functions