Vision

Question 1

Intro facts

Answer 1

• retina: light capturing device
  
  • 100million photoreceptors
  • only 1 million afferent axons
  • requires convergence
• binocular: 2 images that must be merged
  
  • lazy eyes
• the version of the world we see is not always accurate

Question 2

Conscious visual perception

Answer 2

1. Retina
2. LGN (lateral geniculate nucleus) in thalamus
3. Area 17/V1/Striate Cortex
4. Many other cortical areas (occipital, temporal, parietal)

Question 3

Information flow in retina

Answer 3

1. Photoreceptors
2. Bipolar cells
3. Ganglion cells

• horizontal cells between photoreceptors and bipolar
• amacrine cells between bipolar and ganglion cells
• horizontal and amacrine for lateral communication (gate keepers)
  
  • modulate excitement in response to NTs in cleft
• when photoreceptors become hyperpolarized, bipolar and ganglion become depolarized and vice versa
• only AP in ganglionic cells… all others use chemical NT
• calls are translucent so light can pass through
• melanin in pigment epithelium deflects light

Question 4

Rods and Cones

Answer 4

• more rods than cones
• rods: more sensitive to light
  
  • acramatic
  • scoptic (colourless)
  • low light
  • no rods at fovea
• cones: colour vision
  
  • photopic
  • intense/bright light
  • more cones at fovea (only cones at fovea)

Question 5

Cornea

Answer 5

-most refractive power

Question 6

Lens

Answer 6

• also has refractive power
• Important for crisp images (9m or closer)
• ACCOMODATION: additional power provided by changing of lens shape
• far object require less refraction
• near objects require more refraction

Question 7

Visual field

Answer 7

• left visual field imaged on right side of retina

- upper visual field imaged on lower retina

Question 8

Visual acuity

Answer 8

• ability to distinguish 2 points near eachother
• poor at distinguishing colour on peripheral retina
• peripheral retina most sensitive to low light light conditions (due to many rods)

Question 9

Receptive field

Answer 9

• area of the retina that changes firing rate for specific neuron
• specified by the pattern of light on retina that elicits neural response
• input from centre —> ganglion cell = DIRECT
• input from surround —> ganglion cell = INDRIECT (via amacrine cells)
• centre vs surround have antagonistic effects

Question 10

Ganglion cell receptive fields

Answer 10

• ON centre ganglion cells will increase AP fire rate when light hits centre
• OFF centre will fire fewer AP when light hits centre
  
  • fire more AP if dark spot covers centre
• in ON/OFF, response to stimulation of centre is cancelled by response to surround
• centre/surround organization of receptive fields leads to neural response that emphasizes contrast at light-dark edges

Question 11

M type ganglion cells

Answer 11

• large cells
• 5% of cell population
• large receptive fields
• conduct AP more rapidly in optic nerve
• more sensitive to low contrast stimuli
• respond to stimulation of receptive field centres with transient bursts of APs

Question 12

P type ganglion cells

Answer 12

• smaller
• 90% of cell population
• smaller receptive field (good for discrimination of fine detail)
• respond with sustained discharge of AP as long as stimulus is on

Question 13

Retinofugal prjection

Answer 13

-eye —> optic nerve —> optic chiasm —> optic tract

• optic tracts: formed from axons of retinofugal projection
• binocular visual field: central portion of both visual field viewed by both retinas
• left visual field viewed by right hemisphere

Question 14

Targets of optic tract

Answer 14

• most innervate LGN of dorsal thalamus
• neurons in LGN give rise to axons that project to area 17
  
  • OPTIC RADIATION
• legions anywhere in retinofugal projection cause blindness
  
  • pathway mediates conscious visual perception

Question 15

Non-thalamic targets

Answer 15

• projections to HYPOTHALAMUS play important tole in synchronizing sleep cycle
• direct projections to MIDBRAIN control size of pupil/eye movement
• 10% of ganglion cells project to SUPERIOR COLLICULUS to generate saccadic eye movement (reading a page)
  
  • RETINOTECTAL PROJECTION = ganglion —> superior colliculus

Question 16

LGN

Answer 16

• has 6 layers (1=ventral, 6=dorsal)
• layers are curved
• receives input from retinal cells from the contralateral visual field
• output to area 17
• ipsilateral synapses: 2,3,5
• contralateral synapses: 1,4,6
• MAGNOCELLULAR layers:1,2
• PARVOCELLULAR layers: 3-6
• KONIOCELLULAR layers: inbetween

-neurons in LGN all MONOCULAR

Question 17

Retinotopy

Answer 17

• organization whereby neighbouring cells in retina feed info to neighbouring places in their target structures
• 2D surface of retina mapped onto 2D surface of LGN and A17
• central degrees of visual field are magnified/overprepresented
• discrete point of light can activate many cells due to overlap of receptive fields

Question 18

Striate cortex

Answer 18

• 6 layers
  
  • layer IV subdivided in IVA, IVB, and IVC
  • layer IVC subdivided into IVCalpha and IVCbeta
• spiny stellate cells: layer IVC
• pyramidal cells: layers III, IVB, V, VI
  
  • only ones that send axons to other cortical areas
• magnocellular project to IVCalpha
• parvocellular project to IVCbeta
• koniocellular project to I and III

-radial connections maintain retinotopic organization

Question 19

Ocular dominance columns

Answer 19

• Hubel + Weisel (1970)
• inputs to layer IV laid out in alternating bands
• layer IVC stellate cells project radially to IVB + III where info from right and left eye begin to mix

Question 20

Outputs from Striate cortex

Answer 20

• layers II, III and IVB —> other cortical areas
• layer V —> superior colliculus + pons
• layer VI —> back to LGN

Question 21

Receptive fields of different cells

Answer 21

• retinal/ganglion = monocular, centre/surround
• Striate cortex = monocular, centre/surround
• other layers = binocular, orientation selective

Question 22

Orientation column

Answer 22

• preferred orientation remains the same perpendicularly through layers II->VI
• as electrode passes parallel through layers, optimal orientation shifts
• complete 180 degree shift requires a transverse of 1mm within layer III

Question 23

Direction selectivity

Answer 23

• response when a bar of light at optimal orientation moves perpendicular to orientation in one direction but not the other
• subset of orientation selective

Question 24

Simple cells

Answer 24

• binocular
• orientation selective
• elongated
• ON/OFF flanked with opposite ON/OFF
• constructed from multiple neurons with circular receptive fields

Question 25

Complex cells

Answer 25

• binocular
• orientation selective
• elongated
• ON/OFF responses bu no distinct regions

Question 26

Extra Striate areas

Answer 26

• dorsal stream: motion + visual control of action

- ventral stream: perception of visual world + recognition of object

Question 27

Dorsal stream

Answer 27

• motion and visual control of action
• V1–>V2–>V3–>MT->MST
• MT (temporal):
  
  • large receptive fields
  • direction selective
  • respond to object motion
• MST (medial superior temporal):
  
  • linear, radial, circular motion
  • important for navigation, directing eye movement, motion perception

Question 28

Ventral stream

Answer 28

• perception of visual world and recognition of objects
• V1–>V2–>V3–>V4–>IT
• V4: large receptive fields
  
  • orientation selective
  • colour selective (achromatopsia)
• IT: complex stimuli
  
  • facial recognition (esp right hemisphere)