Vision III

Question 1

What is the pathway from the retina to the brain?

Answer 1

-pathway serving conscious visual perception originates in the retina
-progresses to lateral geniculate nucleus (LGN)
-then from the LGN to the Primary visual cortex
-ending in higher order visual areas in occipital, temporal, and parietal lobes
(retina, optic nerve, optic chiasm, optic tract, LGN, optic radiation, visual cortex)

Question 2

How does our full visual field work?

Answer 2

-the entire region of space that can be seen with both eyes looking straight ahead
-divided in the middle into a left and right hemifield which are partly imaged on both retinas
-we have contralateral vision, so the left visual field is processed by the right visual cortex

Question 3

What are receptive vs visual fields?

Answer 3

-the central portion of the visual field is seen by both eyes is the binocular visual field
-to allow the images from either side of the field for both eyes to be transmitted to the appropriate side of the brain, some fibers cross over to the other hemisphere at the optic chiasm

Question 4

What do the optic chiasm, nerve, and tract do?

Answer 4

-at the optic chiasm, a partial decussation (crossing over) turns the optic nerve into optic tracts
-optic nerve: temporal & nasal fibers from the same eye
-optic tract: ipsilateral temporal fibers & contralateral nasal fibers from the opposite visual field

Question 5

Where do temporal and nasal fibers project to?

Answer 5

-an object in right visual hemisfield projects to right nasal half and left temporal half of the retain, and vice versa
-temporal fibers from each eye project to ipsilateral hemisphere
-nasal fibers from each eye project to contralateral hemisphere

Question 6

How do we have contralateral vision?

Answer 6

-axons from ganglion cells serving inner half of retina (nasal sides) cross through chiasm and ascend to LGN
-neurons in the LGN project to the primary visual cortex –> optic radiation
-the result is contralateral vision

Question 7

What does transection of left optic nerve result in?

Answer 7

-lose all input from left eye; loses the monocular part (only what the left eye sees);
-can still see a lot, just have to turn head;
-however, we don’t have binocularity, so there is no depth

Question 8

What does transection of left optic tract result in?

Answer 8

-damage on the left side, but loss of entire right visual field;
-however, we have binocularity so we still have good depth

Question 9

What does transection of optic chiasm result in?

Answer 9

-cuts off everything that crosses over, meaning that we lack vision on the sides (tunnel vision);
-but we have binocularity so we have depth

Question 10

What did David & Goliath find?

Answer 10

-pituitary gland disease –> gigantism & tunnel vision

Question 11

What are the nonthalamic (LGN) targets of the optic tract?

Answer 11

-retinofugal projection
-pretectum
-hypothalamus
-superior colliculus

Question 12

What is Retinofugal Projection?

Answer 12

-retinal ganglion cells send their message away from the eye
-80% project to LGN (thalamus) –> primary visual cortex
-10-20% project to hypothalamus and midbrain structures (superior colliculus)

Question 13

What is Pretectum?

Answer 13

-control size of the pupil, certain types of eye movement

Question 14

What is Hypothalamus?

Answer 14

-role in biological rhythms, including sleep and wakefulness

Question 15

What is Superior Colliculus?

Answer 15

-orients the eyes in response to new stimuli - moves fovea to objects of interest

Question 16

Where do retinal inputs go?

Answer 16

-the optic tracts project to the left and right LGN
-each LGN is arranged in 6 distinct numbered layers stacked as bent pancakes
-retinal information is segregated into the different layers
-M-type, P-type, nonM-nonP type

Question 17

What are the retinal inputs to the LGN layers?

Answer 17

-the left visual field is viewed by both the nasal left retina and the temporal right retina
-at the LGN, input from the two eyes is kept separate
-in the right LGN, the right ete (ipsilateral) axons synapse on LGN cells in layers 2,3, and 5
-the left eye (contralateral) axons synapse on cells in layers 1,4, and 6

Question 18

What is the organization of the LGN?

Answer 18

-layers 1 & 2 contrain larger neurons: Magnocellular LGN layers (M-type RGCs)
-layers 3-6 contain smaller neurons: Parvocellular LGN layers (P-type RGCs)
-Koniocellular (dust) layers are tiny cells ventral to each layer (K1-K6)
-separate parallel streams of visual information

Question 19

What are the receptive fields of LGN?

Answer 19

-receptive fields of LGN neurons are almost identical to the ganglion cells that feed them
-Magnocellular LGN neurons: large center-surround receptive fields with transient response (insensitive to wavelength)
-Parvocellular LGN neurons: small center-surround receptive fields with sustained response (express color-opponency)
-neurons of the LGN are monocular (activated by just one eye)

Question 20

What is the LGN’s primary synaptic target?

Answer 20

-the LGN’s primary synaptic target is the primary visual cortex (area 17) –> also known as V1 or striate cortex

Question 21

What is Retinotopy?

Answer 21

-the organizing principles for the visual system –> neighbouring cells in the retina project to neighbouring cells in the LGN and V1

Question 22

What are Retinotopic Maps?

Answer 22

-the retinal map is distorted
-cortical magnification of fovea due to many more RGC inputs
-many more V1 neurons receive input from the central retina than from the peripheral retina
-perception is based on the brain’s interpretation of distributed patterns of activity - not literal map

Question 23

What does radioactive tracing show?

Answer 23

-radioactive tracing experiments shows alternating inputs to layer IV from both eyes
-adjacent columns receive input from neighbouring areas of the visual field from left vs right eye

Question 24

What are Ocular Dominance Columns?

Answer 24

-ocular dominance columns are columns of the visual cortex that receive primarily input from one eye
-inputs from either eye in layer IV are monocular
-first binocular neurons are found in layer III neurons of V1
-these neurons have binocular receptive fields

Question 25

What is Orientation Selectivity?

Answer 25

-Hubel and Wiesel discovered that many neurons in V1 respond best to bars of light
-greatest response to a bar of a particular orientation - Orientation Selectivity

Question 26

What is Direction Selectivity?

Answer 26

-a subset of orientation selective neurons also show direction selectivity
-these neurons fire action potentials in direction-dependent manner in response to a moving bar of light
(we get movement information - depth and speed)

Question 27

What are Simple and Complex cells?

Answer 27

-Hubel and Wiesel classified these different types of cortical neurons as simple or complex cells
-Simple cells, such as orientation selective cells, have defined ON and OFF areas in their receptive field
-Complex cells have more complex receptive fields, such as is the case with direction selective cells
-simple and complex cells are typically binocular and sensitive to orientation

Question 28

What is Parallel Processing?

Answer 28

-some evidence suggests that there are parallel streams of information going into the visual cortex
-Magnocellular pathway for motion
-Parvocellular pathway for shape
-Koniocellular pathway for color
-likely not as simple as this

Question 29

What is Cortical Module?

Answer 29

-Hubel and Wiesel found that a small stimulus in the retina would activate a 2 x 2 mm chunk of striate cortex
-this chunk is referred to as a cortical module
-it is believed to be able to process all the information necessary for that portion of the visual field

Question 30

Where does the Dorsal Stream project visual info beyond V1?

Answer 30

-Dorsal stream: projects towards the parietal lobe
* analysis of visual motion and the visual control of action
* mostly magnocellular pathway
*projects to: V1, V2, V3, MT, MST, other dorsal areas

Question 31

Where does the Ventral Stream project visual info beyond V1?

Answer 31

-Ventral stream: projects towards the temporal lobe
* perception of the visual world and the recognition of objects
* mostly parvocellular pathway
*projects to: V1, V2, V3, V4, IT, other ventral areas.

Question 32

What is Area MT (dorsal stream) responsible for?

Answer 32

-Area MT (temporal lobe)
* most cells are direction-selective, respond more to the motion of objects than their shape
* involved in the perception of motion-based optical illusions

Question 33

What is the area beyond MT (dorsal stream) responsible for?

Answer 33

-Beyond area MT—three roles proposed for cells in area MST (medial superior temporal; parietal lobe)
* navigation
* directing eye movements
* motion perception

Question 34

What is Akinetopsia?

Answer 34

-motion blindness
-stroboscopic vision –> to total motion blindness (very rare)

Question 35

What is Area V4 (ventral stream) responsible for?

Answer 35

-Area V4—shape and color perception
* Achromatopsia: clinical syndrome caused by damage to area V4—partial or complete loss of color vision – suggests area specialized for color

Question 36

What is Area IT (ventral stream) responsible for?

Answer 36

-Area IT (inferotemporal cortex)
* major output of V4
* receptive fields respond to a wide variety of colors and
abstract shapes
* may be important for both visual perception and visual memory (such as faces)

Question 37

Which area deals with facial details?

Answer 37

-Fusiform Face Area - FFA
* may contain specific face-selective neurons
* other areas in IT seem to be sensitive to facial information
-Prosopagnosia: face blindness