Vision ll

Question 1

retinal ganglion cells project to many places

Answer 1

~90% of retinal projection is to the lateral geniculate nucleus
and it is also projected to pretectum:reflexive eye movements and pupil size.

Question 2

lateral geniculate nucleus

Answer 2

LGN is a nucleus (cluster of neurons) in the thalamus
~90% of retinal projections are to the LGN, which subsequently sends significant projections to the cortex.
Cortex is associated with “conscious” vision.

Question 3

Why do you want binocular vision

Answer 3

– if you’ve seen a 3D movie you know – it lets you easily judge relative depth and perceive distance.
So primary visual cortex is the first time that you get converging information from the two eyes – binocular cells

Question 4

The image on the right (nasal) portion of the left eye projects to the

Answer 4

right side of the brain

- decussation of the optic fiber

Question 5

the image on the right (temporal) portion of the right eye, projects to

Answer 5

the right side of the brain
the image projected to the temporal portion of each eye do not cross at the optic chiasma, it goes to that side of the brain where the eye is located

Question 6

Contralateral vs Ipsilateral

Answer 6

Contralateral – on the opposite side of the body (used for left-vs-right)
Ipsilateral – on the same side

Question 7

Decussation vs Partial decussation

Answer 7

Decussation – crossing of the axons from one side of the body to the other
Partial decussation – when only some of the axons in a nerve cross from one side to the other
Partial decussation – each eye receives information both sides of the visual world.
However, each side of the brain only receives information from one side of the visual world.

Question 8

Fibres from each nasal hemiretina

decussate in the

Answer 8

optic chiasm:

The left visual cortex represents the right visual field.
The right visual cortex represents the left visual field.

Question 9

Transection of optic nerve

Answer 9

• Functionally equivalent to closing one eye

left optic nerve cut => monocular vision, only see with right eye – but both sides

Question 10

Transection of optic tract (or LGN or V1)

Answer 10

Only see things in one hemifield
Both eyes still functional
left optic tract cut => retain binocular vision – but only for left visual hemifield.

Question 11

Transection of optic chiasm

Answer 11

Only crossing fibres are affected
Both eyes still functional
Lose binocular stereoscopic vision
Bitemporal hemianopia (e.g. due to pituitary tumour)
optic chiasm cut=> only central vision. Remember, nasal portion of retina – which “sees” temporal visual field decussates. This is lost, so we are left with central vision.
For bonus points – processing of each part of the visual field is monocular, because only get R visual field from L eye and L visual field from R eye.

Question 12

what happens if pituitary gland swells?

Answer 12

Optic tract & chiasm wrap around pituitary gland, so swelling of the pituitary can block nerve transmission

Question 13

what are two main visual “streams?

Answer 13

Parietal / dorsal / where pathway

Temporal / ventral / what pathway

Question 14

Role of Parietal / dorsal / where pathway

Answer 14

cortical areas are specialised for processing object position and motion
vision for action / interacting with environment

Question 15

Temporal / ventral / what pathway

Answer 15

• Cortical areas are specialised for processing object form and identification
• vision for perception

Question 16

what is Brodmann areas – anatomical classification (cytoarchitecture

Answer 16

Brodmann areas – anatomical classification (cytoarchitecture
discrete regions of the brain could be distinguished based purely on anatomical criteria:
the density of cell bodies in different layers
The patterns of myelination
In these pictures, each anatomically defined cortical area is shaded differently.
How are they connected?

Question 17

Serial vs Parallel processing

Answer 17

Many computations are carried out simultaneously (in parallel)
e.g. Photoreceptors all work in parallel; MT and V4 act in parallel

Some processing increases in complexity and must occur serially

e. g. in the ventral stream
- V1 contains neurons that encode orientation
- V4 contains neurons that encode simple shapes
- TE contains neurons that encode objects and faces

Question 18

Different ganglion cell types tile the retina:

Answer 18

parallel processing with functional segregation
>12 parallel circuits, with unique classes of ganglion cell
Each circuit receives inputs from the same cone photoreceptors, but the inputs are processed in different ways.

Question 19

Parasol cells (M-type cells)

Answer 19

• large cell bodies, dendritic arbors, receptive fields
  sensitive to rapidly changing stimuli
  not colour sensitive (inputs from all cone types)
  project to Magnocellular LGN layers

Question 20

Midget cells (P-type cells)

Answer 20

• small cell bodies, dendritic arbors, receptive fields
  sensitive to fine stimulus features
  colour sensitive (selective cone inputs)
  project to Parvocellular LGN layers
  predominantly in fovea

Question 21

Size of cell body reflects function

Answer 21

Midget cells are able to detect fine spatial features because they don’t integrate across as many photoreceptors
parasol cells – sensitive to lower contrasts – larger regions of space
• Larger cells => faster conduction velocities / higher temporal resolution.
•Intrinsically photosensitive RG cells – project to suprachiasmatic nucleus – circadian rhythms; also control pupillary light reflex.

Question 22

• Prevailing theory

Answer 22

– Parvo = object recognition / shape; Magno = motion.

Question 23

Major retinal projection is to the

Answer 23

lateral geniculate nucleus (LGN) in the thalamus
The six layers of the LGN each represent the contralateral visual field.

Roughly speaking, receptive field properties of neurons in LGN are similar to those in the retina.
LGN receives more inputs from cortex than from retina – feedback is clearly important.
•LGN is first region where attention can influence sensory processing.

Question 24

LGN is a six-layered structure: parallel processing with functional segregation (again)

Answer 24

Parvocellular Layers (3-6)---Inputs from midget RGC
Magnocellular Layers (1 & 2)----Inputs from parasol RGC
there are also intermediate, or Koniocellular layers between each layers

Question 25

where do each layer receives information from?

Answer 25

Each layer is retinotopically organised and has a complete representation of the contralateral visual field. Layers 1, 4, 6 = receive inputs from contralateral eye. Layers 2, 3, 5 = receive inputs from ipsilateral eye. If you drive an electrode perpendicular to the layers, you will encounter neurons with the same spatial receptive fields. • Magno – low contrast, high temporal acuity, poor spatial acuity, monochromatic • Parvo – colour, fine spatial acuity

Question 26

The major projection from the LGN is to

Answer 26

Primary visual cortex = V1 = striate cortex = area 17

Question 27

functional segregation in V1

Answer 27

Functional segregation evident in retina & LGN is also evident in V1. M-cells project to layer IVCα; P-cells project to layer IVCβ. Complex connectivity then occurs within a column of neurons. (long axon connection ) Outputs from V1 are separated into two streams - Parietal / dorsal / where pathway - Temporal / ventral / what pathway

Question 28

Retinotopic maps in V1

Answer 28

Adjacent neurons in V1 respond to stimuli in adjacent regions of visual field Receptive field locations are determined by their inputs (photoreceptors => RGC => LGN => V1) - Foveal over-representation (more spatial area = more neurons = higher acuity)

Question 29

Is Spatiotopic organisation is common in sensory systems ? explain

Answer 29

YES!– in somatosensory cortex, adjacent areas of skin are associated with adjacent regions of cortex. Similarly in auditory system. In visual system, adjacent regions of cortex are associated with stimuli that are close in visual space.

Question 30

The terminology “simple” and “complex” comes from Hubel and Wiesel.

Answer 30

Simple and complex cells: - respond best to elongated bars or edges. - are orientation selective

Question 31

Simple cells:

Answer 31

have spatially segregated ON and OFF subregions position of the bar within the RF is important are often monocular (i.e. only respond to inputs from one eye)

Question 32

Complex cells:

Answer 32

have spatially homogeneous receptive fields (i.e. no segregation of ON/OFF subregions). position of the bar within the RF is unimportant are nearly all binocular.

Question 33

What stimulus is most likely to cause an AP in the simple cell?

Answer 33

What stimulus is most likely to cause an AP in the simple cell – something that generates simultaneous APs in the LGN cells, leading to simultaneous EPSPs in the simple cell. •Working backwards, this requires a particular pattern of visual stimuli to affect the photorecteptors … RGC etc. •This highlights the benefits of center-surround receptive field organisation – it’s optimised for encoding edges, borders, and enhancing contrast. It’s also computationally efficient.

Question 34

Putative circuits to explain “Simple cells” and “Complex cells”

Answer 34

Theoretical model:Multiple LGN cells with collinear receptive fields synapse onto a single V1 simple cellto the stimulus that best activates all LGN neuron The V1 simple cell’s preferred stimulus corresponds

Question 35

A map of orientation in V1

Answer 35

1-Cells within the same cortical column have similar tuning properties (i.e. they prefer the same orientation and have similar spatial receptive fields) 2-Cells in adjacent columns prefer similar orientations. 3-The map for orientation exists on a coarser spatial scale than the map for position

Question 36

If you drive an electrode perpendicular to cortical surface – neurons all have

Answer 36

same RF and same orientation preference.

Question 37

If you drive an electrode tangential to cortical surface – neurons have

Answer 37

adjacent RF and slowly changing orientation preferences. • Important for two reasons: suggests there are constraints on the way cortex is ‘wired” up during development. • Secondly, it suggests that lines and their orientations are important, primitive features in the visual world.

Question 38

Tuning in V1

Answer 38

• All neurons respond best to elongated edges and are orientation selective • Neurons respond to specific regions of visual space • Some neurons are dominated by parvocellular LGN inputs => sensitive to colour • Some neurons are dominated by magnocellular LGN inputs => motion-sensitive • Within a V1 cortical column, neurons have the same spatial receptive field and prefer the same orientation • Moving across adjacent V1 cortical columns, neurons systematically change their spatial RF location and preferred orientation

Question 39

Neurons in middle temporal area

Answer 39

underlie motion perception Individual MT neurons are tuned for direction Groups of MT neurons show diverse tuning In humans, MT lesions impair motion perception (akinetopsia) In macaque monkeys, lesions impair eye movements Experimental microstimulation biases motion perception Perception of ambiguous stimuli is predicted from MT activity

Question 40

Neurons in inferotemporal cortex underlie

Answer 40

face perception Individual IT neurons are tuned for direction Groups of IT neurons show diverse tuning In humans, IT lesions impair face perception (prosopagnosia) Experimental microstimulation biases face perception