Lateral Geniculate Nucleus: Superior Colliculus & other Visual Pathways

Question 1

Which fibres from which eye go where in the cortex?

Answer 1

• Each eye is going to opposite hemisphere of visual cortex – each eye is represented on each cortex
  o Ensures you won’t lose your sight – lots of backup
• Nasal – goes to opposite side
• Temporal – goes to same side

Question 2

Where is each part of the visual field represented in the cortex?

Answer 2

• Superior part of VF – is on the inferior part of the cortex
• Inferior part of VF – is on the superior part of the cortex

Question 3

Which cells are in the parvocellular pathway & which are in the magnocellular pathway?

Answer 3

• Parvocellular pathway Cells – midget cells
• Magnocellular pathway cells – parasol cells – umbrella like = large RFs – magnify things

Question 4

What happens after the retina?

Answer 4

• Post-retinal processing
• Nasal (contralateral) fibres cross ‘decussation’ at optic chiasma
• Temporal (ipsilateral) fibres remain uncrossed
• Some fibres (~20%) leave via superior brachium to superior colliculus in midbrain (orientating, eye movements, multisensory cells guide)
• Increase contrast of image – get good response
• Inferior colliculus does for auditory system what superior colliculus does for visual system

Question 5

Describe the human diencephalon?

Answer 5

• Pineal Gland = where IP ganglion cells in retina project to
• Corpus collosum = white matter that goes between the 2 hemispheres which allows them to communicate with each other – myelinated axons
• Hypothalamus = important for basic drives – hunger, thirst, sex etc
• Brain stem – midbrain, pons, medulla
• Movement control loops – basal ganglia – ensures don’t stand still and can still make voluntary actions
• All sensory info is going to thalamus – except smell

Question 6

Describe the diencephalon (thalamus)?

Answer 6

• Diencephalon consists of hypothalamus, subthalamus, dorsal thalamus and epithalamus
  o Surround the 3rd ventricle, & comprise the lateral wall & floor of this ventricle
• Thalamus has 2 basic types of inputs to it:
  o Input that is being relayed to the cortex
  o Modulatory input, which primarily arises from the cerebral cortex, as well as from the reticular thalamic nucleus & various brain stem areas

Question 7

Describe the thalamus & LGN?

Answer 7

• Relay station for all sensory signals on way to cerebral cortex
• LGN receives input from optic tract & sends impulses via area 17 (primary visual cortex) of cerebrum

Question 8

Give all the names for V1?

Answer 8

Area 17
Striate Cortex
Primary Visual Cortex
Calcarine Cortex

Question 9

Describe the layers in the LGN?

Answer 9

• LGN arranged in 6 layers (or laminae)
• Each lamina contains a retinotopic map of half of VF
• Retinotopic = topographic map of retina onto the next level of functioning/ integration – how retina is organised is mapped onto LGN or primary visual cortex
• Layers 1, 4 & 6 receive input from contralateral eye & layers 2, 3 and 5 from ipsilateral eye
• LGN cells have concentric RFs, similar to GCs in retina
  o Layers 1 & 2 have cells with larger bodies (magnocellular) that have monochromatic responses (mediate responses to light and dark)
  o Layers 3 to 6 have cells with small, tight cell bodies (parvocellular) that mediate colour vision – respond to different λ of light

Question 10

Describe the LGN & Visual Fields?

Answer 10

• Optic nerve axons from right temporal retina arrive at LGN on same side of head (ipsilateral) and those from left nasal retina cross over to opposit e side of head (contralateral)
• Both join in same LGN – “PARTIAL DECUSSATION”
• If have lesion at optic chiasm – both sides are affected as this is where the nasal fibres cross over
• Eye on same side sends info to layers 2, 3 and 5
• Eye on opposite side sends info to layers 1, 4 and 6

Question 11

Describe retinal input to LGN?

Answer 11

• X type or ‘P’ cells connect with small cells (parvocellular)
• Y type or ‘M’ cells connect with larger cells (magnocellular)
• GC bodies in layers 1 and 2 are larger
• Basic principle: streams of visual info get segregated into layers at LGN – these layers are retinotopic maps i.e. preserve the topography of the retina
• Cells in LGN have similar RFs to retinal GCs
• Get arousal input from reticular system

Question 12

Describe more info on the LGN?

Answer 12

• Organisation of projections to layers of LGN
• Crossed projections (solid line on image) go to same layers & uncrossed projections (dotted line on image) to others within both magnocellular layers and parvocellular layers
• Stronger surround inhibition means these cells amplify differences in illumination
• Colour opponency (parvocellular layers)
• RF larger representing periphery
• Magnocellular RF bigger than parvocellular and signal motion

Question 13

Describe post retinal processing?

Answer 13

• “Eyes are window to soul”
• Interpretation of image as representation of real world, occurs in brain – not everyone will see colour the same – perceptual experience is different from person to person
• Brain does not receive signals from each eye unilaterally
  o Half of each optical field is directed to contralateral portion of brain
  o Occurs when bundled fibres of optic nerves meet and cross at optic chiasm, located on ventral side of brain
  o If chiasm is split, half of each eye’s input to the brain is lost
• Optic nerve entirely sensory – nothing motor about it
• Oculomotor neve is one that moves most of EOMs – sits in midbrain
• LGN is key intermediate “way station” along route visual signals follow.
  o This region of brain is the site of synapses between fibres coming via optic tracts (axons of GCs) & second set of fibres (geniculocalcarine tract), which carries the signal into visual cortex of the cerebrum
  o At visual cortex, info is interpreted & true vision resides
• Fibres within visual tract also run into other regions of brain, involved in reflex controls of eye movement & behavioural patterns
• Dorsal LGN relays the info in exact point-to-point form
  o There is a faithful spatial representation of the on/off pattern of visual fibres brough from the retina to the visual cortex
• Although visual tract fibres cross at optic chiasm, the dorsal LGN is arranged in layers that keep the signals “parallel” and route the info from each half of each VF to appropriate cerebral hemisphere.
  o The dorsal LGN also controls how much of the signal gets to the cortex.
  o It has internal inhibitory circuits that can selectively turn individual signals off and regulate exactly which visual info is ultimately passed through to the cortex for processing

Question 14

Describe LGN & Koniocellular Pathways? What is flicker? Describe LGN Neurons responding to various stimuli? Describe active vision in LGN?

Answer 14

• LGN has direct koniocellular pathway to V1 and V2/3
• Flicker: white and black squares are alternating – look as if they are moving
• LGN cells respond to all stimuli with different spatial frequencies
• Active vision – moving and seeing things changing in your environment
• Visual attention can have an affect

Question 15

What can LGN also detect even though they have centre surround?

Answer 15

LGN, despite having centre surround, also detect shape, movement, and colours

Question 16

Describe LGN Output?

Answer 16

• Info leaving LGN travels out on optic radiations, which form part of retrolenticular limb of internal capsule
• The axons leaving the LGN go to V1 visual cortex and generally end in layer IV
• Axons from layer VI of visual cortex send info back to LGN
• RFs of numerous cells in LGN are summed to form RF of a simple cell in V1 & RFs of numerous cells in V1 are summed to form RF of a single complex cell

Question 17

Describes LGN’s Function in Visual Perception?

Answer 17

• LGN introduces coding efficiencies by cancelling out redundant info from retina
• Like other areas of thalamus, particularly other relay nuclei, LGN probably helps visual system focus its attention on most important info
  o If hear a sound slightly to left, auditory system probably ‘tells’ visual system, through LGN to direct visual attention to that part of space
  o Recent experiments using fMRI in humans have found both spatial attention and saccadic eye movements can modulate activity in LGN
• RFs of M, P and K cells all correspond closely to their counterparts in retina – they are centre-surround, on-off fields with K cells being insensitive to λ

Question 18

Describe the midbrain or mesencephalon?

Answer 18

• Posterior part (tectum) contains 2 superior and 2 inferior colliculi – posterior part on dorsal side so controls more motor aspects
• Superior colliculi are important subcortical visual centres & are also involved in intersensory & sensory-motor integration for head & eye movements
• Inferior colliculi have similar functions for auditory perception
• Motor signals are also processed here
• The substantia nigra controls subconscious muscle activity
• The red nucleus, basal ganglia & cerebellum co-ordinate movement
• Motor signals flow downward from cortex via cerebral peduncles
• Motor neurons controlling eye movements are in oculomotor nuclei

Question 19

Describe the superior colliculus (SC)?

Answer 19

• Just below LGN
• Relatively posterior structure
• If label axons originating in retina, you find a dense, sharply defined layer of input to SC
• Cortical area V1 sends axons to this layer
• Areas of extra striate cortex send axons to a deeper layer of colliculus
  o Cortical areas further removed from V1 send axons to a deeper zone
• Motor cortex, particularly frontal eye fields (sitting in frontal lobe), sends input to intermediate – deep colliculus
• Also receive auditory input as SC receives input from both eyes and ears – these are called multisensory cells – stimulation must originate from same vicinity in visual space, summation of these properties enhances weak environmental events
  o Hear sound and turn to view it -> all parts working together
• Superior colliculus is a collector of inputs from all over brain
• Like LGN, SC contains precise retinotopic map laid out across its top layer – its basic organisation is same in all vertebrate classes
• RFs are simple rather than ill-defined centre-surround affairs
  o Differ from those in retina as the centre responds to both stimulus onset and offset while surround is strictly inhibitory
  o Respond to any visual stimulus not what but ‘where’ stimulus is
• Cells with vigorous visual responses are found only in upper layers of superior colliculus
  o Some deeper cells do have visual RFs, but their hallmark is motor behaviour
   They fire before particular saccades – hear sound and turn head -> most common saccade

Question 20

Describe the Geniculo-striate & ‘Other’ Pathways?

Answer 20

• Geniculostriate Pathway (retina – LGN – striate cortex) is main direct pathway
• But number of other places in brain that receive direct input from the retinal GCs: non-geniculostriate pathway
  o Most important of these is the collicular pathway
• Some fibres (~20% = 100,000) leave via superior branchium to the midbrain superior colliculus (orientating eye movements, multisensory cells guide)
• In non-mammalian species, the largest visual structure in the brain is the optic/visual tectum (superior colliculus in mammals)
  o It both receives direct retinal input, and sends its own output to brainstem regions that are just a synapse or two away from motor neurons controlling eye muscles
  o Layered structure
• Direct correspondence between a given location in retinotopic map in upper layers of colliculus, and size and direction of saccade “coded” by deeper underlying cells
• Upper layers of superior colliculus have a map in retinotopic coordinates but the deeper layers have a motor map in body centred coordinates – info is transmitted accurately from upper map to deeper map
• SC detects objects located away from the point of fixation/interest and guides movement of eyes and head that orientate towards those objects: THIS THERFORE HAS NO PATTERN ANALYSIS (not doing anything central vision cells are doing -> interested in where something is happening rather than what it is)

Question 21

Describe the superior colliculus and ‘other’ visual areas?

Answer 21

• Importance of these areas is shown by fact that removal of striate cortex (V1) in non-primates does not lead to blindness
• In humans and monkeys its removal does not cause blindness but v limited visual function remains
  o Surviving visual capability is called BLINDSIGHT

Question 22

Describe Visual & Emotional Pathways?

Answer 22

• Primary visual pathway originates from retina and projects to V1 in occipital lobe via intermediate station in LGN of thalamus
• From V1, visual info reaches the extrastriate cortex along ventral (occipitotemporal) and the dorsal (occipitoparietal) stream.
• Minority of fibres originating from retina take secondary route and reach both superior colliculus and pulvinar. These 2 subcortical sites are connected and also send direct projections to the extrastriate visual cortex, bypassing V1.
• ‘Emotion system’ includes several cortical and subcortical areas. Among these subcortical structures is the amygdala (AMG) – buried deeply in temporal lobe – responsible for negative emotions

Question 23

Describe Another Direct Retinal Pathway to Cortex?

Answer 23

• Suprachiasmatic nucleus (aware of generalised light (day) and generalised (dark) – involved in circadian rhythm) -> intergeniculate leaflet (thin structure) -> olivary pretectal nucleus -> Edinger-Westphal nucleus (involved in location of object in order to move eyes) -> ciliary ganglion
• All these are subcortical structures

Question 24

Describe damage & disruption of function in the brain?

Answer 24

• Pretectal area (important visual reflex centre) lies rostral to Superior colliculus where midbrain fuses with thalamus
• This region mediates pupillary reflexes clinically significant ability to differentiate afferent (input) and efferent (output) pathways
  o E.g. direct response absent? Efferent damage?
• 3rd Cranial Nerve unilateral nerve lesion
• Weber’s (3rd CN) palsy and ptosis (drooping) ipsilateral (same side of body)
  o Pupils mid-position to dilated
  o Reactivity: sluggish to fixed
  o Loss of consciousness: varies
  o Movement: abnormal extensor (muscle that extends a part)
  o Respiratory: hyperventilating. 3rd and 4th CN deficits