Vision: Central Pathways Flashcards
Central projections of retinal ganglion cells
- optic nerve carries fires from only one eye
- optic chiasm - split of fibres (60% cross, 40% stay = decussation)
- optic tract carries fibres from both eyes: this is important because it allows us to represent a side of space, not an eye (left visual field goes to r hemisphere, right visual field to l hemisphere)
- major targets are the:
- -> thalamus (dorsolateral geniculate nucleus / LGN)
- -> pretectum (pupillary light reflex)
- -> hypothalamus
Why a decussation at the optic chiasm?
Retinotopic mapping: the decussation at the optic chiasm allows information about space to stay together regardless of which eye detects it.
Temporal retina stays on the same side, nasal retina (60%) goes to the other side.
Peripheral locations fall only on one retina
- Left periphery –> nasal L retina
- Right periphery –> nasal R retina
More central locations fall on both retina
The image on the retina is flipped
flipped vertically and horizontally
Fovea
centre of the eye, when youre fixating it has really high spatial acuity because it has cones packed densely.
With both eyes open, the fovea is directed at the same image, and the centre of the fovea is the boundary between neurons that will cross the chiasm (nasal retina), and those that will stay on the temporal side
You are looking straight ahead and you see, in your peripheral vision, an object to your left, all the way to the side. It will be registered by:
the left nasal retina
Thalamus -
dorsolateral geniculate nucleus (LGN)
Once the info has crossed over, it heads to its major target to the LGN. Then it passes through these optic radiations to V1 (primary visual cortex = striate cortex = brodmann’s area 17)
Most of the info travels through the LGN to V1
Optic radiations from LGN to V1
some go through the temporal lobe (meyer’s loop) and carry infor from the upper visual field (below)
some go through the parietal lobe and carry information about the lower visual field
Visuotopic organisation
Primary visual cortex lies on the banks of the calcarine sulcus.
Central vision (fovea) represented at caudal portion (occipital pole) - disproportionally large area –> supports our high acuity
Peripheral vision more anterior - less cortical magnification –> less acuity
Upper visual field = lower calcarine
Lower visual field = upper calcarine
Damage to the right optic nerve before the optic chiasm would
prevent input from the right eye (right eye blindness)
damage at the optic chiasm:
this is where nasal retinal fibres cross - therefore damage at B should affect vision of space which normally falls on the nasal retina (lose periphery vision)
damage at the right optic tract
carries info from both eyes about the left side of space
therefore blindness to the left side of space
damage at optic radiation (left)
effects the right side of vision - if its meyer’s loop its the upper left
Pretectum pupillary light reflex
- reduction in pupil diameter with increasing light
- the temporal side of retina in left eye gets combined with the nasal retina on right eye - they both come to left pretectum (this ensures that if ur left eye gets stimulated by light, your right also has a connection)
- the pretectal neurons connect to the Edinger-Westphal nuclei –> which has axons in the ocular motor nerve that terminate on neurons in the ciliary ganglion
- the constrictor muscle reduces the amount of light entering the eye, preventing the saturation of those photoreceptors
Pupillary light reflex circuit
Pretectum gets info from both eyes (side of space)
Pretectum connects to Edinger-Westphal nuclei (both sides)
Travels in oculomotor nerves
Via ciliary ganglion on both sides –> controls constrictor muscle in iris –> changes diameter of both pupils
Scenario where light is shone in left eye
- Retina; photoreceptors hyperpolarise, releasing less transmitter onto bipular cells
- This frees the on-centre bipolar cell to depolarise, which releases more transmitter onto the on-centre ganglion cells, which fire action potentials along the axon (which travels in the optic nerve)
- The type of ganglion cells that project to the pretectum and hypothalamus have their light sensitive pigment called melanopsin
- gets to the optic chiasm, offshoot of fibres to the pretectum
- activity in these fibres activate the cells of both the EW nuclei, which activates ciliary ganglia, which stimulates the pupilary constrictor muscles
Input for circadian rhythms
- hypothalamus suprachiasmatic nucleus (SCN)
- offshoot just behind the chiasm
- input from ganglion cells that hold melanopsin gives info about environmental illumination
- melanopsin is a novel photopigment - the opposite of rods and cones, meaning it depolarises by light
Superior colliculus
orients responses
coordinates head and eye movements towards targets
What is special about the visual cortex
6 layers, but the 4th layer is divided into 3
layer 4 is important in terms of inputs and outputs for vision
- -> 4C has spiny stellate neurons (gets LGN info and passes it to other layers)
- -> 4B sends axons to extrastriate areas
Neural responses in V1
V1 neurons respond to light-dark bars/edges and have a preferred orientation.
Orientation tuning (but also motion direction tuning, spatial freq tuning, temporal frequency tuning) Scenes are encoded in combined activity of neurons across the visual field
Vertical columns
respond to similar areas of space and similar orientations
Adjacent columns
neurons have slightly shifted receptive fields and progression in orientation selectivity
Neurons responding to stimulation of both eyes first occur in V1
LGN neurons keep input from the eyes separate - now we have input from both eyes going to the LGN but kept in separate layers
Cortex: special layer 4 - they stat separate by synapsing on neurons that are organised in these ocular dominance columns
Stereopsis
perception of depth from binocular disparity
Magno- parvo- and koniocellular pathways keypoints
a) Retina
- M ganglion cells - large bodies, more dendrites, thicker axons
- P ganglion cells = smaller bodies, less dendrites, thinner axons
b) LGN
M cells –> 2 ventral layers = Magnocellular layers
P cells –> 4 more dorsal layers = parvocellular layers
c) V1
Magnocellular axons –> upper part of layer 4C
Parvocellular axons = lower part of 4C
Magnocellular pathway
M ganglion cells = larger receptive field, faster conduction velocities, transient responses, insensitive to different wavelengths of light
MOTION, not colour
Parvocellular pathway
P ganglion cells = smaller receptive fields, slower ocnduciton, sustained responses, sensitive to differences in wavelengths
COLOUR, not motion