Sensory system - Visual system

22 important questions on Sensory system - Visual system

Why does the light has to cross many layers before reaching the photo receptors?

To make distinction between the direction of the light. When the photo receptors were located at the surface they would be stimulated by light from any direction.

There is no yellow cone, how can this colour still be distinguished?

Convergence, divergence and lateral inhibition

There are two types of axons that leave the eye, which two?

Axons from the nasal retina and from the temporal retina
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What is the target of the retinal projections?

1. Lateral geniculate body (thalamus) = 90%
2. Superior colliculus and pretectum = 10%
3. Suprachiastmatic nucleus (some) 

What is the function of the suprachiasmatic nucleus?

Receives tonic input from light sensitive retinal ganglion cells -> informs about presence of light. This adapts our endogenous rhythm to reality --> production melatonin (pineal gland/epifyse and RF)

What is the function of the pretectum?

Pupillary light reaction (curve of lens) and accommodation of lens

What is special about the lateral geniculate body (thalamus)?

Eyes remain separate
Layered structure (6) --> each receives input from a specific type of ganglion (3 types)

Where is the lateral geniculate nucleus connected to?

Primary visual cortex

--> way more neurons from LGN to V1 than from eye to LGN, because the amount of information increases

How many routes to V1 from LGN exist?

2, one via the temporal (Meyer) and one via parietal lobe (Barum)

--> damaged separately --> quadrant anopsia

What is quadrant anopsia?

If one of the two tracts to V1 from LGN is damaged, this leads to missing one quarter of the visual field
Meyer's loop (via temporal lobe) = missing left/right upper quarter (lower part of retina)
Barum's loop (via parietal lobe) = missing left/right lower quarter  
(upper part retina)

Does the information from the LGN to the cortex still arrives separately for each eye?

Yes. Also retinotopy (map of the eye in the brain)

What is the effect of ventral stream lesion on the right side?
(V1 -> temporal lobe)

Face blindness (prosopagnosia) --> impariment of the recognition of faces and impairment in the recognition of emition's on the face
But Galvanic skin response remains present, the limbic system still recognizes the face (but consciousness is not present)

Can also be the other way around -> Capgras delusion

--> Unilateral right-sided lesion

What happens when there is damage to the left side ventral stream?
V1 -> parietal lobe

Object agnosia = impariment of place recognition and hobby recognition

--> unilateral left sided lesion

What happens when there is damage to the dorsal stream?

Akinetopsia = incapability to perceive motion (beweging)

What are saccadic eye movements?

To obtain a complete “image” the interesting areas of the peripheral field of vision are viewed with central vision using saccadic movements based on information from peripheral vision. The saccades thus allow us to obtain relevant visual information with minimal effort.


  
Only central (macular) vision is sharp and in color, but it has an angle of vision of only a few degrees of arc.


Peripheral vision is unsharp and in black and white, but wide angled.

What is the effect on the visual field of an infartion of the basilar artery?

Tunnel vision

What is the structure of the rods and cones

Inner and outer segment that is connected by cilium
outer segments are renewed continuously

Which receptor is present in the rods and which in the cones?

Rods = rhodopsin
Cones = iodopsin 

G-protein coupled receptors

What happens when iodopsin absorbs light?

The photopsin-retinal breaks down because of a cis --> trans transformation of retinal. Eventually, this leads to less formation of cGMP and thus less influx of Na+ and Ca2+ -> small hyperpolarization (-30 -> -70 mV)

Why is there a constant 'depolarization' in the eyes?

There is an active transport of Na+ to the inside of the cell in the outer segment

What happens in the light and dark with the sensitivity of vision?

Sensitivity of rods/cones is antilogarhythmic to concentration of rhodopsin/iodosin (the higher the concentration, the higher sensitivity)

Light: break down into retinal and opsin happens due to light which reduces sensitivity 
Dark: build up of opsin which increases sensitivity

Tell something about the synthesize speed of cones/rods

Cones = resynthesize fast but sensitivity is low
Rods = resynthesize slow but sensitivity is high

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