Visual processing Flashcards
Describe how does info from right and left side of retina get to V1.
- What stops can we meet on the way? (their function)
- Suprachiasmatic nucleus - gets input from special ganglion cells that don’t even need the outer layer of retina
- Superior colliculi
- Pretectum nuclei
- cilliary muscles, accomodation = focus on retina
Look at a histological slice of V1- what is interesting about it?
- We can see a clear stria of Gennari
- Brodmann’s area 17, straite cortex
Recall what were the ACC of cortical columns?
- Amplification, Computation, Communication
- First established by studying visual cortex
Who were the pioneers of studying V1?
- Daid Hubel, Torsten Wiesel
- Won a Nobel prize in Physiology and Medicine in 1981
What are the main properties of neurons in V1? How did Hubel&Wiesel research this?
- Orientation selectivity
- Direction selectivity
Procedure: anaesthetized (no pain, but still brain activity) -> implant microelectrodes into V1 -> measuring activity when showing bars of light moving in particular axis
Finding:
=> some n. react to certain orientation or direction but not the other
=> We may also compute Tuning function
What pattern of activity would we find if we penetrated visual cortex 1) perpendicular, 2) steeper angle with an electrode?
The same orientation -> moving through orientation function (e.g. from horizontal -> vertical)
- Columns build in systematic fashion
What method is shown in the picture - how does it work? What subjects to we use?
Optical imaging
- Area becomes active -> needs oxygen -> draws hemoglobin from the blood -> oxygen debt -> arteries dilate to supply the depletion
- For experimental animals, patients undergoing surgery
- Procedure: illuminate the brain directly -> observe subtle changes in absorption of light
- Make an image of active and control (no visual stimulus shown) -> subtract these to see the changes (i.e. image on the left)
Picture:
- Dark spots = cortical columns activated
- Subtraction = selectivity
What are pinwheel centers?
= maps of orientation preferences where the sensitivity changes around the center
- If we were to play a film composed of optical imaging for continuous change of orientation -> they would appear to spin
Look at the map of orientation preferences
Using colour to represent distinct orientations e.g. horizontal = red
- Dark = we cannot distinguish preferences because the different cells are too close together (we would need a different method i.e two-photon calcium imaging)
- Arrows = centers of the pinwheels
What do we mean by direction selectivity?
- We may find domains specialized in horizontal lines -> these could be subdivided into those responding to lines of upward motion and those of downward motion
What is meant by ocular dominance? How long does info stay monocular (+structure of LGN)?
= notion that different layers/part of the brain are modulated by a different eye
- LGN has layered structure - keeping nputs from retina devided
- Contra = 1, 4, 6
- Ipsi = 2, 3, 5
=> info remains monocular even up to layer 4 of V1 = ocular dominance columns
- Only after this -> does info get mixed
=> e.g. stereopsis
What is stereopsis? What’s the mechanism of far and near disparity?
= differences between images of eyes that enable perception of depth
- We fixate on a point (b) -> anything closer will be seen by the non-corresponding regions of retina -> don’t project to the same point in thalamus = slightly off => near disparity in inputs
What types of ganglion cells do we have
Parasol (M), Midget (P), Konio ganglion cells
How do these types of ganglion cells project on LGN? And how about V1? (+their function)
Note: K ganglion cells project to blobs
- coloured vision (probably)
Magnocellular layer
- motion, location
Parvocellur layer
- colour, detail
What are extrastriate visual cortex? What sort of pathways do we have here? Give example
= all vision-related areas higher up from V1
-> increasingly more abstract representation
- parallel pathways e.g. what (detail, shape, colour) x where (location, motion)
- MT (middle temporal area)
- motion sensitivity
-> parietal lobe
In what way is image projected to the brain? (does it arrive just like we see it)
- Image is reversed due to optical properties of the lens
Look how the visual representation arriving to each retina.
Horizontal line = horizontal meridian
Vertical line = vertical meridian
There are portions of visual scene observed by both eyes (binacular) - BUT also monocular i.e. peripheries visible to only one eye
Explain picture of pathways.
Fixation point is seen by both foveas.
- Objects to the right
-> seen by nasal portion of the right eye and temporal portion of the left eye
=> need to transfer info together to create a cohesive image
- Thus, nasal path crosses optic chiasm
- nasal = contra
- temporal = ipse
= creates left/right optic tract which corresponds to the contralateral side
Where does the visual info continue from thalamus?
From LGN axons have to go around the lateral ventricles towards V1 = Optic radiation
- Dorsal portion -> posterior bank of calcurine sulcus
- Ventral portion -> sweeping through temporal lobe -> lower bank of the calcarine sulcus = Meyer’s loop
Which sulcus houses V1 - which gyri is it composed of + which portion of the visual field does it process?
- Calcarine s. is formed by the folding of lingual and cuneus gyrus
- Cuneus gyrus represents lower part of the visual field
- lower visual world -> upper retina -> LGN - Lingual = upper visual field (from inferior retina which gets the upper)
Fovea is very limited within retina (about a thumb upon arm’s length) -> BUT much greater cortical space
Explain this picture - use the proper vocabulary.
A) Right anopsia (complete blindness)
B) Often happens after a injury to head or tumour from pituirary
- cuts of nasal pathways -> disrupts vision of peripheral portions
= Bitemporal heteronymous hemianopsia
C) Blindness of one side = Homonymous hemianopsia
D) Homonymous superior quandrantanopsia
E) Notice Macular sparing
- Probably, because we have large representation of fovea which escapes the injury (some circuits are spared)
Which structure is responsible for pupillary light reflex? Which portion of visual field does it get info from?
Which nucleus does it project to? The rest of the pathway and why?
+ PNS or SNC?
Pretectum gets information from temporal side of one eye and nasal side of the other eye
-> projections to motor nuclei are bilateral = Edinger-Westphal nucleus
-> grows axons through the oculomotor nerve (unilateral, ipselateral)
-> synapse onto ciliary neurons
-> Pupillary constrictor muscle
Parasympathetic neurons - contriction
Sympathetic neurons - dilation
- cervical ganglions
