Yuste C6

Question 1

Domination of visual cortex

Answer 1

dominates in terms of its size, occupying a significant portion or our cortex, and also in terms of the amount of information it brings to the brain, probably more than all the other senses put together. We use vision for many purposes: we determine the location, shape and size of an object, its movement, direction, speed, and its color, which gives us information about its surface.

Question 2

Sensitivity of our eyesight

Answer 2

vision is sensitive over 10 orders of magnitude in light level.

Question 3

Organisation of the visual system

Answer 3

1. As a hierarchical pathway. – It starts with the retina, at the back of the eye, then progresses to the lateral geniculate nucleus of the thalamus, and then continues to the cortex, first to the primary visual cortex area and then to secondary visual areas that spread out through the dorsal and ventral regions of the back or far brains. This pathway is hierarchical, meaning that as you move to the next step, neurons do something more sophisticated, i.e. have more complex receptive fields, than the neurons before them, forming a hierarchy of processing units.
2. Topographic organisation – has a series of maps; each step of the visual pathway has maps of the visual world. Objects next to each other are detected by neurons next to each other = faithful representation.

Question 4

Parallel pathways

Answer 4

The visual system has parallel pathways; each visual stream processes a particular type of information and ends at a different place in the brain. Not completely isolated from one another.

Question 5

Benefit of shape of human eye

Answer 5

Spherical, so can move smoothly and invert and focus light through the cornea and the lens onto the retina.

Question 6

Retina

Answer 6

Thin layer of cells at the back of the eye that contains the photoreceptors.

Question 7

Photoreceptors

Answer 7

Located at the back of the retina, where the retinal pigment epithelium (RPE) can provide them nourishment and support.

Have inner segments that contain the Golgi apparatus, the ER, lots of mitochondria and a synaptic terminal where NT glutamate is released.

Elongated in the direction of light.

Question 8

Retinal pigment epithelium (RPE)

Answer 8

provides photoreceptors sources of photopigment and prevents scattering of light and prevents vision distortion by absorbing stray photons not absorbed by the retina.

Question 9

Types of photoreceptors

Answer 9

Rods and cones.

Question 10

Rods

Answer 10

Extremely sensitive to light; low light conditions. Structure: outer segment that contains stacks of discs with very high densities of the visual pigment (rhodopsin).

Question 11

Purpose of structure of outer segment of photoreceptors

Answer 11

Outer segment discs are stacked precisely perpendicular to the light, to ensure that every photon that passes by can get absorbed. It does this by maximising the change it encounters a photopigment.

Question 12

Phototransduction

Answer 12

The absorption of a photon leads to the hyper-polarisation of the cell.

In the dark, photoreceptor cells have high [ ] of cGMP. cGMP binds and causes the opening of cGMP-gated ion channels, so NA+ flows into the cell and depolarises it.

When a photon hits the outer segment, it is absorbed by trans-retinal (the photopigment in the stacks’ membrane and located in a pocket of the opsin), which bends the retinal and thereby activates the rhodopsin. Activated rhodopsin activates transducin (G protein in disc membrane), which diffuses and activates phosphodiesterase. Phosphodiesterase “chews” up cGMP, reducing its [ ]. The cGMP-gated ion channels close, reducing the influx of Na+. Since the K+ channels remain open, the cell hyper polarises.

Question 13

Relationship between light and NT release

Answer 13

Inverted; in the dark, the photoreceptor is depolarised and the glutamate is released. In the light, the photoreceptor hyper polarises and transmitter release stops.

Question 14

Why so many steps in phototransduction

Answer 14

Amplification; absorption of a single photon ends up causing a significant change in the current fluxed by the channels.

Question 15

Location of phototransduction – benefit

Answer 15

By constraining it to the membrane of the stacks, which is 2D, instead of the cytoplasm which is 3D, the reactions happen much more quickly.

Question 16

Four types of cells in the retina

Answer 16

Horizontal cells, amacrine cells, bipolar cells, ganglion cells

Photoreceptors connect reciprocally to horizontal cells and bipolar cells; bipolar cells connect to amacrine cells; amacrine cells connect to ganglion cells.

Only ganglion cells have axons that leave the retina and fire APs.

Question 17

Discovery of visual receptive fields

Answer 17

Made possible by by single-cell (unit) recordings. Fine tipped micro electrode is inserted into the tissue and carefully placed either within the cell membrane of a target neuron of just outside of it to allow for IC or EC recordings. Measure the activation/inactivation of a neuron by measuring the increase of decrease in the frequency of APs.

Discovered by Stephen Kuffler; recording APs from retinal ganglion cells of cats. Used modified ophthalmoscope to study the responses of retina to a diffused background light and a highly focused stimulus stop. Found two types of ganglion cells with antagonistic centre-surround properties: ON-centre cells and OFF-centre cells.

Question 18

Receptive field

Answer 18

The stimulus that activates the neuron.

Question 19

Two types of ganglion cells with antagonistic centre-surround properties

Answer 19

ON-centre cell and OFF-centre cells.

Question 20

ON-centre cells

Answer 20

Shining light in the centre of the receptive field produces strong responses, but this response is inhibited when the stop of light increases, encompassing the surround of the receptive field of the cell.

Question 21

OFF-centre cells

Answer 21

The cell is inhibited if the light is shined the centre of the receptive field, but stimulated when there is light in the surround.

Question 22

Receptive fields of ganglion cells

Answer 22

ON/OFF centre-surround receptive fields. They have antagonistic surrounds, so they respond more vigorously to small spots of light. They respond to changes in light intensity between their centre and surrounds; measuring the difference in luminance in the borders of objects. Measuring CONTRAST, the relative difference between stimuli (stimulus 1 - stimulus 2/sum of stimulus 1 and 2).

The brain thereby obtains most information about object boundaries.

Question 23

Contrast

Answer 23

Function we compute; relative differences in intensity of stimuli; the brain is interested in measuring changes.

Question 24

Why half of retinal ganglion cells are turned on by light rather than off

Answer 24

Some bipolar cells flip the sign of the signal from the photoreceptors, so they get turned ON by light rather than OFF. Then ON bipolar cells then excite ON ganglion cells.

Question 25

Horizontal cells

Answer 25

Reciprocally connected to the photoreceptors and get excited by them. They also inhibit photoreceptors, and bipolar cells too. So, they flip the sign of whatever signals they receive, so an excitatory signal form a photoreceptor becomes inhibitory etc. They created a blanket of inhibition that surrounds the area that was turned on; or a blanket of excitation that surrounds the area that was turned off. This is why you get centre and surround regions of the receptive fields with opposite polarity.

Question 26

Effect of retina computing spatial contrast

Answer 26

Turning the visual scene into an outline of the borders of objects; useful to recognising objects

Question 27

Retina pathways

Answer 27

Each retinal ganglion cell tube starts a pathway that propagate through the brain; retina begins the parallel pathways of visual processing. It processes info about changes in lightness (ON pathway) and changes in darkness (OFF pathway) (retinal ganglion cells that respond to stimulus contrast); processes info about from visual scenes during the day (cone pathway) vs at night (rod pathway). Some ganglion cells also compute temporal contrast.

Some ganglion cells have their own photopigment and care about the absolute luminance of the visual scene (used to reset and control circadian clock).

All retinal ganglion cells adapt to changes in visual stimulation, meaning responses change when you change the stimulus feature (ie. light level).

Question 28

Benefit of all retinal ganglion cells adapting to changes in visual stimulation (ie. light level).

Answer 28

On Earth, there are massive changes in luminance across the day-night cycle.

Question 29

LGN

Answer 29

Retinal ganglion cells project to the LGN where they make connections with neurons that project to the 1º visual cortex. Their axons make contact with thalamic neurons which themselves send axons to the 1º visual vortex via the optic radiation.

Question 30

Optic chiasm

Answer 30

The point at which the ganglion cell axons from the medial half of the retina project to the other side of the brain, while the lateral retina axons stay on the same side.

The eye inverts the image; this crossover ensures that all the info from the left side of the visual field ends up in the right side of the brain and vice versa.

Question 31

Optic tract

Answer 31

Axons from one temporal hemiretina join the axons from the contralateral nasal hemiretina to form the optic tract.

Then the axons go together to the LGN.

Question 32

Function of LGN

Answer 32

Has similar receptive fields as those of the retinal ganglion cells. 10X more neurons in the LGN.
Connections from the retina to the LGN pathway are all “afferent” (go into the brain) whereas for every afferent connection from the LGN to the brain there are 10 efferent connections that come to the LGN. So, LGN driven by the cortex not by the sensory periphery. Possible that the cortex is selecting and turning on certain neurons that it is interested in “listening to” as a selective filter. => attention, brain constructs the world.

Question 33

Organisation of LGN cells

Answer 33

Arranged in layers, with different ones being innervated by different eyes, with ordered retinotopic maps of the visual field that are all aligned on top of each other.

Question 34

Hubel and Wiesel

Answer 34

Discovered that most neurons in the 1º visual cortex respond to oriented bars.

Electrode was placed in the middle of the visual cortex of the cat; tried to stimulate the visual field of the animal with a small dark circle of metal glued to a glass microscope slide. Noticed a nlruon started firing when they were removing the slide from the projector => cell was responding to the shadow the slide cast.

Conclusion: neurons in the visual cortex have receptive fields that are elongated (not concentric anymore).

Observed different receptive fields => diff orientations of the bar, wider or thinner centres, different centre-surround relationships.

Question 35

Selectivity of neurons in visual cortex

Answer 35

V1 neurons have orientation selectivity;

Question 36

Receptive fields of V1 neurons

Answer 36

Elongated; constructed from inputs of several LGN neurons added together; a bunch of circles lined up.

Question 37

Simple cells

Answer 37

Discovered by Hubel and Wiesel.

Only respond to a particularly oriented bar in a particular part of the visual field.

Question 38

Complex cells

Answer 38

Discovered by Hubel and Wiesel.
Respond to the orientations of bars positioned anywhere within the visual field of the animal. Could be generating larger receptive fields by summing up inputs of different simple cells => builds a natural hierarchy of processing.

Simple cells allow you to see an object only if it is positioned in. particular part of the visual field, while complex cells can detect that object in multiple locations (tracking movement?).

Question 39

Hierarchy in visual cortex

Answer 39

Continues as you get higher up, reaching neurons that fire in response to specific objects like the grandmother cell in V2. As you go up you are abstracting from the world. => Hierarchy of info.

Question 40

1º visual cortex organisation

Answer 40

Hubel and Wiesel also found it was organised in columns. Push an electrode straight down through the cortex and record the different layers in the same column => these neurons have very similar receptive fields and similar orientations.

These columns are arranged forming a map of orientations; move from one orientation to the other when you move horizontally.

Question 41

Orientation map

Answer 41

Looking at reflectance of haemoglobin optically (its spectroscopy is related to oxygen consumption so neural activity). Assign diff colours to neurons according to the diff orientations of the bar. => structured representation of orientations selectivity territories, organised around non-oriented centres (pinwheels).
Grinvald and Bonhoeffer

Question 42

Other maps of the visual cortex

Answer 42

Maps are superimposed on top of each other; also have an ocular dominance column map (dividing the visual cortex into regions preferentially responding to one eye or the other) and also regions in the middle of these ocular dominance columns (blobs) that respond preferentially to colour.

Question 43

Module hypothesis

Answer 43

The visual cortex is built out of modules. Module is a part of the cortex that houses all the neurons that respond to a particular area of the visual field; hyper column that would have two ocular dominance columns (one from each eye) and a pinwheel of orientation and blobs that have information about colour.

Question 44

Visual cortex and principles of neuroscience

Answer 44

Hierarchical processing, modular organisation, topographic maps.

Question 45

Visual primitives

Answer 45

The 1º visual cortex appears to be decomposing the image into visual primitives – essential building blocks. Location, edges, depth, colours, motion and shapes.

Question 46

Ventral pathway

Answer 46

“What” pathway. Runs downwards towards the temporal lobe. Processes shapes.

Small retinal ganglion cells, parvocellular P cells, have smaller dendrites so smaller receptive fields. Map the world with precision. Slow, determining position. Feed into Parvo layers of the LGN (3-6); these layers project to particular layers of V1, with both simple and complex receptive fields; then the P pathway generates neurons that project to V2….

Inferior temporal area (IT) - neurons that respond particularly to faces.

Agnosia. Prosopagnosia = patient cannot recognise faces; see a person and describe the face but cannot identify it. Vegetable agnosias = people cannot recognise vegetables.

Neurons that recognise particular objects in the world must be arranged in maps in our temporal lobe; must be ordered in some way since agnosias tend to be selective.

Question 47

Doral pathways

Answer 47

“Where” pathway, runs from V1 to the parietal lobe on top of the head. Processes motion.

Large retinal ganglion cells, magnocellular M cells, have a larger receptive field so focus on larger areas; fast in their responses, needed to detect change of position over time. Project to 2 M layers of the LGN (1 and 2, for each eye). They then go to layer 4 of V1, then onto V2.

MT - mediotemporal, where neurons are selective to the direction of motion. If lesion in this area, become blind to movement, cannot detect the movement of correlated dots at levels that people without such lesion can => akinetopsia.

Question 48

Cones

Answer 48

3 types, with different opsin, tuned to absorb light of diff wavelengths. Don’t see colour - their response to many photons of the wavelength that they are less sensitive to will be the same as to fewer photons but from a more sensitive spectrum. Our brain has to compare the responses of different cones to light to be able to calculate which colour it is.

Question 49

Colour

Answer 49

Starts with Parvo retinal ganglion cells that receive inputs from cones (colour opponent retinal cells); pathway continues to the P LGN layers, then to the blobs in the middle layers of V1. Then to the temporal lobe, along the what pathway.

In V4, cells respond to our perception of colour.

Question 50

Colour receptive fields

Answer 50

Their receptive fields are ON/OFF centre and also respond to specific colours. The surround is always the opposite colour. “Colour opponent” cells compute the colour contrast.

Question 51

What our perception of colour is

Answer 51

The light that hits the earth changes in wavelength throughout the day and depends on cloud coverage, so we don’t just measure wavelengths emitted by objects. The brain computes the contrast in wavelength of an object and its background (centre-surround) and thereby measures a property of the object “colour” that is independent of the illuminating wavelength. Colour is actually the measure of the reflectance of the objects and it’s chemical property of their surface. Canceling out the effect of the illuminating wavelength, performing a spectroscopic analysis of the chemical properties of the surface which is invariant.

Question 52

Blind spot

Answer 52

Where all axons from the retinal ganglion cells exit. No space for photoreceptors; when the image of an object hits one blind stop it also hits the normal part of the retina in the other eye, so we can detect objects that are in front of one of the blind spots.

What we see in the blind spot resembles the neighbouring areas of the visual field, the visual cortex filling in the blind spot with a made up image that resembles the area around it.