Vision- from retina to cortex

Question 1

“The problem of vision”
What is there in the visual signals our eyes receive?

Answer 1

There is inherent ambiguity eg. in shape

Question 2

What are some examples of ambiguity?

Answer 2

Cars presented at different viewpoints but its the same car. This will pass different shapes onto your retina. You might see different features as well. But we figure out these viewpoint and conclude its the same car.

You can have 2 different shapes that cast the same shape onto the retina. Due to the way light and perspective works, those 2 shapes (although different in their orientation and identity) might be cast onto the retina the same shape at the back of the eye.

Spinning dancer is also an example of ambiguity

Question 3

List structures of the eye and their function

Answer 3

Pupil: where light enters eye (it passes through the lens)

Iris: (coloured part around pupil) adjustable aperture, constricts in bright light to make pupil smaller. The aperture determines how much light enters the eye.

Cornea and lens: focuses light on retina

Accommodation: ciliary muscles change shape of lens to bring objects into focus at different distances

Question 4

List other structures of the eye and their function

Answer 4

Lens: helps focus light onto the retina

Ciliary muscles: allow for accommodation - when focusing on something change the shape of the lens

Retina: red layer which runs along the back of the eye

Fovea: high definition vision

Question 5

What are photoreceptors? + photo transduction

Answer 5

Cells with light sensitive photopigments in outer segments

Photo transduction- production of AP so nerves begin to fire

Question 6

What are the 2 broad classes of photoreceptors?

Answer 6

Rods:
* Contain rhodopsin, respond in dim light (generally used at night), none in fovea
* Features of rods- colour blind in rod vision

Cones:
* Split into three types with photopigments sensitive to different wavebands (long, medium, short) - daytime vision
* Daytime vision and colour vision

Question 7

How does light enter the retina?

Answer 7

Due to the way our retina is organised, light comes in through the bottom and passes through all the neural machinery before it reaches the parts of the eye that are sensitive to light.

Question 8

When staring at a dot on a black and white figure, what is happening to the photoreceptors?

Answer 8

Whilst staring at the dot, the photoreceptors on the parts of your retina which are underneath the light areas are adapting. They’re firing because light is landing on them, but they’re going to gradually reduce the amount of activity they produce (this is called adaptation).

Question 9

• where do Retinal Ganglion Cells sit?
• what stage are the of retinal processing?

Answer 9

• downstream of photoreceptors
• last stage of retinal processing

Question 10

Retinal Ganglion Cells:
What are the cells and what do they have?

Answer 10

Large Parasol ganglion cells: they have large receptor fields and connect to large areas of retina

Small Midget ganglion cells: smaller receptor fields- connect to a small number of photoreceptors

Cells have ‘Receptive Fields’ – the part of the retina from which the ganglion cell receives input

Question 11

What are the parasol and midget cells linked to?

Answer 11

The parasol cells are connected to the magnocellular

The midget ganglion cells are connected to the parvocellular system

Question 12

Single cell recording form retinal ganglion cells (RGCs)

Answer 12

• no light- baseline activity
• if we fill the light- baseline activity again (no change)
• For lots of classes of retinal ganglion cells, they have an arrangement where if light hits the centre then their activity will increase. This means lots of AP’s will be sent out.
• If you fill the surrounding light, then the activity decreases below baseline.
• This is called a centre surround receptive field
• On centre- off surround
• Off centre- on surround

Question 13

Whats the difference between On centre- off surround and Off centre- on surround?

Answer 13

On centre- off surround: theres excitation (activity increases) when light hits the middle and inhibition if light hits the outside
Lateral inhibition

Off centre- on surround: excitation when light hits the outside and inhibition when it hits the middle

Question 14

Retinal ganglion processing:
List 3 features

Answer 14

1- Poor at spotting gradual change

2- Good at picking out sharp edges:
When you have multiple on centre-off surround cells which span the visual field, you’ll get signals from the ones that correspond to where the edges are and no signal from the ones where theres no edge. This is because when theres an edge, there will likely be a difference between the centre and surround of the receptor field- no edge the centre and surround will be more the same.

3- Filters the input for useful information

Question 15

Retinal ganglion processing:
Gradual example

Answer 15

If you spread the on centre receptor field, the only place you get activity is where the edge is there.

If you do this to a naturalistic scene eg. in a parking lot, it points out all the edges.
Being filtered by receptive fields similar to those of the retinal ganglion cells. The edges are picked out, whereas areas of uniform brightness (whether light or dark) look the same mid-grey.

Question 16

What helps us with recognising things?

Answer 16

Edges

Question 17

Perceptual effects of retinal ganglion cells……
Hermann Grid? (Baumgartner, 1960)

Answer 17

If you focus on one intersection- you will find that the dot disappears. But in the periphery there will be dots between these intersections. It was thought this was due to on centre- off surround visual fields.

Theres a difference between the junctions and the bits in between in terms of how much inhibition there is and the result is that is that you perceive these darker spots.

Question 18

BUT, is it really due to retinal ganglion cells?

Answer 18

Wiggly version makes illusion go away-
Making it wiggly makes the effect go away so it probably isn’t due to retinal ganglion cells.

Question 19

What else can retinal ganglion cells can help you detect?

Answer 19

Contrast

Question 20

Perceptual effects of retinal ganglion cells……
Simultaneous contrast

Answer 20

image- black square on left, white square on right with 2 small squares in both

The patch (small square) on the right looks relatively darker than the left. This is because on the left hand side, theres less inhibition from the surround due to less light coming from the surround.

Question 21

Simultaneous contrast
However it’s not this simple, theres something more going on…

Answer 21

The 2nd and 3rd tiles are physically the same but the one on the right looks darker. The difference in the overall picture is the presence of the table.

Question 22

Where is the Lateral Geniculate Nucleus? + feature

Answer 22

In thalamus- sensory gateway (all senses except smell go through it)

Question 23

What does the LGN have?

Answer 23

Layers (which are folded over). These have different roles visually.

Question 24

What are the 3 distinct pathways running through the LGN?

Answer 24

The parvocellular system exists in some of these layers. (the lighter stripes)

The magnocellular system go into the sublayers which are closer to the middle of the structure.

The Koniocellular cells exist in between the layers.

Question 25

What are the 3 distinct pathways running through the LGN linked to?

Answer 25

Magnocellular cells: movement & flicker

Parvocellular cells: colour & detail

Koniocellular cells: Blue-yellow

Question 26

Visual Cortex:
- how much of the cortex is dedicated to vision
- what type of visual cortex are we concerned with

Answer 26

• Up to or more than 50% of cortex dedicated to vision (visual cortex and beyond)
• Primary visual cortex / V1 / striate cortex (has stripes in it- discovered by staining)

Question 27

Retinotopy

Answer 27

V1 contains a retinatopic map. Over the surface of the cortex in V1, there is a point by point representation of all the points in your visual field.

Question 28

Cells in V1 and orientation finding in cats

Answer 28

Visual images to anaesthetised cat

Slipping glass into projector moving black dot produced a faint line which lead to cats reponding.

They though maybe cells in V1 are a bit like retinal ganglion cells (like to see spots in their visual field and not spots in surround)

Instead the cells in V1 they were measuring from were sensitive to lines and lines of particular orientations

They found orientation selectivity in V1.

Question 29

Detailed investigation of visual cortex

Answer 29

Lines indicate the sending of APs

For different orientations of lines, you get different responses

Eg. vertical line shows to be very responsive

Question 30

Experimental setup used by Hubel & Wiesel.

Answer 30

Across V1 there are pin wheels. There are cells all round which respond to all of the orientations.

This tells us, across V1’s retinatopic map, you have a representation of every point and orientation of every point

Question 31

Detailed investigation of visual cortex:
Orientation selectivity

Answer 31

Making a V1 receptive field from LGN inputs. The elongated bar detector has an excitatory central strip flanked by 2 inhibitory regions. It will respond best to a bright bar, tilted to the left, on a dark background.

V1 simple cell- if you take the output from centre-surround receptors - some on centre and some off centre- you can get a rectangular receptive field which is responsive to a certain or orientation of line and it won’t be responsive to other orientations of line.

Question 32

Role of development/critical periods

Blakemore & Cooper (1970): task and results

Answer 32

Procedure:
* Prevalence of neuron types shaped by environment experienced early on
* Kittens raised in striped tubes from birth, vertical or horizontal stripes
* 5 hours per day in tube
* 5 months later: no response to orientation not in tube

Results:
* cats don’t show any behavioural response to other orientations.
* Recorded from cells in visual cortex: no neurons that respond to orientation absent in tube. Cats show no response to any other orientations.
* Example of neural plasticity, but with a critical period - ‘use it or lose it’

Question 33

Beyond V1 has 2 streams of processing: what are they?

Answer 33

Ventral (runs along the bottom) and dorsal (runs along the top)

Question 34

Beyond V1: Streams of processing
What is the difference between dorsal and ventral?

Answer 34

1- Dorsal (where)- used to plan movements, it projects up to the motor system

2- Ventral (what)- objects, colour, memory

Question 35

Colour (ventral) & motion (dorsal)
Video illusion- Suchow & Alvarez, 2011 (Curr. Biol.)

Answer 35

Coloured dots appear in a cloud around a central white fixation point. The dots change colour smoothly and quite rapidly so the perception is of a twinkling changing set of colours. Then the cloud of dots all rotate together around the fixation point. Provided the observer looks at the white fixation point, when the dots are in motion the colour change is much less salient, even though the dots are still changing colour at the same rate.

Effect happens because colour and motion are separated in terms of those 2 streams.

Question 36

What happens to processing of some features?

Answer 36

Processing of some features (i.e. edges) begins at the retina, and gets more complex “higher” in the visual processing stream.

Question 37

What is V1?

Answer 37

brain area/ part of the cortex/ primary visual cortex

Question 38

What is meant by the phrase “retinotopic map” in this context?

Answer 38

There is a representation of each point in the visual field (or hemi-field) arranged so that neighbouring parts of the retina are represented by neighbouring parts of V1/ the cortex/ brain.