lecture 11: from eye to (further into) the brain

Question 1

multimodal

Answer 1

all sensory sections project to them (frontal, parietal, temporal)
occipital = purely visual

Question 2

brain organisation

Answer 2

1. 4 lobes = frontal, parietal, occipital and temporal
2. cortex = all the convolutions
3. subcortex = everything below those convolutions

Question 3

visual pathway

Answer 3

1. eyes –> subcortex (lateral geniculate nucleus - LGN)
2. LGN –> primary visual cortex (V1 in occipital lobe)
3. cortical visual pathways beyond V1

Question 4

where is V1 located

Answer 4

occipital lobe

Question 5

mishkin and ungerleider (1982)

Answer 5

• monkeys were trained on object discrimination task and a landmark discrimination task
  object discrimination = involves telling shapes apart
  landmark discrimination = involved discriminating the position of a cylinder
• once the monkeys learned both tasks half received lesions of the parietal lobe and half received lesions of the temporal lobe

Question 6

mishkin and ungerleider results

Answer 6

performance on object task:
1. damage to the temporal lobe = impaired
2. damage to the parietal lobe = ok
perfomance on landmark task:
1. damage to temporal lobe = ok
2. damage to parietal lobe = imparied

Question 7

what is temporal lobe responsible for

Answer 7

distinguishing shapes and not spatial vision

Question 8

what is parietal lobe involved in

Answer 8

determining where things are in space

Question 9

ventral stream

Answer 9

–> related to temporal lobe
- involved in pattern visual-also known as the what pathway

Question 10

dorsal stream

Answer 10

–> related to parietal lobe
- involved in spatial vision also known as the where pathway

Question 11

what does a change in firing rate of neurons tell us

Answer 11

• all neurons fire at a baseline rate, if a cell is interested in something it will either increase or decrease its firing rate relative to baseline

Question 12

electrophysiology studies

Answer 12

• drop wires into the brain and listen in to the action potentials of neurons
• all neurons (cells) fire at a baseline rate
• if a cell is interested in something it will either increase or decrease its firing rate relative to baseline

Question 13

what do rods and cones respond to

Answer 13

changes in illumination

Question 14

what do retinal ganglion cells respond to

Answer 14

spots of light
eg: could be excited by black spots against a white background and could be inhibited by white spots against a black background

Question 15

what do LGN cells respond to

Answer 15

spots of light

Question 16

what do V1 cells respond to

Answer 16

lines of different orientations

Question 17

what does the IT cortex (beyond V1) respond to

Answer 17

jagged objects
–> the smoother it is the less IT cortex responds to it

Question 18

grandmother cells

Answer 18

• cells that seem to respond to discrete features (complex objects)
• also known as feature detectors

Question 19

retinotopic mapping

Answer 19

= point-to-point mapping of external world onto a brain area
V1 and before: there is retinotopic mapping
After V1: there is no retinotopic mapping

Question 20

what changes as you go higher into the visual system (away from the eyes)

Answer 20

• the info that is processed becomes more complex
• the features that drive a cell change from basic illumination levels (rods and cones) to spots of light (RG cells, LGN cells) to lines (V1) to complex features (IT cortex cells)
• in line with these changes, the complexity with respect to where information has to be changes from small specific areas of space (V1) to very large areas of space (IT cortex cells)

Question 21

lateral inhibition

Answer 21

• every cell in the brain is not only trying to relay info its recieving, it is also trying to inhibit its neighbour at the same time
• This mechanism enhances the contrast and sharpness of sensory signals, helping the brain to better distinguish between different stimuli

Question 22

what does it mean when you say that a retinal ganglion cell likes dots

Answer 22

• RG cells has a certain receptive field (centre surround architecture), shows what info has to fall on the receptive field to activate the cell
• something has to happen in the inside which is different to what is happening on the outside = maximal response from a retinal ganglia cell
• the receptive fields of some retinal ganglion cells are organized in a centre surround fashion, where they are more responsive to localized, circular stimuli rather than to larger, more diffuse patterns

Question 23

what is the point of center-surround architecture

Answer 23

• enhances contrast
• brightness contrast

Question 24

the hermann grid illusion

Answer 24

–> see black dots in the middle of the intersections where there are none
centre = responds excitatorily (increases firing) when light hits it
surround = inhibits the response when it is stimulated
At the intersections:
centre = exposed to white lines = strong response
surround = exposed to black areas = inhibits response
–> bc white lines around the intersection are surrounded by black, the contrast create a situation where ganglion cells near intersection have increased inhibition in surrounding areas = perceived dark spots

Question 25

fovea

Answer 25

this is where all your cones and retinal ganglia cells are packed more densely

Question 26

why does the dot vanish when you foveate the intersection

Answer 26

- at the fovea, the cones are packed more tightly than at the periphery, and so the RGC assembly is smaller, and falls completely within the intersection and street - bc this region is more compressed you are looking at it with a compressed receptvie field such that now all the centre cells and all those surround cells are bathed in light