Visual System- Sterzer

Question 1

what are the 2 optical functions of the eye?

Answer 1

1. adaptation- gather light reflected from surfaces

2. accommodation- focus it on an a clear image on the retina

Question 2

what types of photo receptors do we have? where are they located?

Answer 2

rods and cones

located in the back of the retina and oriented away from the retina to propagate info to the brain

Question 3

characteristics of rods….

Answer 3

• very light sensitive
• more photopigement
• high amplification, single photon detection
• sensitive to scattered light
• not preset in the central fovea
• achromatic (no colour distinction)

Question 4

characteristics of cones….

Answer 4

• not so light sensitive
• less photopigement
• low amplification
• most sensitive to direct axis rays
• most preset in the central fovea
• trichromatic (3 types: S cones - short WL, M cones - medium WL, L cones- long WL)

Question 5

Photoreceptors respond w. graded changes in membrane potential which generates greater response from foveal and ganglion cells
T/F?

Answer 5

T

Question 6

which of the following statements is/are correct?

a. M have lower spatial resolution than P cells but have a faster frequency
b. M and P ganglion cells convey the same type of information
c. P cells are more sensitive to contrast than M cells
d. P cells respond to lower temporal frequency than M cells
e. M cells are more sensitive to contrast than P cells

Answer 6

a, c, d

Question 7

which of the following statements is/are correct?

a. the organisation of the LGN is by eye (some cells respond to right eye and some to left eye)
b. the organisation of the LGN is by hemifield i.e. left LGN receives info from right hemifield and right LGN from left hemifield
c. the LGN has layers for Ipsy- and contralateral inputs which project to P and M channels
d. the LGN projects to the optic chiasm
e. the LGN is organised in columns

Answer 7

b, c

Question 8

why can lesions in the temporal lobe cause visual disturbances?

Answer 8

optic graduation doesn’t go straight from LGN to occipital lobe, but curves through the temporal lobe–> lesions can damage subcortical pathways

Question 9

what are the different visual-cortical pathways V1-V5?

Answer 9

the visual cortex is composed from 5 regions.

• V1- V3 have high phototopic organisation
• V4-V5 sensitive to very specific functional aspects (colour, motion…)

Question 10

what does retinotopy mean?

Answer 10

• mapping of the visual field from the retina to cortical subregions.
• spatial organisation of neurons in the visual cortex corresponds to the visual field.
• the fovea is reflected to the back of the brain (posterior regions of the visual cortex) whereas the periphery is mapped more anteriorly.

Question 11

what is polar angle mapping?

Answer 11

polar angle mapping is a method for imaging retinotopic mapping using fMRI.

• subject looks at object–> fMRI imaging shows which position of the wedge of the object activates which part of the visual cortex.
• flattening the fMRI data of the cortex –> detection of exact borders of the activated regions in the visual cortices

Question 12

which of the following statements is/are correct?

a. P cells project more to the ventral/’what’ stream and M-cells project more dorsally (‘where’) stream
b. The WHAT stream is the ventral stream and cannot act without information from the WHERE stream which is dorsal
c. P cells project to the where/ventral stream and M-cells project to the what/dorsal stream
d. the dorsal (WHERE) stream can act without information from the ventral (WHAT) stream and vice versa
e. there is only one stream in the visual system

Answer 12

a, d

Question 13

P-cells are more sensitive to shapes and colours and thus project to the ventral stream and M cells are more sensitive to motion and project to the dorsal stream
T/F?

Answer 13

T

Question 14

what happens to a person with achromatopsia? which visual subregion is damaged?

Answer 14

the person cannot perceive colours due to lesions in V4

Question 15

what happens to a person with akinetopsia? which visual subregion is damaged?

Answer 15

subject cannot perceive motion due to lesion in V5

Question 16

which cortical subregion is responsible for perception of objects? what pathological conditions are associated with lesions in these areas?

Answer 16

the fusiform gyrus, fusiform face area (FFA) in the ventral stream and PPA are the subregions where visual processing of objects takes place.

• -> information about objects is not specific to each object, but rather it depends on activation pattern.
• -> example for pathological condition- prosopagnosia- patient can’t recognise faces (lesions in FFA)

Question 17

the visual cortex is organised in…

a. layers
b. columns
c. columns and layers
d. hemifields
e. none of the above

Answer 17

c

Question 18

why is the primary visual cortex (V1) called striate cortex?

Answer 18

because of the stria of Gunnari, a heavily myelinated group of cells. (stria = stripe)

Question 19

how is the primary visual cortex organised?

Answer 19

V1 is organised in layers and columns:

Layers:
there are 6 layers in V1 (similar to the neocortex):
I- molecular layer
II-III- mainly pyramidal cells, receive info from LGN and project it to higher areas
IV- mainly interneurons
V-VI- cells receive info from II and III and send it to lower level areas–> feedback mechanism, info from V and VI is transmitted back to LGN.

Columns:
clusters of cells in V1 are organised across layers from top to bottom in a columnar fashion. This organisation is important for orientation selectivity - graded orientation selectivity of cells in columns.

Question 20

definition if receptive fields…

Answer 20

a region in space in which the presence of a stimulus will alter the firing rate of a specific neuron (either increase or decrease)

Question 21

the structure of receptive fields….

a. starts in the retina
b. starts in LGN
c. starts in V1
b. only occurs in the retina

Answer 21

a

Question 22

what are the types of receptive fields at the level of retinal ganglion cells?

Answer 22

• On centre- respond to objects on the centre

- Off centre- higher response to objects in the periphery

Question 23

response to stimuli…

a. determined by the borders and not by the light of the pattern.
b. response depends on the light reflected from the pattern to the retina
c. determined both by the light and the borders of a oattern
d. depends on the contrast of the image

Answer 23

a

Question 24

at the level of V1 there is direction selectivity, as well as orientation selectivity. what cell types are responsible for the orientation selectivity in V1? what determines their response?

Answer 24

• simple cells- receive horizontal input from on- and off- cells in LGN. –> the orientation selectivity of simple cells is determined by the group of concentric cells in LGN which is projecting the information to it.
• Complex cells- simple cell population converge onto 1 complex cell. complex cells respond to vertical bars (don’t respond to horizontal/diagonal like simple cells) –> strength of the response changes according to the position of the stimulus (good for motion processing as position of stimulus changes)

Question 25

what are the functional characteristics of V1 cells?

Answer 25

• orientation selectivity (most cells)
• size selectivity (M- and P- cells)
• position selectivity
• colour selectivity
• direction selectivity
• binocular selectivity

Question 26

when looking at a screen with lines in contrasting colours (black/white) at a certain orientation (e.g. tilted to the right) and then the stimulus suddenly changes to completely vertical lines - what would you expect to happen? why?

Answer 26

In the first few seconds, the lines after changing orientation will appear as if they are oriented to the opposite side, due to overcompensation of the eyes that are trying to adapt to the sudden change.

The reason for it is that the cells with the orientation for the original stimulus are less active after a while and right after the orientation changes, the cells with the opposite orientation will react with great intensity, and therefore at first the image seems tilted.

Question 27

which of the following statements about visual perception is/are correct?

a. visual perception is based solely on the sensory input we see
b. the brain needs additional information to construct visual perception which comes from prior knowledge
c. experience affects perception greatly
d. colour is the only aspect of vision that does not change according to context

Answer 27

b, c

Question 28

what are Gestalt principles?

Answer 28

Gestalt principles of visual perception are rules by which we construct our vision:

1. figure-ground- we perceive objects either as in be background or in the fore-ground
2. similarity- when things appear to be similar to each other, we group them together
3. proximity- things that are close together appear to be more related than things that are spaced farther apart
4. common region - similar to proximity; things that are close together are grouped
5. continuity - elements in a line are perceived to be more related than ones that are not in the line
6. closure- when we look at a complex arrangement of visual elements, we tend to look for a single, recognizable pattern (pattern completion)
7. focal point- whatever stands out visually will capture and hold the viewer’s attention first.

Question 29

what is the principle of perceptual inference?

Answer 29

A perceptual system is an inference engine whose function is to infer the probable causes of sensory input-
influenced by: 
- built-in neural wiring
- context
- experience
- knowledge
- expectation

Question 30

what is the predictive coding theory?

Answer 30

hierarchical system, in which info is coded so the brain makes predictions about the sensory input –> top-down processing

• if prediction is not met –> error signal –> prediction changes
• feedback connections from a higher- to a lower-order visual cortical area carry predictions of lower-level neural activities
• feedforward connections carry the residual errors between the predictions and the actual lower-level activities

Question 31

What two GROUPS of theories aim to explain conscious perception ? what is the difference between them?

Answer 31

a. anatomical location theories- one/ a set of structures that can generate consciousness, and the info can be first process subconsciously but will reach consciousness only when arrives at this area.
b. state change theories- special state of neural activity resulting from a specific processing pattern that leads to consciousness (not a matter of location but a matter of activation pattern)

Question 32

give an example for an anatomical location theory

Answer 32

The global workspace theory:

• global availability of cognitive content for diverse cognitive processes (attention, working memory, verbal report)
• automatic sensory processing
• mobilisation of global activity pattern among workspace neurons
• any global pattern can inhibit alternative patterns
• conscious perception as a non-linear function of stimulus salience

Question 33

give an example for a state change theory

Answer 33

Neural synchronisation:

• synchronous oscillations in the gamma band
• activation of synchronisation by central activating systems
• synchronisation within & across distant cortical areas
• temporal signature of neural signals makes info available to other brain centres
• potential solution of the “binding problem”

Question 34

what is the binding problem in conscious perception? how can it be potentially solved?

Answer 34

the binding problem is a problem of differently contrasted percept that is also moving may be difficult to process if we only have one “consciousness centre”
–> can be solved by synchronous activity of neurons in different cortical areas that are firing together to reach conscious perception