Vision

Question 1

Describe roughly how we see

Answer 1

Light enters our eyes where it acts on photoreceptors in the retina, this triggers processes where neural impulses are generated then travel through pathways and networks of the visual brain. Separate pathways to the midbrain and cerebral cortex mediate different visual functions. In the cortex, neurons in a large number of distinctive visual areas are specialised for making different kinds of visual decisions

Question 2

How does light enter the eye?

Answer 2

Light enters the eye through the pupil and is focused by the cornea and the lens, on to the retina at the back of the eye

Question 3

How does pupil become larger or smaller as light levels vary?

Answer 3

Pupil is surrounded by a pigment iris that can expand or contract

Question 4

How many photoreceptors do humans have?

Answer 4

125 million

Question 5

How does light become signals to the brain?

Answer 5

Photoreceptors arranged across the surface of retina respond to light that hits them by generating tiny electrical potentials, these signals pass via synapses through a network of cells in the retina, activating retinal ganglion cells whose axons collect together to form the optic nerve, these enter the brain where they transmit action potentials to different visual regions with distinct functions

Question 6

What is the most numerous photoreceptor?

Answer 6

Rods

Question 7

What is the least numerous photoreceptor?

Answer 7

Cones

Question 8

How many times is rods more sensitive than cones?

Answer 8

1000

Question 9

How do you use the types of photoreceptors?

Answer 9

You see at night with rods but day with cones

Question 10

How many types of cones are there?

Answer 10

3, sensitive to different wavelengths of light

Question 11

What are cones vital for?

Answer 11

They are vital for producing colour vision

Question 12

What happens when one is overexposed to one colour of light?

Answer 12

The pigments in the cones adapt and make a lesser contribution to out perception of colour for a short while

Question 13

What is phototransduction?

Answer 13

The conversion of light into electrical signals in the rods and cones

Question 14

What causes the genetic basis of colour blindness?

Answer 14

The absence of certain visual pigments, function of retinal network and presence of two different types of ganglion cells

Question 15

What are the types of ganglion cells?

Answer 15

90% very small P-type cells, 5% large M-type or mango cellular cells and 5% others

Question 16

How are optic nerves arranged?

Answer 16

The fibres of each nerves meet at a structure called the optic chiasm, half of them cross to the other side where they joined another half that have stayed uncrossed. Together theses bundles of fibres form the optic tract which projects to the cerebral cortex.

Question 17

How are optic tracts projected to the cerebral cortex?

Answer 17

Via synaptic relay in a structure called the lateral geniculate nucleus

Question 18

Where are internal representations of visual space around us created in the brain?

Answer 18

Cerebral cortex

Question 19

Where is the left-hand side of the visual world in?

Answer 19

In the right hemisphere

Question 20

What is binocularity?

Answer 20

Neural representation has inputs from each eye and so the cells in the visual areas at the back of the brain can fire in response to an image in either eye

Question 21

How are cells arranged in the visual cortex?

Answer 21

Columns

Question 22

What is receptive field?

Answer 22

The region of retina over which the cell will respond to the preferred kind of image

Question 23

What is V1?

Answer 23

The first stage of cortical processing

Question 24

What does the neurons respond to best in V1?

Answer 24

Lines and edges in a particular orientation

Question 25

What does the neurons fire in V1?

Answer 25

All neurons in any one column of cells fire in the same orientation while the neighbouring column of cells fire best to a slightly different orientation and so on across the surface of V1

Question 26

What does the way neurons fire in V1 mean?

Answer 26

Cortical visual cells have an intrinsic (natural) organisation for interpreting the world

Question 27

What do all sensory systems in visual cortex display?

Answer 27

Plasticity

Question 28

How does neurons in V1 display plasticity?

Answer 28

The extent to which an individual cell can be driven by activity in the eyes is modified by experience

Question 29

What is blindsight?

Answer 29

Ability of primary visual cortex (V1) to respond to visual stimuli that they do not consciously see

Question 30

What is the circuitry of visual cortex like?

Answer 30

Different types of neurons are arranged across the six cortical layers, connected together in very precise local circuits, the circuitry in one visual area has many similarities to that in another, but there could be subtle differences that reflect the different ways each bit of visual brain interprets different aspects of the visual world

Question 31

What is the key function of the cerebral cortex?

Answer 31

Ability to form and act upon sensory information received from many sources

Question 32

How does brain make decisions about vision?

Answer 32

Available sensory evidence is weighed up and uses internal knowledge to interpret the sensory scene

Question 33

What does cells in V5 do?

Answer 33

Activity of cells in V5 accurately reflects the strength of movement signal

Question 34

What is interesting about movements detecting movement?

Answer 34

Some individual neurons perform just as well at detecting movement of dots, micro stimulation of such neurons of these neurons through recording electrode can even bias the judgement of relative movement. There are very large numbers of neurons sensitive to visual motion but just a few can make decisions

Question 35

What does area V5 do other than register motion of visual stimuli?

Answer 35

It registers perceived motion

Question 36

How does area V5 register perceived motion?

Answer 36

Neurons corresponding to area of illusion will fire differently to rightwards or leftwards perceived motion. If movement is completely random, neurons that normally prefer rightwards will fire slightly more when the random motion signal is moving rightwards and vice versa

Question 37

What does the decision of rightwards or leftwards reflect?

Answer 37

It reflect what the observer judges about the appearance of motion and not the absolute nature of moving stimulus

Question 38

How do observer react to perceptual targets that are genuinely ambiguous?

Answer 38

The observer is in a state of indecision, constantly fluctuating from one interpretation to another

Question 39

What is binocular rivalry?

Answer 39

When two dissimilar images are presented to the eye, they compete for perceptual dominance so each image is visible in term for a few seconds while the other is suppressed

Question 40

Which area decides whether something in the visual world in moving?

Answer 40

Area V5