light, eye, retina - first steps in seeing

Question 1

Process through eye

Answer 1

Light hits cornea
Then hits iris and goes through hole in pupil
Hits lens and lens helps eye to focus the light to back of the eye
Retina at the back of the eye is a sheet of sensors to pick up the light

Question 2

Eye and optics - visual neighbours

Answer 2

Eyes refract light in such a way that the light that comes from one point in visual space is mapped on the same point in the retina
Bent by the cornea

Question 3

Retina

Answer 3

Earliest sensory unit that interacts with the light that comes from the environment
Has a ridiculously complicated structure

Question 4

Photoreceptors

Answer 4

Sensory cells that have certain molecules that change their configuration when they are hit by light

Question 5

Vertical connectivity

Answer 5

Photoreceptors that are connected vertically to interneurons
Interneurons connected to ganglion cells
Ganglion cells take info from eye and feed it into brain

Question 6

Horizontal cells

Answer 6

Amacrine cells help connect the cells horizontally

Question 7

Rods

Answer 7

Used when light is dim
Many rods (100 million)
Colourblind
Very sensitive to light and used when very dim
Poor spatial resolution, very blurry
High temporal resolution, used to perceive motion

Question 8

Cones

Answer 8

Used when light is very bright
Fewer cones (6 million)
S-, M- and L- cones with peak sensitivities at different wavelengths
Less sensitive to light and used in bright light
High spatial resolution, need to use cones when you want to see in high resolution
Poor temporal resolution

Question 9

Duplex retina

Answer 9

Two systems for seeing

Question 10

Rod system sensitive to light

Answer 10

Four rod cells feed info into one bipolar cell and the one bipolar cell feeds info to ganglion cell
Only one rod needs to pick up light in order to detect it
Brain doesn’t know where in retina light came from because ganglion cell covers a large cell

Question 11

Cone system

Answer 11

One cone mapped to one ganglion cell
Brain knows where in visual space and where in retina light comes from due to there only being one

Question 12

Distribution of photoreceptors

Answer 12

Rods and cones aren’t equally distributed across the whole retina

Question 13

Fovea

Answer 13

Directly behind iris opening
Lots of cones and no rods
This is why we can’t see stars when we look directly at them because cones are less sensitive to light - so we use our peripheral

Question 14

Blind spot

Answer 14

No cones or rods
Part of the retina where all the axons of the photo receptors leave our eye
Don’t experience the blind spot due to our other eye filling in the gap

Question 15

Receptive fields

Answer 15

Specific region of sensory space in which an appropriate stimulus leads to a response in a sensory neuron
Each ganglion cell has a set spatial extent to which it can get info from
Has to be an appropriate stimulus

Question 16

Receptive fields of ganglion cells

Answer 16

One photoreceptor directly linked to a bipolar cell which feeds directly to a ganglion cell - facilitory

Other photoreceptors surround the central one and link to a HORIZONTAL cell which links to a bipolar cell which then feeds to the ganglion cell - inhibitory

Activity of the horizontal cell inhibits the activation of the bipolar cell

Question 17

Centre-surround receptive field

Answer 17

Ganglion cell acts like a ballot box - adds up faciliatory and inhibitory votes from photoreceptors
If there are more positive than negative votes then the threshold becomes active
If there is an equal number nothing happens
If it takes in more negative then it will reduce its baseline responding

Question 18

Receptive field structure

Answer 18

Stimulus has to hit the receptive field of the ganglion cells in order to activate it

Question 19

Light in centre of receptive field

Answer 19

Ganglion cells will become more active
The photoreceptors in the middle of the receptive field have a facilitatory link to the ganglion cells

Question 20

Light in edge of receptive field

Answer 20

Ganglion cells become less active
The photoreceptors on the outside of the receptive field have an inhibitory link to the ganglion cells

Question 21

Light across whole receptive field

Answer 21

Ganglion cells stay at baseline
Not an appropriate stimulus

Question 22

Local differences in light intensity

Answer 22

Mainly set up to process contrast
Very efficient way to process environment

Question 23

Edges and receptive fields

Answer 23

With one small part of receptive field in a darker grey area, ganglion cell will respond higher than baseline due to more light hitting the faciliatory centre
Receptive field is exactly in between two greys then the faciliatory and inhibitory cancel each other out
Periphery hits more of the light and the centre is in the dark grey then it will be lower than baseline

Question 24

Cornsweet effect and receptive fields

Answer 24

When the baseline changes, the ganglion cells tell our brain that there is an edge from light to dark
Our brain fills in the information and assumes that the two different surfaces in the illusion are completely different colours
Efficient way to fill in the information