Light, Eye and Brain Spatial Frequency

Question 1

Define light

Answer 1

Wave of electromagnetic radiation
- fluctuating amounts of electric and magnetic charge at the same time
- form of energy (when it varies over time and space it results in a wave)
- measured in nanometres

Can also be thought of as a particle
- light particles = photons
- smallest amount of light you can have is one quantum of light

Question 2

Visible spectrum

Answer 2

between 400-700 wavelengs mm

Question 3

Light intensity

Answer 3

Luminance: scale that only includes detectable light on the visible spectrum
- measured in candelas/m2 or cd m^-2

Vision works in a wide range of different luminances

Objects reflect different percentages of the incident light

Question 4

Contrast

Answer 4

Contrast = the comparison between the lightest thing and the darkest thing
C = (Lmax - Lmin) / (Lmax + Lmin)
If small difference in luminance, the number will be a small number, and if large difference, the number will be larger. ANSWER ONLY BETWEEN 0 and 1

Question 5

Parts of the eye

Answer 5

Cornea = transparent window into the eyeball
Pupil = dark circular opening at the centre of the iris in the eye (where light enters the eye)
Lens = enables changing focus using ciliary muscles
Retina = a light sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve
Aqueous / vitreous humor: squishy bits

Question 6

Process of light entering the eye

Answer 6

• light energy enters the eye through the cornea
• cornea bends the light to focus on the retina (does 2/3 of the focusing)
• light goes through aqueous humour
• goes through pupil
• light goes through the lens and the lens does a bit of focusing (tension due to zonules of zinn)
• light contines through the vitreous humour and then hits the back of the eyeball, forming an image on the retina

Question 7

focussing

Answer 7

Focussing = recombining rays from various directions to form a single point on the imaging surface
Cornea - is curved, light refracts a constant amount and the cornea has greater refractive power.
Lens - refracts light by a variable amount –> accommodation = lens can be stretched to allow focussing of far objects

If light source is close - need lots of refraction
If far away - you’ll need less refraction

Question 8

Focussing errors

Answer 8

Emmetropia: normal refraction (appropriate focus)
Myopia: near / short-sightedness
- focal length too short
- light is focussed in front of the retina
- need concave corrective lenses
Hyperopia: far / long sightedness
- focal length is too long
- light focussed behind retina
- need convex corrective lenses
Presbyobia
- inability to change accommodation (old age)
Astigmatism
- different focal lengths for different orientations

Question 9

Transduction - rods and cones

Answer 9

Retina contains light-sensitive photoreceptors
- Rods: high sensitivity (night vision)
- Cones: low sensitivity (daytime)
Rod/cones pass electrical impulses to ganglion cells
- light goes through the ganglion cells, amacrine, bipolar, horizontal cells, and then falls on the rods and cones (which are photosensitive)
- ganglion cells have long axons that exit the eyeball via the ‘optic nerve’ (blindspot)

Question 10

Define fovea and optic disk

Answer 10

Fovea = region packed with photoreceptors (highest acuity), many receptors and no blood vessels
Optic disc = ganglion cells axons leaving the eye through the optic nerve (no receptors here = blind spot)

Question 11

Ganglion cell selectivity

Answer 11

One ganglion cell receives input from many photoreceptors

The firing of a ganglion cell could be affected by light falling over a range of locations on the retina

These photoreceptors represent the receptive field for these ganglion cells.

Question 12

Describe the receptive fields

Answer 12

• receptive fields for foveal vision are smaller (more densely packed = higher acuity)
• in the periphery, receptive fields are larger and less dense
• cortical magnification (larger area of cortex processing foveal vision than for peripheral

Question 13

Pathway from eye to brain

Answer 13

• signals from the retinae go along the optic nerve
• retinal ganglion cell axons terminate in lateral geniculate nucleus
• crosses over at the optic chiasm - partial decussation (left receptive field goes to the right side of your brain, right receptive field goes to the left side of your brain)
• LGN projects to primary visual cortex in the occipital lobe via optic radiations

Question 14

Centre surround antagonism

Answer 14

Ganglion cell receptive field has 2 concentric areas (two zones which are spatially next to eachother in RF that work in opposing ways)

ON CENTRE: light falling on inner part is excitatory, light falling on outer part is inhibitory
OFF CENTRE: light falling on inner part is inhibitory, light falling on outer part is excitatory

For on-centre:
- the optimal stimulus is a central spot of light
- light all over or no light causes spontaneous activity
- stimulation in the surround reduces firing rate

Question 15

Why does centre surround antagonism occur?

Answer 15

2 reasons
- tells us where the changes are in the image (exaggerates edge, defining boundaries)
- allows us to compensate for intensity of the light source (sensitivity to CONTRAST)

Question 16

What do OFF centre cells show us?

Answer 16

• shows us how dark an area is, helps detect local luminance decrements
• they compliment the on centre cells
• ensures that dark spots are detected as easily as light ones

Question 17

Properties of the LGN

Answer 17

• 6 layers
• each is retinotopically organised (neighbouring cells have retinal RFs next to each other)
• all cells are monocular (both eyes have inputs to LGN, but each eye goes to separate layers –> 1,4,6 from contralateral eye, 2,3,5 from ipsilateral eye)

Magnocellular v Parvocellular
- large / small RFs (low/high resolution)
- fast/small response
- high / low sensitivity
- process motion / R-G colour

Question 18

The cortex: V1 cell properties - SELECTIVITY

Answer 18

Selective orientation tuning is when cells respond to an edge or bar with a particular preferred orientation within its receptive field.
- bandwidth = the range of orientations to which the cell fires lots is a measure of its bandwidth (small bandwidth = sharp tuning)

Question 19

What are filters

Answer 19

• filters separate things on the basis of a given property (audition –> frequency)

Question 20

Discuss the V1 organisation:

Answer 20

V2 is organised into orientation columns
In a column, cells take inputs from the same eye

Each cube will have two sides –> one is driven principally by signals coming from the right eye, one from the left

Question 21

What are the different types of V1 cells?

Answer 21

Simple cells: respond to an oriented stimulus in a particular location within their receptive field (bar / edge detectors) –> excitation in the lighter colour

Complex cells: respond to an oriented stimulus anywhere within their RF (doesn’t matter where the edges are in their receptive field)

Hypercomplex cells: respond to an oriented stimulus but prefer stimuli with an end within the RF (end-stopped)

Question 22

Discuss the building of cells for selectivity –> simple, complex and hypercomplex cells

Answer 22

AND way
Simple cells: By connecting LGN cells in a line, we get orientation selective cells. The inputs of ganglion cells whose RFs are appropriately arranged.

OR way
Complex cells: connecting several simple cells, same orientation preference and different edge locations (doesn’t matter where the bar ends)

Hypercomplex cells: ‘endstopping’ by joining complex cells

• results in illusory contours and orientation aftereffect

Question 23

Orientation channels: orientation aftereffect
How does this occur?

Answer 23

Orientation aftereffect: where prolonged exposure to tilted bars biases vertical lines to appear tilted in the opposite direction
- because orientation utilises population coding and adaptation occurs (decrease in sensitivity and reduced firing rate in comparison to the other side of the population coding)

Question 24

Size after effect

Answer 24

cells are tuned to the width of bars/stripes
adapting to fat stripes makes mediums stripes look thin

Question 25

What are gratings? How does this correspond with spatial frequency?

Answer 25

Gratings have a sine wave luminance profile
Rather than stripes being fat or thin, its the amount of stripes that fit into a set space on the retina.

This is the spatial frequency
i.e. fat stripes have a low spatial frequency
thin stripes have a high spatial frequency

Question 26

Fourier analysis

Answer 26

SF = size
Phase (starting with a peak / trough) - are you starting with a black stripe or a white stribe
Orientation = rotation
Contrast

Question 27

Spatial scales

Answer 27

Low SFs carry course scale information (background)

High SFs carry fine scale information (fine details)

i.e. low pass filter takes out the fine details (high spatial freq) and high pass filter takes out the background (low spatial freq)

Question 28

Contrast sensitivity function

Answer 28

Tend to plot sensitivity and spatial frequency
Resolution limit is the point that it doesn’t matter how much you turn up the contrast, you won’t be able to see them as distinct lines because the spatial frequency is too high.