Week 4-Low Level/Visual Perception

Question 1

What is the structure of the eye?

Answer 1

• Cornea: focuses light, fixed
• Pupil: the aperture (coloured part) in the iris, changes size according to light levels

-Photoreceptor cells (rods and cones)

• Lens: focuses light, adjustable ——> accommodation
• Retina: rods and cones are cells in the retina that convert light into an electrochemical signal

-Light enters the eye through the pupil and hits the back of the retina

-Hits the retina and then triggers the photoreceptors which in turn triggers the action of other cells e.g., ganglion cells, then going off into the optic nerve to different parts of the brain

Question 2

What is the Retinal Structure?

Answer 2

-Light-sensitive receptors (rod and cone cells) that convert light into electrical activity ~125 million rods per eye and ~ 6 million cones per eye

-Various types of nerve cells (neurons)

-Nerve fibres (axons of neurons) - around 800,000 per eye; these POOL (i.e., combine) signals from multiple rod or cone receptors

Question 3

How do the rod and cone receptors send signals?

Answer 3

-When enough light falls on a rod
or cone cell in the retina, that cell
sends an electrical signal down
its nerve fibre (an axon).

-This signal is like an action
potential. It triggers a chemical
signal to another neuron which,
in turn, can send signals to other
neurons.

-The rods and cones convert light
into neural signals - the language
of the brain.

Question 4

What is the electromagnetic spectrum?

Answer 4

-Is a spectrum containing short, medium and long wavelengths of varying kinds e.g., X-rays, UV rays, TV rays etc.,

-Visible light is only a tiny part of it (roughly 380-700nm)

-nm = nanometres = 10-9 metres or one billionth of a metre, so very, very small; 1000 nm = one millionth of a metre

-400nm=short wavelengths

-500nm=medium wavelengths

-700nm=long wavelengths

Question 5

What are the roles of rod cells?

Answer 5

-Achromatic night vision

-1 type

*Are mostly in the periphery, outside the fovea.

*Are specialized for vision under low light conditions.

*Do not distinguish between different colours (check for yourself at night). The periphery is
low acuity (function at low light levels) as outputs from hundreds of rod cells are pooled together.

Question 6

What are the roles of cone cells?

Answer 6

-Daytime, achromatic and chromatic vision

3 types:
1. Long-wavelength-sensitive (L) or “red” cone

2. Middle-wavelength-sensitive (M) or “green” cone
3. Short-wavelength-sensitive (S) or “blue” cone

*Are mostly in the fovea (little dent in the eye)

• The eye has short, middle and long wavelength cones, which detect different colours.
• The fovea is high acuity as outputs from only a few cone cells are pooled together (pupil is smaller with light and larger in the dark)
• Cones are used in normal lighting conditions and in most situations. We move our eyes to bring important things onto the
  fovea (as we don’t tend to look at things peripherally unless sneaking a peek!)

Question 7

Define what is meant by the Blind spot

Answer 7

Where the optic nerve leaves the retina so no rod or cone cells to detect light (Brain FILLS IN the gap, so we don’t SEE a black spot!)

Question 8

What is meant by acuity high?

Answer 8

-Acuity high: fine detail visible at fovea Information processed by CONE cells

-The central dot shows the fixation point in the image, which is where information in the image would reach retinal receptors in the fovea.

-We are constantly moving our eyes to control what information falls on the fovea.

-Stimuli need to be much larger to be identified at periphery

-Information processed by ROD cells

-This shows the SIZE that letters need to be so they are recognised
equally well at different distances from the fixation point.

Question 9

How does information go from the eye to the cortex?

Answer 9

• Most of the back of the brain (‘posterior regions’) are dedicated to visual processing.
• Nerve fibers go from the eye to the Lateral Geniculate Nucleus in the Thalamus (LGN), and then to the primary visual cortex (V1)
• The right visual field projects to the left cerebral hemisphere
• And the left visual field projects to the right cerebral hemisphere.

Question 10

What are the rod and cone pathways to the visual cortex? (Solomon, 2021)

Answer 10

1. Cone cells (L & M)=Parvocellular pathway (sensitive to colour and fine detail)
2. Cones and rod=Magnocellular pathway (sensitive to luminance and motion in the periphery)
3. Cones (S & M)=Koniocellular pathway (mediates mostly colour vision: Yellow-blue pathway)

Question 11

How is the visual brain divided?

Answer 11

-The visual brain is divided into different anatomical regions, with each area having a distinct function.

-Most cells in V1 and V2 only detect simple information (eg oriented lines) whereas cells at
later stages may, for example, respond to complex shapes and motion patterns:
V4=Responds to Colour
V5=Responds to Motion
LO=Responds to Shape

Question 12

How does the visual information travel to the brain? (Mather, 2009)

Answer 12

-Processing cascades up through the different areas of the brain, starting from the LGN and then to the visual cortex (V1, V2, V3, V5 and V5) and to IT (inferotemporal cortex).

-Information from the LGN to the V1 takes 10ms.

Question 13

What are some methods which have allowed us to learn everything we know about the visual system?

Answer 13

• We can disrupt processing in brain areas with Transcranial Magnetic Stimulation (TMS).
• We can record brain activity using functional Magnetic Resonance Imaging (fMRI).
• We can measure the effect of accidental damage to specific brain regions in humans.
• We can probe individual neurons in live animal brains with microelectrodes. When the ‘preferred’ stimulus is presented, the neurons fire.
• When it comes to the visual system, all these techniques provide converging evidence about the specialization of function.

Question 14

What can we disrupt with TMS?

Answer 14

-With Transcranial Magnetic Stimulation, we can temporarily disrupt neural processing in
specific visual maps.

-TMS disrupting V5 produces a
sense of motion.

-TMS disrupting V4 produces
colored phosphines.

-TMS is non-invasive, reversible and only disrupts the brain temporarily

Question 15

Define what is meant by Brain lesions

Answer 15

• Brain lesions = holes in the brain
  caused by accidents, strokes, tumors etc.,
• Lesions often have specific effects, loss of language, difficulty seeing faces.
• Some lesions can change personality dramatically (As in the famous case of Phineas Gage)

Question 16

What is Akinetopsia? (Zihl et al., 1983).

Answer 16

-Akinetopsia means difficulty seeing and perceiving motion. It is caused by discrete damage to area MT (also known as V5). This is a description of a patient with Akinetopsia:

-“She had difficulty .. In pouring tea or coffee into a cup because
the fluid appeared to be frozen, like a glacier. In addition, she
could not stop pouring at the right time since she was unable to
perceive the movement in the cup or pot when the fluid rose”.

-In a room where more than two people were walking… “people
were suddenly here or there but I have not seen them moving.”

Question 17

What is the consequence when the MST is damaged?

Answer 17

• Damage to area MST selectively impairs optic flow processing which we use for walking (Vaina, 1998).
• First order motion and second order motion are different. First order motion = luminance (shadows) defined motion. Second order motion = Contrast defined motion

Question 18

What happens with V4 lesions?

Answer 18

-Cells in V4 fire are to some extent selective to colours (but broadly tuned).

-There are no neurons that are only driven by red, yellow, green etc.,

-Bouvier and Engel (2006) ran a “meta analysis” of all lesions which create achromatopsia (trouble seeing colours).

-The most common lesion site was V4. Some colour processing is spared after V4 lesions – so this is not the only colour center in the brain.

Question 19

What can we measure with microelectrode recordings?

Answer 19

• We can actually pick up various types of neural activity with
  microelectrodes.
• Most simply, we can measure action potentials = neurons firing electrical pulses down the long axon.
• Firing rate increases when the neuron is active.

-When we say a neuron ‘responds’ to something, we mean there is an increase in firing rate when an animal looks at one stimulus, but not at another.

Question 20

When do neurons fire in the inferior temporal cortex?

Answer 20

-In the inferior temporal cortex, there are neurons that fire when we see certain shapes but not others.

-A cell might fire very strongly when a monkey is looking at a star, but not at a circle.

Question 21

What is the “binding problem”?

Answer 21

• If we had different groups of neurons responding to different features, and these neurons are in different neuroanatomical regions, how we group the features which belong to a particular object.
• We have color cells in V4 and shape cells in the inferotemporal cortex.
• How do we know that the blue goes with the circle shape, yellow goes with the star, and red goes with the square?
• Neurons switch between intervals of low and high excitability rapidly. All neurons that code features of a single object oscillate in synchrony (Singer and Gray, 1995). There is a synchronized assembly of thousands of neurons.
• But how does the brain synchronize the right neurons?

Question 22

What is the ventral and dorsal stream?

Answer 22

1. Ventral stream is also called the ‘what stream’. It is involved in identifying objects. (Is it a car? Is it a plane?)
2. The Dorsal stream is called the ‘where stream’ (It controls actions).

Question 23

What is meant by double dissociation? (Milner and Goodale, 1995)

Answer 23

• Milner and Goodale (1995) are famous for work on the dissociation between ventral and dorsal streams:

Double dissociation: Damage to ventral stream should impair
object recognition but not action planning, while damage to dorsal stream should impair action planning but not object recognition.

Question 24

What is Optic ataxia?

Answer 24

Damage to the posterior parietal lobe (part of the dorsal stream). Patient profile partially fits with
the theory as they have some difficult action planning.
E.g., inserting a card through an oriented slot (although they are not a simple group). Can label objects
reasonably well.

Question 25

What do Visual form agnosia patients experience?

Answer 25

Visual form agnosia patients have the opposite problem. DF cannot name drawings of objects. However, she is very good at in inserting a card in a slot.

Question 26

How are optic ataxia and visual form agnosia not the perfect double dissociation?

Answer 26

Optic ataxia and visual form agnosia patients are not the perfect double dissociation the theory demands. Both have some impairments both with action planning and object recognition.

Question 27

What are the 4 characteristics of the dorsal and ventral stream? (Schenk & McIntosh, 2010)

Answer 27

1) The ventral stream underlies vision for perception, while the dorsal stream underlies vision for action.

2) Coding in the ventral stream is allocentric (object-centered, independent of observers perspective), while coding in the dorsal stream is egocentric (body-centered, dependent on the observers perspective).

3) Representations in the ventral stream are sustained over time,
representations in the dorsal stream are short-lasting.

4) Processing in the ventral stream typically (but by no means always) leads to conscious awareness, whereas processing in the dorsal stream does not.

Question 28

How might visual illusions be unique to the ventral stream?

Answer 28

-When people grab the central circle quickly, they supposedly used the dorsal stream. On average, grip aperture is only 5.5% wider when approaching circle 2 than circle 1.

-When people verbally estimate the diameter of the central
circle, they supposedly use the ventral stream. The illusion is
22% (i.e., they only use their perceptual system not their action system).

Q. Why is the illusion zero in the grabbing and pointing
tasks?
If the action is slow and measured, the effect of the illusion is
larger. It is likely that representations in the ventral stream can be used to guide some types of more deliberate action.

Question 29

What do the three cone types respond to in the retina?

Answer 29

1. S cones respond best to blue light.
2. M cones respond best to green
   light.
3. L cones respond best to red light.

Question 30

Give an example of how colour isn’t just physics

Answer 30

-These two metamers of yellow (example on slide).

-Though physically very different (one pure yellow, another made from red and green to get yellow), can appear identical (i.e., it’s not a 1-to-1 mapping).

Question 31

How does colour vision work in the retina?

Answer 31

-The human retina contains only 3 kind of cone cells (L, M, S), Human colour vision is trichromatic. The cones form specific excitatory and inhibitory connections on bipolar cells.

• One kind of bipolar cell is excited by red cones, but inhibited
  by green cones.
• If faced with a white patch, the bipolar cell fires at a baseline
  rate. If faced with a red patch, the cell is excited, and fires rapidly.
• If faced with a green patch, it is inhibited, and fires at below
  baseline rate.

-Another is the opposite way round (excited by green and inhibited by red).

Question 32

In the brain the colour signals are recombined into which opponent channels?

Answer 32

RED-GREEN BLUE-YELLOW

But there is no ‘greenish red’ or ’bluish yellow’ Why?
-This is because of colour opponent channels. When we stare at a colour for a long time, the response of cones fatigues. When we look at a white area, the opposite colour dominates. This
gives us coloured after images.

Question 33

What is colour constancy?

Answer 33

How can your brain tell that things are the same colour when the lighting conditions vary dramatically?

• The wavelengths reflected from objects are very different in the morning and the evening, but the colour looks about the same.
• Likewise, some artificial illuminants are yellow, others are white. This doesn’t have a dramatic effect on our sense of object colour.
• Chromatic adaptation: We slowly adjust to stable properties of light. Electric light doesn’t look yellow after a while.
• When problems like colour constancy start to look difficult, you are mastering vision science! We take vision for granted. But it is deeply mysterious.

Question 34

Give an example of colour constancy

Answer 34

• Part of the story is cone excitation ratios.

Imagine you look at a mug on a table:
* At dusk, Surface 1 makes red cones fire 1 time per second, +
Surface 2 makes them fire 3 times per second.

• In the day, surface 1 makes the red cones fire 10 times per second and surface 2 makes them fire 30 times per second.
• The ratio is 1:3 both at night and at day.
• We still perceive the same colour, because your visual system makes use of relative excitation across different surfaces.

Question 35

What are Monocular or pictorial cues?

Answer 35

Monocular or pictorial cues are those we can use use when
one eye is shut.

Question 36

What is Linear perspective?

Answer 36

Parallel lines converge on the horizon.

Question 37

What are Texture gradients?

Answer 37

These can be perceived as a surface receding into the distance. The finer texture is further away.

Question 38

What is Occlusion or interposition?

Answer 38

Near things occlude far things.

Question 39

What is familiar size?

Answer 39

If we have a sense of typical size, and it looks small on the retina, we can assume its far away (Ittelson,
1951).

Question 40

What is Motion Parallax?

Answer 40

f I move my head, everything moves across the retina. Near things move further than far things.

Question 41

Binocular and oculomotor cues: What is vergence?

Answer 41

If we are looking at something really close, our eyes turn inwards.

Question 42

Binocular and oculomotor cues: What is accommodation?

Answer 42

The lens changes shape to focus on near and far things, once something is in focus. Lens shape tells us the distance of the object in focus.

• Both vergence and accommodation are very limited
  (can only tell me about the position of a single object)

Question 43

Binocular and oculomotor cues: What is Binocular disparity?

Answer 43

The image on each eye is slightly
different. The nearer something is the greater the difference in retinal position on each eye.

• 3D cinema works by presenting slightly different images to each eye. This ‘really looks 3D’

Question 44

Depth perception: What is cue combination?

Answer 44

• Usually there are a lot of different depth cues available – How does the brain integrate them?
• Average estimate based on all cues? Always rely on just one of the cues?
• What we actually do is something like weighted average (Jacobs, 2002). We put more
  weight on the more reliable cues, and reliability estimates can be updated. Note how effortless the visual system does this this complex operation.
• How do we actually make a weighted average estimate out of neurons? How would you
  wire up a load of neurons to do mathematical operations like this? When problems like this start to look difficult, you are mastering vision science! We take vision for granted. But it is deeply mysterious.