Karteikarten
Lecture 7 Vision
Karten in diesem Stapel (44)
- 1Name the structures of the eye and their functionCornea: focuses light, fixed
Pupil: the aperture in the iris, changes size according to light levels
Lens: focuses light, adjustable accommodation
Retina: rods and cones are cells in the retina that convert light into an electrochemical signal - 2Label the diagram of the eye
- 3—
- 4What is the electromagnetic spectrum and how much is comprised of visible light?The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength.
- 5Name the properties of a cone cell...Cones are mostly in the fovea. 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. - 6Name the properties of a rod cellRods are mostly in the periphery, outside the fovea. Rods are specialized for vision under low light conditions, and rods to not distinguish between different colours (check for yourself at night). The periphery is low acuity as outputs from hundreds of rod cells are pooled together.In summary, Rods are specialised for low light vision, movement detection and deciding where to look next by moving your eyes.
- 7What is the blind spot?Blind spot
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! - 8Why do people move their eyes about?
- 9What pathway does the rod cell contribute to? What pathway does the cone cell contribute to?Rod cells contribute to the Magnocellular pathway, which is most sensitive to luminance and motion in the periphery.
Cone cells contribute to the Parvocellular pathway. Most sensitive to colour and fine detail. - 10Describe the retinotopic organisation of in V1?Neurons in V1 have receptive fields: They fire when stimulus is present in a particular part of the visual field. Neurons with nearby receptive fields are anatomically close to each other in V1.
- 11—Most of the back of the brain, the ‘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
- 12The visual brain is divided into different anatomical regions, with each area having a distinct function. Name the main function of the V4, C5 and LO regionsV4 = Colour
V5 = Motion
LO = Shape
- 13Describe the way processing occurs in the brain. What are the areas which detect simple information?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). 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.
- 14How do we know about the visual system? What are the techniques which provide evidence about the specialisation of function?1. We can record brain activity using functional Magnetic Resonance Imaging (fMRI). 2. We can disrupt processing in brain areas with Transcranial Magnetic Stimulation (TMS). 3. We can measure the effect of accidental damage to specific brain regions in humans. 4. We can probe individual neurons in live animal brains with microelectrodes. When the ‘preferred’ stimulus is presented, the neurons fire.
- 15What is TMS and what effect does it have on areas V4 and V5?With Transcranial Magnetic Stimulation, we can temporarily disrupt neural processing in specific visual maps.
TMS to V5 produces a sense of motion
TMS to V4 produces coloured phosphines.
- 16What is fMRI?Functional magnetic resonance imaging or functional MRI (fMRI) is a functional neuroimaging procedure using MRI technology that measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled
- 17What is microelectrode recording?We can actually pick up various types of neural activity with microelectrodes.
Most simply, we can measure action potentials = neuros 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.
- 18According to the concept of functional specialization (Zeki, 1993) what do the areas V1 & V2 contain?V1 and V2 contain different groups of cells that respond to edges. Hubel and Wiesel are famous for recording orientation tuning of V1 cells in the cat. V1 has small receptive fields. Cells in these area only fire when their preferred stimulus is presented in a very small part of the visual field (e.g. near the fovea, or in the top left and so on).
- 19According to the concept of functional specialization (Zeki, 1993) what are the areas V3, V3a, V4 and V5 sensitive to?V3 and V3a are sensitive to form and the shapes of objects in motion. V4 is sensitive to colour. Cells in V4 fire when particular colours and presented. V5 is sensitive to motion. V5 cells respond to particular velocities (speed and direction). For example, a cell might fire when an object moves leftward across its receptive field at 20 degrees per second, but not at all when an object moves rightwards at 10 degrees per second. In humans V5 is called the MT.
MST cells respond to optic flow, the pattern of motion produced by self motion. - 20What area fires when colours are presented?Cells in V4 fire when certain colours are presented. 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 centre in the brain.
- 21What is Akinetopsia?Akinetopsia means difficulty seeing 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.” Zihl et al. (1983). 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 defined motion. Second order motion = Contrast defined motion. (this is quite difficult to describe without seeing demos, look up the terms online
- 22What is the binding problem?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?
- 23What is a receptive field????
- 24What does it mean to say cells in V1 are organized retinotopically?Retinotopic organization in V1 : Neurons in V1 have receptive fields: They fire when stimulus is present in a particular part of the visual field. Neurons with nearby receptive fields are anatomically close to each other in V1.
- 25What are Hubel and Wiesel famous for?They are famous for recording orientation tuning of V1 cells in the cat.
They recorded from neurons in the visual cortex of a cat, as they moved a bright line across its retina. During their recordings, they noticed a few interesting things: (1) the neurons fired only when the line was in a particular place on the retina, (2) the activity of these neurons changed depending on the orientation of the line, and (3) sometimes the neurons fired only when the line was moving in a particular direction.
- 26What is second order motion?Second order motion = Contrast defined motion. You can't distinguish the rectangle until it starts moving
- 27What is the ventral stream also known as?The 'What' stream as it is involved in identifying objects, is it a car?
V4 and LOC are the early parts of the ventral stream. This is an elaboration of the Parvocellular pathway.
Milner and Goodale (1995) are famous for work on the dissociation between ventral and dorsal streams.
- 28What is the dorsal stream also known as?Also known as the 'Where' stream as it controls actions. MT is the early part of the dorsal stream. This is an elaboration of the Magnocellular pathway. Milner and Goodale (1995) are famous for work on the dissociation between ventral and dorsal streams.
- 29What are Schenk and McIntosh four characteristics of the dorsal stream?1.Dorsal stream underlies vision for action
2. Coding in the dorsal stream is egocentric (body-centered, dependent on the observers perspective)
3. Representations in the dorsal stream are short-lasting
4. Whereas processing in the dorsal stream does not lead to conscious awareness - 30What are Schenk and McIntosh four characteristics of the ventral stream?1. The ventral stream underlies vision for perception
2. Coding in the ventral stream is allocentric (object-centered, independent of observers perspective)
3. Representations in the ventral stream are sustained over time
4. Processing in the ventral stream typically (but by no means always) leads to conscious awareness - 31What is double dissociation in regards to the dorsal and ventral streams?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.
- 32What symptoms do optic ataxi patients have in relation to the dorsal and ventral streams?Patients have damage to the posterior parietal lobe (part of the dorsal stream). Their profile partially fits with the theory. They have some difficult action planning, for example inserting a card through an oriented slot (although they are not a simple group). They can label objects reasonably well.
- 33What is visual form agnosia? Is it a double dissociation with optic ataxi?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.
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. - 34Which visual stream is most sensitive to visual illusions?Visual illusions might be unique to ventral stream
- 35What is achromatopsia?Trouble seeing colours
The most common lesion site was V4 indicating this area may be one of the colour centers of the brain - 36Which two researchers are known for the two visual streams theory?Milner & Goodale 1995
- 37A patient has damage to the posterior parietal cortex: Would they be poor at object recognition or rapid action?Patients with posterial parietal damage have optic ataxia - they have problems with making visually guided movements in spite of reasonably intact visual perception
A = rapid action
- 38Q) Which stream would be activated in an fMRI experiment where participants discriminate pictures of animals?
- 39List four distinctions between ventral and dorsal streams1. V stream underlies vision-for-perception whereas D stream underlies vision-for-action
2. Coding in V system is allocentric (object centered, independent of observers perspective), D coding is egocentric (body-centred, dependent on observers perspective)
3. Representations in V stream are sustained over time, whereas in D stream they are short lasting
4. Processing in V stream typically leads to conscious awareness, processing in D stream does not. - 40What wavelengths do each cone respond to?Blue = Short wavelength
Green = Medium wavelength
Red = Long wavelength
- 41What is a colour opponent channel? Why is it useful to for cones to be wired up in this way?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 some 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 the baseline rate. Another is the opposite way round (excited by green and inhibited by red). This gives us the red-green pathway.
We also have the yellow-blue pathway. - 42If I stare at a red dot, then look away, what colour after image is likely?Green
- 43Can you describe the concept of colour constancy?V4 cells respond to colour. They respond to colour of surfaces, independent of changes in the illuminant. Thus V4 cells show some colour constancy.
Colour constancy is the tendency for a surface or object to be perceived as having the same colour when the wavelengths contained in the illuminant (the light source illuminating the surface or object) change
Part of the story is cone excitation ratios. Say, at dusk, surface 1 makes red cones fire 1 time per second, and 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 3 makes them fire 30 times per second. The ratio is 1:3 both at night and at day. We still perceive the same color, because your visual system makes use of relative excitation across different surfaces.
Familiarity also matters. A perfectly grey banana looks slightly blue, because we expect it to be yellow (Hansen et al., 2006).
Chromatic adaptation: We slowly adjust to stable properties of light. Electric light doesn’t look yellow after a while. - 44What are cone ratios?Part of the story is cone excitation ratios. Say, at dusk, surface 1 makes red cones fire 1 time per second, and 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 3 makes them fire 30 times per second. The ratio is 1:3 both at night and at day. We still perceive the same color, because your visual system makes use of relative excitation across different surfaces.
Familiarity also matters. A perfectly grey banana looks slightly blue, because we expect it to be yellow (Hansen et al., 2006).
Chromatic adaptation: We slowly adjust to stable properties of light. Electric light doesn’t look yellow after a while.
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- Degree Psychology (Psych 105) Karteikarten am Lecture 7 Vision, erstellt von Molly Macgregor am 19/11/2016.
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