SENSATION & PERCEPTION EXAM QUESTIONS 1. Which of the following best describes the Platonic view of vision and the world? Platonic view: There is a real world out there, but due to our limited senses, we can only take in/absorb some of it. There is a 'real world' but our mortal senses are only capable of sampling a small subsection of it. The world - the ‘shadows on a cave wall’ analogy. Our own physical experience of the world makes up our reality. Vision is limited to particular spatial dimensions (that is, we cannot see everything in the world at the same time). THERE IS AN OBJECTIVE WORLD. Link to Aldous Huxley’s utilitarian filtered vision. The owl’s infra-red vision vs. human visible spectrum (we only see what is biologically useful to us). 2. Which of the following is the alternative view which is illustrated in the writing of Friedrich

Nietzsche? There is no real world, only the world inside your head. The world is a construction of the brain. We construct our reality. Our reality is subjective. Quote: “The apparent world and the true world means – ‘the world’ and ‘nothing.’” – Nietzsche. Vision = purely a constructive process.

3. The world that we visually sense is entirely dependent upon… LIGHT and our detection of it. We experience light as color, brightness, and saturation. The photoreceptors in the retina detect this light, and are the only form of direct contact between the external visual world and our internal visual system. 4. Excitation refers to……. The triggering of neurons, such as photoreceptors, which causes an electrical impulse to be transmitted. The production of an output/response. The detection of a stimulus. Occurs at the level of the retina, where the detection of photons converts the light message into an electrical form within neurones.

5. Sensation refers to……. What you experience. The output of excitation. Sensation is the process by which physical energy from objects in the world or in the body stimulates the sense organs. The brain interprets and organizes this sensory information in a process called perception. 6. At what stage in the visual pathway is an action potential first generated? The retinal ganglion cells. This output is then received by the bipolar neurons and photoreceptors. Sparknotes: The very first stage, where the photoreceptors of the retina convert the detection of photons into an electrical impulse (action potential). Transduction (conversion from electromagnetic to electrochemical energy) occurs at the level of the photoreceptors. Visual pathway: Photoreceptors (rods/cones) of retina Bipolar neurones Ganglion cells (axons make up optic nerve) Lateral Geniculate Nucleus (LGN) Primary visual cortex. Notes: The outputs of the photoreceptors are received by the bipolar neurones which themselves do not 'care' or 'know' that those signals originated as energy in a photon, they are neurones which respond in some specific way to their inputs.

The retina has millions of photoreceptor cells called rods and cones. Rods and cones connect via

synapses to bipolar neurons, which connect to ganglion cells. The axons of the ganglion cells make up

the optic nerve, which connects to the eye at the optic disk, also called the blind spot.

7. What do L,M and S denote when applied to cones? The different wavelengths of light (long, medium and short) that are detected by the cones.

We have 3 types of cones, each sensitive to a different wavelength of light. There are cones that are most sensitive to short wavelengths, cones that are most sensitive to long wavelengths, and other cones that are most sensitive to middle wavelengths. A cone has an area of space over which it will absorb photons within a specific range of wavelengths. The sensitivity range is L/M/S, over which the receptive field detects photons.

Notes: Trichromatic theory, or the Young-Helmholtz theory, states that there are three types of cones in the retina, which

are sensitive to light of different wavelengths corresponding to red, green, or blue. This theory accounts for colour

blindness.

8. What reason may you have for expecting the photoreceptors to be physically anchored on the retina despite the observation that this means that there is neural matter between the light source and the photoreceptor? If the photoreceptors were free to move and were not anchored on the retina, the image captured by the retina would be constantly moving, and no full concrete image at one point in time over a particular space could be observed. We wouldn’t be able to make meaning out of such a dynamic view of the world, so the photoreceptors must be physically anchored on the retina to allow the full picture to be detected smoothly. Also if the cones are moving, the sharpness and edges of the image cannot be observed (as the L/M/S cones would be running all over the place), and the image would be blurry. 9. A colour space is…… A colour space is a research tool or a method used to represent a particular light and colour in a way that is unique and replicable. It provides a quantitative framework on which to build a view of the system’s first stages. Eg. Conespace, Cardinal space (Both relate to the EXCITATION stage of processing), and Munsell space (relates to the sensation of colour, and can be further specialised into a ‘Unique Hue’ – red-green, blue-yellow). Each of these colour spaces relates best to a particular level of processing within the system. It is the PERCEIVED colour that is important, not the cone excitation involved. 4 primary colours which are the main hues, while the remaining colours are perceived as mixtures of these 4 primary colours and are more subjective as they are based on cultural/abstract/language values. Language restricts sensations eg. names given to colours.

10. Orthogonality in the vector representation of signal-coding confers which of the following

properties upon that stage of the system?

A receptive field smaller than the edge it detects will only be able to signal motion

perpendicular/orthogonal to the orientation of the edge.

The aperture problem = the receptive fields are very restricted to being sensitive to only a tiny region of

space, so by themselves they cannot detect any motion as there is no reference point present, but when

we look at the outputs of all the receptive fields (esp. the edges/ends of a pen for example), we can then

get a sense of motion occurring.

11. Which statement best describes a receptive field? The concept of a receptive field is like a leaf on the web. It helps you to see it is there, to understand its structure, but it is not the system itself. So whilst there is an area of the visual field and a temporal period that relates to the receptive field dimensions, and that space-time volume is critical to the properties of the receptive field, the receptive field itself does not exist in space or time. Therefore, there is no problem with the receptive fields overlapping in their sensitivity (in fact it is critical to the operation of the system) and there is no problem 'fitting them in' to the visual system. The overlap is functional rather than physical. Ehile the number of receptive fields within the visual system is ultimately limited by the number of neurones available for vision, the location and size of the receptive field is not related to the physical size of the neurone. The second point is that, related to this issue of spatiotemporal volume, it does not matter to the properties of a receptive field where the neurone it describes is actually located. 12. What is the most likely format of the first spatially structured (or differentiated) receptive fields in the visual system? Centre-Surround format, with centre = increases output while periphery reduces the output (inhibitory).

Striate Cortical arrangement - Regular arrangement of primary visual cortex allows/facilitates interaction

between neurons because they’re close together. As you go across the cortex, the receptive fields change

in orientation, and as you do down, the size of the receptive fields get bigger.

13.The receptive field arrangement referred to in Question 12 confers what properties to the system? Centre- surround arrangement results in cone- opponency. 14. The term “opponency” refers to…..

The opponent process theory states that receptors act in opposite ways to wavelengths associated with 2

pairs of colours: red vs. green, blue vs. yellow. The theory accounts for the perception of four primary

colours. It also accounts for afterimages (the colours perceived after the complementary colours are

removed).

Commentary (‘What is Red’):

Hering suggested that the first stage of vision involved 2 complementary phases, trichromacy and

Opponency. Trichromacy = visual system is made up of just 3 receptors. The outputs of these receptors

are compared with each other to reconstruct all the colours we see. (Young & Helmholtz)

Opponency = visual system is made up of 4 receptors that are wired into pairs that act in opposition to

one another: red-green and blue-yellow. Hering’s opponent mechanism exists at both levels of excitation

and sensation. o Pairing system = Centre and surround (cone Opponency) Colour

o Different wavelengths of light absorbed in centre and surrounding regions of receptive

field.

5. Why, when considering the processing of the neural signal, is the actual physical location of any visual neurone other than the photoreceptors, arbitrary? Because the neurons are just dealing with information, it doesn’t matter how they are arranged and

where they are located, as long as the connections between the neurons are kept. The only component of

the visual system that HAS to be where it is, is the retina, because it’s the thing that has to receive the

input from the outside world. Beyond the retina, the parts of the visual system could be anywhere.

Once the photoreceptors have encoded the relevant information it doesn't make any difference where

that info is processed.

While the regular arrangement of cortex allows/facilitates interaction between neurons as they’re close

together, it IS possible for all the neurons in the brain to be arranged in an irregular, complex pattern as

long as the connections are all effectively maintained We would still get the same result (we would see

perfectly well).

(But in a biological system it is more likely that the brain is structurally organised in a particular way as it

is easier to understand the arrangement of neurons this way).

16. Why, however, is the relative location of any sensory neurone potentially important? Because sensory neurones have to receive the input from the outside world, and this input is dependent on the nature of the stimulus (where it is for eg.). The relative positions of the sensory neurons allow different information to be taken in from the external world from different perspectives/angles, so in order to get the full picture, the arrangement of sensory neurones is important.

This is because any other imagery that we perceive is first and foremost detected by the photoreceptors/neuron. Without this initial point of contact, everything else does not exist since all direct contact with the image is lost; the relative location of any sensory neuron is important because lines/contours next to each other in the 'real world' have to be represented next to each other in the mind, otherwise they would just be random lines and we wouldn't be able to work out for example the outline and consequently the shape of something. 17. What is meant by the term “retinotopic mapping”? The neurons are organized in an orderly fashion forming a 2D representation of the visual image formed

on the retina in such a way that neurons adjacent to each other in the cortex are sensitive to adjacent

areas in visual space.

Retinotopic maps = the external world as it is projected on the retina has a 2-dimensional arrangement, a

particular x,y coordinate map, where there’s something happening at every point. That representation is

topographically (relatively) maintained early on in the visual system.

The retina has a topographic map (retinotopic map of the world) as it is a sheet of photoreceptors. Each

layer of the LGN, cortex (v1-5 etc.) also has this 2-D version of the world such that neurones adjacent to

each other in cortex are sensitive to adjacent areas in visual space.

18. The term “phase-coherence” refers to the hypothesis that….. An edge or border is signalled consistently across all scales of analysis. 19. The idea of modularity in visual processing refers to…… After v1 there appears to be some segregation of processing and the presence of multiple retinotopic maps (versions of the world). But the idea of modularity is that it is more efficient to analyse a complex input in modules in parallel than it is to do it serially. It appears that this is how the visual system works (to a first approximation). Marr, D. (1982). How various components of an input (eg. Colour, depth, motion) are processed separately but at the same time (instead of in a sequence, one after the other). Cortical modularity While there is specialisation of the cortex in terms of function, there isn’t complete

separation of those brain parts – interactions are maintained even though different brain parts may be

doing different things with the input.

Different properties of the visual world (colour, motion, form etc.) require different processing solutions

which are implemented in anatomically/functionally distinct regions that operate independently – that is,

in a modular fashion.

20.What reason may you have for questioning the assumption that the LGN is just a relay station for signals traveling from the retina to the cortex? LGN = part of thalamus. Most signals that enter the cortex pass through the thalamus (LGN), indicating

that it might be doing something important. Simplest reason is that there needs to be a cell body in

between a retinal cell and cortical cell as the axon would be too long. But, LGN may do something more

than just relay the information. Also, there are more connections carrying signals from the cortex back to

the LGN than from the LGN to the cortex (so more stuff going into the LGN than coming out of it).

Idea: The LGN could be changing the information/outputs of the neurons as a group (instead of

independently) Combining output.

Note:

LGN has centre and surround receptive fields. No change in sensitivity profile in retina and LGN

therefore viewed as useless/no function as it seems to do the same thing as retinal ganglion cells. The

LGN cells receive inputs from retinal ganglion cells whose receptive fields are similar in size and shape to

the LGN receptive fields. They are also not sensitive to the orientation of the input same output

received regardless of orientation of input.

21. Taken as a population, primary visual cortical (v1) neurones have what critical property? Primary visual cortex represents every aspect of the image that you see, but in a sparse (spread out) fashion. (It receives all the visual inputs at this point, and begins to get specialised). The receptive fields are selective for rudimentary properties of colour, motion, depth and form. NOTE: Primary Visual cortex = V1 = Striate cortex. 22. An example of context-dependency in vision is...... Straight matchsticks arranged into a curve gives the whole impression of a curve even though individual matchsticks are straight. The image is obtained depending on the context/circumstance/positions the individuals components are in. 23. What is red? Linking to the Platonic view, we can’t describe red without using the word colour to someone who hasn’t seen red. The Qualia problem = We have different experiences of a particular colour – How do we know that when

you and I see a colour that we both call ‘red,’ we are actually having the same colour experience?

Red is a colour that is subjective and its perception varies depending on the experience of each individual. The world we perceive is a construct of the operation of the system and hence red to me would be a different hue/shade to someone else just because of the perceptions and associations we form to the colour.

24. An attentionally-controlled motion system may…. Attention allows you to ‘select’ a particular aspect of the input – Is this a critical link between seeing something move and doing something with it? Allow you to see a moving object in amidst a sea of other moving objects, and essentially as the retina moves with the moving object, the object itself does not represent itself as moving. 25. The visual system appears to dissociate motion-signals elicited by eye-movements or from retinal motion by..... There are two potential mechanisms to allow the brain to deal with the conflicting signals – Inflow Hypothesis (Sherrington 1857-1952) – Outflow Hypothesis (Helmholtz 1821-1894)

Inflow Hypothesis

Send a message to the eye. The muscles in the eye send that info back to the brain. Brain uses this info

discriminates between eye movement and motion in the environment. (Using feedback from eyes). The

Inflow hypothesis relies on the feedback from the eye movement to account for the effect on retinal

motion.

The outflow Hypothesis

The Outflow hypothesis uses the same signal that ‘commands’ the eye movement. So the effect of retinal motion is ‘subtracted’ from the motion in the environment. The eyes and the brain use the same signal to account for motion of the eyes. It is therefore quicker and probably more accurate.

27. “Adaptation” refers to...

Reduced sensitivity to stimulation as a result of repeated exposure to stimulus. The stimulus doesn’t

disappear—we just become less sensitive to it as it is unchanging and therefore not as important as a

stimulus that is dynamic.

28. What is meant by the term “parallel processing”... Parallel processing is the ability of the brain to simultaneously process incoming stimuli of differing quality. This becomes most important in vision, as the brain divides what it sees into four components: color, motion, shape, and depth. These are individually analysed and then compared to stored memories, which helps the brain identify what you are viewing. The brain then combines all of these into the field of view that you see and comprehend. (The ability of the brain to do many things/processes simultaneously).

29. The terms M and P in the context of the visual system refer to.... The 6 layers of the LGN include: Retinal P cells (parvocellular – 4 layers) and retinal M cells (Magnocellular – 2 layers). There are larger cell bodies in the magnocellular layers than there are in the parvocellular layers. 30. Information is... Surroundings, image (depth, motion colour) which is processed by the visual system 31. A vector is... A motion vector is a signal indicating the direction and speed of some aspect of an image on the retina.

Signal is most likely to be a luminance-defined border or a colour border. The vector provides the initial seed or fragment of what ultimately becomes our sense of motion in the image. 32. The theoretical hierarchy established by David Marr is……

In the 1970s David Marr developed a multi-level theory of vision, which analysed the process of vision at

different levels of abstraction. In order to focus on the understanding of specific problems in vision, he

identified three levels of analysis: the computational, algorithmic and implementational levels.

The computational level addresses, at a high level of abstraction, the problems that the visual system must

overcome. The algorithmic level attempts to identify the strategy that may be used to solve these

problems. Finally, the implementational level attempts to explain how solutions to these problems are

realized in neural circuitry.

Marr suggested that it is possible to investigate vision at any of these levels independently. Marr

described vision as proceeding from a two-dimensional visual array (on the retina) to a three-

dimensional description of the world as output. His stages of vision include:

A 2D or primal sketch of the scene, based on feature extraction of fundamental components of the

scene, including edges, regions, etc.

A 2½ D sketch of the scene, where textures are acknowledged, etc. Eg. provide depth.

A 3 D model, where the scene is visualized in a continuous, 3-dimensional map.

Retina LGN Cortex (INCREASING SPECIALISATION).

OR IS IT: Hierarchy:

1) First kernel of a signal which becomes our idea of motion and change in the env. Could be

considered as A) glue – the combining of the separate frames into a continuous flow. Evidence:

kenitopsia patient who saw world frame by frame as she lacked this first stage of hierarchical

system or a B) Motion vector that is directionally selective.

Motion -an example system • Basic 1-dimensional motion vectors

– Cortical (v1) receptive fields are directionally selective – Maybe either from a correspondence-type detector or a spatiotemporal gradient detector

• Both equally likely in humans (i.e. we don’t know!) • Retinal vs eye motion

– Keeping the world ‘still’ when you move your eyes – Attentionally-modulated motion

• Higher level motion – Optic flow – Structure from motion – Biological motion

Lecture 19: An informational processing system where there are 3 ways to approach a

problem. Define the problem clearly than look for solution, then look for implementation.

Do not limit yourself when you might not know everything about the problem.

33. “Retino-cortical expansion” refers to..... We need more neurons/machinery for the centre of the visual field than in the peripheral regions of our

view. More neurons sensitive to the central visual field than there are detecting the peripheral visual

areas which have a relatively greater spatial area. So the visual system is structured in a way that it is

mostly sensitive to what it NEEDS to deal with.

34. The two most likely kinds of motion detector in the human visual system are called…. o The correspondence or Reichardt detector (simplest form of motion detector): Assuming that the first thing that stimulated one receptor is the same thing that’s stimulating another

receptor later on in time. The timing needs to be well managed for motion to be perceived. This is

consistent in flies.

o Spatiotemporal gradient detector (mathematically different to above process) More complicated.

But we don’t know the real answer. We just look at these 2 possibilities. Simon’s view = both systems combined as their pros and cons complement each other. 35. The three critical dimensions of vision are… The three critical dimensions of vision are those 2 dimensions of space on your 2 retinae and one dimension of time. You construct everything else from those dimensions. The third (temporal) dimension

and how it relates to space tells us about motion. 36. The term “Biological motion” describes….. A type of motion integration where a complex interpretation/experience is received from just a simple

input like moving dots.

Harmonic motion (moving in a pattern) of dots where the dots move repetitively at particular time

intervals in relation to one another gives us the sense that as a whole, these moving dots represent a

moving person.

MOTION INTEGRATION

Motion is a critical cue for depth. Eg. Baby bowls calibrate disparity selection detectors.

Eg. Sense of a sphere created by dots moving in a characteristic pattern. You integrate those dots into

something which you gives you a much more whole experience (but the movement of each independent

dot itself has no meaning).

Independent and isolated motion vectors (signals) from simple direction selective receptive fields are integrated into a global percept. 37. Interaction between V1 orientation-selective receptive fields follows what ʻlawsʼ? Orientation selectivity Detection of orientation does not occur in the human visual system until the primary visual cortex. Only

some orientations are detected.

Retinal cells (photoreceptors) get excited, then bipolar cells and LGN cells, and finally cortical cells. Just

before cortical cells, the orientation is detected. Specialisation starts to occur in the cortex.

38. The visual system is sensitive to…… A very narrow section of the electromagnetic spectrum. The wavelength which we can perceive is between 400-700 nanometres in length. This is known as the visual spectrum. 39. What critical neural interaction is affected by, among other things, hallucinogenic drugs?

The ability to inhibit certain signals is weakened. Results in more noise as all signals are received whether they are useful or not, and meaning is gained from them. Increases an individuals’ awareness of background noise.

Lecture 21: Hallucinogenic drugs increases the amount of serotonin in the body, which

increases cortical activity to reduce the inhibition of signals (therefore a lot of information is

entering perception) Ability to inhibit some signals is weakened.

Hallucinogenic drugs directly affect the serotonin receptors (specifically the serotonin

receptor subtype, 5-HT2), which is what eventually results in a complex pattern of action

potentials and activity.

40. Your experience of reality is….. Subjective. It is in between the theories of Plato and Nietzsche Subjective and the taking in of only a portion of the external world. (There is a world out there, but we only take in a part of it and make meaning out of it so that our reality is essentially personal and subjective).

41. Synaethesia is…

• The involuntary conjoint/linked perception across two modalities. – A ‘common’ example is letter/colour – A more interesting example is sound/colour • Famous synaesthetes; – Vladimir Nabokov, David Hockney

For example, a person with synethesia may see colours when they hear a sound, or can actually taste

words; stimulation of one sense, it seems, causes an inappropriate stimulation of another.

42. One fish, two fish… Red fish, blue fish.

The Qualia problem = We have different experiences of a particular colour – How do we know that when

you and I see a colour that we both call ‘red,’ we are actually having the same colour experience? Color is

a psychological experience created when the eyes and the brain interpret light.

26. The spatial structure of natural textures is consistent with the properties of the system because……

The visual system consists of a plethora of receptive fields of varying size, and in combination, those

receptive fields allow different textures and edges to be distinguished. Smaller receptive fields are

more sensitive to fine edges and complex, intricate textures, whereas larger receptive fields are

better at detecting change over a large space (so thicker edges for eg.).

The system has tiny receptive fields sensitive to tiny fragments of texture and uses the same

component input but interaction between them is different.

Motion = Discrete analysis of change in position over time. Signal = noise. What we consider as noise is the decision we make (which one we choose is important).

Sparknotes: http://www.sparknotes.com/psychology/psych101/sensation/section2.rhtml Seeing Colour Lecture Stanford: http://www-psych.stanford.edu/~lera/psych115s/notes/lecture5/ Students’ perspectives: http://www.atarnotes.com/forum/index.php?topic=133336.0