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Final

PSYC10003 Study Guide - Final Guide: Visual Cortex, Receptive Field, Retinal Ganglion Cell


Department
Psychology
Course Code
PSYC10003
Professor
Scott
Study Guide
Final

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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.

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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.

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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).
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