PSYB51H3 Study Guide - Independent Component Analysis, Optic Chiasm, Ganglion Cell

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1 Mar 2013
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Lec 2 b51
Sa ex. Which kinds of psychophysical methods exist mention at least 4. Describe one of them in more
detail.
- Magnitude of estimation: assign a certain number to stimulus intensity. Relationship between
intensity of how you perceive stimuli to how you perceive it
- Method of constant stimuli
- Method of limits
- Method of adjustment
Pinhole eyes cavity with a small aperture which you can restrict direction of light quite a bit to create
finer directional sensitivity
To have acute vision, small hole. Not a lot of light will be allowed to go inside. What you see is a very
low contrast version of reaity.
Transparent humor in the cavity.
Sclera envelopes the eye. The front is the transparent cornea, then behind iti s the aqueous humor to
the lens. The pupil is on the irist which is part of the lengs. Conules of zinn are the tendons that connect
the ciliary muscle to the lens and help it contract and relaxes. When it contracts it gets smaller. When
this happens the zonules relax. Then it is the virtuous humor between the lens and the retina. The
virtuous humor keeps the eye healthy. Choroid is the pigmented area around all of this.
Refraction is needed to focus light because the light bounces off different objects at different distances.
All the light that ever has the chance of entering the light will be parallel
Accommodation: change of lens strcture by the cilliary muscles to have the light refracted the way it is
needed for the retina to see
Myopia: refractory power of the lens is stronger than it should be. Diverge when it gets close to the
retina. Cant stretch lens enough to see thigns at large distances. Concave lenses correct this so focus os
moved back to the retina
Hyperopia: too low of refractory power. With age our lenses lose refractive properties. Eye can be too
short, or it can be that the lense loses its elasticity
Astigmatism: cornea is misshaped.
Short wave lengths are bluish
L s redish (more red than others)
And m is green
In darkness, photoreceptors are depolarized
Shining light on them, they get hyperpolarized, (they become silent)
Graded potentials are more sensitive and are able to lump information. Short distances. Don’t lost
information
Amacrine cells: keeping information across retina even involved in lateral pathway
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Veritcal pathway : just things inside the retina.
- It passes on information from pr to ganglion cells to beyond engaged n converging information
esp in periphery of the retina.
Midget bipolar: use less energy
Receptive fields.
Light indirectly guides the bipolar cells because they hit the photoreceptors
2 diff types of ganglion cells: Parvocellular and Magnocellular
Receptive fields
Kuffler: on ganglion cells. Simple structure for receptive field of ganglion. Circular region in a concentric
fashion. If spot of light gets bigger, the response is reduced
Visual neurons are filters, filtering out what cannot be processed
- What the cells are most specialized for
- See differences in light and psotting contrast
Machbands result of lateral inhibition- -see boundaries that don’t really exist
M cells: transient responses- a flash or flicker of light and motion information.
- More sensitive to motion in periphery than in fovea
Lec 3
The retina processes information similar to a computer. Which neurons in which pathways play a role?
Describe some of the functions of the pathways. In which important way does retinal information
processing differ from that of a usual computer?
- Horizontal and the vertical pathway
- Horizontal: horizontal cells and amacrine cells. Functions are lateral inhibition. They have centre
surround organization.
- Vertical: photoreceptors, bipolar cells, ganglion cells. Photoreceptors transform physical energy
into neuro signlas which passes on the info. Pass on information. Convergence of information.
- Difference between this and computer, computers have about 4 central processing units. The
retina has millions of the central processing units (nucleus of every cell.
- Why care about gratings?
Printing press: 1439, with movable types. Gutenberg.
3 types of gratins: rectangular, sine wave, gabor (combination of sine an Gaussian)
Frequency, phase, amplitude
Frequency is important to recognize a scence and helps us with efficient codings. Similar to basic
alphabets of vision. Cnan creae a computer code with less memory effort.
Info on lower frequency channels tell us that it is lincoln’s face,
Higher spatial frequencies tell us that we are looking at squares.
This inhibits our ability to recognize the face. The two are interfering
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Mona lisa: when you look at her mouth directly, is she smiling? Not really, but if you look ar the corner
of face she looks like she is smiling. This happens because da vinci used many layers to superimpose the
paints. The paint is transparent. Makes it seem like the objects are blurred, 3D. also applied it to the
face. Amplifying lower frequencies when looking through periphery. When looking straight on it is the
fovea and we are seeing higher frequency so we don’t see the smile.
Bell and sejnovski independent component analysis: basic components of natural images.
Independent component analysis: takes a stimulus and breaks it down into its components. Even if no
knowledge of languuae, you can still do so.
- Decompose photos of natura scenes.
- Finds aspects that are independent of one another.
- All pictures are composed of little lines and grating. Just superimpose the lines onto eachother
and you can recreate almost any kind of photo
- Can create an alphabet to describe all sorts of images
- It is efficient
- Reason that it happens in the brain is that the visual system learn, train and adapt
Certain neurons are specialized for fine edges and lines
Limits of cosing images in terms of oriented stripes: visual acuity and contrast. The two intereact in a
non linear way
Acuity: smallest sspatial detail that can be resolved (20-visual acuity, smallest detail you can see in 20
feet.) the second 20 is what normal ppl see)
Limit of visual acuity: spacing of photoreceptorsi n the retina
- Convergence
- Sampling
Aliasing: signal reconstructed fro msamples is different from the original continuous signal.
As you go further away you see thigns differently, at higher frequency (smaller)
Really, contrast changes uniformly, its just that our visual system is nto as good at seeing these visual
lines
Best tuned to see ntermediate freuqnccies reather than higher or lower.
Csf: sensitvity is inversion of threshold . when high sensitivity, then low thresholds
Low threshold mean high sensitivity
Everything gets inverted
- Not all information goes to contralateral side of the brain. This means that it is the visual fields.
Separation in terms of visual fields.
- Before optic chiasm, separation in terms of eyes. Right brain sees the left visual field. Left brain
sees the right visual fields. Happens after optic chiasm
- LGN, part of thalamus. Plaace where senses pass thorugh except for olfaction also place where
inhibits senses when sleeping.
- LGN-pulvinar-superior colliculus
- Primary visual pathway:**eye, optic nerve, optic chiasm, optic tract, lgn, V1.
- If you have a lesion at the optic nerve, before the optic chiasm, the result is that one eye is
blind.
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