BIOL 455 Visual
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1. How is visual system backwards? (3)
2. What are the functions of the eyes? (2)
3. How does brain perceive backwards image on retina as rightside up? What would slow it down if things changed?
4. Diff sensitivity vs. acuity?
1. (a) Images are focused on retina backwards and upside down (b) Sensory cells (photoreceptors) are in the back of the eye (c) Increased light causes hyperpolarization of photoreceptor cell, leading to decreased glutamate release.
2. (a) capture and focus light from cornea onto retina (b) to begin neural processing (retina's photoreceptors)
3. Perception and experience - based on age. The older the orgnaism, the slower it adapts.
4. Sensitivity - ability to respond at low levels of light vs. sharpness of vision
1. What are cornea and lens responsible for? Which primarily?
2. Define retina
3. What is it made of?
4. Which responses in retina are graded potentials? Which aren't?
5. All info collected by photoreceptors converges where?
- 1. Capturing and focusing light (primarily cornea)
- 2. Retina - receptive surface inside eye containing neurons.
- 3. Neural tissue
- 4. Photoreceptors, bipolar cells, and horizontal cells. ganglion cells (action potential)
- 5. Ganglion cells
1. Where is the first stage of visual information processing?
2. What does the retina contain layers of? (4)
3. What is the fovea? What is special about this? (3)
4. What causes our blind spot? Why?
5. What is a saccade? What does it focus on? 3
- 1. Retina
- 2. Ganglion cells, bipolar cells, photoreceptors (rods/cones) and pigmented epithelium.
- 3. Fovea - central part of retina. Packed with most receptors, center of our gaze, region of highest acuity
- 4. Optic disc - no photoreceptors, that's where ganglion axons and blood vessels exit.
- 5. Saccade - rapid eye movement to make up for blind spot and lack of peripheral vision. Eyes, lips, edges.
Scotopic vs. photopic
1. When is it on?
2. What type of cell does it act on?
3. Do they work independently of each other?
5. Receptive field
6. Visual acuity
7. Response time
8. Color vision?
- 1. Dark, light
- 2. Rods, cones
- 3. Yes
- 4. Single bipolar cell collects lots of info from rods. Given numerous sources will respond to low levels of light; opposite
- 5. Large receptive field because info comes in through numerous rods and is integrated (cannot distinguish exactly which rod stimulus its from); small - preserves info from only a few cells.
- 6. Low visual acuity (b/c of large receptive field); high because of small receptive field
- 7. Slow (collects broad set of info over long time; also contributes to sensitivity); does not require lots of time to exceed threshold since responding to high levels of light.
- 8. No. Only collects one wavelength and all info gets integrated anyway; yes.
1. What determines whether receptive field is large or not?
2. What determines response time?
3. Which is larger in number? Cones or rods
4. What happens if you integrate all info from cones?
5. Why are photoreceptors stacked?
6. Why is the above important?
- 1. Photoreceptor:bipolar cell ratio (if large, then large receptive field)
- 2. How long it collects info for - if it needs a long time to collect enough glutamate to release
- 3. Rods (100 mil) vs. cones (4 mil)
- 4. No color vision either!
- 5. To increase probability of capturing photons
- 6. Because in eye, light is reflected in so random directions by surface of eyeball, lens, and fluid in eye, so only a small fraction actually reaches retina.
1. What does light cause in photoreceptor cell?
2. Describe mechanism of signal transduction cascade (5)
What is important to note?
- 1. Hyperpolarization, instead of depolarization
- 1. Light hits rhodopsin, causing conformational change of retinal, causing rhodopsin to split into retinal and opsin, exposing active site on opsin.
- 2. Activated opsin activates transducin
- 3. Transducin + phosphodiesterase convert cGMP (which keeps ion channels open) to 5'GMP.
- 4. Less cGMP --> ion channels close, hyperpolarization, less glutamate release.
THIS IS GRADED.
Draw a graph of voltage vs. time comparing intensities of light.
When does peak intensity occur?
How does this compare to auditory system?
1000x slower than auditory system (auditory system worked on us).
Draw all four scenarios of off-center, on-center with or without light bipolar cells.
What are the things to memorize about photoreceptors, bipolar cells (on- and off center differences) and ganglion cells?
- 1. Photoreceptor cells are always hyperpolarized by light (less glutamate)
- 2. Ganglion cells are always depolarized by glutamate (releasing AP signal)
- 3. Increased depolarization of BC always leads to increased glutamate release (needed to send action potential).
In off-center, more glutamate causes depolarization, while in on-center, less glutamate causes depolarization.
How does visual system adapt as a sensory system? (6)
1. Pupils - open and close via ciliary muscles to control amount of light entering eye; Responsible for 1/16th range; immediate reaction because its muscular
2. Rods and cones - broad division of range fractionation intensity- different receptors handle different intensities (rods - low, cones - high)
3. Photoreceptor adaptations
- 1. Internal range fractionation - each photoreceptor adjusts its sensitivity to match average level of ambient light (concerned with CHANGES in brightness, not absolute levels)
- - range of intensities over 100fold - will be completely depolarized by 1/10th and completely hyperpolarized by 10x
- - Beyond these ranges, will be completely hyper/depolarized --> no info.
4. Calcium - photoreceptors reglulate release/storage of IC Ca2+ ions to control sensitivity to light
5. Level of photopigment - balance between rate of breakdown and rate of recombination determines how much is available to respond to light
6. Availability of retinal chemicals - low retinal at low levels of light; high retinal at high levels of light. Requires more photons to increase [retinal] to hyperpolarize receptors
Describe pathway of vision (6)
- 1. Retina's image is inverted and reversed
- 2. Axons of retinal ganglino cells form optic nerves
- 3. At optic chiasm, axons from temporal halves of each retina continue epsilaterally, while axons from nasal halves decussate to optic tracts on opposite sides.
- 4. Most axons terminate in lateral geniculate nucleus/others in superior colliculus
- 5. Info converges at layer 4 of primary visual cortex topographically
- 6. left primary visual cortex gets info from both retinas, but from right visual field and vice versa
Draw projections from photoreceptors to cortex (5)
1. For lateral geniculate nucleus, how many layers does it have?
2. Which layers (other name) are for magnocellular? Which (other name) are for parvocellular?
3. Which receive input from eye on opposite sides? same side?
4. What type of receptive field to LGNs have?
5. Which out of MGCs or PVCs show differnetiation in wavelength?
6. What was a puzzle in regards to LGN stimuli vs. cortical stimuli?
- 1. 6
- 2. 2 main ventral (1 & 2); 4 main doral (3-6)
- 3. 1,4,6; 2,3,5 (this is right notes are wrong)
- 4. Concentric
- 5. PVCs (MGCs work with rods - no diff and get lots of info from lots of cells integrated together)
- 6. Why some stimuli that activated LGn didn't activate cortical cells
In the cat LGN, cortical cell experiment, in each case, when did they see response and when did they not see response?
- 1. LGN (on-center - response; off-center - inhibition (not just no response)
- 2. Cortical cells sensitive to orientation (vertical - response, slanted - less response; horizontal - no response)
- 3. Cortical cells sensitive to direction of movement: horizontal bars moving down --> YES, horizontal bars moving up, kind of. Vertical bars moving left or right, none.
1. What is the difference between simple and complex cortical cells?
2.. What are the two opposing hypothesis in coding for vision?
1. Simple cortical cells elicit response edge/bar of particular size/orientation in particular location in space.
Complex cortical cells elicit response for edge/bar of particular size/orientation ANYWHERE in space
2. Grandmother neuron vs. spatial frequency analysis
In vision, describe grandmother theory vs. spatial frequency analysis - what does it apply? what system is this like
Problems w/ grandma? 2
1. Grandmother neuron - integration of successive levels of analysis would enable someone to recognize something as complex and specific as their grandma.
Problems (1) requires infinite number of cells to recognize each object ever (2) neurons respond only to entire face, not facial features ---- required alternate hypothesis
- SpAtiAL fReqUENCY
- - Applies fourier analysis to spatial field, breaking it down into spatial frequencies. Like auditory system.
- - changes in spatial area in lightness and darkness or color OR number of light-dark or color cycles that the stimulus shows per degree of visual space.
1. Describe what a picture would look like with high spatial frequencies filtered out?
2. With low spatial frequencies filtered out?
- 1. High - appears blurry - small details and contrasts are lost
- 2. Low - without low, large uniform areas and gradual transitions are lost.
Describe V1 (aka 4), V2 (2), V4 (4), and V5 (1) in terms of function
What does inferior temporal cortex do? (3)
V1 - primary visual cortex - perceives objects, events and formals mental images, spatial frequency, orientation of bars)
V2 - Has more complex receptive fields and is responsible for perceiving illusory contours
V4 - Receives axons from V2, important for responding to sinusoidal frequency gratings, concentric/radii stimuli, responds strongly to wavelength differences
- V5 - perception of motion.
It has (1) complex receptive fields responding to (2) complex forms and (3) recognized objects
Compare and contrast trichromatic hypothesis (def, example, problem)
opponent-process hypothesis (what photopigments are needed?, what photoreceptors/color detectors? - 6)
Draw curves of red, blue, green, yellow
- Trichromatic hypothesis:
- Hypothesis of color perception stating that there are 3 different types of cones (red-, blue-, green-sensitive), each excited by a different region of the spectrum and each having a separate pathway to the brain.
- - Beginning of labeled line system for seeing color
- - Example - seeing purple means firing of red and blue cones at the same time.
- - Problem? Only having 3 cones in response to one wavelength gives poor acuity and sensitivity, because you're using very small fraction of photoreceptors at one time.
- Opponent process
- The theory that color vision depends on systems that produce opposite responses to light or different wavelengths. Neurons compare blue vs. yellow, green vs. red, and black vs. white.
Photopigments: S (420), M (530) L (560), rods (495).
Color detectors: +S/-(L+M) (blue) vs. +(L+M)/-S (yellow); red (+L/-M) vs. green (+M/-L).
Black (-M/-L), white (+M/+L)
1. Draw pigments (absorbance vs. wavelength)
2. What do broad curves mean? (4)
3. What does almost any visual object stimulate at least? To ensure what? (4)
- 2. Peak sensitivity, responds to wide range of wavelengths, broadly tuned/lots of overlap, and you can only extract color info by comparing responses of different cones.
- 3. AT least two cones to ensure high visual acuity and good perception of form and color extraction.
1. Are LGNs color cells? Why or why not? (2)
2. Despite the fact that the peaks of sensitivity curves of M and L cones are not very different, how do they yield distinctive neural curves?
3. Name four stages of color perception
- 1. No, because they send outputs to higher circuits for detection of form, depth, movement, and huge.
- 2. Peak wavelength sensitivities do not correspond to wavelengths we see as principal hues.
- 3. (1) Cones receive visual info ---> (2) neurons in local circuits process info --< ganglion --< LGN (3) ---< V1 (4) ---< V4
1. When do V4 cells respond best?
2. What are V4 cells important for? (3)
3. What happens when you have color blindness? Why do men have this more often.
- 1. When color outside of receptive field is different from color preferred in the receptive field.
- 2. Color perception, color constancy and discsriminatino between figure and background
- 3. Lacking photoreceptor, because photoreceptor genes are on X chromosome (men only have one, so instantly recessive genes are taken in)
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