Lecture 1 The Eye

imissyou419's version from 2017-10-24 01:07


Question Answer
When ciliary muscle contractsits diameter decreases, springs less taut, lens spring back to its normal round shape, close object focused onto the retina
When ciliary muscle relaxedits diameter increases, springs more taunt, lens become more flat, distant object focused onto retina
What shape of eyeball and lens to create nearsightedness, what lens to fix it?eye too long, lens too round; concave lens
What shape of eyeball and lens create farsightedness, what lens to fix it?eye too short, lens too flat; convex lens
As one gets older, what happens? What kind of lens do you prescribe?lens lose its elasticity and become too flat even when ciliary muscles are completely contracted. When this happens, image of near objects are blurred; prescribe convex lens
When the lighting is bright, what happens to the iris?iris constricts and the aperture (pupil) becomes smaller so less light enters the eye. Prevents light sensitive rods and cones from becoming saturated. The iris improves the focus of the image on the retina (when pupil becomes smaller in diameter, the area of blur on the retina becomes smaller)
Ganglion cellsthe only output from the eye
Bipolar cellsconnect the receptors to the ganglion cells
Horizontal cellsconverge signals from several photoreceptors. They determine how many receptors each ganglion "sees"
Amacrine cellsconverge signals from peripheral rods via bipolar cells
What does light and darkness do to rods and cones?light hyperpolarizes these cells (voltage inside drops), darkness depolarizes these cells (voltage inside rises) thus dark acts like a stimulus
Which cells produce APs?some amacrine and all ganglion cells; photoreceptors, horizontal cells, and bipolar cells only produce graded potentials
Why do most cell types in the eye show only graded changes in potential?graded changes allow for continuous and rapid transmission of information (rather than APs that are slow at transmitting info - must wait until next AP to see if frequency increased or decreased)
Why must ganglion cells generate APs?Ganglion cells must transmit info over a long distance to LGN and Superior Colliculus; must convert visual info coded by graded potential changes in bipolar cells into discrete code consisting of APs
Why is reading difficult in low illumination?Peripheral retina contains primarily rods, fovea contains only cones. Cones are less sensitive to light. (in low illumination, we can only see with our rods and see greys). Periphery has poor acuity.
Why do ganglion cells in the peripheral retina have poor visual acuity?1. large ganglion cells integrate information from a large area of retina. 2. Large spacing (low density) and large convergence
Why do ganglion cells in the fovea have high visual acuity?1. small ganglion cells integrate information from a small area of retina. 2. Small spacing (high density) and low convergence
In the daylight, what does the eye see?only the central fovea sees in detail and in colour
On a dark night, what does the eye see?only the periphery sees, only in black and white, with poor acuity; the proper functioning of rods is essential for night vision
What does the fovea see?the central 2 degrees (1%) of the visual field but it takes up 50% of the optical nerve
How would you measure the receptive field of a ganglion cell?1. record from a ganglion cell. 2. shine a small beam of light over different parts of the retina in sequence and map those that produce a change in firing rate (changes can be excitatory or inhibitory)
ON centre OFF surround measuresrelative brightness
OFF centre ON surround measures relative darkness
What happens in and ON centre OFF surround when 1) light in the centre, 2) light in the surround, 3) light completely fills the on region1) increase in firing frequency in the ganglion cell, 2) decrease in firing frequency, 3) maximum firing
Rapidly adapting or phasic ganglion cellWhen light is turned on, cell rapidly fires for a short period of time. When light is turned off, the cell activity is briefly inhibited. Good cells for detecting changes such as a flashing light
Light in the on centre cascade1) light decreases the cone voltage (hyperpolarize) and the cone releases less inhibitory NT.
2) less inhibition causes the voltage inside the bipolar cell to increase (depol) and it releases more NT.
3) the ganglion cell is excited and it fires more often
Light in the off surround cascade1) light decreases the surround cone's voltage (hyperpolarize) and the cone releases less excitatory NT.
2) The voltage inside the horizontal cell decreases (hyperpol) and it releases less inhibitory NT.
3) the voltage inside the centre cone increases (depol) and it releases more inhibitory NT.
4) the voltage inside the bipolar cell decreases (hyperpol) and releases less excitatory NT.
5) the ganglion cell fires less often
How does the brain compute what it must with so little neurons?activating only ganglion cells that sense a change i.e. the edges, colour, or brightness (activity of ganglion cells far from the edge show basal rates b/c both centres and surrounds cancel and the ganglion are the edges does firing rate increase or decrease) so many receptors in retina activate only a few neurons in the cortex
What is the consequence of edge extraction by ganglion cells?perception of illusory bands at edges to accentuate the shape of objects
Explain how the ganglion cells also provide constancy by measuring changeThe contrast remains constant independent of light conditions
Cones and colour visioncones respond best to a particular wavelength of light and respond, but less, for a large range of colours (cones is responsive to light over a wide range of colours so if activity of cones change, is it due to change in colour or brightness? resolved by special color sensing cells in visual cortex). 3 types of cones - Blue, red, green
Red + Green lightyellow light
Red + Green + Blue lightwhite or grey coloured light
Red + Blue lightmagenta light
Green + Blue lightcyan light
How are cones distributed in the retina?In the fovea,
1) red cones > green cones > blue cones #s,
2) relative #s vary from person to person,
3) cones of the same type form clusters.
The very centre of the fovea has NO blue type cones.
As you move to the periphery,
1) the # of cones drop and # of rods increase,
2) the size of both rods and cones increases and thus their density decreases,
3) cones become larger than rods
Colour blindnessdefect in the gene that produce the photopigment 1) missing 1 cone type, 2) missing 2-cone types, 3) missing all 3-cone types, vision is limited to the rods, the patient has only peripheral, not foveal, vision, 4) a cone type is made with a photo pigment different from normal
If you lost all 3 cone receptors, you would NOT be able to 1. see, 2. see a red apple, 3. see in dim light, 4. read4. read because need cones/fovea for reading - visual acuity. Rods have some response to red light
How many gradations of colour can the human brain distinguish?200 hues, 20 levels of saturation (combination of 2 or more wavelengths), 500 brightness levels = 2 million gradations of colour with 3 cone types
What happens when complementary colours are combined?get grey
Red circle leaves a ___ after image, green leaves a ___ after image, yellow leaves a ___ after imagered - cyan, green - magenta, yellow - dark blue (complementary)
What causes an after image?during prolonged viewing of 1 colour, the signal from that cone adapts. If you then look at white which activates all cones equally, the push from the 1 cone is weaker than the pull from the other cones
Where is the double opponent cell found?in the cerebral cortex V1 (NOT THE EYE)
It combines info from the ganglion cells
Why does a blue object against a yellow background stand out so well?blue spot in centre, a yellow spot (red and green) in the surround activates the double opponent cell = maximum response; the double opponent cell responds best to the change from blue to yellow while other double opponent cells would be maximally activated by a yellow tie against a blue background
What happens when you shine diffuse yellow light over the whole receptive field and how does this maintain colour constancy?diffuse yellow light over the centre activates both red and green receptors and so does yellow light in surround so cancels the input from the centre. Same thing with diffuse any colour; Since the centre and surround produce opposite responses, the double opponent cell is unaffected by any background colour so this maintains colour constancy
What happens if you do not adjust the white balance in your camera?pictures will have a yellow hue b/c cameras measure absolute colour and do not have colour constancy

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