Retinal Processing

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What is light?
A wave; stream of photons, tiny particles that each consist of one quantum of energy
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What is sight?
Part of the electromagnetic spectrum is visible - results in the sensation of seeing
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what is the unit of visual angle?
1. Light rays which travel through the nodal point of the eye, unlike all other rays, do not change direction
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What can we use this point for?
To make all our measurements since we can trace a straight line from the edge of any object to the retina
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What happens to the lines?
From each extreme of the object intersect an angle is formed. the size of this angle depends on the size of the object
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What is the anatomy of the retina?
- Cornea and crystalline lens focus the image on the retina. Retinal images are rotated through 180 degrees
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What does the retina act like?
A film in a camera, for every point on the retina there is a corresponding point in the visual space from which it receives light, referred to as the visual frield
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Demonstration of a physiological blind spot and filling in?
1. Close LE 2. Fixate letter 3. Alter distance from eye until middle dot vanishes 1. Repeat above 2. Find distance where line appears continuous
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What happens at the Fovea?
1. At the fovea, the retina thins and a pit is formed 2. The nerve fibres are displaced to one side and no blood vessels are present 3. At this point the density of photoreceptors increases dramatically
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How many layers does the retina contain?
8
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What 5 type of neurons does the retina contain?
- Photoreceptors - Bipolar cells - Ganglion cells - Horizontal cells - Amacrine cells
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Where are the rods concentrated in?
Absent from the fovea and reach a maximum density just nasal to the optic nerve head
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Where are the cones concentrated in?
The Fovea
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What is the goal of the duplex theory of retinal processing?
To catch photons and signify the presence of light
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What is the problem?
system must operate in all the different environments we are likely to encounter
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when is it brightest?
looking at or near the sun (solar retinopathy)
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When is it the dimmest?
Starless, moonless nights
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What must the eye deal with?
Eye must deal with vast environmental range of luminance variation (~ 10,000,000,000,000) Photometer (light meter) - 10,000 fold change in light level
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How does the visual system do it?
Pupil constriction
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Cones?
Driven by phototopic light (Light) system high acquity, low sensitivity
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Rods
Driven by scotopic (dark) system low acuity, high sensitivity
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What does Rod outer segment have?
Rhodopsin
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What happens inside a rod?
It is electrically negative compared to fluid surrounding cell,- When light strikes rhodopsin, ionic channels are blocked and cell becomes more negatie and is said to ‘hyperpolarise’
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What happens to the electrical potential?
carried to later synapses with bipolar and horizontal cells
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What is the photopigment in cones?
Iodopsin
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Bipolar cells?
form the inner nuclear layer and connect to the photoreceptors - they simply transfer information on to ganglion cells
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rod Bipolars
connect many rods to between 1-4 ganglion cells
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Flat Bipolars
connect many cones to many ganglion cells
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Midget Bipolars
connect a single cone to a single ganglion cell (found only in the fovea
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Horizontal cells
collect signals together from a number of different photoreceptors. Neurotransmitter blocks hyperpolarisation of neighboring bipolar cells – ‘lateral inhibition’
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Amacrine cells
serve a similar function for the ganglion cell layer that the horizontal cells serve for the photoreceptor layer, i.e. they transmit signals from one ganglion cell to another a short distance away
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Retinal ganglion cells
The ganglion cells must collate information from all the photoreceptors, with the help of horizontal, amacrine and bipolar cells, and summarise it in a way that retains the essential features of the image.
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How do Ganglion cells acomplish this?
This is done using a technique called single cell recording. This allows us the measure the firing rate of a ganglion cell to visual stimuli placed in various parts of the visual field
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On region
A region where the spot causes an increase in firing rate
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Off regions
- A marked reduction of the firing rate when the light is on, followed by a rapid burst when light is switched off
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Ganglion cell response
- ON responses can be elicited from a restricted circular retinal region - OFF responses form a ring surrounding the ON response region - The arrangement is called ‘centre surround antagonism
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Centre surround antagonism
What purpose does this organisation serve?
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Answer
It helps the retina find edges in images. Objects are usually distinguished from their background by sudden in reflected light.
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What do receptive fields have?
smallest at the fovea (0.01mm) and have a low neural convergence factor - they provide high spatial resolution
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WHat does the fovea provide?
poor spatial resolution but good light sensitivity
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a problem of centre surround antagonism?
- Excitatory receptive field centres receive equivalent stimulation but the surrounds receive different amounts. A cell centered on intersection will respond less and thus dimming (grey spots) is experienced in these regions relative to other region
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If you look directly at the spot the smallest foveal receptive fields available will what?
will be used - in this situation both centre and surround will fit within the intersection
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Other cards in this set

Card 2

Front

What is sight?

Back

Part of the electromagnetic spectrum is visible - results in the sensation of seeing

Card 3

Front

what is the unit of visual angle?

Back

Preview of the front of card 3

Card 4

Front

What can we use this point for?

Back

Preview of the front of card 4

Card 5

Front

What happens to the lines?

Back

Preview of the front of card 5
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