Lecture 6: Retina Flashcards
Name teh 3 main parts of a photoreceptor
Outer segment
Inner segment
Synaptic terminal
Describe inner segment
Contains cell nucleus and synaptic terminals
Describe outer segment
Contains stack of disks packed with light sensitive pigment molecules (photopigemnts)
Responsible for phototrasnduction
Define phototransduction
Process by which photoreceptor cells in retina convert light into electrical signals - neural acitvity = can be processed by brain
Define photopigemnts
1st step visual process= chemical process in photoreceptors
Ligth sensitive molecule in photoreceptors composed of opsin and chromophore
Define opsin
Protein sensitive to a specific wavelength of light
Protein component of photopigemnts, determines wavelength sensitivity of photoreceptor - enables colour and light detection, sensitivity to certain type of light
Define chromophore
Light absorbing molecule within photopigments
Mammals = 11 cis retinal - changes shape upon light absorption
Describe photopigemnt cycle - whole thing
Cycle of photoisomerization and regeneration = lets us see
11 cis retinal = receptive state —light—> photoisomerization = light changes shape of chromophore, isomerized = now all trans retinal - can no longer absorb light = bleached —> photopigment regeneration to get back to normal state —> 11 cis retinal
Define photoisomerization
Proces by which 11 cis retinal chromophore changes shape to all trans retinal when exposed to light = initiates phototransduction
Results in photoreceptor being bleached - renders it temporality unable to absorb more light
Define photopigemnt regeneration
Process fo converting all trans retinal back to 11 cis retinal form
Allows photoreceptors to recover from bleaching and resume light detection
Occurs in retinal pigment epithelium
How does light become transduced into neuronal activity
When number of bleached photopigments reaches a THRESHOLD = photoreceptor will change rate at which it releases nts to indicate it has received light = light transduced into neural activity
How many types of opsin does each photoreceptor
Rods = have rhodopsin (498 nm)
Cones = 3 types of photopsin = wavelength specific= basis of colour perception
Describe types of photopsin for cones
Short wave length specific cones = s-cones - perception of blue, 437nm
Medium = m-cones = green 533nm
Long = l-cones = red 564nm
What do some photoreceptor cells have
Melanopsin = can monitor ambient light levels = influence circadian rhythm
Unconscious perception of light cycle
Describe rods - when best
Sensitive to light intensities that are a million times smaller than the smallest intensity perceivable by cones
More sensitive
Fucntion better in low ligth conditions
Become overwhelmed = saturated in bright Environment
Describe cones - when best
Fucntion well in bright environment
Ineffective in low light conditions
Describe duplex retina idea
2 visual subsystems = bc differnce in light intensity between day and night = v large
Define photopic vision
Vision under well lit conditions
Primialry mediated by cones
Enable colour perception and high visual acuity
Define scotopic vision
Vision in low light conditions
Mediated by rods
Provide high sensitivy to light but no colour perception
Hard to see detail
Define dark adaptation
Process by which eyes adjust to low light conditions
Involves increased sensitivity of rods and shift from cone dominated to rod dominated vision
Can take up to 30mins
Name mechanism that allow us to see in dimly lit situations
Pupil dilation
Gain in sensitive of photoreceptors
Normalization through lateral inhibition
Describe pupil dilation - see in dimly lit
Allows entry of more light = see Bette
Fastest mechanism
Effects relatively limited
Describe gain in light sensitivity of photoreceptors - see in dimly lit
Dark environment associated with more regeneration than bleaching
Renders photoreceptors more receptive to light
Less receptors in bleached state in dark = more will be a viable to capture light
Maximum sensitivity of cones
5-8mins in dark = cones stop adapting
Limit to sensitivy of light cones can have
Maximum sensitivity of rods
After 25 mins
Rods = more sensitive
Rod cone break
After 7-8 mins inf ark
Vision becomes dominated by rods
Switch between photopic —> scotopic vision = duplex retina
When rods become more sensitive than cones = because larger reservoir of photopigemnts than cones
Describe whole graph - for rod and cones and duplex retina idea
Cones = more sensitive at first,
Rods = small number, all photopigments bleached in light
BOTH increase sensitivity since less light can regenerate sensitive before they bleached
But then rod cone break = rods become more sensitive
Maximal visual sensitivity = due to rods
Define normalization through lateral inhibition
Neural mechanism in which active neurons suppress activity of neighbouring neurons
Enhances contrast and improves detection fo edges and fine details
Changes in luminence v large throughout day but visual system adapts fast via this mechanism
What is general principle lateral inhibition
Useful for enhancing contrast
Subtracting and dividing by teh average luminance
Describe how we see outside vs inside
Measure background
Not much contrast
Would see this if visual system directly perceiving absolute intensity of light = would be very sensitive to big variations in light
Imrpoatnt part = relative distance in luminence between surfaces = disregard illuminant
Describe normalzation through lateral inhibition
Put everything on common scale
Remove average luminance
= percieve difference in luminence between every point in visual field and average luminence
Also used by decision making system - v widespread, relative value =important
What is enhancing contrast for
To see objects regardless of overal level illuminance
Define receptive field
Region on retina in which stimuli influence a neurons firing rate = increase or decrease it
Define ON CENTER OFF SURROUND cells
Activated by light in Center
Inhibit by light in surround
Define OFF CENTER ON SURROUND CELLS
Opposite response =
Activated by light in surround
Inhibited by light in center
Light on center only - on center cells
Ganglion cell fires rapidly
Light on surround only - on center cells
Cell does not fire
Bc on center off surround does not want light in surround
No light in center or surround - on center cells
Cell does not fire
Light on center and surround - on centre cells
Weak response = low frequency firing
Light on center only - off center cells
Ganglion cell does not fire
Light on surround only - off center cells
Cell fires rapidly
No light on center or surround - off center cells
Cell does not fire
Light on center and surround - off center cells
Weak response - low frequency firing
Describe enhancing contrast - 1
Once photoactivation starts
Photoreceptors become hyperpolarized (negatively charged)
Describe enhancing contrast - 2
Changes in photoreceptor activation are communicated to the bipolar cells In form of graded potentials
Describe enhancing contrast - 3
Bipolar cells synapse with retinal ganglion cells
Fire an all or non fashion, not like graded potentials
Define graded potentials
Gradual
Not all or nothing = not like aps
More nt= signal gradually increase as transmitted to bipolar cells
Describe what bipolar cells do generally for enhancing contrast
If light = photoreceptors hyperpolarized (-)
Surround = no light = photoreceptors release nts
On Center bipolar cells, activated, and reverse response = tell ganglion cell to fire ap
If lots activity off center bipoalr cell will block it
Describe receptive fields and lateral inhibition - 1
To enhance contrast = use horizontal cells
Light hyperpolarizes = renders polarity more negative of center cone
Surround cone = connected to one horizontal cell
Center cone connected to 2 horizontal
Horizontal cells = communicate with many photoreceptors at once
Describe receptive fields and lateral inhibition - 2
On center bipoalr cells reverse the sign of the cone
Off Center bipoalr cells would keep same sign
Describe receptive fields and lateral inhibition - 3
Dark depolarizes - renders polarity more positive of surround cones = + +
Describe receptive fields and lateral inhibition - 4
++ of surround cones = Activates horizontal cells = in turn inhibit all the cones
Because center cone placed in middle of 2 horizontal cells = receives more inhibition than surround cones
Center cone gets - - (- og) = 2 minus signs added bc connected to 2 horizontal cells
Describe receptive fields and lateral inhibition - 5
Amplifies bipoalr on cell activity = amplify retinal ganglion cell activity
Specific configuration of center on off surround, activates these ganglion cells more
3 negs on center cone, additional inhibition = then reversed , so more activation =
= bipolar cell gets 2 more +
= INCREASES CONTRAST
Describe receptive fields and lateral inhibition - WHEN LIGHT ON CENTER AND SURROUND
All cones hyperpolarized
Horizontal cell not activated = on center cone = - and on center bipoalr cell = + only (not super good)
Describe types of ganglion and bipolar cells
On center and off center
On center bipoalr cells = reverse sign of photoreceptor
Off Center bipoalr cells do not reverse sign of photoreceptor
Allows black on white to stand out as much as white on black
Describe enhancing contrast - Mach bands
Greys are teh same but look diff
Perceive more contrast than is
Bc percieve light intensity diff
Ex = B fire more than a, bc on Center, has highest ratio of light in Center and ark in surround = responsds more
Same for D, = respond more than c bc n complement darkness, want more dark in surround
Describe enhancing contrast - Hermann grid
See dark in middle of lines when there is no dark
Central vs peripheral vision - as move eyes = ones in periphery more affected by illusion
In foveal vision = central vision = highest acuity, receptive field neurons = smaller
More active on center off surround just to left pf gap = so the area right in center perceived as darker but then
Respective fields smaller when look right at it = foveal vision
