Vision Flashcards
What is the signal in vision?
Electromagnetic waves: photons
Packets of electromagnetic energy (quanta)
What is the range of frequencies the eyes can see?
Light particles fall within certain range of frequencies
400-700 nm: range that human visual system response to
frequency = ?
amplitude = ?
Frequency (related to wavelength) of light waves: tells something about the color (hue)
Amplitude of light waves: tells how bright something appears
How is the signal collected for vision? (dilation, constriction, refraction/accommodation)
Amount of light entering the eye is controlled by the pupil.
Wider dilation (controlled by sympathetic ns) means more light which = means more information
Constriction (controlled by the parasympathetic ns) to bring the amount of light coming in back down after a stressful event
Refraction (bending of light rays) is done by cornea and lens to form an image on the retina (is upside down)
Accommodation: ciliary muscles in the eye adjust the focus by changing the shape of lens. Flattened lens for a distant object bc the rays of light are more parallel and dont need as much refraction. Rounded lens for closer object bc light rays diverge and need more refraction
dilation is controlled by?
what does wider dilation mean for the animal?
Dilation is controlled by sympathetic ns
Wider dilation means more light means more information
constriction is controlled by
parasympathetic NS
refraction
bending of light rays
Done by cornea and lens to form an image on the retina (is upside down)
Accommodation
ciliary muscles in the eye adjust the focus by changing the shape of lens
general principles of vision
- Image is upside down
- is in a right/left reverse on the retina
- doesn’t have a one-to-one representation of senses in cortex, so there is a distortion
Example: fovea has 35x more cortical representation than peripheral part of retina; foveal input takes up ~25% of primary visual cortex
retina: which are the photoreceptor cells
rods and cones
what are the differences between rods vs cones? where are they located?
Rods: Highly sensitive to lower levels of light → associated to night vision
Cones: Sensitive to higher levels of light & capable of color vision → mediate daytime vision
Both located at back of retina
Cones located in the middle of the retina, rods located in the periphery (around)
what causes the blind spot in the retina? why do we not see a blind spot in our vision
Optic nerve interrupts the retina → makes a blind spot
Compensation from the cells and other signals to fill in the blind spot
The other eye also maps another part of the visual field to compensate
Range fractionation
- some receptors have high sensitivity (rods) and others low sensitivity (cones); but each receptor has a range of adaptation
each receptor being able to encode varying levels of stimulus
Bipolar cells
receive input from photoreceptors and synapse onto/pass to ganglion cells
Ganglion cells
cells whose axons form the optic nerve
Horizontal cells
makes contact btw photoreceptors and bipolar cells.
Important in lateral inhibition
- Horizontal cells are inhibitory interneurons that release GABA upon depolarization.
- are depolarized by the release of glutamate from photoreceptors, which happens in the dark.
modulate the output of photoreceptors and play many roles in early visual processing contributing to contrast enhancement, color opponency, and the generation of centre–surround receptive fields in cone photoreceptors (cones) and BCs.
Amacrine cells contact bipolar and ganglion cells.
- major carriers of rod signals to the ganglion cells in the retina
- play a role in speeding up the slow potential rod messages for presentation to ganglion cells
make contact with bipolar cells and ganglion cells. They assist in intraocular visual processing. Amacrine cells in particular are important for lateral inhibition.
visual acuity
sharpness of vision
Influenced by density of receptors and mapping to ganglion cells
Greatest at fovea (High concentration of cones)
- every cone is communicating with a ganglion cell
Falls off towards the periphery of the visual field
what kind of potentials do all except ganglion cells produce?
All cell types except ganglion cells generate only graded local potentials
ganglion cells generate action potentials
photoreceptors
Transduce (convert) light into chemical reactions
Disks capture photons
Modulate the transmitter release at the base of the rod
In dark, photoreceptors are really active
Light inhibits activity of photoreceptors
How do photoreceptors (rods) work in the dark?
Na+ channels kept open by cGMP (cGMPgated)
- cGMP is a second messenger molecule that modulates various downstream effects, including vasodilation, retinal phototransduction, calcium homeostasis, and neurotransmission
Na+ ions flow into photoreceptor which depolarizes photoreceptor (slightly negative state)
“Dark current”
Rods continually release glutamate
How do photoreceptors (rods) work in the light?
Na+ channels close which blocks Na+ flow into photoreceptor. This the causes hyperpolarization (more negative) of the photoreceptor, reducing glutamate release (excitatory NT)
Why do Na+ channels close in photoreceptors when in the light?
Photoreceptors (rods) contain light sensitive photopigment (rhodopsin) that captures photons. Rhodopsin is made up of retinal and opsin.
When light hits rhodopsin, the retinal isomer unbinds and changes confirmation (cis → trans) which activates transducin which then activates phosphodiesterase. Phosphodiesterase destroys cGMP on Na+ channel, so it will no longer keep the gate open for Na+ to flow in. This causes a drop in Na+ bc ion channels are closing more.
One photon of light can block entry of more than 1 million Na+ molecules
What is rhodopsin made of?
is made up of a combo of retinal (small light absorbing lipid) and opsin (large protein:348 AA)
configurations of retinal
Cis isomer
Trans isomer
process of light activation
- Light activates rhodopsin
- Retinal dissociates
There is a conformational change from 11-cis-retinal attached to opsin to → all-trans-retinal & activates G protein called transducin. This process is called “Bleaching” (which is when things looked washed out in bright light) - Transducin activates phosphodiesterase
- Phosphodiesterase destroys cGMP on Na+ channel, so it will no longer keep the gate open for Na+ to flow in. This causes a drop in Na+ and ion channels are closing more. Thus the cGMP as well. More & more of the channels close
- Closes Na+ channel → Membrane hyperpolarizes
Influences amount of glutamate released (rods release less glutamate)
When light hits, how does the retinal change?
Retinal cis isomer bound to opsin in the dark. When light hits it, it unbinds and changes confirmation. This change is what sends signal cascade which changes ion channel
How does hyperpolarization depend on light? What kind of potential (action or graded) happens at the photoreceptor level?
Light activation produces grades change in membrane potential (graded potential); Hyperpolarization magnitude is dependent on the amount of light (larger if more light)
- bright light hyperpolarizes more than dim light
At the photoreceptor level, there is no action potential
how do photoreceptors work in the dark? (include cGMP and depolarization in explanation)
Photoreceptors, specifically rods, are specialized cells in the retina that detect light and play a crucial role in vision, particularly in low light conditions. In the dark, rods are constantly releasing a neurotransmitter called glutamate, which inhibits the activity of bipolar cells and prevents them from sending signals to the ganglion cells in the retina.
The process by which rods release glutamate is mediated by a second messenger molecule called cyclic guanosine monophosphate (cGMP). When rods are in the dark, cGMP levels are high and this keeps ion channels in the outer segment of the rod cell membrane open. These channels allow positively charged ions, such as sodium and calcium, to flow into the cell, causing it to depolarize and release glutamate.
However, when light enters the eye and strikes the rods, it activates a molecule called rhodopsin, which triggers a cascade of events that leads to the breakdown of cGMP. This causes the ion channels in the outer segment of the rod to close, which prevents the influx of positively charged ions and results in hyperpolarization of the cell. This hyperpolarization decreases the release of glutamate, which in turn reduces inhibition of bipolar cells and allows them to send signals to the ganglion cells in the retina.
In other words, when rods are in the dark, high levels of cGMP keep ion channels open and cause the release of glutamate, which inhibits bipolar cells and prevents them from sending signals to ganglion cells. When light enters the eye and strikes the rods, it triggers the breakdown of cGMP, which closes the ion channels, hyperpolarizes the cell, and reduces the release of glutamate. This disinhibits bipolar cells and allows them to send signals to ganglion cells, which can then transmit visual information to the brain.
When moving from dark to LIGHT, transmitter release from On-center bipolar cells is ___________ and transmitter release from Off-center bipolar cells is __________. (increased/decreased?)
increased; decreased
off center ganglion cells (dark to light)
hyperpolarizes photoreceptor
off bipolar hyperpolarizes
off-center ganglion cell decreases firing rate
on center ganglion cells (dark to light)
hyperpolarizes photoreceptor
on bipolar depolarizes
on center ganglion cell increases firing rate
Magnitude of hyperpolarization in response to light determines a rate of reduction in neurotransmitter release.
What does this look like in photoreceptors from dark to light and vice versa?
Dark → light : photoreceptor will hyperpolarize and glutamate release is decreased
Light → dark : photoreceptors will depolarize and glutamate release is increased
what is the difference between on vs off bipolar cells?
unlike photoreceptors which are always inhibited by light, bipolar cells can be
On: activated by light
Off: inhibited by light
Action from bipolar cells (inhibited or activated by light) depends on photoreceptor present (depolarizes or hyperpolarizes others depending on receptor)
what effect does glutamate have on the bipolar cell?
has opposite effects depending on the kind of bipolar cell it is bc they have different glutamate receptors
on-center bipolar cells in light
Has metabotropic glutamate receptors so glutamate released by photoreceptors is inhibitory which → depolarizes bipolar cell thus increasing transmitter release to ganglion cell which → increased firing rate
off-center bipolar cells in light
Has ionotropic glutamate receptors so glutamate released by photoreceptors is excitatory which → hyperpolarizes bipolar cells thus decreasing transmitter release to off-center ganglion cells which → decreased firing rate
on vs off ganglion cells
On-Center Ganglion Cells: Activated by light
Off-center Ganglion Cells: Activated when light turned off
Glutamate released from bipolar cells always depolarizes ganglion cells
Influence of glutamate is translated into an action potential: rate of discharge (neural code). Relative firing rate encodes info about light
The first station that processes visual information from the _______ is the _______ and ___________ then get to the ________________
The first station that processes visual information from the optic chiasm is the thalamus and superior colliculus then get to the primary visual cortex (V1)
what is the visual pathway (from ganglion cell to primary visual cortex)
Ganglion cells (the axons of it) exit the retina and form the optic nerve and cross at the optic chiasm. The first station this hits is the thalamus to start processing visual information.
The first station that processes visual information from the optic chiasm in the thalamus and superior colliculus (uses somatosensory info to initiate motor movement) then get to the primary visual cortex (V1)
Most axons synapse on cells in the lateral geniculate nucleus (LGN) of the thalamus (also superior colliculus )
Axons of postsynaptic cells terminate in the primary visual cortex (V1) of the occipital lobe
what is the visual pathway (from ganglion cell to primary visual cortex)
Ganglion cells (the axons of it) exit the retina and form the optic nerve and cross at the optic chiasm. The first station this hits is the thalamus to start processing visual information.
The first station that processes visual information from the optic chiasm in the thalamus and superior colliculus (uses somatosensory info to initiate motor movement) then get to the primary visual cortex (V1)
Most axons synapse on cells in the lateral geniculate nucleus (LGN) of the thalamus (also superior colliculus )
Axons of postsynaptic cells terminate in the primary visual cortex (V1) of the occipital lobe
How is the visual pathway from the retina (photoreceptors) similar/different from auditory pathway from basilar membrane (hair cells)?
both go through the thalamus and cross (at different stages information. They both have a crossing of info to different hemispheres. Preference to left side of brain for auditory signals.)
Auditory pathway goes to the brainstem but visual goes to the thalamus.
More relay station to brainstem in auditory system than visual
More complex set of cells (multi-step) graded potential in visual field
what is a receptive field and what influences its size?
specific areas in the retina and LGN that respond to light stimuli
(look at what cells activate when light hits the retina)
Receptive field sizes vary depending on the number of photoreceptors that synapse onto a given bipolar cell and number of bipolar cells that synapse on ganglion cell (i.e. the amount of convergence)
what is the difference between fovea vs periphery receptive fields?
Fovea: small receptive field bc each bipolar cell receives input from only one photoreceptor and then synapses on only one ganglion cell (1 to 1)
Periphery: larger receptive field bc more bipolar cells synapse onto each ganglion (1 to many)
what happens when light hits the on center receptive field vs surround receptive field?
If light hits the center receptive field, the photoreceptor is hyperpolarized and the bipolar cell is depolarized.
If the surround area of receptive field is illuminated, then the photoreceptor is hyperpolarized which hyperpolarizes the horizontal cell thus hyperpolarizing the bipolar cell (opposite effect on bipolar cell when periphery is illuminated bc there is inhibition)
what happens when light hits the on center ganglion cell vs surround?
On-center vs off-surround cell:
- Excited by an increase of illumination in the center of its receptive field
- inhibited by illumination in the surrounding
Changes in illumination have opposite effects on off-center/on-surround cell:
- inhibited by illumination in center
- Excited by illumination in surround
what is the significance of light hitting the on center vs surround ganglion cells?
shows our system is set to compare adjacent areas of visual field; contrast light and dark (falling on center vs surround regions)
Helps us identify patterns of light and dark in our visual field
As information moves from _______ to _______, receptive fields become larger and more complex.
Receptive fields in LGN (thalamus) continue to be ______ like _________ neurons
Once in primary visual cortex (V1), circular receptive fields combine to create receptive fields activated by ________
As information moves from retina to cortex, receptive fields become larger and more complex
Receptive fields in LGN (thalamus) continue to be circular like retinal neurons
Once in primary visual cortex (V1), circular receptive fields combine to create receptive fields activated by lines
What are orientation columns? Where does this take place?
Cells in column respond to different stimuli of particular orientation
Neurons in the primary visual cortex show increased firing rates in response to a preferred line orientation. (neurons change firing rates depending on orientation or edge). lines rotated away from the preferred orientation will not cause activity
what gives the cortex its striped appearance?
the ocular dominance columns
what are the two major visual processing streams from V1?
Dorsal stream: assess location (where info is coming from)
Ventral stream: identify objects (what is it)
what happens if dorsal pathway is lesioned
Disoriented; not able to locate places/space
Imbalance
Cant adjust grip to different items in their environment (or reach out)
