Visual system 1 Flashcards
What is the sense organ for vision?
The eye- responds to light energy
What is a quantum of light energy called?
Photon
How does the number of photons affect brightness?
Greater number of photons= brighter the light appears
what are the packets called that electromagnetic energy come in?
Electromagnetic energy comes in very small packets called Quanta (plural) or Quantum (singular)
what is electromagnetic energy?
Electromagnetic energy is radiant energy that travels in waves at the speed of light
What is visible light?
A band of electromagnetic radiation with oscillating electric and magnetic waves
How is visible light measured?
In nanometers (nm)
How does wavelength relate to colour?
Short wavelengths → blue
Long wavelengths → red
where does light enter the eye?
through the cornea
what does the lense of the eye do?
Light is focussed by the lens on to the retina
What do specialised retinal cells do?
Specialised cells in the retina convert light energy into an electrical signal
What is the optic disc?
The optic disc is where the optic nerve leaves the eye.
This part of the retina has no specialised neurons and gives rise to a blind spot.
Why don’t we notice our blind spot? (optic disc)
We are not normally aware of this blind spot because we have two eyes, and they compensate for each other’s blind spot.
where does an electrical signal leave the eye?
This electrical signal leaves the eye via the optic nerve.
What are the two types of photoreceptors
Rods and cones
Where are rods and cones located?
Rods → Peripheral retina
Cones → Central retina (fovea)
what are the 3 layers of the retina?
- input cell layer
- interneuron cell layer
- output cell layer
What are the functions of rod photoreceptors?
Best in dim light
More sensitive than cones
Black & white vision (no colour)
More rods than cones
what is the function of photoreceptors?
They take light that enters your eyes and convert it into a form your brain can use for your sense of vision
What are the functions of cone photoreceptors?
Best in bright light
Detect colour
Provide detailed visual acuity
Fewer cones than rods
More densley packed in central retina
What are the three types of cones and their wavelength sensitivity?
Short-wavelength cones → Blue
Medium-wavelength cones → Green
Long-wavelength cones → Red
Do cones only respond to one wavelength?
No, they respond best at a specific wavelength but can still respond to others
What determines cone sensitivity to light?
Photopigments in cones
what is trichromatic/trichomacy
there are 3 independent channels for conveying colour info
3 different cone types in the eye for processing colour
What are the three vertical layers of the retina?
Photoreceptors → Bipolar cells → Ganglion cells
what are the input cells in the retina?
Photoreceptors (rods and cones) are the input cells in the retina
what do rods and cones synapse with in the retina?
synapse with bipolar cells
what do photoreceptors release?
neurotransmitters
What do bipolar cells do?
Connect photoreceptors to ganglion cells
what do bipolar cells and photoreceptors produce?
a graded response
what is a graded response?
The change in membrane potential depends on how much neurotransmitter is received and can be depolarizing or hyperpolarizing. This type of change in potential is typically called a graded response since it varies with the amount of neurotransmitter received.
what are the output cells in the retina?
ganglion cells
what do ganglion cells produce?
action potentials
What do ganglion cells do?
Output cells that send information to the brain
what forms the optic nerve?
ganglion cells axons in the outer layer of retina
what is the role of the optic nerve ?
carry sense info from the eye towards the brain for interpretation and organisation
what rate of firing do neurons have?
baseline Level which allows neurons to be either excited, inhibited or resting
what to the three states of neuron firing allow for?
allow for the use of inhibition as a method of providing info
can neurons have a zero rate of firing when resting?
no, inhibition would not be possible because you can’t have less than a zero rate of firing
what does rate of firing mean?
Firing rate is a measure of neuronal activity, indicating how frequently a neuron is sending electrical signals.
conveying information to other neurons
What is a receptive field?
The receptive field of a neuron reflects the features of a stimulus that the neuron will respond to (rate of firing)
features= shape, colour, and movement
what is the excitatory state of firing in a neuron?
The neuron receives enough excitatory input to reach the threshold for an action potential, causing it to fire and send a signal down the axon.
When ganglion cells receive enough excitatory input from bipolar cells, they fire action potentials, sending signals to the brain.
what is the Inhibitory State of firing in a neuron?
The neuron receives inhibitory input that makes it less likely to fire.
When inhibitory input dominates, the ganglion cell’s firing rate decreases or stops.
hyperpolarization, making the inside of the neuron more negative and preventing an action potential.
what is the Resting Membrane Potential of firing in a neuron?
The neuron is not actively firing but is maintaining a steady resting membrane potential (~ -70 mV).
Ganglion cells maintain a spontaneous firing rate even when there’s no strong excitatory or inhibitory input. This allows them to respond dynamically to changes in light patterns.
what receptive field do retinal ganglion cells have?
concentric circular receptive fields that are antagonistic (centre responds to light in a opposite way to the outside) - work in opposite was
either have an ‘on’ centre and ‘off’ surround or vice versa
what are the two types of ganglion cells?
ON-center, OFF-surround → Likes light in the center
- Excited when light hits the center of their receptive field.
-Inhibited when light hits the surrounding area.
- Best respond to a bright spot of light surrounded by darkness.
- if full receptive field flooded with light= baseline level- excitation n d inhibition cancel out
OFF-center, ON-surround → Likes darkness in the center
- Inhibited when light hits the center.
- Excited when light hits the surrounding area.
- Best respond to darkness in centre and light in surrounding (maximum excitation)
- entire centre of receptive field lit up and surround in darkness (maximum inhibition)
-whoe receptive field flooded with light to darkness (baseline level of firing)
why is Antagonistic Center-Surround Organization in ganglion cells important?
This setup enhances contrast sensitivity, making it easier to detect edges, shapes, and changes in light intensity.
helps you see edges and contrasts better instead of just detecting light.
Why are ganglion cells only responsive to small changes within their receptive field?
Their receptive fields are relatively small, so they can only detect local contrast changes.
How does the visual system handle global illumination changes?
It largely ignores them, focusing instead on detecting contrast and edges.
Ganglion cells don’t respond strongly to global illumination because their center-surround receptive fields balance out.
They detect contrast (edges and differences in brightness), not absolute brightness levels.
what is opponent theory of colour vision?
the opponent theory of colour vision proposes that responses from 3 colour channels (red, green, blue) are compared in pairs to allow colour vision
pairings are antagonistic- excitation of one channel inhibits the opponent cone type
How do the centre and surround areas of a retinal ganglion cell’s receptive field respond to colour?
The centre area responds to one colour, while the surround area responds to the opponent colour.
What are the two types of single-colour opponent retinal ganglion cells?
Red-green opponent cells
Blue-yellow opponent cells
Besides colour opponency, what other opponency exists in retinal ganglion cells?
Black and white opponency for luminance detection.
How do red-green opponent retinal ganglion cells respond to light?
Some are excited by long-wavelength (red) light in the centre and inhibited by medium-wavelength (green) light in the surround.
Others are excited by medium-wavelength (green) light in the centre and inhibited by long-wavelength (red) light in the surround.
How do blue-yellow opponent retinal ganglion cells function?
They have the same centre-surround organisation as red-green opponent cells, responding to blue and yellow light in an antagonistic way.
How is the perception of yellow achieved
By summing the outputs of medium-wavelength and long-wavelength cones.
adding them together
What do retinal ganglion cells respond best to?
The contrast between two opponent colours.
what are the 2 classes of retinal ganglion cell?
magnocellular and parvocellular
what do magnocellular cells (M-cell) respond to?
Magnocellular cells respond to large objects and appear to be concerned with the analysis of the gross features of a stimulus, depth and movement
what do parvocellular cells respond to?
Parvocellular cells are smaller and more numerous. P-cells can provide information required for colour vision and are responsible for the analysis of fine detail in the visual image (form)
does inhibition stop the firing ?
Inhibition reduces the firing rate but doesn’t necessarily stop it completely. Instead, it helps fine-tune neural responses by modulating activity relative to the baseline firing rate.
