Module 29 - Vision Flashcards
What is the sensory system hierarchy for the visual system?
Basic neurons in the eye and the retina such as the rods and cones → go through the optic nerve to a specialized area in thalamus called the lateral geniculate nucleus and ultimately lies in the primary visual cortex.
What are the 3 main steps for the information to reach higher processing areas in the visual system?
- Eye and retina
- Primary visual cortex
- Visual association areas
What is the Retina?
- Retina = receives light from the lens and converts it into neural signals that the brain can understand.
- It is a laminated neural component of the eye that contains photoreceptors = which are the specialized neurons that respond to light.
- It is the initial processing machinery for the visual pathway.
What is the optic disc?
- This is the region where the axons leaving the retina to gather to form the optic nerve.
- There are no photoreceptors here → blind spot
- The blindspots are not superimposed, they are slightly different for each eye, therefore you have no functional deficits when you are using both eyes at once.
What is the Fovea?
- The fovea is the central fixation point for each eye.
- Region of the retina with the highest visual acuity.
What is the Macula?
- Oval region surrounding the fovea.
- It also has high visual acuity.
- Occupies the central 5 degrees of visual space.
- Detailed vision
- Small in size, but provides input to about half of the optic nerve fibers and cells in the visual cortex.
- It corresponds to the central 1-2 degrees of visual space
What is the general pathway of the light to travel in the fovea and macula to be perceived as information?
- As you can see in the image the light is coming in a different direction than the information.
- Light will enter the area and hit the photoreceptors, and then the information will travel backward (back to where the light came from = as you can see in the image).
What are photoreceptors?
- They are cells in the retina that respond to light
- They are specialized neurons.
What are the two types of photoreceptors?
Rods and cones
What are a few of the characteristics of RODS photoreceptors?
- Primarily found in the peripheral retina
- Achromatic → they do not see color
- High light sensitivity
- Nighttime vision, when light levels are low
- More rods than cones (20:1)!!!
What are a few of the characteristics of CONES photoreceptors?
- High-acuity and colour vision during day-time when light levels are higher
- Primarily found in the central retina (fovea)
- Cones are less numerous than rods overall but there are more of them located in the fovea. This makes sense because the fovea has the highest level of acuity, therefore we need cells that are specialized for high acuity and color.
Why do we have an acuity grading of the visual field?
- The two systems of photoreceptors both the Rods and the Cones allow the visual system to meet the conflicting demand for sensitivity and acuity.
- The way the photoreceptors are laid out allows for our visual field to meet the visual demands.
- The optic disk = the blind spot = where the axons are coming out
- Blurred peripheral vision = A high number of Rods for the peripheral vision and barely any for the fovea/central part
- Central detailed vision = A high number of Cones for the fovea/central vision and barely any at the peripheries.
What are the main players of the eyeball
Retina
Fovea
Optic disk
Photoreceptors
Rods/cones
What are the main players for phototransduction?
Retina
Fovea
Optic disk
Photoreceptors
- Rods/cones
How is light translated into a neural signal our brain can understand?
The photoreceptors will respond to light, and we know that photoreceptors provide information to ganglion cells.
What is the receptive field for photoreceptors?
- All photoreceptors (cones and rods) CANNOT respond to everything at the same time.
- Otherwise, everything would look the exact same all the time → it is similar to touch, if you hit your pinky finger with a hammer your whole hand isn’t going to feel it.
- But if you hit your finger in space where there isn’t a receptor field (which isn’t necessarily possible but for argument sake) you aren’t going to feel it.
- For the eye or the visual system, it is the same but instead of this mechanical system or the hammer, you need to think of the visual system responding to light
- But if you hit your finger in space where there isn’t a receptor field (which isn’t necessarily possible but for argument sake) you aren’t going to feel it.
- The receptive field of a neuron in the visual pathway is defined as the portion of the visual field where light causes excitation or inhibition of the cell.
- Each cone and rod has its own receptive field.
What is a graded potential?
A graded potential is a change in membrane potential that varies in size. It is NOT an all or none phenomenon.
Recap: If the information that is felt on the skin, is sufficient in magnitude an AP will happen and the information will travel up to the brain. This is how the sensory system, or in terms of touch works. NOW: There are a few differences when we talk about phototransduction. Talk about these differences…
- Photoreceptors (rods and cones) use graded potentials, not action potentials, to control the release of neurotransmitters onto post-synaptic neurons.
- In the eye, membrane potential changes are done in a graded response with varying degrees of light.
- Photoreceptors respond to light by hyperpolarizing. NOT depolarizing.
- Light = hyperpolarized = low NT release onto post-synaptic neuron
- Dark = relatively depolarized = high NT release onto post-synaptic neuron
What are the two main differences between photoreceptors and other sensory afferents?
- Photoreceptors use GRADED POTENTIALS
- More light (more stimulus) causes HYPER-POLARIZATION (less NT release)
Where do interneurons fit in the light pathway in the eyeball?
- Once photoreceptors (cones and rods) have responded to the light, the information will then travel through the retina through the bipolar cell layer and ganglion cell layer before it goes out the axon to the optic neuron. As they go through the bipolar and ganglion cell layer there are these interneurons called horizontal cells and amacrine cells
- The interneurons are quite similar to the stellate and basket cells in the cerebellum –> they have a lot of lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells
How are the interneurons in the eyeball similar to the stellate and basket cells in the cerebellum?
The interneurons are quite similar to the stellate and basket cells in the cerebellum –> they have a lot of lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells
Where do the horizontal and amacrine cells reside?
In the bipolar cell layer.
What are the two types of cells within the ganglion cell layer?
Parasol (ganglion) cells
Midget (ganglion) cells
Describe the parasol (ganglion) cells.
- Gross stimulus features and movement
- Large cell bodies
- Large receptive fields
- Large axons
Describe the midget (ganglion) cells.
- Fine visual detail and color
- Small cell bodies
- Small receptive fields
- Smaller axons
- More NUMEROUS → compared to the parasol cells
The photoreceptors will respond to light, and we know that photoreceptors provide information to ganglion cells. BUT how do these ganglion cells respond to light?
- Retinal ganglion cells have receptive fields (receptive field = area of the visual field that cell monitors/responds to)
- There are on-center (off surround) and off-center receptive fields
- On-centre (off surround): a light spot in the centre of receptive field excites
- Off-center (on surround): a light spot in the center of the receptive field inhibits
What happens to when light hits an on-center (off surround) ganglion cell?
- If we think of a hammer as the light (the stimulus) hitting an on-center cell, the light spot in the center of the receptive field will want to excite the cell and information is going to want to travel through the afferent and continue to the optic nerve to the brain.
What happens to when light hits an off-center (on surround) ganglion cell?
- If we think of a hammer as the light (the stimulus) hitting an on-center cell, the light spot in the center of the receptive field will want to inhibit the cell and information is going to stop and will NOT travel through the afferent and continue to the optic nerve to the brain.
- The cell is not receptive to information coming its way, it does not want to transmit any information any further if there is a light spot in there.
How is glutamate involved in the off-center cell response?
- Light hyperpolarizes cone
- Less glutamate is released
- Glutamate is EXCITATORY to off-center bipolar cell
- This results in less excitation of ganglion cell = fewer action potentials = less
- *Removing its influence means bipolar cell now less excited (hyperpolarized)
What is age related macular degenerations (AMD)?
- Damage to macula.
- Central vision affected
- Objects are obstructed and unclear
What is diabetic retinopathy?
- Excess blood sugar damages retinal blood vessels.
- There is no specific to which blood vessel is damaged.
- Patchy vision loss throughout the visual field.
- There are areas where the rods and cones don’t receive blood supply and are not able to work
- Vision is obstructed by macular edema.
What is glaucoma?
Increased intraocular pressure = damage to optic nerve
Often peripheral deficits
What are cataracts?
Loss of lens transparency
Objects are obstructed and unclear
How does information travel to the primary visual cortex? Describe the whole pathway.
- As light enters the eyes, it forms an image on the retina, and then this image is inverted and reversed.
- The light will pass through the rods and cones and afterwards pass into the opposite direction through the ganglion cells.
- The information then comes out from the optic nerve.
- The information is crossed at the optic chiasm. Only 60% of the fibers of each eye is crossed (for example, the optic tract on the left has 60% of fibers from the right eye and 40% of fibers from the left.)
- As soon as the fibers pass through the optic chiasm, we now call this the optic tract. (the information that is contained in the tract is from BOTH eyes)
- Then the information goes through the lateral geniculate nucleus, which is in the thalamus
- Then it goes through the optic radiation which is within the internal capsule.
- And then it ends up in the primary visual cortex, this is called V1 or brodmann’s area 17
Explain what does it mean that the image in the retina is inverted and reversed?
- Inverted and reverse, meaning that the image has been flipped sideways and then up/down.
- Information from the upper visual space is projected on the lower retina
- Information from the lower visual space is projected on the upper retina
- Information has the same concept for left and right
What are the four retinal projections that project to important subcortical regions?
Lateral geniculate nucleus = the primary one
Pretectum
Superior colliculus
Hypothalamus
What are homonymous field deficits? What is a possible cause? What are some examples?
- Homonymous field deficits means deficits within the same visual field.
Cause = following a stroke
What are the response characteristics of cells in the primary visual cortex?
- Rather than responding only to light, we see that the cells within the primary visual cortex respond to different characteristics within a higher level characteristic.
- For example,
- Preferred orientation = these cells all respond to hard edges. Within all of these cells that respond to lines, some cells respond preferentially to a preferred orientation.
- Therefore if you take the graph on the right hand side and look at the neuron that is in the middle, you can see that where it responds the most, its spike rate is when the stimulus is at its preferred orientation.
- All edge orientations are roughly equally represented in the visual cortex, all these lines are assigned a cell, and have a given orientation.
- Different cells have different characteristics which are encoded into their activity. There are also neurons that act in a similar fashion for direction of motion, right versus left and others.
What kind of information is processed through the parietal/Dorsal stream?
- It is processing WHERE?
- The analysis of motion and spatial relations
- *Stroke = will typically happen in the parietal lobe because this is where things like motion and spatial relations are analysed
What kind of information is processed through the Temporal/Ventral stream?
It is processing the WHAT?
Analysis of form and color
Explain the visual pathway of colour.
If information was colour it would be processed in the midget cells –> they would go to the parvocellular layers of the thalamus which then goes to V1 and V2 in the what pathway = the inferior occipital-temporal cortex.
Explain the visual pathway of form.
If information was form it would be processed in the midget cells → they would go to the parvocellular layers of the thalamus which then goes to V1 and V2 in the what pathway = the inferior occipital-temporal cortex.
Explain the visual pathway of motion/spatial analysis.
If information is motion/spatial analysis it was processed by the parasol cells, it would go into the magnocellular layers and then to V1 and V2 and then the where pathway = dorsolateral parieto-occipital cortex.
Give an example of the WHAT pathway to explain how it works
- We have talked about that within the primary visual cortex, we have some cells responding to different orientations of a line.
- We also have recognition neurons within the temporal lobe, and these recognition neurons preferentially respond to different visual stimuli. So for example, FACES
- Study on monkeys:
- There are recognition neurons that preferentially respond to different visual stimuli (faces)
- Response is maximal when preferred stimuli (faces) are present
- A population of cells with similar response characteristics allows us to perceive complex stimuli
- If you look at the images from 1-5… we see that these recognition cells fired when a picture of a monkey face was shown 1 and 4… There was a little firing when a human was shown since we do look like monkeys. BUT there was no firing of these neurons for a hand.
- As an example, a lesion in the right inferior temporal lobe produces prosopagnosia which is an inability to recognize faces
What are the general cells and corresponding structures of how the visual information is progressively more complex when moving from the retina to higher visual processing centres?
Answer = image
Explain how the sensory system for touch and the sensory system for vision works the same way.
Answer = image
