What can be said about animal models when using them to understand human vision?
1) Animal models provide insight into the human visual system (evolutionarily the same)
2) Consideration of the visual system in the animal model we are utilizing (i.e., considering “unwelt’, what is the animal’s capacity for vision?)
What are the simple cells in V1 responsible for?
- Each simple cell is tuned to respond to a specific orientation in space
- A stimulus must match this orientation (ex. positioned at 45 degrees) for the simple cell to respond highly
What are the simple cells if V1 connected to?
- Retinal Ganglion cells (RGCs) in the retina
- They have their own receptive fields that correspond to those in V1
- They are located on the topmost layer of the retina
What’s the order of neural structures that a light stimulus passes through?
1) The receptive fields in retinal ganglion cells (RGCs)
2) The lateral geniculate nucleus (LGN) in the thalamus
3) Simple cells in the primary visual cortex (V1)
What’s the difference between complex cells and hypercomplex cells?
- Complex cells - Tuned to a specific orientation and motion
- Hypercomplex cells - Tuned to a specific orientation, motion, plus inhibition areas
- Both contain all this info in a firing rate
What methodology did Hubel and Wiesel use to determine the presence of receptive fields in V1?
1) Anesthetized a cat and then fixed its head into position and propped its eyes open so that it was facing a screen that would contain a stimulus of light
2) A recording electrode was placed into V1 of the cat
3) Flashed an angled band of light onto a screen and moved it around to determine the position of that specific simple cell
4) The recording electrode would emit a continuous clicking sound indicating that they had found the simple cells that were tuned to that specific orientation
5) They were able to trace the the receptive field of the cell on the screen they were projecting the light to.
Who discovered ocular dominance columns?
- Also Hubel and Wiesel
How were ocular dominance columns discovered?
1) Injected a radioactive tracer into the eye of a normal cat which eventually makes its way to V1
2) Sacrificed the cat and then observed the striped pattern in the cat’s V1, indicating which cells were activated
- The patterned distribution of columns indicated how visual information from each eye is represented in each hemisphere
T/F: Ocular dominance columns are also observed in humans.
- TRUE
- Illustrates how simple systems can be observed across multiple species
T/F: Rats do not have ocular dominance columns
- TRUE
- Their vision is very poor
What’s the chemoaffinity hypothesis?
- Describes how neurons and their axons/dendrites are drawn towards a signalling chemical that indicates the correct pathway
What are the two general ways that the chemoaffinity hypothesis works?
- Can provide the general location of where neurons to go. Like placing neuronal pathways in the correct neighbourhood (ex. Retinal ganglion cells going to the optic tectum, then to the striate cortex)
- Also experience-dependent modification where neuronal connections are fine-tuned once in the correct locations. Largely depends on the types of inputs they receive
What’s one major way to understand developmental cortical plasticity?
- Can be illustrated by monocular deprivation (i.e., removal of visual input via an eyelid suture
What are the cortical changes that can be observed in a monocular deprivation experiment?
- Major reduction in size of ocular dominance columns for the closed eye
- Columns associated with the good eye encroach on those of the bad eye
What are considered the critical periods for ocular dominance columns to develop in cats and humans?
- 2 years for humans
- 4 months for cats
Why do the seeing eye columns in monocular deprivation take up a lot more space?
- Since this eye is responsible for taking in all the visual inputs, it also requires a lot of resources to adapt its structuring to the heightened volume of stimuli relative to the sutured eye
- Considering the dormancy in neuronal activity of the sutured eye, the plastic capacities of the seeing eye allow it to encroach upon the columns of the sutured eye
- Regardless, the general structure of the columns is still present (brain knows not to be too plastic)
How do sensory-dependent modifications at the axonal level differ among the seeing eye and the non-seeing eye during monocular deprivation?
- Axons in the seeing-eye tract are much more complex and have more surface area compared to those in the non-seeing-eye tract.
What happens to the ocular dominance columns when a third eye is inserted into the frogs head?
- While the visual input from both eyes completely crosses over to the other side in a frog, there is no need for the presence of ocular dominance columns
- When a third eye is inserted, it creates competition for resources between the two other eyes since visual input from this eye is now being directed to both hemispheres
- To accommodate these new connections, columns start to form due to new competition for cortical real-estate
What would happen to the ocular dominance columns if the third frog’s eye was covered for a while?
- Over time the ocular dominance columns would start to fade over time
- Since many behaviours are mediated by vision, and our behaviours impact brain function, having a completely dormant eye that is not taking in visual input would require minimal resources and would not be exercising the neurons in the optic tectum. This would allow the seeing eye to begin to encroach upon the columns developed by the third eye
Which receptor is involved in experience-dependent modification in frogs?
- NMDA a type of glutamate receptor which is the primary excitatory neuron in the nervous system.
- Plays a role in further organizing the ocular dominance columns
What’s amblyopia?
- “Lazy eye”
- There’s decreased vision in one eye, which may be due to a host of reasons (irregular shape; nearsightedness; improper alignment
What’s a major solution for amblyopia?
- Cover the good eye with a patch for a period of time (on a developing child)
- This will make the lazy more active and responsible for processing all visual input and letting the other eye lay dormant
- This reduces the competition for resources required between the two eyes for vision and gives the lazy eye time to catch up in terms of developing its visual tract so it may develop areas such as its ocular dominance columns in the striate cortex in both hemispheres (i.e., V1) .
