Lecture 2- Visual System Flashcards
Where do the photoreceptor cells lie in relation to the rest of the retina?
-Ganglion cells at front are the output
-Middle layer connecting the ganglion cells to the photoreceptor cells
-Photoreceptor cells are at the back of the retina
What are the two kinds of photoreceptors? What type of vision are each responsible for?
-Rods= scotopic (achromatic vision, low-levels of light, low resolution/ acuity)
-Cones= photo topic (colour, high levels of light, high acuity)
How does the distribution of rods and cones differ according to where you are in the retina? What does this mean for vision?
Fovea= highest distribution of cones: drops away very quickly within a few degrees of the fovea .This means the fovea has the highest acuity vision (resolution is lower in the periphery)
As you get to the periphery there are more rods. This means in low light/ night central vision isn’t very good (not much rods only cones) but peripheral vision is fairly strong.
Which of rods and cones is low convergence and which is high convergence in terms of the ganglion cells that lead into it?
-Cones= low convergence. Means 1 cone activates a small number of bipolar cells and retinal ganglion cells. High resolution, know the direct source of the information.
-Rods= high convergence. Means multiple rods activate many of the same bipolar cells and in turn retinal ganglion cells. Means low resolution in vision as it is easy to confuse the source of information (don’t know which rod led to activation).
Why is the mona Lisa so confusing?
The features that characterize her as smiling are in the periphery/ low spatial frequency or resolution parts of the image. This means when you look away and using rods you see smile but when you turn head and focus those parts on the fovea you are using high resolution cones and don’t see the smile.
Are photoreceptors (rods and cones) the only cells in the eye sensitive to light?
No, the eye also contains intrinsically photosensitive retinal ganglion cells
What do intrinsically photosensitive retinal ganglion cells do?
These cells utilize the photopigment melanopsin which is
maximally sensitive to blue light.
They have connections through to the superchiasmatic nucleus
(SCN) and are critical for entraining circadian rhythms
Helps then make melatonin crucial for triggering sleep
How does light work in terms of resetting our body’s internal clock? How does the position of intrinsically photosensitive retinal ganglion cells aid this?
-Light synchronizes our internal body clock with the external world
-Intrinsically photosensitive retinal ganglion cells are located largely in the lower portion of retina meaning we are most sensitive to light coming from the sky
What is the blind spot in your retina caused by?
-It is caused by the optic disc where all the nerve fibers in the eye (ganglions) exit
-Obviously there are no photoreceptors here which causes the blind spot
-Usually we accommodate
How is an EPSP produced as opposed to an IPSP at the level of ion channels?
-EPSP produced by binding of glutamate/ neurotransmitter onto a sodium channel (let sodium in leads to Depolarisation)
-IPSP produced by binding of glutamate/ neurotransmitter onto potassium channel (let potassium out means goes more negative, hyperpolarization)
What are the role of amacrine and horizontal cells?
-Release GABA/ neurotransmitters that mediate the connections between ganglion cells and bipolar cells and bipolar cells and photoreceptors
-Mediate the responses of the surrounding cells
What is lateral inhibition/ how does it work in terms of contrast?
-Intensity of two squares is identical on right and left but perceive the one of the right (with darker background) as brighter
-This is because the cells away from the fovea in periphery are not activated at all by the dark background and so they do not inhibit the function of the fovea cells in picking up the light square
-Definition of lateral inhibition: the phenomenon in which a neuron’s response to a stimulus is inhibited by the excitation of a neighboring neuron.
What is lateral inhibition known as?
An opponent process
How do Mach bands shown lateral inhibition? What is happening at the more cellular level?
At edge of colour bands= dark band on left of junctions and light band on right. In other words, edges of darker objects next to lighter objects will appear darker and vice versa.
We just perceive this, it is not actually there
As a result of lateral inhibition/ cells competing for representation. At the structural level this is possible because of horizontal cells connecting various different cells. Strong activity in one area/ group of cells causes the horizontal cells connected to inhibit other cells that aren’t being stimulated quite as much.
Does colour exist external to the mind?
No, we experience colour as a result of perception by nervous system
Colour is simply different electromagnetic frequencies being picked up photoreceptors
