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Sensation and Perception Exam 2 Questions.docx

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University of Guelph
PSYC 2390
Lana Trick

Sensation and Perception Exam 2 Questions1 How the retina works a What is the difference between lateral summation and lateral inhibition Why is it useful to have both in the visual system How do lateral summation and inhibition work in the retinaLateral summation is the convergence of rods into the same ganglion and it is an additive response The ganglion will only fire if a threshold is reached and rods can have both an excitatory and an inhibitory response Each rod contributes to the overall firing of the cell Lateral inhibition is when 1 cell inhibits its neighbours when activated Cells are connected by the lateral plexus which transmits the inhibitory neurotransmitters Lateral summation helps us see in low illumination and is good for vision in dark conditions Lateral inhibition highlights edges and changes in light This is good for recognizing objects It makes the light edges look brighter than bright and makes dark edges look darker than dark b Describe 2 illusions that are caused by lateral inhibition and explain what happens at the level of the retina to cause the illusion One illusion in lateral inhibition is the mach bands In the mach bands we perceive an exaggerated edge where the light looks brighter than bright and the dark looks darker than dark For example say neurons that are being stimulated with light have a resting action potential of 100 action potentials per second and the neurons in the dark are 20 action potentials per second The amount of inhibition is proportional to the neurons action thpotentials per second eg 110 Therefore a neuron exposed to light will inhibit its neighbours by 10 and it will be inhibited by 20 by each light neuron beside it This means its action potentials decrease from 100 to 80 action potentials per second In a neuron in the dark it will decrease its neighbours by 2 and it will also be decreased by 4 dark neuron on each side This means its action potential will decrease from 20 to 16 action potentials per second A light neuron on the edge of light and dark will not be inhibited as much 10 by light neuron neighbour and 2 by dark neuron neighbour therefore it is decreased from 100 to 88 action potentials per second A dark neuron on the edge of light and dark is inhibited more 10 by light neighbour and 2 by dark neighbour therefore its action potential goes from 20 to 8 Because of this we perceive the dark as darker than dark and the light as brighter than bright even though it is physically the same intensity Another illusion from lateral inhibition is simultaneous contrast A grey box on a white background will look darker than the same grey box on a black background This is because on a white background the grey box will be inhibited more and look darker than a black background which inhibits the grey box less thus making it look brighter c A retinal ganglion cell has a circular surround oncenter off surround receptive field What does that meanThis means that to maximize the action potentials of the cell you want to shine the light in the middle of its receptive field If you were to shine light all over the receptive field the cell would continue with spontaneous activity because the on and off of the cell would cancel out d If the neuron in part c had inhibitory and excitatory areas that were equal in size and had a spontaneous activity level of 30 action potentials per second how would the cell react ifi A small white dot illuminated the center of the receptive field and the rest of the area was blackThe cell would increase in action potentials over 30 action potentialssecond ii A white light illuminated the entire receptive field The cell would continue at 30 action potentials per second inhibitory and excitatory cancel out iii A small black dot illuminated the center of the receptive field but outside that area is white
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