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Chapter 3: Vision, Part 2

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Western University
Psychology 2115A/B
Christine Tsang

Chapter 3 Review: Intro to Vision, Part 2 By the end of this section, you should know about: - Neural convergence: the connections expand laterally across the retina - Lateral inhibition: what happens after phototransduction  from receptor to the ganglion cell out of the retina to the cortex Neural Responses to Light  The output of the photoreceptors (hyperpolarization) is transmitted to the retinal ganglion cells  Convergence of rod and cone signals onto the ganglion cells o Way more rods than cones in the retina o As a result, rods show greater convergence than cone cells o A lot more rods feed into one ganglion cells than cones o This means that rods do greater spatial summation: many rods summate (add up together) their responses and feed them to a single ganglion cell, while this doesn’t often happen in cones o Greater spatial summation means that we can detect specks of light all across our visual fields and it doesn’t have to be particularly intense for us to sense that it is there o The cones don’t have this sensitivity to dim spots of light o This great spatial summation we trade off for detail – we can detect that spot of light but we aren't very good at knowing what or where the spot of light is o Cones give us a lot of very good detail/acuity Neural Convergence  120 million rods and 5 million cones convergence to 1 million ganglion cells  Higher convergence of rods than cones o Average of 120 rods to one ganglion cell o Average of six cones to one ganglion cell o Cones in fovea have one to one relation to ganglion cells  The reason why the fovea is the most visually acute part of the eye Rod Sensitivity > Cone Sensitivity?  Why is it the case that the sensitivity of the rods to light in comparison to the rods?  Rods take less light to respond – fewer photons of light to generate a response in a rod than we would in a cone  Rods have to add together all of their responses to create a response in a ganglion cell – increase likelihood of the ganglion cell responding Visual Acuity  The ability to see fine detail  Foveal acuity vs. peripheral acuity o Best acuity in the fovea since it is full of cones o Acuity largely reduced in low light levels Cone Vision and Visual Acuity  One to one wiring between cones and ganglion cells allows for detailed vision, but the trade off to that is low sensitivity  Testing whether you can individuate two spots of light in a rod circuit vs. cone circuit  Showing two spots of light to two adjacent receptors  Rods are unable to tell the ganglion cell whether there are two different spots of light or one big spot of light  Because there is a one to one connection in the cones, two separate ganglion cell fires when the two spots of light are shining and we are able to tell there are two spots of light  Rod system Allows us to see as much light as possible but is traded off by not being able to differentiate the separate spots of light  The cone system needs intense light to sense it but once they transmit the signal they can give you more information about the signal  Low convergence in cone system results in high acuity, but cones at a cost of sensitivity  High convergence in rod system allows for high sensitivity, but comes at a cost for acuity Lateral Inhibition  Inhibition transmitted across the retina  Where our bipolar cells come into play – do a lot of the connecting across one level of the retina Lateral Inhibition and the Hermann Grid  When you look at the grid, you see flashing grey dots at the intersections of the white lines  The grey spots are entirely perceptual – are not there in the image, we impose them from our mind into the image  The spots are thought to arise form bipolar cell activity – receptors that correspond to the white corridors send inhibitory signals to receptors that are responding to the black squares in the intersections  Inhibition causing grey to arise at the intersections How Does the Hermann Illusion Work?  Corridors of white and black boxes around then that create the corridors themselves  Different receptors in each of the intersections of the grids  Each bipolar c
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