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Chapter 3

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Gautam Ullal

Psych 2E03: Sensory Processes Chapter 3: Neurons and Perception Convergence: Many Neurons Sending Signals to One Neuron - Convergence in the retina:  Rod and cone receptors send their signals to bipolar cells, which synapse onto ganglion cells which send the signal to the optic nerve  Receptor, bipolar, and ganglion cells transmit signals that originate in the receptors vertically trough the retina and out the back of the eye  Horizontal and amacrine cells: transmit cells horizontally across the retina  Each ganglion cell receives signals from 126 receptors  Signals from rods converge more than the signals from cones  Many of the foveal cones have private lines to ganglion cells  Rods result in better sensitivity than cones and cones result in better detail vision than rods - Why rods result in greater sensitivity than cones:  Takes less light to generate a response from an individual rod  Rods have greater convergence than cones  Receiving excitatory transmitter from a number of neurons increases the chances that a neuron will fire - Why do we use our cones to see details:  Only all-cone foveal vision has good visual acuity  Foveal cones are tightly packed whereas peripheral cones are more widely spaced  Acuity changes during dark adaptation Inhibition: How Activity In One Neuron Can Decrease Activity In Another One - What the horseshoe crab teaches us about inhibition:  Limulus eye is made up of hundreds of tiny structures called ommatidia, and each ommatidium has a small lens on the eye’s surface that is located directly over a single receptor.  Very large receptors compared to humans so it is possible to illuminate and record from a single receptor without illuminating its neighbouring receptors  Illumination of neighbouring receptors inhibited the firing of receptor A  Lateral inhibition: inhibition transmitted laterally across the retina  Lateral inhibition is transmitted across the Limulus’ eye by the fibres of the lateral plexus.  In humans lateral inhibition is transmitted by the horizontal and amacrine cells. - Lateral inhibition and lightness perception:  The Hermann grid – seeing spots at intersections:  Ghostlike gray images at the intersection of white corridors  Reduces or vanishes when you look directly at an intersection or by masking the four squares that surround an intersection  Size of the bipolar cell response is depends on how much stimulation each one receives from its receptor and on the amount that this response is decreased by the lateral inhibition it receives from its neighbouring cells.  Depends on whether the other receptors are located in the corridors or in the boxes  A’s bipolar cell receives more lateral inhibition that B’s bipolar cell, so it will fire less than B, and this lower rate of firing in translated into the gray spots wee see at the intersections.  We perceive what is out there as filtered through the properties of our visual system.  Mach bands – seeing borders more sharply:  Mach number: ratio of an object’s speed to the speed of sound  All our knowledge is caused by the stimulation of our senses  Mach bands: tendency to see light and dark bands near the borders between light and dark areas not actually present in the pattern of light  Receptors A, B, and C receive intense illumination, and D, E, and F receive dim illumination.  Lateral inhibition and simultaneous contrast:  Simultaneous contrast occurs when our perception of brightness or colour of one area is affected by the presence of an adjacent or surrounding area  Viewing squares of apparently different contrast through holes  Simultaneous contrast is caused by the backgrounds surrounding the central squares  Cells receiving signals from receptors under the lighter surround are intensely stimulated, so they send a large amount of inhibition toward the cell in the center. Receive more inhibition, their response is decreased more, they fire less and the square therefor looks darker  Cells receiving signals from receptors under the darker surround are less intensely stimulated, so they send only a small amount of inhibition to the cells in the center  Expect that the square would look lighter near the border and darker in the center. The fact that this does not occur suggest that lateral inhibition cannot be the whole story behind simultaneous contrast. - Displays that can’t be explained by lateral inhibition:  Benary cross: triangle B looks lighter than triangle A even though they both reflect the same light. Both triangles are surrounded by dark areas on two sides and a light area on one side, receiving the same amount of lateral inhibition.  Perception of light and dark may not be determined solely by lateral inhibition in the retina  White’s illusion: light gray rectangle and dark gray rectangle. The bars that create each rectangle reflect exactly the same amount of light. The areas receiving the most inhibition look the lightest.  Belongingness: an area’s appearance is influenced by the part of the surrounding to which the area appears to belong to.  The mechanism behind many contrast effects appears to be based not only in the retina, but probably in the cortex Neural Processing: Interactions Between Neurons - Neural circuits: groups of interconnected neurons - Excitation, inhibition, and neural responding:  Neural circuits can be very simple, or extremely complicated  No convergence with excitatory synapses: the firing of neuron B simply indicates that its receptor has been stimulated and doesn’t provide any information about the length of the bar of light  Convergence with excitatory synapses: when we monitor the firing rate of neuron B, we find that each time we increase the length of the stimulus, neuron B’s firing rate increases. This occurs because stimulating more receptors increases the amount of excitatory transmitter released onto neuron B providing information about the length of the stimulus.  Convergence with two inhibitory synapses: neuron B fires weakly to small stimuli or longer stimuli and fires best to a stimulus of medium length responding best to a light stimulus of a specific size. Neurons that synapse with neuron B act as part of a neural circuit that enables the firing of neuron B to indicate the size of the stimulus falling on the receptors. - Introduction to receptive fields:  Receptive field: area on retina that influences firing rate of neuron  Position electrode to pick up signals from single nerves then small spot of light is flashed on screen and plotted  Excitatory area: increase in neuron’s firing rate  Inhibitory area: decrease in firing rate  Center-surround receptive field: areas of receptive field are arranged in a center region that responds one way and a surround region responding in the opposite way  Center-surround antagonism: center and surround of the receptive field respond in opposite ways  Bigger stimulus on the excitatory area increases cell’ response  Stimulation of inhibitory surround counteracts center’s e
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