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psych-Lateral_Inhibition_handout

psych-Lateral_Inhibition_handout - 1 Lateral Inhibition...

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1 Lateral Inhibition, Mach Bands, and the Herman Grid Psych 1101 F – Fall 2008 The first step in understanding lateral inhibition is to understand receptive fields. Receptive fields refer to the fact that many rod receptors (r) can be wired to one ganglion cell (G) via the layer of horizontal cells (not pictured here for the sake of simplicity). The ganglion cell, like any neuron, is constantly summing the inputs from its synapses from the rods of the receptive field when “deciding” whether or not to fire and thus pass along a message up the visual pathway. Remember that the inputs to the ganglion cell from the rod cells can either be excitatory —making the ganglion cell more likely to fire, or inhibitory , making the ganglion cell less likely to fire. Remember that all rods are sensitive to light energy. When light falls on the rods, they send a message to the Ganglion cell. IMPORTANT: Not all rods send the same message, even though the same light may be falling on the rods. In a center-surround receptive field with an excitatory center (shaded rod cells here), the rods in the center will tell the synapsed ganglion cell to fire when light falls on them When the same light falls on the surrounding (i.e., the outer circle of) inhibitory cells (unshaded here), the opposite message is sent to the ganglion cell, namely don’t fire. Both the excitatory center and the inhibitory surround are stimulated by the same light, but they send different messages and have different effects on the ganglion cell.
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2 So we have a couple of layers of action to consider. First, light will cause the receptor cells to fire, with center cells sending excitatory messages and surround cells sending inhibitory messages to the ganglion cell. Second, we have the ganglion cell summing up all of these excitatory and inhibitory inputs that determine what message the ganglion cell will send up the visual pathway. Let’s say this sphere represents light energy falling on the receptive field. In this example, the light is only falling on and activating the excitatory center
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