PSYC 213 - Chapter 2

PSYC 213 - Chapter 2 - How Stimuli Are Represented by the...

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How Stimuli Are Represented by the Firing of Neurons As explained in the last section, some neurons respond best to specific types of stimuli. But how does neural firing enable us to recognize specific faces? The neural code of an object, or in this case of a face, is the information contained in the neural firing to that face. The Neural Code for Perceiving Faces A specific face is represented by the firing of neurons in the temporal cortex. Specificity coding is one of the possible ways that this may happen. Specificity coding is when specially tuned neurons respond to just one specific stimulus. Figure 2.13 (p. 41) shows how neuron 1 responds to Bill’s face, neuron 2 to Samantha’s and neuron 3 to Roger’s. The idea of specificity coding came up in the 1960s by Konorski and Lettvin. Lettvin further developed this theory by using the term grandmother cell to define a neuron that responds only to a specific stimulus. It could be a specific image or concept, like a picture of your grandmother or the idea of grandmothers in general. There are problems with this idea: 1. There are too many different objects in the environment for there to be a specific neuron for each one. 2. Even though there are neurons that respond best to specific shapes (simple cells, complex cells, end-stopped cells etc.) and to different types of stimuli (faces), even these respond to a number of different forms. So, the specific grandmother type neuron has NOT been accepted by researchers. The generally accepted solution to the question of sensory coding is that a particular face (object) is not represented by the firing of a specific neuron, but by the firing rate of groups of neurons.
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The different faces in this figure cause all 6 neurons to fire, though at different rates. It is the combination of firings of the group that indicates a specific face. Each face is represented by a different pattern of firing. This phenomenon is called distributed coding because the code that indicates a certain face is distributed across many neurons. Advantages of distributed coding: - The firing of just a few neurons can signal a large number of stimuli. The 6 neurons of the figure indicate three different faces in this example, but they are capable of signalling many more. So our capacity of easily identifying and recognizing the objects our environment is the end result of distributed cooperation between the neurons. The Neural Code for Other Cognitive Capacities
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This note was uploaded on 02/13/2011 for the course PSYC 213 taught by Professor Levitin during the Winter '08 term at McGill.

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PSYC 213 - Chapter 2 - How Stimuli Are Represented by the...

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