visual Cortex 2-1 - Emergent Property is a property built...

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Unformatted text preview: Emergent Property is a property built up through a circuit composed of several lower neurons that themselves have specific properties and whose convergence onto a higher order cell creates a new (emergent) property. Hierarchical Processing is the successive construction of new emergent properties that are built up progressively by combining properties created previously. Orientation Simple cell of simple cortex visual cortex Simple specificity in visual cells in is an emergent property that is built thru hierarchical processing built from convergence of several cells in layer 4C that have center surround receptive fields which are slightly offset in space - +- +- +- +- All geniculate cells make excitatory connections with stellate cells in layer 4C + + + + + All stellate cells make excitatory connections with simple cortical neuron + + + + + 3 simple cells Cell fires + + Complex cell + + + + + 3 simple cells Cell fires + + Complex cell + + + + - + + + + + 3 simple cells Cell fires Complex cell + + + + - + + + + - + + + + Cell fires 3 simple cells + Complex cell - + + + + - + + + + - + + + + No response 3 simple cells Complex cell Receptive fields of each simple cell overlap and are tiled Neuron fires strongly Neuron fires weakly No response Neuron fires strongly 1 2 3 4A 4B 4C 5 6 Complex and End-Stopped Simple and Complex Simple and Complex Center-surround Complex Complex From lateral geniculate The Two Emergent Properties that We Consider Today are: 1) Motion Directional Selectivity 2) Binocularity . We shall see how these properties are created through Hierarchical Processing We will next show that orientation and binocularity have orderly arrangements in the primary visual cortex (V1). Those arrangements create functional units of neuronal organization, the Hypercolumns We consider first the property of Directional Selectivity for Motion . Some cortical cells respond much better to a moving bar of light than a stationary bar. However, some cells do not care whether the bar moves up or down. Thus, these cells are not directionally selective. Place microelectrode in visual cortex to record action potentials from neurons in visual cortex Weak response Receptive Field Some cortical cells respond much better to a moving bar of light than a stationary bar. However, some cells do not care whether the bar moves up or down. Thus, these cells are not directionally selective. Strong response Receptive Field Cortical cells respond more vigorously to moving bars of light than to stationary bars but are not directionally selective. Stationary bar flashed in receptive field Receptive Field Bar moved downward through receptive field Bar moved upward through receptive field Directionally selective neurons fire much more vigorously to a bar of light moving in one direction than when the bar moves in the other direction or when the bar of light is stationary Strong response No response Receptive Field Cortical cells that are Directionally Selective for motion respond much more vigorously or only to bars of light that move in the preferred direction. Stationary bar flashed in receptive field Receptive Field Bar moved downward through receptive field Bar moved upward through receptive field Same receptive field Same receptive field 1 2 1 2 3 4 Circular center-surround receptive fields 3 4 Inject tracer into one eye Inputs from injected eye in layer 4C of monkey cortex Inputs from adjacent layers creates Binocularity Convergence from 4C creates Orientation Specificity This is how binocularity is demonstrated First, I show monocular cells and then contrast them with binocular cells Monocular Cells (cells driven by only one eye) Record from neuron in layer 4 C Both Eyes are Open COVER EYE B A BB A B A A B A Record from neuron in layer 4 C NO RESPONSE NOW COVER EYE A A BB A B A A B A Record from neuron in layer 4 C Neurons in layer 4C are innervated only by one eye. In this case from eye A Both eyes A BB A B A A Eye B covered Eye A covered B A Binocular Cells (cells driven by both eyes) Record from neuron in layer 3 Very strong response COVER EYE A A BB A B A A B B A Binocular Cells (cells driven by both eyes) Record from neuron in layer 3 Neurons in all other layers are binocular. However, they are dominated by one eye, in this case eye B NOW COVER EYE B A BB A B A A B EYE A ONLY B EYE B ONLY BINOCULAR A Ocular Dominance Binocular Cells (cells driven by both eyes) are not driven equally by both eyes. Rather one eye has a stronger excitatory drive than the other eye. Thus binocular cells are dominated by one or the other eye, although they are driven by both eyes. A BB A B A A B EYE A ONLY B EYE B ONLY BINOCULAR A Receptive fields of Binocular Cells The receptive field location and type (simple, complex or end-stopped) evoked by either eye is the same in binocular cells. This similarity is illustrated in the following slides. A BB A B A A B EYE A ONLY B EYE B ONLY BINOCULAR A Cover left eye and flash bar of light first in ON portion of receptive field and then in the OFF portion of the receptive field A Left eye +++++++++ B Right eye ++++++++ RIGHT EYE ALONE A BB A B A A B A Cover Left eye and flash bar of light first in ON portion of receptive field and then in the OFF portion of the receptive field A Left eye +++++++++ B Right eye ++++++++ Now the OFF region RIGHT EYE ALONE A BB A B A A RIGHT EYE ALONE Now the OFF region B Light on A Cover RIGHT eye and flash bar of light first in ON portion of receptive field and then in the OFF portion of the receptive field A Left eye +++++++++ B Right eye ++++++++ RIGHT EYE ALONE A BB A B A A RIGHT EYE ALONE Light on LEFT EYE ALONE B A Cover RIGHT eye and flash bar of light first in ON portion of receptive field and then in the OFF portion of the receptive field A Left eye +++++++++ B Right eye ++++++++ Now the OFF region RIGHT EYE ALONE A BB A B A A RIGHT EYE ALONE Now the OFF region B Light on LEFT EYE ALONE A Now present bar of light to both eyes, binocularly A Left eye +++++++++ B Right eye ++++++++ RIGHT EYE ALONE A BB A B A A Light on LEFT EYE ALONE B A BINOCULAR Now present bar of light to both eyes, binocularly A Left eye +++++++++ B Right eye ++++++++ RIGHT EYE ALONE A BB A B A A Light on LEFT EYE ALONE B A BINOCULAR Record neurons in a single column Record neurons across columns All cells in a cortical column have the same orientation specificity (except for cells in layer 4C) Orderly change in orientation specificity as columns are traversed Cortex contains neurons having all orientation specificities Dominated by left eye Dominated by right eye { { Equal dominance Inputs from adjacent layers creates Binocularity Convergence from 4C creates Orientation Specificity Aggregate fields surface of cortex cortical layers 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 The Hypercolumn is the Basic Unit that Forms the Functional Organization of the Visual Cortex ...
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