Lecture14_notes.pdf

Primary Visual Cortex The diagram below illustrates the visuotopic/retinotopic map in primary visual cortex: The visual field divided into quadrants that are coded by letters, numbers and colors 3 2 A B C D 4 Calcarine fissure D 4 2 C B A 3 1 Left and right primary visual cortex showing the visuotop...

Primary Visual Cortex The diagram below illustrates the visuotopic/retinotopic map in primary visual cortex: The visual field divided into quadrants that are coded by letters, numbers and colors 3 2 A B C D 4 Calcarine fissure D 4 2 C B A 3 1 Left and right primary visual cortex showing the visuotopic map. That is, neurons at location “A” respond only to objects at location “A” in the visual field. Although there is a one-to-one relationship between locations in the visual field and locations in the cortical areas that encode them, almost 1/2 of the neurons in V1 code for the fovea region even though this is only a very tiny portion of the visual field. Apparently, this is because it takes a lot of neural machinery to decode the complex and detailed information that we can perceive from this area of highest visual acuity. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. How a face centered in the visual field is mapped onto primary visual cortex: + Calcarine fissure Left and right primary visual cortex showing the visuotopic map. How is this visual information processed? How do you recognize this as a face? 1. Visual processing occurs in stages. 2. Retina processing followed by V1 processing followed by V2, V3 processing.... In V1 (primary visual cortex) how is information analyzed? 1. The visual field is broken up into approximately 1000 parts and information from each location is analyzed by neurons in ~ 1000 different cortical modules. The diagram below shows both the visuotopic/retinotopic map in V1 and how it is parceled into cortical modules: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. How the visual field is subdivided into regions in primary visual cortex + Each square area in primary visual cortex above represents one of the ~ 1000 cortical modules. The purple area in the visual field would be analyzed by the purple cortical module and all the neurons in that cortical module would respond only to light from the purple area in the visual field. Orientation tuning in V1 simple cells What drives the activity of V1 neurons (i.e. what is their adequate stimulus?) 1. A spot of light or a spot of darkness will only weakly drive these neurons. 2. An elongated stimulus is what drives these neurons (a bar of light or dark of a particular orientation). V1 simple cells have well-defined receptive field - antagonistic center surround receptive fields, but they are not circular, rather they are elongated. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Below is a diagram of two simple cell receptive fields from two neurons in V1 (they can be either on- or off-center). Also, see Figure 10.23. - + - + - + - + - + - + - + + - + - - _ + _ + _ + + _ + _ + _ _ _ _ _ _ _ 3. These neurons are most sensitive to bars that fill their center without impinging on the surround. They aren't sensitive to spots of light because spots don't fill much of the long center. 4. Why are these neurons sensitive to orientation of a bar of light? They are sensitive to orientation because if the light stimulus isn't aligned with the receptive field center, then the light stimulus will fall on parts of both the center and the antagonistic surround. 5. This specific type of receptive field is called a "simple-cell" receptive field. 6. For comparison, there are also cells that have complex receptive fields, in which the stimulus can be anywhere within the receptive field, but it still must have a particular orientation. (Cells with simple receptive field response properties are © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. most concentrated close to layer four in V1. Cells with “complex” receptive fields are most concentrated further above and below layer four and cells with even more complex receptive fields are concentrated even further above and below layer four). 7. How do these receptive fields arise? They are believed to arise because these neurons receive highly organized patterns of input from neurons from the previous stage of processing. For example, a “simple cell” that specifically responds to a vertical bar of light is thought to receive inputs from a set of neurons each of which has a circular center-surround organization with the centers of the input neurons’ receptive fields being aligned vertically with each other. See Figure 10.23. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed.