2The Primary Visual Cortex A multi-layered structure located in the occipital lobe (AKA, Area 17, V1, Striate cortex).Receives axons from the LGN.Each hemisphere represents the contralateral visual field.
3The Visual Cortex The visual cortex is topographically organized. A large area of the visual cortex is mapped to the central portion of the visual field (retina)Cortical magnification.
4The Visual CortexIn all, it contains 6 layers and the layers are distinguished by the types of cells they contain.LGN connections are sent to layer 4C.
5Magnocellular cells input to Layer 4C alpha. The Visual CortexMagnocellular cells input to Layer 4C alpha.Parvocellular cells inputs to Layer 4C beta.
6The Visual Cortex Layer IV projects to II, III, and IVB. Connections run from II and III to V and VI.
7The Primary Visual Cortex Hubel and Weisel discovered simple, complex and hypercomplex cells in the striate cortex.Most neurons in V1 are eitherSimple – respond to edges at particular locations and orientations within the visual fieldComplex – represent a more abstract type of visual information, at least partially independent of location within the visual field.Hypercomplex or end-stopped – cells that are selective for a certain length of contour
8Receptive Fields of Simple and Complex Cells in the Visual Cortex Simple – receptive fields are rectangular with “on” and “off” regionsComplex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field
9Receptive Field of Simple Cells Note that the central region is oblong and not circular as was the case for the center-surround receptive field of the retinal ganglion cells.Also, the surround region is now located only on the sides. In this particular cell, the inhibitory region is located in the center, not on the sides
11The Visual CortexSo here, we’re trying to understand the orientation-specific response of a simple cell.Hubel and Weisel show that the response pattern of simple cells can be explained by interaction of receptive fields of cells further back in the visual system.Each of these four cells has an on center/off surround organization, and all four feed into the same simple cell.The receptive field of these cells overlap in a diagonal pattern such that the simple cell is excited by light stimulation that covers the four receptive field centers in the same diagonal pattern while leaving the surround regions unstimulated.Hubel and Weisel show that the response pattern of simple cells can be explained by interaction of receptive fields of cells further back in the visual system.
12Complex and Hypercomplex Cells Complex cells respond to orientation but location is less specific.Do not have center surround arrangementHypercomplex cells are like complex cells, but respond to edges that stop in the receptive field and might be thought of as ‘corner’ detectors or ‘end of the line’ detectors.
13Orientation ColumnsSimple and complex cells have preferred orientationElectrode penetration that is exactly perpendicular to the cortical sheet will encounter cells with the same orientation preference (Columns).
14Orientation ColumnsMoving the electrode – column with a different orientation preferencePreferences for neighboring columns are NOT randomOrientation selectivity changes only slightly as you move from column to column and there is a specific and consistent order.
15Ocular Dominance Columns Visual signals from the two eyes remain segregated in the LGN and primary visual cortex.As the recording electrode is moved within layer 4C, there is an abrupt shift as to which eye drives the unit. In layer 4C, the shift from one eye to the other takes place over a distance of less than 50 microns
16Ocular Dominance Columns Thus, layer 4c is organized into columns based on ocular dominance.i.e., ocular dominance columns.
18HypercolumnsTogether, orientation columns and ocular dominance columns form hypercolumns.18-20 columns to represent all orientations for both eyesabout 1 mm square
19HypercolumnEach hypercolumn encodes information about one small sector of the retina, and neighboring hypercolumns represent neighboring sectors of the retina
20Binocular CellsAlthough cells in layer IVC are exclusively monocular, cells in other layers are binocular.Respond to stimulation in either eye.Binocular cellsAlthough these cells are binocular, some are driven primarily by one eye.
21Disparity Selective Cells Binocular disparity: difference in the location between the right eye's and left eye'simage. Amount of disparity depends on the depth (i.e., the difference in distance tothe two objects and the distance to the point of fixation), and hence it is a cue that the visual system uses to infer depth.
22Disparity Selective Cells We have cells that are disparity selective.They respond to specific amounts of disparity.Crucial for stereopsis (3-D vision).
23Cytochrome Oxidase Blobs Found primarily in Layers 2, 3, 4. Hubel and Wiesel called them ‘blobs’.Blobs are located in the centers of ocular dominance columns.They are not orientation selective.Sensitive to color.