Psychology 4051 The Primary Visual Cortex
The 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.
The 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.
The Visual Cortex • In all, it contains 6 layers and the layers are distinguished by the types of cells they contain. • LGN connections are sent to layer 4C.
The Visual Cortex Magnocellular cells input to Layer 4C alpha. Parvocellular cells inputs to Layer 4C beta.
The Visual Cortex • Layer IV projects to II, III, and IVB. • Connections run from II and III to V and VI.
The Primary Visual Cortex • Hubel and Weisel discovered simple, complex and hypercomplex cells in the striate cortex. • Most neurons in V1 are either • Simple – respond to edges at particular locations and orientations within the visual field • Complex – 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
Receptive Fields of Simple and Complex Cells in the Visual Cortex • Simple – receptive fields are rectangular with “on” and “off” regions • Complex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field
Receptive 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
The Visual Cortex 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.
Complex and Hypercomplex Cells • Complex cells respond to orientation but location is less specific. • Do not have center surround arrangement • Hypercomplex 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.
Orientation Columns • Simple and complex cells have preferred orientation • Electrode penetration that is exactly perpendicular to the cortical sheet will encounter cells with the same orientation preference (Columns).
Orientation Columns • Moving the electrode – column with a different orientation preference • Preferences for neighboring columns are NOT random • Orientation selectivity changes only slightly as you move from column to column and there is a specific and consistent order.
Ocular 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
Ocular Dominance Columns • Thus, layer 4c is organized into columns based on ocular dominance. • i.e., ocular dominance columns.
Hypercolumns • Together, orientation columns and ocular dominance columns form hypercolumns. • 18-20 columns to represent all orientations for both eyes • about 1 mm square
Hypercolumn • Each hypercolumn encodes information about one small sector of the retina, and neighboring hypercolumns represent neighboring sectors of the retina
Binocular Cells • Although cells in layer IVC are exclusively monocular, cells in other layers are binocular. • Respond to stimulation in either eye. • Binocular cells • Although these cells are binocular, some are driven primarily by one eye.
Disparity Selective Cells Binocular disparity: difference in the location between the right eye's and left eye's image. Amount of disparity depends on the depth (i.e., the difference in distance to the two objects and the distance to the point of fixation), and hence it is a cue that the visual system uses to infer depth.
Disparity Selective Cells • We have cells that are disparity selective. • They respond to specific amounts of disparity. • Crucial for stereopsis (3-D vision).
Cytochrome 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.