Transcript Document
Brodmann’s Areas The Primary Visual Cortex Hubel and Weisel discovered simple, complex and hypercomplex cells in the striate cortex. Simple cells respond to edges at particular locations and orientations in the visual field. They have on and off components Complex and Hypercomplex Cells Complex cells respond to orientation but location is less specific. 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 line’ detectors. Layered organization aspect of 6 layered neocortex Magnocellular cells input to Layer 4C alpha. Parvocellular cells inputs to Layer 4C beta. Output to other cortical areas goes from Layers 2, 3 and 4B. Output to other parts of brain goes from Layers 5 and 6. Ice cube model Pinwheel reality Hypercolumn Orientation columns (18-20) cover all orientations and the ocular dominance columns. All orientations for both eyes for small sector of retina is a ‘hypercolumn’ (about 1 mm square). Hypothesis: This arrangement allows similar information from similar locations to be pooled, improving signal discriminability. Ocular dominance and cytochrome oxidase distributions Cytochrome oxidase patches Discovered by Margaret Wong-Riley. Mark: areas high in mitochondria which make ATP. Found in Layers 2 and 3. Hubel and Wiesel called them ‘blobs’. They have colorsensitive cells. More about blobs Each ocular dominance column location has a blob in layers 2 and 3, cells in the blobs do not have orientation preferences, are monocular, and are responsive to low, not high spatial frequencies. But they respond to wave length. Color sensitive, often with center surround organization to complementary colors (red/green; blue/yellow). V5 Livingstone color vision. and Hubel’s sweeping hypothesis of form, motion Interblobs and 4B Interblob cells are less sensitive to wave length, but responsive to high spatial frequencies (fine lines), and are orientation selective and binocular. Layer 4B cells are sensitive to binocular disparity (stereopsis); orientation, not wavelength, selective; and highly selective for direction of motion. Livingstone color vision. and Hubel’s sweeping hypothesis of form, motion Beyond V1 4B properties (in thick stripes of cytochrome oxidase), blob properties (in thin stripes of cytochrome oxidase) and interblob properties (little CO stain) are also seen in V2. Livingstone and Hubel suggest movement and stereoscopic vision go on to be represented in area MT (V5) while color goes to V4. Livingstone and Hubel’s hypothesis Layer 4B is a continuation of the magnocellular stream (4C alpha) and is specialized for motion perception and stereoscopic depth perception. Blobs are a continuation of the parvocellular stream (4C beta) and specialized for color perception. Interblobs are a continuation of the parvocellular stream (4Cbeta) and specialized for form or shape perception. Hebb’s rule Cells that fire together, wire together Experience alters connections For example, kittens raised with vertical stripes have many vertical orientation columns, but few horizontal orientation columns Also true for eye dominance columns, connections lost in critical period if there is no input Modifications to the Hubel and Wiesel model under discussion While there is segregation as we will see, they are not single function areas. There is more mixing of inputs than the original model predicted. So both M and P inputs are seen in V4 for example. But M does dominate in MT(V5). Segregation view dominates Ventral visual areas are concerned with the ‘appearance’ of objects. Dorsal visual areas are concerned with spatio-temporal processing (including attention), so location and movement coding here. What (ventral) and where (dorsal).