Primary visual cortex(a.k.a. V1, striate cortex, area 17) • ~1.5-2.0 mm thick • ~100 million cells per hemisphere (though highly variable)
Primary visual cortex • Magnocellular LGN cells project onto 4Cα upper level of layer 4 of V1. • Parvocellular LGN cells project onto 4Cβ lower level of layer 4 of V1. • Koniocellular LGN cells project onto layers 1, 2 & 3 of V1 (“blob” regions)
Cortical magnification • A disproportionate number of ganglion cells represent the fovea. • A disproportionate number of LGN cells represent the fovea. • A disproportionate number of V1 cells represent the fovea.
V1 cells classified by receptive field types • Simple cells: Oriented lines or bars at a precise location. • Complex cells: Oriented lines or bars at any location in the receptive field. • Hypercomplex cells: Oriented bars of a given length and width in the receptive field.
The oblique effect • [For most people] It is perceptually easier to resolve horizontal and vertical lines than it is to resolve diagonal lines. • (There can be improvement in this ability.)
What causes the oblique effect? • The carpentered environment. • It is an innate tendency.
For more on the illusion see: http://www.michaelbach.de/ot/sze_muelue/ http://www.essex.ac.uk/psychology/experiments/muller.html http://coglab.wadsworth.com/experiments/MullerLyer/
Experiments reported in 1966 by Segall, Campbell and Herskovitz suggested that the Müller-Lyer illusion may be absent or reduced amongst people who grow up in certain environments. They tested some Zulu people in South Africa who, at the time, lived in circular huts with arched doorways and had little experience of Western rectangular buildings. The Zulus seemed less affected by the Müller-Lyer illusion.
Motion sensitive V1 cells • Typically, such cells have a directional preference. • These motions are indifference to whether the eye moves or an object moves. • Some prefer slow; some prefer fast. • There is size selectivity with increasing ecentricity. • There is size selectivity at each eccentricity.
Binocularity in V1 • Although, in principle, cells in the LGN could be binocular, V1 is where binocularity begins.
Orientation sensitivity • Motion sensitivity • Binocularity • Color sensitivity
Color sensitivity in V1 • Color sensitive cells are found in blob regions. • Blobs are innervated by parvocellular cells. • Blob cells show no orientation selectivity. • Blob cells show color opponency.
Columns • As you proceed down through the layers of V1, all orientation sensitive cells are sensitive to the same orientation. • As you proceed down through the layers of V1, all orientation sensitive cells have the same occular dominance. (There can be variation in receptive field size and edges versus bars.)
Columns • Laterally adjacent regions (~0.05 mm apart) • have similar orientation preferences. (~10 degree difference.) • have either same or different occular dominance patterns
Hypercolumns • Each hypercolumn contains tens of thousands of cells. • Each hypercolumn serves one portion of the retina. • Analyzes orientation, direction of motion, binocularity, and size.
Beyond V1 • The visual system grows more complicated in going from the retina to the LGN. • The visual system grows still more complicated in going from the LGN to V1. • The visual system grows still more complicated in going beyond V1.
Some generalizations • P pathway continues to V4, a color processing area. • M pathway continues to MT (a.k.a. V5), a motion processing area.
The Duality of Visual Processing(a.k.a. the second bifurcation) • Parietal stream and temporal stream • Goodale and Milner: There is a perceptual stream and an perception-action stream