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BrodmannBrodmann’’ss AreasAreas

The Primary Visual CortexThe Primary Visual Cortex

��Hubel and Hubel and WeiselWeisel discovered discovered

simple, complex and simple, complex and

hypercomplexhypercomplex cells in the striate cells in the striate

cortex.cortex.

Simple cells respond to edges at Simple cells respond to edges at

particular locations and orientations particular locations and orientations

in the visual field. They have on and in the visual field. They have on and

off components off components

Complex and Complex and HypercomplexHypercomplex CellsCells

�� Complex cells respond to orientation Complex cells respond to orientation

but location is less specific. but location is less specific.

�� HypercomplexHypercomplex cells are like complex cells are like complex

cells, but respond to edges that stop cells, but respond to edges that stop

in the receptive field and might be in the receptive field and might be

thought of as thought of as ‘‘cornercorner’’ detectors or detectors or

‘‘end of lineend of line’’ detectors. detectors.

Layered organization aspect of 6 layered Layered organization aspect of 6 layered neocortexneocortex

MagnocellularMagnocellular cells input to Layer 4C alpha.cells input to Layer 4C alpha.ParvocellularParvocellular cells inputs to Layer 4C beta.cells inputs to Layer 4C beta.

Output to other cortical areas goes from Layers 2, 3 and Output to other cortical areas goes from Layers 2, 3 and 4B. 4B.

Output to other parts of brain goes from Layers 5 and 6.Output to other parts of brain goes from Layers 5 and 6.

Ice cube model

Pinwheelreality

HypercolumnHypercolumn

�� Orientation columns (18Orientation columns (18--20) cover all 20) cover all

orientations and the ocular dominance orientations and the ocular dominance

columns. All orientations for both eyes for columns. All orientations for both eyes for

small sector of retina is a small sector of retina is a ‘‘hypercolumnhypercolumn’’

(about 1 mm square).(about 1 mm square).

�� Hypothesis: This arrangement allows Hypothesis: This arrangement allows

similar information from similar locations similar information from similar locations

to be pooled, improving signal to be pooled, improving signal

discriminabilitydiscriminability..

Ocular dominance and Ocular dominance and cytochromecytochromeoxidaseoxidase distributionsdistributions

CytochromeCytochrome oxidaseoxidase patchespatches

�� Discovered by Discovered by

Margaret WongMargaret Wong--Riley. Riley.

Mark: areas high in Mark: areas high in

mitochondria which mitochondria which

make ATP.make ATP.

�� Found in Layers 2 and Found in Layers 2 and

3. Hubel and Wiesel 3. Hubel and Wiesel

called them called them ‘‘blobsblobs’’. .

They have colorThey have color--

sensitive cells.sensitive cells.

More about blobsMore about blobs

�� Each ocular dominance column Each ocular dominance column location has a blob in layers 2 and 3, location has a blob in layers 2 and 3, cells in the blobs do not have cells in the blobs do not have orientation preferences, are orientation preferences, are monocular, and are responsive to monocular, and are responsive to low, not high spatial frequencies. But low, not high spatial frequencies. But they respond to they respond to wave lengthwave length. Color . Color sensitive, often with center surround sensitive, often with center surround organization to complementary organization to complementary colors (red/green; blue/yellow). colors (red/green; blue/yellow).

��Livingstone and HubelLivingstone and Hubel’’s sweeping hypothesis of form, motion s sweeping hypothesis of form, motion

color vision.color vision.

V5

InterblobsInterblobs and 4Band 4B

�� InterblobInterblob cells are less sensitive to cells are less sensitive to

wave length, but responsive to high wave length, but responsive to high

spatial frequencies (fine lines), and spatial frequencies (fine lines), and

are orientation selective and are orientation selective and

binocular.binocular.

�� Layer 4B cells are sensitive to Layer 4B cells are sensitive to

binocular disparity (binocular disparity (stereopsisstereopsis); );

orientation, not wavelength, orientation, not wavelength,

selective; and highly selective for selective; and highly selective for

direction of motion. direction of motion.

��Livingstone and HubelLivingstone and Hubel’’s sweeping hypothesis of form, motion s sweeping hypothesis of form, motion

color vision.color vision.

Beyond V1Beyond V1

�� 4B properties (in thick stripes of 4B properties (in thick stripes of

cytochromecytochrome oxidaseoxidase), blob properties ), blob properties

(in thin stripes of (in thin stripes of cytochromecytochrome

oxidaseoxidase) and ) and interblobinterblob properties properties

(little CO stain) are also seen in V2. (little CO stain) are also seen in V2.

Livingstone and Hubel suggest Livingstone and Hubel suggest

movement and stereoscopic vision movement and stereoscopic vision

go on to be represented in area MT go on to be represented in area MT

(V5) while color goes to V4.(V5) while color goes to V4.

Livingstone and HubelLivingstone and Hubel’’s s hypothesis hypothesis

�� Layer 4B is a continuation of the Layer 4B is a continuation of the magnocellularmagnocellular

stream (4C alpha) and is specialized for stream (4C alpha) and is specialized for motion motion

perception and stereoscopic depth perception.perception and stereoscopic depth perception.

�� Blobs are a continuation of the Blobs are a continuation of the parvocellularparvocellular

stream (4C beta) and specialized for stream (4C beta) and specialized for colorcolor

perception.perception.

�� InterblobsInterblobs are a continuation of the are a continuation of the parvocellularparvocellular

stream (4Cbeta) and specialized for stream (4Cbeta) and specialized for formform or or

shape perception.shape perception.

HebbHebb’’ss rulerule

�� Cells that fire together, wire togetherCells that fire together, wire together

�� Experience alters connectionsExperience alters connections

�� For example, kittens raised with vertical For example, kittens raised with vertical

stripes have many vertical orientation stripes have many vertical orientation

columns, but few horizontal orientation columns, but few horizontal orientation

columnscolumns

�� Also true for eye dominance columns, Also true for eye dominance columns,

connections lost in critical period if there is connections lost in critical period if there is

no inputno input

Modifications to the Hubel and Modifications to the Hubel and Wiesel model under discussionWiesel model under discussion

�� While there is segregation as we will While there is segregation as we will

see, they are not single function see, they are not single function

areas. There is more mixing of inputs areas. There is more mixing of inputs

than the original model predicted. So than the original model predicted. So

both M and P inputs are seen in V4 both M and P inputs are seen in V4

for example. But M does dominate in for example. But M does dominate in

MT(V5). MT(V5).

Segregation view dominates Segregation view dominates

�� Ventral visual areas are concerned Ventral visual areas are concerned

with the with the ‘‘appearanceappearance’’ of objects. of objects.

Dorsal visual areas are concerned Dorsal visual areas are concerned

with with spatiospatio--temporal processing temporal processing

(including attention), so location and (including attention), so location and

movement coding here. movement coding here.

�� What (ventral) and where (dorsal).What (ventral) and where (dorsal).