Lecture 14: Primary Visual Cortex and Cortical Modules

How do you recognize this as a face?

1. visual processing occurs in stages

2. Retina processing followed by V1 processing followed by V2, V3 processing

In V1 (primary visual cortex) how is information analyzed?

• visual field is broken up into approximately 1000 parts

• information from each location is analyzed by neurons in 1000 different cortical modules

What does this diagram show?

• shows both visuotopic/retinotopic map in V1 and how it is parceled into cortical modules

What are cortical modules?

• hypothetical constructs but they have a physiological basis

• can be though of as computer containing all of the neural machinery needed for the initial analysis of the visual info in a single location in the visual field

What do different neurons in each cortical module do?

• they analyze different aspects of a visual stimulus

  • Some in Blobs: analyze the color of an object

  • Some in Orientation Columns: analyze the shape or form of an object

What are some features of Cortical modules?

1. there’s about 1000 of them in the primary visual cortex

2. they are cubes (2mm x 2mm x 2mm)

3. There are about 300,000 neurons in each cortical module

4. Cortical module contains:

   1. 4 ocular dominance columns (shaped like slabs)

   2. 36 orientation columns (also shaped like slabs)

   3. 16 blobs (shaped like columns)

What does figure 10.26 show?

• shows a diagrammed example of a cortical module

• neurons in a single cortical module all respond to visual input from the same particular location in the visual field

  • their receptive fields have similar locations

What defines an ocular dominance column?

• all neurons in it respond primarily and preferentially to input from the same eye

Explain what happens to neurons in layer 4C?

• neurons in this layer respond to input from only one eye (one hemiretina)

• While neurons in the layers above and below are driven by input from both eyes ( the ipsilateral temporal hemiretina, and contralateral nasal hemiretina)

• Input from one hemiretina has the dominant influence

What are some characteristics of cells in blobs?

• they are color sensitive but insensitive to shape

• there are 16 blobs in each cortical module

• 4 per ocular dominance column

• Neurons in blobs mainly project to V4 which is specialized for color perception
  

Where does information about the form of an object arise from?

• mostly from cells in the orientation column

• info about color of an object arises mostly from cells in blobs (somehow integrated)

What does this diagram depict ?

• depicts a cortical module with neurons and the stimuli that drives them

What drives the activity of neurons in an orientation column?

1. A spot of light or a spot of darkness will only weakly drive these neurons

2. An elongated stimulus is what drives these neurons, a bar of light or dark of a particular orientation)

3. All the neurons in a particular orientation column respond to a line of the same particular orientation

4. Neurons in adjacent orientation columns respond to lines with a 10 degree difference in rotation

   1. Progressive Rotation of Line Orientation: drives these neurons as you move across the columns

   2. recording activity across all 36 orientation columns there’ll be a complete 360 degree rotation

5. Your perception on a visual stimulus is thought to depend upon which subsets of cells within the relevant cortical modules are active

What are the mechanisms generating orientation columns?

1. Neurons in orientation columns have antagonistic center surround receptive fields, but they aren’t circular rather they are elongated

What does this diagram depict?

• two so-called “simple cell” receptive fields from 2 neurons in different orientation columns in V1 (they can be either on or off center)

What are these neurons more sensitive to ?

• most sensitive to bars that fill their center without impinging on the surround

• They aren’t sensitive to spots of light b/c spots don’t fill much of the long center

Why are these neurons sensitive to orientation of a bar of light?

• “Simple- cell”receptive field

  • this is because if the light stimulus isn’t aligned with the receptive field center then the light stimulus will fall on parts of both the center and the antagonistic surround

Where could the stimulus be if cells have complex receptive fields?

• stimulus can be anywhere within the receptive field

• BUT still must have a particular orientation

Where are cells with simple receptive field response properties located?

• Most concentrated closer to layer 4 in V1

• Complex Receptive Field: Most concentrated further above and below layer 4

• Cells With Even More Complex receptive fields: are concentrated further above and below layer 4

How do these receptive fields arise?

• because these neurons receive highly organized patterns of input from neurons from the previous stage of processing

What happens to a “simple cell” that specifically responds to a vertical bar of light?

• it is thought to receive inputs from a set of neurons each of which has a circular center surround organization

• with centers of the input neurons receptive fields being aligned vertically

What does all this suggest?

• visual world is broken down into a mosaic

• each portion of the mosaic being analyzed by a diff group of cells

• one piece of the mosaic and the adjacent cortical module analyses an adjacent piece of the mosaic)

What we perceive is dependent on what?

• dependent upon which subset of neurons in each cortical module is active

what type of processing is the processing of visual info in V1?

• an intermediate level of processing

What does the IT cortex contain?

• a subset of neurons that respond selectively to highly complex visual stimuli (hands, faces, food, etc)

What are the 3 questions to keep in mind?

1. How do our perceptions of stimuli in different categories (hands vs faces vs chairs) arise

2. How do we do this despite the fact that the image of the same object can be very different? (open vs closed book or frontal vs profile view of a face)

3. How do you recognize different individuals within a category (one book vs another, and one face vs another)