Vision III: Cortical mechanisms of vision

First you tell them what you’re gonna tell them

See the triangle?

See the white bar?

See the wavy line?

Which small square is darker?

So

Roughly 40% of cerebral cortex is involved in vision

Remember

The striate cortex – V1 – builds more sophisticated receptive fields from these basic building blocks. Cells describe specific

V1 simple cell is most responsive to an oriented line

Orientation tuning in a V1 simple cell

V1 complex cells are sensitive to orientation of stimuli

But not particularly to stimulus position within the receptive field

Complex cells can be constructed from an array of similarly oriented simple cells

The cerebral cortex is organized in a columnar manner

Within a column

Cells with similar orientation preferences lie in the same column

Geniculate cells representing the same area of the visual field but arising from different eyes project to adjacent areas of V1

Orientation columns with the same monocular lateral geniculate input lie in the same ocular dominance column.

The actual topology of orientation and ocular dominance columns

Color sensitive cells lie at the center of the pinwheels, in cytochrome oxidase containing ‘blobs.’

Depth perception starts with the detection of binocular disparity

Random dot stereograms generate structure from disparity

Disparity selectivity in a V1 neuron

Motion selectivity in a V1 neuron

Two cortical visual streams subserve two different visual functions.

Patients demonstrate this functional segregation

Functional separation begins in the retina and continues through the LGN

And continues in V1

V2 (Area 18) also is divisible by cytochrome oxidase staining

Functional separation continues in V2

After V2, different functions are performed by anatomically different areas:
The dorsal stream provides vision for action –”where and how”

After V2, different functions are performed by anatomically different areas:
The ventral stream provides vision for object identification

After V2, different functions are performed by anatomically different areas:
But the areas are interconnected

MT – the analysis of motion

Human MT

Structure from motion

MT Cells are tuned for direction

Perceived motion in a plaid

Striate neurons respond to the components of the plaid

MT  responds to the direction of the plaid, and not the components

MT has columns for direction of motion

MT has disparity columns

Electrically stimulating an orientation column in MT induces the perception of motion described by that column

Electrically stimulating an orientation column in MT induces the perception of motion described by that column

Electrically stimulating an orientation column in MT induces the perception of motion described by that column

Electrically stimulating an orientation column in MT induces the perception of motion described by that column

The parietal lobe describes the world for action, location, and attention.

There are multiple representations of the visual field in the intraparietal sulcus

Within the dorsal stream there is further functional segregation –

An example of a dorsal stream function

A patient with a dorsal stream lesion cannot orient her hand with respect to a slot

Neurons in AIP specialized for grip

The inferior temporal lobe describes the visual world for object recognition

A patient with a ventral stream lesion can move her hand to a slot, but can’t mimic the position.

Neurons in inferior temporal cortex are selective for complex patterns like faces

Slide 58

Patients with inferior temporal lesions have visual agnosia

Ventral stream patients

Prosopagnosia “face blindness” is the most dramatic ventral stream deficit

Finally, you tell them what you told them