Vision III: Cortical mechanisms of vision

	Please sit where you can examine a partner.
	Michael E. Goldberg, M.D.

First you tell them what you’re gonna tell them

	The cortical visual system is composed of multiple visual areas with different functions.
	V1 neurons describe object features.
	The principle of columnar organization.
	Two visual streams – ‘what’ and ‘how’ (or ‘where’).
	MT neurons describe motion and depth (dorsal stream).
	IT neurons describe objects (ventral stream).

See the triangle?

See the white bar?

See the wavy line?

Which small square is darker?

So

	Your visual system does not measure and report the exact physical nature of the visual world.
	It collects some data, and makes guesses.
	Optical illusions take advantage of the guessing strategies.

Roughly 40% of cerebral cortex is involved in vision

Remember

	Receptive fields in the retina and the lateral geniculate are circular, with center – surround organization.

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

	Contour orientations.
	Binocular interaction.
	Speed and direction of motion.
	Color.

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

	Information is processed and transformed from monocular, center-surround,non-directionally selective input to
			Orientation-
			Binocular disparity-
			Direction-selective output
	Processed information is distributed
			Layers 2-3 to other cortical areas
			Layer 5 to the superior colliculus
			Layer 6 to the lateral geniculate nucleus
	This general arrangement of columnar processing is maintained throughout the cortex, not just visual cortex.

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

	Patients with V1 lesions generally have total visual field deficits in the affected field.
	Patients with dorsal stream lesions have deficits in sensory location (and attention), motion perception, color perception, and the performance of visually-guided movements.
	Patients with ventral stream lesions have visual agnosia, the inability to associate a visual stimulus with a name or function.

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

	Neurons in MT are selective for speed and direction of motion, and retinal disparity.
	Neurons in MT report the perceptual aspects of motion.
	Electrical stimulation of MT affects the perception 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 –

	MT is specialized for depth and motion.
	LIP is specialized for attention in far space.
	MIP is specialized for providing visual. information for reaching.
	AIP is specialized for providing visual. information for grasping.
	VIP is specialized for providing visual. information for mouth and head movements.

An example of a dorsal stream function

	When you reach for something, your grip opens to accommodate the size of your target.
	Patients with dorsal stream lesions can’t do this.
	They can, however, describe the size of the object.

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

	Cannot identify objects
	But they can make appropriate visually-guided movements.
	The patient who could not set her grip can still tell you which cylinder is thicker.
	The patient who cannot tell you which cylinder is thicker can set still her grip.

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

	Term first used by Bodamer, 1947
	Inability to recognize familiar faces
	Visual acuity is normal
	Caused by lesion to right inferior temporal lobe
	May be congenital (“developmental prosopagnosia”)
	Patients compensate by using other recognition cues: clothing, gait, voice, etc.

Finally, you tell them what you told them

	The striate cortex (V1) uses unoriented, monocular input from the lateral geniculate to assemble cells selective for orientation,motion, and retinal disparity. Complex cells generalize the orientation information found in simple cells.
	Striate cortex is organized in columns with similar orientation and ocular dominance.
	Two visual streams emanate from V1: a dorsal stream concerned with analyzing the visual world for location and action, and a ventral stream concerned with analyzing the nature of objects in the visual world.  Different areas subsume different spatial and object attribute functions.
	Clinical deficits include specific deficits for color, faces, motion, visual targeting of motion, and spatial localization.