Timothy Gawne

Dept. Vision Sciences, School of Optometry

Worrell Bldg, Rm. 664 (Office) 235/236 (Lab)

Office Phone: 934-5495

Lab Phone: 934-2567

Email: Tgawne@uab.edu

Higher Visual ProcessingNeuroscience 2009

So what is the visual system anyhow?

-> Optical System

-> Phototransduction (rods and cones)

-> Computation ***** Largest part by far *****

Something like 1/3 of the brain required for our full visual capability.

The Inverse Problem!! No formal solution!!

So how does the visual system work?

-> Don’t really know! But learning fast – this lecture hardly even skims the surface.

-> One of the most fundamental and exciting problems in modern science

-> Ultimately bears on the issue of what is the mind, and what is consciousness.

-> But for now, robots that ‘see’ are still very much science fiction. A video camera can RECORD an image, but it can not understand what it means and is thus perceptually blind…

The major targets of the retinal ganglion cells:

RETINA LGN CORTEX (VISION)

Superior Colliculus (saccades)

Suprachiasmatic Nucleus (circadian)

Pretectum (pupillary light reflex)

Accessory Optic System (stabilize gaze)

Only about 10% of retinal ganglion cells do not terminate in the Lateral Geniculate Nucleus (LGN).

The Retina -> LGN -> Cortex pathway is known as the geniculostriate or thalamocortical system, it’s responsible for conscious vision.

‘BLINDSIGHT’: preservation of very limited ability to perform visually guided tasks after destruction of the retina to LGN to cortex pathway, in the apparent absence of conscious perception.

Bottom line: take out the LGN-cortical system and you are for all practical purposes completely blind. Lesion anywhere in retina/LGN/primary visual cortex and you get complete loss of conscious vision in specific parts of the visual field from one or both eyes.

Lesioning in extrastriate visual cortex can cause more subtle defects in the ability of a person to understand what they are looking at, but with normal visual fields and acuity.

The LGN is part of the thalamus, which itself is part of the diencephalon.

The thalamus contains sensory relay nuclei for all the senses: sometimes called “the gateway to the cortex”.

(Sometimes referred to as “LGNd”).

Know this slide! This is basic: real clinical lesions not usually so clean, there are a LOT of subtleties. Key point: pre- vs. post chiasmal lesions. There might, or might not, be sparing of macular vision with lesion #5.

Chiasm crosses fibers from left and right, no crossing up/down.

Monocular crescent located deep in the calcarine sulcus

Note: this is only the MOST BASIC description of visual fields, you will have to do a neuro-optometry or neurology etc. residency to really get all the fine points of clinical diagnosis.

Migraine Visual Aura!

Thought to be due to spreading depression on the surface of primary visual cortex.

Can you guess the visual field properties?

Parvocellular System: Originates with the midget ganglion cells in the retina, connects to parvo cells in LGN, is most strongly associated with extrastriate visual areas in the inferior temporal lobe. Neurons respond well to color and fine detail, not so strongly to rapid motion or low contrast.

Magnocellular System: originates with the parasol ganglion cells in the retina, connects to magno cells in LGN, more strongly associated with extrastriate visual areas in the posterior parietal lobe. Neurons respond well to rapid motion and low contrasts, not so well to color or fine detail.

1,4,6, cross the river styx

2,3,5, stay on the same side!

LGN relay neurons project (I.e., send axons) to cortex.

LGN relay neurons have response properties that are very similar to the ganglion cells that contact them.

LGN interneurons make only local connections.

There are more interneurons than relay neurons!

LGN neurons get feedback connections from cortex. (The one-way connection from retina to rest of brain is unique in the visual system).

LGN gets other inputs as well. For example: from brainstem and perigeniculate.

-> In primates (nearly) all visual information that gets to cortex must first go through primary visual cortex. Four main synonyms for this part of cortex:

-> Primary Visual Cortex

-> Area V1

-> Brodmann’s area 17

-> Striate cortex

It’s called striate cortex because of a heavy band of myelinated axons in layer 4, the stria of Gennari, about the only landmark you can see in cortex without special stains.

The LGN to V1 pathway is known as the thalamocortical or geniculostriate pathway for vision.

Points: pial surface vs. white matter border, ‘vertical’ vs. ‘horizontal’ organization, three major classes of cortical neurons (pyramidal, spiny stellate, smooth stellate), feedback connections.

V1 SIMPLE CELLS have receptive fields with discrete excitatory and inhibitory subregions, but these regions are elongated into strips. Thus, simple cells are selective for orientation.

V1 COMPLEX CELLS have NO discrete excitatory and inhibitory subregions in their receptive fields. Selective for orientation regardless of small shifts in position or changes in contrast (i.e., switching black and white).

Artists have long known that lines - edges and contours - are of great importance in vision (think how expressive a few lines in a cartoon can be). The V1 neurons that respond to oriented lines are presumably analyzing an image for contours and edges, which is likely critical for vision.

Orientation-selective neurons in V1 area arranged in cortical columns specialized for analyzing a specific orientation, region of space, and eye of origin (Ipsi or Contra).

Grossly simplified block-diagram of the cortical visual system

Extrastriate visual cortex is much larger than striate cortex

Many Bothans died to give us this information.

Even more grossly simplified block-diagram of the cortical visual system!

Like all really complicated systems, never drawn the same way twice!

Very crudely:

DORSAL PATHWAY: “WHERE” stream of visual processing: magno.

VENTRAL PATHWAY: “WHAT” stream of visual processing: parvo.

V2-> Closely associated with V1

-> BOTH magno and parvo sections

-> Neurons in V2 have, at first glance, roughly similar properties to those of neurons in V1. Small receptive fields, often selective for orientation of a visual stimulus.

Neurons in V2 seem to have receptive field properties a lot like those in V1, but are somehow better able to infer the existence of ‘illusory’ or hidden contours from contextual information.

V4-> One of the larger and more important of the post-V1/V2 visual cortical areas. Identified in Rhesus monkeys, but probably has a homolog in humans.

-> More of a ‘parvo’ sort of pathway. ‘Ventral Stream’

-> More complex responsivities to shapes than just oriented lines (real or imagined). Larger RF size.

-> Complex color properties (used to be thought of as the color center, now know that it does more than just color)

-> Attentional effects become easier to elicit

-> Lesions cause more subtle problems than just visual field defects.

V4 neurons often have more complex shape selectivities. Most V1 cells can be excited by an oriented line, but many V4 cells respond best to spirals, curves, angles, and other things a little more complicated than just a single line.

IT-> Inferior temporal cortex (IT, TE, TEO, etc.)

-> Last of the hierarchy of (more-or-less) purely visual areas in the ventral (‘parvo’, ‘what’) stream.

-> Neurons can have very large receptive fields…

-> …but specificity for visual stimuli can be VERY high

-> Lesions of IT can have devastating consequences for the ability to recognize specific objects (e.g. faces: PROSOPAGNOSIA) with no corresponding loss of acuity or visual field deficits.

-> Large attentional effects

-> Lesions of temporal cortex can cause visual field deficits by interrupting the passing fibers of Myer’s loops.

MT/V5-> Middle Temporal cortex. MT is widely studied but is only a small (although very important) part of the dorsal stream. MT does motion! Cells respond well to motion, activity correlates with perception of motion, lesions interfere with motion perception, stimulation biases direction of perceived motion.

Important Things to Know

• Targets of the retinal ganglion cells, laminations of LGN, pupillary light reflex.

• Visual field defects resulting from lesions in the retina-LGN-striate pathway

• Receptive field properties of neurons in LGN, Striate cortex (simple + complex), and IT cortex.

• What is V1 (+synonyms), V2, V4, IT, MT• Never fight a land war in asia.