Visual Pathways:The Road to VisionAnthony DeSimone LDO
Visual Pathway “Focus on the Eye” • Concerned about Cornea Lens Retina There is more to vision than meets the eye
Retinal Fields vs. Visual Fields What’s the difference • Retinal Field – describes the area that includes neural fibers of the retina that are receiving light from some object • Visual Field – describes the area in space where the object lies They are the reversal of one another • The nasal retinal field receives light from the temporal visual field • The temporal retinal field receives light from the nasal visual field
Temporal TemporalVisual field Visual FieldNasal Visual Nasal VisualField Field Temporal Temporal Retinal field Retinal Field Nasal Retinal Nasal Retinal Field Field
Optic Chiasm Partial decussation (cross-over) of Optic Nerve fibers occurs at the level of the Optic Chiasm • Only nasal retinal fibers (from the nasal retinal field) cross over • Temporal nasal fibers (from the temporal retinal field) do not.
Optic Tract Optic tract It is important for the sense of sight. By convention, the optic tract is defined as that extent of the visual system pathway from the optic chiasm to the lateral geniculate nucleus of the thalamus. Each optic tract contains axons from ganglion cells in the retinas of both the left and right eyes, but information from only one half (i.e either left or right) of each eyes visual field
Lateral Geniculate Body After the optic tract, the next stop is the Lateral Geniculate Body (or Lateral Geniculate Nucleus)
LGN• Optic nerve fibers from the optictracts terminate at two bodies in thethalamus (a structure in the middleof the brain) known as the LateralGeniculate Nuclei (or LGN forshort).• One LGN lies in the lefthemisphere and the other lies in theright hemisphere.• Each has six layers
The optic tract wraps around thecerebral peduncles of themidbrain to get to the lateralgeniculate nucleus (LGN), whichis a part of the thalamic sensoryrelay system.There are two geniculate nuclei,located on either side of the rearend of the thalamus. They eachconsist of six cellular layers,forming a threefoldrepresentation of the oppositebinocular visual hemifield inexact anatomic registration.
This apparentlycomplicated arrangementis engineered so that theright LGN receivesinformation about the leftvisual field, and the leftLGN receives informationabout the right visualfield.
LGN This layered structure is exquisitely precise in two ways. • First, cells in different layers that align (like the numbers in the picture) have receptive fields in the same area of retina. • Second, optic nerve fibers from the two eyes are segregated in different layers. If you look carefully at the projections to the LGN, you will see that ipsilateral fibers arrive in layers 2, 3, and 5, while contralateral fibers arrive in layers 1, 4, and 6 (no-one knows why).
LGN Cell Types All cells in the LGN have concentric receptive fields, just like the ganglion cells whose fibers terminate in the LGN. Layers 1 and 2 are made up of cells with large bodies ("magnocellular") that have monochromatic responses (ie. mediate responses to light and dark) Layers 3 to 6 are made up of cells with small bodies ("parvocellular") that mediate color vision.
Optic Radiations Leaving the LGN are optic radiations Optic radiations are a collection of axons from relay neurons in the lateral geniculate nucleus of the thalamus. They carry visual information to the visual cortex (also called striate cortex) along the calcarine fissure. There is one such tract on each side of the brain.
Meyer’s Loop The optic radiations follow a very wide three dimensional arc. Here is how the radiations are conventionally drawn, and how they look from the side The longer loop actually dives into the temporal lobe before it heads back to the occipital lobe. This group of fibers is called Meyers loop. Recall that, since the lens inverts all images, the lower half of the retina sees the upper half of the world. This orientation is preserved through the pathway, so that the lower optic radiations, or Meyers loop, are carrying information from the upper visual world.
Retinotopic Mapping In lower visual areas (e.g., V1 through V5) the neurons are organized in an orderly fashion called topographic or retinotopic mapping • they form a 2D representation of the visual image formed on the retina in such a way that neighboring regions of the image are represented by neighboring regions of the visual area But this retinotopic representation in the cortical areas is distorted. The foveal area is represented by a relatively larger area in V1 than the peripheral areas.
Visual Cortex Much of the primate cortex is devoted to visual processing. • In the macaque monkey at least 50% of the neocortex appears to be directly involved in vision, with over twenty distinct areas. • Some of the areas concerned are quite well understood, others are still a complete mystery.
Visual Cortex Nearly all visual information reaches the cortex via V1, the largest and most important visual cortical area. Because of its stripey appearance this area is also known as striate cortex, amongst other things. Other areas of visual cortex are known as extrastriate visual cortex • the more important areas are V2, V3, V4 and MT (also known as .....V5!).
V1 In primates nearly all visual information enters the cortex via area V1. This area is located in the occipital lobe at the back of the brain. It is also known as: - primary visual cortex - area 17 - striate cortex.
V1 This region represents about 5% of the neocortex in man. It is the most complex region of the cortex with at least 6 identifiable layers • layer 1 is close to the cortical surface, layer 6 adjoins the white matter below
Simple Cells Simple cell receptive fields contain sub-regions that exert an excitatory influence on the cells response (light grey in the picture), and sub- regions that exert an inhibitory influence (dark grey in the picture). The blue lines in the picture are time traces that plot the onset and offset of stimulation. The black vertical lines below them indicate individual nerve impulses. The most effective stimulus for this particular receptive field (left) is one that puts a lot of light in the excitatory region, and only a little in the inhibitory region.
Simple Cells It must have the right orientation, the right position, and the right size. Stimuli that are non-optimal in terms of position (middle left), or orientation (middle right), or size (right) are less effective. Simple cell receptive fields could be built in the cortex by collecting responses from LGN cells whose receptive fields fall along a line across the retina, but the exact wiring is still the subject of debate.
Complex Cells Complex cells are the most numerous in V1 (perhaps making up three-quarters of the population). Like Simple cells, they respond only to appropriately oriented stimuli, but unlike Simple cells, they are not fussy about the position of the stimulus, as along as it falls somewhere inside the receptive field (left and middle- left examples above). Many complex cells are also direction-selective, in the sense that they respond only when the stimulus moves in one direction and not when it moves in the opposite direction
Orientation Cells Hubel and Wiesel were the first to discover that cells in V1 are arranged in a beautifully precise and orderly fashion. Hubel and Wiesel found that as one advances deeper into the cortex through successive layers perpendicular to the surface, all cells that have orientation tuning prefer the same orientation. On the other hand, moving across the surface of the cortex, orientation tuning mostly changes in an orderly fashion (as shown by the small lines in the picture). Hubel and Wiesel used the term "orientation columns" to describe this arrangement, but they are really slabs rather than columns.