Color visionDifferent cone photo-receptors have opsin molecules which are differentially sensitive to certain wavelengths of light – these are the physical basis of color. The cones are classified as S, M or L, depending on their color sensitivity.

Fig 15-23

Color visionWe already know that rods are far more sensitive to light, but rods preferentially respond to light at 500 nm wavelength. Light at other wavelengths must be stronger to elicit the same response.

Fig 15-24

Color visionHow do cones signal color?For a stimulus of any wavelength, each type of cone will have a different response, and the combination of these signals will signal the perceived color.

Fig 15-25

Color visionHow do cones signal color?Cones send color information centrally by causing center-surround stimulation in retinal ganglion cells; in these cases, the center and surround are responding to different colors, and receive inputs from M (medium) and L (long) wavelength cones.

Fig 15-26

Color visionHow do cones signal color?The other class of color retinal ganglion cells encodes the antagonism of yellow-blue, and this ganglion cell receives information from all 3 types of cones.

Fig 15-26

Ganglion cells project the visual field Retinal ganglion cells from the retina transmit information centrally in parallel pathways. They are divided into subtypes, called W, X and Y in cats. The W cells are the oldest cells phylogenetically, and send projections to the older portions of the brain (mostly midbrain) and not to the cortex.

Fig 16-5

Ganglion cells project the visual field Y cells have large somata and large dendritic branching fields (and are viewing a relatively larger portion of the visual field). They are called M cells, for magnocellular, in primate cortex. They respond rapidly, thus specializing in the movement of a stimulus Fig 16-5

Ganglion cells project the visual field X cells have smaller somata and less branching fields. They are called P cells, for parvocellular, in primate cortex. They have smaller receptive fields and are more concerned with color, texture, detail and form

Fig 16-5

The visual fieldAt the optic chiasm, visual information from the two sides of the head cross.In animals with eyes on the sides of the head, the entire visual field for each side is sent to the opposite side of the brain (to the tectum).

Fig 16-2

The visual fieldIn forward-looking animals, the visual image is split

An object on the right side of the visual field is seen by both left hemi-retinae (but not by the right hemi-retinae). The optic nerves leave the retinae, and at the optic chiasm, the two left hemi-retinae projections go left, while the two right hemi-retinae go right.

The visual fieldThere are binocular and

monocular zonesWhen both eyes are fixed on a central point, much of the visual field is seen by both eyes and is therefore binocular. However, in the temporal regions, only one eye can perceive the peripheral fields on each side; that is the monocular zone.

Central projectionsTargets – LGN and cortex

The retinal output then travels to the mid-brain (where pupillary reflexes are mediated) and to the thalamus, before going on to the cortex.

Fig 16-1

Central projectionsEach LGN serves the contralateral visual field

In this example, the left nasal retina and the right temporal retina view the same visual field (except for the monocular zone). Because the ganglion cell projections from the left nasal retina cross to the right side, all the ganglion cells serving the left visual field go to the right LGN.

Fig 16-3

Projection to LGNEach LGN layer is eye-specific

The projections from the retinal ganglion cells maintain the field of view as it was seen - this is called a retinotopic map. The LGN contains 6 layers of cell bodies; each layer receives input from only one eye. The two most ventral layers receive M (magno) ganglion cell inputs, while the other 4 receive P (parvo) inputs.

Projection to LGNDifferences between M cells and P cells in LGN

M cells do not see color differences, but perceive differences in brightness. M cells are able to resolve images quickly, but not precisely. P cells are sensitive to color, and are also able to resolve detail in stimuli.

So, in general, the M cells respond to movement in the visual field while P cells are better at discriminating color and detail. The M and P cells project to different cortical layers.

Each LGN neuron receives input from only a few ganglion cells. The receptive fields of LGN neurons are center-surround, much like those in the retina.

Projection to cortexThe visual field is projected in a retinotopic

way

The right visual field is projected to the left cortex, while the left visual field is represented on the right.

The region of the fovea, because of its high sensitivity and density of cones, is represented on a huge amount of the cortex!

Projection to cortexThe striate cortex is a six-layered structure - layer 4 is the major input

layer.

LGN inputs Cortical cells

Projection to cortexLocal cortical neurons distribute the inputs

Stellate cells in layers 4 receive the input and project it to layers 2/3, which then project to layers 5 and 6. The output of the cortex is via pyramidal cells.

LGN inputs Cortical cells

Projection to cortexLGN input is segregatedThe magnocellular (Y) cells of the LGN are located in layers 1 and 2 (layer 1 for the contralateral eye, layer 2 for the ipsilateral). The parvocellular (X) cells are in layers 4 and 6 for the contralateral eye, layers 3 and 5 for the ipsilateral eye. These inputs remain strictly segregated when they arrive at the cortex.

Fig 16-6

Projection to cortexLGN input is segregatedThe inputs from the magnocellular layers synapse on stellate cells in layer IVc in the cortex, but in different positions, called ocular dominance columns.

The parvocellular (X) layers synapse in layers IVa and c, but still in separate ocular dominance columns.

So, within layer IV, the visual information is still eye-specific.

Fig 16-6

Cortical receptive fields• Simple cells respond best to oriented bars

Recording from cells in the cortex demonstrated that many “simple” cells respond to bars of light of particular orientation, not spots as in the retina or LGN.

Fig 16-7

Cortical receptive fields• Simple cells respond best

to oriented barsThe simple cell response can be constructed by having several on-center cells from the LGN project onto one cell, causing the simple cell to to respond to a bar of light. Complex cell receptive fields are composed of converging simple cells.

Orientation columns• Cells with similar orientation

responses are in columns

This figure shows an area of the cortex with orientation columns. Each column through the cortex responds to the same orientation, and represents a small portion of the visual field. Adjacent columns are from neighboring areas in the visual field.

Fig 16-8

Ocular dominance columns• Each visual field point is

represented by two eyes

The input from the right visual field, which is coming through the left LGN, is represented on the left cortex by both eyes.

Fig 16-8

Color Color information from specific LGN cells in input to layers other than layer IV, and cells responding to color are organized into regions called “blobs”.

Fig 16-11

Higher order visual processingInformation from the primary visual cortex (V1) is passed via several pathways, to higher-order portions of the visual cortex (V2-V5).

Fig 16-14

Higher order visual processingDifferent areas of the visual cortex are thus concerned with different aspects of vision.

Fig 16-15

Higher order visual processingCells in V5 are concerned with processing of visual motion, and respond to oriented movement with changes in action potential frequency.

Fig 16-16