The visual system

The visual system is the best understood system in the brain. In the visual system, there are two neural paths from the retina, one through the superior colliculus, which controls automatic eye movements, one through the lateral geniculate, which leads to cortex and most visual processing.

Much of the structure of the visual system is revealed by maps, in which patterns of neural activation reflect the spatial pattern of some feature of the input. The slide of the monkey brain showing a distinct dart board pattern is an example of a visual topographic map. Other brain systems have maps as well; for instance, we have tonotopic maps.

The retina is actually part of the brain. Rods are good at detecting low levels of light. The cones are the color sensors. The functional properties of rods and cones constrain the color systems of human language. Other cells in the retina take care of dark adaptation and contrast enhancement. Ganglion cells transfer output from the retina to the lateral geniculate body. Processing within the retina is analog, but ganglion cells produce standard spike trains.

There are two kinds of ganglion cells. The smaller parvo cells are tuned for spatial resolution and object form detection. The larger magno cells are concerned with temporal resolution and motion detection.

The first cortical visual processing area V1 is the primary visual cortex, located at the back of the occipital lobe. The cells in V1 are arranged in patterns including, e.g., blobs of cells which respond to color and ocular dominance columns of cells with input mainly from one eye.

In the V2 area of the visual processing system, cells with different functional properties are arranged in stripes. >From these areas, visual information is sent out into higher level processing systems, the ventral or "what" system specialized more for object identification and the dorsal "where" system for motion and spatial cognition.

Many cells in the visual system have a center-surround receptive field, responding best to contrast. These are circular in early stages, but become more elaborate at higher levels. In primary visual cortex (V1) the characteristic RF is an oriented line, moving with constant velocity.

Other higher processing areas of the visual system include the V4 area for color constancy, the MT area for simple motion detection, MST area for motion contrast detection, 7A for spatial maps and FEF (frontal eye fields) which control attentional eye movements. Although this was not explicitly pointed out in lecture, it is important that the connections among visual areas is bi-directional, higher processing areas always have extensive connections back.

Development of the brain.

In any biological approach to studying a particular trait, four questions must be asked. Neglecting to address any of these questions may lead to a theory which is absurd in one of these respects.

Mechanical: How does it work?
Functional: What does it do for the animal?
Ontogenetic: How does it develop?
Phylogenetic: How did it evolve?

The development of the brain is an ontogenetic perspective. Many of the slides for this lecture and detailed discussion of the development of the human brain can be found in the coursepak reading #1. It is also important that brain development as well as the properties of neurons remains similar across evolution, so much can be learned from studying simple system. The basic tubular structure of vertebrate brains provides a good starting point for considering the development of the brain.

In comparison with human brains, the brains of smaller animals differ in that they have a much smaller cerebrum. Brains of primates have a larger cerebrum and cerebellum. Cats have visual systems similar to the human system. Macaque monkeys and chimpanzees have brains quite similar to human brains. Human brains have by far the largest frontal lobe of any animal.

In humans, developing brains create 250,000 neurons per minute from conception to birth. After birth, brain connections continue to be fine-tuned. The embryo first differentiate into 3 layers. The ectoderm which become the skin, the mesoderm which becomes the muscles, and the endoderm which becomes the organs. Between the ectoderm and the mesoderm a neural induction layer develops which becomes the nervous system.

The central nervous system begins as a neural plate which develops into a groove, then a neural tube, which elongates into the brain and spinal cord.

There are four overlapping stages for the development of neuronal processes:
Genetic/Chemical
Prenatal Activity Dependent
Childhood
Learning and Recovery in Adulthood

From the reader: the phases of neural development :
Induction of neural plate
Localized proliferation of cells in different regions
Migration of cells from original regions to final regions
Aggregation of cells to form identifiable parts of the brain
Differentiation of immature neurons
Formation of connections with other cells
Selective death of certain cells
Elimination/stabilization of links

Cowan's stages do not include a specific treatment of how nerve cell migration is guided and controlled. Cortical nerve cell migration is guided by elongated glial cells. Nerve cells have growth cones on one end which spread out in the direction of movement. The direction of growth and synapsing with appropriate cells is determined by chemical tags on the neurons and in the environment. Experimental evidence illustrates that neurons will grow in the direction of the concentration gradient of nerve growth factor.

The control of growth and direction of neurons involves chemicals which repulse and attract neurons at surfaces in contact with neurons and also through longer distance gradients. Neurons may also be induced to form fiber bundles. In this process, fasciculation, neural fibers are attracted to each other by chemicals in the outer layers of the neuron. As a result of the attraction, the neurons will grow in a bundle. When the bundle reaches its destination, surrounding polysialic acid causes the formerly attracted fibers to repulse each other. The fibers then split and spread out. Depending on the specific type of chemical reaction involved, particular fibers may be selected to split off from the bundle which others remain in the bundle.

Chemical gradients are used for even more finely tuned neural placement. For instance, cells beginning from the nasal side of the retina end in the posterior of the tectum. Cells from the temple side of the retina end in the anterior of the tectum. The chemicals which attract these cells have a higher concentration in the posterior, which attracts cells from the nasal side of the retina. The lower gradient in the anterior likewise attracts the cells from the temple side of the retina.

In the monkey and the human cortical areas, pyramidal cells develop horizontal collaterals in addition to vertical connections. Horizontal collaterals extend out side to side and make connections with cells with similar functions. For instance, in the visual cortex, horizontal collaterals may connect cells which respond to similar kinds of visual input, such as edges with a particular orientation. The horizontal collaterals are guided to make connections with the right neighboring cells by chemical tagging. These cells may also make connections with inhibitory neurons. This simultaneous excitatory and inhibitory connection may serve to keep the neurons primed or keep them from firing on the basis of minimal evidence.

Chemical tagging remains a viable process into adulthood. Mature brains in recovery are guided by such tagging.

These processes are only responsible for getting neurons in generally the right areas with generally the right connections. Finer tuning is activity dependent. Muscles fibers, for instance, are originally connected to several neurons, but through activity the connection to one neuron is strengthened and other connections are weakened and eliminated. In mature systems, one muscle fiber is connected to one neuron. This competitive elimination of connections is also an important process for learning.

There is a evidence that prenatal brains experience waves of firings across visual maps and this provides prenatal experiential tuning of connections so that experiential. Early postnatal experience is also essential to the proper wiring. In the visual system, immature brains have connections leading from the left and right eyes to overlapping areas in the lateral geniculate and cortex. After experiential activation the cells leading from the left eye make connections in an area separate from the cells leading from the right eye. The process establishes ocular dominance columns in visual cortex.

If one kind of experience is missing during this experiential fine-tuning, the neurons which do get activation will establish more connections than they normally would. In the visual system, for instance, if one eye is missing, the cells from the remaining eye will establish connections in the areas which would have been targeted by the missing eye. There are similar deprivation results involving motion, orientation, etc. The invasion of vacated neural map territory is also seen in adults where damage has occurred. There is also good evidence that extensive stimulation can cause the receptive field associated with e.g. a patch of skin, to grow.

Selected slides:

References: Reader 1 -4, Regier Ch 1