Term Paper: Development and Plasticity of the Brain Vision and Other Sensory Systems

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Biologists can develop antibodies against nerve growth factor (i.e., molecules that inactivate nerve growth factor). What would happen if someone injected such antibodies into a developing nervous system?

The nerve growth factor is a neurotrophin that promotes the survival and growth of neurons. If antibodies to that growth factor were to be injected into an organism that had a developing nerve system the nerve growth would slow or end, based on no development of axons, dendrites and new synapses. The neuron needs to receive neurotrophins from target cells and axons. Previous experiments indicate that the brain and nervous system assemble normally, but neurons die rapidly. This is like a domino effect -- more neurons die, fewer neurotransmitters are released, and the system collapses (Kalat, p. 130).

Ordinarily, patients with advanced Parkinson's disease (who have damage to dopamine-releasing axons) move very slowly if at all. However, during an emergency (e.g., a fire in the building), they may move rapidly and vigorously. Suggest a possible explanation.

During stressful times adrenalin is released by the brain. This is typically a self-regulating stress response that works to allow for short bursts of energy and flight. Research suggests that when a stressor occurs, dopamine releasing axons also product adrenalin and move faster to ensure that the organism is prepared to handle the situation. Thus, dopamine levels are induced by stress factors that may facilitate certain behaviors (pp. 211, 254).

3. Drugs that block dopamine synapses tend to impair or slow limb movements. However, after people have taken such drugs for a long time, some experience involuntary twitches or tremors in their muscles. Based on material in this chapter, propose a possible explanation.

A tremor is an involuntary muscle contraction and relaxation of one of the body parts. Sometimes dopamine blockers like haloperidol cause resting tumors. One possible explanation holds that neurons make adjustments to maintain a constant level of stimuli (arousal). If some of the axons that transmit dopamine become inactive (or even die) the remaining dopamine synapses actually become more responsive and can cause stimulation resulting in involuntary tics or tremors. This is called denervation super sensitivity. Another explanation is that the blocking of dopamine synapses damages those synapses and when the axons are regenerated (healed), they attach to different muscles than intended (p. 145).

1. How could you test for the presence of color vision in a bee? Examining the retina does not help because invertebrate receptors resemble neither rods nor cones. It is possible to train bees to approach one visual stimulus and not another. However, if you train bees to approach, say, a yellow card and not a green card, you do not know whether they solved the problem by color or by brightness. Because brightness is different from physical intensity, you cannot assume that two colors equally bright to humans are also equally bright to bees. How might you get around the problem of brightness to test color vision in bees?

One could use a double blind study like zoologist von Frisch did. For example, putting sugar water over a color and then training the bees to find the water (blue). The double blind would be non-sugared water on dishes with a different color. If the bees can see color, they would always go to the colored dish (blue, for instance). If they cannot see color, they would randomly go to both dishes regardless of the study. If we add the variable of brightness to the equation, we would also see if the bees responded to the original color and light, or if light were added to a new color, more randomly. It does make sense that bees "see" differently than humans, since evolutionarily they would need to distinguish between nectar-bearing and non-nectar bearing flowers. Experiments could be done by using different levels of light in the experiment; say low, medium and high with the blue dish; then low, medium and high with another color. Observe the results and test for randomness (pp. 165-7).

2. After a receptor cell is stimulated, the bipolar cell receiving input from it shows an immediate strong response. A fraction of a second later, the bipolar's response decreases, even though the stimulation from the receptor cell remains constant. How can you account for that decrease? (Hint: What does the horizontal cell do?)

The decrease in the bipolar cell's response, even though stimulation from the receptor cell remains constant, may have something to do with light. Red, green and yellow light inhibit the bipolar cell through the horizontal cell -- with yellow light stimulating the strongest in long and medium-wavelengths. The bipolar cell remains excited, causing the peak and valley responses. Chemically, the horizontal cells become depolarized with the release of glutamate from photoreceptors (during absence of light) Depolarization of this horizontal cell causes it to release GABA on an adjacent photoreceptor causing positive and negative feedback loops (p. 161-3).

3. A rabbit's eyes are on the sides of its head instead of in front. Would you expect rabbits to have many cells with binocular receptive fields -- that is, cells that respond to both eyes? Why or why not?

A rabbit's eyes are at the side of their face, meaning that each of the eyes can see all around them, above them, side to side and below. The rabbit has two blind spots that are directly in front of, and behind the head. We would not expect the rabbit to have many binocular receptive cells because, as a prey animal, it is more useful to have surrounding monocular vision in order to escape predation. The rabbit's visual system is designed to quickly detect a predator from any direction as well as more farsighted (p. 168).

4. Would you expect the cortical cells of a rabbit to be just as sensitive to the effects of experience as are the cells of cats and primates? Why or why not?

We would not expect a rabbit's cortical cells to be as sensitive to experience as those of felines or primates because of the ecological niche a rabbit occupies -- a prey animal. Evolution in prey animals codes for different strengths, for instance, being able to see shapes or detect movement from far away or to see all around them. However, felines and other predators had to learn to judge distance and strength of surfaces when moving through trees and to spot prey. Experience (or memory) in cortical cells would be more advantageous to a predator or scavenger. Additionally, research shows that cortical memory is determined not only by the need for certain sensory development from the environment, but also the cognitive abilities of the animal (with predators being higher on the developmental chain than prey animals or grazers) (pp. 173-80).

5. The visual system has specialized areas for perceiving faces, bodies, and places, but not other kinds of objects. Why might we have evolved specialized areas for these functions but not others?

Just as with other evolutionary traits, human vision would code for the most advantageous attributes. Research suggests that the first proto-humans were likely scavengers who moved from the trees to the savannas in search of food. Coding for faces and bodies would help these early humans distinguish clan or tribe members from strangers, assist with potential mating, and provide a sense of family and kinship for protection. Coding for places would orient the group in terms of places for savaging, places to avoid, places for safety and rest, and to be able to extrapolate data from safe spot a into characteristics they might find for a new safe or savaging site. This is likely due to specialization, and over time, humans became better and better at recognizing the things that helped them survive (faces, bodies, places) (pp. 188-9).

6. Why is it advantageous to become motion blind during voluntary eye movements? That is, why might we have evolved this mechanism?

Motion induced blindness is a visual phenomenon that means that certain stationary visual stimuli disappear in front of the observer's eyes when masked with a moving background. Research shows that this has evolved to allow the brain to process reality. From a genetic perspective, humans likely evolved this in order to be able to differentiate objects from complex backgrounds that had a great deal of movement. This would be advantageous for humans to be able to pick out food, prey, or predator within a fast and complex environment. It also may have evolved to allow humans to focus more clearly on specific objects, for instance, when learning to hunt with a spear or bow and arrow - and to more effectively block out other stimuli (pp. 187-8).

1. Why do you suppose that the human auditory system evolved sensitivity to sounds in the range of 20 to 20,000 Hz instead of some other range of frequencies?

In experiments with other mammals, humans have a relatively low high range cut off. This implies that the human ear evolved to hear lower frequencies in order to communicate… [END OF PREVIEW]

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