Learning and the Diversity of Behavior

Learning and the Diversity of Behavior
Another aspect of behavior is learning, which we define as the modification of behavior through experience. An excellent model system for studying learning processes has been the marine opisthobranch snail Aplysia (Figure 38-7), a subject of intense experimentation by E. R. Kandel and his associates. The gills of Aplysia are partly covered by the mantle cavity and open to the outside by a siphon (Figure 38-8). If one prods the siphon, Aplysia withdraws its siphon and gills and folds them in the mantle cavity. This simple protective response, called gill withdrawal reflex, will be repeated when Aplysia extends its siphon again. But if the siphon is touched repeatedly, Aplysia decreases the gillwithdrawal response and finally comes to ignore the stimulus altogether. This behavior modification illustrates a widespread form of learning called habituation. If now Aplysia is given a noxious stimulus (for example, an electric shock) to the head at the same time the siphon is touched, it becomes sensitized to the stimulus and withdraws its gills as completely as it did before habituation occurred. Sensitization, then, can reverse any previous habituation.

The mechanisms of habituation and sensitization in Aplysia are known because these behaviors constitute a rare instance in which the nervous pathways involved have been completely revealed. Receptors in the siphon are connected through sensory neurons (black pathways in Figure 38-8) to motor neurons (blue pathway in Figure 38-8) that control the gill-withdrawal muscles and muscles of the mantle cavity. Kandel found that repeated stimulation of the siphon diminished the release of synaptic transmitter from the sensory neurons. Sensory neurons continue to fire when the siphon is probed but, with less neurotransmitter being released into the synapse, the system becomes less responsive.

Sensitization requires the action of a different kind of neuron called a facilitating interneuron. These interneurons make connections between sensory neurons in the animal’s head and motor neurons that control muscles of the gill and mantle (see Figure 38-8). When sensory neurons in the head are stimulated by an electric shock, they fire the facilitating interneurons, which end on the synaptic terminals of the sensory neurons (red pathways in Figure 38-8). These endings in turn cause an increase in the amount of transmitter released by the siphon sensory neurons. This release increases the state of excitation in the excitatory interneurons and motor neurons leading to the gill and mantle muscles. The motor neurons now fire more readily than before. The system is now sensitized because any stimulus to the siphon will produce a strong gill-withdrawal response.

Figure 38-8 Neural circuitry concerned with habituation and sensitization of the gill-withdrawal reflex in the marine snail Aplysia. See text for explanation.

Kandel’s studies indicate that strengthening or weakening of the gillwithdrawal reflex involves changes in levels of transmitter in existing synapses. However, we know that more complex kinds of learning may involve formation of new neural pathways and connections, as well as changes in existing circuits.

Imprinting
Another kind of learned behavior is imprinting, the imposition of a stable behavior in a young animal by exposure to particular stimuli during a critical period in the animal’s development. As soon as a newly hatched gosling or duckling is strong enough to walk, it follows its mother away from the nest. After it has followed the mother for some time it follows no other animal. But, if the eggs are hatched in an incubator or if the mother is separated from the eggs as they hatch, the goslings follow the first large object they see. As they grow, the young geese prefer the artificial “mother” to anything else, including their true mother. The goslings are said to be imprinted on the artificial mother.

Imprinting was observed at least as early as the first century A.D. when the Roman naturalist Pliny the Elder wrote of “a goose which followed Lacydes as faithfully as a dog.” Konrad Lorenz was the first to study imprinting objectively and systematically. When Lorenz handreared goslings, they formed an immediate and permanent attachment to him and waddled or swam after him wherever he went (see Figure 38-9). They could no longer be induced to follow their own mother or another human being. Lorenz found that the imprinting period is confined to a brief sensitive period in the individual’s early life and that once established the imprinted bond usually is retained for life.

What imprinting shows is that the brain of the goose (or the brain of numerous other birds and mammals that show imprinting-like behavior) accommodates the imprinting experience. Natural selection favors evolution of a brain that imprints in this way, in which following the mother and obeying her commands are important for survival. The fact that a gosling can be made to imprint to a mechanical toy duck or a person under artificial conditions is a cost to the system that can be tolerated because goslings seldom encounter these stimuli in their natural environment. The disadvantages of the system’s simplicity are outweighed by the advantages of its reliability.

We will cite one final example to complete our consideration of learning. The males of many species of birds have characteristic territorial songs that identify the singers to the other birds and announce territorial rights to other males of that species.Like many other songbirds, the male white-crowned sparrow must learn the song of its species by hearing the song of its father. If the sparrow is handreared in acoustic isolation in the laboratory, it develops an abnormal song (Figure 38-10). But if the isolated bird is allowed to hear recordings of normal white-crowned sparrow songs during a critical period of 10 to 50 days after hatching, it learns to sing normally. It even imitates the local dialect it hears.

Figure 38-10 Sound spectrograms of songs of white-crowned sparrows, Zonotrichia leucophrys. Top, natural songs of wild bird; bottom, abnormal song of isolated bird.

It might appear from this result that characteristics of the song are determined by learning alone. However, if during the critical learning period, the isolated male whitecrowned sparrow is played a recording of another species of sparrow, even a closely related one, it does not learn the song. It learns only the song appropriate to its own species. Thus although the song must be learned, the brain is constrained to recognize and to learn vocalizations produced by males of its species alone. Learning the wrong song would result in behavioral chaos, and natural selection favors a system that eliminates such errors.