BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.9. Learned Behavior

Class="center">17.9.1. Memory

Memory is The ability to store and retrieve information about past experiences, and learning is impossible without memory. Past experiences, including cues and responses to them, are "recorded" by the Organism in the form of "memory traces," or engrams. Since the capacity for learning in mammals is proportional to The Development of the cerebral hemispheres, it is likely that engrams are formed and stored precisely there.

The Nature of engrams remains unknown, and there are only hypotheses based on controversial data regarding this matter. In principle, two main hypotheses exist: one suggests that memory is based on changes in The Structure of Neurons and their Organization within the Central Nervous system, while the other proposes lasting changes in Brain biochemistry.

Histological studies of brain tissue indicate the presence of closed neural circuits, which led to METABOLISM/2.html">THE CONCEPT OF reverberating circuits as components of engrams. According to this view, reverberating circuits allow excitation to circulate in a loop, thereby preserving information in memory. However, it seems doubtful that such activity could be sustained for any significant length of time, and experimental evidence suggests that memory possesses much greater capacity and stability than could be achieved by this mechanism alone. For example, when a rat's brain is cooled to 0 °C, all electrical activity ceases, but no disruption of memory traces is observed once the Temperature returns to normal. Nevertheless, it is generally accepted that such circuits may play a role in forming short-term memory—which lasts for a matter of minutes—and in facilitating signal transmission along specific Neural Pathways. The predominantly physiological nature of short-term memory is evidenced by its susceptibility to easy loss, such as following a concussion or HEAD trauma (retrograde amnesia), and its decline during Aging. Long-term memory is far more robust, leading to the hypothesis that it is underpinned by more permanent structural changes in brain tissue.

It is conceivable that The formation of memory traces is associated with a biochemical mechanism involving the Synthesis of specific substances in the brain. For instance, extracts from the anterior ganglia of trained Flatworms or from the brains of trained rats, when administered to untrained worms or rats respectively, reduced the time required to master the same tasks. RNA appeared to be the active factor in all these experiments.

There is also evidence suggesting that the RNA composition of neurons changes during the learning process, which may lead to the synthesis of specific "memory Proteins" associated with specific life experiences. It has been demonstrated that the administration of a Protein Synthesis inhibitor, such as puromycin, causes memory impairments. For example, mice recently trained to navigate a maze lost this ability after being injected with puromycin in the brain, whereas control subjects injected with a saline solution retained it.

In Conclusion, while we are still far from fully understanding the Mechanisms of Memory, it appears that the formation of memory traces involves Changes in the electrical Properties of Neurons, the permeability of synaptic membranes, and the synthesis of Enzymes related to synaptic transmission. There is little doubt that memory is intimately linked to processes occurring at the synaptic level.

17.9.2. Learning

Learning is an adaptive modification of individual behavior resulting from prior life experience (Fig. 17.70). The scope and persistence of skills acquired in this manner depend on the capacity of memory to accumulate and store information. In turn, this is determined by the Nature of the information itself. In humans, the memorization of "facts," such as for an examination, may be short-lived, whereas coordinated motor skills (such as personal hygiene, bicycle riding, or swimming) persist throughout life. Although we commonly associate the capacity for learning with vertebrate behavior—mammals in particular—it has been found in all animals except Protozoans, Coelenterates, and Echinoderms, which lack a nervous system or possess a very primitive one. Psychologists have attempted to establish universal "laws of learning," but all such attempts have thus far been unsuccessful. Learning appears to be an individualized process that proceeds differently across various species and circumstances.

Fig. 17.70. A typical "learning curve." The graph illustrates how the number of attacks by a cuttlefish on a shrimp placed in a Glass container changes as a function of accumulated experience. The predator was given eight separate 3-minute trials to seize the inaccessible prey. As the number of unsuccessful strikes increased, the cuttlefish "learned" that the shrimp was out of reach and consequently reduced its hunting activity. Results are based on data obtained from 40 cuttlefish (From Messenger J.B. (1977), Symp. Zool. Soc. Lond., 38, 347-376.)

The Classification of forms and features of learned behavior presented in this chapter is admittedly artificial, and this should be kept in mind. Nevertheless, it encompasses the full spectrum of current views regarding types of learning and is supported by experimental data. The MAIN TYPES OF learned behavior are summarized in Table 17.16, which provides introductory information.