HERMISSENDA: THE GOOD-LOOKING ONE IN THE FAMILY
Even a dedicated neurobiologist would be hard-pressed to describe Aplysia as aesthetically attractive, but another popular invertebrate species used in studies of learning and memory is a clear winner in any molluscan beauty contest. With its bright coloration and striking profile, Hermissenda is the closest thing to a poster child available among the invertebrate species commonly studied by neurobiologists.
HERMISSENDA: THE GOOD-LOOKING ONE IN THE FAMILY
Hermissenda is not just all looks and no brains, however. This system has been used to study the cellular and molecular basis of a particular form of associative learning exhibited by the animal. Hermissenda are normally phototactic; that is, they will move toward a lighted area. However, if the animal is trained that light predicts an upcoming aversive stimulus, in this case turbulence in the water surrounding the animal, the normal phototactic response is suppressed. The laboratories of Dan Alkon and Terry Crow have been instrumental in discovering the neuronal circuitry, cellular physiology, and molecular mechanisms underlying this form of associative conditioning. Image copyrighted by Mike Johnson. Used with permission.
4fe will discuss several of the biochemical mechanisms underlying the short- and long-term modification of this behavioral response.
Motor learning, skills, and habits are the classic examples of unconsciously learned and unconsciously recalled memories. Walking is a good example. Walking is an extremely complex task involving intricate motor movements, which we generally perform automatically and with great facility. We learned to walk unconsciously as small children, and if anything trying to exert conscious control over our walking as adults likely leads to an awkward gait.
Another example of unconscious learning is learning to play an instrument such as the guitar or piano, at least as concerns the motor components. Repetition allows the development of finely tuned motor patterns that can be recalled without conscious thought. Learning of the motor components also occurs without much conscious control, although certainly there is conscious involvement when the initial motor patterns are beginning to be laid down. Even in this case, though, one does not consciously work out the pattern of firing of individual muscles; indeed, by and large, we don't have very much control over the contraction of single muscles and are not really conscious of them as single units. When we learn to play an instrument, a multitude of complex muscle contractions and hand movements are taking place completely below the level of conscious thought.
While complex unconscious processes go into the initial establishment of learned motor patterns, in some cases such as speech and walking, there is probably also a complicated interaction of developmental processes with signals generated in response to environmental stimuli. As mentioned earlier, in early stages of many types of motor learning, there is conscious involvement, the need for which disappears over time as part of the learning process. The circuitry and cellular mechanisms underlying motor learning are quite complex, involving the motor cortex, basal ganglia including the neostriatum, and cerebellum. We will not deal much with mechanisms of motor learning in this book. However, in later chapters, we will touch briefly on some forms of cerebellar synaptic plasticity, which is probably relevant to some forms of motor learning.
The site of memory storage for most types of motor memory are not clear but, of course, in some way, must involve or have access to the principal circuits that mediate the behavioral motor pattern, such as the motor cortex, basal ganglia, and spinal cord motor neurons. A discussion of these systems is not within the scope of this book, so I refer the reader to any of a number of good reviews and textbooks dealing with this area (Chapter 13 of reference 4 is a good place to start).
Some motor memories are subject to limited conscious recall, but in most cases trying to replay a motor memory with too much conscious control simply messes things up. This is likely a component of the common "choking" component of sports, although stress-induced release of modu-latory neurotransmitters, which affect performance, is also certainly a factor. It is interesting that the unconscious aspect of motor recall has made it into popular sports lingo. When athletes are at the top of their game, they are typically referred to as being "unconscious."
C. Unconscious Learning and Subject to Conscious Recall
The forms of learning we have talked about so far are nonassociative. In habitua-tion, sensitization, and the like, nothing is learned about the relationship or association of one event with another. We next move on to a more complex form of learning where a predictive relationship is learned—an animal learns that one environmental stimulus reliably predicts another.
An important set of nomenclature in this area arose out of the pioneering work of Ivan Pavlov. Pavlov and his co-workers studied associative conditioning of the salivary response of dogs—studies indeed so classic that the terms classical conditioning and Pavlovian conditioning are now used synonymously with associative conditioning. Pavlov knew, as does anyone that has ever owned a dog, that when a dog is presented with a food stimulus, a strong salivatory response is elicited (see Figure 9). This is a natural response, of course, and this salivation is referred to as the unconditioned response (UR), and correspondingly the food stimulus is referred to as the unconditioned stimulus (US). Pavlov's breakthrough realization, which he subsequently
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