LEARNING DEFINED
If you ask a philosopher, a psychologist, and a neurobiologist to define “learning,” you are likely to get three very different answers. You may even get different answers if you ask researchers in the same field of study. Because most of what is known about learning is based on observing changes in behav-ior, the definition of learning can have very different meanings, depending on the behavior being studied. For example, some definitions state that learning does not occur instantaneously.
Some theories define learning as a long lasting change to dis-tinguish it from immediate, automatic responses to sensory stimuli. Other definitions restrict learning to only those types of behaviors that are ecologically relevant for an organism’s sur-vival. For our purposes, we will consider the broadest possible meaning and simply define learning as an experience-depen-dent change in behavior (see “Reflexes” box).
FORMS OF LEARNING
At its most basic level, learning is a change in an organism’s be-havior that occurs in response to experience. We use the word organism and not human (or, for that matter, nonhuman primate, mammal, or vertebrate) because experienced-based behavior changes can be observed in almost all members of the animal kingdom—from certain types of bacteria that move
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toward or away from a particular chemical (a phenomenon called chemotaxis) to the man who picks up the telephone when it rings.
That so many different organisms display this quality suggests that learning is a very important biological characteristic.
Learning can be studied at multiple levels in many different organisms. For example, learning can be explored at the molec-ular or cellmolec-ular levels. In addition to humans, worms, flies, and rodents are commonly used to study learning. Researchers ex-ploit the similar behaviors exhibited by these organisms. In this chapter, we examine three forms of learning—nonassociative learning, associative learning, and incidental learning—and the behavioral paradigms that are commonly used to explore them.
Nonassociative Learning
Nonassociative learning may be the most basic form of learning.
It has been observed in most of the eukaryotesstudied to date.
Nonassociative learning has two components: habituationhabituationand
sensitization.
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Reflexes
Immediate responses to sensory stimuli include reflex circuits.
For example, when your hand encounters pain (e.g., fire, electricity, or a sharp object), it is immediately retracted away from the harmful environment. Some definitions of learning exclude this behavioral response because it is an innate behav-ior, a genetically preprogrammed response that is not voluntar-ily controlled. This protective mechanism is due to the senses forming autonomic (automatic) sensory-motor circuits with the muscle, ensuring a rapid response that is independent of the brain. Other definitions maintain that if the reflex can be modi-fied in an experience-dependent manner, it reflects learning.
Habituation is a gradual reduction in a behavioral response that occurs with repeated presentation of the same (or similar) stimulus. By definition, a habituated response is defined by a loss in an existing response and excludes the effects of fatigue, injury, or drugs. Habituation can be commonly observed all around you. In fact, one reason why you do not respond to many stimuli throughout the day is that you are habituated to them. To illustrate habituation, try this: The next time you catch your cat napping, clap your hands. It is likely that your pet will raise its head and possibly even rise to investigate the noise. If you continue to clap every time your cat takes a nap, however, you will eventually notice that your pet no longer responds and may sleep right through the disturbance.
In the laboratory, habituation is commonly studied in simple organisms, such as the marine mollusk Aplysia or the marine worm Nereis pelagica. When touched, Aplysia withdraws its siphon, or tail. Nereis, which lives in the seabed, withdraws read-ily to the protection of its lair in response to vibration or visual stimuli. Both of these behaviors can be readily habituated with repeated presentation of the appropriate stimulus. Rodents are also easily habituated with relevant biological stimuli. Rats have an acute sense of smell and new smells initiate a change in their sniff frequency. Rats have been observed to increase their breathing rates (a measure of sniffing) from once per second to 8–10 times per second when presented with a new odor. If the smell is repeatedly presented, the rat’s olfactory (smell-based) response becomes habituated. Importantly, presentation of a new odor produces an increase in breathing rate. This experi-ment illustrates that a behaviorally relevant response (rats rely on smell as a primary way to interact with their environment) is stimulus specific. Habituated responses do not readily transfer across stimuli.
These examples focus on lower organisms. Do humans habit-uate to repetitive stimuli? Yes. Parents often observe that
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born babies devote far more attention to a new stimulus than to familiar ones (this has been verified in experiments). The Aus-trian composer Joseph Haydn—known as the father of the sym-phony—used repetitive components in his opening theme in Symphony No. 94 in G major “Surprise” to lull the audience, who found themselves startled out of their seats during the dishabituating full orchestra fortissimo, featuring kettle-drums, which Haydn included in the second movement (see
“Music and Human Performance” box).
Sensitization is the process of becoming highly sensitive to specific stimuli. Where learning is concerned, the stimuli are generally events or situations. Like habituation, the sensitization response is widespread throughout the animal kingdom. The sensitizing stimulus, unlike the stimuli that produce habitua-tion, is usually strong or noxious. Another important difference is that sensitizing stimuli usually produces an increase in the re-sponse to a wide variety of stimuli. Sensitization can be viewed as a priming mechanism, increasing arousal and attention, and lowering the threshold for defensive responses. For example, a leaf falling on a worm that has recently escaped a pecking bird produces a much greater escape response than if the worm had encountered the leaf in the absence of any previous physical as-sault. Presumably, sensitizing stimuli that generate an enhanced defensive threshold—in this case, an enhanced wiggle response by the worm, immediately following a pecking bird—can be viewed as a beneficial behavior, since the bird may have returned for a second try at its meal.
Associative Learning
Associative learning is a more complex process than nonasso-ciative learning in that it requires two stimuli to be presented closely in time. Both habituation and sensitization are learned nonassociative responses that lack stimulus specificity. After the earthworm is pecked by the bird, it wiggles vigorously when any
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subsequent physical contact is made, whether it is another attack by the bird or a falling leaf. Unlike nonassociative learning, associative learning involves discrimination between different stimuli. This discrimination permits an organism to draw causal relationships from its environment. For example, a bee sam-pling nectar in a diverse field of flowers can, upon discovering
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