Non-associative learning is a change in the strength of the response to a single stimulus that occurs with repeated exposure to the stimulus. Non-associative learning occurs in all animals and is considered to be the simplest form of learning. The two types of non-associative learning are habituation and sensitization. Habituation is a decreased response with repeated exposure to the stimulus. Sensitization is an increasingly strong response with repeated exposure to the stimulus.
Non-associative learning usually involves exposure to a single stimulus or event, in contrast to associative learning, which relates multiple stimuli or events. Non-associative learning occurs via sensory and sensorimotor nerve pathways. It is an important mechanism for suppressing or amplifying the sensory information that reaches the higher brain or other organs.
Deficits or overexpression of habituation or sensitization have been implicated in some mental disorders, such as schizophrenia, autism, and attention deficit/ hyperactivity disorder (ADHD). They also may contribute to cognitive defects in neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease.
Habituation is an eventual decline or discontinuation of a response to a highly repetitive stimulus; thus, habituation is learning to ignore innocuous stimuli that have no meaning. Humans are habituated to many types of stimuli such as common background noises. Habituation is the best-studied type of learning because it can be readily measured in babies or animals. Fetal habituation was first reported in 1925, when fetal movements in the womb were shown to decline after repeated exposure to a car horn. The orienting response is often used to measure habituation in infants: when presented with a new visual stimulus, an infant will turn to look at it, but with repeated presentations, the infant will look at it for shorter and shorter periods of time. Habituation can also reveal whether a baby or animal distinguishes between two types of stimuli. For example, a baby who becomes habituated to a toy becomes bored and looks for a new toy or other stimulus. However, if the old and new toys are very similar, the habituation may carry over to the new toy. Habituation is specific for the stimulus and does not generalize to other stimuli unless they are similar. Habituation increases with increasing repetitions of the stimulus and decreases with more intense stimulation.
There are two types of habituation—short-term and long-term. Short intervals between repetition of the stimulus (the inter-stimulus interval or ISI), such as 15 seconds, are better at producing short-term habituation: the habituation occurs faster, with response to the stimulus rapidly decreasing, but lasts only a few minutes or hours before the response strength recovers. Long ISIs are better at producing long-term habituation. Long-term habituation occurs with repeated short-term habituation training, with periods of several hours or days between training sessions. Long-term habituation lasts for days or weeks, but is generally weaker than short-term habituation.
Habituation is not due to either motor fatigue or sensory adaptation, since the process can be reversed by dishabituation—the recovery of responsiveness to a stimulus following habituation training. Rather, habituation is believed to be due to a change between sensory and motor neurons in the central nervous system (brain and spinal cord) that links sensory input to motor output, such that repeated stimulation of the neural circuit reduces processing of information. Dishabituation can be spontaneous or can occur via exposure to a new stimulus, especially if the new stimulus is somewhat noxious.
Sensitization is an increase in responsiveness or behavior caused by exposure to a noxious or painful stimulus. Whereas habituation occurs with a weak, repeated stimulus, sensitization occurs with a strong or painful sensory stimulus. It may only require one exposure and may intensify responses to all stimuli. Sensitization is believed to occur through a general arousal process. An example of sensitization in rats is an increased startle response to a tone after a sensitizing event such as an electric shock. An example of a strong sensitizing stimulus in humans is a sudden blackout on a well-lit street that results in a potentiated (increased) startle response to car lights or footsteps. An example of a weak sensitizing stimulus is the sound of a dripping faucet while trying to fall asleep— a sound that becomes increasingly hard to ignore.
Repetition of noxious stimuli or a stronger stimulus creates more sensitization. A longer interval between sensitization training results in longer-lasting sensitization.
Sensitization differs from dishabituation, because an event can cause dishabituation without causing sensitization to other stimuli. Likewise, sensitization does not necessarily cause dishabituation to other stimuli. Sensitization can underlie both adaptive and maladaptive forms of learning.
Perceptual learning may be considered a third type of non-associative learning. Perceptual learning is the faster processing of stimuli—learning to recognize stimuli faster—and/or learning to distinguish between similar stimuli. Priming—previous exposure to a stimulus— results in faster recognition of the stimulus the next time it is presented. Discrimination training can teach animals to respond to only one of two similar stimuli. Perceptual learning appears to occur simply through exposure to new stimuli—it does not require conscious effort.
In the comparator model of non-associative learning, each stimulus is compared to stored memory. If the stimulus differs from memory, attention is paid to the stimulus, and its information is stored. With repeated stimulation and a buildup of memory, less and less attention is paid to the stimulus.
In the model of non-associative learning proposed by Chi-Sang Poon, habituation and primary sensitization are localized in a sensorimotor pathway, and desensitization and secondary sensitization are modulated by other pathways. In this model, failure of the sensory gating or “firewall” mechanism can distort the sensory information or the behavioral response, as in non-associative-learning abnormalities in various mental, neurophysiologic, and neurodegenerative disorders.
Non-associative learning in reflex sensorineural pathways has been studied in a wide variety of animals. However, non-associative learning also involves cognitive pathways, such as human orienting responses, visual attention, and pain perception. Screens of animals with non-associative-learning deficits or overexpression have identified genes and molecules that are involved in habituation and sensitization.
See also Associative learning; Sensitization .
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