RESPONDING TO A SINGLE STIMULUS
Repeated presentation of a stimulus that elicits a particular UR will result in habituation – a gradual reduction in the magnitude of the response. [habituation waning of the unconditioned response with repeated presentation of the eliciting stimulus] A good instance in vertebrates is the startle response produced by a sudden loud noise, a response that reliably declines if the noise is regularly repeated. Which also shows the phenomenon of dishabituation, [dishabituation restoration of a habituated response by presentation of a strong extraneous stimulus] whereby the response returns when a salient extraneous stimulus (e.g. a flashing light) is presented just before a trial with the habituated noise. The observation that the response can be easily restored in this way shows that habituation is not solely a matter of sensory or motor fatigue – it is a genuine case of learning. And since habituation occurs as a consequence of the presentation of a single event, it is difficult to interpret t his form of learning in terms of association formation. The most likely explanation, at least for simple instances of the phenomenon, is that changes occur in the neuronal pathway connecting the S and R that make transmission of nervous impulses less likely to occur. A series of elegant neurophysiological studies by Kandel and colleagues (e.g. Kandel, 1979) using the marine mollusc Aplysia has gone some way towards establishing which synaptic connection loses effectiveness during habituation, and the biochemical basis of this loss. (For this work Kandel was awarded the Nobel prize for medicine.) Loss of the UR is not the only effect produced by stimulus exposure. Consider the phenomenon of imprinting, [imprinting the development of filial responses by newly hatched birds to an object (usually the mother) experienced early in life, or more generally the early formation of social attachments in animals] in which a chick becomes attached to a conspicuous object experienced early in life. This behaviour pattern is found only in some species, but other features of the imprinting process appear to be more general. Most animals exposed to complex objects are able to learn the characteristics of the object, and subsequently to distinguish more easily the object from other similar things. This phenomenon is known as perceptual learning. The nature of the mechanism responsible for it is not fully known, but it seems likely that associative processes are involved, in that learning the characteristics of a complex object involves learning that its various features go together. This is achieved by the formation of associative links among its component parts. The perceptual learning process, [perceptual learning exposure to events, increasing subsequent ability to discriminate between them] which enables the animal to build up an accurate representation of the stimulus, probably plays a role in some instances of habituation. When animals are habituated to a complex event, the response can be restored if some element of that complex is omitted or changed. Such dishabituation occurs, it has been suggested (Sokolov, 1963), because animals are sensitive to any mismatch between incoming stimulation and the central representations of events they have already experienced.
Repeated presentation of a stimulus that elicits a particular UR will result in habituation – a gradual reduction in the magnitude of the response. [habituation waning of the unconditioned response with repeated presentation of the eliciting stimulus] A good instance in vertebrates is the startle response produced by a sudden loud noise, a response that reliably declines if the noise is regularly repeated. Which also shows the phenomenon of dishabituation, [dishabituation restoration of a habituated response by presentation of a strong extraneous stimulus] whereby the response returns when a salient extraneous stimulus (e.g. a flashing light) is presented just before a trial with the habituated noise. The observation that the response can be easily restored in this way shows that habituation is not solely a matter of sensory or motor fatigue – it is a genuine case of learning. And since habituation occurs as a consequence of the presentation of a single event, it is difficult to interpret t his form of learning in terms of association formation. The most likely explanation, at least for simple instances of the phenomenon, is that changes occur in the neuronal pathway connecting the S and R that make transmission of nervous impulses less likely to occur. A series of elegant neurophysiological studies by Kandel and colleagues (e.g. Kandel, 1979) using the marine mollusc Aplysia has gone some way towards establishing which synaptic connection loses effectiveness during habituation, and the biochemical basis of this loss. (For this work Kandel was awarded the Nobel prize for medicine.) Loss of the UR is not the only effect produced by stimulus exposure. Consider the phenomenon of imprinting, [imprinting the development of filial responses by newly hatched birds to an object (usually the mother) experienced early in life, or more generally the early formation of social attachments in animals] in which a chick becomes attached to a conspicuous object experienced early in life. This behaviour pattern is found only in some species, but other features of the imprinting process appear to be more general. Most animals exposed to complex objects are able to learn the characteristics of the object, and subsequently to distinguish more easily the object from other similar things. This phenomenon is known as perceptual learning. The nature of the mechanism responsible for it is not fully known, but it seems likely that associative processes are involved, in that learning the characteristics of a complex object involves learning that its various features go together. This is achieved by the formation of associative links among its component parts. The perceptual learning process, [perceptual learning exposure to events, increasing subsequent ability to discriminate between them] which enables the animal to build up an accurate representation of the stimulus, probably plays a role in some instances of habituation. When animals are habituated to a complex event, the response can be restored if some element of that complex is omitted or changed. Such dishabituation occurs, it has been suggested (Sokolov, 1963), because animals are sensitive to any mismatch between incoming stimulation and the central representations of events they have already experienced.
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