Nondeclarative Memory Tasks

Many neurobiologists interested in the cellular analysis of learning and memory have employed paradigms that involve the conditioning of some motor response. These forms of learning are defined by the procedures used to produce them. Among these are associative and nonassociative learning paradigms. Associative learning includes classical conditioning and operant conditioning. In both, two stimuli or events that occur in a temporally paired fashion result in the formation of an association

Stimulus 8

FIGURE 2 Positron emission tomography (PET) analysis of memory localization. (A1) Subjects were instructed to attend to each object in the sequence and remember its location. (A2) Following a 10-minute rest period, the subjects were presented with the objects in two locations and asked to decide which of the two possible locations was correct. (B) During the retrieval phase of the test, the subjects had elevated blood flow (a measure of neural activity) in the right anterior parahippocampal gyrus in the region corresponding to the entorhinal cortex. The extent of blood flow is quantified with the color scale on the left, with red indicating the greatest levels. (Modified from Owen et al., 1996.)

FIGURE 2 Positron emission tomography (PET) analysis of memory localization. (A1) Subjects were instructed to attend to each object in the sequence and remember its location. (A2) Following a 10-minute rest period, the subjects were presented with the objects in two locations and asked to decide which of the two possible locations was correct. (B) During the retrieval phase of the test, the subjects had elevated blood flow (a measure of neural activity) in the right anterior parahippocampal gyrus in the region corresponding to the entorhinal cortex. The extent of blood flow is quantified with the color scale on the left, with red indicating the greatest levels. (Modified from Owen et al., 1996.)

between the two events. In contrast, nonassociative learning is not dependent on pairing. Examples of this form of learning include habituation and sensitization. Habituation has been defined as a decrease in a response as a result of repeated stimulation, while sensitization has been defined as an increase in response magnitude as a result of a stimulus that increases arousal (Groves and Thompson, 1970; Thompson and Spencer, 1966).

Oftentimes, scientists study nonassociative learning of reflexive behaviors. These learning paradigms are attractive because the stimuli can be precisely controlled and there is generally a well-defined behavioral response. Moreover, the neural pathways of many reflexive behaviors have been described. This knowledge of the circuit enables powerful cell biologic approaches to be applied to the analysis of the underlying mechanisms.

Habituation

Habituation, perhaps the simplest form of nonasso-ciative learning, refers to a decrement of responsiveness caused by repetition of a stimulus. It is generally distinguished from simple fatigue, as responsiveness can be rapidly restored (dishabituated) by the presentation of a novel stimulus to the animal. The parametric features of habituation have been described by Thompson and Spencer (1966).

Sensitization

Sensitization is also a form of nonassociative learning and refers to the enhancement of a behavioral response to a test stimulus as a result of applying a novel stimulus to the animal. The sensitizing stimulus may be of the same modality as the test stimulus and applied at the same site as a test stimulus used to elicit the response, or it may be of a different modality, applied to a different locus. The same stimuli that lead to habituation may also lead to sensitization. Consequently, the strength of a behavioral response elicited by a repeated stimulus may be the net outcome of the two processes of habituation and sensitization (Groves and Thompson, 1970).

Classical Conditioning

Classical conditioning is an example of associative learning in which the presentation of a reinforcing (unconditioned) stimulus is made contingent on that of a preceding (conditioned) stimulus. The change in behavior produced by repeated pairing of the two stimuli can be measured in a number of ways. An example of classical conditioning involves the training procedure originally described by Pavlov (1927) to condition salivation in dogs (Fig. 3). Before training, meat powder (referred to as the unconditioned stimulus, or US) reliably elicited salivation (referred to as the unconditioned response, or UR). The signal for the presentation of the US is called the conditioned stimulus (CS). In the original experiments of Pavlov, the CS was a bell. Traditionally, the CS does not evoke a response similar to the UR and is typically referred to as a neutral stimulus. During training, the US was made contingent on the CS by repeatedly pairing the presentations of CS and US. After training, the response to the CS alone had changed such that the bell elicited salivation (the conditioned response, CR). The persistence of a CR after training is called retention. When the contingency between CS and US was eliminated by repeatedly presenting the bell in the absence of the meat powder (US), the ability of the CS to elicit the CR (salivation) gradually diminished. This process is called extinction.

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