Considerable evidence suggests that various discrete nuclei within the amygdala complex are critically involved in the assignment of emotional significance or value to events through associative learning. Much of this evidence comes from aversive conditioning procedures. For example, lesions of either basolateral amygdala (ABL) or the central nucleus (CN) interfere with the acquisition or expression of conditioned fear. The present study examined the effects of selective neurotoxic lesions of either ABL or CN on the acquisition of positive incentive value by a conditioned stimulus (CS) with two appetitive Pavlovian conditioning procedures. In second-order conditioning experiments, rats first received light-food pairings intended to endow the light with reinforcing power. The acquired reinforcing power of the light was then measured by examining its ability to serve as a reinforcer for second-order conditioning of a tone when tone-light pairings were given in the absence of food. Acquisition of second-order conditioning was impaired in rats with ABL lesions but not in rats with CN lesions. In reinforcer devaluation procedures, conditioned responding of rats with ABL lesions was insensitive to postconditioning changes in the value of the reinforcer, whereas rats with CN lesions, like normal rats, were able to spontaneously adjust their CRs to the current value of the reinforcer. The results of both test procedures indicate that ABL, but not CN, is part of a system involved in CSs' acquisition of positive incentive value. Together with evidence that identifies a role for CN in certain changes in attentional processing of CSs in conditioning, these results suggest that separate amygdala subsystems contribute to a variety of processes inherent in associative learning. Key words: basolateral amygdala; amygdala central nucleus; second-order conditioning; reinforcer devaluation; classical conditioning; ratsMuch evidence indicates that neural processing in the amygdala complex is important for assigning emotional value or significance to events through associative learning. For example, cues that signal an aversive event elicit freezing and potentiate startle reactivity and produce characteristic autonomic responses in rodents. Discrete lesions or inactivation of basolateral amygdala (ABL) or the central nucleus (CN) produce deficits in the acquisition and/or expression of a range of conditioned fear behaviors (for review, see Davis, 1992;LeDoux, 1992). Similarly, unlike normal rats, rats with ABL lesions typically fail to acquire preferences for environmental locations paired with positively reinforcing events, such as food or certain drug states (Everitt and Robbins, 1992;Everitt et al., 1991;McDonald and White, 1993).Recent research indicates that the amygdala also serves an attentional function in Pavlovian appetitive conditioning. Rats with selective neurotoxic lesions of the CN show a pronounced deficit in the acquisition of conditioned orienting behavior to visual and auditory conditioned stimuli (CSs) paired with...

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Neurons with somatic sensory receptive fields were examined electrophysiologically in the thalamic reticular

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Small extracellular injections of HRP were placed into a stratum of corticocortical axons situated immediately deep to area 3b of the monkey somatic sensory cortex. This stratum had previously been demonstrated to contain corticocortical fibers linking the cytoarchitectonic fields of the somatic sensory cortex to one another and certain of them to the motor cortex. This method resulted in extremely successful filling of pyramidal cells, their axons, collateral axon branches, and terminations in areas 3b, 1, and 2 posterior to the injection and in areas 3a and 4 anterior to it. The major finding was that cells with somata situated in any one of these fields and with principal axons traversing the injection site have long collaterals, primarily in layers III and V, which can extend throughout their own cytoarchitectonic field and into one or more other fields. In these fields they give off focused, columnlike concentrations of terminal boutons, which can be separated from one another by 800 micron or more. The anterogradely labeled, primary corticocortical fibers, traced forwards into areas 3a and 4, have virtually identical focal terminations. These findings indicate that interareal connectivity in the sensory-motor cortex can be effected by the axon branches of single cells rather than by separate groups of cells, and this may form a basis for the convergence of place and modality information on single cells in the sensorimotor cortex, a convergence that is not seen in the thalamic input to this cortex.