Evidence of plasticity in the pontocerebellar conditioned stimulus pathway during classical conditioning of the eyeblink response in the rabbit.

Electrical stimulation thresholds required to elicit eyeblinks with either pontine or cerebellar interpositus stimulation were measured before and after classical eyeblink conditioning with paired pontine stimulation (conditioned stimulus, CS) and corneal airpuff (unconditioned stimulus, US). Pontine stimulation thresholds dropped dramatically after training and returned to baseline levels following extinction, whereas interpositus thresholds and input-output functions remained stable across training sessions. Learning rate, magnitude of threshold change, and electrode placements were correlated. Pontine projection patterns to the cerebellum were confirmed with retrograde labeling techniques. These results add to the body of literature suggesting that the pons relays CS information to the cerebellum and provide further evidence of synaptic plasticity in the cerebellar network.

Layer-specific somatosensory cortical activation during active tactile discrimination.

Ensemble neuronal activity was recorded in each layer of the whisker area of the primary somatosensory cortex (SI) while rats performed a whisker-dependent tactile discrimination task. Comparison of this activity with SI activity evoked by similar passive whisker stimulation revealed fundamental differences in tactile signal processing during active and passive stimulation. Moreover, significant layer-specific functional differences in SI activity were observed during active discrimination. These differences could not be explained solely by variations in ascending thalamocortical input to SI. Instead, these results suggest that top-down influences during active discrimination may alter the overall functional nature of SI as well as layer-specific mechanisms of tactile processing.

Inhibiting the expression of a classically conditioned behavior prevents its extinction.

The underlying neuronal substrates and behavioral properties that might mediate extinction of the classically conditioned eye-blink response (CR) were examined. Four groups of rabbits were trained to perform the CR. Two of the groups then received either three or six sessions of tone-alone extinction training while the motor nuclei that mediate expression of the CR (facial nucleus and accessory abducens) were reversibly inactivated with microinjections of the GABA agonist muscimol. After these inactivation extinction sessions, rabbits received four more extinction sessions without inactivation. Two groups of controls received either three or six extinction sessions while saline vehicle was infused into the motor nuclei, followed by four sessions with no infusions. Saline infusions had no effect on extinction, and controls extinguished the CR normally over the first three to four sessions. In contrast, muscimol inactivation of the motor nuclei completely prevented any performance of CRs during the three or six inactivation extinction sessions. At the start of the four extinction sessions without inactivation, rabbits performed CRs at the same rate and amplitude as controls on their first extinction sessions. The muscimol rabbits then extinguished the CR normally over the four sessions without inactivation. In short, inactivation of the motor nuclei completely prevented any extinction of the eye-blink CR with no effect on subsequent extinction without inactivation. These results are discussed in terms of possible neuroanatomical loci that might mediate the extinction process as well as how effects of manipulating CR performance during extinction may affect the extinction process.

Integration of bilateral whisker stimuli in rats: role of the whisker barrel cortices.

Recently, we demonstrated that neural responses within the whisker region of the primary somatosensory cortex (SIw) of rats are profoundly influenced by the spatiotemporal attributes of ipsilateral, as well as contralateral, whisker stimuli. As inactivation of one SIw eliminates in the intact SIw both ipsilaterally evoked responses and the influence of ipsilateral stimulation on contralaterally evoked activity, we proposed that interhemispheric interactions between the SIws may be important for integrating bilateral whisker information. To test whether rats can recognize the bilateral nature of a whisker stimulus, we developed a tactile discrimination task that required rats to conjointly determine distances to a left and a right discriminandum as equidistant or non-equidistant using only their facial whiskers. All rats trained in this task achieved performance levels indicative of an ability to integrate bilateral whisker information. Testing during unilateral, as well as bilateral, inactivation of the SIws indicated that rats rely on both SIws for detecting the bilateral nature of a whisker stimulus. Rats were unable to perform the task without both sets of whiskers, a fact that indicates that the whiskers (and not other modalities) were used to perform this task. The findings presented here indicate that rats can solve a task that requires the conjoint detection of left and right whisker-mediated distance information and implicate the SIws as central to this ability.