Timing of conditioned responses utilizing electrical stimulation in the region of the interpositus nucleus as a CS

Establishment and transfer of classical eyeblink conditioning using electrical microstimulation of the hippocampus as the conditioned stimulus

The present experiment was designed to determine whether classical eyeblink conditioning (EBC) can be established by using electrical microstimulation of the hippocampus as a conditioned stimulus (CS) paired with an air-puff unconditioned stimulus (US). We intended to examine whether EBC transfer could occur when a CS was shifted between microstimulation of the hippocampus as a CS (Hip-CS) and tone as a CS (tone-CS) and to compare the difference in transfer effectiveness between delay EBC (dEBC) and trace EBC (tEBC). Eight groups of guinea pigs, including 4 experimental groups and 4 control groups, were included in the study. First, the experimental groups received either a Hip-CS or a tone-CS paired with a US; then, these groups were exposed to a shifted CS (tone-CS or Hip-CS) paired with the US. The control groups received the corresponding Hip-CS or tone-CS, which was, however, pseudo-paired with the US. The control groups were then shifted to the tone-CS (or Hip-CS) paired with the US. The results show that EBC can be successfully established when using microstimulation of the hippocampus as a CS paired with an air-puff US, and that the acquisition rates of EBC are higher in the experimental groups than in the control groups after switching from the Hip-CS to the tone-CS or vice versa, indicating the occurrence of learning transfer between EBC established with the Hip-CS and tone-CS. The present study also demonstrated that the EBC re-acquisition rates were remarkably higher in dEBC than in tEBC with both types of transfer, which suggests that the saving effect was more evident in dEBC than tEBC. These results significantly expand our knowledge of EBC transfer as well as the functional neural circuit underlying EBC transfer.

Classical eyeblink conditioning using electrical stimulation of caudal mPFC as conditioned stimulus is dependent on cerebellar interpositus nucleus in guinea pigs

AIM

To determine whether electrical stimulation of caudal medial prefrontal cortex (mPFC) as conditioned stimulus (CS) paired with airpuff unconditioned stimulus (US) was sufficient for establishing eyeblink conditioning in guinea pigs, and whether it was dependent on cerebellar interpositus nucleus.

METHODS

Thirty adult guinea pigs were divided into 3 conditioned groups, and trained on the delay eyeblink conditioning, short-trace eyeblink conditioning, and long-trace eyeblink conditioning paradigms, respectively, in which electrical stimulation of the right caudal mPFC was used as CS and paired with corneal airpuff US. A pseudo conditioned group of another 10 adult guinea pigs was given unpaired caudal mPFC electrical stimulation and the US. Muscimol (1 μg in 1 μL saline) and saline (1 μL) were infused into the cerebellar interpositus nucleus of the animals through the infusion cannula on d 11 and 12, respectively.

RESULTS

The 3 eyeblink conditioning paradigms have been successfully established in guinea pigs. The animals acquired the delay and short-trace conditioned responses more rapidly than long-trace conditioned responses. Muscimol infusion into the cerebellar interpositus nucleus markedly impaired the expression of the 3 eyeblink conditioned responses.

CONCLUSION

Electrical stimulation of caudal mPFC is effective CS for establishing eyeblink conditioning in guinea pigs, and it is dependent on the cerebellar interpositus nucleus.

Evaluation of bidirectional interstimulus interval (ISI) shift in auditory delay eye-blink conditioning in healthy humans

Delay eye-blink conditioning is an associative learning task that can be utilized to probe the functional integrity of the cerebellum and related neural circuits. Typically, a single interstimulus interval (ISI) is utilized, and the amplitude of the conditioned response (CR) is the primary dependent variable. To study the timing of the CR, an ISI shift can be introduced (e.g., shifting the ISI from 350 to 850 ms). In each phase, a conditioned stimulus (e.g., a 400- or 900-ms tone) coterminates with a 50-ms corneal air puff unconditioned stimulus. The ability of a subject to adjust the CR to the changing ISI constitutes a critical timing shift. The feasibility of this procedure was examined in healthy human participants (N = 58) using a bidirectional ISI shift procedure while cortical event-related brain potentials were measured. CR acquisition was faster and the responses better timed when a short ISI was used. After the ISI shift, additional training was necessary to allow asymptotic responding at the new ISI. Interestingly, auditory event-related potentials to the CR were not associated with conditioning measures at either ISI.

Decremental effects of context exposure following delay eyeblink conditioning in rabbits

The conditioning context arises from the relatively static features of the training environment. In rabbit eyeblink conditioning, procedures that retard acquisition (conditioned stimulus [CS] preexposure, unconditioned stimulus preexposure, blocking manipulations) are attenuated by context changes. In this article the authors investigate the effect of context exposure after initial delay conditioning. After conditioned responses (CRs) were established, one group received 6 sessions of context exposure, whereas control groups either remained in their home cages or received exposure to handling and a novel context. Thereafter, all groups received CS-alone testing. The expression of CRs was substantially reduced following context exposure relative to any retention loss in the home-cage control. Exposure to handling and a novel context facilitated the CRs rather than reducing them.

Eyeblink conditioning during an interstimulus interval switch in rabbits (Oryctolagus cuniculus) using picrotoxin to disrupt cerebellar cortical input to the interpositus nucleus

The role of the cerebellar cortex in eyeblink classical conditioning remains unclear. Experimental manipulations that disrupt the normal function impair learning to various degrees, and task parameters may be important factors in determining the severity of impairment. This study examined the role of cerebellar cortex in eyeblink conditioning under conditioned stimulus?unconditioned stimulus intervals known to be optimal or nonoptimal for learning. Using infusions of picrotoxin to the interpositus nucleus of the rabbit cerebellum, the authors pharmacologically disrupted input from the cerebellar cortex while training with an interstimulus interval (ISI)-switch procedure. One group of rabbits (Oryctolagus cuniculus) was 1st trained with a 250-ms ISI (optimal) and then switched to a 750-ms ISI (nonoptimal). A 2nd group was trained in the opposite order. The most striking effect was that picrotoxin-treated rabbits initially trained with a 250-ms ISI learned comparably to controls, but those initially trained with a 750-ms ISI were severely impaired. These results suggest that functional input from cerebellar cortex becomes increasingly important for the interpositus nucleus to learn delay eyeblink conditioning as the ISI departs from an optimal interval.

The neuroscience of mammalian associative learning

Mammalian associative learning is organized into separate anatomically defined functional systems. We illustrate the organization of two of these systems, Pavlovian fear conditioning and Pavlovian eyeblink conditioning, by describing studies using mutant mice, brain stimulation and recording, brain lesions and direct pharmacological manipulations of specific brain regions. The amygdala serves as the neuroanatomical hub of the former, whereas the cerebellum is the hub of the latter. Pathways that carry information about signals for biologically important events arrive at these hubs by circuitry that depends on stimulus modality and complexity. Within the amygdala and cerebellum, neural plasticity occurs because of convergence of these stimuli and the biologically important information they predict. This neural plasticity is the physical basis of associative memory formation, and although the intracellular mechanisms of plasticity within these structures share some similarities, they differ significantly. The last Annual Review of Psychology article to specifically tackle the question of mammalian associative learning ( Lavond et al. 1993 ) persuasively argued that identifiable "essential" circuits encode memories formed during associative learning. The next dozen years saw breathtaking progress not only in detailing those essential circuits but also in identifying the essential processes occurring at the synapses (e.g., Bi & Poo 2001, Martinez & Derrick 1996 ) and within the neurons (e.g., Malinow & Malenka 2002, Murthy & De Camilli 2003 ) that make up those circuits. In this chapter, we describe the orientation that the neuroscience of learning has taken and review some of the progress made within that orientation.