The neurobiology of learning and memory

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Biologically predisposed learning and selective associations in amygdalar neurons

Modern views on learning and memory accept the notion of biological constraints-that the formation of association is not uniform across all stimuli. Yet cellular evidence of the encoding of selective associations is lacking. Here, conditioned stimuli (CSs) and unconditioned stimuli (USs) commonly employed in two basic associative learning paradigms, fear conditioning and taste aversion conditioning, were delivered in a manner compatible with a functional cellular imaging technique (Arc cellular compartmental analysis of temporal gene transcription by fluorescence in situ hybridization [catFISH]) to identify biological constraints on CS-US convergence at the level of neurons in basolateral amygdala (BLA). Results indicate coincident Arc mRNA activation within BLA neurons after CS-US combinations that yield rapid, efficient learning, but not after CS-US combinations that do not.

The cerebellum: a neural system for the study of reinforcement learning

In its strictest application, the term “reinforcement learning” refers to a computational approach to learning in which an agent (often a machine) interacts with a mutable environment to maximize reward through trial and error. The approach borrows essentials from several fields, most notably Computer Science, Behavioral Neuroscience, and Psychology. At the most basic level, a neural system capable of mediating reinforcement learning must be able to acquire sensory information about the external environment and internal milieu (either directly or through connectivities with other brain regions), must be able to select a behavior to be executed, and must be capable of providing evaluative feedback about the success of that behavior. Given that Psychology informs us that reinforcers, both positive and negative, are stimuli or consequences that increase the probability that the immediately antecedent behavior will be repeated and that reinforcer strength or viability is modulated by the organism's past experience with the reinforcer, its affect, and even the state of its muscles (e.g., eyes open or closed); it is the case that any neural system that supports reinforcement learning must also be sensitive to these same considerations. Once learning is established, such a neural system must finally be able to maintain continued response expression and prevent response drift. In this report, we examine both historical and recent evidence that the cerebellum satisfies all of these requirements. While we report evidence from a variety of learning paradigms, the majority of our discussion will focus on classical conditioning of the rabbit eye blink response as an ideal model system for the study of reinforcement and reinforcement learning.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

The biochemistry of memory: The 26year journey of a 'new and specific hypothesis'

This Special Issue of Neurobiology of Learning and Memory dedicated to Dr. Richard Thompson to celebrate his 80th birthday and his numerous contributions to the field of learning and memory gave us the opportunity to revisit the hypothesis we proposed more than 25years ago regarding the biochemistry of learning and memory. This review summarizes our early 1980s hypothesis and then describes how it was tested and modified over the years following its introduction. We then discuss the current status of the hypothesis and provide some examples of how it has led to unexpected insights into the memory problems that accompany a broad range of neuropsychiatric disorders.

Impaired delay and trace eyeblink conditioning in school-age children with fetal alcohol syndrome

BACKGROUND

Classical eyeblink conditioning (EBC) involves contingent temporal pairing of a conditioned stimulus (e.g., tone) with an unconditioned stimulus (e.g., air puff). Impairment of EBC has been demonstrated in studies of alcohol-exposed animals and in children exposed prenatally at heavy levels.

METHODS

Fetal alcohol syndrome (FAS) was diagnosed by expert dysmorphologists in a large sample of Cape Coloured, South African children. Delay EBC was examined in a new sample of 63 children at 11.3 years, and trace conditioning in 32 of the same children at 12.8 years. At each age, 2 sessions of 50 trials each were administered on the same day; 2 more sessions the next day, for children not meeting criterion for conditioning.

RESULTS

Six of 34 (17.6%) children born to heavy drinkers were diagnosed with FAS, 28 were heavily exposed nonsyndromal (HE), and 29 were nonexposed controls. Only 33.3% with FAS and 42.9% of HE met criterion for delay conditioning, compared with 79.3% of controls. The more difficult trace conditioning task was also highly sensitive to fetal alcohol exposure. Only 16.7% of the FAS and 21.4% of HE met criterion for trace conditioning, compared with 66.7% of controls. The magnitude of the effect of diagnostic group on trace conditioning was not greater than the effect on short delay conditioning, findings consistent with recent rat studies. Longer latency to onset and peak eyeblink CR in exposed children indicated poor timing and failure to blink in anticipation of the puff. Extended training resulted in some but not all of the children reaching criterion.

CONCLUSIONS

These data showing alcohol-related delay and trace conditioning deficits extend our earlier findings of impaired EBC in 5-year-olds to school-age. Alcohol-related impairment in the cerebellar circuitry required for both forms of conditioning may be sufficient to account for the deficit in both tasks. Extended training was beneficial for some exposed children. EBC provides a well-characterized model system for assessment of degree of cerebellar-related learning and memory dysfunction in fetal alcohol exposed children.

Trace eyeblink conditioning is impaired in α7 but not in β2 nicotinic acetylcholine receptor knockout mice

Nicotinic acetylcholine receptors (nAChRs) are essentially involved in learning and memory. A neurobiologically and behaviorally well-characterized measure of learning and memory, eyeblink classical conditioning, is sensitive to disruptions in acetylcholine neurotransmission. The two most common forms of eyeblink classical conditioning - the delay and trace paradigms - differentially engage forebrain areas densely-populated with nAChRs. The present study used genetically modified mice to investigate the effects of selective nAChR subunit deletion on delay and trace eyeblink classical conditioning. α7 and β2 nAChR subunit knockout (KO) mice and their wild-type littermates were trained for 10 daily sessions in a 500-ms delay or 500-ms trace eyeblink conditioning task, matched for the interstimulus interval between conditioned stimulus and unconditioned stimulus onset. Impairments in conditioned responding were found in α7 KO mice trained in trace - but not delay - eyeblink conditioning. Relative to littermate controls, β2 KO mice were unimpaired in the trace task but displayed higher levels of conditioned responding in delay eyeblink conditioning. Elevated conditioned response levels in delay-conditioned β2 KOs corresponded to elevated levels of alpha responding in this group. These findings suggest that α7 nAChRs play a role in normal acquisition of 500 ms trace eyeblink classical conditioning in mice. The prominent distribution of α7 nAChRs in the hippocampus and other forebrain regions may account for these genotype-specific acquisition effects in this hippocampus-dependent trace paradigm.

Functional magnetic resonance imaging of cerebellar activation during the learning of a visuomotor dissociation task

We have used functional magnetic resonance imaging (fMRI) to study the changes in cerebellar activation that occur during the acquisition of motor skill in human subjects presented with a new task. The standard paradigm consisted of a center-out movement in which subjects used a joystick to superimposed a cursor onto viusual targets. Two variations of this paradigm were introduced: (1) a learning paradigm, where the relationship between movement of the joystick and cursor was reversed, requiring the learning of a visuomotor transformation to optimize performance and (2) a random paradigm, where the joystick/cursor relationship was changed randomly for each trial. Activation in the cerebellum was highest during the random paradigm and during the early stages of the learning paradigm. In the early stages of learning and during the random paradigm performance was poor with a decrease in the number of completed movements, and an increase in the time and length of movements. With repeated practice at the learning paradigm performance improbed and reached the same level of proficiency as in the standard task. Commensurate with the improbement in performance was a decrease in cerebellar activation, that is, activation in the cerebellum changed in a parallel, but inverse relationship with performance. Linear regression analysis demonstarated that the inverse correlation between cerebellar activation and motor performance was significant. Repeated practice at the random paradigm did not produce improvements in performance and cerebellar activity remained high. The data support the hypothesis that the cerebellum is strongly activated when motor performance is inaccurate, consistent with a role for the cerebellum in the detection of, and correction for visuomotor errors.

Intrastriatal inhibition of extracellular signal-regulated kinases impaired the consolidation phase of motor skill learning

It is well known that motor skill learning is characterized by rapid improvement in performances within the first training session and a slower progression in the following sessions that is correlated to the consolidation phase. Our goal was to establish the regional mapping of neural activity in relation to the motor skill learning included in the accelerating rotarod task using Zif268, c-Fos and ERK 1/2. As ERK 1/2 activity is also a marker of adaptive response to synaptic activation for newly learned events, its role was also verified. Learning the rotarod task did not affect levels of Zif268, but induced a selective upregulation of c-Fos in the cerebellum, motor cortex M1 and M2, cingulate cortex CG1 and CG2 as well as dorsal striatum. Notably, levels of phosphorylated ERK 1/2 were selectively increased in this later region during consolidation phase. To further study this effect, we injected inhibitors of ERK activation, the SL327 intraperitoneally or the PD98059 directly into the dorsal striatum, and observed that motor performances were exclusively impaired in this phase. These findings indicate that ERK 1/2 activity of the dorsal striatum is critical for the consolidation of late but not early phase of motor skill memory.

Classical conditioning: The new hegemony

Converging data from different disciplines are showing the role of classical conditioning processes in the elaboration of human and animal behavior to be larger than previously supposed. Restricted views of classically conditioned responses as merely secretory, reflexive, or emotional are giving way to a broader conception that includes problem-solving, and other rule-governed behavior thought to be the exclusive province of either operant conditiońing or cognitive psychology. These new views have been accompanied by changes in the way conditioning is conducted and evaluated. Data from a number of seemingly unrelated phenomena such as relapse to drug abuse by postaddicts, the placebo effect, and the immune response appear to involve classical conditioning processes. Classical conditioning, moreover, has been found to occur in simpler and simpler organisms and recently even demonstrated in brain slices and in utero. This target article will integrate the several research areas that have used the classical conditioning process as an explanatory model; it will challenge teleological interpretations of the classically conditioned CR and offer some basic principles for testing conditioning in diverse areas.

Unimpaired trace classical eyeblink conditioning in Purkinje cell degeneration (pcd) mutant mice

Young adult Purkinje cell degeneration (pcd) mutant mice, with complete loss of cerebellar cortical Purkinje cells, are impaired in delay eyeblink classical conditioning. In the delay paradigm, the conditioned stimulus (CS) overlaps and coterminates with the unconditioned stimulus (US), and the cerebellar cortex supports normal acquisition. The ability of pcd mutant mice to acquire trace eyeblink conditioning in which the CS and US do not overlap has not been explored. Recent evidence suggests that cerebellar cortex may not be necessary for trace eyeblink classical conditioning. Using a 500 ms trace paradigm for which forebrain structures are essential in mice, we assessed the performance of homozygous male pcd mutant mice and their littermates in acquisition and extinction. In contrast to results with delay conditioning, acquisition of trace conditioning was unimpaired in pcd mutant mice. Extinction to the CS alone did not differ between pcd and littermate control mice, and timing of the conditioned response was not altered by the absence of Purkinje cells during acquisition or extinction. The ability of pcd mutant mice to acquire and extinguish trace eyeblink conditioning at levels comparable to controls suggests that the cerebellar cortex is not a critical component of the neural circuitry underlying trace conditioning. Results indicate that the essential neural circuitry for trace eyeblink conditioning involves connectivity that bypasses cerebellar cortex.

The cerebellum and eye-blink conditioning: learning versus network performance hypotheses

Classical conditioning of the eye-blink reflex in the rabbit is a form of motor learning that is uniquely dependent on the cerebellum. The cerebellar learning hypothesis proposes that plasticity subserving eye-blink conditioning occurs in the cerebellum. The major evidence for this hypothesis originated from studies based on a telecommunications network metaphor of eye-blink circuits. These experiments inactivated parts of cerebellum-related networks during the acquisition and expression of classically conditioned eye blinks in order to determine sites at which the plasticity occurred. However, recent evidence revealed that these manipulations could be explained by a network performance hypothesis which attributes learning deficits to a non-specific tonic dysfunction of eye-blink networks. Since eye-blink conditioning is mediated by a spontaneously active, recurrent neuronal network with strong tonic interactions, differentiating between the cerebellar learning hypothesis and the network performance hypothesis represents a major experimental challenge. A possible solution to this problem is offered by several promising new approaches that minimize the effects of experimental interventions on spontaneous neuronal activity. Results from these studies indicate that plastic changes underlying eye-blink conditioning are distributed across several cerebellar and extra-cerebellar regions. Specific input interactions that induce these plastic changes as well as their cellular mechanisms remain unresolved.

Silent trace eliminates differential eyeblink learning in abstinent alcoholics

Chronic alcoholism has profound effects on the brain, including volume reductions in regions critical for eyeblink classical conditioning (EBCC). The current study challenged abstinent alcoholics using delay (n = 20) and trace (n = 17) discrimination/reversal EBCC. Comparisons revealed a significant difference between delay and trace conditioning performance during reversal (t (35) = 2.08, p < 0.05). The difference between the two tasks for discrimination was not significant (p = 0.44). These data support the notion that alcoholics are increasingly impaired in the complex task of reversing a previously learned discrimination when a silent trace interval is introduced. Alcoholics' impairment in flexibly altering learned associations may be central to their continued addiction.

Functional imaging of stimulus convergence in amygdalar neurons during Pavlovian fear conditioning

Background

Associative conditioning is a ubiquitous form of learning throughout the animal kingdom and fear conditioning is one of the most widely researched models for studying its neurobiological basis. Fear conditioning is also considered a model system for understanding phobias and anxiety disorders. A fundamental issue in fear conditioning regards the existence and location of neurons in the brain that receive convergent information about the conditioned stimulus (CS) and unconditioned stimulus (US) during the acquisition of conditioned fear memory. Convergent activation of neurons is generally viewed as a key event for fear learning, yet there has been almost no direct evidence of this critical event in the mammalian brain.

Methodology/Principal Findings

Here, we used Arc cellular compartmental analysis of temporal gene transcription by fluorescence in situ hybridization (catFISH) to identify neurons activated during single trial contextual fear conditioning in rats. To conform to temporal requirements of catFISH analysis we used a novel delayed contextual fear conditioning protocol which yields significant single- trial fear conditioning with temporal parameters amenable to catFISH analysis. Analysis yielded clear evidence that a population of BLA neurons receives convergent CS and US information at the time of the learning, that this only occurs when the CS-US arrangement is supportive of the learning, and that this process requires N-methyl-D-aspartate receptor activation. In contrast, CS-US convergence was not observed in dorsal hippocampus.

Conclusions/Significance

Based on the pattern of Arc activation seen in conditioning and control groups, we propose that a key requirement for CS-US convergence onto BLA neurons is the potentiation of US responding by prior exposure to a novel CS. Our results also support the view that contextual fear memories are encoded in the amygdala and that the role of dorsal hippocampus is to process and transmit contextual CS information.

An investigation of auditory dimensional interaction in a bivariate bilateral conditioning paradigm in the rabbit

The current study adapted the Garner paradigm for diagnosing separable versus integral perceptual dimensions to the eye-blink classical conditioning paradigm using rabbits. Specifically, this study examined the ability of rabbits to categorize stimuli based on one auditory dimension while ignoring a second, irrelevant dimension by displaying an appropriate eye-blink for bilaterally conditioned discriminative responses. Tones used in training varied along two dimensions, starting frequency and magnitude of frequency sweep upwards from the start. Rabbits first learned to categorize along a single dimension (blinking one eye for one category response and the other eye for the other response) and then continued to categorize tones in a second phase in which the irrelevant dimension was varied. The variation of the irrelevant dimension did not disrupt performance, indicating that rabbits perceive these dimensions as separable.

Functional recoveries of giant interneurons in the early period after unilateral cercal ablation in the cricket Gryllus bimaculatus

Wind-sensitive giant interneurons (GIs) in the cricket Gryllus bimaculatus show functional recovery after unilateral cercal ablation. Physiological properties such as threshold velocity and response magnitude (number of action potentials elicited) of GIs 8-1, 9-1, 9-2 and 9-3 to an air puff stimulus were investigated in crickets reared under the condition that permitted free walking for 6 days after unilateral cercal ablation ("6-day-free" crickets). The results were compared to those previously obtained from crickets 1 day after unilateral cercal ablation ("1-day-free" crickets) to clarify functional changes during an early 5-day period after the sensory deprivation. Each GI showed a large functional change, despite the short period after the ablation. However, the degree of physiological change was different from one GI to another and from one stimulus direction to another. The direction at which the GIs showed physiological change during the 5-day period coincided well with that at which the GIs received excitatory sensory inputs from filiform hairs on the remaining cercus. It seems that the synaptic connection or efficacy between sensory neurons of filiform hairs with particular directionality on the one remaining cercus and GIs increased during the 5-day recovery period.

Interactions between prefrontal cortex and cerebellum revealed by trace eyelid conditioning

Eyelid conditioning has proven useful for analysis of learning and computation in the cerebellum. Two variants, delay and trace conditioning, differ only by the relative timing of the training stimuli. Despite the subtlety of this difference, trace eyelid conditioning is prevented by lesions of the cerebellum, hippocampus, or medial prefrontal cortex (mPFC), whereas delay eyelid conditioning is prevented by cerebellar lesions and is largely unaffected by forebrain lesions. Here we test whether these lesion results can be explained by two assertions: (1) Cerebellar learning requires temporal overlap between the mossy fiber inputs activated by the tone conditioned stimulus (CS) and the climbing fiber inputs activated by the reinforcing unconditioned stimulus (US), and therefore (2) trace conditioning requires activity that outlasts the presentation of the CS in a subset of mossy fibers separate from those activated directly by the CS. By use of electrical stimulation of mossy fibers as a CS, we show that cerebellar learning during trace eyelid conditioning requires an input that persists during the stimulus-free trace interval. By use of reversible inactivation experiments, we provide evidence that this input arises from the mPFC and arrives at the cerebellum via a previously unidentified site in the pontine nuclei. In light of previous PFC recordings in various species, we suggest that trace eyelid conditioning involves an interaction between the persistent activity of delay cells in mPFC-a putative mechanism of working memory-and motor learning in the cerebellum.

Visualizing stimulus convergence in amygdala neurons during associative learning

A central feature of models of associative memory formation is the reliance on information convergence from pathways responsive to the conditioned stimulus (CS) and unconditioned stimulus (US). In particular, cells receiving coincident input are held to be critical for subsequent plasticity. Yet identification of neurons in the mammalian brain that respond to such coincident inputs during a learning event remains elusive. Here we use Arc cellular compartmental analysis of temporal gene transcription by fluorescence in situ hybridization (catFISH) to locate populations of neurons in the mammalian brain that respond to both the CS and US during training in a one-trial learning task, conditioned taste aversion (CTA). Individual neurons in the basolateral nucleus of the amygdala (BLA) responded to both the CS taste and US drug during conditioning. Coincident activation was not evident, however, when stimulus exposure was altered so as to be ineffective in promoting learning (backward conditioning, latent inhibition). Together, these data provide clear visualization of neurons in the mammalian brain receiving convergent information about the CS and US during acquisition of a learned association.

Hippocampal response patterns during discriminative eyeblink/jaw movement conditioning in the rabbit

Rabbits were given concurrent training in eyeblink (EB) and jaw movement (JM) conditioning in which 1 tone predicted an airpuff and another tone predicted water. After 10 days of discrimination training, the animals were given 10 days of reversal training. In the discrimination phase, acquisition of the 2 conditioned responses was not significantly different; however JM discrimination errors were much more frequent than were EB errors. In the reversal phase, correct performance on EB trials increased gradually, as was expected, whereas there was immediate behavioral reversal on JM trials. Differences in size and topography of dorsal CA1 multiple-unit responses reflected the ability of the hippocampus to discriminate between stimuli in trained animals, corresponding to the performance of the behavioral discrimination. During JM trials, the rhythmicity of the neural response was further modulated by the type of the prior trial, suggesting the coding of sequential events by the hippocampus. Thus, hippocampal conditioned activity can rapidly change its magnitude and pattern depending on the specific trial type during a concurrent EB/JM discrimination task and its reversal. (PsycINFO Database Record (c) 2008 APA, all rights reserved).

Rearing under different conditions results in different functional recoveries of giant interneurons in unilaterally cercus-ablated crickets, Gryllus bimaculatus

The effects of rearing conditions on the functional recovery of wind-sensitive giant interneurons (GIs) after unilateral cercal ablation were investigated in the cricket, Gryllus bimaculatus. Crickets were reared in a glass vials to prohibit free walking for 14 days after unilateral cercal ablation ("14-day vial" crickets). Other crickets were reared in an apparatus called a "walking inducer" (WI) to increase the walking distance during the same 14-day period ("14-day WI" crickets). In these crickets, the response properties of GIs 8-1, 9-1, 9-2, and 9-3 to air currents from various directions were investigated. From the intensity-response curves obtained, directionality curves expressed in terms of threshold velocity and response magnitude were made independently. To understand changes in the functional recovery of GIs more thoroughly, the directional characteristics of GIs in crickets 1 day after unilateral cercal ablation ("1-day free" crickets) were also compared. Between the 1-day free and 14-day vial crickets, all the GIs showed differences in both threshold velocity and response magnitude for some stimulus directions. Between the 14-day vial and 14-day WI crickets, differences in the threshold velocities of GIs 9-1, 9-2, and 9-3, and in the response magnitudes of GIs 8-1, 9-1, and 9-3 were detected. Because the rearing condition after unilateral cercal ablation largely affects the compensatory recovery in some parameters of wind-evoked escape behavior, such as relative occurrence and escape direction, we discuss the functional differences in GIs revealed here in relation to the roles of GIs in the neural system that controls escape behavior.

Eyeblink classical conditioning in the preweanling lamb

Classical conditioning of eyeblink responses has been one of the most important models for studying the neurobiology of learning, with many comparative, ontogenetic, and clinical applications. The current study reports the development of procedures to conduct eyeblink conditioning in preweanling lambs and demonstrates successful conditioning using these procedures. These methods will permit application of eyeblink conditioning procedures in the analysis of functional correlates of cerebellar damage in a sheep model of fetal alcohol spectrum disorders, which has significant advantages over more common laboratory rodent models. Because sheep have been widely used for studies of pathogenesis and mechanisms of injury with many different prenatal or perinatal physiological insults, eyeblink conditioning can provide a well-studied method to assess postnatal behavioral outcomes, which heretofore have not typically been pursued with ovine models of developmental insults.

Cannabis use disrupts eyeblink conditioning: evidence for cannabinoid modulation of cerebellar-dependent learning

While the cerebellum contains the highest density of cannabinoid receptor (CB1) in the brain, no studies have assessed the effect of exogenous cannabinoids on cerebellar-dependent learning in humans. The current study, therefore, examined the effect of chronic cannabis use on classical eyeblink conditioning (EBC), a cerebellar-mediated task which has been shown to be disrupted in CB1 knockout mice. Chronic cannabis users (24 h abstinence before study; positive THC urine drug test) free of DSM-IV Axis-I or -II disorders, were evaluated. A delay EBC task was utilized, in which a conditioned stimulus (CS; 400 ms tone) co-terminated with a corneal air puff unconditioned stimulus (US; 50 ms), thus eliciting a conditioned blink response (CR). The cannabis group exhibited markedly fewer, and more poorly timed CRs as compared to drug-naive controls. There were no differences between the groups in either the unconditioned response (UR) or an EEG measure of selective attention to the CS (N100 auditory ERP), indicating that the disruption observed in the cannabis group was specific to CR acquisition. These results suggest that cannabis use is associated with functional deficits in the cerebellar circuitry underlying EBC, a finding which corroborates the recent work in CB1 knockout mice.

Differential prefrontal-like deficit in children after cerebellar astrocytoma and medulloblastoma tumor

Background

This study was realized thanks to the collaboration of children and adolescents who had been resected from cerebellar tumors. The medulloblastoma group (CE+, n = 7) in addition to surgery received radiation and chemotherapy. The astrocytoma group (CE, n = 13) did not receive additional treatments. Each clinical group was compared in their executive functioning with a paired control group (n = 12). The performances of the clinical groups with respect to controls were compared considering the tumor's localization (vermis or hemisphere) and the affectation (or not) of the dentate nucleus. Executive variables were correlated with the age at surgery, the time between surgery-evaluation and the resected volume.

Methods

The executive functioning was assessed by means of WCST, Complex Rey Figure, Controlled Oral Word Association Test (letter and animal categories), Digits span (WISC-R verbal scale) and Stroop test. These tests are very sensitive to dorsolateral PFC and/or to medial frontal cortex functions. The scores for the non-verbal Raven IQ were also obtained. Direct scores were corrected by age and transformed in standard scores using normative data. The neuropsychological evaluation was made at 3.25 (SD = 2.74) years from surgery in CE group and at 6.47 (SD = 2.77) in CE+ group.

Results

The Medulloblastoma group showed severe executive deficit (≤ 1.5 SD below normal mean) in all assessed tests, the most severe occurring in vermal patients. The Astrocytoma group also showed executive deficits in digits span, semantic fluency (animal category) and moderate to slight deficit in Stroop (word and colour) tests. In the astrocytoma group, the tumor's localization and dentate affectation showed different profile and level of impairment: moderate to slight for vermal and hemispheric patients respectively. The resected volume, age at surgery and the time between surgery-evaluation correlated with some neuropsychological executive variables.

Conclusion

Results suggest a differential prefrontal-like deficit due to cerebellar lesions and/or cerebellar-frontal diaschisis, as indicate the results in astrocytoma group (without treatments), that also can be generated and/or increased by treatments in the medulloblastoma group. The need for differential rehabilitation strategies for specific clinical groups is remarked. The results are also discussed in the context of the Cerebellar Cognitive Affective Syndrome.

Neural activities of tactile cross-modal working memory in humans: an event-related potential study

In the present study, we examined the neural mechanisms underlying cross-modal working memory by analyzing scalp-recorded event-related potentials (ERPs) from normal human subjects performing tactile-tactile unimodal or tactile-auditory cross-modal delay tasks that consisted of stimulus-1 (S-1, tactile), interval (delay), and stimulus-2 (S-2, tactile or auditory). We hypothesized that there would be sequentially discrete task-correlated changes in ERPs representing neural processes of tactile working memory, and in addition, significant differences would be observed in ERPs between the unimodal task and the cross-modal task. In comparison to the ERP components in the unimodal task, two late positive ERP components (LPC-1 and LPC-2) evoked by the tactile S-1 in the delay of the cross-modal task were enhanced by expectation of the associated auditory S-2 presented at the end of the delay. Such enhancement might represent neural activities involved in cross-modal association between the tactile stimulus and the auditory stimulus. Later in the delay, a late negative component (LNC) was observed. The amplitude of LNC depended on information retained during the delay, and when the same information was retained, this amplitude was not influenced by modality or location of S-2 (auditory S-2 through headphones, or tactile S-2 on the left index finger). LNC might represent the neural activity involved in working memory. The above results suggest that the sequential ERP changes in the present study represent temporally distinguishable neural processes, such as the cross-modal association and cross-modal working memory.

Procedural memory system supports single cue trace eyeblink conditioning in medial temporal lobe amnesia

A number of studies investigating trace eyeblink conditioning have found impaired, but not eliminated, acquisition of conditioned responses (CRs) in both animals and humans with hippocampal removal or damage. The underlying mechanism of this residual learning is unclear. The present study investigated whether the impaired level of learning is the product of residual hippocampal function or whether it is mediated by another memory system that has been shown to function normally in delay eyeblink conditioning. Performance of bilateral medial temporal lobe amnesic patients who had a prior history of participating in eyeblink conditioning studies was compared to a control group with a similar training history and to an untrained control group in a series of single cue trace conditioning tasks with 500 ms, 250 ms, and 0 ms trace intervals. Overall, patients acquired CRs to a level similar to the untrained controls, but were significantly impaired compared to the trained controls. The pattern of acquisition suggests that amnesic patients may be relying on the expression of previously acquired, likely cerebellar based, procedural memory representations in trace conditioning.

Delay discrimination and reversal eyeblink classical conditioning in abstinent chronic alcoholics

Evidence has shown that alcoholism leads to volume reductions in brain regions critical for associative learning using the eyeblink classical conditioning paradigm (EBCC). Evidence indicates that cerebellar shrinkage causes impairment in simple forms of EBCC, whereas changes in forebrain structures result in impairment in more complex tasks. In this study, the ability of abstinent alcoholics and matched control participants to acquire learned responses during delay discrimination and discrimination reversal was examined and related to severity of drinking history and neuropsychological performance. During discrimination learning, one tone (CS+) predicted the occurrence of an airpuff (unconditioned stimulus), and another tone (CS-) served as a neutral stimulus; then the significance of the tones was reversed. Alcoholics who learned the initial discrimination were impaired in acquiring the new CS+ after the tones reversed; this is a function that has previously been linked to forebrain structures. It is suggested that a factor important to alcoholic addiction may be the presence of alcoholic-related associative responses that interfere with the ability to learn new more adaptive associations.

Cerebellar dysfunction explains the extinction-like abolition of conditioned eyeblinks after NBQX injections in the inferior olive

Classical conditioning of the eyeblink response is a form of motor learning that is controlled by the intermediate cerebellum and related brainstem structures. The inferior olive (IO) is commonly thought to provide the cerebellum with a "teaching" unconditioned stimulus (US) signal required for cerebellar learning. Testing this concept has been difficult because the IO, in addition to its putative learning function, also controls tonic activity in the cerebellum. Previously, it was reported that inactivation of AMPA/kainate receptors in the IO produces extinction of conditioned responses (CRs), suggesting that it blocks the transmission of US signals without perturbing the functional state of the cerebellum. However, the electrophysiological support for this critical finding was lacking, mostly because of methodological difficulties in maintaining stable recordings from the same set of single units throughout long drug injection sessions in awake rabbits. To address this critical issue, we used our microwire-based multiple single-unit recording method. The IO in trained rabbits was injected with the AMPA/kainate receptor blocker, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), and its effects on CR expression and neuronal activity in the cerebellar interposed nuclei (IN) were examined. We found that NBQX abolished CR expression and that delayed drug effects were independent of the presentation of the conditioned stimulus and were therefore not related to extinction. In parallel to these behavioral effects, the spontaneous neuronal activity and CR-related neuronal responses in the IN were suppressed, suggesting cerebellar dysfunction. These findings indicate that testing the role of IO in learning requires methods that do not alter the functional state of the cerebellum.

Musicians have enhanced subcortical auditory and audiovisual processing of speech and music

Musical training is known to modify cortical organization. Here, we show that such modifications extend to subcortical sensory structures and generalize to processing of speech. Musicians had earlier and larger brainstem responses than nonmusician controls to both speech and music stimuli presented in auditory and audiovisual conditions, evident as early as 10 ms after acoustic onset. Phase-locking to stimulus periodicity, which likely underlies perception of pitch, was enhanced in musicians and strongly correlated with length of musical practice. In addition, viewing videos of speech (lip-reading) and music (instrument being played) enhanced temporal and frequency encoding in the auditory brainstem, particularly in musicians. These findings demonstrate practice-related changes in the early sensory encoding of auditory and audiovisual information.

Long-term depression: multiple forms and implications for brain function

Long-term potentiation (LTP) and long-term depression (LTD) remain widely accepted vertebrate models for the cellular and molecular mechanisms that underlie synaptic changes during learning and memory. Although LTD is a phenomenon that occurs in many regions of the CNS, it is clear that the mechanisms recruited in its induction and expression can vary, depending on many factors, including brain region and developmental time point. LTD in the hippocampus and cerebellum is probably the best characterized, although there are also other brain areas where mechanisms of LTD are well understood, and where it is thought to have a functional role.

Inferior olivary inactivation abolishes conditioned eyeblinks: extinction or cerebellar malfunction?

The inferior olive (IO) is a required component of neural circuits controlling the classical conditioning of eyeblink responses. Previous reports indicated that lesioning or inactivating the IO abolishes conditioned eyeblinks (CRs), but there was disagreement regarding the timing of the CR performance deficit. As a result, it was not clear whether IO inactivation produces unlearning of CRs or a non-specific dysfunction of cerebellar circuits. Since most of these studies used methods that could block unrelated axons passing through the IO region, additional experiments are required to further elucidate IO function, using inactivating agents that act selectively on cell bodies. In the present study, the IO was inactivated using the glutamate receptor antagonist DGG and the GABA-A receptor agonist muscimol in rabbits performing well-learned CRs. Effects of inactivating the IO on CR expression and on neuronal activity in the anterior cerebellar interposed nucleus (IN) were examined. We found that either blocking excitatory glutamate inputs or activating inhibitory GABA inputs to the IO abolished CRs. This effect occurred with variable delay following drug injections. Additional experiments, in which post-injection testing was delayed to allow for drug diffusion, revealed invariably immediate suppression of CRs. This demonstrated that suppressing IO activity using DGG or muscimol does not induce unlearning of CRs. Single-unit recording during DGG injections revealed that CR suppression was paralleled by a dramatic suppression of IN neuronal activity. We concluded that inactivating the rostral parts of the IO complex abolishes CRs by producing a tonic malfunction of cerebellar eyeblink conditioning circuits.