Children with autism spectrum disorders show abnormal conditioned response timing on delay, but not trace, eyeblink conditioning

Children with autism spectrum disorder (ASD) and age-matched typically-developing (TD) peers were tested on two forms of eyeblink conditioning (EBC), a Pavlovian associative learning paradigm where subjects learn to execute an appropriately-timed eyeblink in response to a previously neutral conditioning stimulus (CS). One version of the task, trace EBC, interposes a stimulus-free interval between the presentation of the CS and the unconditioned stimulus (US), a puff of air to the eye which causes the subjects to blink. In delay EBC, the CS overlaps in time with the delivery of the US, usually with both stimuli terminating simultaneously. ASD children performed normally during trace EBC, exhibiting no differences from TD subjects with regard to the learning rate or the timing of the conditioned response. However, when subsequently tested on delay EBC, subjects with ASD displayed abnormally-timed conditioned eye blinks that began earlier and peaked sooner than those of TD subjects, consistent with previous findings. The results suggest an impaired ability of children with ASD to properly time conditioned eye blinks which appears to be specific to delay EBC. We suggest that this deficit may reflect a dysfunction of the cerebellar cortex in which increases in the intensity or duration of sensory input can temporarily disrupt the accuracy of motor timing over short temporal intervals.

Characterization and clinical application of human CD34+ stem/progenitor cell populations mobilized into the blood by granulocyte colony-stimulating factor

A phase I study was performed to determine the safety and tolerability of injecting autologous CD34(+) cells into five patients with liver insufficiency. The study was based on the hypothesis that the CD34(+) cell population in granulocyte colony-stimulating factor (G-CSF)-mobilized blood contains a subpopulation of cells with the potential for regenerating damaged tissue. We separated a candidate CD34(+) stem cell population from the majority of the CD34(+) cells (99%) by adherence to tissue culture plastic. The adherent and nonadherent CD34(+) cells were distinct in morphology, immunophenotype, and gene expression profile. Reverse transcription-polymerase chain reaction-based gene expression analysis indicated that the adherent CD34(+) cells had the potential to express determinants consistent with liver, pancreas, heart, muscle, and nerve cell differentiation as well as hematopoiesis. Overall, the characteristics of the adherent CD34(+) cells identify them as a separate putative stem/progenitor cell population. In culture, they produced a population of cells exhibiting diverse morphologies and expressing genes corresponding to multiple tissue types. Encouraged by this evidence that the CD34(+) cell population contains cells with the potential to form hepatocyte-like cells, we gave G-CSF to five patients with liver insufficiency to mobilize their stem cells for collection by leukapheresis. Between 1 x 10(6) and 2 x 10(8) CD34(+) cells were injected into the portal vein (three patients) or hepatic artery (two patients). No complications or specific side effects related to the procedure were observed. Three of the five patients showed improvement in serum bilirubin and four of five in serum albumin. These observations warrant further clinical trials.

In vitro effects of interferon-γ and tumor necrosis factor-α on CD34+ bone marrow progenitor cells from aplastic anemia patients and normal donors

Acquired aplastic anemia is characterized by loss or dysfunction of hematopoietic stem and progenitor cells. The proinflammatory cytokines Tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) may be responsible for the immune-mediated pathology observed in some patients. The CD34+ population of bone marrow mononuclear cells contains primitive cells responsible for hemopoiesis. We investigated the response of CD34+ cells from aplastic anemia patients to a combination of IFN-gamma and TNF-alpha, and compared them to cells from normal volunteer donors. This was to determine whether aplastic CD34+ cells are more sensitive than normal cells to IFN-gamma/TNF-alpha-mediated effects, and whether cytokine-induced CD95 expression can explain the high levels of apoptosis observed in CD34+ cells from aplastic patients. CD34+38- cells were most affected by overnight incubation with these cytokines, their proportion and numbers being reduced in both normal donors and patients. There was no evidence for increased apoptosis, suggesting that this effect may be due to differentiation. IFN-gamma/TNF-alpha induced upregulation of CD95 on both normal and aplastic CD34+ cells, although the basal level of CD95 expression was increased in aplastic cells. However, CD95 induction did not make cells from normal donors or aplastic anemia patients susceptible to induction of apoptosis by agonistic anti-CD95 antibodies, soluble CD95 ligand, or membrane-bound CD95L. In vivo CD95L is required for CD95 induced apoptosis. No forms of this protein were detectable in lymphocytes from aplastic patients. We conclude that increased apoptosis in aplastic CD34+ cells is not due to increased sensitivity to IFN-gamma/TNF-alpha. We further show that normal and aplastic CD34+ cells are resistant to CD95 apoptosis, even in the presence of mCD95L.

Fidelity and reproducibility of antisense RNA amplification for the study of gene expression in human CD34+ haemopoietic stem and progenitor cells

Microarrays provide a powerful tool for the study of haemopoietic stem and progenitor cells (HSC). Because of the low frequency of HSC, it is rarely feasible to obtain enough mRNA for microarray hybridizations, and amplification will be necessary. Antisense RNA (aRNA) amplification is reported to give high-fidelity amplification, but most studies have used only qualitative validation. Before applying aRNA amplification to the study of HSC, we wished to determine its fidelity and reproducibility, and whether statistically significant results can be obtained. We found that aRNA amplification introduced biases into relative RNA abundance. However, these biases were extremely consistent, and valid comparisons could be made, if amplified RNA was compared with amplified RNA. By applying this method to the effect of interferon-gamma and tumour necrosis factor-alpha on normal primary CD34+ HSC, biologically significant differences could be detected, including potential mechanisms for resistance of CD34+ cells to CD95-mediated apoptosis and evidence of the differentiating effects of the cytokines. Differences of twofold or less were detected, and most of these differences attained statistical significance after triplicate experiments. These data demonstrate that aRNA amplification can be used with microarray hybridization to study the transcriptional profiles of small numbers of primary CD34+ HSC.

Dynamic modulation of mossy fiber system throughput by inferior olive synchrony: a multielectrode study of cerebellar cortex activated by motor cortex

We investigated the effects of climbing fiber synchrony on the temporal dynamics of mossy fiber system throughput in populations of cerebellar Purkinje cells (PCs). A multielectrode technique was used in ketamine-anesthetized rats that allowed both complex and simple spikes (CSs and SSs) to be recorded from multiple PCs simultaneously in lobule crus IIa. Stimulation of the tongue area of the primary motor cortex (TM1) was used to evoke cerebro-cerebellar interaction. At the single PC level, robust short-term interactions of CSs and SSs were observed after TM1 stimulation that typically consisted of an immediate depression and subsequent enhancement of SS firing after the occurrence of a CS. Such modulations of SS rate in a given PC were as robustly correlated to the CSs of simultaneously recorded PCs as they were to the CS on its own membrane-and did not require a CS on its own membrane-indicating a network basis for the interaction. Analyses of simultaneously recorded PCs using the normalized joint perievent time histogram demonstrated that CS and SS firing were dynamically correlated after TM1 stimulation in a manner that indicated strong control of mossy fiber system throughput by CS synchrony. For < or =300 ms after TM1 stimulation, most PCs showed episodic modulations in SS rate that appeared to be entrained by the population rhythm of climbing fiber synchrony. SS rhythmicity also was modulated dynamically by CSs, such that it was depressed by CSs and facilitated by their absence. Like the modulations in SS rate, a given PC's modulation in SS rhythmicity did not require it to fire a CS but was, on those instances, equally correlated to the synchronous CSs of other PCs. The data indicate that the climbing fiber system controls the temporal dynamics of SS firing in populations of PCs by using synchrony to engage intracerebellar circuitry and modulate mossy fiber system throughput.

Serotonin suppresses subthreshold and suprathreshold oscillatory activity of rat inferior olivary neurones in vitro

The effect of serotonin on membrane potential oscillations of inferior olivary neurones was studied in brainstem slices from 10- to 19-day-old rats. Serotonin at 50 and 5 microM induced a mean depolarization of 9.4 and 7.7 mV, respectively, that was preceded by a reversible suppression of subthreshold membrane potential oscillations. These effects were not changed by 1 microM tetrodotoxin and the suppression of subthreshold oscillations persisted after current-mediated restoration of resting potential. In spontaneously active neurones, serotonin abolished the rhythmicity of action potential firing without affecting spike frequency. Serotonin reduced the slope of the calcium-mediated rebound spike and both the duration and amplitude of the subsequent afterhyperpolarization. Serotonin also shifted the voltage dependence of the rebound spike to more negative values. Hyperpolarizing current pulses (200 ms) revealed that serotonin increased the pre-rectification and steady-state components of membrane resistance by 37 and 38 %, respectively, in 66 % of neurones, but decreased these parameters by 14 and 20% in the remaining cells. The serotonin effects were antagonized by 5 microM methysergide or 1-5 microM ketanserin and were mimicked by 10-20 microM dimethoxy-4-iodoamphetamine but not 10 microM 8-hydroxy-2-(di-N-propylamino)-tetralin. The data indicate that serotonin suppresses the rhythmic activity of olivary neurones via 5-HT2 receptors by inhibition of the T-type calcium current in combination with membrane depolarization due to activation of a cation current (Ih) and block of a resting K+ current (fast IK(ir)). This modulatory action of serotonin may account for the differential propensity of olivary neurones to fire rhythmically during different behavioural states in vivo.

Patterns of spontaneous purkinje cell complex spike activity in the awake rat

The olivocerebellar system is known to generate periodic synchronous discharges that result in synchronous (to within 1 msec) climbing fiber activation of Purkinje cells (complex spikes) organized in parasagittally oriented strips. These results have been obtained primarily in anesthetized animals, and so the question remains whether the olivocerebellar system generates such patterns in the awake animal. To this end, multiple electrode recordings of crus 2a complex spike activity were obtained in awake rats conditioned to execute tongue movements in response to a tone. After removal of all movement- and tone-related activity, the remaining data were examined to characterize spontaneous complex spike activity in the alert animal. Spontaneous complex spikes occurred at an average firing rate of 1 Hz and a clear approximately 10 Hz rhythmicity. Analysis of the autocorrelograms using a rhythm index indicated that the large majority of Purkinje cells displayed rhythmicity, similar to that in the anesthetized preparation. In addition, the patterns of synchronous complex spike activity were also similar to those observed in the anesthetized preparation (i.e., simultaneous activity was found predominantly among Purkinje cells located within the same parasagittally oriented strip of cortex). The results provide unequivocal evidence that the olivocerebellar system is capable of generating periodic patterns of synchronous activity in the awake animal. These findings support the extrapolation of previous results obtained in the anesthetized preparation to the waking state and are consistent with the timing hypothesis concerning the role of the olivocerebellar system in motor coordination.

Systemic harmaline blocks associative and motor learning by the actions of the inferior olive

Is there a role for the inferior olive in learning? Novel paradigms of conditioning involving tongue protrusion were developed using the rat to test whether: (a) the indole alkaloid harmaline blocks associative learning via actions within the inferior olive, and (b) the inferior olive is required for associative and motor learning. Harmaline blocked associative learning as measured by the absence of conditioned responses to a tone over six daily sessions of conditioning and the absence of retention without harmaline. Harmaline's effect on associative learning was completely blocked by prior removal of the inferior olive with 3-acetylpyridine. Rats whose inferior olives were chronically lesioned showed normal associative learning, normal associative memory, and could learn to modify tongue protrusion via a motor learning paradigm involving response shaping. Removal of the inferior olive degraded the performance of the licking motor system by increasing the latency of conditioned tongue protrusions and by increasing the temporal variability of rhythmic licking elicited by intraoral water. The experiments raise doubt as to whether the inferior olive encodes memory in the cerebellum but demonstrate that the inferior olive is essential for the temporal precision of movement. The results indicate that harmaline's antilearning action is produced by its ability to exaggerate the normal propensity of olivary neurons to fire rhythmically, a process that must be constrained under physiological conditions for normal learning to occur. It is concluded that there may be an important role for the rhythmic activity of inferior olivary neurons in the temporal processes that underlie both motor control and learning.

Acute inactivation of the inferior olive blocks associative learning

Can acute inactivation of the inferior olive block associative learning? We anaesthetized the inferior olive with lidocaine while rabbits simultaneously: (i) performed conditioned nictitating membrane responses to a flashing light to which they had already been trained; and (ii) underwent their first experience with classical conditioning of the same response to a tone. Inactivation of the inferior olive immediately and reversibly abolished the performance of conditioned responses and prevented learning during rabbits' initial conditioning with a tone-conditioned stimulus. When olivary function was restored, rabbits showed no signs of having learned under olivary anaesthesia. The experiment demonstrates that an acute disruption in olivary function can block learning, in addition to severely degrading motor control. The results are interpreted to indicate the importance of the inferior olive in optimizing learning, perhaps through a general role in regulating temporal processing.

Removal of the inferior olive abolishes myoclonic seizures associated with a loss of olivary serotonin

Several lines of clinical evidence suggest that myoclonus is caused by a reduction of serotonin in the brain and hyperactivity of the inferior olive. We determined whether a change in serotonin content within the olivocerebellar system accompanied a predisposition to myoclonus and investigated the necessity of the inferior olive for a myoclonic seizure. The experiments employed the genetically epilepsy-prone rat that exhibits a profound myoclonic seizure in response to an auditory stimulus. We found that these animals demonstrated a significant reduction in the serotonergic innervation of the inferior olive without a significant change in the serotonergic innervation at any other level of the olivocerebellar circuit. The deficit in olivary serotonin was verified physiologically and pharmacologically by a reduced sensitivity of the genetically epilepsy-prone rat to the tremorogenic effect of harmaline, which is known to produce tremor through a mechanism that requires serotonergic innervation of the inferior olive. We quantified the timing of the myoclonic seizure of the genetically epilepsy-prone rat and found that its large amplitude 2-6 Hz clonus was always preceded by 9-10 Hz tremor that was synchronized among limbs. Ablation of the inferior olive by 3-acetylpyridine abolished the myoclonic seizure. The specificity of the deficit in olivary serotonin, the timing of the seizure, and the demonstration of the necessity of the inferior olive for myoclonus suggest that pathological inferior olivary activity contributes to the genesis of a myoclonic seizure.

The cerebellum, LTD, and memory: alternative views

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Some organizing principles for the control of movement based on olivocerebellar physiology

Motor control is defined as the process of restricting the output of the motor nervous system so that meaningful and coordinated behavior ensues. The high dimensionality of the computation underlying motor control is presented and a simplifying framework is outlined. Evidence that movements are performed non-continuously is reviewed as is the construct of the 'motor synergy' as a fundamental unit of control. It is proposed that the pulsatile nature of movement and the tendency of muscle collectives to be activated as synergies reflect processes that the nervous system has evolved to reduce the dimensionality of motor control. We propose that the inferior olive simplifies the computation underlying motor control by biasing the activities of spinal and cranial motor systems so that discrete collectives of muscles are predisposed to contract at specific times during movement. The well-characterized oscillatory activity of olivary neurons is postulated to provide a pacemaking signal and to restrict the control process to particular moments in time while the process of electrotonic coupling and uncoupling of assemblies of olivary neurons is proposed to underlie the spatial distribution of synergic muscle activations. It is proposed that the olivocerebellar contribution to the control process is to allow movements to be executed rapidly in a feedforward manner, so that the need for sensory guidance and feedback is minimized.

Dynamic organization of motor control within the olivocerebellar system

What is the role of the cerebellum in motor coordination? Such coordination depends upon the integrity of the inferior olive, a major cerebellar afferent, as its lesion produces ataxic and dysmetric movement abnormalities. Using multiple-microelectrode recordings, we report here that there are domains of Purkinje cell activity that are generated by olivary input during skilled tongue movements in rats. Such activity domains are highly rhythmic and time-locked to movement. Patterns of synchronous olivocerebellar activity are geometrically complex and can change during a sequence of movements. The results support the view that the inferior olive organizes movement in time, by entraining motor-neuronal firing through rhythmic activation of the cerebellum, and in space, by synchronously activating cell ensembles that allow the use of individual muscles. Dynamic repatterning of olivocerebellar synchrony may allow different combinations of muscles to be used for movements intended to have varying spatial structures.

On the cerebellum and motor learning

A critical review of the role of the cerebellum in motor learning is presented. Specifically, the hypothesis that the climbing fibers that issue from the inferior olive serve to modify the responsiveness of cerebellar Purkinje cells is evaluated. It is concluded that there is no convincing evidence, at this time, to support the view that a long-term modification of Purkinje cell activity is either the basis of motor learning or an authentic mechanism of cerebellar function. An alternative view, based on the biophysical, anatomical and ensemble properties of olivary neurons, suggests an important role for the olivocerebellar system in the coordination of movements. Future work in this interesting area of neuroscience will distinguish these two hypotheses.

Recoverable and nonrecoverable deficits in conditioned responses after cerebellar cortical lesions

This study reexamined the effects of unilateral damage to cerebellar hemispheral lobule VI on the rabbit's conditioned nictitating membrane (NM) response. Extensive unilateral removal of hemispheral lobule VI in 11 rabbits impaired ipsilateral conditioned responses as reflected by reductions of 52% in mean frequency and 53% in mean amplitude during test trials on the first postoperative session. The decreases in the amplitude and frequency of conditioned responses were highly correlated (r = 0.82). The frequency of conditioned responses recovered to control levels but their amplitudes remained reduced such that the correlation between these two measures of responding was no longer significant by the 12th postoperative conditioning session. The decrease in the amplitude of conditioned responses was not accompanied by changes in onset latency or rise time. There was no significant impairment of conditioned responses in surgical controls and animals with only partial damage to hemispheral lobule VI. It was concluded that hemispheral lobule VI plays an important role in the regulation of motor centers in the brainstem so as to facilitate the initiation and optimum execution of the conditioned NM reflex. This cortical regulation of the conditioned NM response may contain learned elements; however, these cannot be resolved with lesion methods, nor has their existence been proven in this or other lesion studies. Nevertheless, the results of this study do demonstrate that the cerebellar cortex cannot be considered as the single locus necessary for NM conditioning.

Changes in the motor pattern of learned and unlearned responses following cerebellar lesions: a kinematic analysis of the nictitating membrane reflex

Kinematic and dynamic analyses were employed to study the effects of cerebellar lesions on conditioned and unconditioned nictitating membrane responses in the rabbit. It was found that conditioned responses acquired to an auditory stimulus accelerated in two bursts as indicated by two distinct peaks of acceleration. The second peak of acceleration was very weak during the early portions of conditioning but became a prominent feature of the conditioned response over 16 sessions of conditioning. The second peak of acceleration in the conditioned response was more sensitive to cerebellar damage than was the first peak. When lesions of the cerebellum permanently reduced the amplitude of conditioned responses, but did not affect their frequency, the second peak of acceleration was nearly abolished while the first peak was unaffected. When cerebellar lesions profoundly impaired both the amplitude and frequency of conditioned responses, large and permanent impairments occurred in both peaks of acceleration. Lesions of the anterior interpositus nucleus most severely impaired both peaks of acceleration in the conditioned response and significantly reduced the acceleration of unconditioned responses across a wide range of intensities of corneal air puff. The deficit in the acceleration of unconditioned responses became manifest only after membrane extension exceeded 0.12 mm. The impairment in the amplitude of the unconditioned response after cerebellar lesions more closely approximated the impairment in the amplitude of the conditioned response when the force-generating properties of the conditioned and unconditioned stimuli were equated. It was hypothesized, therefore, that one reason why conditioned responses are so easily disrupted by cerebellar lesions is because they are of low force and not simply because they are learned. It was proposed that the two peaks of acceleration that characterize the conditioned response represent the function of two distinct anatomical systems. The first, a short-latency system, initiates the response and is most likely mediated by circuits that traverse the pontomedullary reticular formation. The second, a longer-latency system, amplifies response amplitude and its neural basis remains to be elucidated. The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles. It was concluded that the vulnerability of the second component of the conditioned response to cerebellar damage reflects an important role for the cerebellum in modulating the degree to which long-latency neural systems contribute to the ongoing performance of learned and unlearned behaviors.

Pavlovian conditioning in the rabbit during inactivation of the interpositus nucleus

1. We have examined the role of the anterior interpositus nucleus (AIP) of the cerebellum in Pavlovian conditioning of the nictitating membrane response (NMR) of the rabbit with the use of reversible brain lesions produced by the local anaesthetic lidocaine. Previous experiments have demonstrated that destructive lesions of the AIP prevent the performance of conditioned NMRs (CRs). Microinjections of lidocaine into the AIP were used in the present experiment to determine whether the deficit in the performance of CRs resulted from a deficit in learning or memory. 2. A 3-phase procedure was employed to determine whether associative learning required the function of the AIP. In phase 1, rabbits were trained to make CRs to a flashing-light conditioned stimulus (CS) that was paired with an air-puff unconditioned stimulus (UCS) directed at the cornea. In phase 2, the AIP was anaesthetized during a session of conditioning in which a tone CS was paired with the UCS. Presentations of the light CS were interpolated throughout the tone conditioning in order to monitor the degree to which CRs were impaired by lidocaine. Phase 3 occurred after the effects of the lidocaine had dissipated and consisted of a test of retention to determine whether learning occurred during phase 2 but could not be expressed because of a performance deficit resulting from the inactivation of the AIP. 3. Infusion of lidocaine into the AIP abolished CRs to the light CS and prevented the performance of CRs to the tone CS in phase 2. The effect of the infusion was specifically due to a conduction block of neurons and/or fibres in the lateral aspect of the AIP. The infusion of lidocaine into regions surrounding the AIP did not affect CRs elicited by the light CS or prevent acquisition of CRs to the tone. Infusions of saline directly into the AIP did not impair the performance of CRs to either the tone or light CS. Quantitative analysis of diffusion revealed that the abolition of CRs was accompanied by anaesthetization of the AIP. 4. The retention test in phase 3 indicated that learning occurred normally during phase 2 when the AIP was inactivated and performance was abolished. When the function of the AIP was restored and performance had recovered, the subjects demonstrated a frequency of CRs to the tone CS that was indistinguishable from control subjects whose performance had never been impaired. 5. The CRs observed during the retention test provided an unequivocal measure of associative learning.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar lesions and the nictitating membrane reflex: performance deficits of the conditioned and unconditioned response

Unilateral cerebellar lesions abolished the occurrence of ipsilateral conditioned nictitating membrane responses during the 285 msec interval between onset of the conditioned and unconditioned stimuli on paired trials. This effect was obtained in 15 animals sustaining damage to the dorsolateral aspects of the interpositus nucleus and the adjoining white matter. However, conditioned responses did occur during the 800 msec observation interval employed on tone-alone test trials, and these responses exhibited the classic performance deficits normally associated with cerebellar damage: a low frequency of occurrence (14%, as compared with 96% before the lesion); a 3.1 mm decrease in amplitude; a 236 msec increase in onset latency; a 563 msec increase in latency of peak amplitude; and a 327 msec increase in rise time. Four of the 15 animals failed to demonstrate greater than 5% responding during the test trials. These performance deficits were not specific to the learned, conditioned response. Unconditioned responses were also reduced in frequency and increased in latency of peak amplitude and rise time, especially when elicited at lower air-puff intensities. These deficits in the unconditioned response were observed in animals that failed to exhibit conditioned responses on either paired or test trials, as well as in animals demonstrating conditioned responses only during test trials. We conclude that the cerebellum has a general role in regulating the nictitating membrane reflex so that deficits in learned responses observed after cerebellar lesions are secondary to a broader deficit in performance. The performance deficits appear to consist of a sensory component, as reflected by an increase in stimulus threshold for elicitation of the nictitating membrane reflex, and a motor component, as reflected by the altered topography of the evoked response. The results of this study thus reaffirm the role of the cerebellum in regulating the sensorimotor processes necessary for the optimal performance of both conditioned and unconditioned responses and extends this role to the expression of a simple cranial nerve reflex.

Asymmetric uptake of 2-deoxy-D-[14C]glucose in the dorsal cochlear nucleus during Pavlovian conditioning in the rabbit

Uptake of 2-deoxy-D-[14C]glucose was measured during Pavlovian conditioning of the rabbit's nictitating membrane response by both qualitative autoradiography and by quantitative measurement of radioactivity in samples of brain tissue. Conditioning was accomplished by pairing a tone stimulus delivered to both ears with an air-puff stimulus delivered to the right eye. Infusion of 2-deoxy-D-[14C]glucose during the first day of conditioning when there was no evidence of acquisition or during the 7th day of conditioning when animals demonstrated 68% conditioned responses resulted in a significantly greater uptake of radioactivity by the caudal portions of the left as compared with the right dorsal cochlear nucleus. Similar changes were not observed in other auditory and non-auditory nuclei. Rabbits that had acquired conditioned responses across 6 days of training and were exposed only to the tone-conditioned stimulus on the 7th day of testing exhibited 69% conditioned responses but no asymmetry in the uptake of 2-deoxy-D-[14C]glucose. Control animals receiving unpaired presentations of tone and air puff or no stimulation did not acquire conditioned responses and did not demonstrate asymmetric uptake of radioactivity in the dorsal cochlear nucleus. These results indicate that the asymmetric uptake of radioactivity by the dorsal cochlear nucleus did not result from the effects of stimulation per se or the prior occurrence of learning but was due to the explicit pairing of the tone stimulus with the asymmetric delivery of the air puff. It would appear that the caudal dorsal cochlear nucleus not only serves as a signal transducer for auditory stimuli but also receives inputs from other sensory systems thus allowing it to both recognize when an auditory stimulus is followed by a biologically significant event and to transmit such information to other brain regions that are, in turn, responsible for learning.