Frequency sensitivities of auditory neurons in the cerebellum of the cat

Central Processing in Tinnitus: fMRI Study Outlining Patterns of Activation Using an Auditory Discrimination Task in Normal Versus Tinnitus Patients

OBJECTIVE

Elucidate brain activity differences between patients with tinnitus and controls.

STUDY DESIGN

Cross-sectional cohort study.

SETTING

Outpatient Otolaryngology clinic.

PATIENTS

Three cohorts; 8 controls, 12 with subjective idiopathic tinnitus (tinnitus without hearing loss), and 12 with both tinnitus and hearing loss.

INTERVENTION

An auditory oddball identification task was performed in fMRI scanner.

MAIN OUTCOME MEASURES

Task performance and Tinnitus Handicap Inventory (THI) scores were recorded. Brain activation maps were generated comparing deviant and standard tones as well as at rest. One-way and two-way T-contrasts were generated in addition to multiple regression modeling which identified significant brain regions predicting tinnitus, disease severity, duration, and task performance.

RESULTS

Task performance worsened in tinnitus patients with increased auditory workload, in terms of additional hearing loss. THI score and grade correlated with false alarms. The limbic system, heschel's gyrus, angular gyrus and cerebellum have a significant effect on both brain behavior in patients with tinnitus, and predictability of tinnitus and its behavioral implications.

CONCLUSION

Increased auditory workload resulted in poorer task performance. Moreover, it is possible to predict auditory task performance in patients with tinnitus by looking at the activity of specific regions of interest. Heschl's gyrus, angular gyrus, cerebellar, and limbic system activity are important contributors to neurological activity associated with tinnitus. Finally, predictive modeling may influence further research surrounding tinnitus treatment.

Auditory thalamus dysfunction and pathophysiology in tinnitus: a predictive network hypothesis

Tinnitus is the perception of a 'ringing' sound without an acoustic source. It is generally accepted that tinnitus develops after peripheral hearing loss and is associated with altered auditory processing. The thalamus is a crucial relay in the underlying pathways that actively shapes processing of auditory signals before the respective information reaches the cerebral cortex. Here, we review animal and human evidence to define thalamic function in tinnitus. Overall increased spontaneous firing patterns and altered coherence between the thalamic medial geniculate body (MGB) and auditory cortices is observed in animal models of tinnitus. It is likely that the functional connectivity between the MGB and primary and secondary auditory cortices is reduced in humans. Conversely, there are indications for increased connectivity between the MGB and several areas in the cingulate cortex and posterior cerebellar regions, as well as variability in connectivity between the MGB and frontal areas regarding laterality and orientation in the inferior, medial and superior frontal gyrus. We suggest that these changes affect adaptive sensory gating of temporal and spectral sound features along the auditory pathway, reflecting dysfunction in an extensive thalamo-cortical network implicated in predictive temporal adaptation to the auditory environment. Modulation of temporal characteristics of input signals might hence factor into a thalamo-cortical dysrhythmia profile of tinnitus, but could ultimately also establish new directions for treatment options for persons with tinnitus.

Tinnitus and hyperacusis: Contributions of paraflocculus, reticular formation and stress

Tinnitus and hyperacusis are common and potentially serious hearing disorders associated with noise-, age- or drug-induced hearing loss. Accumulating evidence suggests that tinnitus and hyperacusis are linked to excessive neural activity in a distributed brain network that not only includes the central auditory pathway, but also brain regions involved in arousal, emotion, stress and motor control. Here we examine electrophysiological changes in two novel non-auditory areas implicated in tinnitus and hyperacusis: the caudal pontine reticular nucleus (PnC), involved in arousal, and the paraflocculus lobe of the cerebellum (PFL), implicated in head-eye coordination and gating tinnitus and we measure the changes in corticosterone stress hormone levels. Using the salicylate-induced model of tinnitus and hyperacusis, we found that long-latency (>10 ms) sound-evoked response components in both the brain regions were significantly enhanced after salicylate administration, while the short-latency responses were reduced, likely reflecting cochlear hearing loss. These results are consistent with the central gain model of tinnitus and hyperacusis, which proposes that these disorders arise from the amplification of neural activity in central auditory pathway plus other regions linked to arousal, emotion, tinnitus gating and motor control. Finally, we demonstrate that salicylate results in an increase in corticosterone level in a dose-dependent manner consistent with the notion that stress may interact with hearing loss in tinnitus and hyperacusis development. This increased stress response has the potential to have wide-ranging effects on the central nervous system and may therefore contribute to brain-wide changes in neural activity.

Acute ethanol exposure increases firing and induces oscillations in cerebellar Golgi cells of freely moving rats

BACKGROUND

Alcohol is a widely abused substance and is responsible for significant morbidity and mortality worldwide. The precise mechanisms underlying ethanol (EtOH)'s actions in the central nervous system (CNS) remain elusive. In vitro studies suggest that GABAergic interneurons are important targets of EtOH action in the CNS. Although EtOH generally acts to inhibit CNS neurons, it appears to cause an increase in GABAergic interneuron excitability. However, it has yet to be demonstrated that EtOH produces this effect in the brain of behaving animals. Here, we demonstrate for the first time that acute EtOH exposure excites a subtype of GABAergic interneuron (cerebellar Golgi cell [GoC]) in a freely moving animal.

METHODS

Electrophysiological recordings were made from microwire arrays implanted in the anterior cerebellum of freely moving rats.

RESULTS

Cerebellar GoCs display a slow, irregular, spontaneous action potential firing pattern under control conditions. EtOH caused dramatic and consistent increases in the rate and regularity of GoC discharges, including a redistribution of the power in the GoC spike train, such that power became concentrated in the 26.7 ± 7.3 Hz region.

CONCLUSIONS

Taken together with our previous findings, these data suggest that a major mechanism of EtOH actions on cerebellar function is an EtOH-induced de-afferentation at the input stage of the cerebellar cortex in the form of granule cell inhibition, and that this inhibition is caused by an increase in GoC firing. It is likely that GoCs may play a significant role both in the gating of information transmission to granule cells and in the modulation of the overall excitability of the cerebellum by tonically controlling granule cell activity.

Dichotic listening test in patients with chronic cerebellar disease

PURPOSE

The aim of the study was to identify alterations in the auditory processing of patients with chronic cerebellar disease using a dichotic listening test with alternating dissyllables, also known as the Staggered Spondaic Word (SSW) test.

MATERIALS AND METHODS

A study involving a control group of 20 subjects and a study group of 18 patients with chronic cerebellar disease of both sexes aged between 9 and 56 years was performed. The SSW test was conducted in accordance with strict standard protocols along with the analysis procedures.

RESULTS

Findings revealed a statistically significant difference in the quantitative alterations on the SSW test in the study group compared with the control group (P < .001). Results of the qualitative evaluation showed no statistically significant differences between the study and control groups for order or auditory effects. However, a statistically significant difference for presence of inversions was identified, with the worse result in the study group.

CONCLUSION

The present study identified quantitative and qualitative changes in auditory processing for decodifying, gradual memory loss, and organization modes on the dichotic listening test with alternating dissyllables (SSW) in individuals with chronic cerebellar disease.

Music and the Auditory Brain: Where is the Connection?

Sound processing by the auditory system is understood in unprecedented details, even compared with sensory coding in the visual system. Nevertheless, we do not understand yet the way in which some of the simplest perceptual properties of sounds are coded in neuronal activity. This poses serious difficulties for linking neuronal responses in the auditory system and music processing, since music operates on abstract representations of sounds. Paradoxically, although perceptual representations of sounds most probably occur high in auditory system or even beyond it, neuronal responses are strongly affected by the temporal organization of sound streams even in subcortical stations. Thus, to the extent that music is organized sound, it is the organization, rather than the sound, which is represented first in the auditory brain.

Implications on cerebellar function from information coding

One function of the cerebellar cortex is to process information. There are at least two types of information. Temporal information is encoded in the timing pattern of action and synaptic potentials, whereas structural information is encoded in the spatial pattern of the cerebellar synaptic circuitry. Intuitively, analysis of highly complex information in the time domain would require a cerebellar cortex with structural complexity to match. Information theory offers a way to estimate quantitatively both types of information and thereby helps to test hypotheses or advance theories of cerebellar neurobiology. These estimates suggest: (i) That the mossy-fiber-granule-cell system carries far more (temporal) information than the climbing fiber system, (ii) that Purkinje cells extract only a fraction of the (temporal) information from their afferents, and (iii) that the cerebellar cortex has a large (spatial) information coding capacity. Concerning information, one can argue that the cerebellar cortex analyzes temporal information in its afferents as a search engine, in search of coincidental mossy fiber events based on timing cues provided by climbing fiber events. Results of successive searches are continuously being converted into structural information encoded in the spatial distribution pattern of granule-cell-Purkinje-cell synapses along granule cell axons, thereby providing an adaptive and indeed self-correcting dimension to the structural information code. The search engine operation involves cellular mechanisms acting on temporal events and is part of an associative learning process. The conversion and generation of structural information involves neuroplasticity mechanisms acting at the synaptic level, with electrophysiological as well as structural consequences, and may be part of the short- and long-term memory process. These and other attributes qualify the cerebellar cortex as a dynamic information processing center, contributing to memory and learning while linking motor output with sensory events.

Participação do cerebelo no processamento auditivo

O cerebelo era tradicionalmente visto como um órgão coordenador da motricidade, entretanto é atualmente considerado como um importante centro de integração de sensibilidades e coordenação de várias fases do processo cognitivo. OBJETIVO: é sistematizar as informações da literatura quanto à participação do cerebelo na percepção auditiva. MÉTODOS: foram selecionados na literatura trabalhos em animais sobre a fisiologia e anatomia das vias auditivas do cerebelo, além de trabalhos em humanos sobre diversas funções do cerebelo na percepção auditiva. Foram discutidos os achados da literatura, que há evidências que o cerebelo participa das seguintes funções cognitivas relacionadas à audição: geração verbal; processamento auditivo; atenção auditiva; memória auditiva; raciocínio abstrato; timing; solução de problemas; discriminação sensorial; informação sensorial; processamento da linguagem; operações lingüísticas. CONCLUSÃO: Foi constatado que são incompletas as informações sobre as estruturas, funções e vias auditivas do cerebelo.

Ethanol inhibits the sensory responses of cerebellar granule cells in anesthetized cats

BACKGROUND

Granule cells occupy a strategic position in the transmission of afferent information to the cerebellar cortex. They are also the most abundant type of neurons in the cerebellum. The functions of the cerebellum are thought to be sensitive to acute alcohol intoxication. The effects of acute alcohol intoxication on the in vivo physiology of cerebellar granule cells are, however, not completely known.

METHODS

We studied chloralose-anesthetized cats at ethanol doses relevant to human drinking (0.3-1.2 g/kg). We recorded the electrophysiological responses of granule cell clusters to auditory and visual stimulation, and simultaneously monitored the concentration of ethanol in the cerebrospinal fluid (CSF).

RESULTS

At an intravenous ethanol dose of 0.3 g/kg, CSF ethanol concentration peaked in 10 minutes at 17 mM, equivalent to a blood alcohol concentration (BAC) of about 0.08 g/dL. Ethanol quickly and almost completely abolished both auditory and visual responses from granule cells. Complete or near-complete inhibition lasted 15 to 20 minutes; approximately 50% recovery required an additional 15 minutes, and a full recovery yet another 15 minutes. A higher ethanol dose at 1.2 g/kg resulted in a more severe inhibition and required longer time for recovery. The relationship between ethanol dose, CSF ethanol concentration, and granule cell responses was dynamic and nonlinear, critically depending upon the elapsed time.

CONCLUSIONS

Cerebellar granule cell sensory responses are highly sensitive to ethanol inhibition. A rapid development of acute tolerance appears to be a major factor contributing to the dynamic and nonlinear relationship among ethanol dosage, CSF ethanol concentration, and granule cell responses. It is likely that a generalized de-afferentation of the cerebellum from its mossy fiber afferents, followed by the subsequent development of acute tolerance may play major roles by which alcohol intoxication affects cerebellar functions.

Participation of the Cerebellum in Auditory Processing

The cerebellum, traditionally conceived as a controlling organ of motricity, it is today considered an all-important integration center for both sensitivity and coordination of the various phases of the cognitive process.

AIM

This paper aims at gather and sort literature information on the cerebellums role in the auditory perception.

METHODS

We have selected animal studies of both the physiology and the anatomy of the cerebellum auditory pathway, as well as papers on humans discussing several functions of the cerebellum in auditory perception. As for the literature, it has been discussed and concluded that there is evidence that the cerebellum participates in many cognitive functions related to hearing: speech generation, auditory processing, auditory memory, abstract reasoning, timing, solution of problems, sensorial discrimination, sensorial information, language processing, and linguistic operations.

CONCLUSION

It has been reported that all information about cerebellum structure, functions and auditory pathways is incomplete.

Information coding capacity of cerebellar parallel fibers

Understanding synaptic connectivity is a prerequisite to gaining insight on how the central nervous system processes information. Cerebellar parallel fibers make an impressive number of synapses with the Purkinje cells. These synapses are the major structural elements of a large information processing system. The objective of the present report is to describe a method to estimate the coding capacity of this information processing system. We propose to derive the coding capacity from the linear distribution pattern of synaptic varicosities along parallel fibers in a manner consistent with Shannon's information theory formalism. The coding capacity of an average parallel fiber synapse is S=-kappaSigmaP(l(i))lnP(l(i)), where kappa=1/ln2, P(l(i)) is the probability of observing a particular inter-varicosital distance l(i), and ln is the natural logarithm to the base e. In the cerebellar parallel fibers of the mouse, and in a number of other unmyelinated axonal systems, the distribution pattern of P(l(i)) as a function of l(i) is exponential-like. According to information theory, the exponential-like distribution pattern suggests that information transmission in these axonal synaptic systems is operating at near-optimal coding capacity. This optimization in information coding may be the result of a stochastic-like process regulating the formation or elimination of parallel fiber synapses during development and maturation. In the adult nervous system, neuroplasticity-mediated synaptic remodeling may also regulate the coding capacity of axonal synapses via a similar stochastic-like process. The conceptual framework herein may be applicable to other axonal systems in the nervous system.

The neuroanatomy and neuroendocrinology of fragile X syndrome

Fragile X syndrome (FXS), caused by a single gene mutation on the X chromosome, offers a unique opportunity for investigation of gene-brain-behavior relationships. Recent advances in molecular genetics, human brain imaging, and behavioral studies have started to unravel the complex pathways leading to the cognitive, psychiatric, and physical features that are unique to this syndrome. In this article, we summarize studies focused on the neuroanatomy and neuroendocrinology of FXS. A review of structural imaging studies of individuals with the full mutation shows that several brain regions are enlarged, including the hippocampus, amygdala, caudate nucleus, and thalamus, even after controlling for overall brain volume. These regions mediate several cognitive and behavioral functions known to be aberrant in FXS such as memory and learning, information and sensory processing, and social and emotional behavior. Two regions, the cerebellar vermis, important for a variety of cognitive tasks and regulation of motor behavior, and the superior temporal gyrus, involved in processing complex auditory stimuli, are reported to be reduced in size relative to controls. Functional imaging, typically limited to females, has emphasized that individuals with FXS do not adequately recruit brain regions that are normally utilized by unaffected individuals to carry out various cognitive tasks, such as arithmetic processing or visual memory tasks. Finally, we review a number of neuroendocrine studies implicating hypothalamic dysfunction in FXS, including abnormal activation of the hypothalamic-pituitary-adrenal (HPA) axis. These studies may help to explain the abnormal stress responses, sleep abnormalities, and physical growth patterns commonly seen in affected individuals. In the future, innovative longitudinal studies to investigate development of neurobiologic and behavioral features over time, and ultimately empirical testing of pharmacological, behavioral, and even molecular genetic interventions using MRI are likely to yield significant positive changes in the lives of persons with FXS, as well as increase our understanding of the development of psychiatric and learning problems in the general population.

Long- and short-term habituation of acoustic startle is not frequency specific in the rat

Two experiments examined the frequency specificity of habituation of the acoustic startle response in the rat. Following the long-term habituation of startle to one of two pure tone stimuli in Experiment 1, animals were presented with the other stimulus. Startle response asymptotes were unaffected by this change in stimulus frequency. Short-term habituation of startle also was insensitive to stimulus frequency. In Experiment 2, pure tone stimuli were used to provoke both a startle response and the interruption of drinking. Long-term habituation of startle to either stimulus was unaffected by a change in frequency. Animals that received the two stimuli on alternating days showed as rapid a habituation as did the groups receiving only one stimulus frequency during acquisition. Conversely, the lick suppression measure was found to be frequency specific. Lick suppression durations rose to pre-habituation levels when the frequency of the stimulus was changed. Animals that received the two stimuli on alternating days showed retarded habituation compared to those groups presented with only one stimulus frequency during acquisition. Although long-term habituation of startle is not stimulus specific, it is mediated by central processes and thus remains a valuable model in the study of neurophysiological mechanisms of behavioral change.

Autistic behaviors among girls with fragile X syndrome

Reports of autistic behaviors were examined for 30 school-age girls with fragile X (fraX) and 31 age- and IQ-matched controls through a structured interview administered to each girl's parent(s). IQ scores were obtained for each participant; anxiety, neuroanatomical, and molecular-genetic data were derived for girls with fraX. Girls with fraX had significantly more autistic behaviors than controls. These behaviors were qualitatively similar to those reported for boys with fraX, but were not correlated with IQ. Anxiety in girls with fraX was positively correlated with abnormal social and communication behaviors; posterior cerebellar vermis area was negatively correlated with measures of communication and stereotypic/restricted behaviors. Severity of stereotypic/restricted behaviors was negatively correlated with the prevalence of active non-fraX chromosomes. Thus anxiety and posterior cerebellar area measures had distinct associations with subsets of autistic behaviors; these associations may have important implications for understanding the neurobiology of autism.

Identification of cell types from action potential waveforms: cerebellar granule cells

We have recorded 404 single units extracellularly in the cerebellar cortex of the rat with tungsten microelectrodes. Waveforms of action potentials were analyzed in order to develop criteria for on-line identification of cell types. Two of the four most frequently recorded waveforms were simple and complex spikes from Purkinje cells. The other two originated from granule cells and glomeruli. Presumed granule cells showed biphasic action potentials with half-widths (0.78 +/- 0.14 ms, n = 51) broader than those of the simple spikes of Purkinje cells (0.22 +/- 0.06 ms, n = 54), whereas presumed glomerular potentials had complex action potentials with narrower half-widths (0.14 +/- 0.05 ms, n = 35). The mean inter-spike interval of presumed granule cells (333.3 +/- 195.4 ms, n = 53) was longer than that of Purkinje cells (47.3 +/- 31.8 ms, n = 59) and the presumed glomerular potentials (77.7 +/- 50.8 ms, n = 20). Results were virtually identical from 17 cerebellar units recorded extracellularly in the cat. Intracellular recording and staining of 20 granule cells with HRP-filled microelectrodes provided further support for our assessment. These results suggest that action potentials from granule cells may be identified on-line by waveform.

The effects of X monosomy on brain development: monozygotic twins discordant for Turner's syndrome

Monosomy for the X chromosome is the most frequent cause of Turner's syndrome, a common clinical syndrome associated with particular physical and neurobehavioral features. The results from comprehensive assessment of prepubertal monozygotic female twins discordant for X monosomy are presented. Zygosity was established with DNA Fingerprinting and no evidence of chromosomal mosaicism was seen in either child. Physical features in the affected twin were relatively mild with respect to the full spectrum of physical malformations and disabilities associated with Turner's syndrome. The neurobehavioral phenotypes of the twins were compared. Although both sisters scored in the superior range of intelligence, the affected twin's Performance IQ was 18 points less than her sister, whereas Verbal IQ showed only a 3-point difference between the sisters. Other relative differences were noted within the executive, visuospatial, and visuomotor domains of function. Behavioral evaluation indicated greater problems with attention, hyperactivity, and anxiety in the affected twin. Quantitative analysis of brain anatomy revealed evidence of both general and regional effects of X monosomy on neurodevelopment. Cerebrospinal fluid volume was increased by 25% in the affected twin compared with her sister with a corresponding decrease in gray matter volume. The right frontal, right parietal-occipital, and left parietal-perisylvian regions showed the greatest discrepancy between the sisters with respect to increased cerebrospinal fluid and decreased gray matter volumes in twin with X monosomy. Differences in the posterior fossa were also noted with a 50% relative increase in the volumes of the fourth ventricle and cisterna magna and a 10 to 15% relative reduction in size of the cerebellar vermis, pons, and medulla in the affected twin. The association between the neurobehavioral and neuroanatomical findings in the affected twin is discussed. The unique nature of the naturally occurring genetic phenomenon seen in this twin pair provides an opportunity to more fully elucidate the neurobehavioral phenotype associated with X monosomy and Turner's syndrome.

Electrophysiology of nitrous oxide on cerebellar granule cells: a single-cell study

We recorded 18 single cells in the granule cell layer of the cat. Each single cell was screened and identified as a granule cell based on a set of criteria derived from known electrophysiological properties of granule cells. We then monitored the effects of nitrous oxide on the spontaneous activities and the auditory responses of these cells. Nitrous oxide consistently caused a severe inhibition of spontaneous activities as well as responses to sound in these cells. Furthermore, the amplitudes of their action potentials decreased during the inhibition. Three of the 18 cells were subsequently injected intracellularly with HRP. All three were verified to be granule cells. In those granule cells we recorded intracellularly, nitrous oxide did not change the resting membrane potentials. The gradual decrease in the amplitudes of action potentials suggested that some of the mechanisms leading to the genesis of action potentials were being altered by nitrous oxide. It is also possible that nitrous oxide may act on synaptic transmission at a site located postsynaptically on the granule cells.

Auditory responses in the posterior vermis of the cat: the buried cerebellar cortex

Although an auditory area has been found over the exposed area of lobules VI-VIII in the posterior cerebellar vermis of the cat, there has been little information on its buried portion. This point should be of some concern, since very little of the brain tissue in lobules VI-VIII of the cerebellar cortex is exposed. We have systematically explored the posterior vermis with long electrode tracks and compared auditory responses recorded from neurons within the granule cell layer in the folial crown with those in the buried cortex. There was a similarity between neural activities recorded from the exposed surface area and those from the buried portion both in the temporal discharge pattern of neurons to sound and in the spatial distribution pattern of auditory responses.

Nitrous oxide inhibits auditory and visual responses of granule cells in the cerebellum

Much of the laboratory investigation on the auditory and visual areas of the posterior vermis has been carried out under barbiturate anesthesia. It is now known that barbiturates potentiate GABA inhibition by binding directly to the GABA receptor protein. Since GABAergic receptors are present in many cell types of the cerebellar cortex, barbiturate anesthesia is likely to interfere severely with cerebellar physiology. We have examined auditory and visual responses in granule cells in the cerebellum of the cat under nitrous oxide anesthesia. To our surprise, nitrous oxide abolished auditory as well as visual responses in the granule cell layer in the posterior vermis. However, both auditory and visual responses recovered after the cessation of nitrous oxide.

Auditory receptive area in the cerebellar hemisphere is surrounded by somatosensory areas

We mapped the neuronal discharges in response to sound in the granule cell layer of the cerebellar hemisphere of the rat. An auditory receptive area was located in the lateral part of Crus IIb. The size of the auditory area was approx. 1 mm2. It was surrounded by somatosensory receptive areas representing the regions in and around the mouth, particularly the lips, the incisors, and areas inside the mouth. Frequency selectivity of neurons in the auditory area was so broad that it resembled the audiogram of the ear of the animal. The auditory responses were not particularly sensitive to binaural intensity differences. On the basis of the response properties of these neurons to sound and the receptive field properties of the adjacent somatosensory areas, we suggest that the function of the auditory area in the cerebellar hemisphere may be in the control of movements involved in vocalization.

Neuroanatomy of fragile X syndrome: the posterior fossa

The occurrence and specificity of posterior fossa abnormalities as measured from magnetic resonance images of the brain were investigated in a group of 14 males with fragile X syndrome and comparison groups consisting of 17 males with other causes of developmental disability and 18 males with normal IQs. The size of the posterior cerebellar vermis was significantly decreased and the fourth ventricle significantly increased in the group of males with fragile X syndrome compared with males in both comparison groups. These neuroanatomical abnormalities appeared to be secondary to hypoplasia rather than atrophy.

Target areas of presumed auditory projections from lateral and dorsolateral pontine nuclei to posterior cerebellar vermis in rat

Small injections of the retrograde tracer horseradish peroxidase were made into lobules V through X in the posterior cerebellar vermis of the rat. Labeled cells were counted in the auditory subdivisions of the pontine nuclei, the lateral and dorsolateral pontine nuclei. The results suggest that, within the posterior vermis, lobules VI, VII, and VIII together receive over 90% of projections from the lateral and dorsolateral pontine nuclei.

Interactions of visual and auditory mossy fiber inputs in the paraflocculus of the rat: a gating action of multimodal inputs

We have demonstrated previously that visual and auditory inputs reach the rat paraflocculus via dorsolateral pontine gray from the secondary regions of the visual and auditory cortices. It has also been demonstrated that mossy fiber responses can be evoked to physiological acoustic stimuli in unanesthetized preparations. In this study, we investigated the interaction of auditory and visual inputs in the paraflocculus. Activity of parafloccular neurons was recorded in immobilized, locally anesthetized hooded rats. Selected images and tones were presented to animals. Orientation, position, and velocity of visual stimuli and different parameters of acoustic stimuli were controlled by a computer. Visual and auditory stimuli were also presented in combination or in different temporal sequences. We discovered that visual and auditory stimuli have coextensive termination zones in paraflocculus, and influence the same neurons in 60% of the cases. Combined auditory and visual stimuli produced synergistic responses in parafloccular neurons in comparison with single modality stimuli. Augmentation of responses could be observed even when one of the stimuli was subthreshold and did not alter per se the neuronal activity. Our findings suggest that within the cerebro-cerebellar system, subthreshold inputs are capable of powerful control over the neuronal activity and may alter responses to subsequent stimuli when properly biased by inputs from paired modalities.

Organization of the auditory area in the posterior cerebellar vermis of the cat

We mapped the topographic distribution of auditory responses in the posterior cerebellar vermis of the cat under barbiturate anesthesia. Auditory neurons in the granule cell layer of lobules VI and VII appeared to be arranged in columns perpendicular to the surface of the cerebellar cell layers. Mapping the surface of the cerebellum, auditory responses were found as separated patches of the order of a square millimeter. Neurons on these patches responded to auditory stimuli but neurons between patches did not respond to sound. In decerebrated cats, the entire granule cell layer within the cerebellar auditory area responded to acoustic stimulation without a patchy pattern. Responses to tonal stimuli from single neurons in the granule cell layer were studied before and after the induction of barbiturate anesthesia. Some neurons showed no change in their responses to sound before and under barbiturate. But other neurons showed dramatically attenuated responses or essentially stopped responding as a result of barbiturate anesthesia. These results suggest that there may be two types of granule cells distinguishable in their auditory responses and therefore possibly in function.