Differences in responses to 70 dB clicks of cerebellar units with simple versus complex spike activity: (i) in medial and lateral ansiform lobes and flocculus; and (ii) before and after conditioning blink conditioned responses with clicks as conditioned stimuli

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.

Time course of classically conditioned Purkinje cell response is determined by initial part of conditioned stimulus

Classical conditioning of a motor response such as eyeblink is associated with the development of a pause in cerebellar Purkinje cell firing that is an important driver of the overt response. This conditioned Purkinje cell response is adaptively timed and has a specific temporal profile that probably explains the time course of the overt behavior. It is generally assumed that the temporal properties of the conditioned Purkinje cell response are determined by the temporal pattern of the parallel fiber impulses generated by the conditioned stimulus at the time of the conditioned response. We show here in the decerebrate ferret preparation that a very brief conditioned stimulus, consisting of only one or two impulses in the mossy fibers, can be sufficient to elicit a full conditioned Purkinje cell response with normal time course. The finding suggests that parallel fiber input to the Purkinje cell influences the firing rate several hundred milliseconds later. It poses a serious challenge to the standard view of the role of parallel fiber impulses in response timing.

Endogenous neuromagnetic activity for mental hierarchy of timing

The frontal-striatal circuits, the cerebellum, and motor cortices play crucial roles in processing timing information on second to millisecond scales. However, little is known about the physiological mechanism underlying human's preference to robustly encode a sequence of time intervals into a mental hierarchy of temporal units called meter. This is especially salient in music: temporal patterns are typically interpreted as integer multiples of a basic unit (i.e., the beat) and accommodated into a global context such as march or waltz. With magnetoencephalography and spatial-filtering source analysis, we demonstrated that the time courses of neural activities index a subjectively induced meter context. Auditory evoked responses from hippocampus, basal ganglia, and auditory and association cortices showed a significant contrast between march and waltz metric conditions during listening to identical click stimuli. Specifically, the right hippocampus was activated differentially at 80 ms to the march downbeat (the count one) and approximately 250 ms to the waltz downbeat. In contrast, basal ganglia showed a larger 80 ms peak for march downbeat than waltz. The metric contrast was also expressed in long-latency responses in the right temporal lobe. These findings suggest that anticipatory processes in the hippocampal memory system and temporal computation mechanism in the basal ganglia circuits facilitate endogenous activities in auditory and association cortices through feedback loops. The close interaction of auditory, motor, and limbic systems suggests a distributed network for metric organization in temporal processing and its relevance for musical behavior.

Synaptic shunting by a baseline of synaptic conductances modulates responses to inhibitory input volleys in cerebellar Purkinje cells

When processing synaptic input in vivo, large neurons in the brain must cope with thousands of events each second. Much work has focused on the specific processing of synchronous excitatory input volleys, both in cerebellar and cerebral cortical research. Here we pursue the question of how a continuous background of ongoing 'noise' inputs interacts with the processing of synchronous inhibitory input volleys. Specifically we examine the processing of inhibitory input transients in cerebellar Purkinje cells, which by inducing pauses in Purkinje cell spike activity may lead to a disinhibition of the deep cerebellar nuclei and thus to cerebellar motor command signals. We use the technique of dynamic clamping in vitro to simulate controlled patterns of in vivo like background inputs. We use electrical stimulation of inhibitory interneurons in the deep or upper molecular layer to create inhibitory input transients that lead to spike pauses in Purkinje cell activity. These pauses were much longer in the absence than in the presence of background inputs applied with dynamic clamping. We found that a significant amount of the synaptic current elicited by electrical stimulation was shunted by the background inputs. The overall amount of background conductance as well as the pattern of background inputs modulated spike pause duration in a specific manner. This modulation by shunting may be employed in vivo to evaluate the salience of specific sensory input received by cerebellar cortex.

Identification of differently timed motor components of conditioned blink responses

Electromyographic recordings were made from the orbicularis oculi muscles of cats in order to identify differently timed motor components of conditioned eye blink responses (CRs). Conditioning was established rapidly by pairing electrical stimulation of the hypothalamus (HS) with a click conditioned stimulus (CS) and a glabella tap unconditioned stimulus (US). Analysis of the EMG responses disclosed five different motor components of the CR that could be distinguished and characterized according to their latencies of occurrence. Four were associated with an increase in EMG activity elicited by the CS (16-48 ms, alpha(1); 48-80 ms, alpha(2); 80 to 120 ms, beta; >/=120 ms, gamma), and one was associated with a decrease in activity (16 to 60 ms, alpha(i)). Analysis of the amplitudes of the different components of the CR during the course of conditioning and extinction disclosed that short latency, alpha(1) components of the CRs were acquired and extinguished in a manner equivalent to longer latency components of the CRs. The observations supported the hypothesis that short and long latency components of blink responses represented comparable rather than substantially different forms of Pavlovian conditioning. The alpha(2) response was present before conditioning began, and increased with other components after conditioning. The alpha(i) response component was also observed prior to conditioning, and represents a previously undetected, inhibitory consequence of presenting weak (70 dB) acoustic stimuli. It could play a role in conditioned inhibition, latent inhibition and blocking as well as suppression of the conditioned motor response during extinction.