A long-range, wide field-of-view infrared eyeblink detector

Relationship Between Ocular Surface Epithelial Damage, Tear Abnormalities, and Blink in Patients With Dry Eye

PURPOSE

Considering that tears play the role of a lubricant, it is speculated that in the pathophysiology of dry eye, increased friction during blinking results in corneal and conjunctival damage, which may subsequently affect the blink. The purpose of this study was to investigate the relationship between ocular surface epithelial damage, tear abnormalities, and blinks in patients with dry eye.

METHODS

This study involved 45 eyes of 45 female patients with dry eye (mean age: 57.6 years). In all eyes, tear meniscus radius (mm), spread grade of the tear film lipid layer (SG: 1-5: 1 being the best), fluorescein breakup time (FBUT, seconds), corneal and bulbar conjunctival epithelial damage (CED: 15 points maximum and CONJUNCTIVAL EPITHELIAL DAMAGE (CjED): 6 points maximum, respectively), and Schirmer I test (ST1, mm) were evaluated. Blink rate (BR, blinks per minute), palpebral aperture height (mm), upper-eyelid opening-phase amplitude/upper-eyelid closing-phase amplitude (mm), upper-eyelid opening-phase duration/upper-eyelid closing-phase duration (ms), and upper-eyelid opening-phase maximum velocity/upper-eyelid closing-phase maximum velocity (mm/s) were measured using a custom-made high-speed blink analyzer. Finally, the factors that determine CED and CjED were investigated by multiple regression analysis, in which the parameters were chosen using the stepwise procedure.

RESULTS

CED and CjED were found to be described as 2.687 + (1.816 × SG) - (0.937 × FBUT) (R = 0.656, P < 0.0001) and 0.684 + (0.801 × SG) - (0.526 × FBUT) - (0.041 × ST1) + (0.010 × upper-eyelid closing-phase maximum velocity) (R = 0.714, P < 0.0001), respectively.

CONCLUSIONS

Although CED was significantly related to only tear abnormalities, CjED was significantly related to tear abnormalities and blinking.

Cerebellar implementation of movement sequences through feedback

Most movements are not unitary, but are comprised of sequences. Although patients with cerebellar pathology display severe deficits in the execution and learning of sequences (Doyon et al., 1997; Shin and Ivry, 2003), most of our understanding of cerebellar mechanisms has come from analyses of single component movements. Eyelid conditioning is a cerebellar-mediated behavior that provides the ability to control and restrict inputs to the cerebellum through stimulation of mossy fibers. We utilized this advantage to test directly how the cerebellum can learn a sequence of inter-connected movement components in rabbits. We show that the feedback signals from one component are sufficient to serve as a cue for the next component in the sequence. In vivo recordings from Purkinje cells demonstrated that all components of the sequence were encoded similarly by cerebellar cortex. These results provide a simple yet general framework for how the cerebellum can use simple associate learning processes to chain together a sequence of appropriately timed responses.

Imagine a gymnastics competition in which participants take turns to cartwheel and somersault across the floor. The routines on display comprise sequences of precisely timed movements learned through practice. This is also true for many of the actions we perform every day, such as reaching for a cup of coffee. A region of the brain called the cerebellum helps us learn sequences of movements. But how does it do this?

To find out, Khilkevich et al. came up with a new version of an old experiment. Rabbits were first trained to blink their eye in response to a specific external cue. This type of learning, called associative learning, has been shown before in the cerebellum. But Khilkevich et al. wondered whether the cerebellum could also use internal feedback signals from the eyeblink as a cue to learn the next movement? If so, this might explain how the cerebellum can chain movements together in a sequence.

As predicted, Khilkevich et al. found that rabbits could learn to blink their eye in response to an initial signal, and then blink again in response to the first blink. Control experiments confirmed that the second eyeblink was coupled to the first, and not to the original cue. Moreover, on many trials the rabbits showed a third and even fourth eyeblink. This is because feedback signals from the first, second or third blink were the same. Thus, the feedback signals from the first blink triggered the second blink, feedback from the second triggered the third, and so forth. Rabbits could also learn to use a blink of the left eye as a cue for a blink of the right eye. Similar patterns of neuronal activity accompanied each blink, suggesting that the same mechanism generated them all.

The cerebellum can thus use feedback from one movement as a cue to learn the proper timing of the next movement in a sequence. A key question is whether this mechanism of sequence learning extends beyond movement. The cerebellum has extensive connections to the brain’s outer layer, the cortex, including many areas involved in cognition. Future experiments should test whether the cerebellum might help guide sequences of cortical activity during cognitive tasks.

A cerebellar adaptation to uncertain inputs

Noise and variability are inherent and unavoidable features of neural processing. Despite this physiological challenge, brain systems function well, suggesting the existence of adaptations that cope with noise. We report a novel adaptation that the cerebellum implements to maintain correct responses in the face of ambiguous inputs. We found that under these conditions, the cerebellum used a probabilistic binary choice: Although the probability of behavioral response gradually increased or decreased depending on the degree of similarity between current and trained inputs, the size of response remained constant. That way the cerebellum kept responses adaptive to trained input corrupted by noise while minimizing false responses to novel stimuli. Recordings and analysis of Purkinje cells activity showed that the binary choice is made in the cerebellar cortex. Results from large-scale simulation suggest that internal feedback from cerebellar nucleus back to cerebellar cortex plays a critical role in implementation of binary choice.

Links Between Single-Trial Changes and Learning Rate in Eyelid Conditioning

The discovery of single-trial learning effects, where the presence or absence (or the number) of climbing fiber inputs produces measureable changes in Purkinje cell response and in behavior, represents a major breakthrough in cerebellar learning. Among other things, these observations provide strong links between climbing fiber-mediated plasticity and cerebellar learning. They also demonstrate that cerebellar learning is stochastic, with each instantiation of a movement producing a small increment or decrement in gain. The sum of the small changes give rise to the macroscopic properties of cerebellar learning. We used a relatively large data set from another example of cerebellar-dependent learning, classical conditioning of eyelid responses, to attempt a behavioral replication and extension of single-trial learning effects. As a normal part of training, stimulus-alone trials provide instances where the climbing fiber response would be omitted, similar to non-climbing-fiber trials (gain down) during smooth pursuit training. The consequences of the stimulus-alone trial on the amplitude and timing of the conditioned response on the following paired trials were examined. We find that the amplitude of the conditioned response during the trial after a stimulus-alone trial (no climbing fiber input) was measurably smaller than the amplitude on the previous trials, and this single-trial effect on amplitude is larger for longer interstimulus intervals. The magnitude of the single-trial effect parallels the rate of extinction at different interstimulus intervals supporting the previously observed link between single-trial effects and learning.

Relating cerebellar purkinje cell activity to the timing and amplitude of conditioned eyelid responses

How Purkinje cell (PC) activity may be altered by learning is central to theories of the cerebellum. Pavlovian eyelid conditioning, because of how directly it engages the cerebellum, has helped reveal many aspects of cerebellar learning and the underlying mechanisms. Theories of cerebellar learning assert that climbing fiber inputs control plasticity at synapses onto PCs, and thus PCs control the expression of learned responses. We tested this assertion by recording 184 eyelid PCs and 240 non-eyelid PCs during the expression of conditioned eyelid responses (CRs) in well trained rabbits. By contrasting the responses of eyelid and non-eyelid PCs and by contrasting the responses of eyelid PCs under conditions that produce differently timed CRs, we test the hypothesis that learning-related changes in eyelid PCs contribute to the learning and adaptive timing of the CRs. We used a variety of analyses to test the quantitative relationships between eyelid PC responses and the kinematic properties of the eyelid CRs. We find that the timing of eyelid PC responses varies systematically with the timing of the behavioral CRs and that there are differences in the magnitude of eyelid PC responses between larger-CR, smaller-CR, and non-CR trials. However, eyelid PC activity does not encode any single kinematic property of the behavioral CRs at a fixed time lag, nor does it linearly encode CR amplitude. Even so, the results are consistent with the hypothesis that learning-dependent changes in PC activity contribute to the adaptively timed expression of conditioned eyelid responses.

Infrared-based blink-detecting glasses for facial pacing: toward a bionic blink

IMPORTANCE Facial paralysis remains one of the most challenging conditions to effectively manage, often causing life-altering deficits in both function and appearance. Facial rehabilitation via pacing and robotic technology has great yet unmet potential. A critical first step toward reanimating symmetrical facial movement in cases of unilateral paralysis is the detection of healthy movement to use as a trigger for stimulated movement. OBJECTIVE To test a blink detection system that can be attached to standard eyeglasses and used as part of a closed-loop facial pacing system. DESIGN, SETTING, AND PARTICIPANTS Standard safety glasses were equipped with an infrared (IR) emitter-detector unit, oriented horizontally across the palpebral fissure, creating a monitored IR beam that became interrupted when the eyelids closed, and were tested in 24 healthy volunteers from a tertiary care facial nerve center community. MAIN OUTCOMES AND MEASURES Video-quantified blinking was compared with both IR sensor signal magnitude and rate of change in healthy participants with their gaze in repose, while they shifted their gaze from central to far-peripheral positions, and during the production of particular facial expressions. RESULTS Blink detection based on signal magnitude achieved 100% sensitivity in forward gaze but generated false detections on downward gaze. Calculations of peak rate of signal change (first derivative) typically distinguished blinks from gaze-related eyelid movements. During forward gaze, 87% of detected blink events were true positives, 11% were false positives, and 2% were false negatives. Of the 11% false positives, 6% were associated with partial eyelid closures. During gaze changes, false blink detection occurred 6% of the time during lateral eye movements, 10% of the time during upward movements, 47% of the time during downward movements, and 6% of the time for movements from an upward or downward gaze back to the primary gaze. Facial expressions disrupted sensor output if they caused substantial squinting or shifted the glasses. CONCLUSIONS AND RELEVANCE Our blink detection system provides a reliable, noninvasive indication of eyelid closure using an invisible light beam passing in front of the eye. Future versions will aim to mitigate detection errors by using multiple IR emitter-detector units mounted on glasses, and alternative frame designs may reduce shifting of the sensors relative to the eye during facial movements.

Blocking glutamate-mediated inferior olivary signals abolishes expression of conditioned eyeblinks but does not prevent their acquisition

The inferior olive (IO) is considered a crucial component of the eyeblink conditioning network. The cerebellar learning hypothesis proposes that the IO provides the cerebellum with a teaching signal that is required for the acquisition and maintenance of conditioned eyeblinks. Supporting this concept, previous experiments showed that lesions or inactivation of the IO blocked CR acquisition. However, these studies were not conclusive. The drawback of the methods used by those studies is that they not only blocked task-related signals, but also completely shut down the spontaneous activity within the IO, which affects the rest of the eyeblink circuits in a nonspecific manner. We hypothesized that more selective blocking of task-related IO signals could be achieved by using injections of glutamate antagonists, which reduce, but do not eliminate, the spontaneous activity in the IO. We expected that if glutamate-mediated IO signals are required for learning, then blocking these signals during training sessions should prevent conditioned response (CR) acquisition. To test this prediction, rabbits were trained to acquire conditioned eyeblinks to a mild vibrissal airpuff as the conditioned stimulus while injections of the glutamate antagonist γ-d-glutamylglycine were administered to the IO. Remarkably, even though this treatment suppressed CRs during training sessions, the postacquisition retention test revealed that CR acquisition had not been abolished. The ability to acquire CRs with IO unconditioned stimulus signals that were blocked or severely suppressed suggests that mechanisms responsible for CR acquisition are extremely resilient and probably less dependent on IO-task-related signals than previously thought.

Disrupting neural activity related to awake-state sharp wave-ripple complexes prevents hippocampal learning

Oscillations in hippocampal local-field potentials (LFPs) reflect the crucial involvement of the hippocampus in memory trace formation: theta (4–8 Hz) oscillations and ripples (~200 Hz) occurring during sharp waves are thought to mediate encoding and consolidation, respectively. During sharp wave-ripple complexes (SPW-Rs), hippocampal cell firing closely follows the pattern that took place during the initial experience, most likely reflecting replay of that event. Disrupting hippocampal ripples using electrical stimulation either during training in awake animals or during sleep after training retards spatial learning. Here, adult rabbits were trained in trace eyeblink conditioning, a hippocampus-dependent associative learning task. A bright light was presented to the animals during the inter-trial interval (ITI), when awake, either during SPW-Rs or irrespective of their neural state. Learning was particularly poor when the light was presented following SPW-Rs. While the light did not disrupt the ripple itself, it elicited a theta-band oscillation, a state that does not usually coincide with SPW-Rs. Thus, it seems that consolidation depends on neuronal activity within and beyond the hippocampus taking place immediately after, but by no means limited to, hippocampal SPW-Rs.

Assessing the role of inferior olivary sensory signaling in the expression of conditioned eyeblinks using a combined glutamate/GABAA receptor antagonist protocol

The inferior olive (IO) is a major component of the eyeblink conditioning neural network. The cerebellar learning hypothesis assumes that the IO supplies the cerebellum with a "teaching" unconditioned stimulus input required for the acquisition of the conditioned response (CR) and predicts that inactivating this input leads to the extinction of CRs. Previous tests of this prediction attempted to block the teaching input by blocking glutamatergic sensory inputs in the IO. These tests were inconclusive because blocking glutamate neurotransmission in the IO produces a nonspecific tonic malfunction of cerebellar circuits. The purpose of the present experiment was to examine whether the behavioral outcomes of blocking glutamate receptors in the IO could be counterbalanced by reducing GABA-mediated inhibition in the IO. We found that injecting the IO with the glutamate antagonist γ-d-glutamylglycine (DGG) abolished previously learned CRs, whereas injecting the GABA(A) receptor antagonist gabazine at the same site did not affect CR incidence but shortened CR latencies and produced tonic eyelid closure. To test whether the glutamate antagonist-induced behavioral deficit could be offset by elevating IO activity with GABA(A) antagonists, rabbits were first injected with DGG and then with gabazine in the same training session. While DGG abolished CRs, follow-up injections of gabazine accelerated their recovery. These findings suggest that the level of IO neuronal activity is critical for the performance of CRs, and that combined pharmacological approaches that maintain spontaneous activity at near normal levels hold tremendous potential for unveiling the role of IO-mediated signals in eyeblink conditioning.

A trigeminal conditioned stimulus yields fast acquisition of cerebellum-dependent conditioned eyeblinks

Classical conditioning of the eyeblink response in the rabbit is a form of motor learning whereby the animal learns to respond to an initially irrelevant conditioned stimulus (CS). It is thought that acquired conditioned responses (CRs) are adaptive because they protect the eye in anticipation of potentially harmful events. This protective mechanism is surprisingly inefficient because the acquisition of CRs requires extensive training - a condition that is unlikely to occur in nature. We hypothesized that the rate of conditioning in rabbits could depend on CS modality and that stimulating mystacial vibrissae as the CS could produce CR acquisition faster than the traditional auditory or visual stimulation. We tested this hypothesis by conditioning naïve rabbits in the delay paradigm using a weak airpuff CS (vCS) directed to the ipsilateral mystacial vibrissae. We found that the trigeminal vCS yields significantly faster CR acquisition. We next examined if vCS-evoked CRs are dependent on the intermediate cerebellum in the same fashion as CRs evoked by the traditional auditory CS. We found that vibrissal CRs could be abolished by inactivating the cerebellar interposed nuclei (IN) with muscimol. In addition, injections of picrotoxin in the IN shortened the onset latency of vibrissal CRs. These findings suggest that the tone and vCS-evoked CRs share similar cerebellar dependency.

Spontaneous eyeblink activity

Spontaneous blinking is essential for maintaining a healthy ocular surface and clarity of vision. The spontaneous blink rate (SBR) is believed to reflect a complex interaction between peripheral influences mediated by the eye surface and the central dopaminergic activity. The SBR is thus extremely variable and dependent on a variety of psychological and medical conditions. Many different methods have been employed to measure the SBR and the upper eyelid kinematics during a blink movement. Each has its own merits and drawbacks, and the choice of a specific method should be tailored to the specific needs of the investigation. Although the sequence of muscle events that leads to a blink has been fully described, knowledge about the neural control of spontaneous blinking activity is not complete. The tear film is dynamically modified between blinks, and abnormalities of the blink rate have an obvious influence on the ocular surface.

Hippocampal theta activity is selectively associated with contingency detection but not discrimination in rabbit discrimination-reversal eyeblink conditioning

The relative power of the hippocampal theta-band ( approximately 6 Hz) activity (theta ratio) is thought to reflect a distinct neural state and has been shown to affect learning rate in classical eyeblink conditioning in rabbits. We sought to determine if the theta ratio is mostly related to the detection of the contingency between the stimuli used in conditioning or also to the learning of more complex inhibitory associations when a highly demanding delay discrimination-reversal eyeblink conditioning paradigm is used. A high hippocampal theta ratio was not only associated with a fast increase in conditioned responding in general but also correlated with slow emergence of discriminative responding due to sustained responding to the conditioned stimulus not paired with an unconditioned stimulus. The results indicate that the neural state reflected by the hippocampal theta ratio is specifically linked to forming associations between stimuli rather than to the learning of inhibitory associations needed for successful discrimination. This is in line with the view that the hippocampus is responsible for contingency detection in the early phase of learning in eyeblink conditioning.

Inactivating the middle cerebellar peduncle abolishes the expression of short-latency conditioned eyeblinks

The interposed nuclei (IN) of the cerebellum play a crucial role in the classically conditioned eyeblink circuit. It has previously been shown in well-trained animals that injecting the IN with GABA(A) antagonists produces short-latency conditioned responses (SLRs). The mechanism underlying SLR generation is not clear. According to one concept, SLRs originate in cerebellar nuclei in response to direct inputs from collaterals of mossy fibers. An alternate explanation is that SLRs are produced by extra-cerebellar circuits that are excited by increased tonic activity in cerebellar nuclei or by the combined action of inputs to cerebellar nuclei from mossy fiber collaterals and incompletely blocked Purkinje cells. In the present study, we examined whether cerebellar afferent axons in the middle cerebellar peduncle (MCP) participate in SLR expression. We hypothesized that if SLRs are evoked by the sensory mossy fiber input to the IN and cerebellar cortex, then blocking the MCP should abolish these responses. Well-trained animals, which had been implanted with dual injection cannulae in the left IN and the left MCP, were injected with gabazine (GZ) into the IN to produce SLRs followed by an injection of the sodium channel blocker tetrodotoxin (TTX) into the MCP. TTX infusions in the MCP suppressed both CRs and SLRs. These findings suggest that the expression of SLRs depends on both direct and cerebellar cortex-mediated sensory information from the mossy fiber system.

Evidence for facial nerve-independent mechanisms of blinking in the rat

PURPOSE

The rat facial nerve (CN VII) controls the orbicularis oculi (OO) muscle, which contracts to close the palpebral fissure during blinking. It was recently observed that rats are able to achieve nearly complete eye closure shortly after CN VII lesion, and hypothesized that the retractor bulbi (RB) muscle assumes an important compensatory role after CN VII lesion. This study was undertaken to determine the maintenance of rat corneal health and eye closure capability after lesion of the OO, RB, or both.

METHODS

Twenty-two rats underwent RB transection; 12 of them had undergone complete unilateral CN VII transection (OO denervation) 15 weeks earlier. Corneal appearance and ability to blink in response to a corneal air puff was monitored weekly for 9 weeks. An additional 13 rats received CN VII transection and were video recorded (1000 frames/s) during elicited blinks at days 1, 3, 5/6, and 11 after surgery.

RESULTS

Rats achieved nearly full or full eye closure after OO paralysis or RB myotomy, respectively. Ninety-two percent of rats maintained good corneal health after OO denervation over 9 weeks, consistent with compensatory eyelid movement served by the RB muscles. In contrast, only 40% of rats with loss of RB function alone and only 17% of rats with concurrent OO and RB paralysis were able to maintain corneal health by week 3.

CONCLUSIONS

Like other small mammals, the rat RB musculature can support nearly complete eye closure when CN VII is lesioned, and must be carefully considered when using blink as a functional recovery parameter of facial nerve lesion.

Blocking GABAA neurotransmission in the interposed nuclei: effects on conditioned and unconditioned eyeblinks

The interposed nuclei (IN) of the intermediate cerebellum are critical components of the circuits that control associative learning of eyeblinks and other defensive reflexes in mammals. The IN, which represent the sole output of the intermediate cerebellum, receive massive GABAergic input from Purkinje cells of the cerebellar cortex and are thought to contribute to the acquisition and performance of classically conditioned eyeblinks. The specific role of deep cerebellar nuclei and the cerebellar cortex in eyeblink conditioning are not well understood. One group of studies reported that blocking GABA(A) neurotransmission in the IN altered the time profile of conditioned responses (CRs), suggesting that the main function of the cerebellar cortex is to shape the timing of CRs. Other studies reported that blocking GABA(A) neurotransmission in the IN abolished CRs, indicating a more fundamental involvement of the cerebellar cortex in CR generation. When examining this controversy, we hypothesized that the behavioral effect of GABA(A) blockers could be dose-dependent. The IN of classically conditioned rabbits were injected with high and low doses of picrotoxin and gabazine. Both GABA(A) blockers produced tonic eyelid closure. A high dose of both drugs abolished CRs, whereas a less complete block of GABA(A)-mediated inputs with substantially smaller drug doses shortened CR latencies. In addition, low doses of picrotoxin facilitated the expression of unconditioned eyeblinks evoked by trigeminal stimulation. These results suggest that the intermediate cerebellum regulates both associative and non-associative components of the eyeblink reflex, and that behavioral effects of blocking Purkinje cell action on IN neurons are related to collective changes in cerebellar signals and in the excitability of extra-cerebellar eyeblink circuits.

A system for studying facial nerve function in rats through simultaneous bilateral monitoring of eyelid and whisker movements

The occurrence of inappropriate co-contraction (synkinesis) of facially innervated muscles in humans is a common sequela of facial nerve injury and recovery. We have developed a system for studying facial nerve function and synkinesis in restrained rats using non-contact opto-electronic techniques that enable simultaneous bilateral monitoring of eyelid and whisker movements. Whisking is monitored in high spatio-temporal resolution using laser micrometers, and eyelid movements are detected using infrared diode and phototransistor pairs that respond to the increased reflection when the eyelids cover the cornea. To validate the system, 8 rats were tested with multiple 5-min sessions that included corneal air puffs to elicit blink and scented air flows to elicit robust whisking. Four rats then received unilateral facial nerve section and were tested at weeks 3-6. Whisking and eye blink behavior occurred both spontaneously and under stimulus control, with no detectable difference from published whisking data. Proximal facial nerve section caused an immediate ipsilateral loss of whisking and eye blink response, but some ocular closures emerged due to retractor bulbi muscle function. The independence observed between whisker and eyelid control indicates that this system may provide a powerful tool for identifying abnormal co-activation of facial zones resulting from aberrant axonal regeneration.

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.

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.

Inactivation of cerebellar output axons impairs acquisition of conditioned eyeblinks

Acquisition of classically conditioned eyeblink responses (CRs) in the rabbit critically depends on intermediate cerebellum-related neural circuits. A highly efficient method for determining possible sites of plasticity within eyeblink circuits is the reversible inactivation of circuit components during learning. Inactivation of either the HVI region of the cerebellar cortex or the cerebellar interposed nuclei (IN) during learning is known to prevent CR acquisition. In contrast, inactivating cerebellar efferent axons in the brachium conjunctivum (BC) with small injections of tetrodotoxin (TTX) has been reported to have no effect on CR acquisition. This suggested that the intermediate cerebellum is essential for learning CRs and that activity mediated by the BC is not required for this process. Since we previously found that BC inactivation blocks CR extinction we re-examined its role in CR acquisition. To ensure complete and long-lasting inactivation of the BC, we injected before each training session doses of TTX that were larger than those in the previous acquisition study. Contrary to the previous negative findings, we found that this temporary block of axons in the brachium conjunctivum prevented normal acquisition of CRs. Injecting TTX directly in the adjacent lateral lemniscus, which could possibly influence CR acquisition, had no effect on learning. In addition, a functional test of TTX diffusion around the BC indicated that the inactivation did not affect other known parts of eyeblink circuits, such as the cerebellar interposed nuclei, the middle cerebellar peduncle or the contralateral red nucleus. We conclude that this form of associative learning in the rabbit eyeblink system requires extra-cerebellar learning and/or cerebellar learning that depends on the operation of cerebellar feedback loops.