Classically conditioned withdrawal reflex in cerebellar patients. 2. Impaired unconditioned responses

Comparison of the classically conditioned withdrawal reflex in cerebellar patients and healthy control subjects during stance: 2. Biomechanical characteristics

This study addresses cerebellar involvement in classically conditioned nociceptive lower limb withdrawal reflexes in standing humans. A preceding study compared electromyographic activities in leg muscles of eight patients with cerebellar disease (CBL) and eight age-matched controls (CTRL). The present study extends and completes that investigation by recording biomechanical signals from a strain-gauge-equipped platform during paired auditory conditioning stimuli (CS) and unconditioned stimuli (US) trials and during US-alone trials. The withdrawal reflex performance-lifting the stimulated limb (decreasing the vertical force from that leg, i.e. 'unloading') and transferring body weight to the supporting limb (increasing the vertical force from that leg, i.e. 'loading')-was quantified by the corresponding forces exerted onto the platform. The force changes were not simultaneous but occurred as a sequence of multiple force peaks at different times depending on the specific limb task (loading or unloading). Motor learning, expressed by the occurrence of conditioned responses (CR), is characterized by this sequence beginning already within the CSUS window. Loading and unloading were delayed and prolonged in CBL, resulting in incomplete rebalancing during the analysis period. Trajectory loops of the center of vertical pressure-derived from vertical forces-were also incomplete in CBL within the recording period. However, exposing CBL to a CS resulted in motor improvement reflected by shortening the time of rebalancing and by optimizing the trajectory loop. In summary, associative responses in CBL are not absent although they are less frequent and of smaller amplitude than in CTRL.

Comparison of the classically conditioned withdrawal reflex in cerebellar patients and healthy control subjects during stance: I. electrophysiological characteristics

The aim of this study was to demonstrate the involvement of the human cerebellum in the classically conditioned lower limb withdrawal reflex in standing subjects. Electromyographic activity was recorded from the main muscle groups of both legs of eight patients with cerebellar disease (CBL) and eight control subjects (CTRL). The unconditioned stimulus (US) consisted of electrical stimulation of the tibial nerve at the medial malleolus. The conditioning stimulus (CS) was an auditory signal given via headphones. Experiments started with 70 paired conditioning stimulus-unconditioned stimulus(CSUS) trials followed by 50 US-alone trials. The general reaction consisted of lifting and flexing the stimulated (stepping) leg with accompanying activation of the contralateral (supporting) leg. In CTRL, the ipsilateral (side of stimulation) flexor and contralateral extensor muscles were activated characteristically. In CBL, the magnitudes of ipsilateral flexor and contralateral extensor muscle activation were reduced comparably. In CTRL, the conditioning process increased the incidence of conditioned responses (CR), following a typical learning curve, while CBL showed a clearly lower CR incidence with a marginal increase, albeit, at a shorter latency. Conditioning processes also modified temporal parameters by shortening unconditioned response (UR) onset latencies and UR times to peak and, more importantly in CBL, also the sequence of activation of muscles, which became similar to that of CTRL. The expression of this reflex in standing subjects showed characteristic differences in the groups tested with the underlying associative processes not being restricted exclusively to the CR but also modifying parameters of the innate UR.

Differences in unconditioned and conditioned responses of the human withdrawal reflex during stance: muscle responses and biomechanical data

The aim of this study was to characterize differences between unconditioned and classically conditioned lower limb withdrawal reflexes in young subjects during standing. Electromyographic activity in the main muscle groups and biomechanical signals from a strain-gauge-equipped platform on which subjects stood were recorded from 17 healthy subjects during unconditioned stimulus (US)-alone trials and during auditory conditioning stimuli (CS) and US trials. In US-alone trials the leg muscle activation sequence was characteristic: ipsilateral, distal muscles were activated prior to proximal muscles; contralaterally the sequence was reversed. In CSUS trials latencies were shorter. Subjects unloaded the stimulated leg and shifted body weight to the supporting leg. In US-alone and in CSUS trials leg forces on each side were inversely related and asymmetric, due to preparation for unloading, whilst conditioned responses (CR), representing the unloading preparation, were symmetric. The trajectory of the center of vertical pressure during US-alone trials moved initially forward (a preparatory balance reaction) and to the stimulation side, followed by a large lateral shift to the side of the supporting limb. During CSUS trials the forwards shift was absent but the CR (early lateral shift) represented a preponed preparatory unloading. Electrophysiological and biomechanical responses of the classically conditioned lower limb withdrawal reflex in standing subjects changed significantly in CSUS trials compared to US-alone trials with higher sensitivity in the biomechanics. These findings will serve as a basis for a subsequent study on a group of patients with cerebellar diseases in whom the success of establishing procedural processes is known to be impaired.

Comparison of the electrically evoked leg withdrawal reflex in cerebellar patients and healthy controls

The aim of this study was to analyze the contribution of the cerebellum in the performance of the lower limb withdrawal reflexes. This has been accomplished by comparing the electrically evoked responses in cerebellar patients (CBL) with those in sex- and age-matched healthy control subjects (CTRL). The stimulus was applied to the subjects' medial plantar nerve in four blocks of ten trials each with switching the stimulus from one leg to the other after each block. Responses of the main muscle groups (tibial muscle: TA; gastrocnemius muscle: GA; rectus femoris muscle: RF; biceps femoris muscle: BI) of both legs were recorded during each stimulus. The group of CBL patients consisted of both focally lesioned patients (CBLf) and patients presenting a diffuse degenerative pathology (CBLd). (1) For the withdrawal reflex in CTRL subjects, responses were observed in distal and proximal muscles of the ipsilateral side and corresponding concomitant responses on the side contralateral to the stimulation, whereas in CBL patients responses were restricted primarily to distal muscles, particularly the TA of the ipsilateral, i.e. the stimulated, side. (2) The sequence of activation of the different distal and proximal muscles ipsilateral to the stimulation, derived from latencies and times-to-peak, was for the CTRL group: TA-GA-BI-RF. This sequence was found also in the CBLf patients on their unaffected side. However, on their affected side CBLf patients showed very early GA activation, almost simultaneously with TA and RF activations and before BI activation. RF activation before BI activation was also found in CBLd. In the latter group, GA was activated after RF but before BI with all responses typically delayed. (3) The general pattern of the electrically evoked lower limb reflex consisted of an early, excitatory F1 component and a later, excitatory F2 component of larger amplitude observed in the CTRL subjects and the CBLd patients. In contrast to this pattern CBLf patients exhibited large F1 components followed by small F2 components. (4) The characteristic differences in the withdrawal reflex responses of cerebellar patients depended on the type of the lesion, providing evidence for an important involvement of the cerebellum in the control of the performance of withdrawal reflexes.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Classically conditioned postural reflex in cerebellar patients

The aim of the current study was to compare postural responses to repetitive platform-evoked perturbations in cerebellar patients with those of healthy subjects using a classical conditioning paradigm. The perturbations consisted of tilting of the platform (unconditioned stimulus: US) at random time intervals, preceded by an auditory signal that represented the conditioning stimulus (CS). Physiological reactions were recorded biomechanically by measuring the vertical ground forces, yielding the center of vertical pressure (CVP), and electrophysiologically by EMG measurements of the main muscle groups of both legs. The recording session consisted of a control section with US-alone trials, a testing section with paired stimuli and a brief final section with US-alone trials. Healthy control subjects were divided into those establishing conditioned responses (CR) in all muscles tested (strategy I) and those with CR in the gastrocnemius muscles only (strategy II), suggesting an associative motor-related process is involved. Patients with a diffuse, non-localized disease were almost unable to establish CR. This was also true for a patient with a focal surgical lesion with no CR on the affected side but who, simultaneously, showed an essentially normal CR incidence on the intact side. During US-alone trials healthy controls exhibited a remarkable decay of the UR amplitude due to a non-associative motor-related process such as habituation. The decay was most prominent in the paired trials section. In contrast, patients showed no significant differences in the UR amplitude throughout the entire recording session. Analysis of the CVP supported the electrophysiological findings, showing CR in the controls only. The differences between the responses of control subjects and those of the cerebellar patients imply strongly that the cerebellum is involved critically in controlling associative and non-associative motor-related processes.

Single-cue delay and trace classical conditioning in schizophrenia

BACKGROUND

Classical conditioning provides a means of addressing mechanisms of learning and can therefore help understand the pathophysiology of memory alteration in schizophrenia.

METHODS

Single cue delay and trace eyeblink conditioning were used in patients with schizophrenia and matched normal control subjects to explore, respectively, cerebellar and hippocampal integrity during learning. We measured percent of conditioned (CRs) and unconditioned responses (URs), their amplitude, and onset and peak latencies. We also accounted for spontaneous blink rates and stimulus-induced responses before learning.

RESULTS

During delay conditioning, patients showed CRs with longer onset and peak latencies and improved efficiency compared to normal volunteers without there being differences between patients and normal control subjects in the percentage of CRs. During trace conditioning, neither group showed an increase in CRs as a function of conditioned stimulus-unconditioned stimulus pairings, in part because the level of spontaneous blink rates exceeded the level of CRs; however, patients with schizophrenia showed increased responding 150-400 msec after the conditioned stimulus and in the last 100-150 msec before the unconditioned stimulus, whereas normal control subjects showed only the latter type of responses. The former type of response was more frequent in patients with schizophrenia even before either trace or delay conditioning.

CONCLUSIONS

These results suggest integrity of cerebellar mechanisms underlying conditioning, although the altered timing of CRs in patients may indicate differences in the modulation of such responses. Both the greater CR onset latency during delay and the presence of early nonadaptive responses during trace are compatible with the pattern of responding seen in animals with hippocampal damage.

Limb flexion reflex-related areas in human cerebellum

Numerous studies have shown that the cerebellum is involved in classical conditioning of specific aversive reactions. Only few studies, however, have examined the exact localization of cerebellar areas involved in the control of unconditioned reflex responses. The present study investigated cerebellar areas, which are activated during eliciting of the limb flexion reflex in healthy controls using [15O]H2O PET. Limb flexion reflex related areas were found in vermal lobules III-VI with the local maximum in vermal lobule V. Areas of activation were more widespread compared to a voluntary foot movement and a sensory foot stimulation condition. Limb flexion reflex areas probably reflect efferent reflex modulation as well as sensory inputs from the stimulus itself and/or the reflex response.