Papers by André Fabio Kohn
IFMBE Proceedings, 2009
Force fluctuations, even for maintained isometric force, have many origins and are attributed to ... more Force fluctuations, even for maintained isometric force, have many origins and are attributed to noise, errors and variabilities associated with different elements of the sensory-motor system. Part of the random-like fluctuations of the force is due to the motor pathway per se and much work has been focused in quantifying the fluctuations and in determining their dependence on variables such as number of motor units, discharge rate variability of each motor unit, etc. A recently developed simulator of the neuromuscular control system was utilized here to verify stochastic features of the simulated force signal generated by the soleus muscle for different mean force levels. The mean force level increased in a sigmoidal fashion as the intensity of the descending drive increased. The standard deviation of force as a function of mean force level resulted in an approximate parabola, while the coefficient of variation of force decreased monotonically with mean force level. Force power spectrum was strongly concentrated in the 0-5 Hz frequency range. Individual motor unit force signals showed a tendency for decreased force variability as overall muscle force level increased, helping to explain the relation found between muscle force variability and mean muscle force. Comparisons are made with results from the literature of simulations of other muscles and the differences and similarities are discussed.
Biology of Sex Differences, 2020
Background Ischemic preconditioning (IPC) is suggested to decrease fatigability in some individua... more Background Ischemic preconditioning (IPC) is suggested to decrease fatigability in some individuals but not others. Sex differences in response to IPC may account for this variability and few studies systematically investigated the effects of IPC in men and women. The goal of this study was to determine if time to task failure, perception of pain, and neuromuscular mechanisms of fatigability were altered by IPC in men and women. Methods Ten women (29 ± 5 years old) and 10 men (28 ± 6 years old) performed isometric contractions with the plantar flexor muscles of the dominant leg at 20% of maximal voluntary contraction until task failure. We used a repeated measures design where each individual performed 3 randomized and counterbalanced test sessions: (A) IPC session, cuff inflation and deflation (5 min each repeated 3 times) performed before the exercise by inflating cuffs to the non-dominant leg and arm; (B) sham session, cuffs were inflated for a short period (1 min); and (C) contr...
Revista dos Trabalhos de Iniciação Científica da UNICAMP, 2019
Decomposition of intramuscular electromyogram (iEMG) into its constituent motor unit spike trains... more Decomposition of intramuscular electromyogram (iEMG) into its constituent motor unit spike trains is a useful tool for understanding the neurophysiological control of muscle force. Some experimental results have shown that the performance in a force-matching motor task is influenced by the gain of the visual feedback provided to the subject. In this project, the propose was to decompose iEMG signals from the Soleus muscle recorded in a force-matching task (plantarflexion contractions with different intensities). The motor unit spike trains were analyzed in six different conditions of visual feedback. From the results found, the visual feedback gain seems not to influence the discharge properties of MUs recruited in a force-matching task. Force intensity only influenced the number of recruited MUs and the MU FR, which is expected from the recruitment and rate coding mechanisms of force control.
Neuroscience, 2017
The application of subsensory noise stimulation over the lower limbs has been shown to improve pr... more The application of subsensory noise stimulation over the lower limbs has been shown to improve proprioception and postural control under certain conditions. Whereas the effect specificity seems to depend on several factors, studies are still needed to determine the appropriate method for training and rehabilitation purposes. In the current study, we investigated whether the application of subsensory electrical noise over the legs improves proprioceptive function in young and older adults. We aimed to provide evidence that stronger and age-related differential effects occur in more demanding tasks. Proprioceptive function was initially assessed by testing the detection of passive ankle movement (kinesthetic perception) in twenty-eight subjects (14 young and 14 older adults). Thereafter, postural control was assessed during tasks with different sensory challenges: i) by removing visual information (eyes closed) and; ii) by moving the visual scene (moving room paradigm). Tests performed with the application of electrical noise stimulation were compared to those performed without noise. The results showed that electrical noise applied over the legs led to a reduction in the response time to kinesthetic perception in both young and older adults. On the other hand, the magnitude of postural sway was reduced by noise stimulation only during a more challenging task, namely, when the optical flow was changing in an unpredictable (nonperiodic) manner. No differential effects of stimulation between groups were observed. These findings suggest that the relevance of proprioceptive inputs in tasks with different challenges, but not the subjects' age, is a determining factor for sensorimotor improvements due to electrical noise stimulation.
Neuroscience letters, May 27, 2016
The aim of this work was to compare cortical beta oscillatory activity between young (YA) and old... more The aim of this work was to compare cortical beta oscillatory activity between young (YA) and older (OA) adults during the assessment of ankle proprioception. We analyzed the response time (RT) to kinesthetic perception and beta event-related desynchronization/synchronization (ERD/ERS) in response to passive ankle movement applied at a slow speed, 0.5°/s. The relationship between ERD/ERS and RT was investigated by classifying the signals into fast-, medium-, and slow-RT. The results showed a temporal relationship between beta oscillation changes and RT for both groups, i.e., earlier ERD and ERS were obtained for trials with faster response time. ERD was larger and delayed in OA compared to the YA, and beta ERS was present only for OA. These findings suggest that a less efficient proprioceptive signaling reaching the brain of OA requires a higher level of brain processing and hence the differences in ERD potentials between YA and OA. Furthermore, the occurrence of ERS in OA might rep...
BMC Neuroscience, 2015
Figure 1 Dependence of resonance of the high-input conductance MN model on the basal level of inj... more Figure 1 Dependence of resonance of the high-input conductance MN model on the basal level of injected current. A. somatic potential B. impedance amplitude C. impedance phase D. Quality (Q) factor E. resonance frequency (f r) F. total phase advance (Φ L).
Frontiers in Psychology, 2015
Electrophysiological markers of chunking of words during encoding have mostly been shown in studi... more Electrophysiological markers of chunking of words during encoding have mostly been shown in studies that present pairs of related stimuli. In these cases it is difficult to disentangle cognitive processes that reflect distinctiveness (i.e., conspicuous items because they are related), perceived association between related items and unified representations of various items, or chunking. Here, we propose a paradigm that enables the determination of a separate Event-related Potential (ERP) marker of these cognitive processes using sequentially related word triads. Twenty-three young healthy individuals viewed 80 15-word lists composed of unrelated items except for the three words in the middle serial positions (triads), which could be either unrelated (control list), related perceptually, phonetically or semantically. ERP amplitudes were measured at encoding of each one of the words in the triads. We analyzed two latency intervals (350-400 and 400-800 ms) at midline locations. Behaviorally, we observed a progressive facilitation in the immediate free recall of the words in the triads depending on the relations between their items (control < perceptual < phonetic < semantic), but only semantically related items were recalled as chunks. P300-like deflections were observed for perceptually deviant stimuli. A reduction of amplitude of a component akin to the N400 was found for words that were phonetically and semantically associated with prior items and therefore were not associated to chunking. Positive slow wave (PSW) amplitudes increased as successive phonetically and semantically related items were presented, but they were observed earlier and were more prominent at Fz for semantic associates. PSWs at Fz and Cz also correlated with recall of semantic word chunks. This confirms prior claims that PSWs at Fz are potential markers of chunking which, in the proposed paradigm, were modulated differently from the detection of deviant stimuli and of relations between stimuli.
Proceedings of 17th International Conference of the Engineering in Medicine and Biology Society
Clinical Neurophysiology, 2016
During passive ankle movement, older adults showed lower amplitude and longer latency of the N1 c... more During passive ankle movement, older adults showed lower amplitude and longer latency of the N1 component than young adults, which correlated with their delayed response time. Larger beta event-related desynchronization (ERD) in a response time task in older adults may be an indication of a higher cognitive effort to process weaker proprioceptive inputs (lower N1). Attenuated beta event-related synchronization (ERS) in older adults may be related to changes in intracortical inhibitory activity. a b s t r a c t Objectives: The response time (RT) to kinesthetic perception has been used as a proprioceptive measurement, for example, in older individuals. However, the RT cannot provide information on impairments at specific stages of the respective sensorimotor processing. In the present study, electroencephalographic (EEG) signals were recorded during passive ankle movement with and without an associated perceptual task of movement detection. The main purpose was to analyze differences between young and older adults both in terms of RT and cortical responses. Putative differences in the latter were expected to point to changes in the processing associated with neural pathways or cortical regions in the older subjects. Methods: The EEG activity of nineteen older (OA, 65-76 years) and 19 young adults (YA, 21-32 years) was recorded during passive ankle movement, without motor voluntary response (NVR, sensory condition), and during a condition with voluntary motor response (VR, with measurement of the RT). Event-related potentials (ERP) and beta event-related desynchronization/synchronization (ERD/ERS) were recorded and analyzed in both experimental conditions. Results: The RT in OA was larger than in YA (P < 0.0001). EEG analyses showed that the N1 amplitude was larger in the VR than in the NVR condition (P = 0.006), whereas no difference for latency was obtained between conditions (P = 0.376). Comparisons between the groups revealed attenuated (P = 0.019) and delayed (P = 0.001) N1 in the OA group, irrespective of the condition (no interaction group vs condition). Only OA showed correlations between RT and N1, with significant correlation for both amplitude (r = À0.603, P = 0.006) and latency (r = 0.703, P = 0.001). The ERD/ERS analyses revealed a task-dependent group effect: in NVR, significant differences were obtained only for the ERS amplitude, which was attenuated in OA (P = 0.003). In VR, larger (P = 0.004) and delayed (P = 0.003) ERD and attenuated (P = 0.029) and delayed (P = 0.017) ERS peaks were observed in the older group. Conclusions: The results suggest that a larger response time to proprioceptive stimuli in older adults is associated with a weaker and delayed proprioceptive afferent inflow to the cortex. In this scenario, older adults would need a higher cognitive effort (larger ERD) to process the sensory inputs when attempting to properly perform a sensorimotor task.
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2007
The H-reflex is frequently used both in the clinic as well as in research, with the purpose of pr... more The H-reflex is frequently used both in the clinic as well as in research, with the purpose of providing a better understanding of the spinal cord. For repetitive stimuli (e.g. at 1 Hz) the H-reflex depresses, probably due to synaptic depression. Experimental results from the literature provided the basis for the simulations presented here. A large network of motoneurons connected to muscle fibers was modeled and implemented as a computer simulator. Afferent fibers that excited synaptically the motoneurons fired in response to a programmable stimulus. After an initial fitting to the experimental results was achieved, the analysis of which motor units contributed to the H-reflex in two different paradigms pointed to an alternative interpretation of the human data.
Journal of neurophysiology, 2002
Experiments using electrical and mechanical activation of spinal reflexes have contributed import... more Experiments using electrical and mechanical activation of spinal reflexes have contributed important results toward the understanding of neuronal and synaptic dynamics involved in spinal neural circuits as well as their response to different inputs. In this work, data obtained from the simultaneous stimulation of both legs are analyzed to provide information on the degree of symmetry of the respective spinal reflex circuits and on the characteristics of reflex variability. H-reflexes recorded from relaxed muscles show a frequency-dependent amplitude depression when elicited by a train of stimuli. This effect has been attributed to homosynaptic depression. Soleus H-reflexes were recorded in response to trains of simultaneous stimuli applied to both legs in right-handed subjects that were sitting in a relaxed state. The first objective was to verify the existence of asymmetries in H-reflex parameters obtained from the two legs. We measured the mean, variance, and coefficient of variat...
A biologically based neuromuscular simulator was used to produce short-(SLR) and medium-latency r... more A biologically based neuromuscular simulator was used to produce short-(SLR) and medium-latency reflexes (MLR) observed after a stretch of the SO muscle. As expected, SLR was exclusively mediated by the activity of Ia afferents passing through a monosynaptic pathway. The MLR was generated by a summation of effects from Ia and type-II afferents, though it could be generated without the activity of type-II afferents. When the reflex was mediated exclusively by type-II afferents, only MLR was observed as found in patients with a loss of Ia afferents.
Wiley Encyclopedia of Biomedical Engineering, 2006
²A biologically-inspired model of the Soleus neuromuscular system was used to study the influence... more ²A biologically-inspired model of the Soleus neuromuscular system was used to study the influences of musculotendon mechanical properties on the proprioceptive feedback and on the short-latency reflex. The model structure comprised: i) stochastic descending drive to a motoneuron pool; ii) a Hill-type muscle model; and iii) a muscle spindle model with its Ia sensory afferents establishing a feedback loop with the motoneurons. H-reflex and short-latency stretch reflex were simulated considering their possible dependence on musculotendon mechanical properties and on the muscle activation level. The main results showed an intrinsic relationship between the Ia response to muscle stretch and the relative stiffness between muscle and tendon. In addition, fusimotor activation and synaptic gain could modulate the short-latency reflex, indicating that these mechanisms may help to maintain reflexes functionally active during different motor tasks. These preliminary results match some experimental data and provide insights on the intricate relationship between muscular mechanics and neuronal activity. Moreover, the adopted model of the closed-loop neuromuscular system has shown its potential as a tool for studies of complex mechanisms involved in motor control.
Brazilian Journal of Biomedical Engineering, 2013
The learning of core concepts in neuroscience can be reinforced by a hands-on approach, either ex... more The learning of core concepts in neuroscience can be reinforced by a hands-on approach, either experimental or computer-based. In this work, we present a web-based multi-scale neuromuscular simulator that is being used as a teaching aid in a campus-wide course on the Principles of Neuroscience. Methods: The simulator has several built-in individual models based on cat and human biophysics, which are interconnected to represent part of the neuromuscular system that controls leg muscles. Examples of such elements are i) single neurons, representing either motor neurons or interneurons mediating reciprocal, recurrent and Ib inhibition; ii) afferent fibers that can be stimulated to generate spinal reflexes; iii) muscle unit models, generating force and electromyogram; and iv) stochastic inputs, representing the descending volitional motor drive. Results: Several application examples are provided in the present report, ranging from studies of individual neuron responses to the collective action of many motor units controlling muscle force generation. A subset of them was included in an optional homework assignment for Neuroscience and Biomedical Engineering graduate students enrolled in the course cited above at our University. Almost all students rated the simulator as a good or an excellent learning tool, and approximately 90% declared that they would use the simulator in future projects. Conclusion: The results allow us to conclude that multi-scale neuromuscular simulator is an effective teaching tool. Special features of this free teaching resource are its direct usability from any browser (http://remoto.leb.usp.br/), its user-friendly graphical user interface (GUI) and the preset demonstrations.
Brazilian Journal of Physical Therapy, 2012
Background: Evidence of self-sustained muscle activation following a brief electrical stimulation... more Background: Evidence of self-sustained muscle activation following a brief electrical stimulation has been reported in the literature for certain muscles. Objectives: This report shows that the foot muscle (Flexor Digitorum Brevis-FDB) shows a self-sustained increase in muscle activity during upright stance in some subjects following a train of stimuli to the tibial nerve. Methods: Healthy subjects were requested to stand upright and surface EMG electrodes were placed on the FDB, Soleus and Tibialis Anterior muscles. After background muscle activity (BGA) acquisition, a 50 Hz train of stimuli was applied to the tibial nerve at the popliteal fossa. The root mean square values (RMS) of the BGA and the post-stimulus muscle activation were computed. Results: There was a 13.8% average increase in the FDB muscle EMG amplitude with respect to BGA after the stimulation was turned off. The corresponding post-stimulus Soleus EMG activity decreased by an average of 9.2%. We hypothesize that the sustained contraction observed in the FDB following stimulus may be evidence of persistent inward currents (PIC) generated in FDB spinal motoneurons. The post-stimulus decrease in soleus activity may have occurred due to the action of inhibitory interneurons caused by the PICs, which were triggered by the stimulus train. Conclusions: These sustained post-stimulation changes in postural muscle activity, found in different levels in different subjects, may be part of a set of possible responses that contribute to overall postural control.
Progress in Neuro-Psychopharmacology and Biological Psychiatry, 2003
Introduction: Certain features of event-related potentials (ERPs), electroencephalographic (EEG),... more Introduction: Certain features of event-related potentials (ERPs), electroencephalographic (EEG), and behavioural measures vary with differing states of alertness and/or sedation. Purpose: This study was conducted to investigate changes in several measures usually viewed as reflecting states of sedation/sleepiness associated with the use of a range of doses of the hypnotic benzodiazepine (BZD) flunitrazepam (FNZ). Methods: This was a double blind, independent group design study of the effects of acute oral doses of FNZ in young healthy volunteers. Forty-eight subjects were randomly allocated to one of four groups-FNZ (0.6, 0.8, and 1.0 mg) and placebo (PLAC)-and tested prior to treatment and then in a posttreatment session close to the theoretical peak plasma concentration. ERP latencies and amplitudes were measured at midfrontal (Fz), midcentral (Cz), and midparietal (Pz) using a standard auditory oddball paradigm. EEG changes were assessed at Pz. Behavioural measures included the digit-symbol substitution test (DSST), a cancellation task (CT), and subjective ratings of alertness and attentiveness by the subjects (SUB) and the experimenter (EXP). Results: FNZ led to psychomotor impairments and decreased alertness and attention; these effects were consistent with previous findings. A progressive, dose-related increase in P3 latency occurred in Fz, Cz, and Pz, and there was an increase in N1 (Fz, Cz) and N2 (Fz). N2-P3 amplitude decreased in Fz. EEG power bands beta 1 increased for the two highest doses, but no significant differences were noted in theta, delta, and alpha bands. P3 latencies, experimenter-rated levels of alertness, and DSST scores differentiated all three doses of FNZ from PLAC. Conclusion: The most sensitive measures used were P3 latencies of the ERPs (which varied with FNZ dose), DSST, and the experimenter-rated levels of alertness. However, we found no evidence for the assumption that one single phenomenon was reflected in all measures and different mechanisms were probably involved. Further experiments will be needed for more in-depth probing of the finer mechanisms underlying sedation/sleepiness and how they affect behavioural and eletrophysiological measures of the central nervous system (CNS) function.
Pattern Recognition, 1996
This paper presents a new class discriminability measure based on an adaptive partitioning of the... more This paper presents a new class discriminability measure based on an adaptive partitioning of the feature space according to the available class samples. It is intended to be used as a criterion in a classifier-independent feature selection procedure. The partitioning is performed according to a binary splitting rule and appropriate stopping criteria. Results from several tests with Gaussian and non-Gaussian, multidimensional and multiclass computer-generated samples, were very similar to those obtained using a Bayes error criterion function, i.e. the optimal feature subsets selected by both criterion functions-were the same. The main advantage of the new measure is that it is computationally efficient.
Journal of Strength and Conditioning Research, 2012
The purpose of this study was to compare the neuromuscular adaptations produced by strength-train... more The purpose of this study was to compare the neuromuscular adaptations produced by strength-training (ST) and power-training (PT) regimens in older individuals. Participants were balanced by quadriceps cross-sectional area (CSA) and leg-press 1-repetition maximum and randomly assigned to an ST group (n = 14; 63.6 ± 4.0
Journal of Neurophysiology, 2005
Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic refl... more Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic reflexes (MSRs) of variable amplitude. We recorded simultaneous MSRs in the left and right L7 (or L6) ventral roots of anesthetized cats. We analyzed the cross-covariance (CCV) between the amplitudes of bilateral MSRs. Long-time series (5 to 8 h) of these bilateral MSRs exhibited transitory changes in their covariations (as measured by the zero-lag peak of their CCV), thus suggesting the existence of certain neural sources contributing to produce these changes. The aim of the present study was to show that spinal centers producing negative spontaneous cord dorsum potentials (nSCDPs) contribute to maintain correlations in the amplitude of bilateral MSRs. After spinal cord transection at the L1 segment, no significant changes were observed in the correlation between the amplitude of bilateral nSCDPs versus the amplitude of bilateral MSRs. However, this correlation, as well as the peak at zero ...
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Papers by André Fabio Kohn