The viscoelastic properties of passive eye muscle in primates. III: force elicited by natural elongations

Effects of post-saccadic oscillations on visual processing times

Saccadic eye movements enable us to search for the target of interest in a crowded scene or, in the case of goal-directed saccades, to simply bring the image of the peripheral target to the very centre of the fovea. This mechanism extends the use of the superior image processing performance of the fovea over a large visual field. We know that visual information is processed quickly at the end of each saccade but estimates of the times involved remain controversial. This study aims to investigate the processing of visual information during post fixation oscillations of the eyeball. A new psychophysical test measures the combined eye movement response latencies, including fixation duration and visual processing times. When the test is used in conjunction with an eye tracker, each component that makes up the 'integrated saccade latency' time, from the onset of the peripheral stimulus to the correct interpretation of the information carried by the stimulus, can be measured and the discrete components delineated. The results show that the time required to process and encode the stimulus attribute of interest at the end of a saccade is longer than the time needed to carry out the same task in the absence of an eye movement. We propose two principal hypotheses, each of which can account for this finding. 1. The known inhibition of afferent retinal signals during fast eye movements extends beyond the end point of the saccade. 2. The extended visual processing times measured when saccades are involved are caused by the transient loss of spatial resolution due to eyeball instability during post-saccadic oscillations. The latter can best be described as retinal image smear with greater loss of spatial resolution expected for stimuli of low luminance contrast.

A parameter reduced adaptive quasi-linear viscoelastic model for soft biological tissue in uniaxial tension

Mechanical characterisation of soft viscous materials is essential for many applications including aerospace industries, material models for surgical simulation, and tissue mimicking materials for anatomical models. Constitutive material models are, therefore, necessary to describe soft biological tissues in physiologically relevant strain ranges. Hereby, the adaptive quasi-linear viscoelastic (AQLV) model enables accurate modelling of the strain-dependent non-linear viscoelastic behaviour of soft tissues with a high flexibility. However, the higher flexibility produces a large number of model parameters. In this study, porcine muscle and liver tissue samples were modelled in the framework of the originally published AQLV (3-layers of Maxwell elements) model using four incremental ramp-hold experiments in uniaxial tension. AQLV model parameters were reduced by decreasing model layers (M) as well as the number of experimental ramp-hold steps (N). Leave One out cross validation tests show that the original AQLV model (3M4N) with 19 parameters, accurately describes porcine muscle tissue with an average R² of 0.90 and porcine liver tissue, R² of 0.86. Reducing the number of layers (N) in the model produced acceptable model fits for 1-layer (R² of 0.83) and 2-layer models (R² of 0.89) for porcine muscle tissue and 1-layer (R² of 0.84) and 2-layer model (R² of 0.85) for porcine liver tissue. Additionally, a 2 step (2N) ramp-hold experiment was performed on additional samples of porcine muscle tissue only to further reduce model parameters. Calibrated spring constant values for 2N ramp-hold tests parameters k₁ and k₂ had a 16.8% and 38.0% deviation from those calibrated for a 4 step (4N) ramp hold experiment. This enables further reduction of material parameters by means of step reduction, effectively reducing the number of parameters required to calibrate the AQLV model from 19 for a 3M4N model to 8 for a 2M2N model, with the added advantage of reducing the time per experiment by 50%. This study proposes a 'reduced-parameter' AQLV model (2M2N) for the modelling of soft biological tissues at finite strain ranges. Sequentially, the comparison of model parameters of soft tissues is easier and the experimental burden is reduced.

Mathematical models and human disease

Mathematical models of brain function are built from data covering anatomy, physiology, biophysics and behavior. In almost all cases, many possible models could fit the available data. Theoreticians make assumptions that allow them to constrain the number of possible model structures. However, a model that was more useful clinically would result if the constraints came from lesion studies in animals or clinical disorders. Here, we show a few examples of how clinical disorders have led to improvements in models. We also show a few examples of how models could lead to neural prostheses for patients. The best outcomes result when clinicians, basic scientists and theoreticians work together to understand brain function.

Ocular Kinematics Measured by In Vitro Stimulation of the Cranial Nerves in the Turtle

After animals are euthanized, their tissues begin to die. Turtles offer an advantage because of a longer survival time of their tissues, especially when compared to warm-blooded vertebrates. Because of this, in vitro experiments in turtles can be performed for extended periods of time to investigate the neural signals and control of their target actions. Using an isolated head preparation, we measured the kinematics of eye movements in turtles, and their modulation by electrical signals carried by cranial nerves. After the brain was removed from the skull, leaving the cranial nerves intact, the dissected head was placed in a gimbal to calibrate eye movements. Glass electrodes were attached to cranial nerves (oculomotor, trochlear, and abducens) and stimulated with currents to evoke eye movements. We monitored eye movements with an infrared video tracking system and quantified rotations of the eyes. Current pulses with a range of amplitudes, frequencies, and train durations were used to observe effects on responses. Because the preparation is separated from the brain, the efferent pathway going to muscle targets can be examined in isolation to investigate neural signaling in the absence of centrally processed sensory information.

Contractile Force of Human Extraocular Muscle: A Theoretical Analysis

Aim. The length-contractile force relationships of six human extraocular muscles (EOMs) in primary innervations should be determined during eye movement modeling and surgery of clinical EOMs. This study aims to investigate these relationships. Method. The proposal is based on the assumption that six EOMs have similar constitutive relationships, with the eye suspended in the primary position. The constitutive relationships of EOMs are obtained by optimizing from previous experimental data and the theory of mechanical equilibrium using traditional model. Further, simulate the existing experiment of resistance force, and then compare the simulated results with the existing experimental results. Finally, the mechanical constitutive relationships of EOMs are obtained. Results. The results show that the simulated resistance forces from the other four EOMs except for the horizontal recti well agree with previous experimental results. Conclusion. The mechanical constitutive relationships of six EOMs in primary innervations are obtained, and the rationality of the constitutive relationships is verified. Whereafter, the active stress-strain relationships of the six EOMs in the primary innervations are obtained. The research results can improve the eye movement model to predict the surgical amounts of EOMs before EOM surgery more precisely.

The effect of low- and high-velocity tendon excursion on the mechanical properties of human cadaver subsynovial connective tissue

Fibrosis of the subsynovial connective tissue (SSCT) in the carpal tunnel is the most common histological finding in carpal tunnel syndrome (CTS). Fibrosis may result from damaged SSCT. Previous studies found that with low-velocity (2 mm/s), tendon excursions can irreversibly damage the SSCT. We investigated the effect of tendon excursion velocity in the generation of SSCT damage. Nine human cadaver wrists were used. Three repeated cycles of ramp-stretch testing were performed simulating 40%, 60%, 90%, and 120% of the middle finger flexor tendon superficialis physiological excursion with an excursion velocity of 60 mm/s. Energy and force were calculated and normalized by values obtained in the first cycle for each excursion level. Data were compared with low-velocity excursion data. For high-velocity excursions, a significant drop in the excursion energy ratio was first observed at an excursion level of 60% physiological excursion (p < 0.024) and that for low-velocity excursions was first observed at 90% physiological excursion (p < 0.038). Furthermore, the energy ratio was lower at 60% for high velocities (p ≤ 0.039). Increasing velocity lowers the SSCT damage threshold. This finding may be relevant for understanding the pathogenesis of SSCT fibrosis, such as that accompanying CTS, and a relationship with occupational factors.

Components of the neural signal underlying congenital nystagmus

Congenital nystagmus is an involuntary bilateral horizontal oscillation of the eyes that develops soon after birth. In this study, the time constants of each of the components of the neural signal underlying congenital nystagmus were obtained by time series analysis and interpreted by comparison with those of the normal oculomotor system. In the neighbourhood of the fixation position, the system generating the neural signal is approximately linear with 3 degrees of freedom. The shortest time constant was in the range of 7-9 ms and corresponds to a normal saccadic burst signal. The other stable time constant was in the range of 22-70 ms and corresponds to the slide signal. The final time constant characterises the unidentified neural mechanism underlying the unstable drift component of the oscillation cycle and ranges between 31 and 32 ms across waveforms. The characterisation of this unstable time constant poses a challenge for the modelling of both the normal and abnormal oculomotor control system. We tentatively identify the unstable component with the eye position signal supplied to the superior colliculus in the normal eye movement system and explore some of the implications of this hypothesis.

Assessment of quantitative perfusion parameters by dynamic contrast-enhanced sonography using a deconvolution method: an in vitro and in vivo study

OBJECTIVES

The purpose of this study was to investigate the impact of the arterial input on perfusion parameters measured using dynamic contrast-enhanced sonography combined with a deconvolution method after bolus injections of a contrast agent.

METHODS

The in vitro experiments were conducted using a custom-made setup consisting of pumping a fluid through a phantom made of 3 intertwined silicone pipes, mimicking a complex structure akin to that of vessels in a tumor, combined with their feeding pipe, mimicking the arterial input. In the in vivo experiments, B16F10 melanoma cells were xenografted to 5 nude mice. An ultrasound scanner combined with a linear transducer was used to perform pulse inversion imaging based on linear raw data throughout the experiments. A mathematical model developed by the Gustave Roussy Institute (patent WO/2008/053268) and based on the dye dilution theory was used to evaluate 7 semiquantitative perfusion parameters directly from time-intensity curves and 3 quantitative perfusion parameters from the residue function obtained after a deconvolution process developed in our laboratory based on the Tikhonov regularization method. We evaluated and compared the intraoperator variability values of perfusion parameters determined after these two signal-processing methods.

RESULTS

In vitro, semiquantitative perfusion parameters exhibited intraoperator variability values ranging from 3.39% to 13.60%. Quantitative parameters derived after the deconvolution process ranged from 4.46% to 11.82%. In vivo, tumors exhibited perfusion parameter intraoperator variability values ranging from 3.74% to 29.34%, whereas quantitative ones varied from 5.00% to 12.43%.

CONCLUSIONS

Taking into account the arterial input in evaluating perfusion parameters improves the intraoperator variability and may improve the dynamic contrast-enhanced sonographic technique.

Pediatric orbital floor trapdoor fractures: outcomes and CT-based morphologic assessment of the inferior rectus muscle

INTRODUCTION

Trauma to the pediatric orbit may produce a unique fracture in which entrapment of the periorbital tissue and/or inferior rectus muscle may occur due to a "trap-door" effect of the compliant orbital floor. This study was designed to assess the outcome following the surgical management of orbital trapdoor fractures in children and to examine alterations in the morphology of the inferior rectus (IR) muscle.

METHODOLOGY

Outcome assessment on patients undergoing surgery at the Hospital For Sick Children, Toronto with symptomatic orbital floor trapdoor fractures over a 10-year period and a CT-based morphometric analysis of the inferior rectus muscle were performed.

RESULTS

18 patients (5F, 13M) mean age 12.6 years (range 8.3-16.6 years) underwent surgical exploration (average time to surgery 9.7 ± 3.5 days (range 1-45 days). Follow-up was 15.4 months (range 6-36 months). All patients noted improvement in extra-ocular muscle (EOM) range of motion post-operatively: 7 patients had normal EOM with no diplopia; 9 patients had minimal diplopia on extreme secondary (upwards) gaze and 2 patients had residual significant diplopia with upward gaze. CT morphologic assessment (8 patients) demonstrated: a) zone of bony injury was posterior to the equator of the globe; b) minimal to no extra-conal fat exists to protect the IR muscle; c) a trend toward increased length in the injured IR muscle.

CONCLUSIONS

With surgical intervention, improvement of diplopia (complete or near-complete resolution) occurred in 16/18 (89%) of patients presenting with symptomatic trapdoor orbital floor fractures. CT-based assessment demonstrated the vulnerability of the inferior rectus muscle with close proximity to the orbital floor and lack of periorbital fat for protection. Alteration of the length of the IR muscle may impact the force-length relationship and play a role in the outcomes. Early surgical intervention for symptomatic trapdoor fractures is recommended.

A nonlinear model of passive muscle viscosity

The material properties of passive skeletal muscle are critical to proper function and are frequently a target for therapeutic and interventional strategies. Investigations into the passive viscoelasticity of muscle have primarily focused on characterizing the elastic behavior, largely neglecting the viscous component. However, viscosity is a sizeable contributor to muscle stress and extensibility during passive stretch and thus there is a need for characterization of the viscous as well as the elastic components of muscle viscoelasticity. Single mouse muscle fibers were subjected to incremental stress relaxation tests to characterize the dependence of passive muscle stress on time, strain and strain rate. A model was then developed to describe fiber viscoelasticity incorporating the observed nonlinearities. The results of this model were compared with two commonly used linear viscoelastic models in their ability to represent fiber stress relaxation and strain rate sensitivity. The viscous component of mouse muscle fiber stress was not linear as is typically assumed, but rather a more complex function of time, strain and strain rate. The model developed here, which incorporates these nonlinearities, was better able to represent the stress relaxation behavior of fibers under the conditions tested than commonly used models with linear viscosity. It presents a new tool to investigate the changes in muscle viscous stresses with age, injury and disuse.

The nonlinearity of passive extraocular muscles

Passive extraocular muscles (EOMs), like most biological tissues, are hyperelastic, that is, their stiffness increases as they are stretched. It has always been assumed, and in a few occasions argued, that this is their only nonlinearity and that it can be ignored in central gaze. However, using novel measurement techniques in anesthetized paralyzed monkeys, we have recently demonstrated that EOMs are characterized by another prominent nonlinearity: the forces induced by sequences of stretches do not sum. Thus, superposition, a central tenet of linear and quasi-linear models, does not hold in passive EOMs. Here, we outline the implications of this finding, especially in light of the common assumption that it is easier for the brain to control a linear than a nonlinear plant. We argue against this common belief: the specific nonlinearity of passive EOMs may actually make it easier for the brain to control the plant than if muscles were linear.

Disorders of vergence eye movements

PURPOSE OF REVIEW

The aim is to re-interpret disorders of vergence in the light of recent studies that view disjunctive eye movements as but one component of three-dimensional gaze control.

RECENT FINDINGS

Most natural eye movements combine vergence with saccades, pursuit and vestibular eye movements. Electrophysiological studies in epileptic patients, as well as evidence from monkeys, indicate that frontal and parietal cortex govern vergence as a component of three-dimensional gaze. Clinicians apply Hering's law of equal innervation to interpret disjunctive movements as the superposition of conjugate and vergence commands. However, electrophysiological studies indicate that disjunctive saccades are achieved by programming each eye's movement independently. Patients with internuclear ophthalmoplegia (INO) may have preserved vergence, which can be recruited to compensate for loss of conjugacy. Vergence may also enable gaze shifts in saccadic palsy. Some forms of nystagmus suppress or change with convergence; co-contraction of the horizontal rectus muscles does not appear to be the explanation. Rather, effects of near viewing on central vestibular mechanisms or differential activation of specific types of extra-ocular muscle fiber may be responsible.

SUMMARY

Interpretation of disorders of vergence is aided by applying a scheme in which their contributions to three-dimensional gaze control is considered.

Measurement of the extraocular spike potential during saccade countermanding

The stop signal task is used to investigate motor inhibition. Several groups have reported partial electromyogram (EMG) activation when subjects successfully withhold manual responses and have used this finding to define the nature of response inhibition properties in the spinal motor system. It is unknown whether subthreshold EMG activation from extraocular muscles can be detected in the saccadic response version of the stop signal task. The saccadic spike potential provides a way to examine extraocular EMG activation associated with eye movements in electroencephalogram (EEG) recordings. We used several techniques to isolate extraocular EMG activation from anterior electrode locations of EEG recorded from macaque monkeys. Robust EMG activation was present when eye movements were made, but no activation was detected when saccades were deemed canceled. This work highlights a key difference between the spinal motor system and the saccade system.