Russell S. A. Brinkworth

A method for quantifying reflex responses from intra-muscular and surface electromyogram.

Measuring human reflex responses from electromyogram (EMG) traces in an accurate, repeatable and reliable way with a high degree of specificity has traditionally been a difficult task. This paper describes a new method that can be used to quantify reflex responses from both surface and intra-muscular EMG. This technique extends the classical cumulative sum (CUSUM) calculations by defining precise points for the calculation of latencies, durations and strengths to facilitate automatic reflex detection and permit the strength of a reflex to be defined in absolute units. The effect of varying the pre-stimulus time, the number of trials averaged and the amount of filtering used on the identification and classification of reflex parameters are also investigated. Furthermore, the effect of noise on these values, and how to remove it, is discussed. The new method, which is an expansion of the CUSUM analysis, is compared and contrasted with the more common threshold-crossing method in two different muscles: masseter and first dorsal interosseous (FDI), in experiments utilizing both mechanical and electrical stimulation. There are a number of advantages to using the new method; not only does the modified CUSUM method detect reflexes earlier than threshold-crossing methods but also the strength and duration are less susceptible to averaging and filtering parameters while giving a better indication of the reflex size. The data suggests that a pre-stimulus analysis period of at least 100 ms be used to correctly identify the variability inherent in EMG traces. It is also concluded that for subtle reflexes, 50 stimuli should be the minimum number used when spike trigger averaging is employed as lower numbers are associated with much greater pre-stimulus variability. Zero-phase filtering the rectified averaged EMG traces is recommended as this makes it easier to identify significant changes in the electrical activity of the muscle in question. In addition, noise estimation and removal from averaged rectified EMG recordings yields results that are a more accurate representation of the synaptic activity of the motor units in question.

Robust models for optic flow coding in natural scenes inspired by insect biology.

The extraction of accurate self-motion information from the visual world is a difficult problem that has been solved very efficiently by biological organisms utilizing non-linear processing. Previous bio-inspired models for motion detection based on a correlation mechanism have been dogged by issues that arise from their sensitivity to undesired properties of the image, such as contrast, which vary widely between images. Here we present a model with multiple levels of non-linear dynamic adaptive components based directly on the known or suspected responses of neurons within the visual motion pathway of the fly brain. By testing the model under realistic high-dynamic range conditions we show that the addition of these elements makes the motion detection model robust across a large variety of images, velocities and accelerations. Furthermore the performance of the entire system is more than the incremental improvements offered by the individual components, indicating beneficial non-linear interactions between processing stages. The algorithms underlying the model can be implemented in either digital or analog hardware, including neuromorphic analog VLSI, but defy an analytical solution due to their dynamic non-linear operation. The successful application of this algorithm has applications in the development of miniature autonomous systems in defense and civilian roles, including robotics, miniature unmanned aerial vehicles and collision avoidance sensors.

Standardization of H-reflex analyses

Variability in the H-reflex can make it difficult to identify significant changes using traditional pooled analysis techniques. This study was undertaken to introduce a normalisation approach to calculate both the relative size and the relative stimulus intensity required to elicit the H-reflex response so that comparisons can be made not only with results obtained during different experimental session but also between different subjects. This normalisation process fits the size of the measured M-responses and H-reflexes over the entire stimulus range with model curves to better facilitate the calculation of important parameters. This approach allows normalisation of not only the size of the response but also the relative stimulus intensity required to elicit the response. This eases the comparison of the reflex responses under various situations, and is capable of bringing out any genuine differences in the reflex in a reliable manner not previously possible. This study illustrates that comparison of the reflex between days is problematic, even in the same subject, as both the reflex size and the relative stimulus intensity required to obtain this reflex changed in all subjects. We suggest that H-reflex studies need to use normalisation not only for size of the reflex but also for the stimulus intensity, and also that all experiments for a single subject should be performed in the same session or during the same day using some level of background muscle activity in the muscle concerned as the variability of the muscle at rest was found to be larger.

Photoreceptor processing improves salience facilitating small target detection in cluttered scenes.

Target detection amidst clutter is a challenging task for both natural and artificial vision, yet one solved at the level of neurons in the 3rd optic ganglion of insects. These neurons are capable of responding to the motion of small objects, even against complex moving backgrounds. While the basic physiology has been investigated, little is known about how these cells are able to reject background motion while robustly responding to such small stimuli. By recording intracellularly from fly photoreceptors stimulated with natural image sequences containing a target viewed against a complex moving background, we show that the process of target detection begins at the earliest stages of vision. The temporal processing by photoreceptors alone, in the absence of any spatial interactions, improved the discrimination of targets (essentially a spatial task) by around 70%. This enhancement of target salience can be explained by elaborate models of photoreceptor temporal non-linear dynamics. The application of the functional principals outlined in this work could be utilized in areas such as robotics and surveillance, medical imaging, or astronomy, anywhere it is necessary to detect a small item from a cluttered surround.

Response of human jaw muscles to axial stimulation of the incisor

The role of periodontal mechanoreceptors (PMRs) in the reflex control of the jaw muscles has thus far been mainly derived from animal studies. To date, the work that has been done on humans has been limited and confined to orthogonal stimulation of the labial surface of the tooth. The purpose of this study was to investigate the response of the masseter and digastric muscles in humans to controlled axial stimulation of the upper left central incisor, both before and during a local anaesthetic block of the PMRs. Ten neurologically normal young adult females were tested, each on two separate occasions to confirm the reproducibility of the results. It was found that the reflex response in the masseter was modulated by the rate of rise of the stimulus used and, to a lesser degree, the level of background muscle activity. There was little detectable change in the activity of the digastric muscle under the tested conditions and what was found could be attributed to cross‐talk with the masseter. The reflex responses obtained were significantly different between subjects; however retesting the same subject on a different occasion yielded similar results. The results indicate that the most common response of the masseter muscle to brisk axial stimulation of the incisor is a reflex inhibition at 20 ms, followed by a late excitation at 44 ms. However, it is possible that this late excitation could be due to delayed action potentials and hence be artefactual. As the application of a local anaesthetic block removed or significantly reduced both of these responses, it was concluded that they originated from the PMRs. Unlike during orthogonal stimulation, slowly rising stimuli did not produce any excitatory reflex activity. This indicated a difference in jaw reflexes to forces applied in different directions, possibly due to the activation of different receptor types when stimulating the tooth in either the orthogonal or axial directions.

Implementation of an elaborated neuromorphic model of a biological photoreceptor

We describe here an elaborated neuromorphic model based on the photoreceptors of flies and realised in both software simulation and hardware using discrete circuit components. The design of the model is based on optimisations and further elaborations to the mathematical model initially developed by van Hateren and Snippe that has been shown to accurately simulate biological responses in simulations under both steady-state and limited dynamic conditions. The model includes an adaptive time constant, nonlinear adaptive gain control, logarithmic saturation and a nonlinear adaptive frequency response mechanism. It consists of a linear phototransduction stage, a dynamic filter stage, two divisive feedback loops and a static nonlinearity. In order to test the biological accuracy of the model, impulses and step responses were used to test and evaluate the steady-state characteristics of both the biological (fly) and artificial (new neuromorphic model) photoreceptors. These tests showed that the model has faithfully captured most of the essential characteristics of the insect photoreceptor cells. The model showed a decreasing response to impulsive stimuli when the background intensity was increased, indicating that the circuit adapted to background luminance in order to improve the overall operating range and better encode the contrast of the stimulus rather than luminance. The model also showed the same change in its frequency response characteristics as the biological photoreceptors over a luminance range of 70,000 cd/m2, with the corner frequency of the circuit ranging from 10 to 90 Hz depending on the current state of adaptation. Complex naturalistic experiments have also further proven the robustness of the model to perform in real-world scenario. The model showed great correlation to the biological photoreceptors with an r2 value exceeding 0.83. Our model could act as an excellent platform for future experiments that could be carried out in scenarios where in vivo intracellular recording from biological photoreceptors would be impractical or impossible, or as a front-end for an artificial imaging system.

Biomimetic Motion Detection

We present the results of a biomimetic model of motion detection in the insect visual system based on an elaborated correlational elementary motion detector. This model incorporates a number of elements known, or predicted, to be in the insect motion processing pathway. The results show that this model greatly diminishes velocity ambiguity across images from different environments. Due to the nature of the algorithms underlying the model it lends itself to implementation in either digital or analogue hardware including neuromorphic analogue VLSI. The successful application of this algorithm has applications in the development of miniature autonomous systems in defence and civilian roles, including robotics, miniature unmanned aerial vehicles and collision avoidance sensors.

Threshold for Detection of Incisal Forces Is Increased by Jaw Movement

Current knowledge regarding the sensitivity of the teeth to forces is based on psychophysical experiments that measured touch detection thresholds under static jaw conditions. It is not known whether jaw movements alter the perception of forces applied to the teeth, but, based on limb movement studies, it is hypothesized that the perception of mechanoreceptor outputs will be downwardly modulated by jaw movements. We predicted that, compared with static jaw conditions, rhythmic jaw movements would be associated with significantly higher psychophysical thresholds for the detection of incisally applied forces. In eight participants, mechanical pulses were delivered to an incisor during static jaw holding or during cyclic jaw opening and closing. Analogous to findings in human limbs, the psychophysical salience of periodontal mechanoreceptor feedback was downwardly modulated by physiologically relevant movements; detection thresholds for mechanical pulses applied to a central incisor were significantly higher during jaw-closing movements than during static jaw positioning.

Bio-inspired pixel-wise adaptive imaging

The range of luminance levels in the natural world varies in the order of 108, significantly larger than the 8-bits employed by most digital imaging systems. To overcome their limited dynamic range traditional systems rely on the fact that the dynamic range of a scene is typically much lower, and by adjusting a global gain factor (shutter speed) it is possible to acquire usable images. However in many situations 8-bits of dynamic range is insufficient, meaning potentially useful information, lying outside of the dynamic range of the device, is lost. Traditional approaches to solving this have involved using nonlinear gamma tables to compress the range, hence reducing contrast in the digitized scene, or using 16-bit imaging devices, which use more bandwidth and are incompatible with most recording media and software post-processing techniques. This paper describes an algorithm, based on biological vision, which overcomes many of these problems. The algorithm reduces the redundancy of visual information and compresses the data observed in the real world into a significantly lower bandwidth signal, better suited for traditional 8-bit image processing and display. However, most importantly, no potentially useful information is lost and the contrast of the scene is enhanced in areas of high informational content (where there are changes) and reduced in areas containing low information content (where there are no changes). Thus making higher-order tasks, such as object identification and tracking, easier as redundant information has already been removed.

When touch predicts pain: predictive tactile cues modulate perceived intensity of painful stimulation independent of expectancy

Abstract Aims Non-nociceptive somatosensory input, such as tactile or proprioceptive information, alway precedes nociceptive input during a painful event. This relationship provides clear opportunities fo predictive associative learning, which may shape future painful experiences. In this differential classica conditioning study we tested whether pain-associated tactile cues (conditioned stimuli; CS) could altei the perceived intensity of painful stimulation, and whether this depends on duration of the CS—seeing that CS duration might allow or prevent conscious expectation. Methods Subjects underwent a classical differential conditioning task in which a tactile cue at locatior A (CS+) preceded painful electrical stimulation at location B (UShigh), whereas a tactile cue at location C (CS–) preceded non-painful electrical stimulation at location B (USlow). At test, we compared the pain evoked by a moderately painful stimulus (USmed) when preceded by either the CS+ or CS–. CS duration was manipulated between subjects. Participants were assigned to one of three groups: Long CS (4s, allowing conscious expectation), Short CS (110 ms) and CS-US indistinguishable (20 ms), preventing conscious expectation). We hypothesised that more pain would be evoked by the US when preceded by the CS+ relative to the CS-, and that the effect would be independent of CS duration. Results Fifty-four healthy participants (31 females, age = 26, SD = 9) were included in the analysis. The hypotheses were supported in that more intense pain was evoked by the USmed when paired with the tactile CS+, than when paired with the tactile CS-; mean difference 3 mm on a 150 mm VAS (C 0.4-4.8 mm). CS duration did not moderate the effect. The effect was greater in those participants where calibration was optimal, as indicated by a relatively more painful UShigh. Conclusions We conclude that pain-associated tactile cues can influence pain, and that this effect i: not dependent on stimulus duration. This suggests that explicit expectation is not a requirement for predictive cues to modulate pain. That the presence of the CS+ resulted in only a 5.3% higher intensity rating compared with the CS- may reflect a limitation of laboratory studies, where a limited number o trials, an artificial context and the use of experimental pain are likely to reveal only glimpses of what i: clinically possible. Implications Pain-associated visual and auditory cues have been shown to enhance pain in laboratory and clinical scenarios, supposedly by influencing expectation of impending harm. We show that pain-associated somatosensory cues can also modulate pain and that this can occur independently of expectation. This points to a larger potential role for associative learning in the development and treatment of pain than has previously been considered. We suggest that research into associative mechanisms underpinning pain, as distinct from those that link pain to pain-related fear and avoidance, is worthwhile.