Biology of sensory systems

A comprehensive and highly illustrated text providing a broad and invaluable overview of sensory systems at the molecular, cellular and neurophysiological level of vertebrates, invertebrates and prokaryotes. It retains a strong focus on human systems, and takes an evolutionary and comparative approach to review the mechanosenses, chemosenses, photosenses, and other sensory systems including those for detecting pain, temperature electric and magnetic fields etc. It incorporates exciting and significant new insights provided by molecular biology which demonstrate how similar the molecular architecture and physiology of sensory cells are across species and across sensory modality, often indicationg a common ancestry dating back over half a billion years. Written by a renowned author, with extensive teaching experience in the biology of sensory systems, this book includes: - Over 400 illustrations - Self–assessment questions - Full bibliography preceded by short bibliographical essays - Boxes containing useful supplementary material. It will be invaluable for undergraduates and postgraduates studying biology, zoology, animal physiology, neuroscience, anatomy, molecular biology, physiological psychology and related courses.

Normative MEG data to flash and pattern reversal stimuli

We have attempted to establish normative values of components of the magnetic evoked field to flash and pattern reversal stimuli prior to clinical use of the MEG. Full visual field, binocular evoked magnetic fields were recorded from 100 subjects 16 to 86 years of age with a single channel dc Squid (BTI) second-order gradiometer at a point 5-6cm above the inion. The majority of subjects showed a large positive component (out going magnetic field) of mean latency 115 ms (SD range 2.5 -11.8 in different decades of life) to the pattern reversal stimulus. In many subjects, this P100M was preceeded and succeeded by negative deflections (in going field). About 6% of subjects showed an inverted response i.e. a PNP wave. Waveforms to flash were more variable in shape with several positive components; the most consistent having a mean latency of 110ms (SD range 6.4-23.2). Responses to both stimuli were consistent when measured on the same subject on six different occasions (SD range 4.8 to 7.3). The data suggest that norms can be established for evoked magnetic field components, in particular for the pattern reversal P100M, which could be used in the diagnosis of neuro-opthalmological disease.

Functional localisation of human sensory-motor cortex using magnetoencephalography

The 19 channel Neuromagnetometer system in the Clinical Neurophysiology Unit at Aston University is a multi-channel system, unique in the United Kingdom. A bite bar head localisation and MRI co-registration strategy which enabled accurate and reproducible localisation of MEG data into cortical space was developed. This afforded the opportunity to study magnetic fields of the human cortex generated by stimulation of peripheral nerve, by stimulation of visceral sensory receptors and by those evoked through voluntary finger movement. Initially, a study of sensory-motor evoked data was performed in a healthy control population. The techniques developed were then applied to patients who were to undergo neurosurgical intervention for the treatment of epilepsy and I or space occupying lesions. This enabled both validation of the effective accuracy of source localisation using MEG as well as to determine the clinical value of MEG in presurgical assessment of functional localisation in human cortex. The studies in this thesis have demonstrated that MEG can repeatedly and reliably locate sources contained within a single gyrus and thus potentially differentiate between disparate gyral activation. This ability is critical in the clinical application of any functional imaging technique; which is yet to be fully validated by any other 'non-invasive' functional imaging methodology. The technique was also applied to the study of visceral sensory representation in the cortex which yielded important data about the multiple cortical representation of visceral sensory function.

Neuro-electromagnetic imaging of the human visual cortex

Methods of solving the neuro-electromagnetic inverse problem are examined and developed, with specific reference to the human visual cortex. The anatomy, physiology and function of the human visual system are first reviewed. Mechanisms by which the visual cortex gives rise to external electric and magnetic fields are then discussed, and the forward problem is described mathematically for the case of an isotropic, piecewise homogeneous volume conductor, and then for an anisotropic, concentric, spherical volume conductor. Methods of solving the inverse problem are reviewed, before a new technique is presented. This technique combines prior anatomical information gained from stereotaxic studies, with a probabilistic distributed-source algorithm to yield accurate, realistic inverse solutions. The solution accuracy is enhanced by using both visual evoked electric and magnetic responses simultaneously. The numerical algorithm is then modified to perform equivalent current dipole fitting and minimum norm estimation, and these three techniques are implemented on a transputer array for fast computation. Due to the linear nature of the techniques, they can be executed on up to 22 transputers with close to linear speedup. The latter part of the thesis describes the application of the inverse methods to the analysis of visual evoked electric and magnetic responses. The CIIm peak of the pattern onset evoked magnetic response is deduced to be a product of current flowing away from the surface areas 17, 18 and 19, while the pattern reversal P100m response originates in the same areas, but from oppositely directed current. Cortical retinotopy is examined using sectorial stimuli, the CI and CIm ;peaks of the pattern onset electric and magnetic responses are found to originate from areas V1 and V2 simultaneously, and they therefore do not conform to a simple cruciform model of primary visual cortex.

Right hemisphere contributions to imitation tasks

Humans imitate biological movements faster than non-biological movements. The faster response has been attributed to an activation of the human mirror neuron system, which is thought to match observation and execution of actions. However, it is unclear which cortical areas are responsible for this behavioural advantage. Also, little is known about the timing of activations. Using whole-head magnetoencephalography we recorded neuronal responses to single biological finger movements and non-biological dot movements while the subjects were required to perform an imitation task or an observation task, respectively. Previous imaging studies on the human mirror neurone system suggested that activation in response to biological movements would be stronger in ventral premotor, parietal and superior temporal regions. In accordance with previous studies, reaction times to biological movements were faster than those to dot movements in all subjects. The analysis of evoked magnetic fields revealed that the reaction time benefit was paralleled by stronger and earlier activation of the left temporo-occipital cortex, right superior temporal area and right ventral motor/premotor area. The activity patterns suggest that the latter areas mediate the observed behavioural advantage of biological movements and indicate a predominant contribution of the right temporo-frontal hemisphere to action observation–execution matching processes in intransitive movements, which has not been reported previously.

Magnetohydrodynamic stability of stochastically driven accretion flows

We investigate the evolution of magnetohydrodynamic (or hydromagnetic as coined by Chandrasekhar) perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations and experiments. The mismatch seems to have been resolved, at least in certain regimes, in the presence of a weak magnetic field, revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It is found that such stochastically driven flows exhibit large temporal and spatial autocorrelations and cross-correlations of perturbation and, hence, large energy dissipations of perturbation, which generate instability. Interestingly, autocorrelations and cross-correlations appear independent of background angular velocity profiles, which are Rayleigh stable, indicating their universality. This work initiates our attempt to understand the evolution of three-dimensional hydromagnetic perturbations in rotating shear flows in the presence of stochastic noise. © 2013 American Physical Society.

Magneto-vibratory separation of glass and bronze granular mixtures immersed in a paramagnetic liquid

A fluid-immersed granular mixture may spontaneously separate when subjected to vertical vibration, separation occurring when the ratio of particle inertia to fluid drag is sufficiently different between the component species of the mixture. Here, we describe how fluid-driven separation is influenced by magneto-Archimedes buoyancy, the additional buoyancy force experienced by a body immersed in a paramagnetic fluid when a strong inhomogeneous magnetic field is applied. In our experiments glass and bronze mixtures immersed in paramagnetic aqueous solutions of MnCl2 have been subjected to sinusoidal vertical vibration. In the absence of a magnetic field the separation is similar to that observed when the interstitial fluid is water. However, at modest applied magnetic fields, magneto-Archimedes buoyancy may balance the inertia/fluid-drag separation mechanism, or it may dominate the separation process. We identify the vibratory and magnetic conditions for four granular configurations, each having distinctive granular convection. Abrupt transitions between these states occur at well-defined values of the magnetic and vibrational parameters. In order to gain insight into the dynamics of the separation process we use computer simulations based on solutions of the Navier-Stokes' equations. The simulations reproduce the experimental results revealing the important role of convection and gap formation in the stability of the different states.

Source localization methods

One of the most pressing demands on electrophysiology applied to the diagnosis of epilepsy is the non-invasive localization of the neuronal generators responsible for brain electrical and magnetic fields (the so-called inverse problem). These neuronal generators produce primary currents in the brain, which together with passive currents give rise to the EEG signal. Unfortunately, the signal we measure on the scalp surface doesn't directly indicate the location of the active neuronal assemblies. This is the expression of the ambiguity of the underlying static electromagnetic inverse problem, partly due to the relatively limited number of independent measures available. A given electric potential distribution recorded at the scalp can be explained by the activity of infinite different configurations of intracranial sources. In contrast, the forward problem, which consists of computing the potential field at the scalp from known source locations and strengths with known geometry and conductivity properties of the brain and its layers (CSF/meninges, skin and skull), i.e. the head model, has a unique solution. The head models vary from the computationally simpler spherical models (three or four concentric spheres) to the realistic models based on the segmentation of anatomical images obtained using magnetic resonance imaging (MRI). Realistic models – computationally intensive and difficult to implement – can separate different tissues of the head and account for the convoluted geometry of the brain and the significant inter-individual variability. In real-life applications, if the assumptions of the statistical, anatomical or functional properties of the signal and the volume in which it is generated are meaningful, a true three-dimensional tomographic representation of sources of brain electrical activity is possible in spite of the ‘ill-posed’ nature of the inverse problem (Michel et al., 2004). The techniques used to achieve this are now referred to as electrical source imaging (ESI) or magnetic source imaging (MSI). The first issue to influence reconstruction accuracy is spatial sampling, i.e. the number of EEG electrodes. It has been shown that this relationship is not linear, reaching a plateau at about 128 electrodes, provided spatial distribution is uniform. The second factor is related to the different properties of the source localization strategies used with respect to the hypothesized source configuration.

The functional neuroanatomy of auditory sensory gating and its behavioural implications

Auditory sensory gating (ASG) is the ability in individuals to suppress incoming irrelevant sensory input, indexed by evoked response to paired auditory stimuli. ASG is impaired in psychopathology such as schizophrenia, in which it has been proposed as putative endophenotype. This study aims to characterise electrophysiological properties of the phenomenon using MEG in time and frequency domains as well as to localise putative networks involved in the process at both sensor and source level. We also investigated the relationship between ASG measures and personality profiles in healthy participants in the light of its candidate endophenotype role in psychiatric disorders. Auditory evoked magnetic fields were recorded in twenty seven healthy participants by P50 ‘paired-click’ paradigm presented in pairs (conditioning stimulus S1- testing stimulus S2) at 80dB, separated by 250msec with inter trial interval of 7-10 seconds. Gating ratio in healthy adults ranged from 0.5 to 0.8 suggesting dimensional nature of P50 ASG. The brain regions active during this process were bilateral superior temporal gyrus (STG) and bilateral inferior frontal gyrus (IFG); activation was significantly stronger in IFG during S2 as compared to S1 (at p<0.05). Measures of effective connectivity between these regions using DCM modelling revealed the role of frontal cortex in modulating ASG as suggested by intracranial studies, indicating major role of inhibitory interneuron connections. Findings from this study identified a unique event-related oscillatory pattern for P50 ASG with alpha (STG)-beta (IFG) desynchronization and increase in cortical oscillatory gamma power (IFG) during S2 condition as compared to S1. These findings show that the main generator for P50 response is within temporal lobe and that inhibitory interneurons and gamma oscillations in the frontal cortex contributes substantially towards sensory gating. Our findings also show that ASG is a predictor of personality profiles (introvert vs extrovert dimension).

The topographic distribution of the magnetic P100M to full- and half-field stimulation

Visual evoked magnetic responses were recorded to full-field and left and right half-field stimulation with three check sizes (70′, 34′ and 22′) in five normal subjects. Recordings were made sequentially on a 20-position grid (4 × 5) based on the inion, by means of a single-channel direct current-Superconducting Quantum Interference Device second-order gradiometer. The topographic maps were consistent on the same subjects recorded 2 months apart. The half-field responses produced the strongest signals in the contralateral hemisphere and were consistent with the cruciform model of the calcarine fissure. Right half fields produced upper-left-quadrant outgoing fields and lower-left-quadrant ingoing fields, while the left half field produced the opposite response. The topographic maps also varied with check size, with the larger checks producing positive or negative maximum position more anteriorly than small checks. In addition, with large checks the full-field responses could be explained as the summation of the two half fields, whereas full-field responses to smaller checks were more unpredictable and may be due to sources located at the occipital pole or lateral surface. In addition, dipole sources were located as appropriate with the use of inverse problem solutions. Topographic data will be vital to the clinical use of the visual evoked field but, in addition, provides complementary information to visual evoked potentials, allowing detailed studies of the visual cortex. © 1992 Kluwer Academic Publishers.

The visual evoked magnetic response to a pattern onset stimulus

The topography of the visual evoked magnetic response to a pattern onset stimulus was studied in four normal subjects. The topography of th CIIm component was consistent when measured on the same subject nine months apart. Full field responses were more variable than half field responses. With decreasing check size, the field pattern changes from a simple distribution with one outgoing and one ingoing area of field to a more complex pattern with in and outgoing fields over each hemisphere of the brain. The source may originate at the pole or from within the calcarine fissure.

The use of flash and pattern evoked fields in the diagnosis of Alzheimer's disease

Subjects with Alzheimer's disease (AD) exhibit normal visually evoked potentials (VEP) to pattern reversal stimuli but a delayed P2 flash response. The pattern response may originate in the primary visual cortex via the geniculo-calcarine pathway while the flash P2 may originate in the association areas via the cholinergic-tectal pathway. We now show: a) that the pathology of AD is more prominent in the visual association areas B18/19 than in B17 and b) that the magnetic signal to flash and pattern may originate from B18/19 and B17 respectively.

Topography of the visual evoked magnetic response and hemispherical specialization

The visual evoked magnetic response CIIm component to a pattern onset stimulus presented half field produced a consistent scalp topography in 15 normal subjects. The major response was seen over the contralateral hemisphere, suggesting a dipole with current flowing away from the medial surface of the brain. Full field responses were more unpredictable. The reponses of five subjects were studied to the onset of a full, left half and right half checkerboard stimuli of 38 x 27 min arc checks appearing for 200 ms. In two subjects the full field CIIm topography was consistent with that of the mathematical summation of their relevant half field distribution. The remaining subjects had unpredictable full field topographies, showing little or no relationship to their half or summated half fields. In each of these subjects, a distribution matching that of the summated half field CIIm distribution appears at an earlier latency than that of the predominant full field waveform peak. By examining the topography of the full and half field responses at 5 ms intervals along the waveform for one such subject, the CIIm topography of the right hemisphere develops 10 ms before that of the left hemisphere, and is replaced by the following CIIIm component 20 ms earlier. Hence, the large peak seen in full field results from a combination of the CIIm component of the left hemisphere plus that of the CIIIm from the right. The earlier peak results from the CIIm generated in both hemispheres, at a latency where both show similar amplitudes. As the relative amplitudes of these two peaks alter with check and field size, topographic studies would be required for accurate CIIm identification. In addition. the CIIm-CIIIm complex lasts for 80 ms in the right hemisphere and 135 ms in the left, suggesting hemispherical apecialization in the visual processing of the pattern onset response.

The visually evoked magnetic response (VEMR) to a pattern onset/offset stimulus

This study characterizes the visually evoked magnetic response (VEMR) to pattern onset/offset stimuli, using a single channel BTi magnetometer. The influence of stimulus parameters and recording protocols on the VEMR is studied with inferences drawn about the nature of cortical processing, its origins and optimal recording strategies. Fundamental characteristics are examined, such as the behaviour of successive averaged and unaveraged responses; the effects of environmental shielding; averaging; inter- and intrasubject variability and equipment specificity. The effects of varying check size, field size, contrast and refractive error on latency, amplitude and topographic distribution are also presented. Latency and amplitude trends are consistent with previous VEP findings and known anatomical properties of the visual system. Topographic results are consistent with the activity of sources organised according to the cruciform model of striate cortex. A striate origin for the VEMR is also suggested by the results to quarter, octant and annulus field stimuli. Similarities in the behaviour and origins of the sources contributing to the CIIm and CIIIm onset peaks are presented for a number of stimulus conditions. This would be consistent with differing processing event in the same, or similar neuronal populations. Focal field stimuli produce less predictable responses than full or half fields, attributable to a reduced signal to noise ratio and an increased sensitivity to variations in cortical morphology. Problems with waveform peak identification are encountered for full field stimuli that can only be resolved by the careful choice of stimulus parameters, comparisons with half field responses or with reference to the topographic distribution of each waveform peak. An anatomical study of occipital lobe morphology revealed large inter- and intrasubject variation in calcarine fissure shape and striate cortex distribution. An appreciation of such variability is important for VEMR interpretation, due to the technique's sensitivity to source depth and orientation, and it is used to explain the experimental results obtained.