Optic flow in human vision: MEG reveals a foveo-fugal bias in V1, specialization for spiral space in hMSTs, and global motion sensitivity in the IPS

Abstract We recorded MEG responses from 17 participants viewing random-dot patterns simulating global optic flow components (expansion, contraction, rotation, deformation, and translation) and a random motion control condition. Theta-band (3–7 Hz), MEG signal power was greater for expansion than the other optic flow components in a region concentrated along the calcarine sulcus, indicating an ecologically valid, foveo-fugal bias for unidirectional motion sensors in V1. When the responses to the optic flow components were combined, a decrease in MEG beta-band (17–23 Hz) power was found in regions extending beyond the calcarine sulcus to the posterior parietal lobe (inferior to IPS), indicating the importance of structured motion in this region. However, only one cortical area, within or near the V5/hMT+ complex, responded to all three spiral-space components (expansion, contraction, and rotation) and showed no selectivity for global translation or deformation: we term this area hMSTs. This is the first demonstration of an exclusive region for spiral space in the human brain and suggests a functional role better suited to preliminary analysis of ego-motion than surface pose, which would involve deformation. We also observed that the rotation condition activated the cerebellum, suggesting its involvement in visually mediated control of postural adjustment.

Fabrication of micro-channels in optical fibers using femtosecond laser pulses and selective chemical etching

The fabrication of micro-channels in single-mode optical fibers is demonstrated using focused femtosecond laser processing and chemical etching. Straight line micro-channels are achieved based on a simple technique which overcomes limitations imposed by the fiber curved surface.

Genetics of optic atrophy: recent progress in understanding

This article considers the clinical symptoms associated with hereditary optic atrophy and reviews recent progress in our understanding the genetics of the disorder. The major genes linked to optic atrophy are identified and how defects in these genes could lead to the optic disc pathology is discussed.

Large diameter optic nerve axon fibres in Alzheimer's disease

A variety of visual symptoms have been associated with Alzheimer's disease (AD). These include delays in flash visual evoked potentials which indicate a disruption of the integrity of the visual pathway. Examination of the visual cortex has revealed the presence of both senile plaques and neurofibrillary tangles. The purpose of this study was to determine whether there were differences in the number and/or size of optic nerve axons between AD patients and non-demented age-matched controls. Five optic nerves from AD patients and five from age-matched controls were embedded in epon resin and 1 micron sections prepared on a Reichert ultramicrotome. The sections were then stained in toluidine blue and examined at x400 magnification. The numbers of axons were counted in photographs of three fields taken at random from each section. To evaluate the axon diameters, 70 axons were chosen at random from each patient and measured using a calibrated eyepiece graticule. The total axon counts revealed no significant differences between the AD optic nerves and the age-matched controls. However, the frequency distribution of axon diameters was significantly different in the two groups. In particular, there were fewer larger diameter axons in patients with AD as previously reported. Degeneration of the large diameter axons suggests involvement of the magnocellular as opposed to the parvocellular pathways. Hence, there could be differences in visual performance of AD patients compared with normals which could be important in clinical diagnosis.

The incidence of corpora amylacea (CA) in the optic nerve of patients with Alzheimer's disease

Corpora amylacea (CA) are spherical or ovoid bodies 50-50 microns in diameter. They have been described in normal elderly brain as well as in a number of neurodegenerative disorders. In this study, the incidence of CA in the optic nerves of Alzheimer's disease (AD) patients was compared with normal elderly controls. Samples of optic nerves (MRC Brain Bank, Institute of Psychiatry) were taken from 12 AD patients (age range 69-94 years) and 18 controls (43-82 years). Optic nerves were fixed in 2% buffered glutaraldehyde, post-fixed in osmium tetroxide, embedded in epoxy resin and then sectioned to a thickness of 2 microns. Sections were stained with toluidine blue. CA were present in all of the optic nerves examined. In addition, a number of similarly stained but more irregularly shaped bodies were present. Fewer CA were found in the optic nerves of AD patients compared with controls. By contrast, the number or irregularly shaped bodies was increased in AD. In AD, there may be a preferential decline in the large diameter fibres which may mediate the M-cell pathway. Hence, the decline in the incidence of CA in AD may be associated with a reduction in these fibres. It is also possible that the irregualrly shaped bodies are a degeneration product of the CA.

Fiber Bragg gratings inscribed using 800nm femtosecond laser and a phase mask in singleand multi-core mid-IR glass fibers

For the first time, Fiber Bragg grating (FBG) structures have been inscribed in single-core passive germanate and three-core passive and active tellurite glass fibers using 800nm femtosecond (fs) laser and phase mask technique. With fs peak power intensity in the order of 1011W/cm2, the FBG spectra with 2nd and 3rd order resonances at 1540 and 1033nm in the germanate glass fiber and 2nd order resonances at ~1694 and ~1677nm with strengths up to 14dB in all three cores in the tellurite fiber were observed. Thermal responsivities of the FBGs made in these mid-IR glass fibers were characterized, showing average temperature responsivity ~20pm/°C. Strain responsivities of the FBGs in germanate glass fiber were measured to be 1.219pm/µe.

Modified hydrogel matrices in fibre optic sensors

Common problems encountered in clinical sensing are those of non-biocompatibility, and slow response time of the device. The latter, also applying to chemical sensors, is possibly due to a lack of understanding of polymer support or membrane properties and hence failure to optimise membranes chosen for specific sensor applications. Hydrogels can be described as polymers which swell in water. In addition to this, the presence of water in the polymer matrix offers some control of biocompatibility. They thus provide a medium through which rapid transport of a sensed species to an incorporated reagent could occur. This work considers the feasibility of such a system, leading to the design and construction of an optical sensor test bed. The development of suitable membrane systems and of suitable coating techniques in order to apply them to the fibre optics is described. Initial results obtained from hydrogel coatings implied that the refractive index change in the polymer matrix, due to a change in water content with pH is the major factor contributing to the sensor response. However the presence of the colourimetric reagent was also altering the output signal obtained. An analysis of factors contributing to the overall response, such as colour change and membrane composition were made on both the test bed, via optical response, and on whole membranes via measurement of water content change. The investigation of coatings with low equilibrium water contents, of less than 10% was carried out and in fact a clearer signal response from the test bed was noted. Again these membranes were suprisingly responding via refractive index change, with the reagent playing a primary role in obtaining a sensible or non-random response, although not in a colourimetric fashion. A photographic study of these coatings revealed some clues as to the physical nature of these coatings and hence partially explained this phenomenon. A study of the transport properties of the most successful membrane, on a coated wire electrode and also on the fibre optic test bed, in a series of test environments, indicated that the reagent was possibly acting as an ion exchanger and hence having a major influence on transport and therefore sensor characteristics.

The temperature dependence of the electro-optic kerr effect in solutions

A novel Kerr cell of greater optical path length and temperature stability has been designed and built. The Kerr effect experimental has been substantially automated using an Apple IIe computer. Software has been written allowing the computer to partially control the Kerr effect equipment and to acquire and analyse the relevant data.The temperature dependent electro-optic Kerr effect of 2-methyl-4-nitroaniline, p-nitroaniline, nitrobenzene, aniline, and toluene as solutions in 1, 4-dioxane has been studied. The Kerr effect measurements combined with dipole moment, depolarisation ratio, dielectric, and electronic polarisation measurements have been used to calculate the first hyperpolarisability of the solute.Although first hyperpolarisabilities for the compounds studied have been measured in various physical states using a variety of experimental techniques, it is gratifying to find that the values presented in this thesis have a linear relationship with values reported by other workers.

The combination of smart hydrogels and fibre optic sensor technology for analyte detection

The subject of investigation of the present research is the use of smart hydrogels with fibre optic sensor technology. The aim was to develop a costeffective sensor platform for the detection of water in hydrocarbon media, and of dissolved inorganic analytes, namely potassium, calcium and aluminium. The fibre optic sensors in this work depend upon the use of hydrogels to either entrap chemotropic agents or to respond to external environmental changes, by changing their inherent properties, such as refractive index (RI). A review of current fibre optic technology for sensing outlined that the main principles utilised are either the measurement of signal loss or a change in wavelength of the light transmitted through the system. The signal loss principle relies on changing the conditions required for total internal reflection to occur. Hydrogels are cross-linked polymer networks that swell but do not dissolve in aqueous environments. Smart hydrogels are synthetic materials that exhibit additional properties to those inherent in their structure. In order to control the non-inherent properties, the hydrogels were fabricated with the addition of chemotropic agents. For the detection of water, hydrogels of low refractive index were synthesized using fluorinated monomers. Sulfonated monomers were used for their extreme hydrophilicity as a means of water sensing through an RI change. To enhance the sensing capability of the hydrogel, chemotropic agents, such as pH indicators and cobalt salts, were used. The system comprises of the smart hydrogel coated onto an exposed section of the fibre optic core, connected to the interrogation system measuring the difference in the signal. Information obtained was analysed using a purpose designed software. The developed sensor platform showed that an increase in the target species caused an increase in the signal lost from the sensor system, allowing for a detection of the target species. The system has potential applications in areas such as clinical point of care, water detection in fuels and the detection of dissolved ions in the water industry.

Fibre Bragg grating sensor interrogation using an acousto-optic tunable filter and low-coherence interferometry

We describe how an acousto-optic tunable filter can be used to both demultiplex the signals from multiple fibre Bragg grating sensors and simultaneously provide wide bandwidth signal demodulation in a system using interferometric wavelength shift detection. In an experimental demonstration, the approach provided a noise limited strain resolution of 24.9 n epsilon Hz(-1/ 2) at 15 Hz.

Autonomic dysfunction and systemic oxidative stress associated with glaucomatous optic neuropathy

There were four principal sections to the work: 1. Investigation of ocular and systemic vascular risk factors in POAG. The principal findings of this work were: a). Glaucoma patients exhibit an anticipatory reaction to the physical stress, similar to subjects at risk for cardiovascular diseases; a blunted BP response and a reduction in ONH blood flow in response to cold provocation was also recorded. b). Silent myocardial ischaemic episodes occurred during peaks in systemic BP and HR. c). Independent of a positive history for cardiovascular diseases, patients suffering from POAG demonstrate a blunt circadian rhythm of the ANS. 2. Assessment of the relationship between vascular and systemic vascular risk factors in GON. The principal findings of this work were: a). POAG patients demonstrate a high sympathetic tonus over a 24-h period. b). POAG patients with lower OBF demonstrate both 24-h systemic BP and HRV abnormalities. c). OBF alterations observed in some glaucoma patients could be either primary or secondary to systemic haemodynamic disturbances and not a consequence of ONH damage. 3. Assessment of the level of systemic anti-oxidant defence in POAG patients. The principal finding of this work was: Patients suffering from POAG demonstrated significantly lower GSH and t-GSH levels than normal controls. 4. Investigation of the effect of treatment with latanoprost 0.005% on visual function and OBF. The findings of this work were: a). Treatment with latanoprost 0.005% resulted in a significant decrease in IOP and increase in OPP. VF damage progression has also been stopped. b). Treatment with latanoprost 0.005% resulted in a significant increase in the OBF parameters measured at the ONH and peripapillary retina levels. Finally, the importance of a clear protocol for managing new POAG cases is highlighted and a clinical conduit is proposed.

Topographic, haemodynamic and psychophysical investigation of glaucomatous optic neuropathy

Loss of optic nerve head (ONH) axons in primary open angle glaucoma (POAG) has been attributed to both mechanical and vascular factors. Confocal scanning laser ophthalmoscopy (cSLO) provides a promising tool for the topographic follow-up of the ONH in glaucoma, while scanning laser Doppler flowmetry (SLDF) facilitates the rapid non-invasive assessment of retinal capillary blood flow. The purposes of these investigations were to optimise the techniques and explore their potential to classify and monitor disease. Preliminary investigations explored the reproducibility and validity of cSLO and SLDF and showed that: For cSLO: In a model eye, measurements are accurate over a range of axial lengths. For best reproducibility, seven images per visit are required, with a contour line located on Elschnig's scleral ring and transferred automatically between images. For SLDF: Three perfusion images are required for optimum reproducibility. Physiological changes induced by gas perturbation can be measured. Cross-sectional comparison of groups of normal subjects and early POAG patients showed that: cSLO parameters differentiate the early POAG group. Blood volume measured by SLDF showed group differences in superior nasal retina only. Longitudinal investigation of ONH topography, haemodynamic and visual field indices in normal subjects and POAG patients showed that: cSLO detects topographical change over time more frequently in the POAG group. Important parameters include: C:D area ratio, cup and rim area, mean depth in contour, volumes above and below reference and surface. Factor analysis identified "cup" and "rim" factors that can be used to detect change over time in individual patients. Blood flow changes were most apparent in the inferior nasal peripapillary retina of the POAG group. Perimetry is of clinical value for the identification of glaucoma but is less sensitive than cSLO for monitoring glaucomatous change.

Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80° tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80° tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs.