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Biomedical subjects

F Wilkinson

Publications and source records attributed to F Wilkinson.

At least 19 recordsLinked to original sources
PubMed2006

Ocular motor measures in migraine with and without aura.

The purpose of this study was to examine basic ocular motor function in individuals with migraine. We used an infrared eye-tracking system to measure horizontal smooth pursuit to a sinusoidal target, saccades to horizontal target displacements of 5-20 degrees , and the stability of fixation in 19 migraine without aura (MoA), 19 migraine with aura (MA) and 19 headache-free control (C) subjects. Eye movement measurements were made at two target displacement rates and against both homogeneous grey and patterned backgrounds. We found no statistically significant differences between migraine and control subjects in any of the eye movement parameters measured, but did find highly significant effects of both target speed and background pattern in all groups. Our results do not provide support for subclinical cerebellar impairment in migraineurs, and do provide evidence that previously described visual abnormalities in migraine are not artefacts of abnormal fixation or eye movements.

Adult↗
PubMed2001

An inverse oblique effect in human vision.

In the classic oblique effect contrast detection thresholds, orientation discrimination thresholds, and other psychophysical measures are found to be smallest for vertical or horizontal stimuli and significantly higher for stimuli near the +/-45 degrees obliques. Here we report a novel inverse oblique effect in which thresholds for detecting translational structure in random dot patterns [Glass, L. (1969). Moiré effect from random dots. Nature, 223, 578-580] are lowest for obliquely oriented structure and higher for either horizontal or vertical structure. Area summation experiments provide evidence that this results from larger pooling areas for oblique orientations in these patterns. The results can be explained quantitatively by a model for complex cells in which the final filtering stage in a filter-rectify-filter sequence is of significantly larger area for oblique orientations.

Contrast Sensitivity↗
PubMed2001

Photophysical properties of lumichromes in water.

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 micros have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 micros has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, phi(Delta), are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D(2)O.

Journal Article↗
PubMed2001

Perceived scintillation rate of migraine aura.

OBJECTIVE: To measure the perceived rate of flicker (temporal frequency) observed during visual auras. BACKGROUND: The flickering or scintillating quality of aura elements is a commonly described characteristic of visual migraine auras. Hypotheses about the neural mechanisms involved in aura have rarely taken this feature into account, perhaps because of a lack of quantitative data on this aspect of the aura. While a rate of 10 Hertz had been suggested in the literature, estimates have been speculative due to the difficulty of judging temporal frequencies subjectively. METHODS: Eleven participants were given portable devices that contained an adjustable light-emitting diode with which to match the flickering of their auras. Observers were asked to make flicker matches at two time points so that rate change during aura progression could be analyzed. RESULTS: Data were obtained for 36 aura episodes. The mean rate of flicker across individuals was 17.8 Hertz. Rates varied widely between individuals, but were more consistent across multiple episodes in the same observer. Rate of flicker did not appear to relate to aura side or type, or to individual characteristics such as migraine history. When episodes were analyzed for change in flicker rate over time, patterns of increase (n = 7), decrease (n = 4), and no change (n = 22) were all observed. CONCLUSIONS: When measured with an objective task, aura scintillation rates were found to be somewhat higher than previous anecdotal observations had suggested. These data are discussed in the context of two competing hypotheses concerning the neural mechanism underlying the flicker percept during migraine aura.

Adult↗
PubMed2000

An fMRI study of the selective activation of human extrastriate form vision areas by radial and concentric gratings.

The ventral form vision pathway of the primate brain comprises a sequence of areas that include V1, V2, V4 and the inferior temporal cortex (IT) [1]. Although contour extraction in the V1 area and responses to complex images, such as faces, in the IT have been studied extensively, much less is known about shape extraction at intermediate cortical levels such as V4. Here, we used functional magnetic resonance imaging (fMRI) to demonstrate that the human V4 is more strongly activated by concentric and radial patterns than by conventional sinusoidal gratings. This is consistent with global pooling of local V1 orientations to extract concentric and radial shape information in V4. Furthermore, concentric patterns were found to be effective in activating the fusiform face area. These findings support recent psychophysical [2,3] and physiological [4,5] data indicating that analysis of concentric and radial structure represents an important aspect of processing at intermediate levels of form vision.

Brain Mapping↗
PubMed2000

Methylene blue based protein solder for vascular anastomoses: an in vitro burst pressure study.

BACKGROUND AND OBJECTIVE: Attempts at sutureless anastomoses have used protein-based solders containing chromophores [Oz et al., J Vasc Surg 1990;11:718; Poppas et al., J Urol 1998150:1052] to enhance the strength of laser anastomoses. Reports have described the use of indocyanine green [Oz et al., Surg Forum 1989;316.], fuschin, and fluorescein isothiocyanate as chromophores [Chuck et al. , Lasers Surg Med 1989;9:471; Vance et al., Lasers Med Sci 1988;3:219]. Methylene blue (MB) is a chromophore with absorption peaks in the 600-700 nm region whose use has not been reported in laser-assisted vascular anastomoses. Therefore, we set out to produce and characterise a MB-containing protein solder. The absorption and burst pressure characteristics have been investigated and described as well as a brief review of the chemical and biological properties of MB. STUDY DESIGN/MATERIALS AND METHODS: The MB and porcine serum albumin (PSA)-based solder was produced and used to form end-to-end anastomoses in porcine splenic arteries. The solder was activated using a laser diode emitting at 670 nm. The burst pressures of the anastomoses were tested, and the results analysed as a function of MB concentration and absorption. In addition, the relationship between MB concentration and absorption was examined. RESULTS: A dose-response relationship was found between the measured absorption of the solder and the burst pressure of the anastomoses formed. Burst pressures exceeding physiological levels were found. Changes in MB concentration revealed a marked negative deviation from Beer's law at 670 nm, owing to the monomer-dimer-trimer equilibria. CONCLUSION: PSA with MB solder is able to form high-quality end-to-end anastomoses, with immediate burst pressure profiles similar to those previously described for sutured [Quigley et al., Microsurgery 1985;6:229], lasered [Quigley et al., Microsurgery 1985;6:229], and soldered anastomoses [Small et al., J Clin Laser Med Surg 1997;15:205]. The relationship between burst pressure strength and chromophore absorption is discussed.

Absorption↗
PubMed2000

Perception of head orientation.

There are two visual components to gaze: head orientation and orientation of the eyes relative to the head. This study explores the accuracy with which subjects can discriminate head orientation when the eyes are centered in the head. Discrimination thresholds averaged 1.9 degrees of head rotation for base head orientations of 0 degree and 15 degrees, but discrimination was markedly poorer around a 30 degrees head orientation. Results were independent of spatial frequency and size over a 4-fold range. Neither negative contrast nor head inversion affected discrimination. Experiments dissociating the internal features from head outline revealed the presence of two main cues to discrimination: deviation of the head profile from bilateral symmetry, and deviation of nose orientation from vertical. Simulations show that model V4 units revealed in previous experiments with Glass patterns can extract the relevant head orientation information. The data are consistent with neurological data indicating a selective loss of face recognition in prosopagnosia with spared gaze discrimination.

Discrimination, Psychological↗
PubMed2000

Dynamics of perceptual oscillations in form vision.

Certain periodic dot patterns (Marroquin patterns) generate a percept of dynamically oscillating circles, and analogous effects were explored by op artists in the 1960s. Here we show psychophysically that circles are perceived in these patterns only around specific points that are quantitatively predicted by a neural model of configural units hypothesized to reside in cortical area V4. Circles superimposed on the pattern mask perception of illusory circles. A neural model of lateral inhibitory interactions among V4 configural units showing spike-frequency adaptation quantitatively accounts for the human data. The model is consistent with ideas on the neural basis of attention in V4, and it suggests that attention may be biased via neuromodulation of slow hyperpolarizing potentials in cortical neurons.

Action Potentials↗
PubMed2000

Orientation discrimination thresholds in migraine: a measure of visual cortical inhibition.

Orientation discrimination is a visual task dependent on inhibitory mechanisms in the visual cortex. In this study, orientation discrimination thresholds for bar and grating patterns were measured at two visual field locations in subjects with migraine with (n = 20) and without aura (n = 20) and in migraine-free control subjects (n = 20). No statistically significant differences were found between migraine groups and the control group on either task at foveal or peripheral visual field locations. No significant correlations were found between psychophysical thresholds and age, total lifetime auras or total lifetime migraine episodes. However, a trend was seen toward slightly impaired performance on the two foveal tasks in a subgroup of subjects with the highest total lifetime aura count. Thus we have found no convincing evidence that impaired cortical inhibitory mechanisms are a predisposing characteristic in migraine, but cannot rule out the possibility that cortical inhibitory mechanisms may be adversely affected by repeated visual auras.

Adult↗
PubMed2000

Visual contrast gain control in migraine: measures of visual cortical excitability and inhibition.

The present study examined the extent to which migraineurs demonstrate interictal visual cortical hyperexcitability as a result of poor inhibitory control in the visual system. We employed a well-established psychophysical measure of inhibition, visual contrast gain control. The task involved detecting a briefly presented target that was superimposed on a highly excitable high contrast masking pattern. The strength of inhibition was assessed by comparing target detection thresholds with and without the operation of gain controls. Migraineurs with and without aura (n=25, n=22, respectively) were compared with those with no history of migraine (n=25). Our results do not indicate a loss of inhibition in migraine; the strength of inhibitory feedback contrast gain controls was similar between migraineurs and controls. We did however, find a statistically greater masking effect in migraineurs compared with controls in the zero delay condition, suggesting cortical hyperexcitability in migraine. Possible mechanisms of cortical hyperexcitability are discussed in light of the results.

Adolescent↗
PubMed1999

A deficit in strabismic amblyopia for global shape detection.

Using a task which relied upon the detection of sinusoidal deformations from circularity, we show that strabismic amblyopes exhibit deficits which are not critically dependent on either the scale of deformation or the spatial frequency characteristics of the stimulus (circular D4) itself. We show that this loss is not due to the restricted passband of the amblyopic eye. Furthermore, in a pedestal distortion experiment, we show that the suprathreshold form of this loss is consistent with an elevated level of 'intrinsic noise' rather than a loss in 'sampling efficiency'.

Amblyopia↗
PubMed1999

Modulation frequency and orientation tuning of second-order texture mechanisms.

Modulation frequency and orientation tuning of second-order mechanisms underlying the detection of modulation in local spatial-frequency information are assessed by using an oblique-masking paradigm. Stimuli were Gabor-filtered noise patterns in which the local carrier spatial frequency was modulated about an average value of 4.7 cycles per degree (cpd) according to sinusoidal function. Thresholds were determined for spatial-frequency modulated test patterns (0.2 and 0.8 cpd) with fixed vertical carrier and modulation orientations presented alone and in the presence of spatiotemporally superimposed masks. Mask modulation frequency (0.1, 0.2, 0.4, 0.8, or 1.6 cpd), modulation orientation (0 degree, 45 degrees, or 90 degrees relative to vertical), and carrier orientation (18.5 degrees or 90 degrees relative to vertical) were manipulated independently while the mask modulation amplitude remained fixed at 0.25. Manipulating the modulation frequency of the mask revealed some modulation frequency specificity, particularly at lower test modulation frequencies. Spatial-frequency modulated masks produced threshold elevations regardless of the local carrier orientation. However, there was no evidence of threshold elevation when the mask modulation orientation was orthogonal to that of the test pattern. These results suggest a second-order texture mechanism that is tuned to both modulation frequency and modulation orientation but is not selective in terms of the orientation of first-order inputs.

Contrast Sensitivity↗
PubMed1998

Detection and recognition of radial frequency patterns.

Detection thresholds for radial deformations of circular contours were measured using a range of radii and contour peak spatial frequencies. For radial frequencies above two cycles, thresholds were found to be a constant fraction of the mean radius across a four-octave range of pattern radii and peak spatial frequencies (mean Weber fraction: 0.003-0.004). At low radial frequencies, thresholds were unaffected by contrast reduction. In 167 ms presentations, subjects were able to identify radial frequencies of six cycles and below with an accuracy of over 90% correct even when phase was randomized. The extreme sensitivity of subjects to these radial deformations (as low as 2-4 s of arc) cannot be explained by local orientation or curvature analysis, and points instead to the global pooling of contour information at intermediate levels of form vision.

Female↗
PubMed1998

Detection of global structure in Glass patterns: implications for form vision.

Glass (Nature 1969;223:578-580) patterns are random dot stimuli that generate a percept of global structure. To study the mechanisms underlying this global form perception, concentric, radial, hyperbolic, and parallel Glass patterns were constructed. Thresholds for detecting each type of pattern were measured by degrading the patterns through the addition of noise. Concentric patterns yielded the lowest thresholds for all subjects, while radial and hyperbolic patterns produced somewhat higher thresholds. For all subjects the parallel patterns produced the highest thresholds. Threshold measurements as a function of the area containing pattern structure provided evidence for global pooling of orientation information in the detection of radial and concentric Glass patterns but only local pooling in the detection of parallel patterns. Monte-Carlo simulations demonstrate that plausible neural models can accurately predict the data. These models indicate that the visual system contains networks that pool orientation information within regions 3.5-4.5 degrees in diameter in central vision. This pooling is organized to extract cross-shaped, X-shaped, and quasi-circular forms from the retinal image. The results are in good agreement with recent single unit physiology of primate area V4, an intermediate level of the form vision pathway.

Computer Graphics↗
PubMed1998

Measurement of the texture-coherence limit for bandpass arrays.

Casual observation suggests that when the elements of a visual array are packed at a sufficiently high density they cohere to generate the percept of a texture. This 'texture-coherence limit' has been quantified by using arrays composed of Gabor functions, sixth Gaussian derivatives, or differences of Gaussians. In all cases the texture-coherence limit was a power-law function of the size of the elements as quantified by their space constants with an exponent averaging 0.7. Furthermore, the texture-coherence limit was independent of both element spatial frequency and contrast over a considerable range. A quantitative fit to the data is provided by a model in which the texture-coherence limit is determined by activation of complex cells, which pool a spatial range of subunit inputs, throughout the stimulus region. Possible extensions to two dimensions are considered.

Female↗
PubMed1998

Apparent contrast and spatial frequency of local texture elements.

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.

Contrast Sensitivity↗