Evidence for antibiotic induced Clostridium perfringens diarrhoea.
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Aged↗
Biomedical subjects
T Carney
Publications and source records attributed to T Carney.
Electrophysiological estimate of human cortical magnification.
OBJECTIVE: The cortical magnification factor characterizes the area of human primary visual cortex activated by a stimulus as a function of angular distance from an observer's line of sight. This study estimates human cortical magnification using an electrophysiological method with excellent temporal resolution: visual evoked potential (VEP) dipole source localization. METHODS: For each of 60 independently modulated checkerboard patches within the central 18 deg of the visual field, location, orientation, magnitude, and time-course of the dipole current source that best described the VEP distribution across a multi-electrode array was obtained. At numerous eccentricities, cortical magnification was determined using two different techniques: (1) the distance between each pair of adjacent stimulus patches was matched to the corresponding distance between adjacent cortical sources; and (2) the area of each stimulus patch was matched to the magnitude of the corresponding cortical source (which was assumed to be proportional to cortical area). RESULTS: The estimates of human cortical magnification using our electrophysiological method were similar to previous estimates from psychophysics, cortical stimulation, and functional magnetic resonance imaging. CONCLUSIONS: The concordance of results provided by these disparate technologies, with differing spatial and temporal limitations, supports their combination in studying the spatio-temporal dynamics of human brain function.
Adult↗
Unmasking the mechanisms for Vernier acuity: evidence for a template model for Vernier acuity.
The goal of this study was to evaluate the mechanisms underlying Vernier acuity, over a range of spatial scales using narrow-band Vernier stimuli and oblique masking. Specifically, the test stimuli consisted of a pair of vertical ribbons of horizontal cosine grating with a vertical Vernier offset between the ribbons. These stimuli have two important advantages for studying Vernier acuity: (1) they are relatively well localized in vertical spatial frequency, and (2) they are localized in their horizontal extent (width). We measured the orientation, spatial frequency and width tuning of Vernier acuity over a wide range of ribbon spatial frequencies, using a simultaneous oblique masking paradigm. Our masking results suggest that the mechanisms underlying Vernier acuity are tuned to the orientation, spatial frequency and width of the ribbon stimuli. The peak of the bimodal orientation tuning function varies systematically with the spatial frequency of the ribbon. The peak of the spatial frequency tuning function varies systematically with both the ribbon spatial frequency, and the ribbon width (i.e. the grating length). A 'template' model, in which the 'mechanism' is a windowed version of the stimulus is able to account for many features of the data, including results which cannot be easily accounted for by standard multi-scale filter models. Specifically, the template model can account for: (i) the bimodal orientation tuning function, (ii) the systematic variation in the peak of the orientation and spatial frequency tuning functions with spatial frequency, and (iii) the systematic effect of ribbon width on spatial frequency tuning.
Contrast Sensitivity↗
Setting up a pain management programme. The Ayrshire experience.
A controlled trial of an outpatient cognitive behavioural pain management programme for sufferers of non-cancer chronic pain is described. A multidisciplinary team set up a programme of ten half day sessions for groups of ten to fourteen patients aiming to improve activity levels and control over pain; to reduce maladaptive pain behaviours and drug intake; to mitigate negative mood; to modify unhelpful beliefs and to maintain treatment gains by operant and cognitive methods. Self report questionnaires were employed before and six weeks, six months and one year after the programme. Fifty-eight patients entered the study group and 39 patients completed the programme and initial follow up with further attrition in long term follow up. There were no changes in the waiting list control group of twelve subjects but the study group made significant short and long term improvements in pain severity, activity levels, mood, coping and experienced fewer catastrophizing thoughts.
Activities of Daily Living↗
Optimal spatial localization is limited by contrast sensitivity.
Bisection is one of several spatial localization tasks that achieve hyperacuity performance levels. We find that optimal bisection thresholds, and hyperacuity tasks in general, are no better than might be expected from simple contrast detection and discrimination performance. The three-line bisection task can be described in terms of the test-pedestal paradigm where the test pattern is a horizontal dipole and the pedestal is a horizontal three-line pattern with equal spacing between the lines. When the dipole test is added to the center line, the line shifts up or down, depending on the test polarity. For low contrast pedestal lines at the optimal separation, the bisection threshold falls between the observer's own dipole contrast detection threshold and the bottom of the dipole contrast discrimination dipper function. At higher pedestal strengths performance degrades with a slope of about 0.5-0.7, similar to that found in contrast discrimination tasks. Therefore, bisection performance is compatible with expectations based on contrast discrimination data. At large pedestal line separations (> 10 min) bisection thresholds in min are about 1/60 the separation and relatively independent of pedestal strength. These findings are consistent with the idea that two processes are involved in limiting bisection performance; the first limit is based on contrast sensitivity of the system and the second limit to performance is based on a local sign or position tag processing. Finally, when bisection is compared with Vernier acuity and blur resolution tasks, where the test is also a dipole, bisection performance falls roughly midway, better than Vernier acuity but worse than blur resolution.
Contrast Sensitivity↗
Using multi-stimulus VEP source localization to obtain a retinotopic map of human primary visual cortex.
OBJECTIVE: The goal of this study was to acquire a detailed spatial and temporal map of primary visual cortex using a novel VEP stimulus and analysis technique. METHODS: A multi-stimulus array spanning the central 18 degrees of the visual field was used where each of 60 checkerboard stimulus 'patches' was simultaneously modulated with an independent binary m-sequence (Sutter, 1992). VEPs corresponding to each patch were recorded from 3 subjects using a dense posterior electrode array. For each stimulus patch, single dipole source localization was conducted to determine the location, magnitude, and time-function of the underlying neural activation. To reduce ambiguity in the solution, a common time-function was assumed for stimulus patches at the same visual eccentricity (defining an annulus). The analysis was conducted independently for each annulus composed of 4-12 patches. RESULTS: The loci of the dipole solutions followed a smooth retinotopic pattern across annuli consistent with the classical organization of primary visual cortex. Specifically, each dipole was found contralateral to the corresponding stimulus patch and field inversion was observed for all subjects. CONCLUSIONS: Using this technique, the most detailed spatial and temporal retinotopic map of primary visual cortex to date has been obtained.
Adult↗
Adult guardianship: narrative readings in the "shadow" of the law?
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Adult↗
Evidence for an early motion system which integrates information from the two eyes.
In one type of cyclopean motion stimulus one eye views a counterphase flickering grating while the other eye views the same pattern in spatio-temporal quadrature. Algebraic summation of the two image sequences results in a drifting grating. Upon binocular (cyclopean) combination of the two patterns a drifting grating is perceived even though neither monocular pattern is moving. While this appears to support the position that the motion system is binocular, it has been suggested that such demonstrations involve higher level feature tracking rather than early motion system activation. The perceived direction of motion could result from the tracking of features after neural summation of left and right eye images. However, by adding a static, in-phase, pedestal grating to the left and right eye flickering test gratings, the direction information based on feature tracking is removed while leaving the motion energy information unchanged. We have found that when such stimuli are presented for several seconds, direction discrimination performance is significantly better than chance for pedestal grating contrasts several times the test grating contrast. Therefore, in the absence of a feature tracking cue, the direction of motion is identified using a binocular motion energy mechanism. The results do not exclude the existence of a binocular feature tracking system. Both systems are likely to exist.
Humans↗
Resolution acuity is better than vernier acuity.
Our impressive sensitivity to vernier offsets as compared to resolution acuity has long inspired vision researchers to study the phenomena in great detail. In this study we use the test-pedestal framework to compare resolution and vernier acuity. In these experiments the test stimulus is the same for both tasks, only the pedestals differ. When thresholds are expressed in common units of test strength, vernier acuity thresholds are higher (worse) than for resolution and contrast discrimination tasks over the range of pedestal strengths tested. This apparent reversal of sensitivity is actually consistent with expectations based on the presumed underlying visual mechanisms involved in the tasks.
Contrast Sensitivity↗
Introduction: competence.
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Decision Making↗
Caught between two systems? Guardianship and young people with a disability.
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Adolescent↗
Motion thresholds can be predicted from contrast discrimination.
The detection thresholds for oscillatory motion and flicker were compared across a wide range of pedestal contrasts, spatial frequencies, and temporal frequencies in both foveal and peripheral vision. Motion and flicker stimuli were both generated by summation of a counterphase test grating with a static pedestal grating. For the oscillatory motion task the test and the pedestal were presented 90 deg out of phase to each other, whereas for flicker the two gratings were presented in phase. Since detection of both stimuli would depend on the same test stimulus, detection thresholds could be similar even though the tasks differ. Although there was a slight elevation in flicker detection thresholds compared with motion thresholds, our main finding is that oscillatory motion and flicker thresholds for suprathreshold sinusoidal gratings are similar. This finding supports the idea that motion and flicker have a common underlying detection mechanism. Our finding that flicker thresholds are slightly higher than jitter thresholds indicates that the contrast gain control (or saturating energy transducer) has a weak phase dependence. The ability to discriminate motion from flicker was elevated relative to their detection thresholds, particularly at high temporal frequencies. We offer two models to account for this behavior. The discrimination of motion from flicker may require a temporal comparison of the outputs of directionally selective filters tuned to opposite directions or to the population statistics of a bank of separable mechanisms.
Contrast Sensitivity↗
The adjacent pixel nonlinearity: problems and solutions.
The adjacent pixel nonlinearity refers to the dependence of the luminance of a given pixel on the preceding pixel or pixels. We measured this nonlinearity for two CRT displays by measuring the average luminances of a variety of test patterns with different luminance jumps. A two-stage model proposed by Mulligan and Stone was used to fit the data [Mulligan, J.B. & Stone, L. S., (1989). Journal of the Optical Society of America A, 6, 1217-1227 (1989)]. The results show that the model predicts our data well. Based on our measurements and the modeling results, a double-entry look-up table was created to compensate for this nonlinearity. This compensation method works even if the current pixel depends on more than one preceding pixel. Observers commented that at small pixel sizes the compensation results in a sharp, accurate image. Advantages and problems of this compensation will be discussed.
Computer Graphics↗
The usefulness of the Laplacian in principal component analysis and dipole source localization.
Evoked potentials are difficult to analyze because multiple sources are active simultaneously. Principal component analysis and dipole localization are two techniques that have been used to disentangle overlapping sources. Both of these techniques have problems. Principal component analysis suffers from a rotation ambiguity. Dipole localization suffers from biases when the model used to derive the sources from the scalp potentials is misspecified. Using computer simulations we demonstrate that by applying both of these techniques to the Laplacian of the voltages rather than to the raw voltages the problems associated with the two techniques are reduced. Computer programs for the analyses are presented in an Appendix.
Brain Mapping↗
Vernier acuity during image rotation and translation: visual performance limits.
Our capacity to detect spatial misalignments a fraction of the distance between retinal receptors in the presence of image motion challenges our understanding of spatial vision. We find that vernier acuity, while robust to image translation, rapidly degrades during image rotation. This indicates that orientation is a critical cue utilized by the visual system in vernier acuity tasks. Moreover, vernier acuity is robust to translational motion only at high target strengths. Vernier acuity for translating 3-dot targets over midrange velocities can be predicted from vernier acuity data derived from static targets of different presentation durations. However, the degradation observed at higher velocities is greater than predicted. The high velocity degradation reveals that performance is limited by a 1 msec asynchrony sensitivity. The moving vernier stimulus appears to constitute an optimal configuration for the visual system to achieve a 1 msec asynchrony sensitivity by making use of an orientation cue.
Cues↗
Effect of structured postgraduate medical education on the care of patients with diabetes.
BACKGROUND: Diabetes can be effectively treated and monitored in general practice. Postgraduate medical education at a local level is required to support this strategic shift of medical care from hospital to general practice. AIM: This study set out to determine whether a structured educational programme initiated by and led by general practitioners, but involving all health professionals, leads to improved care for diabetic patients. METHOD: The primary health care teams from 12 of the general practices in Tynedale, Northumberland who attend the local postgraduate centre and refer patients to the local general hospital were involved in the educational initiative which led to a locally agreed protocol for diabetic care. A comparison was made of data for all diabetic patients, registered with the general practitioners in 1991, in the years 1986 and 1991 (before and after the educational initiative) in order to determine the prevalence of diabetes, place of care and treatment received, and to collect clinical information. RESULTS: More patients received general practice care only or shared care in 1991 than in 1986. There was a reduction in the use of oral hypoglycaemic agents among non-insulin dependent diabetic patients and more patients were maintained on diet alone. A greater proportion of patients were referred to dietitians, ophthalmologists and chiropodists in 1991 than 1986, and there was increased recording of, examination of, and identification of, diabetic complications. Little change was found in the mean values for clinical parameters between the two years. CONCLUSION: Structured educational programmes involving all professionals are an effective way of producing protocols that are adhered to by all concerned, and lead to improved clinical care for diabetic patients.
Clinical Protocols↗
The topography of visual evoked response properties across the visual field.
Visual evoked potentials (VEPs) to luminance and pattern reversal stimulation were derived for a large number of small areas throughout the central visual field. In one study, the field was tested with a stimulus array consisting of 64 equal-area patches. Local response components were extracted by independent m-sequence modulation of the patches. Field topographies were compared between and within subjects using different electrode placements. The subject-dependent local variability observed in response characteristics is attributed to contributions from two or more cortical representations of the visual field and to inter-subject variations in gross cortical anatomy. The second study used luminance modulation of 56 patches across a 15 degrees field, scaled to activate approximately equal cortical areas in area V1. This produced many robust signals at all eccentricities. Bipolar and double differential ("1-dimensional Laplacian") signals were compared. The double differencing reduced contributions from distant or distributed sources, enhancing nearby current source activity, and greatly improved S/N for many stimulus locations. The high-resolution visual field maps demonstrated that clinical field testing using the VEP is not feasible because of effects of cortical convolutions on responses. However, the vast improvement in data quality and quantity make it a useful tool for VEP source localization and identification.
Brain↗
Dichoptic activation of the early motion system.
The short range or early motion system has long been considered incapable of binocular integration. We have developed dichoptic motion stimuli which are based upon the decomposition of traveling sinewave gratings into the sum of two standing waves in spatial and temporal quadrature. The monocular views of such displays appear as counterphase flicker but when presented dichoptically the perception is of movement in a unique direction. Two lines of evidence are presented for the binocularity of early motion mechanisms in human vision. First, adaptation to dichoptic motion sinewave gratings is found to result in a motion aftereffect. Second, random texture motion displays based on the quadrature decomposition are found to support dichoptic perception of motion direction, but not figure/ground. Unlike random dot kinematograms, these displays do not necessitate alternating the direction of motion during dichoptic presentation. This encumbrance, and the reliance on figure/ground discrimination, may have been responsible for prior failure to achieve dichoptic motion perception with short range stimuli.
Adaptation, Ocular↗