Low bulk milk urea in dairy herds.
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Animals↗
Biomedical subjects
David Whitaker
Publications and source records attributed to David Whitaker.
Functional evidence for cone-specific connectivity in the human retina.
Physiological studies of colour vision have not yet resolved the controversial issue of how chromatic opponency is constructed at a neuronal level. Two competing theories, the cone-selective hypothesis and the random-wiring hypothesis, are currently equivocal to the architecture of the primate retina. In central vision, both schemes are capable of producing colour opponency due to the fact that receptive field centres receive input from a single bipolar cell - the so called 'private line arrangement'. However, in peripheral vision this single-cone input to the receptive field centre is lost, so that any random cone connectivity would result in a predictable reduction in the quality of colour vision. Behavioural studies thus far have indeed suggested a selective loss of chromatic sensitivity in peripheral vision. We investigated chromatic sensitivity as a function of eccentricity for the cardinal chromatic (L/M and S/(L + M)) and achromatic (L + M) pathways, adopting stimulus size as the critical variable. Results show that performance can be equated across the visual field simply by a change of scale (size). In other words, there exists no qualitative loss of chromatic sensitivity across the visual field. Critically, however, the quantitative nature of size dependency for each of the cardinal chromatic and achromatic mechanisms is very specific, reinforcing their independence in terms of anatomy and genetics. Our data provide clear evidence for a physiological model of primate colour vision that retains chromatic quality in peripheral vision, thus supporting the cone-selective hypothesis.
Adaptation, Physiological↗
A psychophysical investigation of ocular expansion in human eyes.
PURPOSE: To investigate spatial anisotropies in the peripheral visual field of axially myopic eyes, in an attempt to distinguish between models of isotropic and anisotropic ocular stretching. METHODS: Stimuli consisted of two high-contrast Gaussian patches presented in one of four orientations (90 degrees, 180 degrees, 45 degrees, or 135 degrees). For each orientation, perceived separation was established relative to that for all other orientations. The experiment was conducted with central fixation and at 15 degrees in the nasal and inferior visual fields. Eleven myopes and nine emmetropes participated in the study. Biometric data were collected from all subjects. RESULTS: For foveal fixation, the magnitude of the spatial anisotropy (approximately 5%) was consistent with the well-documented horizontal-vertical illusion (HVI), and unrelated to axial length. In the nasal visual field, much larger misperceptions were found (approximately 19%), the magnitude of which increased significantly with increasing axial length. Inferiorly, a reversal of the traditional HVI is found in most subjects (approximately 7%), with a tendency for a larger reversed illusion with increasing axial length. Differences between nasal and inferior misperceptions were significantly correlated with axial length. CONCLUSIONS: Isotropic stretching, such as globe expansion, should preserve the aspect ratios of receptive fields, predicting a separation misperception which is independent of axial length. In contrast, the magnitude of the misperception is significantly correlated with axial length, supporting anisotropic stretching models of myopic growth.
Elastic Tissue↗
An electronic chart prompt to decrease proprietary antibiotic prescription to self-pay patients.
OBJECTIVES: Emergency physicians unaware of patients' insurance status may prescribe expensive proprietary antibiotics for patients who cannot afford them. The objective of this study was to develop a clinical decision support system to display patient insurance status before prescription writing for outpatient conditions. METHODS: This was a 26-week "before-and-after" trial at an urban emergency department (ED) with 78,000 visits/year treating a medically underserved population. Sixty-one prescribers, including attending physicians, residents, and physician assistants, participated. All patients older than 18 years of age discharged from the ED receiving antibiotic prescriptions were eligible. The electronic ED chart is linked to prescription-writing software, which includes a menu of 74 antibiotics. The system was programmed so that when an emergency physician accessed the prescription menu, a prompt appeared displaying insurance status. Prescribers also received educational interventions. The main outcome measure was the percentage of prescribers who reduced their prescription writing of proprietary antibiotics to self-pay patients. Data were analyzed with cluster techniques using SPSS 10.0 (SPSS Inc., Chicago, IL). RESULTS: Of 594 prescriptions written for self-pay patients before prompt insertion, 158 (26.6%) were for proprietary antibiotics. After the intervention, self-pay patients received 564 antibiotic prescriptions, of which 117 (20.7%) were for proprietary drugs. Analyzed by prescriber, the reduction in the prescription rate for proprietary antibiotics was statistically significant (p = 0.03, chi(2) test). Patients with respiratory or urinary infections also had a statistically significant reduction in proprietary antibiotic prescription (p = 0.03). CONCLUSIONS: A clinical decision support system, integrated into a prescription-writing program, can decrease the prescription of proprietary antibiotics for self-pay patients in the ED.
Adult↗
Positional adaptation reveals multiple chromatic mechanisms in human vision.
Precortical color vision is mediated by three independent opponent or cardinal mechanisms that linearly combine receptoral outputs to form L/M, S/(L+M), and L+M channels. However, data from a variety of psychophysical and physiological experiments indicate that chromatic processing undergoes a reorganization away from the basic opponent model. Frequently, this post-opponent reorganization is viewed in terms of the generation of multiple "higher order" chromatic mechanisms, tuned to a wide variety of axes in color space. Moreover, adaptation experiments have revealed that the synthesis of these mechanisms occurs at a level in the cortex following the binocular integration of the inputs from each eye. Here we report results from an experiment in which the influence of chromatic adaptation on the perceived visual location of a test stimulus was explored using a Vernier alignment task. The results indicate that not only is positional information processed independently within the L/M, S/(L+M), and L+M channels, but that when adapting and test stimuli are extended to non-cardinal axes, the existence of multiple chromatically tuned mechanisms is revealed. Most importantly, the effects of chromatic adaptation on this task exhibit little interocular transfer and have rapid decay rates, consistent with chromatic as opposed to contrast adaptation. These findings suggest that the reorganization of chromatic processing may take place earlier in the visual pathway than previously thought.
Adaptation, Ocular↗
Pulling the other one: 1st- and 2nd-order visual information interact to determine perceived location.
We demonstrate that the 1st- and 2nd-order characteristics of a visual stimulus can have a profound influence on each other in terms of perceived position. We use the parameter of spatial separation to selectively manipulate the effect of one characteristic upon the other. 1st-order features have their largest effect upon the perceived position of 2nd-order structure when separation is small, whilst the reciprocal effect is maximal at large separations. Implications for models of 1st- and 2nd-order interaction are discussed.
Cues↗
A comparison of vernier acuity for narrowband and broadband stimuli.
This study investigates the influence of contrast and exposure duration on vernier acuity thresholds for abutting and separated narrowband stimuli, and asks whether these data can predict broadband vernier performance. Vernier thresholds were determined for sinusoidal grating stimuli at two spatial frequencies (1 and 8 c/deg) across a range of contrasts (0.05-0.8) and exposure durations (35-2100 ms). Performance was assessed for the abutting configuration, and when a gap equivalent to 0.5 to 1.5 times the spatial period of the grating was introduced between the upper and lower halves of the grating. Vernier thresholds were also determined for a square-wave stimulus as a function of contrast (0.06 to 0.78). Exposure duration was fixed at 2100 ms. In addition, thresholds were determined at the appropriate contrast levels for the fundamental frequency (1.8 c/deg) of the square-wave, and for a number of the harmonics (3F, 5F, 7F, 9F). Our results provide support for filter models of vernier acuity by showing that vernier performance for abutting and closely-separated broadband stimuli represents the envelope of vernier sensitivity of those spatial frequency mechanisms that are activated by the broadband stimulus. In the case of high frequency grating stimuli presented for long exposure durations, vernier performance can be invariant across much of the contrast range. Despite this, however, contrast independence is not exhibited for abutting broadband stimuli because, within the broadband stimuli, the contrast of the higher harmonic components never reaches a level to reveal this plateau.
Contrast Sensitivity↗
Neither here nor there: localizing conflicting visual attributes.
Natural visual scenes are a rich source of information. Objects often carry luminance, colour, motion, depth and textural cues, each of which can serve to aid detection and localization of the object within a scene. Contemporary neuroscience presumes a modular approach to visual analysis in which each of these attributes are processed within ostensibly independent visual streams and are transmitted to geographically distinct and functionally dedicated centres in visual cortex (van Essen & Maunsell, 1983; Zihl, von Cramon & Mai, 1983; Maunsell & Newsome, 1987; Tootell, Hadjikhani, Mendola, Marrett & Dale, 1998). In the present study we ask how the visual system localizes objects within this framework. Specifically, we investigate how the visual system assigns a unitary location to objects defined by multiple stimulus attributes, where such attributes provide conflicting positional cues. The results show that conflicting sources of visual information can be effortlessly combined to form a global estimate of spatial position, yet, this conflation of visual attributes is achieved at a cost to localization accuracy. Furthermore, our results suggest that the visual system assigns more perceptual weight (Landy, 1993; Landy & Kojima, 2001) to visual attributes which are reliably related to object contours.
Attention↗
Motion adaptation distorts perceived visual position.
After an observer adapts to a moving stimulus, texture within a stationary stimulus is perceived to drift in the opposite direction-the traditional motion aftereffect (MAE). It has recently been shown that the perceived position of objects can be markedly influenced by motion adaptation. In the present study, we examine the selectivity of positional shifts resulting from motion adaptation to stimulus attributes such as velocity, relative contrast, and relative spatial frequency. In addition, we ask whether spatial position can be modified in the absence of perceived motion. Results show that when adapting and test stimuli have collinear carrier gratings, the global position of the object shows a substantial shift in the direction of the illusory motion. When the carrier gratings of the adapting and test stimuli are orthogonal (a configuration in which no MAE is experienced), a global positional shift of similar magnitude is found. The illusory positional shift was found to be immune to changes in spatial frequency and to contrast between adapting and test stimuli-manipulations that dramatically reduce the magnitude of the traditional MAE. The lack of sensitivity for stimulus characteristics other than direction of motion suggests that a specialized population of cortical neurones, which are insensitive to changes in a number of rudimentary visual attributes, may modulate positional representation in lower cortical areas.
Adaptation, Ocular↗
Low blood urea levels in cattle.
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Animal Nutritional Physiological Phenomena↗
Isolation of stimulus characteristics contributing to Weber's law for position.
To examine the independent contribution of various stimulus characteristics to positional judgements, we measured vernier alignment performance for three types of Gabor stimuli. In one, only the contrast envelope of the upper and lower stimulus elements was offset, with the luminance-modulated carrier grating remaining in alignment. In the second, only the carrier grating was offset. In the third, both carrier and envelope were offset together. Performance was examined over a range of element separations. When both cues are available, thresholds for small separations are dominated by carrier offset information and are inversely proportional to carrier frequency. At large separations, thresholds are governed by the spatial scale characteristics of the envelope. For broad-band stimuli such as lines, bars or dots typically used for vernier acuity, their higher frequency content can be used when separations are small, but as separation increases a smooth transition between the scales that determine threshold results in the continuum known as Weber's law for position. That is, with increasing separation, larger scales must be used, and thresholds increase in direct proportion to 1/frequency.
Contrast Sensitivity↗
The importance of spatial scale in determining illusions of orientation.
The twisted-cord illusion is a powerful demonstration of interaction between 1st-order (luminance-defined) and 2nd-order (contrast-defined) orientation processing. The perceived orientation of contrast-defined objects is pulled towards their 1st-order orientation content when the difference in orientation is small (Fraser effect), yet is pushed away from the 1st-order content at large orientation differences (Zöllner effect). Here we show that the relative spatial scale of carrier and envelope represents a decisive factor in determining the magnitude and direction of such interactions. We conclude that the perceived 2nd-order structure of a stimulus is biased by the properties of the 1st-order structure in a manner that depends on relative, rather than absolute spatial scale.
Form Perception↗