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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Orientation, masking, and vernier acuity for line targets.

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured.

Can sinusoidal vernier acuity be predicted by contrast discrimination?

A test-pedestal approach, with a test grating superimposed on a masking pedestal, was used to compare sinusoidal grating vernier acuity and contrast discrimination thresholds. The goal is to develop a simple model for vernier acuity without assumptions about underlying mechanisms. In the contrast discrimination task, subjects were asked to detect contrast increments in the presence of a base pedestal.

Binocularity of early motion mechanisms: comments on Georgeson and Shackleton.

The demonstration of compelling dichoptic illusions of motion using a variety of stimuli, all of which share the property that left and right eye patterns were spatio-temporal quadrature pairs, has been interpreted as evidence for binocular integration in the early motion system (short range motion). Georgeson and Shackleton (1989, Vision Research, 29, 1511-1523) have recently challenged this position based on results using 3 types of stimuli, sinusoidal gratings, random line kinematograms and missing fundamental squarewaves. For each class of stimuli motion was perceived during dichoptic presentation, but certain limitations led them to conclude that early motion mechanisms are defeated when no motion is present monocularly.

Vernier acuity as line and dipole detection.

The vernier judgment is commonly thought of as discriminating the displacement of a portion of a pattern. However, we have found it revealing to consider vernier stimuli in another light; as the composite of a test pattern superimposed on a masking pedestal. The pedestal is the pattern with zero spatial offset, and the test pattern is the luminance distribution which, when added to the pedestal, produces the offset.