Target Dot Research Articles

Vernier acuity with plaid masks: the role of oriented filters in vernier acuity

Superimposition of oriented grating masks on vernier targets results in bimodal patterns of vernier threshold elevation, with peaks occurring on either side of vernier target orientation. These bimodal masking effects suggest a contribution to vernier acuity from spatial filters tuned to orientations on either side of the target [Findlay, Nature, 241, 135–137 (1973); Waugh et al., Vision Research, 33, 1619–1638 (1993)]. We report similar bimodal threshold elevation with plaid masks composed of symmetrically-oriented pairs of gratings. Since filters oriented to either side of the vernier stimulus will be affected similarly by plaid masks, it is unlikely that threshold elevation reflects disruption of relative filter activity that is used to code for change in target orientation [ cf Regan & Beverley, Journal of the Optical Society of America A, 2, 147–155 (1985)]. Instead, the results support the proposition that misalignments are detected on the basis of differential (i.e. absolute rather than relative) activity of spatial filters [ cf Wilson, Vision Research, 26, 453–469 (1986)]. Our plaid-mask data also rule out the possibility that: (i) “off-channel” looking [ cf Blake & Holopigian, Vision Research, 25, 1459–1468 (1985)]; or (ii) detection of orientation shifts (e.g. tilt illusions), underlie bimodal masking effects. The finding that weak bimodal threshold elevation occurs with dot targets separated by 40 min arc further suggests that the mechanisms involved in detecting misalignments over large regions [possibly collator/collector-type mechanisms; cf Morgan & Hotopf, Vision Research, 29, 1005–1015 (1989); Moulden, Higher-order processing in the visual system, Ciba Foundation Symposium 184 (pp. 170–192); Chichester: Wiley (1994); Mussap & Levi, Vision Research, (in press)] also do so via analysis of their differential activity.

A belongingness principle of motion perception.

Four experiments are reported that investigated the role of the perceived coplanarity of a moving target with respect to a frame of reference in the third dimension on the perceived path of that target. When a target dot and small moving frame appeared coplanar, the dot's perceived trajectory was governed entirely by its changing position relative to the moving frame. However, when the target and a large stationary frame appeared in a different plane than the small moving frame, the motion of the dot was seen independently of the moving frame. The results support a belongingness principle of motion perception: The displacement of an object relative to a frame of reference to which it belongs governs its perceived path of motion.

The Visual Function of Professional Baseball Players

To measure the visual acuity, stereoacuity, and contrast sensitivity of professional baseball players. Three hundred eighty-seven professional baseball players underwent several tests of visual function including distance visual acuity. Stereoacuity was evaluated at near by the Randot test and at distance by both contour and random dot targets. Distance stereoacuity was also tested under timed and untimed conditions. Contrast sensitivity was evaluated by the Vision Contrast Test System, Contrast Sensitivity Viewer, and Binocular Visual Acuity Tester. Visual acuity (measured with players' regular distance correction) in 774 eyes ranged from 20/8.89 to 20/100. Near stereoacuity ranged from 23 to 37 seconds of arc, mean untimed distance contour stereoacuity from 55 to 35 seconds of arc, and mean untimed distance random dot stereoacuity from 98 to 76 seconds of arc. The results under timed conditions were 86 to 65 seconds of arc (timed distance contour stereoacuity) and 104 to 83 seconds of arc (timed distance random dot stereoacuity). Statistically significant differences were found between major and minor league players on tests of untimed distance contour and random dot stereopsis, and on contrast sensitivity testing with the 3.0- and 6.0-cpd gratings using the Contrast Sensitivity Viewer. Professional baseball players have excellent visual skills. Mean visual acuity, distance stereoacuity, and contrast sensitivity are significantly better than those of the general population.

Hierarchical Motion Organization in Random-Dot Configurations

Motion fields differ in the degree to which they can serve as hierarchical frames of reference. To assess the strength of hierarchical motion organisation, we measured the ability of human observers to extract the motion of a target dot relative to a background of moving dots. The task was to discriminate circular from elliptical relative motions for the target dot (2AFC). Observers performed above chance for translational and divergence backgrounds, but were at chance level for rotational backgrounds. This finding, however, was based on random configurations of dots, where locations and trajectories were unrelated. When spatial constraints were introduced, observers used a different strategy and performed above chance also for rotational backgrounds. Both the effect of motion transformations and the effect of spatial information can be understood if we assume that the task is to find the most likely mechanical description of an object in a three-dimensional environment.

Perceptual Grouping of Dots in Noise

Observers were asked to report the orientation of alignment (horizontal vs vertical) of a group of target dots embedded in randomly placed noise dots. For this perceptual grouping task, sensitivity (the number of noise dots giving 75% correct performance) was measured as a function of the number of target dots. We report surprising tolerance to positional jitter of target dots, with far less tolerance exhibited to alignment jitter than separation jitter. In this respect, perceptual grouping in our task resembles line detection. In a further series of experiments the target was produced by positional shifts of an appropriate number of noise dots, rather than by addition of target dots to the noise dots (this procedure minimised density cues). The improvement in sensitivity with increasing number of target dots was found to be linear (when plotted on log — log axes) with a slope of 0.5, as opposed to bilinear [cf B Moulden, 1994, in Higher-Order Processing in the Visual System Eds G R Bock, J A Goode (Chichester: John Wiley) pp 170 – 192]. This linearity suggests that, over the range of target dot numbers tested, a single mechanism may be sufficient to explain performance.

Theoretical predictions and experimental findings of visual deficits in glaucoma

Purpose: To investigate the motion and colour processing in normal subjects and in patients with primary open angle glaucoma, and to develop theoretical models in order to explain present findings and to understand the processes involved.Methods: Movement processing was investigated using an optical motion projection system, capable of generating continuous motion of a single dot target with controlled speed, displacement and motion direction. Displacement and direction discrimination thresholds (i.e. target displacement at a constant speed needed for 75% probability of detection of motion or correct discrimination of its direction) were measured for both foveal and peripheral stimulus presentations and for a range of speeds. Colour vision was tested using a novel method which employs a random luminance modulation technique to mask out achromatic contrast changes and allowing detection of a spatially structured object based on the processing of chromatic signals only (Barbur et al. 1993).Results: Changes in the parameters of the motion detection model (e.g., delay time and receptor regularity) that was developed to predict the experimental data in normal subjects also predicts an increase in displacement threshold and poorer sensitivity for discrimination of motion direction in glaucoma patients. The results obtained show that patients may report stimulus motion in 80% of presentations whilst performing at chance when required to discriminate the direction of motion. Chromatic discrimination ellipses measured for foveal stimulus presentations in glaucoma patients were often similar to those measured in normal subjects. In the periphery, however, much larger chromatic displacement thresholds were measured in all directions.Conclusions: The technique used to measure the chromatic discrimination ellipses is sensitive to glaucomatous nerve damage and the results suggest a larger threshold elevation in the periphery than at the fovea. The motion model developed to explain performance in normal vision also predicts the experimental findings in glaucoma patients and may be used to predict degraded motion discrimination in patient groups with similar pathologies.

On knowing which way to trace: direction errors during visual curve tracing.

McCormick and Jolicoeur's (1991; 1994) zoom lens model of visual curve tracing proposes that curve tracing involves tracking a curve with a variable size local operator. Unspecified in their model is how the executive function guiding the processing field of this operator initially knows which direction to trace. An experiment was conducted to determine whether observers occasionally curve trace in the wrong direction. A typical curve tracing task involving the determination of whether two dots occur on the same line in a display consisting of two dots and two intertwining lines was used. Lengthening the curve segment at the beginning of a to-be-traced curve resulted in a slowing of response times only to dot locations close to the end of the to-be-traced line. Apparently, observers calculate curve tracing direction based on the spatial location of the second (i.e., target) dot, which can result in erroneously tracing in the wrong direction at dot locations farther along the line.

Instability of ocular torsion during fixation: Cyclovergence is more stable than cycloversion

We investigated spontaneous variation of binocular torsion. Variation was expressed as SD of torsional eye positions measured over periods up to 32 sec. Subjects viewed a single dot target for periods of 32 sec. In half of the trials a large random-dot background pattern was superimposed on the dot. The movements of both eyes were measured with scleral induction coils. Spontaneous torsional movements were largely conjugate: cyclovergence was much more stable than cycloversion. This difference was not due to roll head movements. Stability of cyclovergence was improved by the background pattern. Although overall stability (SD of position) of cycloversion was unaffected by a background, the background induced or enhanced a small-amplitude torsional nystagmus in 3 out of 4 subjects. We hypothesize that the difference in stability of cycloversion vs cyclovergence reflects the greater importance of torsional retinal correspondence, compared to absolute torsional position. In two subjects we found evidence for the existence of cyclophoria, manifested by systematic shifts in cyclovergence caused by the appearance and disappearance of the background.

DNA typing with digoxigenin-11-dideoxyuridinetriphosphate-labelled oligonucleotide probes enables non-radioactive analysis using a dual detection system: application for screening HLA-DQA1 polymorphisms.

We describe a standardized, highly sensitive, nonradioactive detection procedure for HLA class-II typing of DQA1 alleles and suballeles which has important and cost-effective application in studying HLA disease associations. The procedure involves polymerase chain reaction-based target DNA sequence amplification and dot blotting followed by stringent hybridization with digoxigenin-11-dideoxyuridine triphosphate 3'-end-labelled allele-specific oligonucleotide probes. The dual detection system described here makes this procedure very versatile.

Extrapolation of linear motion

We investigated observers' ability to extrapolate a linear trajectory of a moving point, in order to determine how effectively the visual system can combine orientation and position information for moving stimuli. Observers saw a probe dot moving along a straight line toward a stationary target dot. The probe dot extinguished before reaching the target, and the observers' task was to judge whether an extrapolation of the trajectory of the probe would pass to the left or right of the target. Performance was measured as a function of probe velocity, length of the visible trajectory, and location of the target. The empirical results indicated that over a range of conditions, performance on this task is qualitatively similar to, but somewhat less accurate than, that on an analogous task with static stimuli. A four-component model is presented to account for the results. The model specifies an accurate extraction of probe motion parameters, extrapolation of the motion by an ideal observer, and limitations on the input to these processes in the form of visual field spatial inhomogeneity and temporal decay of position information.

The role of directionally selective neurons in the perception of global motion

Dynamic random dot targets consisting of many localized motion vectors have been used to study the pooling of local motion signals into a global motion percept (Williams and Sekuler, 1984). In such displays, the dots are displaced with a constant step size and the direction of motion for each dot is chosen at random from a specified distribution. When the distribution extends over 360 deg, the display consists only of local random motion of individual dots and no coherent motion is reported. However, when the distribution is less than 360 deg (biased), the stimulus appears to flow in a single direction. We examined the effects of reducing the number of directionally selective (DS) cortical neurons on this integration process. Normal cats and cats with severely reduced proportions of DS neurons were trained on 2 direction discrimination tasks. The discrimination of opposite directions was examined while varying either the range of directions of local motion, or the proportion of dots moving with biased distribution. When all dots in the display were directionally biased, cats with reduced numbers of DS neurons performed the task as well as normal cats and humans (threshold range: 280-320 deg). However, when the proportion of biased dots decreased, these animals had severe deficits. Thus, in the absence of noise, even a very small number of DS neurons can perform spatial pooling of local directional signals, and support normal discrimination of opposite directions. However, a full complement of directional detectors appears necessary when the motion signal is masked by noise. The discrimination of small differences in direction revealed far more severe deficits, even when all the dots in the display were directionally biased (no noise).(ABSTRACT TRUNCATED AT 250 WORDS)

Proof-of-principle experiment for a hot electron pumped XUV amplifier

An X-ray laser scheme was recently proposed based on the pumping of neon-like 3s-3p transitions using inner-shell ionization of sodium-like ions by suprathermal electrons. Since this laser operates in principle with no population in the neon-like charge-state power requirements can be reduced from the standard collisionally driven amplifier, and the problem of line-trapping can be ameliorated. Model calculations using a quasi-steady state collisional-radiative model indicated that measurable gains could be obtained in practical experiments. We propose a proof-of-principle experiment to demonstrate the pumping of neon-like excited states by hot electrons. A ~ 1 ns long, 0.53 µ laser beam, will be focussed on a solid dot target of dilute (5%) bromine, to create a cool, dense plasma. The pulse and target will be optimized to produce as much sodium-like population as possible in the corona. This low-fluence green pulse does not produce suprathermals. A second, ~ 100 ps, 1.06 µm pulse of 1015 W/cm2 will be "piggybacked" on the first, to create hot electrons. The timing of the red beam will be optimized with respect to the sodium-like fraction of the cool plasma. The hot electrons create a transient population of neon-like ions, coincident with the red beam, that are detected by their characteristic n=3-n=2 resonant X-ray emission. If, as expected while the red beam is present, the neon-like excited states are produced by inner-shell ionization (as opposed to collisional excitation), n = 3-2 line ratios will carry a distinctive signature indicating a statistical feeding mechanism. The same signature will imply the existence of a population inversion. Proposed detection systems include space and time resolving crystal X-ray spectrometers for measuring the neon-like lines, and an array of filtered X-ray diodes to measure suprathermal electron temperature and number.

Vernier Acuity with Opposite-Contrast Stimuli

Vernier acuity has usually been tested with stimuli of the same contrast polarity (SC). This traditional vernier acuity was compared to that obtained with stimuli of opposite-contrast (OC) in which one target was brighter than the background and the other was darker. For both bar and dot targets vernier acuity with OC stimuli was about half as good as with SC stimuli. There were large individual differences in the size of the disadvantage with OC stimuli, although thresholds remained within the hyperacuity range. There were also individually-differing biases to see a dark vernier stimulus on one or the other side of a bright stimulus. Differences between OC and SC vernier acuities persisted over a wide range of interstimulus spacings, widths, and contrasts. At extremes of these spatial manipulations acuities became similar, but only because SC acuities were degraded to the level of OC acuities. Subjects showed little improvement in OC vernier acuity, even after 50,000 trials. It is concluded that finest judgements of spatial position arise in a level of the visual system at which light and dark stimuli are treated independently.

Spatial properties of mechanisms for detection of moving dot targets in dynamic visual noise.

The maximum displacement threshold for direction discrimination (dmax) was determined for single or paired dot targets moving discretely against a background of dynamic visual noise. dmax rose as the spatial density of noise was reduced, or when the interframe interval was decreased. dmax was greater for dot pairs than for single dots, and rose progressively as the distance between the dots was reduced. dmax was also greater if the orientation of the target dot pairs differed from the orientation of paired dots in the background noise. Dichoptic presentation of the target and background noise allowed the target to be detected with an accuracy that did not depend on displacement.

Saccadic tracking of a periodic target motion in the cat.

Cats were tested for their ability to follow a dot target which periodically jumps horizontally from side to side. They can learn to perform saccadic tracking of various stimulus frequencies, and often they produce saccades predicting the target displacements. The preferred stimulus frequency for response to synchronize was in the 0.5-Hz range. Cats tend to undershoot the target. They also tend to adopt a strategy of locking predominantly to one direction of displacement, but they can be trained to reverse this preferred side or to lock to both sides. Saccades to the preferred side are more accurate, and longer and slower in comparison to saccades to the other side, thus demonstrating a velocity-accuracy trade-off phenomenon in the cat responses. Saccadic characteristics as expressed by the main sequence are apparently a stable property independent of the stimulus frequency.

Bromine dot target plasma spectroscopy: A detailed analysis

A detailed simulation of the spectra produced from a bromine dot target by laser irradiation is presented. This study incorporates the results of a two-dimensional hydrodynamic calculation and an extensive atomic rate data base to generate synthetic spectra under conditions that match those of an earlier experiment. A dynamic picture of the plasma’s evolution is given that is consistent with experimental results. Overall plasma characteristics are calculated that produce spectra largely in agreement with those observed. Also, sensitive dielectronic recombination features are reproduced corroborating the adequacy of this approach.

Constant errors in judgements of collinearity due to the presence of neighbouring objects.

If a line (the pointer) is aligned with a dot (the target) that stands on another line (the induction line) which is at an angle to the pointer, the pointer and the dot may no longer appear collinear. Whether they do or not depends upon the angle formed by the pointer with the induction line: the smaller the angle, the greater the misalignment effect. Misalignment is always in the direction of the induction line, which is why this alignment illusion is called attraction-caused misalignment (attraction misalignment for short). Three experiments are described in which this illusion is explored further. In the first it is shown that the induction line can exert its influence even when not contiguous with the target, though the size of the effect varies inversely with the distance of the induction line from the target. In the second experiment it is demonstrated that a dot as well as a line can induce attraction misalignment and that similarity between the induction and target items increases misalignment. Evidence in support of the theory that the termination of the induction line, as well as the part contiguous with the target dot, may induce attraction misalignment is provided in the third experiment.

XUV spectral observations with two-wavelength laser irradiation

XUV diagnostic equipment was designed and utilized on the OMEGA target chamber at the University of Rochester to study high atomic number plasma generation by two-wavelength laser excitation. Spectral data were collected from silver tracer dot targets irradiated with 1/3 TW of 0.35-μm laser light of the multiple-beam OMEGA laser and the single synchronized 1.06-μm beam of the GDL laser for generating energetic electrons. XUV spectral data in the 30–300-Å region were obtained with both a 3-m grazing incidence spectrograph and a compact 1-m grazing incidence spectrograph designed for reentrant mounting in the OMEGA chamber. High-resolution x-ray spectra were acquired in the 3.6–4.2-Å region with a dual, flat-diffraction crystal spectrograph. A low-resolution x-ray spectrum of silver was recorded with a curved mica spectrograph. Some x-ray spectral lines appeared only when both OMEGA and GDL beams were used. These were identified as 2p–3s,3d transitions in F-like Ag xxxix. F-, Na-, and Mg-like lines were found in the grazing incidence spectra, with F-like lines appearing only with 1.06-μm irradiation.

Distribution of spatial attention in position recognition

Spatial limitation in visual information processing was examined with dot-in-matrix patterns by using a probe recognition procedure. The independent variables were the number (1-16 dots) and the position of target dots. Subjects were four undergraduate students. The data were analyzed and discussed from three points of view; span of attention, spatial limitation of recognition and visual attention. The following became clear: First, the span of position recognition was 4.8. Second, "spatial span of attention" was defined as the range of dot positions at which subjects can perceive target dots with 75% or more accuracy. It extended around the fixation point and shrinked with the increase of the number of target dots. Finally, the distribution of spatial attention was estimated for each target dot condition under the assumption that the hit RT at each probe position reflects the amount of attention allocated there. Distributions estimated were cone-shaped, and the height and extent changed with the number of target dots. It was suggested that spatial limitation (i.e. spatial span of attention) in the processing of spatial positions can be explained by the notion of distribution of spatial attention.

Diagnostic Characterization of Laser-Irradiated dot Targets

AbstractWe have conducted a series of experiments at the KMS CHROMA facility using dot spectroscopy techniques to characterize uncertainties associated with spectral line ratio models commonly applied in determining electron temperatures and densities. Temperatures determined from the slope of the H-like free-bound continuum and densities via holographic interferometry, are compared to line ratio methods. Dot targets of (typically 100µmD Mg or Al) are irradiated with 2 to 40 × 1013W/cm2of 0.53µmlight. Time and spatial gradients are resolved using 4 diagnostics: a 4-frame holographic interferometer, an x-ray streak crystal spectrograph with a spatial imaging slit, a framing crystal x-ray spectrometer, and a conventional space-resolved time-integrating crystal spectrograph used for survey and calibration purposes. Preliminary results indicate the ionization distribution of these laser produced plasmas is not steady-state which plays an important role in measuring the temperature and and density. Electron temperatures derived from line-ratio techniques, assuming steady state conditions, disagree dramatically from simultaneous measurements using the slope of the H-like continuum. Electron densities using He-like triplet to singlet line ratios also differ from densities measured interferometrically.