Thresholds For Targets Research Articles

A clinical evaluation of contrast–detail analysis for ultrasound images

We report on the reproducibility of human observers' vanishing detection thresholds for visual targets in contrast-detail (C/D) analysis of ultrasound B-mode images. The images used in this study contain visual targets which are circular cross sections of constant-contrast conical structures in the C/D phantom. The vanishing threshold diameters for these targets vary as a function of the perceived size of the imaged target, target-to-background contrast, image noise content, and reproducibility of the decision levels of human observers for repeated observations. Our study indicates that the determination of absolute vanishing threshold diameter values for several targets of different contrast by human observers yields a high degree of error that is not predicted by existing theoretical assumptions based on a static threshold detector. We find that systematic error is introduced by the observers during the course of the experiment and that the levels of sensitivity of the observers differ widely at all times, and increase the amount of total observer error. These results suggest that, due to the large total observer error, C/D analysis may be impractical in a clinical environment, unless there is access to a team of observers specifically and extensively trained in this task. We suggest that a computer-based observer may be more reliable for the objective performance of contrast-detail analysis as a method for evaluating ultrasound image quality and comparison of imaging systems.

The imprecision of stereopsis

In comparison to lateral judgments of distance, stereoscopic judgments are not precise. Although stereoacuity thresholds for targets presented in the fixation plane can equal the best thresholds for the monocular hyperacuities i.e. a few sec arc, the increment thresholds for disparity are substantially larger than the increment thresholds for lateral separation (width). We measured the minimum detectable change in the three-dimensional distance separating two features, one presented in the fixation plane and the other some distance in front of it, i.e. with a significant standing disparity between the two features. For briefly-presented targets (150 msec), the Weber fraction for disparity was 10–20% over the range from 1 to 20 min arc, while the Weber fraction for width was only 2–3% under comparable conditions. The disparity thresholds were substantially improved for a longer duration target (1000 msec), but they were still a factor of two worse than the monocular width thresholds. In a related experiment, the vernier acuity for a standard vernier target was profoundly degraded by pairing the offset upper line presented to one eye with a disparate Une in the other eye; the vernier threshold was elevated for disparities ranging from 3 to 30 min arc. This finding shows that the more precise monocular signals are actively suppressed in fused or partially-fused stereoscopic images.

Spatial summation in young and elderly observers.

We measured detection thresholds for targets over a range of sizes at both photopic and scotopic luminance levels in young and elderly observers, and used these data to estimate spatial summation areas 10 degrees in the retinal periphery. There were differences in detection thresholds between the young and old groups at photopic and scotopic luminances, but no differences in spatial summation areas at either background luminance level.

Aging, senile miosis and spatial contrast sensitivity at low luminance

The purpose of this study was to determine how aging affects spatial contrast sensitivity at low light levels and to examine whether senile miosis, which reduces retinal illuminance in the aged eye, underlies any observed sensitivity loss. Contrast thresholds for targets having a range of spatial frequencies were measured in young ( n = 13,M age= 24) and older ( n = 11,M age= 73) adults who were free from identifiable ocular pathology. Measurements were carried out at three luminance levels spanning a three log unit range. Results indicated that older adults' loss in contrast sensitivity not only increased with increasing spatial frequency, but also became more pronounced with decreases in luminance level. Additional threshold measurements where pupil diameter was varied indicated that senile miosis was not responsible for older adults' loss in spatial vision at any light level tested. Rather, older adults' miotic pupil tended to have a positive effect on their spatial vision in that it slightly improved their contrast sensitivity.

Contrast sensitivity, acuity, and the perception of 'real-world' targets.

A major assumption underlying the use of contrast sensitivity testing is that it predicts whether a patient has difficulty seeing objects encountered in everyday life. However, there has been no large-scale attempt to examine whether this putative relationship actually exists. We have examined this assumption using a clinic based sample of adults aged 20-77 years. Contrast thresholds were measured for both: (1) gratings of 0.5-22.8 cycles/degree; and (2) real-world targets (faces, road signs, objects). Multiple regression techniques indicated that the best predictors of thresholds for real-world targets were age and middle to low spatial frequencies. Models incorporating these variables accounted for 25-40% of the variance. Although acuity significantly correlated with thresholds for real-world targets, the inclusion of acuity as a predictor variable did not improve the model. These data provide direct evidence that spatial contrast sensitivity can effectively predict how well patients see targets typical of everyday life.

Contrast thresholds versus border enhancement: effect of scattered light.

Contrast thresholds with sine wave grids, spatial frequency thresholds and border enhancement spread were measured while the amount of scattering in the retinal image was varied. Narrow-angle scattering similar to that produced by the cornea and crystalline lens was generated by specially designed filters. The purpose was to assess the effectivity of these widely different responses as measures of small order scattering effects. As noted by other investigators, spatial frequency thresholds proved insensitive to small amounts of scattering. On the other hand, contrast thresholds for high frequency targets and border enhancement spread both showed significant changes with small amounts of scattering. Of these two criteria, border enhancement proved to be the more consistent scattering index.

Visual masking of targets displayed against two- and three-dimensional backgrounds.

Detection thresholds for targets displayed against two- and three-dimensional backgrounds were measured under backward masking and non-masking conditions. The results indicate that planar ring targets displayed against a two-dimensional ground are easier to mask than identical targets portrayed against a three-dimensional background. Also, the detectability of a planar ring target is enhanced when it is included within a three-dimensional rather than an identical but two-dimensional visual display. These results confirm and extend previous findings and suggest a processing asymmetry biased toward three-dimensional visual displays.

Color perimetry.

Seven subjects were studied to determine the reproducibility of color isopters utilizing a Tubingen perimeter with targets equated for radiant energy and separate for heterochromatic flicker luminance. Achromatic threshold recognition of targets for equal luminance gave smaller isopters with longer wavelengths (red). Color recognition thresholds, on the other hand, showed large blue, midzone red and green, and small yellow isopters. The target recognition and color recognition thresholds for equal energy targets gave smaller red isopters. The data support Traquair's contention that all color isopters would be equivalent if hue, saturation, and intensity were equated. Clinically, the detection of subtle peripheral and central field defects might reside in the use of appropriately selected equally bright-colored targets.

Luminance thresholds for circular and bar-shape stimuli

Studies involving the contribution of stimulus form to the detection threshold for light are equivocal, but theoretical considerations suggest that circular stimuli should yield lower thresholds than bars. This hypothesis was tested by obtaining luminance thresholds for circular and bar-shaped targets presented from 5 to 60 deg into the periphery. Thresholds for circles were significantly lower than thresholds for bars. The difference ranged from about.4 log units at 5 deg to.2 log units at 60 deg eccentricity.

Physiological correlates of adaptation to a rotated visual field.

1. Perceptual adaptation to a 30 or 45 degrees visual tilt was induced in human subjects by means of prismatic spectacles worn for 5-7 days. Relative contrast thresholds and the amplitudes of the occipital potentials to vertical and oblique gratings were studied.2. During continuous exposure to the tilted visual environment the normal differences between contrast thresholds for vertical and oblique targets decreased or were no longer significant. In all subjects the change from control threshold differences was significant at the 0.005 level.3. During the course of adaptation the difference in amplitude of the potentials evoked by vertical and oblique oscillating gratings also decreased. The changes with respect to control differences were significant at the 0.005 level in each subject.4. Intermittent exposure to tilt in one subject resulted only in minor adaptation of the apparent vertical, and the evoked potential differences did not change from control levels.5. The changes are interpreted as suggesting that plastic changes in the extraretinal mechanism responsive to target orientation occur during adaptation to prismatic tilt.

Visual-perception models.

Several theoretical models for the prediction of liminal contrast are studied. One model that employs the known excitation and inhibition spread functions of the eye accurately predicts, at 10 mL, thresholds for circular targets, sine waves, and rectangles, including the line as a special case. Semiempirically, this model is extended to include thresholds at low light levels.

Luminance Requirements for Hue Identification in Small Targets

Foveal luminance thresholds for identification of red (642 nm), yellow (584 nm), green (521 nm), and blue (468 nm) were determined by the method of constant stimuli for a range of small stimulus sizes and for two exposure durations. The luminances necessary for chromatic and absolute thresholds were also specified. The luminance required for chromatic threshold was found to be lowest for the 642-nm stimulus and highest for the 468-nm stimulus; the threshold for correct hue identification was found to be inversely related to wavelength. The ratios of the luminance necessary for hue perception to the absolute threshold ranged from 1.14 for the 642-nm stimulus to 1.51 for the 584-nm stimulus. The ratio of the luminance required for correct hue identification to the absolute threshold ranged from 1.32 for the 642-nm stimulus to 3.33 for the 521-nm stimulus. More reliable responses and fewer confusions in the hue identification were associated with the 642-nm stimulus than with the other wavelengths. The identification thresholds of this study are compared with the detection thresholds for similar targets.

How Large is a Point Source?

A source of light having many times the angular subtense of the sun or moon can usually be treated as a point when making photometric calculations based on the inverse square law. When viewed by the eye, such sources are not perceived as points (like stars) but rather as extended surfaces. In this paper, some typical data on contrast thresholds for targets, both large and small, are converted into threshold stellar magnitudes. This is done for two important cases which, unless carefully distinguished, can lead to erroneous results.

Separation thresholds for colored bars with and without luminance contrast

Separation thresholds for rectangular bar targets were obtained for certain combinations of black, white and colored targets and grounds. Relatively low threshold separation scores were obtained with colored targets against white grounds with targets and ground equated in luminance. The results suggest that color contrast is sufficient but less efficient than luminance contrast for visual acuity.

Separation thresholds for bar targets presented with color contrast only

Separation threshold scores for rectangular bar targets with combination of red, yellow, green and blue targets and grounds were obtained. Threshold scores were low, with small differences between scores obtained with the different color combinations.

Neural formulation of the effects of target size and shape upon visual detection.

A hypothesis is presented, the chief assumptions of which are that neural impulses originating in retinal receptors converge upon neurons in a central area, and that the excitation of the most excited neuron in that area determines the response. The relation of this hypothesis to earlier ideas along similar lines is discussed. It is shown that the hypothesis leads to testable relationships between thresholds for circular targets and for targets of other shapes. Evidence is presented that diffuse neural connections are relatively more important at low background levels, and an interpretation is suggested.

Visual thresholds for line-shaped targets.

Measurements of absolute and contrast thresholds values 7° nasal from the fovea for 6500 and 5250A for line-shaped targets 2′ in width, length l between 2′–256′ and time of observation t between 0.02-1 seconds are presented.The absolute thresholds agree with what was to be expected from the two-quanta theory, For l > 32′ for green flashes and for l > 8′ for red ones, N¯60 percent∾l12 and for t > 0.1 second, N¯60 percent∾t12. When t < 0.1 second or l < 32′ for green and l < 8′ for red light, Bunsen-Roscoe’s and Ricco’s law, respectively, is valid. Also, predictions for the behavior of the contrast thresholds for line-shaped sources are made which are also based on the quanta theories for these visual functions. The dependence of the contrast threshold 〈ΔN¯60 percent〉Av on l for large l for line-shaped targets is equal to the dependence of 〈ΔN¯60 percent〉Av on t and corresponds with the dependence of 〈ΔN¯60 percent〉Av on t and on the diameter d of circular flashes in the way predicted by theory.