Terrain influences the accurate judgement of distance

Mathematically, three-dimensional space can be represented differently by the cartesian, polar, and other coordinate systems. However, in physical sciences, the choice of representation system is restricted by the need to simplify a machine's computation while enhancing its efficiency. Does the brain, for the same reasons, 'select' the most cost-efficient way to represent the three-dimensional location of objects? As we frequently interact with objects on the common ground surface, it might be beneficial for the visual system to code an object's location using a ground-surface-based reference frame. More precisely, the brain could use a quasi-two-dimensional coordinate system (x(s), y(s)) with respect to the ground surface (s), rather than a strictly three-dimensional coordinate system (x, y, z), thus reducing coding redundancy and simplifying computations. Here we provide support for this view by studying human psychophysical performance in perceiving absolute distance and in visually directed action tasks. For example, when an object was seen on a continuous, homogeneous texture ground surface, the observer judged the distance to the object accurately. However, when similar surface information was unavailable, for example, when the object was seen across a gap in the ground, or across distinct texture regions, distance judgement was impaired.

Fourier analysis of nerve fiber layer measurements from scanning laser polarimetry in glaucoma: emphasizing shape characteristics of the 'double-hump' pattern

PURPOSE

The pattern of the distribution of nerve fiber layer (NFL) thickness values across the retina may provide an early anatomic indication of glaucomatous disruption. We developed a method of analyzing polarimetry measurements that emphasizes the shape of the pattern of NFL thickness values. Sensitivity and specificity for detecting glaucoma was obtained for these measures and compared with those for conventional measures.

METHODS

Nerve fiber thickness was inferred from retardation shift measured by a scanning laser polarimeter (Laser Diagnostic Technologies, Inc., San Diego, CA) in 34 healthy subjects (68 eyes) and 34 patients with glaucoma (68 eyes). Fourier analysis was performed on the polarimetry data to emphasize the shape in the evaluation of the distribution of thickness values around the optic disc (along a 1.7-disc diameter ring). This was computed separately on superior and inferior hemiretinas.

RESULTS

Significant differences were found in the Fourier shape measures between healthy subjects and patients with glaucoma. The sensitivity and specificity using Fourier coefficients with this particular sample was 96% and 90%, respectively.

CONCLUSION

The evaluation of NFL measurements with Fourier analysis to emphasize the holistic shape of the "double-hump" pattern was found to be a useful tool as an analysis strategy.

Diffuse and localized nerve fiber layer loss measured with a scanning laser polarimeter: sensitivity and specificity of detecting glaucoma

PURPOSE

To differentiate normal from diseased retinal nerve fiber layers (NFL) using a new method of analyzing polarimetry data that specifically targets patterns of diffuse and localized NFL loss.

METHODS

The NFL from a sample of 34 patients with primary open-angle glaucoma (POAG), 34 patients with ocular hypertension, and 34 normal subjects were imaged using a scanning laser polarimeter (GDx; Laser Diagnostic Technologies, Inc., San Diego, CA). Diffuse loss was defined as a reduction in the peak-to-trough amplitude of the double-hump NFL pattern, and localized loss was defined as a lowering of the correlation of thickness values between local regions shown previously to correspond in normal subjects.

RESULTS

Significant differences were found between the groups of normal subjects, patients with hypertension, and patients for both the amplitude and the correlational measures. The sensitivity and specificity calculated using optimal criterion values were 94% and 91%, respectively.

CONCLUSIONS

These results suggest that NFL analysis targeting specific patterns of loss may be beneficial for differentiating normal NFL patterns from diseased NFL patterns, as well as for identifying patients at high risk.

Perceptual ability with real-world nighttime scenes: image-intensified, infrared, and fused-color imagery

We investigated human perceptual performance allowed by relatively impoverished information conveyed in nighttime natural scenes. We used images of nighttime outdoor scenes rendered in image-intensified low-light visible (i2) sensors, thermal infrared (ir) sensors, and an i2/ir fusion technique with information added. We found that nighttime imagery provides adequate low-level image information for effective perceptual organization on a classification task, but that performance for exemplars within a given object category is dependent on the image type. Overall performance was best with the false-color fused images. This is consistent with the suggestion in the literature that color plays a predominate role in perceptual grouping and segmenting of objects in a scene and supports the suggestion that the addition of color in complex achromatic scenes aids the perceptual organization required for visual search. In the present study, we address the issue of assessment of perceptual performance with alternative night-vision sensors and fusion methods and begin to characterize perceptual organization abilities permitted by the information in relatively impoverished images of complex scenes. Applications of this research include improving night vision, medical, and other devices that use alternative sensors or degraded imagery.

An Evaluation of a Sensor Fusion System to Improve Drivers' Nighttime Detection of Road Hazards

Objective: To determine whether combination of two spectral bands within a single sensor-fused image can improve drivers' detection of road hazards. Methods: Images were collected with visible and short wave infrared sensors, and were combined by an image fusion algorithm derived from a computational model of human retinal processing (Werblin et al., 1997). Test stimuli were single-band and fused images of a nighttime scene, collected with sensors mounted atop a vehicle and facing down a stretch of road into an opposing vehicle's headlights. The intensity of the opposing headlights was varied to produce images of low and of high glare. The subjects' task was to detect the presence of a pedestrian within each image. Results: Sensor-fused imagery reliably produced performance better than or equivalent to that produced by either format of single-band imagery. Conclusions: Sensor fusion may provide an effective method of facilitating the detection of road hazards under low visibility conditions.

Spatial sensitization of increments and decrements: a border-contrast process and a net-excitation process

We investigated the spatially local factors that adjust the sensitivity of the human visual system within a small patch of visual space. A very small adapting field was varied in diameter to map out the strength and extent of the spatially local processes that adjust sensitivity for both increments and decrements. The results demonstrated antagonistic center/surround adaptation regions with a decremental test probe comparable to those demonstrated previously for incremental probes (Westheimer, G., 1965. Spatial interaction in the human retina during scotopic vision, Journal of Physiology 81, 812-894; Westheimer, G., 1967. Spatial interaction in human cone vision, Journal of Physiology 190, 139-154) implying comparable antagonistic regions in the ON and OFF channels. In addition to spatial interactions based on light adaptation, we report a weaker effect that is based on the location of a border (luminance edge) and is governed by the contrast of this edge. Finally, we show that these effects are elicited by both highly localized edges (1' ring pairs) and radial lines (Ehrenstein figure) as well. We conclude that both a border-contrast mechanism and a net-excitation mechanism govern the spatially local adaptation of the visual system and that this view fits well with the behavior of single units reported previously.

An oblique effect of chromatic gratings measured by color-mixture thresholds

Contrast sensitivity is lower for obliquely oriented achromatic gratings than for vertical or horizontal gratings at high spatial and low temporal frequencies. Although this response is suggestive of mediation by P-pathway cortical correlates, no clear sensory (i.e. class 1) oblique effect has been demonstrated with isoluminant chromatic stimuli. In the present experiment, a two-alternative forced-choice detection task was used to measure observers' sensitivity to spatiotemporal sinusoids varying in orientation and color contrast. A maximum-likelihood method fit ellipses to the thresholds, with the length of each ellipse taken as a measure of chromatic contrast sensitivity at isoluminance, and the width as luminance contrast threshold. A chromatic oblique effect was observed at about 3 cycles deg-1 suggesting an orientation bias within the cortical stream conveying P-cell activity.

Interocular symmetry in nerve fiber layer thickness of normal eyes as determined by polarimetry

PURPOSE

The existence of asymmetries between the two eyes in number and distribution of nerve fibers may provide an early anatomic indication of glaucomatous disruption if a symmetrical pattern of nerve fibers can be shown in normal eyes. Normal eyes were tested to determine whether a high degree of correspondence of regional nerve fiber layer thickness exists between fellow eyes.

METHODS

Nerve fiber layer thickness was inferred from retardation shift measured by a scanning laser polarimeter (Laser Diagnostic Technologies, Inc.) in 40 volunteers without glaucoma or ocular disease (80 eyes). Total thickness and the pattern of nerve fiber thickness over 208 regions was compared between the paired eyes.

RESULTS

Inferred nerve fiber layer thickness in companion eyes was highly correlated. Variation of the total nerve fiber thickness was much less between companion eyes than between eyes of different individuals. In terms of regional (i.e., local) interocular correspondence, the measurements in the inferior retina were observed to be highly similar in the two eyes, but a pronounced shift of the location of the superior peak (maximum thickness) in the left eye relative to the right was observed, as were differences between the two eyes in the thickness measurements at the nasal and temporal minima.

CONCLUSION

Significant interocular asymmetries were detected in regions other than the interior hemiretina. These interocular asymmetries may reflect previously unrecognized anatomic nerve fiber differences or systematic angular bias in the instrument for one eye relative to the other. In contrast to the regional measures, the measures of total thickness were very similar between companion eyes, suggesting that the assessment of interocular asymmetries of total thickness determined by polarimetry may offer potential for detecting glaucoma.