Edward A. Essock

A horizontal bias in human visual processing of orientation and its correspondence to the structural components of natural scenes.

Many encoding mechanisms and processing strategies in the visual system appear to have evolved to better process the prevalent content in the visual world. Here we examine the relationship between the prevalence of natural scene content at different orientations and visual ability for detecting oriented natural scene content. Whereas testing with isolated gratings shows best performance at horizontal and vertical (the oblique effect), we report that when tested with natural scene content, performance is best at obliques and worst at horizontal (the horizontal effect). The present analysis of typical natural scenes shows that the prevalence of natural scene content matches the inverse of this horizontal effect pattern with most scene content at horizontal, next most at vertical, and least at obliques. We suggest that encoding of orientation may have evolved to accommodate the anisotropy in natural scene content by perceptually discounting the most prevalent oriented content in a scene, thereby increasing the relative salience of objects and other content in a scene when viewed against a typical natural background.

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 advanced image fusion algorithm based on wavelet transform: incorporation with PCA and morphological processing

There are numerous applications for image fusion, some of which include medical imaging, remote sensing, nighttime operations and multi-spectral imaging. In general, the discrete wavelet transform (DWT) and various pyramids (such as Laplacian, ratio, contrast, gradient and morphological pyramids) are the most common and effective methods. For quantitative evaluation of the quality of fused imagery, the root mean square error (RMSE) is the most suitable measure of quality if there is a “ground truth” image available; otherwise, the entropy, spatial frequency or image quality index of the input images and the fused images can be calculated and compared. Here, after analyzing the pyramids’ performance with the four measures mentioned, an advanced wavelet transform (aDWT) method that incorporates principal component analysis (PCA) and morphological processing into a regular DWT fusion algorithm is presented. Specifically, at each scale of the wavelet transformed images, a principle vector was derived from two input images and then applied to two of the images’ approximation coefficients (i.e., they were fused by using the principal eigenvector). For the detail coefficients (i.e., three quarters of the coefficients), the larger absolute values were chosen and subjected to a neighborhood morphological processing procedure which served to verify the selected pixels by using a “filling” and “cleaning” operation (this operation filled or removed isolated pixels in a 3-by-3 local region). The fusion performance of the advanced DWT (aDWT) method proposed here was compared with six other common methods, and, based on the four quantitative measures, was found to perform the best when tested on the four input image types. Since the different image sources used here varied with respect to intensity, contrast, noise, and intrinsic characteristics, the aDWT is a promising image fusion procedure for inhomogeneous imagery.

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.

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.

Perceptual anisotropies in visual processing and their relation to natural image statistics

The amplitude spectra of natural scenes are typically biased in terms of the amount of content at the cardinal orientations relative to the oblique orientations. This anisotropic distribution has been related to the ‘oblique effect’ (the greater visual sensitivity for simple line/grating stimuli at cardinal compared to oblique orientations). However, we have recently shown that with complex visual stimuli possessing broadband spatial content (i.e. random phase noise patterns), sensitivity for detecting oriented manipulations of amplitude is best for oblique orientations, and worst for horizontal orientations (the ‘horizontal effect’). Here we investigated this effect with respect to the phase spectra of natural scenes. Oriented manipulations of both amplitude and phase were made on a set of natural scene images that were dominated by naturally occurring structure at one of four orientations in order to determine whether the presence of predominant scene content, carried by the Fourier phase spectra, altered the ability to detect an oriented increment of amplitude. The horizontal effect was observed regardless of any scenes content bias. In addition, a content-dependent effect was observed which could be related to the presence of spatial structure conveyed by the phase spectra of this set of natural scenes. Results are evaluated in the context of a divisive normalization model.

An anisotropy of human tactile sensitivity and its relation to the visual oblique effect

SummaryThe ability of humans to detect striated stimuli on the distal phalanges was found to be highly anisotropic. Observers were much more sensitive to stripes presented in the proximal-distal orientation than to stripes in any other orientation. This tactile anisotropy was contrasted with the well-known visual anisotropy in which sensitivity is greatest for stripes at the horizontal and vertical orientations. We suggest that both the tactile anisotropy and the visual anisotropy are caused by corresponding anisotropies in the distribution of preferred orientations of orientation-selective neurons with in the respective modalities.

Influence of scale and orientation on the visual perception of natural scenes

It is well known that the distribution of spatial content with respect to spatial scale in real-world scenes falls in accordance with a 1/ f α relation. Equally well known is the tendency for an orientation bias in scene content with the predominant bias in content at the horizontal and vertical orientations. This has led to the suggestion of a relationship in which the mechanisms of the human visual system are optimized for processing such regularities. Here we review current literature concerning the measurement of these regularities (via Fourier analysis) of natural scenes in the context of other work that has psychophysically assessed the extent to which visual perception exploits such regularities of spatial content. In addition, 2 psychophysical experiments are presented that extend this literature and argue for the importance of these regularities in perceiving orientation in real-world visual stimuli.

Predicting subsequent visual field loss in glaucomatous subjects with disc hemorrhage using retinal nerve fiber layer polarimetry

Purpose:To predict progression of visual field loss after an episode of disc hemorrhage in glaucoma patients on the basis of retinal nerve fiber layer (RNFL) GDx polarimetry measurements analyzed by wavelet-Fourier analysis (WFA). Methods:Retrospective GDx data from 16 subjects (10 progressors and 6 non-progressors based on visual fields) obtained near the time of disc hemorrhage were analyzed to predict which patients would have visual field progression. Polarimetry scans throughout a follow-up period (31 months average) were also analyzed to compare field progression to RNFL thickness change after the hemorrhage. Mean RNFL thickness inferred from the polarimetry data at sixteen 22.5° sectors at distances of 1.6, 1.7, and 1.8 disc diameters were used. Data were analyzed by applying to appropriate regions of disc hemorrhage patients a structural analysis (WFA) we had developed previously. A linear discriminant function (Fischer) was produced and a leave-one-out method using separate training and test data was used to assure validity of the results. Results:Patients who subsequently progressed were successfully predicted with moderate success (sensitivity / specificity was 0.77 / 0.88 with ROC area = 0.858). A separate analysis comparing pre- and post-hemorrhage RNFL sector thickness revealed clear evidence of RNFL thinning at the inferior and superior sectors before progression of visual field. The thinning of RNFL thickness was not restricted to regions corresponding to the location of the hemorrhage. Conclusion:Wavelet-Fourier analysis can differentiate progressors from non-progressors with moderate accuracy. Comparison to a prior study of this same cohort emphasizes that relatively small regions must be considered (as opposed to larger quadrants) to see these significant changes in RNFL.

Thermal Imaging of the Superficial Temporal Artery: An Arterial Pulse Recovery Model

We present a novel model for measurement of the arterial pulse from the superficial temporal artery (STA) using passive thermal infrared (IR) sensors. The proposed approach has a physical and physiological basis and as such is of fundamental nature. Thermal IR camera is used to capture the heat pattern from superficial arteries, and a blood vessel model is used to describe the pulsatile nature of the blood flow. A multiresolution wavelet-based signal analysis approach is used to extract the arterial pulse waveform, which lends itself to various physiological measurements. We validate the results using a traditional contact vital-sign monitor as a ground truth. Eight people of different age, race and gender have been tested in our study consistent with IRB approval. The resultant arterial pulse waveforms exactly matched the ground-truth readings. The essence of our approach is the automatic detection of region of arterial pulse measurement (ROM), from which the arterial pulse waveform is extracted. To the best of our knowledge, the correspondence between non-contact thermal IR imaging based measurements of the arterial pulse in the time domain and traditional contact approaches has never been reported in the literature.