Distribution of content in recently-viewed scenes whitens perception.

Anisotropies in visual perception have often been presumed to reflect an evolutionary adaptation to an environment with a particular anisotropy. Here, we adapt observers to globally-atypical environments presented in virtual reality to assess the malleability of this well-known perceptual anisotropy. Results showed that the typical bias in orientation perception was in fact altered as a result of recent experience. Application of Bayesian modeling indicates that these global changes of the recently-viewed environment implicate a Bayesian prior matched to the recently experienced environment. These results suggest that biases in orientation perception are fluid and predictable, and that humans adapt to orientation biases in their visual environment "on the fly" to optimize perceptual encoding of content in the recently-viewed visual world.

Structural Content in Paintings II: Artists Commissioned to Reproduce a Specific Image Over-Regularize Orientation Biases in Their Paintings.

Measurements of psychophysical performance show that the visual system is biased in ways that counteract statistical regularities of natural scenes thereby allowing efficient coding. Here we consider the perceptual effects of these encoding biases in a "holistic" way by measuring characteristics of the paintings produced by artists making perceptual matches to a natural scene image; 10 artists were asked to produce an exact copy of a single outdoor landscape scene. The structural content of the paintings produced and the "ground truth" image were compared in the frequency domain. The artists were found to over-regularize the orientation content in the paintings: The anisotropy existing only at the lowest spatial scales in the natural scene image was produced across all spatial scales in these commissioned paintings. These results were compared to those from two other methods of comparing paintings and natural scenes reported previously in a companion paper and all three methodologies indicate very similar over-regularization. We suggest that artists may have a general canonical representation of structural relations of scenes that they apply broadly within their creations.

A cortical locus for anisotropic overlay suppression of stimuli presented at fixation.

Human contrast sensitivity for narrowband Gabor targets is suppressed when superimposed on narrowband masks of the same spatial frequency and orientation (referred to as overlay suppression), with suppression being broadly tuned to orientation and spatial frequency. Numerous behavioral and neurophysiological experiments have suggested that overlay suppression originates from the initial lateral geniculate nucleus (LGN) inputs to V1, which is consistent with the broad tuning typically reported for overlay suppression. However, recent reports have shown narrowly tuned anisotropic overlay suppression when narrowband targets are masked by broadband noise. Consequently, researchers have argued for an additional form of overlay suppression that involves cortical contrast gain control processes. The current study sought to further explore this notion behaviorally using narrowband and broadband masks, along with a computational neural simulation of the hypothesized underlying gain control processes in cortex. Additionally, we employed transcranial direct current stimulation (tDCS) in order to test whether cortical processes are involved in driving narrowly tuned anisotropic suppression. The behavioral results yielded anisotropic overlay suppression for both broadband and narrowband masks and could be replicated with our computational neural simulation of anisotropic gain control. Further, the anisotropic form of overlay suppression could be directly modulated by tDCS, which would not be expected if the suppression was primarily subcortical in origin. Altogether, the results of the current study provide further evidence in support of an additional overlay suppression process that originates in cortex and show that this form of suppression is also observable with narrowband masks.

Novel fractal feature-based multiclass glaucoma detection and progression prediction.

We investigate the use of fractal analysis (FA) as the basis of a system for multiclass prediction of the progression of glaucoma. FA is applied to pseudo two-dimensional images converted from one-dimensional retinal nerve fiber layer (RNFL) data obtained from the eyes of normal subjects, and from subjects with progressive and non-progressive glaucoma. FA features are obtained using a box-counting method and a multi-fractional Brownian motion method that incorporates texture and multiresolution analyses. Both features are used for Gaussian kernel-based multiclass classification. Sensitivity, specificity, and area under receiver operating characteristic curve (AUROC) are computed for the FA features and for metrics obtained using wavelet-Fourier analysis (WFA) and fast-Fourier analysis (FFA). The AUROCs that predict progressors from non-progressors based on classifiers trained using a dataset comprised of non-progressors and ocular normal subjects are 0.70, 0.71 and 0.82 for WFA, FFA, and FA, respectively. The correct multiclass classification rates among progressors, non-progressors, and ocular normal subjects are 0.82, 0.86 and 0.88 for WFA, FFA, and FA, respectively. Simultaneous multiclass classification among progressors, non-progressors, and ocular normal subjects has not been previously described. The novel FA-based features achieve better performance with fewer features and less computational complexity than WFA and FFA.

Structural content in paintings: artists overregularize oriented content of paintings relative to the typical natural scene bias.

Natural scenes tend to be biased in both scale (1/f) and orientation (H > V >> O; horizontal > vertical >> oblique), and the human visual system has similar biases that serve to partially 'undo' (ie whiten) the resultant representation. The present approach to investigating this relationship considers content in works of art-scenes produced for processing by the human visual system. We analyzed the content of images by a method that minimizes errors inherent in some prior analysis methods. In the first experiment museum paintings were considered by comparing the amplitude spectrum of landscape paintings, natural scene photos, portrait paintings, and photos of faces. In the second experiment we obtained photos of paintings at the time they were produced by local artists and compared structural content in matched photos which contained the same scenes that the artists had painted. Results show that artists produce paintings with both the 1/f bias of scale and the horizontal-effect bias of orientation (H > V >> O). More importantly, results from both experiments show that artists overregularize the structure in their works: they impose the natural-scene horizontal effect at all structural scales and in all types of subject matter even though, in the real world, the pattern of anisotropy differs considerably across spatial scale and between faces and natural scenes. It appears that artists unconsciously overregularize the oriented structure in their works to make it conform more uniformly to the 'expected' canonical ideal.

Contrast sensitivity for oriented patterns in 1/f noise: contrast response and the horizontal effect.

When observers detect an oriented, broadband contrast increment on a background of 1/f spatial noise, thresholds will be lowest for obliquely orientated stimuli and highest for horizontally oriented stimuli-an anisotropy termed the "horizontal effect." Here, we assessed what spatial frequencies within the broadband increment were relied on by observers in performing the original task and which spatial frequencies contribute to the anisotropic performance. We found that against a background of 1/f noise, contrast thresholds are lowest for content around 8 cycles per degree, and that at this spatial frequency a horizontal effect is seen which closely resembles the anisotropy observed in broadband masking. The magnitude of the horizontal effect decreased at lower and higher spatial frequencies. To allow for a fit to a standard "gain control" model of psychophysical contrast discrimination, threshold-versus-contrast (TvC) functions were measured for the 8-cpd noise broadband content against either an identical pattern (i.e., pedestal) or a broadband 1/f noise pattern, whose contrast was varied. Results and model application indicate that the threshold pattern for oriented noise around 8 cpd, and for oriented broadband content, is best explained as the result of an anisotropic contrast gain control process.

The horizontal effect in suppression: Anisotropic overlay and surround suppression at high and low speeds.

When a pattern of broad spatial content is viewed by an observer, the multiple spatial components in the pattern stimulate detecting-mechanisms that suppress each other. This suppression is anisotropic, being relatively greater at horizontal, and least at obliques (the "horizontal effect"). Here, suppression of a grating by a naturalistic (1/f) broadband mask is shown to be larger when the broadband masks are temporally similar to the target's temporal properties, and generally anisotropic, with the anisotropy present across all spatio-temporal parings tested. We also show that both suppression from within the region of the test pattern (overlay suppression) and from outside of this region (surround suppression) show the horizontal-effect anisotropy. We conclude that these suppression effects stem from locally-tuned and anisotropically-weighted gain-control pools.

Comparison of shape-based analysis of retinal nerve fiber layer data obtained From OCT and GDx-VCC.

PURPOSE: To directly compare in 1 population: (1) the performance of Optical Coherence Tomograph (OCT) and GDx-Variable Corneal Compensator (VCC) when using Wavelet-Fourier Analysis (WFA) and Fast-Fourier Analysis (FFA), (2) the performance of these shape-based and standard metrics, and (3) the shape of the retinal nerve fiber layer (RNFL) temporal, superior, nasal, inferior, temporal (TSNIT) curves obtained by the 2 different devices. METHODS: RNFL estimates were obtained from 136 eyes of 136 individuals (73 healthy and 63 mild glaucoma). WFA and FFA with and without asymmetry measures were performed on the TSNIT RNFL estimates to identify glaucoma from healthy eyes. Performance of WFA, FFA, and the standard metrics of OCT (Inferior Average) and GDX-VCC (Nerve Fiber Indicator) was evaluated by calculating receiver operating characteristic area. Measurements were obtained at a custom radius (33 to 41 pixels) for GDx-VCC to match the OCT radius (1.73 mm). RESULTS: WFA and FFA shape analysis significantly improved performance of both OCT (0.937) and GDx-VCC (0.913) compared with Inferior Average and Nerve Fiber Indicator (0.852 and 0.833, respectively). With either shape-based or standard metrics, OCT performance was slightly, but not significantly, better than GDx-VCC performance. Comparison of RNFL curves revealed that the GDx-VCC curves were more jagged and the peaks shifted more nasally when compared with the OCT RNFL curves. CONCLUSIONS: Performance of both OCT and GDx-VCC devices are improved by shape-based analysis methods. Classification performance was greater when using WFA for the OCT, and greater with FFA for the GDx-VCC. Significant differences between the machines exist in the measured TSNIT thicknesses, possibly because of GDx-VCC's measurements being affected by polarization magnitude varying with angle.

An anisotropy of orientation-tuned suppression that matches the anisotropy of typical natural scenes.

Broadband oriented-noise masks were used to assess the orientation properties of spatial-context suppression in 'general' viewing conditions (i.e., a fixated, large field of 'naturalistic' noise). Suppression was orientation-tuned with a Gaussian shape and bandwidth of 40 degrees that was consistent across test orientation (0 degrees, 45 degrees, 90 degrees, and 135 degrees). Strength of suppression was highly anisotropic following a "horizontal effect" pattern (strongest suppression at horizontal and least suppression at oblique test orientations). Next, the time course of anisotropic masking was investigated by varying stimulus onset asynchrony (SOA). A standard "oblique effect" anisotropy is observed at long SOAs but becomes a "horizontal effect" when a noise mask is present within approximately 50 ms of the test onset. The orientation-tuned masking appears to result from an anisotropic gain-control mechanism that pools the weighted responses to the broadband mask, resulting in a changeover from oblique effect to horizontal effect. In addition, the relative magnitude of suppression at the orientations tested corresponds to the relative magnitudes of the content of typical natural scenes at the same orientations. We suggest that this anisotropic suppression may serve to equalize the visual system's response across orientation when viewing typical natural scenes, 'discounting' the anisotropy of typical natural scene content.

Application of shape-based analysis methods to OCT retinal nerve fiber layer data in glaucoma.

PURPOSE: (1) To evaluate the performance of shape-based analysis [wavelet-Fourier analysis (WFA) and fast Fourier analysis (FFA)] applied to retinal nerve fiber layer (RNFL) thickness values obtained from the optical coherence tomograph (OCT) to discriminate healthy and glaucomatous eyes. (2) To compare the performance of the shape-based metrics to that of the standard OCT output measures (Inferior Average and Average Thickness). METHODS: RNFL values were obtained from 152 eyes of 152 individuals (83 healthy and 69 "mild"-stage perimetric glaucoma). WFA and FFA were performed on the RNFL values and linear discriminant functions for both were obtained using Fisher linear discriminant analysis. Performance was evaluated by calculating sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (ROC area). RESULTS: The ROC area of the shape-based methods [0.94 (WFA) and 0.88 (FFA)] was greater than that of OCT metrics [0.81 (Inferior Average) and 0.74 (Average Thickness)]. Specifically, WFAs performance was significantly better than both the FFA (P=0.009) and the Inferior Average (P=0.001). Inferior average performed significantly better than Average Thickness (P=0.006). CONCLUSIONS: The ability to differentiate glaucomatous from healthy eyes using stratus OCT measurements is improved by using these analysis methods that emphasize the shape of the RNFL thickness pattern.

Illusory bands in orientation and spatial frequency: a cortical analog to Mach bands.

Illusory bands at a luminance transition in space (ie an edge) are well known. Here we demonstrate illusory bands of enhanced orientations or spatial frequencies at transitions between higher-contrast and lower-contrast image content along the orientation and spatial-frequency dimensions--the dimensions of cortical spatial coding. We conclude that this illusion is a consequence of cortical-level suppression of units of similar orientations and spatial frequencies and serves to aid texture segmentation while providing efficient neural coding.

Predicting visual field loss in ocular hypertensive patients using wavelet-fourier analysis of GDx scanning laser polarimetry.

PURPOSE: To predict which ocular hypertensive (OHT) patients later develop a visual field loss by applying shape-based analysis (wavelet-Fourier analysis, WFA) to retinal nerve fiber layer (RNFL) thickness estimates. METHODS: Visual field information and scanning laser polarimetry (SLP) RNFL estimates were obtained from both eyes of 122 patients (73 glaucoma and 49 OHT) and 102 healthy individuals. WFA was applied to RNFL temporal, superior nasal, inferior, and temporal (TSNIT) curves (28 points) of the glaucoma and healthy eyes to obtain a classifier. Without modification, the classifier was then applied to the OHT eyes (16 OHTconverters and 33 OHTnonconverters). The visual fields of the OHT patients (6-month follow-up for a mean period of 4 years) were analyzed using the Advanced Glaucoma Intervention Study (AGIS) criteria to identify eyes which developed subsequent field loss in this period of time (OHT converters) and those that did not (OHT nonconverters). For the OHT converters, the classifier was applied to scans from each of three points in time before the initial visual field damage. For the OHT nonconverters, the last available scan was used. The accuracy of the WFA metric in predicting conversion of OHT eyes was assessed by calculating area under the receiver operating characteristic (ROC) curve (area under the ROC curve, AUC), sensitivity at 80% specificity, and likelihood ratio. RESULTS: The performance (AUC) of WFA in predicting conversion of the OHT eyes from scans taken just before visual field loss was 0.83 with a sensitivity (SD)/specificity (SD) of 0.76 +/- 0.11/0.80 +/- 0.07 and likelihood ratio (+LR +/- SD) of 3.8 +/- 1.4. Performance for scans obtained 6 months before the first signs of visual field defects was 0.77 (AUC), 0.71 +/- 0.11/0.80 +/- 0.07 (sensitivity/specificity), and 3.5 +/- 1.4 (+LR +/- SD). Performance was 0.73 (AUC), 0.59 +/- 0.12/0.8 +/- 0.07 (sensitivity/specificity), and 3.0 +/- 0.12 (+LR +/- SD) using the earliest available RNFL estimates. CONCLUSION: The WFA method of temporal, superior nasal, inferior, and temporal shape analysis offers a means of predicting progression in OHT patients before visual field loss.

Anisotropic local contrast normalization: the role of stimulus orientation and spatial frequency bandwidths in the oblique and horizontal effect perceptual anisotropies.

Visual ability for sine waves and other narrowband stimuli shows an oblique effect--worst performance at obliques, best at horizontal and vertical orientations. Recently, we have shown that with broadband stimuli (either 1/f(alpha) visual noise or natural scenes), performance for detecting oriented content is worst at horizontal, best at the obliques, and intermediate at vertical orientations (a "horizontal effect"). This horizontal effect has been explained by a cortical contrast normalization model that is both local (over orientation and spatial frequency) and anisotropic (due to a numerical bias of neurons with different preferred orientations). Here, the bandwidth of content at which an oblique effect or horizontal effect occurs was assessed in two suprathreshold matching experiments conducted with 1/f(alpha) noise stimuli filtered with a triangle increment function of varied bandwidth (16 levels of orientation and spatial frequency bandwidth). The results provided further support for the local anisotropic normalization model in that an oblique effect was observed when a fairly small range of orientations and high spatial frequencies were tested and the horizontal effect was observed for broadband increments > or = 20 degrees orientation bandwidth and > or = 1-octave in frequency. At intermediate spatial frequency and orientation increment bandwidths, a blend of the two anisotropies was observed.

Analysis of GDx-VCC polarimetry data by Wavelet-Fourier analysis across glaucoma stages.

PURPOSE: The purpose of this study was to apply shape-based analysis techniques of retinal nerve fiber layer (RNFL) thickness to GDx-VCC (variable corneal and lens compensator; Laser Diagnostic Technologies, Inc., San Diego, CA) polarimetry data and to evaluate the techniques' ability to detect glaucoma in its earliest stages. Wavelet-based (wavelet-Fourier analysis [WFA]), Fourier-based (fast Fourier analysis [FFA]), and several previous variations of shape-based analysis were considered, as well as the standard metric nerve fiber indicator (NFI), and all were compared as a function of disease stage. METHODS: GDx-VCC scans of one eye of each of 67 patients with glaucoma and each of 67 healthy age-matched subjects provided RNFL thickness estimates at a fixed distance from the optic disc. Severity of disease was graded according to the Glaucoma Staging System and also by mean deviation (MD) from standard automated perimetry. WFA, FFA, and NFI procedures were performed including the following variations: use of signed or unsigned phase, inclusion of interocular or intraocular asymmetry of analysis parameters, and combination of features by principle components analysis or Wilks lambda. Independent samples (k-fold variation) were used for training and testing. Sensitivity, specificity, and receiver operating characteristic (ROC) area were obtained. RESULTS: Classification performance of WFA (ROC = 0.978) was significantly better than FFA (ROC = 0.938) and NFI (ROC = 0.900). This difference was largest for the earliest stages of glaucoma. Shape-based analysis methods performed better than NFI overall. Adding between-eye asymmetry measures helped FFA but not WFA. CONCLUSIONS: Shape-based analysis, and WFA in particular, makes an important improvement in detecting earliest glaucoma with polarimetry.

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 degrees 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.

Human perceptual performance with nonliteral imagery: region recognition and texture-based segmentation.

In this study the authors address the issue of how the perceptual usefulness of nonliteral imagery should be evaluated. Perceptual performance with nonliteral imagery of natural scenes obtained at night from infrared and image-intensified sensors and from multisensor fusion methods was assessed to relate performance on 2 basic perceptual tasks to fundamental characteristics of the imagery. Specifically, single-sensor imagery and fused multisensor imagery (both achromatic and false color) were used to test performance on a region recognition task and a texture segmentation task. Results indicate that the use of color rendering and type of scene content play specific roles in determining perceptual performance allowed by nonliteral imagery. The authors argue that the usefulness of various image-rendering methods should be evaluated with respect to multiple perceptual tasks.

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.

Fourier analysis of optical coherence tomography and scanning laser polarimetry retinal nerve fiber layer measurements in the diagnosis of glaucoma.

OBJECTIVE: To evaluate a new Fourier-based analysis method for diagnosing glaucoma using retinal nerve fiber layer (RNFL) thickness estimates obtained from the optical coherence tomograph (OCT) (OCT 2000) and the scanning laser polarimeter (GDx). METHODS: We obtained RNFL thickness estimates from 1 eye of 38 healthy individuals and 42 patients with early glaucomatous visual field loss using the OCT and GDx devices. The shape of the RNFL double-hump pattern was assessed using Fourier analysis, and values were entered into a linear discriminant analysis. Receiver operating characteristic (ROC) curves were used to compare the performance of the Fourier-based metrics against other commonly used RNFL analytical procedures. Reliability was assessed on independent samples by the split-half method. Correlations were calculated to determine the extent to which the Fourier discriminant measures and other RNFL measures covaried between the 2 devices and the relationship between these RNFL measures and visual field measures. RESULTS: Sensitivity and specificity for the linear discriminant function (LDF) based on the Fourier analysis of the OCT data were 76% and 90%, respectively, and the area under the ROC curve was 0.925 (SEM, 0.028). For the GDx data, the Fourier-based LDF yielded sensitivity and specificity of 82% and 90%, respectively, with an ROC curve area of 0.928 (SEM, 0.029). These values were better than those determined using the GDx number, a previous discriminant function using GDx variables and OCT thickness values. The Fourier-based LDFs and numerous other measures were significantly correlated between the 2 devices. For each device, the visual field measures correlated most highly with the Fourier-based LDF measure. CONCLUSIONS: For both devices, the LDF based on the output from a Fourier analysis of RNFL data resulted in better diagnostic capability compared with other common RNFL analytical procedures. That this technique improves RNFL analysis is also supported by the better correlations between visual field measures and the Fourier-based LDF measures.

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 scene's 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.

Oblique stimuli are seen best (not worst!) in naturalistic broad-band stimuli: a horizontal effect.

People with normal eyesight typically see horizontal and vertical gratings better than oblique gratings (Psychological Bulletin 78 (1972) 266; Perception 9 (1980) 37). In the present study we investigated whether this oblique effect anisotropy is still observed when viewing more complex visual stimuli that better correspond to the content encountered in everyday viewing of the world. We show that the ability to see oriented structure in an image consisting of broadband spatial content is indeed anisotropic, but that the pattern of this orientation bias is completely different from that obtained with simpler stimuli. Horizontal stimuli are seen worst and oblique stimuli are seen best when tested with more realistic broadband stimuli. We suggest that this "horizontal effect" would be useful in an evolutionary capacity by serving to discount the horizon and other oriented content that tends to dominate natural scenes and thereby increase the salience of objects contained in typical outdoor scenes.