The details of oriented visual stimuli are better resolved when they are horizontal or vertical rather than oblique. This "oblique effect" has been confirmed in numerous behavioral studies in humans and to some extent in animals. However, investigations of its neural basis have produced mixed and inconclusive results, presumably due in part to limited sample sizes. We have used a database to analyze a population of 4,418 cells in the cat's striate cortex to determine possible differences as a function of orientation. We find that both the numbers of cells and the widths of orientation tuning vary as a function of preferred orientation. Specifically, more cells prefer horizontal and vertical orientations compared with oblique angles. The largest population of cells is activated by orientations close to horizontal. In addition, orientation tuning widths are most narrow for cells preferring horizontal orientations. These findings are most prominent for simple cells tuned to high spatial frequencies. Complex cells and simple cells tuned to low spatial frequencies do not exhibit these anisotropies. For a subset of simple cells from our population (n = 104), we examined the relative contributions of linear and nonlinear mechanisms in shaping orientation tuning curves. We find that linear contributions alone do not account for the narrower tuning widths at horizontal orientations. By modeling simple cells as linear filters followed by static expansive nonlinearities, our analysis indicates that horizontally tuned cells have a greater nonlinear component than those tuned to other orientations. This suggests that intracortical mechanisms play a major role in shaping the oblique effect.

https://www.physiology.org/doi/pdf/10.1152/jn.00954.2002

Oblique Effect: A Neural Basis in the Visual Cortex

A new metric based on extended spatial frequency and its application to DWT based fusion algorithms

The contrast sensitivity function is routinely measured in the laboratory with sine-wave gratings presented on homogenous gray backgrounds; natural images are instead composed of a broad range of spatial and temporal structures. In order to extend channel-based models of visual processing to more natural conditions, we examined how contrast sensitivity varies with the context in which it is measured. We report that contrast sensitivity is quite different under laboratory than natural viewing conditions: adaptation or masking with natural scenes attenuates contrast sensitivity at low spatial and temporal frequencies. Expressed another way, viewing stimuli presented on homogenous screens overcomes chronic adaptation to the natural environment and causes a sharp, unnatural increase in sensitivity to low spatial and temporal frequencies. Consequently, the standard contrast sensitivity function is a poor indicator of sensitivity to structure in natural scenes. The magnitude of masking by natural scenes is relatively independent of local contrast but depends strongly on the density of edges even though neither greatly affects the local amplitude spectrum. These results suggest that sensitivity to spatial structure in natural scenes depends on the distribution of local edges as well as the local amplitude spectrum.

Contrast sensitivity in natural scenes depends on edge as well as spatial frequency structure.

Perceptual evaluation of different image fusion schemes

Although orientation columns are less than a millimeter in width, recent neuroimaging studies indicate that viewed orientations can be decoded from cortical activity patterns sampled at relatively coarse resolutions of several millimeters One proposal is that these differential signals arise from random spatial irregularities in the columnar map However, direct support for this hypothesis has yet to be obtained Here, we used high-field, high-resolution functional magnetic resonance imaging (fMRI) and multivariate pattern analysis to determine the spatial scales at which orientation-selective information can be found in the primary visual cortex (V1) of cats and humans We applied a multiscale pattern analysis approach in which fine- and coarse-scale signals were first removed by ideal spatial lowpass and highpass filters, and the residual activity patterns then analyzed by linear classifiers Cat visual cortex, imaged at 03125 mm resolution, showed a strong orientation signal at the scale of individual columns Nonetheless, reliable orientation bias could still be found at spatial scales of several millimeters In the human visual cortex, imaged at 1 mm resolution, a majority of orientation information was found on scales of millimeters, with small contributions from global spatial biases exceeding ∼1 cm Our high-resolution imaging results demonstrate a reliable millimeters-scale orientation signal, likely emerging from irregular spatial arrangements of orientation columns and their supporting vasculature fMRI pattern analysis methods are thus likely to be sensitive to signals originating from other irregular columnar structures elsewhere in the brain

https://www.jneurosci.org/content/jneuro/30/1/325.full.pdf

Multiscale pattern analysis of orientation-selective activity in the primary visual cortex.

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

This study investigated human perceptual performance allowed by relatively impoverished information conveyed in nighttime natural scenes. Researchers used images of nighttime outdoor scenes rendered in image-intensified low-light visible (i-squared) sensors, thermal infrared (ir) sensors, and an i-squared/ir fusion technique with information added. They 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. The study addresses the issue of assessment of perceptual performance with alternative night-vision sensors and fusion methods and begins 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.

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

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.

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

Human Perception of Sensor-Fused Imagery

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.

J. Kevin DeFord

Papers

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

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

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

Human Perception of Sensor-Fused Imagery

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