Dazzle camouflage: benefits and problems revealed.

During WWI, ships were painted in high-contrast 'dazzle' patterns believed to distort, among other things, submariners' perceptions of direction when aiming their torpedoes, but was this strategy effective? Here, we investigated the effects of different camouflage patterns, including versions used in the war, on the perceived direction of travel for a three-dimensional computer model of the RMS Mauretania. The results of this study showed that texture gradients 'twisted' the perceived direction of the ship, the effect being ~10° for a regular pattern of circles. We also found a second, larger effect, 'hysteresis', that biased perceived target directions to parallel the horizon for directions of travel within approximately ±30° of 90° (left-right).

Blurring the boundary between models and reality: Visual perception of scale assessed by performance.

One of the primary jobs of visual perception is to build a three-dimensional representation of the world around us from our flat retinal images. These are a rich source of depth cues but no single one of them can tell us about scale (i.e.

Object Image Size Is a Fundamental Coding Dimension in Human Vision: New Insights and Model.

In previous psychophysical work we found that luminance contrast is integrated over retinal area subject to contrast gain control. If different mechanisms perform this operation for a range of superimposed retinal regions of different sizes, this could provide the basis for size-coding. To test this idea we included two novel features in a standard adaptation paradigm to discount more pedestrian accounts of repulsive size-aftereffects.

A psychophysical performance-based approach to the quality assessment of image processing algorithms.

Image processing algorithms are used to improve digital image representations in either their appearance or storage efficiency. The merit of these algorithms depends, in part, on visual perception by human observers. However, in practice, most are assessed numerically, and the perceptual metrics that do exist are criterion sensitive with several shortcomings.

Binocular summation revisited: Beyond √2.

Our ability to detect faint images is better with two eyes than with one, but how great is this improvement? A meta-analysis of 65 studies published across more than 5 decades shows definitively that psychophysical binocular summation (the ratio of binocular to monocular contrast sensitivity) is significantly greater than the canonical value of √2. Several methodological factors were also found to affect summation estimates. Binocular summation was significantly affected by both the spatial and temporal frequency of the stimulus, and stimulus speed (the ratio of temporal to spatial frequency) systematically predicts summation levels, with slow speeds (high spatial and low temporal frequencies) producing the strongest summation.

Stacking Chairs: Local Sense and Global Nonsense.

We report a confusing stimulus which demonstrates the power of local interpretation of three-dimensional structure to disrupt a coherent global perception.

Perception of global image contrast involves transparent spatial filtering and the integration and suppression of local contrasts (not RMS contrast).

When adjusting the contrast setting on a television set, we experience a perceptual change in the global image contrast. But how is that statistic computed? We addressed this using a contrast-matching task for checkerboard configurations of micro-patterns in which the contrasts and spatial spreads of two interdigitated components were controlled independently. When the patterns differed greatly in contrast, the higher contrast determined the perceived global contrast.

Contrast and lustre: A model that accounts for eleven different forms of contrast discrimination in binocular vision.

Our goal here is a more complete understanding of how information about luminance contrast is encoded and used by the binocular visual system. In two-interval forced-choice experiments we assessed observers' ability to discriminate changes in contrast that could be an increase or decrease of contrast in one or both eyes, or an increase in one eye coupled with a decrease in the other (termed IncDec). The base or pedestal contrasts were either in-phase or out-of-phase in the two eyes.

Grid-texture mechanisms in human vision: Contrast detection of regular sparse micro-patterns requires specialist templates.

Previous work has shown that human vision performs spatial integration of luminance contrast energy, where signals are squared and summed (with internal noise) over area at detection threshold. We tested that model here in an experiment using arrays of micro-pattern textures that varied in overall stimulus area and sparseness of their target elements, where the contrast of each element was normalised for sensitivity across the visual field. We found a power-law improvement in performance with stimulus area, and a decrease in sensitivity with sparseness.

Fourth-root summation of contrast over area: No end in sight when spatially inhomogeneous sensitivity is compensated by a witch's hat.

Measurements of area summation for luminance-modulated stimuli are typically confounded by variations in sensitivity across the retina. Recently we conducted a detailed analysis of sensitivity across the visual field (Baldwin, Meese, & Baker, 2012) and found it to be well described by a bilinear "witch's hat" function: Sensitivity declines rapidly over the first eight cycles or so, but more gently thereafter. Here we multiplied luminance-modulated stimuli (4 cycles/degree gratings and "Swiss cheeses") by the inverse of the witch's hat function to compensate for the inhomogeneity.

Bedazzled: A Blue and Black Ship, Dressed to Deceive.

The blue and black dress that "melted the Internet" is thought to have done so because its perceived color depended on people using different prior assumptions about discounting the illuminant. However, this is not the first monochromatic object to have confused the public. For a brief period during WWI, RMS Mauretania was dressed in (dazzle) camouflage shades of blue and black/grey, yet she is sometimes depicted by artists, modelers, and historians in a much showier dress of red, blue, yellow, green, and black.

Area summation of first- and second-order modulations of luminance.

To extend our understanding of the early visual hierarchy, we investigated the long-range integration of first- and second-order signals in spatial vision. In our first experiment we performed a conventional area summation experiment where we varied the diameter of (a) luminance-modulated (LM) noise and (b) contrast-modulated (CM) noise. Results from the LM condition replicated previous findings with sine-wave gratings in the absence of noise, consistent with long-range integration of signal contrast over space.

Measuring the spatial extent of texture pooling using reverse correlation.

The local image representation produced by early stages of visual analysis is uninformative regarding spatially extensive textures and surfaces. We know little about the cortical algorithm used to combine local information over space, and still less about the area over which it can operate. But such operations are vital to support perception of real-world objects and scenes.

A two-stage model of orientation integration for Battenberg-modulated micropatterns.

The visual system pools information from local samples to calculate textural properties. We used a novel stimulus to investigate how signals are combined to improve estimates of global orientation. Stimuli were 29 × 29 element arrays of 4 c/deg log Gabors, spaced 1° apart.

Paradoxical psychometric functions ("swan functions") are explained by dilution masking in four stimulus dimensions.

The visual system dissects the retinal image into millions of local analyses along numerous visual dimensions. However, our perceptions of the world are not fragmentary, so further processes must be involved in stitching it all back together. Simply summing up the responses would not work because this would convey an increase in image contrast with an increase in the number of mechanisms stimulated.

A common rule for integration and suppression of luminance contrast across eyes, space, time, and pattern.

Visual perception begins by dissecting the retinal image into millions of small patches for local analyses by local receptive fields. However, image structures extend well beyond these receptive fields and so further processes must be involved in sewing the image fragments back together to derive representations of higher order (more global) structures. To investigate the integration process, we also need to understand the opposite process of suppression.

Blobs versus bars: psychophysical evidence supports two types of orientation response in human color vision.

The classic hypothesis of Livingstone and Hubel (1984, 1987) proposed two types of color pathways in primate visual cortex based on recordings from single cells: a segregated, modular pathway that signals color but provides little information about shape or form and a second pathway that signals color differences and so defines forms without the need to specify their colors. A major problem has been to reconcile this neurophysiological hypothesis with the behavioral data. A wealth of psychophysical studies has demonstrated that color vision has orientation-tuned responses and little impairment on form related tasks, but these have not revealed any direct evidence for nonoriented mechanisms.

The attenuation surface for contrast sensitivity has the form of a witch's hat within the central visual field.

Over the full visual field, contrast sensitivity is fairly well described by a linear decline in log sensitivity as a function of eccentricity (expressed in grating cycles). However, many psychophysical studies of spatial visual function concentrate on the central ±4.5 deg (or so) of the visual field.

Theory and data for area summation of contrast with and without uncertainty: evidence for a noisy energy model.

Contrast sensitivity improves with the area of a sine-wave grating, but why? Here we assess this phenomenon against contemporary models involving spatial summation, probability summation, uncertainty, and stochastic noise. Using a two-interval forced-choice procedure we measured contrast sensitivity for circular patches of sine-wave gratings with various diameters that were blocked or interleaved across trials to produce low and high extrinsic uncertainty, respectively. Summation curves were steep initially, becoming shallower thereafter.

The slope of the psychometric function and non-stationarity of thresholds in spatiotemporal contrast vision.

The slope of the two-interval, forced-choice psychometric function (e.g. the Weibull parameter, β) provides valuable information about the relationship between contrast sensitivity and signal strength.

Zero-dimensional noise: the best mask you never saw.

The transmission of weak signals through the visual system is limited by internal noise. Its level can be estimated by adding external noise, which increases the variance within the detecting mechanism, causing masking. But experiments with white noise fail to meet three predictions: (a) noise has too small an influence on the slope of the psychometric function, (b) masking occurs even when the noise sample is identical in each two-alternative forced-choice (2AFC) interval, and (c) double-pass consistency is too low.

Interocular transfer of spatial adaptation is weak at low spatial frequencies.

Adapting one eye to a high contrast grating reduces sensitivity to similar target gratings shown to the same eye, and also to those shown to the opposite eye. According to the textbook account, interocular transfer (IOT) of adaptation is around 60% of the within-eye effect. However, most previous studies on this were limited to using high spatial frequencies, sustained presentation, and criterion-dependent methods for assessing threshold.

The effect of interocular phase difference on perceived contrast.

Binocular vision is traditionally treated as two processes: the fusion of similar images, and the interocular suppression of dissimilar images (e.g. binocular rivalry).