bluuebunny/crossref_metadata_2025 huggingface dataset:
Misinterpreting Changes of Illuminant on Complex Mondrian Patterns
Authors: Nascimento S M C , Foster D H
Published: December 2015

Ratios of cone excitations from different surfaces of the same coloured scene are almost invariant under illuminance changes, and might provide the cue by which the visual system discriminates illuminant from non-illuminant changes in coloured scenes. Previous work with pairs of surfaces showed that observers were able to detect small, naturally occurring, violations in these ratios (Nascimento and Foster, 1995 Perception24 Supplement, 60 – 61). In the present study, sensitivity to violations was assessed with more complex, Mondrian patterns. In a two-interval forced-choice experiment, two colour transformations of the same Mondrian pattern were compared by observers. The patterns comprised 7 × 7 coloured patches. Each patch was a simulation of a Munsell surface, and the whole pattern was illuminated by a Planckian illuminant of variable colour temperature. In one of the intervals only the colour temperature of the illuminant changed; in the other, the same colour-temperature change was made, but, in addition, the spectral reflectances of the surfaces were adjusted such that all cone ratios were exactly preserved for the three classes of cone. The task was to identify which of the intervals contained the pure illuminant change.

Observers could reliably discriminate the intervals but systematically interpreted colour changes with invariant cone-ratios as being illuminant changes, with a probability that increased as the degree of violation of invariance increased. Performance depended mainly on long-wavelength-sensitive cones, less on medium-wavelength-sensitive cones, and little or not at all on short-wavelength-sensitive cones or luminance signals. Cone-excitation ratios, although sometimes unreliable, appear to be the dominant cue for deciding on the nature of colour changes.
Ignoring Sparse Chromatic Context
Authors: Cornelissen F W , Brenner E
Published: December 2015

The apparent colour of a target depends to some extent on the colour of the background. Jennes and Shevell have recently shown that a red background scattered with white or green dots has a different influence on the apparent colour of a target than a uniform background with the same space-averaged chromaticity and luminance (1995 Vision Research35 797 – 805). This could have important implications for our understanding of colour vision. However, it appears to be a very fragile effect. We were only successful in reproducing it when we used the same long dark-adaptation and pre-adaptation times as those used by Jennes and Shevell. One could argue that with shorter adaptation the effect was masked by gradual changes such as shifts in receptor sensitivity. However, when we interleaved trials with and without white dots for well over 20 min there was still no systematic difference between the two kinds of background. Moreover, when we repeated their experiment with a green rather than a red background, adding white dots had no effect. The long adaptation times that are required to produce the effect and the fact that we only found it for a long-wavelength background suggests that the effect may have to do with differences in the extent of rod adaptation. Although it remains an interesting phenomenon, this would probably make it irrelevant for normal colour vision.
Chromatic Intensities with Achromatic Surrounds
Authors: Luque M J , Capilla P , Felipe A , Artigas J M
Published: December 2015

Brightness changes induced in red or green tests by an achromatic surround were studied by a nulling method with a centre — surround spatial configuration. The amplitude of the cancelling modulation was found to increase linearly with the contrast of the inducing field when its mean luminance was constant. The quotient between the amplitudes of the cancelling and the inducing modulations increased with increasing test luminance and decreased with increasing mean luminance of the surround. For a given contrast, the amplitude of the cancelling modulation increased with the amplitude of the inducing modulation, except when the mean luminance of the surround was close to the test luminance, where an abnormally high value was found. With the nulling method, the values of the cancelling modulation were practically the same for the green and the red tests. Nevertheless, with a heterochromatic brightness-matching method, the luminance changes needed to match the test brightness depended on test chromaticity. At equal surround luminance, the greatest changes were for the green test and the smallest for the blue test, the red test being at an intermediate level. The results are consistent with previous studies and were fit by the model recently developed by B Spehar, J DeBonet, and Q Zaidi. In this model the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround with local luminance gain controls in the test and surround fields.
Texture Segregation by Orientation Differences: Colour Sensitive but Not Hue Specific?
Authors: Leonards U , Singer W
Published: December 2015

Segregation of textures on the basis of orientation differences between texture elements is achieved even when these texture elements differ from their surround only by colour (McIlhagga et al, 1990 Vision Research30 489 – 495). This finding seems to contradict the assumption that colour and orientation are extracted in separate feature maps (eg Treisman and Sato, 1990 Journal of Experimental Psychology: Human Perception and Performance16 459 – 478). To examine whether colour information is evaluated in parallel in different processing streams for the assessment of hue and form, we tested whether texture elements can be segregated if they differ only by specific conjunctions of colour and orientation; texture elements consisted of crosses with their two crossing lines differing in colour. Texture elements defining figure and background had the same coloured composition but the conjunction of colour with the two crossing lines was reversed. Different colour combinations were tested under various luminance contrast conditions, irrespective of the colour combination, segmentation was achieved as long as the two crossing lines of the texture elements differed in luminance. If, however, the different colours of the two crossing lines were approximately equiluminant, segmentation was reduced or impossible. Thus, subjects were able to use for texture segregation conjunctions between luminance and orientation but not between colour and orientation. Our results suggest that colour cannot be associated selectively with differently oriented components of the same texture element. This supports the hypothesis that colour contrast is used in parallel by different processing streams to assess the orientation and hue of contours and reveals limitations in the selectivity with which features are subsequently bound together.
Effects of Element Density on Segmentation by Luminance, Colour, and Motion
Authors: Moeller P , Hurlbert A C
Published: December 2015

A target composed of random dots can be easily segmented from a background of random dots when each is defined by a different, broad distribution of dot speeds, colours, or brightnesses, even if these distributions overlap extensively. This shows that segmentation can be based on global properties of the target and background. If average feature value is the relevant global property, segmentation threshold should be a monotonically decreasing function of dot density.

Observers viewed a 6.5 deg × 6.5 deg square array of between 512 and 36 864 randomly positioned dots (dot size 4.7 or 1.6 min arc) in which the target was a vertical band of dots defined by a different distribution of luminance, colour, or motion from the background. The task was to indicate whether the target appeared on the left or on the right of midline. The stimulus was displayed for 67 ms, followed after 233 ms by a 67 ms mask.

We found that motion segmentation thresholds decrease monotonically as a function of density. For luminance and colour, segmentation thresholds initially decrease toward a local minimum, then increase to a local maximum. Thereafter, thresholds decrease monotonically as density increases further. These results suggest that the integrative segmentation process for motion is qualitatively different from those for luminance and colour. The local extrema for luminance and colour suggests that segmentation in these domains is based on forming clusters by linking together dots with similar feature properties. Only when interdot distance is sufficiently small are all dots within an area included in the cluster. Behaviour is then determined by difference in average feature value or target — background contrast.
Simultaneously Judging Contrast in Different Orientation and Frequency Bands
Authors: Olzak L A , Thomas J P , Wickens T D
Published: December 2015

Previous discrimination experiments suggest that suprathreshold contrast signals carried by orthogonally oriented gratings are combined when component frequencies are similar, but contrast signals are not combined across frequency bands regardless of orientation. We investigated neural and attentional processes underlying these findings in a concurrent-response paradigm. Test stimuli were composed of two superimposed sinusoidal gratings. In one condition, two 3 cycles deg−1 gratings were superimposed at orthogonal orientations to form plaids. In the other condition, the component gratings were 3 cycles deg−1 and 15 cycles deg−1, both vertical. In each condition, each component independently took one of two slightly different contrast values, combined all possible ways to create four stimuli. On each trial, one stimulus appeared for 1 s. Observers made two contrast discrimination judgments, one on the vertical (or low frequency) component, the other based on the horizontal (or high frequency) grating. Highly correlated response patterns and the collapse of two decision axes into nearly one confirmed that observers were unable to make independent judgments of contrast on orthogonally oriented components of similar frequency. Analyses suggested that decisions were based primarily on a single neural signal representing the sum or average of the two contrasts present. When cues to discrimination were in different frequency bands, observers showed a marked inability to perform the simultaneous judgment task, choosing idiosyncratic strategies to maximise performance. Analyses indicated that the contrast information was processed through separate and independent pathways, but that information from the two bands was not simultaneously available to the observer.
Dynamic Contrast Perception Assessed by Pattern Masking
Authors: Bowen R W , de Ridder H
Published: December 2015

The perceived contrast of a pulsed grating of about 100 ms duration can exceed that of shorter or longer exposures. We investigated this contrast enhancement effect with pattern masking. The pulsed mask patterns were extended cosines (5 deg square, 35 cd m−2 mean luminance, 0.3 contrast) of 50 to 500 ms duration. Mask spatial frequency was 1 or 6 cycles deg−1. The test patterns (of equivalent spatial frequency) were sixth derivative Gaussians, either positive (ON pathway mediation) or negative (OFF pathway mediation) and of 30 ms duration. The test pattern could be centred on a light bar of the cosine (positive contrast) or a dark bar (negative contrast). Test and mask had simultaneous onset. For a 1 cycle deg−1 test and mask of the same polarity, the test threshold/mask duration function is nonmonotonic (peak at 83 – 100 ms). The function was similar for either positive or negative stimuli. Thus, we measured an analogue to the contrast enhancement effect, and found enhancement for negative as well as positive contrast components. For same-polarity 6 cycles deg−1 test and mask, threshold increased monotonically to 500 ms (no enhancement). For both 1 and 6 cycles deg−1 stimuli of opposite polarity, the threshold/mask duration function is sharply elevated and constant for masks of 83 ms or more. The same-polarity masking functions imply activation of either transient (1 cycle deg−1 stimuli) or sustained (6 cycles deg−1 stimuli) ON or OFF pathways. The opposite-polarity functions suggest that the time course of ON — OFF pathway interaction is similar for sustained and transient pathways.
Mechanisms and Meaning of Devries—Rose Adaptation
Authors: Donner K , Fagerholm P
Published: December 2015

‘Square-root’ or ‘deVries — Rose’ light adaptation is observed over a substantial luminance range in human foveal vision. The classical interpretation is that a detector (presumably in the brain) discriminates the neural signal evoked by the stimulus from the neural noise evoked by quantum fluctuations. It is known, however, that the retina may adjust its gain in inverse proportion to the square root of mean luminance, as observed eg in cat retinal ganglion cells under scotopic or mesopic adaptation. This kind of gain change is approximated even by the primary visual cells, the rods and cones, in at least some vertebrate species up to luminances producing 103 – 104 photoisomerisations per photoreceptor cell, per second. Is square-root adaptation in fact mainly an expression of an inverse-square-root gain in retinal cells? We investigated the roles of gain and noise in human foveal detection of 0.25 deg incremental spots presented for 50 ms on 5 deg steady backgrounds ranging from −0.25 to 2.35 log td, by measuring effects of pixel noise added to the stimulus and background. The results were consistent with the hypothesis that square-root adaptation mainly reflects gain changes, whereas the signal is detected against a constant level of neural noise. They were not consistent with the idea that signals proportional to stimulus intensity are detected against a noise that increases in proportion to quantum fluctuations. Thus, they do not support a simple interpretation of the deVries — Rose law. Still, an inverse-square-root retinal gain may in an evolutionary sense be seen as an adaptation to quantum fluctuations, in view of its functional consequences: (1) that output noise stays constant, independent of luminance level; (2) that light signals of constant statistical significance are encoded by visual responses of constant size.
Isoluminant Colour Contrast Does Not Fill in Surfaces
Authors: Dresp B , Wehrhahn C
Published: December 2015

It has been suggested (Livingstone and Hubel, 1988 Science240 740 – 749) that the ‘colour-blind’ magnocellular pathways generate the neurophysiological basis of surfaces with illusory contours since the latter do not seem to be perceived in inducing configurations of a given colour which is isoluminant with regard to the colour of the background (equiluminant colour contrast). However, psychophysical data allowing us to assess the relative visibility of illusory surfaces in coloured stimuli with luminance contrast compared to configurations with equiluminant colour contrast are not yet available.

We designed a colour-matching experiment where ten naive observers had to adjust the intensity of a red illusory surface so that it appeared to match the intensity of the red background. The configurations used were Kanizsa squares with green inducing elements, isoluminant or not with regard to the background. Isoluminance was assessed individually for each observer by means of a classical flicker test. A brightness-matching procedure was applied to configurations of achromatic inducers on a grey background. In this case, the inducers had either all the same contrast polarity (light), or both polarities (light and dark) within a given configuration. Luminance contrast in the achromatic configuration with only one polarity was the same as in the non-isoluminant colour condition. Luminance contrasts of light and dark inducers in the mixed-polarity condition were physically balanced.

The results show that the mean point of subjective equality (PSE) of the test surface corresponds to the physical intensity of the background with equiluminant colour contrast only, indicating the absence of an apparent surface in this condition. This result supports the idea that magnocellular pathways in the human visual system mediate the neurophysiological genesis of illusory surfaces. In all the other stimulus conditions, the PSE does not correspond to the physical intensity of the background. Matching ‘errors’ are significantly stronger in the achromatic conditions, but, paradoxically, strongest in the condition with balanced contrasts of opposite polarity. This finding suggests that luminance contrast is not the only determinant of the perceived strength of illusory surfaces.
A Hierarchical Dynamic Link Network to Solve the Visual Correspondence Problem
Authors: Würtz R P , von der Malsburg C
Published: December 2015

Conventional neural networks try to solve the problem of object recognition in a single step by building a stimulus — response system that codes its result as cell activities. We take a different approach assuming that recognition is an active process with temporal dynamics and results in an ordered state. We present a structure of neuronal layers, interconnected by dynamic links (von der Malsburg, 1985 Berichte der Bunsengesellschaft für Physikalische Chemie89 703 – 710) that solves the correspondence problem between two images and thus constitutes an important building block for a model of recognition.

Images as well as stored models are represented as Gabor pyramids. This allows the dynamics to proceed from coarse to fine scale and reduces the sequential processing time inherent in the concept. Invariance under background changes is also made possible.

On the lowest frequency level, a single blob of activity moves across the image and model layer, respectively. Dynamic links between these layers are initialised to the (highly ambiguous) feature similarities. Links grow or decline according to a combination of feature similarity and correlated activation. This enforces correct neighbourhood relationships in addition to feature similarity. On the higher levels the established correspondences are refined by several blobs in parallel.

We present an improved version of the dynamical system proposed by Würtz [1995 Multilayer Dynamic Link Networks for Establishing Image Point Correspondences and Visual Object Recognition (Thun, Frankfurt a.M.: Harri Deutsch)] and show, with examples of human faces, that it evolves from an unordered link distribution to any ordered state where only corresponding point pairs are connected by strong links. Correspondences between sample points are population-coded by a set of neighbouring links.
Neural Mechanisms in Boundary Grouping, Illusory Contour Generation, and Spatial Tuning of Receptive Field Selectivity
Authors: Neumann H , Mössner P
Published: December 2015

A model for cortical boundary detection and contour grouping at visual cortical areas V1 and V2 is presented. The functional organisation of the model architecture is based on principles of adaptive resonance theory (Grossberg, 1980 Psychological Review87 1 – 51). The primary visual areas are connected in a bidirectional scheme of feedforward and feedback projections. Signal features are initially measured by orientation-sensitive neurons at the simple and complex cell level. These measurements undergo local competitive processing and subsequent nonlinear bipole integration of coherent activity (von der Heydt and Peterhans, 1988 Journal of Neuroscience9 1731 – 1748). The integration is based on an antagonistic interaction in position and orientation space utilising spatial coupling and compatibility functions for the evaluation of coherent activity distributions. This step can therefore be interpreted as implementing a process of matching neural codes of most-likely contour outlines against the noisy input distribution of local contrast measurements. The result of the integration stage is fed back to the first competitive stages to enhance compatible activations via multiplicative gating. The net effect produces grouping and illusory contour completion. In particular, the results of model simulations demonstrate that (i) the orientation selectivity of V1 cells is sharpened depending on the spatial arrangement and visual context, (ii) orientation-selective contrast measurements are enhanced at boundaries of spatially homogeneous stimulus arrangements and suppressed in the interiors, (iii) oriented cells at model V2 stage only respond to curvilinear input arrangements of activity at both branches of bipole cells, and (iv) model V2 cells show fine tuning for orientation selectivity and generate subjective contours to bridge gaps in arrangements of oriented contrast.
Simulation of Cortex Visual Cells for Texture Segmentation: Foveal and Parafoveal Projections
Authors: Palagi P M , Guérin-Dugué A
Published: December 2015

The objective of this work is to simulate visual cortical cells, their sensitivities to frequencies and orientations, and their part in texture segmentation. The simulation of these cells is realised through band-pass, oriented filters (Gabor filters), and multiresolution image decomposition. By this means, the filter sensitivities represent cell sensitivities to preferred orientations according to their frequency and orientation bandwidths, and multiresolution represents the different band frequencies.

For texture analysis and segmentation, overlaying of band-pass filters is necessary to completely cover the Fourier domain. A continuous sensitivity to frequency and orientation is achieved by the filters overlapping and consequently by their interpolation. We used here four octave frequency bands from 1 to 16 cycles deg−1 and six orientations per band. The results obtained for texture segmentation with these parameters are very promising (up to 97% recognition rate) [Guérin-Dugué and Palagi, 1994 Neural Processing Letters1(1) 25 – 29]. The images analysed cover a multitude of different domains such as psychophysical tests and natural textures of different roughness.

In order to create a cortical cell representation closer to neurophysiological data, and to improve texture segmentation results, we represent cell sensitivities by their foveal and parafoveal projections [R L DeValois, K K DeValois, 1988 Spatial Vision (Oxford: Oxford Science Publications)]. Cells receiving projections from the foveal zone are modeled by five octave frequency bands (from 0.5 to 16 cycles deg−1) and six orientations. Cells receiving projections from the parafoveal zone have the same sensitivities but are modeled by four octave frequency bands (from 0.5 to 8 cycles deg−1). By using these two different resolutions, preliminary tests have shown the capability of detecting textured regions by the parafoveal projection and localisation of boundaries by the foveal projection.
Dynamics in Parvocellular and Magnocellular Pathways: Consequences for Luminance and Colour Processing Streams
Authors: Beaudot W H A
Published: December 2015

An achromatic neuromorphic model of the vertebrate retina has already accounted for X and Y pathways (Beaudot and Hérault, 1994 Perception23 Supplement, 25) and has shown a temporal ‘coarse-to-fine’ processing of spatial information (Beaudot et al, 1995 Perception24 Supplement, 93). This model has been extended to colour vision. By taking into account the chromatic sensitivities of cones, functional properties of the parvocellular pathway are modelled. Approximating the responses of colour-opponent cells, the model provides a spatial multiplexing of luminance and chrominance information: sustained responses show spatial band-pass behaviour to luminance variations and low-pass behaviour to equiluminant colour changes. In addition the spatiotemporal inseparability for luminance in the parvocellular model leads to a temporal multiplexing of spatial luminance information: at higher temporal frequencies the spatial filtering is low-pass, conveying only luminance information.

Demultiplexing this mixed information suggests interactions between retinal channels. By locally combining additive and subtractive mechanisms between opposite parvocellular pathways (eg G+/ R−± R+/ G−), and an inhibition from the magnocellular pathway, the existence of at least three functional subchannels is predicted: (i) a transient, spatially low-pass channel, (ii) a sustained, spatially band-pass channel, dedicated to the analysis of luminance information in a spatiotemporally separable way (eg moving shadows and static textures), and (iii) a spatiotemporally low-pass, colour-opponent channel leading to colour induction, which is little affected by the presence of shadows and is more representative of objects. This hypothesis of spatiotemporal demultiplexing of luminance and chrominance information, which should presumably occur at an early cortical level, is in accordance with the multiple-processing-streams organisation of the primate visual system.
The Orthogonal Motion Aftereffect
Authors: Grunewald A , Lankheet M J M
Published: December 2015

A recent model of motion perception suggests that the motion aftereffect (MAE) is due to an interaction across all directions, rather than just opposite directions (Grunewald, 1995 Perception24 Supplement, 111). According to the model, the MAE is caused by the interaction of broadly tuned inhibition and narrowly tuned excitation, both in direction space. The model correctly suggests that, after adaptation to opposite directions of motion, no MAE results. Unlike other accounts of the MAE, this model predicts that, after adaptation to opposite but broadly defined directions of motion, a MAE orthogonal to the inducing motions is observed.

We tested this counter-intuitive prediction by adapting subjects to two populations of dots, whose average motion vectors were opposite, but which contained motion vectors deviating slightly (up to 30°) from the average direction. During the subsequent test phase, randomly moving dots were displayed. Subjects were asked to indicate whether they perceived any global motion during this phase, and if so, they were asked to indicate the perceived motion axis by aligning a line. Subjects were tested on four pairs of directions: vertical, horizontal, and the two diagonals. In all four conditions subjects reported seeing an MAE, and the axis that they indicated was always orthogonal to the inducing motions (ANOVA: p<0.001, accounted for 95% of variance). This experiment confirms the predictions made by the model, thus further supporting the interaction across all directions of narrowly tuned excitation and broadly tuned inhibition.
Terminator Position Affects the Perceived Speed of Line Segments Tilted with Respect to Direction of Motion
Authors: Scott-Brown K C , Heeley D W
Published: December 2015

We investigated factors producing bias in the perceived speed of tilted lines in horizontal translation. The effects of grouping, collinearity, eccentricity, terminator proximity, and stimulus uncertainty on perceived speed were studied. The matched speed of vertical line compared to an inclined line was estimated with the use of a double random interleaved staircase for speed discrimination with a two-alternative forced choice. The speed of the tilted stimulus was held constant while the speed of the vertical stimulus was modified by the subject's response. Stimuli were single lines and groups of lines. The groups of short lines were arranged either in a collinear or in a randomly scattered fashion. The length of the line stimuli ranged from 0.33 deg to 7.0 deg of visual angle. Speed estimates were obtained for angles of tilt ranging from 0° to 90°. For line segments, collinearity was found to be the critical factor in determining perceived speed. Collinear segments showed a similar bias in perceived speed to single lines of the same overall length. However, randomly scattered segments were not subject to a bias in perceived speed. Random perturbation of the length or vertical position of a single line abolished the bias in perceived speed of a tilted line compared to a vertical line. Current models of the integration of motion measurements should be changed to account for the effects of topological arrangement and terminator position on the perceived speed of inclined lines.
On the Psychophysics of Binocular Visual Space
Authors: Heller J
Published: December 2015

Binocular vision gives rise to a perceptual space that is endowed with a rich geometrical structure, which according to Luneburg [1947 Mathematical Analysis of Binocular Vision (Princeton, NJ: Princeton University Press)] is a non-Euclidean Riemannian geometry of constant curvature. This hypothesis, together with certain psychophysical assumptions, provides a qualitative explanation of classical empirical phenomena. The Luneburg theory has been less successful in quantitatively predicting individual data, a fact that has sometimes been taken as evidence against the presumed geometrical structure. Some of these shortcomings, however, may also be attributed to psychophysical assumptions that do not depend on the geometry of visual space. The locus of perceived egocentric equidistance, for example, has been found to deviate systematically from the Vieth-Müller circle postulated by Luneburg (eg Foley, 1966 Journal of the Optical Society of America56 822 – 827).

To account for these findings a generalisation of Luneburg's psychophysical mapping is proposed that actually realises his original intentions. Because of the theoretical basis of its measurement, the adequacy of this general psychophysical frame of reference may be empirically tested. The results of an experimental test of one of the key conditions, which strongly support the presented theory, are reported. It is also shown that psychophysical invariances lead to restrictions on the possible form of the psychophysical mapping, and the approach is discussed from the viewpoint of retinal correspondence.
Is Stereoscopic Capture Possible with Three-Dimensionally Slanted or Curved Illusory Surfaces?
Authors: Häkkinen J , Liinasuo M , Kojo I , Nyman G
Published: December 2015

The depth of disparate illusory contours affects the perception of background patterns that are enclosed by these contours. If the pattern is repetitive, the depth of the illusory surface is attributed to texture elements even though the texture is in zero disparity. Previous results have suggested that this phenomenon, called stereoscopic capture, is possible only with frontoparallel surfaces (Ramachandran, 1986 Perception & Psychophysics39 361 – 373). We hypothesised that the disruption of stereo capture with three-dimensionally slanted or curved surfaces has been due to the impossibility of consistent rematching of background texture elements. If the texture is designed in such a way that adjacent elements can be rematched to form a surface that is consistent with the three-dimensional structure of the illusory surface, the capture should be possible with complex surfaces.

We investigated our hypothesis by showing three-dimensionally slanted and curved surfaces to subjects and changing the pattern of the background texture. According to our results a texture of a constant period can be captured only by a frontoparallel surface; however, if the texture is designed to be consistent with the three-dimensional structure of the complex surface, the texture elements that initially form a frontoparallel surface form a complex surface when they are captured.

We conclude that (a) stereoscopic capture is possible with complex three-dimensional surfaces; (b) the possible discrete matches within the background texture determine the possible three-dimensional forms that can be captured and therefore stereoscopic capture should not be characterised only as depth interpolation (Mitchison and McKee, 1987 Vision Research27 285 – 294) between large image elements.
Integration of Disparity Information across Multiple Fixations
Authors: Sumnall J H , Cumming B G , Parker A J
Published: December 2015

We measured stereo thresholds for observers viewing noisy, dynamic random-dot stereograms (DRDS) containing either a spatial or a temporal step in depth. Stereograms were presented either with or without a small fixation cross at zero disparity. When the fixation marker was present observers were instructed to maintain fixation throughout the presentation.

For spatial steps in depth, the lowest thresholds were found when there was no fixation marker and observers were free to look around the stimulus. Thresholds were higher with a fixation marker in the stimulus. The opposite pattern of results was found for stimuli that contained a temporal step in depth, ie the lowest thresholds were found with a fixation marker. Furthermore, presentation of a single depth region with a fixation marker gave the lowest thresholds of any configuration tested.

The results with temporal changes in depth are reminiscent of the findings of Westheimer (1979 Experimental Brain Research36 585 – 597). Westheimer argued that sequential presentation in the same location might depress sensitivity to changes in depth, but in DRDS no single dot changes in depth without also changing in retinal location, so this explanation is not applicable.

We conclude that, with a spatial step in depth, it is advantageous to foveate both sides of the step, so multiple fixations help. With a temporal step in depth, the spatial structure of the stimulus is uniform, so multiple fixations may not necessarily help, especially if they result in uncertainty about vergence position.
Assessing the Role of Vergence Changes in the Perception of Random-Dot Stereograms by Using Open-Loop Control of Vergence
Authors: Rogers B J , Bradshaw M F
Published: December 2015

The fact that the 3-D shape of surfaces depicted by random dot stereograms can take several seconds or even tens of seconds to appear has been attributed to the failure to make appropriate vergence changes [B Julesz, 1971 Foundations of Cyclopean Perception (Chicago, IL: University of Chicago Press)]. Alternatively, the long latencies could be a consequence of the processing time needed to match the disparate images. To distinguish between these possibilities we measured perceptual latencies in a situation in which vergence changes had no effect on retinal disparities. To do this, horizontal eye movements were recorded with the aid of close-fitting scleral search coils in both eyes and the difference signal used to shift horizontally the two halves of a random-dot stereogram by equal and opposite amounts. When the amount of shift was equal to the magnitude of the vergence change, changes of vergence had no effect on the pattern of disparities—open-loop vergence. Three observers were presented with a sequence of stereograms depicting both ‘simple’ surfaces (a single square lying in front of the surround) and ‘complex’ surfaces, including spirals, ‘wedding cakes’, and saddle shapes under both normal and open-loop conditions. Under open-loop conditions, the complete 3-D shape was never perceived when the disparity range of the stereogram was large (>40 min arc), demonstrating the necessity of vergence changes, but the 3-D structure of ‘complex’ surfaces did build up over a period of several seconds indicating a separate disparity processing limitation.
Statistical Analysis of Spatial Structure in Natural Images
Authors: Watt R , Carlin P
Published: December 2015

When a sample of natural images is taken and compared with a set of noise images, the two are obviously distinguishable. Even when the noise images are set to have the same power spectral characteristics as the natural images, there is no doubt which is a normal image and which is a noise image. In the study to be reported, we have examined the nature of the spatial characteristics of natural images that allow them to be so discriminated from noise images.

The approach is to process large sets of images belonging to various categories through mechanisms that have some similarities to known operations in biological visual systems. Thus, the images are filtered at various spatial scales and at various orientations; the filter outputs are combined into local energy maps; and features are detected in such processed images. The result of these calculations is the distribution of values of some parameter which describes a particular image characteristics for each set of images. Parameters that could support adequate discrimination between two sets of images, for example natural images and noise images, will have largely non-overlapping distributions.

In practice it is found that no simple parameters can distinguish obviously different sets of images, but parameters that encapsulate spatial patterns, especially those related to non-accidental image properties, can do so. It is concluded that filter outputs must be followed by nontrivial spatial operations. Suggestions are made as to what are the most plausible.
Natural-Image Discrimination in the Periphery: The Importance of Phase and Amplitude Information
Authors: Kennedy G M , Tolhurst D J
Published: December 2015

Previous studies with simplified stimuli such as combinations of sinusoidal gratings have revealed phase identification losses in the periphery that are not eliminated by a scaling factor. How do these phase processing problems influence our ability to discriminate natural images in the periphery?

In this study the ability of an observer to identify the ‘odd-image-out’ when there is either an amplitude-only, phase-only, or amplitude and phase change in one out of three stimuli is compared. Pairs of Fourier-manipulated black-and-white digitised photographs of natural images were used and phase and amplitude spectral exchanges of varying proportions were made between two different images. Measurements were made to determine the smallest phase change needed in order for the observer to reliably discriminate the manipulated image, compared to two reference stimuli, at eccentricities of 0°, 2.5°, 5°, and 10°. This was compared to discrimination thresholds found when amplitude and phase, and amplitude alone were exchanged.

The ability to discriminate images on the basis of phase information alone did fall off quickly with eccentricity (comparable to phase and amplitude discriminations). However, there was a much more rapid decline in amplitude-only discrimination. It appears that phase information in natural scenes remains a relatively more important visual cue in the periphery than amplitude.
Diagnostic Colours Influence Speeded Scene Recognition
Authors: Oliva A , Schyns P G
Published: December 2015

A critical aspect of early visual processes is to extract shape data for matching against memory representations for recognition. Many theories of recognition assume that this is achieved by luminance information. However, psychophysical studies have revealed that colour is being used by low-level visual modules such as motion, stereopsis, texture, and 2-D shapes. Should colour really be discarded from theories of recognition? Here we present two studies which seek to throw light on the role of chromatic information for the recognition of real scene pictures. We used three versions of scene pictures (gray levels, normally coloured and abnormally coloured) coming from two broad categories. In the first category, colour was diagnostic of the category (eg beach, forest, and valley). In the second category colour was not diagnostic (eg city, road, and room). In the second category colour was not diagnostic (eg city, road, and room). Results revealed that chromatic information is being registered and facilitates recognition even after a 30 ms exposure to the scene stimuli. However, influences of colour on speeded categorisations were only observed with the colour-diagnostic categories. No influence of colour was observed with the other categories. A similar pattern of results was observed with 120 ms exposure. However, there was an interference of the wrong colour on recognition in colour-diagnostic categories. In sum, colour, when it is diagnostic of the category, influences speeded scene recognition.
Spatio-Chromatic Information Content of Natural Scenes
Authors: Troscianko T , Parraga C A , Brelstaff G , Carr D , Nelson K
Published: December 2015

A common assumption in the study of the relationship between human vision and the visual environment is that human vision has developed in order to encode the incident information in an optimal manner. Such arguments have been used to support the 1/f dependence of scene content as a function of spatial frequency. In keeping with this assumption, we ask whether there are any important differences between the luminance and (r/g) chrominance Fourier spectra of natural scenes, the simple expectation being that the chrominance spectrum should be relatively richer in low spatial frequencies than the luminance spectrum, to correspond with the different shape of luminance and chrominance contrast sensitivity functions.

We analysed a data set of 29 images of natural scenes (predominantly of vegetation at different distances) which were obtained with a hyper-spectral camera (measuring the scene through a set of 31 wavelength bands in the range 400 – 700 nm). The images were transformed to the three Smith — Pokorny cone fundamentals, and further transformed into ‘luminance’ (r+g) and ‘chrominance’ (r-g) images, with various assumptions being made about the relative weighting of the r and g components, and the form of the chrominance response. We then analysed the Fourier spectra of these images using logarithmic intervals in spatial frequency space. This allowed a determination of the total energy within each Fourier band for each of the luminance and chrominance representations.

The results strongly indicate that, for the set of scenes studied here, there was no evidence of a predominance of low-spatial-frequency chrominance information. Two classes of explanation are possible: (a) that raw Fourier content may not be the main organising principle determining visual encoding of colour, and/or (b) that our scenes were atypical of what may have driven visual evolution. We present arguments in favour of both of these propositions.
Image Region Labelling by Humans and by an Artificial Neural Network
Authors: Clark A , Troscianko T , Campbell N , Thomas B
Published: December 2015

We reported (Troscianko et al, 1995 Perception24 Supplement, 18) that a neural network has been developed which is capable of labelling objects in natural scenes by first segmenting a scene, then obtaining a description of each segment in terms of a set of features. A neural net is then trained to label the segments on the basis of the feature set. The question we are now addressing is: how important is each of these features to overall performance, both in human and machine vision?

We carried out an experiment in which human subjects were trained in the same labelling task as the neural net. Individual segments of scenes (sometimes corresponding to a whole object, eg a car, and sometimes an incomplete region, eg part of the sky) were presented on a screen, and the subject asked to label the scene as one of eleven possible types of object (sky, vegetation, vehicle …). Feedback was given and the learning curve monitored. When the learning curve was flat, each subject's performance was investigated with both intact and degraded stimuli. The degradation consisted of partial representation of the information, such as presenting just the outer boundary, or the average colour, or the average luminance, or randomising the size, position, and texture of the segment.

The results suggest that this degradation produces significant changes in performance (F9,7=4.4, p=0.0005). A posteriori analysis indicates that certain attributes (particularly texture, boundary-only, colour-averaging) are particularly influential in mediating performance. A similar set of results was obtained by training the network on similarly degraded data. The results imply: (1) that a neural net can provide a useful model of human object labelling processes, and (2) that certain features are more important than others in mediating such performance.
Contrast Adaptation and the Spatial Structure of Natural Images
Authors: Webster M A , MacLin O H , Rees A L , Raker V E
Published: December 2015

Contrast (pattern-selective) adaptation influences perception by adjusting sensitivity to the prevailing pattern of stimulation. We asked how the state of adaptation might depend on the patterns of spatial contrast typical of the natural visual environment. In one set of experiments, we examined whether adaptation to the characteristic amplitude spectra of natural images (which tend to decrease with frequency as 1/f) induces characteristic changes in contrast sensitivity. Contrast thresholds and suprathreshold contrast matches were measured after adaptation to random samples from an ensemble of images of natural outdoor scenes, or synthetic images formed by filtering the amplitude spectra of noise over a range of slopes. Adaptation differentially reduced sensitivity at low to medium spatial frequencies, but losses were not strongly dependent on the slope of the adapting spectra. In a second set of experiments, we examined the figural aftereffects induced by adaptation to naturalistic stimuli, by adapting and testing with images of human faces, for which small configural changes are highly discriminable. Observers adapted to frontal-view images of faces that were distorted by local expansions or contractions about the centre, and then adjusted distortions in test images to try to select the original face. Adaptation strongly biased perception in a direction opposite to the adapting distortion, with strongest aftereffects when test and adapting stimuli were derived from the same face image. Our results suggest that adaptation to the stimuli encountered in the course of normal viewing may exert ubiquitous and selective influences that are important in characterising the normal operating state of the visual system.
Perception of Slope and Distance in Photographs and in the Real World
Authors: Davies I , Howes J , Huber J , Nicholls J
Published: December 2015

We report a series of experiments in which spatial judgments of the real world were compared with equivalent judgments of photographs of the real-world scenes. In experiment 1, subjects judged the angle from the horizontal of natural slopes. Judgments of slope correlated with true slope (r=0.88) but judgments were in general overestimates. Equivalent judgments of slope in photographs again correlated with true slope (r=0.91) but judgments tended to be overestimates for small angles (6°) and underestimates for larger angles (up to 25°). In experiment 2 slope judgments were made under laboratory conditions rather than in the natural world. The slopes, which were viewed monocularly, varied from 5° – 45°, and were either plain, or textured, or included perspective information (a rectangle drawn on the surface) or had both texture and perspective. Judgments were overestimates, but the correlation with true slope was high (r=0.97). Slopes with either texture or perspective were judged more accurately than plain slopes, but combining texture and perspective information conferred no further benefit. Judgment of the angle of the same slopes in photographs produced similar results, but the degree of overestimation (closer to the vertical) was greater than for the real slopes. In experiment 3, subjects either judged the distance of landmarks ranging from 200 m to 5000 m from the observation point, or judged distance to the landmarks in photographs. In both cases subjects' judgments were well described by a power function with exponents close to one. Although there are large individual differences, subjects' judgments of slope and distance are accurate to a scale factor, and photographs yield similar judgments to real scenes.