Deg Of Visual Angle Research Articles

Wavelength and ambient luminance dependence of laser eye dazzle

A series of experiments has been conducted to quantify the effects of laser wavelength and ambient luminance on the severity of laser eye dazzle experienced by human subjects. Eight laser wavelengths in the visible spectrum were used (458-647nm) across a wide range of ambient luminance conditions (0.1-10,000 cd·m-2). Subjects were exposed to laser irradiance levels up to 600 μW·cm-2 and were asked to recognize the orientation of optotypes at varying eccentricities up to 31.6deg of visual angle from the laser axis. More than 40,000 data points were collected from 14 subjects (ages 23-64), and these were consolidated into a series of obscuration angles for comparison to a theoretical model of laser eye dazzle. Scaling functions were derived to allow the model to predict the effects of laser dazzle on vision more accurately by including the effects of ambient luminance and laser wavelength. The updated model provides an improved match to observed laser eye dazzle effects across the full range of conditions assessed. The resulting model will find use in a variety of laser safety applications, including the estimation of maximum dazzle exposure and nominal ocular dazzle distance values.

A "blanking effect" for surface features: Transsaccadic spatial-frequency discrimination is improved by postsaccadic blanking.

Although saccadic eye movements occur frequently—about three or four times a second—humans are astonishingly blind to transsaccadic changes. Locational displacements of the saccade target of up to 2 deg of visual angle, and even large changes of a visual scene, can go unnoticed. For a long time, this insensitivity was ascribed to deficits in transsaccadic memory: Only a coarse, (spatially) imprecise representation would be retained across a saccade. This assumption was contradicted by Deubel's and Schneider's (Behavioral and Brain Sciences 17:259-260, 1994) striking finding that locational discrimination performance across a saccade is greatly improved by inserting a short postsaccadic blank. Surprisingly, the question of whether blanking effects occur also for other forms of transsaccadic changes (i.e., surface-feature changes) has been widely ignored. We tested this question by means of a transsaccadic change in spatial frequency. Postsaccadic blanking facilitated spatial-frequency discrimination, but to a smaller amount than the usual blanking effects obtained with locational displacements. This finding bears important implications for models of visual stability and transsaccadic memory.

Harbor seals (Phoca vitulina) can perceive optic flow under water.

Optic flow, the pattern of apparent motion elicited on the retina during movement, has been demonstrated to be widely used by animals living in the aerial habitat, whereas underwater optic flow has not been intensively studied so far. However optic flow would also provide aquatic animals with valuable information about their own movement relative to the environment; even under conditions in which vision is generally thought to be drastically impaired, e. g. in turbid waters. Here, we tested underwater optic flow perception for the first time in a semi-aquatic mammal, the harbor seal, by simulating a forward movement on a straight path through a cloud of dots on an underwater projection. The translatory motion pattern expanded radially out of a singular point along the direction of heading, the focus of expansion. We assessed the seal's accuracy in determining the simulated heading in a task, in which the seal had to judge whether a cross superimposed on the flow field was deviating from or congruent with the actual focus of expansion. The seal perceived optic flow and determined deviations from the simulated heading with a threshold of 0.6 deg of visual angle. Optic flow is thus a source of information seals, fish and most likely aquatic species in general may rely on for e. g. controlling locomotion and orientation under water. This leads to the notion that optic flow seems to be a tool universally used by any moving organism possessing eyes.

P1-13: Color Induction from Surround Color under Interocular Suppression

The effect of surround colors on color appearance is known to subserve color constancy in humans, but how multiple mechanisms in the visual system are involved in this effect is controversial. We used an interocular-suppression technique to examine how the effect occurs at the level higher than the interaction of binocular information. A test color chip (1.7 × 1.7 deg visual angle) was presented in a static surround either with continuous-flash suppression in the dominant eye (CFS condition) to make the surround inperceptible or without the suppression (no-CFS condition). The surround stimulus was either a Mondrian or a uniform field of the same mean chromaticity. Stimuli were simulated OSA color chips under red, white (D65), or green illuminant color and were presented on a CRT display. Unique yellows were measured by asking the subjects to judge whether the test stimulus appeared reddish or greenish. Two sizes of the surround stimuli (widths of 1 deg and 4 deg) were used. Results showed significant shifts in unique yellow even under the CFS conditions, except for the 1 deg uniform-surround condition. Under the no-CFS condition, the shifts showed remarkable difference between subjects, except for the 4 deg Mondrian-surround condition. Interestingly, trends of the shifts showed high consistency within each subject, across conditions. These results indicate that mechanisms at both higher and lower levels than the neuronal site of interocular suppression are involved, and that the color shifts follow each subject's strategy in the higher-order mechanisms when only insufficient clues are available in the surround to estimate illuminant color.

FMRI based Granger causality as a measure of effective connectivity in macaque visual cortex

Event Abstract Back to Event fMRI based Granger causality as a measure of effective connectivity in macaque visual cortex Vesna Vuksanovic1, 2, 3*, Mario Bartolo3, Dave Hunter3, Li Sun3 and Alex Thiele3 1 Bernstein Center for Computational Neuroscience, Humboldt-Universität zu Berlin, Philippstraße 13, 10115 Berlin, Germany, Germany 2 Institut für Theoretische Physik, Technische Universität Berlin, Germany 3 Institute of Neuroscience, Newcastle university, United Kingdom Granger causality (GC) analysis of fMRI blood oxygen level dependent (BOLD) time series has been proposed as a statistical tool to analyse directional influences between remote brain regions. Ongoing BOLD activity is treated as a set of stochastic time series (STS) generated by stochastic processes whose properties can be captured by autoregressive models. GC quantifies the improvement in predicting one brain region STS by inclusion of that from another region. Here, we show how GC can be tested using the ensemble of single-voxel time series in a region of interest (ROI). Granger causality was tested on F statistics derived from linear regression models using BOLD time series. Ongoing BOLD activity was extracted from four ROIs: three in V1 and one in MT/V5 area of the macaque visual cortex. fMRI scans were made under three stimulus conditions, whereby (1) a single (‘centre’) grating was presented, (2) the single grating was flanked by 2 additional gratings, and (3) only the flanking gratings were shown. ROIs were not spatially homogenous in their influence on each other; we found a variable number of voxel pairs with significant F statistics. GC analysis showed that the V1 ROI which represented the ‘centre’ grating was influenced by the MT/V5 ROI when centre-flanker stimuli were shown, but not by the V1 ROIs representing the flanking gratings. It thus appears that centre-flanker interactions (at distances >3 deg of visual angle) arise mainly through feedback, not through lateral intra-areal connections. Keywords: dynamical systems, effective connectivity, fMRI, networks, Neurons, Visual Cortex Conference: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011, Freiburg, Germany, 4 Oct - 6 Oct, 2011. Presentation Type: Poster Topic: neurons, networks and dynamical systems (please use "neurons, networks and dynamical systems" as keywords) Citation: Vuksanovic V, Bartolo M, Hunter D, Sun L and Thiele A (2011). fMRI based Granger causality as a measure of effective connectivity in macaque visual cortex. Front. Comput. Neurosci. Conference Abstract: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011. doi: 10.3389/conf.fncom.2011.53.00200 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 19 Aug 2011; Published Online: 04 Oct 2011. * Correspondence: Dr. Vesna Vuksanovic, Bernstein Center for Computational Neuroscience, Humboldt-Universität zu Berlin, Philippstraße 13, 10115 Berlin, Germany, Berlin, Germany, 10115, Germany, vesna.vuksanovic@bccn-berlin.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Vesna Vuksanovic Mario Bartolo Dave Hunter Li Sun Alex Thiele Google Vesna Vuksanovic Mario Bartolo Dave Hunter Li Sun Alex Thiele Google Scholar Vesna Vuksanovic Mario Bartolo Dave Hunter Li Sun Alex Thiele PubMed Vesna Vuksanovic Mario Bartolo Dave Hunter Li Sun Alex Thiele Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

V1 responses to different types of luminance histogram contrast

It has been suggested that there exist neural mechanisms which are sensitive to luminance histogram skew, and that these mechanisms play a role in estimating surface properties (Motoyoshi, Nishida, Sharan & Adelson, 2007), or texture discrimination (Chubb, Landy, Econopouly, 2004). However, to the best of our knowledge, no systematic measurements of neural activity in early visual cortex in response to stimuli with skewed luminance histogram have been made to corroborate the existence of the postulated mechanisms. Using fMRI, we measured BOLD signal in early visual cortex in response to noise images with either positive, negative or zero skewness of the luminance histogram. Stimuli were circular (radius = 6.35 deg visual angle) random noise images whose histogram mean (M=128) and standard deviation (SD=43) were equated. Histogram contrast was manipulated by adjusting the third moment (skewness) of the luminance histogram to take on one of three values: positive (+1.4), negative (−1.4), or zero (0). Images were presented at a rate of 10Hz, in blocks of 24s separated by 24s presentation of a static image with uniform mean luminance (M=128, SD=0). The order of presentation during each scan (TR=2s) was: blank, positive skew, blank, negative skew, blank, zero skew. This sequence was repeated twice during each scan; there were a total of six scans per session. Observers fixated at the center of each stimulus, and performed a demanding fixation task during each scan. The BOLD signal was analyzed within an independently determined ROI corresponding to an annulus located within the stimulus area (inner radius= 2 deg, outer radius 4 deg). On average V1 responses to noise images with positively and negatively skewed luminance histogram were significantly larger than those to images with zero skew. Furthermore, the V1 BOLD signal in response to images with negative contrast was larger than that to those with positive luminance histogram contrast.

The response dynamics of rabbit retinal ganglion cells to simulated blur

Retinal ganglion cells (RGCs) are highly sensitive to changes in contrast, which is crucial for the detection of edges in a visual scene. However, in the natural environment, edges do not just vary in contrast, but edges also vary in the degree of blur, which can be caused by distance from the plane of fixation, motion, and shadows. Hence, blur is as much a characteristic of an edge as luminance contrast, yet its effects on the responses of RGCs are largely unexplored.We examined the responses of rabbit RGCs to sharp edges varying by contrast and also to high-contrast edges varying by blur. The width of the blur profile ranged from 0.73 to 13.05 deg of visual angle. For most RGCs, blurring a high-contrast edge produced the same pattern of reduction of response strength and increase in latency as decreasing the contrast of a sharp edge. In support of this, we found a significant correlation between the amount of blur required to reduce the response by 50% and the size of the receptive fields, suggesting that blur may operate by reducing the range of luminance values within the receptive field. These RGCs cannot individually encode for blur, and blur could only be estimated by comparing the responses of populations of neurons with different receptive field sizes. However, some RGCs showed a different pattern of changes in latency and magnitude with changes in contrast and blur; these neurons could encode blur directly.We also tested whether the response of a RGC to a blurred edge was linear, that is, whether the response of a neuron to a sharp edge was equal to the response to a blurred edge plus the response to the missing spatial components that were the difference between a sharp and blurred edge. Brisk-sustained cells were more linear; however, brisk-transient cells exhibited both linear and nonlinear behavior.

The response dynamics of primate visual cortical neurons to simulated optical blur

Neurons in visual cortical area V1 typically respond well to lines or edges of specific orientations. There have been many studies investigating how the responses of these neurons to an oriented edge are affected by changes in luminance contrast. However, in natural images, edges vary not only in contrast but also in the degree of blur, both because of changes in focus and also because shadows are not sharp. The effect of blur on the response dynamics of visual cortical neurons has not been explored. We presented luminance-defined single edges in the receptive fields of parafoveal (1-6 deg eccentric) V1 neurons of two macaque monkeys trained to fixate a spot of light. We varied the width of the blurred region of the edge stimuli up to 0.36 deg of visual angle. Even though the neurons responded robustly to stimuli that only contained high spatial frequencies and 0.36 deg is much larger than the limits of acuity at this eccentricity, changing the degree of blur had minimal effect on the responses of these neurons to the edge. Primates need to measure blur at the fovea to evaluate image quality and control accommodation, but this might only involve a specialist subpopulation of neurons. If visual cortical neurons in general responded differently to sharp and blurred stimuli, then this could provide a cue for form perception, for example, by helping to disambiguate the luminance edges created by real objects from those created by shadows. On the other hand, it might be important to avoid the distraction of changing blur as objects move in and out of the plane of fixation. Our results support the latter hypothesis: the responses of parafoveal V1 neurons are largely unaffected by changes in blur over a wide range.

Spatial Extent and Figural Factors in Backward Masking

Spatial and figural characteristics of backward masking were studied, with two collinear arcs presented end-to-end and serving as target and mask, respectively. Stimulus onset asynchrony was 50 ms while interstimulus interval was 0 ms. Mask exposure duration required for masking was determined as a function of target length with mask length as a parameter. The exposure duration of the mask required for complete masking varied directly with target length, but inversely with mask length. The fact that masking strength increased with mask duration while all other parameters were kept constant suggests that masking depended on stimulus termination asynchrony. Maximal masking occurred for target arcs as long as 5.0 deg of visual angle, exceeding previously reported distances. Misaligned or differently shaped stimuli produced less masking, suggesting that figural factors play a role in long-range backward masking.

Spatially asymmetric response to moving patterns in the visual cortex: Re-examining the local sign hypothesis

One of the most fundamental functions of the visual system is to code the positions of objects. Most studies, especially those using fMRI, widely assume that the location of the peak retinotopic activity generated in the visual cortex by an object is the position assigned to that object—this is a simplified version of the local sign hypothesis. Here, we employed a novel technique to compare the pattern of responses to moving and stationary objects and found that the local sign hypothesis is false. By spatially correlating populations of voxel responses to different moving and stationary stimuli in different positions, we recovered the modulation transfer function for moving patterns. The results show that the pattern of responses to a moving object is best correlated with the response to a static object that is located behind the moving one. The pattern of responses across the visual cortex was able to distinguish object positions separated by about 0.25 deg visual angle, equivalent to approximately 0.25 mm cortical distance. We also found that the position assigned to a pattern is not simply dictated by the peak activity—the shape of the luminance envelope and the resulting shape of the population response, including the shape and skew in the response at the edges of the pattern, influences where the visual cortex assigns the object’s position. Therefore, visually coded position is not conveyed by the peak but by the overall profile of activity.

The gap effect is exaggerated in parafovea

In central vision, the discrimination of colors lying on a tritan line is improved if a small gap is introduced between the two stimulus fields. Boynton et al. (1977) called this a "positive gap effect." They found that the effect was weak or absent for discriminations based on the ratio of the signals of long-wave and middle-wave cones; and even for tritan stimuli, the gap effect was weakened when forced choice or brief durations were used. We here describe measurements of the gap effect in the parafovea. The stimuli were 1 deg of visual angle in width and were centered on an imaginary circle of radius 5 deg. They were brief (100 ms), and thresholds were measured with a spatial two-alternative forced choice. Under these conditions we find a clear gap effect, which is of similar magnitude for both the cardinal chromatic axes. It may be a chromatic analog of the crowding effect observed for parafoveal perception of form.

Deficits of spatial localization in children with strabismic amblyopia.

Besides loss of visual acuity and binocularity, spatial localization deficits (comprising both increased spatial uncertainty and spatial distortions) are an important feature of strabismic amblyopia. Although they have been extensively investigated in adult amblyopes, there are still many open questions concerning their substrate and relationship to clinical parameters. Our aim was to develop a procedure for assessing vertical alignment, which enabled us to find out whether children with strabismic amblyopia had similar spatial localization deficits, and their relation to the children's clinical condition. Vertical alignment was assessed in children by comparing the visual direction in space of three loci along the vertical meridian, separated by 5 deg of visual angle. We tested alignment in the amblyopic and dominant eyes of 32 strabismic and in both eyes of 35 control children from 4.5 to 10 years, together with a careful orthoptic examination. In the amblyopic eyes, increased uncertainty and systematic distortions outside the normal range occurred. Large angles of deviation and pathological fixation patterns were necessary, but not sufficient conditions for gross spatial deficits to occur. The fellow dominant eyes showed spatial localization similar to normal eyes. Children with strabismic amblyopia exhibited localization deficits and relationship to clinical data similar to those in adult amblyopes. These data are important for further investigations about the substrate, plasticity and the clinical relevance of perceptual distortions.

Comparison at a distance.

The visual system is known to contain hard-wired mechanisms that compare the values of a given stimulus attribute at adjacent positions in the visual field; but how are comparisons performed when the stimuli are not adjacent? We ask empirically how well a human observer can compare two stimuli that are separated in the visual field. For the stimulus attributes of spatial frequency, contrast, and orientation, we have measured discrimination thresholds as a function of the spatial separation of the discriminanda. The three attributes were studied in separate experiments, but in all cases the target stimuli were briefly presented Gabor patches. The Gabor patches lay on an imaginary circle, which was centred on the fixation point and had a radius of 5 deg of visual angle. Our psychophysical procedures were designed to ensure that the subject actively compared the two stimuli on each presentation, rather than referring just one stimulus to a stored template or criterion. For the cases of spatial frequency and contrast, there was no systematic effect of spatial separation up to 10 deg. We conclude that the subject's judgment does not depend on discontinuity detectors in the early visual system but on more central codes that represent the two stimuli individually. In the case of orientation discrimination, two naive subjects performed as in the cases of spatial frequency and contrast; but two highly trained subjects showed a systematic increase of threshold with spatial separation, suggesting that they were exploiting a distal mechanism designed to detect the parallelism or non-parallelism of contours.

Scene content selected by active vision.

The primate visual system actively selects visual information from the environment for detailed processing through mechanisms of visual attention and saccadic eye movements. This study examines the statistical properties of the scene content selected by active vision. Eye movements were recorded while participants free-viewed digitized images of natural and artificial scenes. Fixation locations were determined for each image and image patches were extracted around the observed fixation locations. Measures of local contrast, local spatial correlation and spatial frequency content were calculated on the extracted image patches. Replicating previous results, local contrast was found to be greater at the points of fixation when compared to either the contrast for image patches extracted at random locations or at the observed fixation locations using an image-shuffled database. Contrary to some results and in agreement with other results in the literature, a significant decorrelation of image intensity is observed between the locations of fixation and other neighboring locations. A discussion and analysis of methodological techniques is given that provides an explanation for the discrepancy in results. The results of our analyses indicate that both the local contrast and correlation at the points of fixation are a function of image type and, furthermore, that the magnitude of these effects depend on the levels of contrast and correlation present overall in the images. Finally, the largest effect sizes in local contrast and correlation are found at distances of approximately 1 deg of visual angle, which agrees well with measures of optimal spatial scale selectivity in the visual periphery where visual information for potential saccade targets is processed.

Scene articulation: dependence of illuminant estimates on number of surfaces.

The ability of observers to detect changes in illuminant over two scenes containing different random samples of reflecting surfaces was determined in an experiment with Mondrian-like patterns containing different numbers of coloured patches. Performance was found to improve as the number of patches increased from 9 to 49. In principle, observers could have used space-average scene colour as the cue ('grey-world' hypothesis) or the colour of the brightest surface in the scene ('bright-is-white' hypothesis), as the two cues generally covary. In a second experiment, observers matched illuminants across different patterns in which the space-average cue and the brightest-patch cue were independently manipulated. The articulation of the patterns was varied: the number of patches increased from 49 (patch width 1 deg visual angle) to over 50000 (patch width 0.03 deg), while the gamut of colours was held constant. Space-average colour was found to be the dominant cue with all patterns except for those with the largest patches.

Interpolation processes in the perception of real and illusory contours.

The spatial and temporal characteristics of mechanisms that bridge gaps between line segments were determined. The presentation time that was necessary for localisation and identification of a triangular shape made up of pacmen, pacmen with lines, lines, line segments (corners), or pacmen with circles (amodal completion) was measured. The triangle was embedded in a field of distractors made up of the same components but at random orientations. Subjects had to indicate whether the triangle was on the left or on the right of the display (localisation) and whether it was pointing upward or downward (identification). Poststimulus masks consisted of pinwheels for the pacmen stimuli or wheels defined by lines. Stimuli were presented on a grey background and defined by luminance or isoluminant contrast. Thresholds were fastest when the triangle was defined by real contours, as for the pacmen with lines (105 ms) and the lines only (92 ms), slightly slower for corners (118 ms) and pacmen (136 ms), and much slower for the amodally completed pacmen (285 ms). For all inducer types localisation was about 20 ms faster than identification. In a second experiment the relative length of the gap between inducers was varied. Thresholds increased as a function of gap length, indicating that the gaps between the inducers need to be interpolated. There was no significant difference in the speed of this interpolation process between the pacman stimuli and the line-segment stimuli. About 40 ms were required to interpolate 1 deg of visual angle, corresponding to about one third of the distance between inducers. In a third experiment, it was found that processing of isoluminant stimuli was as fast as for low-contrast luminance stimuli, when targets were defined by real contours (lines), but much slower for illusory contours (pacmen). The conclusion is that the time necessary to interpolate a contour depends greatly on the spatial configuration of the stimulus. Since interpolation is faster for the line-segment stimuli, which do not elicit the percept of an illusory contour, the interpolation process seems to be independent of the formation of illusory contours.

The effect of spatial frequency and field size on the spread of exclusive visibility in binocular rivalry

We measured binocular rivalry between dichoptic, orthogonal, sinusoidal gratings both having spatial frequencies of 0.5, 1,2,4,8 or 16 c deg −1 in fields ranging from 0.5 to 8 deg of visual angle in diameter. Total time that one or the other grating was exclusively visible had an inverted U-shaped relationship with spatial frequency, with the peak shifting to coarser spatial frequencies as the field size increased. We computed for each spatial frequency the maximum field size over which a criterion duration of exclusive visibility would spread. When expressed as areas, these sizes were inversely proportional to spatial frequency. This dependence of rivalry on spatial frequency is similar to those for stereopsis and fusion, consistent with the notion that all three binocular phenomena have a common mechanism.

Terminator Position Affects the Perceived Speed of Line Segments Tilted with Respect to Direction of Motion

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.

Target Detection and Texture Segmentation in Briefly Presented Displays of Curved-line Elements

Similar pre-attentive processes are often thought to underlie rapid texture segmentation and target ‘pop-out’ in multi-element displays (but see Wolfe, 1992 Vision Research32 757 – 763). Performance in target-detection and texture-segmentation tasks was measured here for briefly presented displays of curved-line elements. In both tasks 49 curved-line elements, each subtending 1 deg of visual angle, were presented in a circular display for 100 ms and followed by a mask. The position of each element in the array was jittered to reduce any possible collinearity or luminance cues. In the target-detection task, observers determined whether the display contained a target which differed in curvature from the other, background elements. In the texture-segmentation task, observers determined the orientation, horizontal or vertical, of a foreground region of 4 × 2 elements which differed in curvature from the background elements. Performance, quantified as percent correct, was measured as a function of target (or foreground) and background curvatures. At small background curvatures, performance in the two tasks was very similar: performance was best when target or foreground curvature was large. Performance differed, however, at large background curvatures: for texture segmentation there was a marked peak in performance when foreground curvature was close to zero, but there was no corresponding peak for target detection. It seems that some additional, global cue can be extracted from a group of straight or slightly curved lines that is not available from a single line, thereby facilitating texture segmentation but not target detection.

Anisotropy in Oriented-Line-Target Detection at High and Low Luminance Contrast

Observers can detect a uniquely oriented line segment (‘target’) in a background field of uniformly oriented line segments (‘nontargets’) even if viewing time is brief. When the lines have high luminance contrast, the variation of orientation increment threshold with nontarget orientation is periodic, generally with a period of about 90° although smaller periods have been found. Do the orientation-sensitive mechanisms giving rise to periodicities function only at high contrast? This question was addressed in a line-target detection experiment. Twenty white line segments of length 1 deg visual angle were presented in a circular field of diameter 20 deg visual angle. Nontarget orientations were in the range 0°, 5°, …, 175°, and the difference between nontarget and target orientations was varied adaptively. Stimulus displays lasting 40 ms were followed by a blank interstimulus interval lasting 60 ms and then a random-line mask. A view-tunnel provided a grey background of luminance 35 cd m−2 and stimulus contrast was 0.1, 0.3, 0.5, 0.7, or 0.9 log unit above the observer's luminance detection threshold for one line segment. When contrast was 0.1 log unit above this threshold, performance was near chance level. As contrast increased from 0.3 to 0.9 log unit above luminance detection threshold, performance improved and orientation increment thresholds decreased, showing that early orientation-processing is most effective at high luminance contrast. Nonetheless, periodicities were found in all conditions where performance was better than chance which suggests that the orientation-sensitive mechanisms associated with periodicities operate at both high and low luminance contrast.