bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoreceptor Nonlinearities Can Account for the MacAdam Ellipses
Authors: Alleysson D , Hérault J
Published: December 2015

Colour perception is known to be nonlinear, as highlighted by D L MacAdam (1942 Journal of the Optical Society of America32 247). He showed that the just noticeable difference (JND) between two colours is not constant and depends nonuniformly on the values of these two colours: under constant illumination, the constant-JND loci, drawn in the xy colour space around various centres, appear as ellipses of different orientations and eccentricities. Though this phenomenon encompasses the whole perception chain, from receptors to higher levels, we show that it can be accounted for mainly by the nonlinearity of receptor transduction.

We start from the colorimetric space XYZ defined by the CIE (1931) and its linear transformation into the cone-excitation space LMS, using data from V C Smith and J Pokorny (1996 Color Research and Applications21 5). Then we consider a perceptual colour space obtained by the nonlinear correspondence between the L, M, and S cone inputs and the l, m, and s cone outputs, through a compression law of Michaelis - Menten type, eg: l = L/( L + L0). In this space, the JNDs are circles, drawn on the surface corresponding to the nonlinear transformation of the constant-luminance plane in LMS, around the corresponding centres. By looking at the projections of those circles onto the constant-luminance plane in LMS, and their representations in the colorimetric space xy, we naturally obtain the ellipses of MacAdam. The formulas, fitted to the data, provide the parameters of the LMS-to- lms transformations.
Sensitivity of LGN Neurons in Infant Macaque Monkeys
Authors: Movshon J A , Kiorpes L , Hawken M J , Skoczenski A M , Cavanaugh J R , Graham N V
Published: December 2015

To understand the neuronal factors limiting visual sensitivity in infant primates, we studied the responses of neurons recorded in parts of the LGN representing the central visual fields in paralysed, opiate-anesthetised 1-week-old and 4-weeks-old macaque monkeys; comparison data were taken from animals older than 6 months. We tested each neuron with achromatic sinusoidal gratings varying in spatial and temporal frequency and contrast, and we also studied the effects of added spatiotemporal white noise.

In agreement with earlier reports, we found that neurons in the infant monkeys had relatively poor spatial resolution; sensitivity to high temporal frequencies was also lower than in adults. When tested with gratings of near-optimal spatiotemporal frequency, however, most LGN neurons in the infant monkeys gave brisk and reliable visual responses that were qualitatively similar to those seen in older animals. Spontaneous and evoked response rates and contrast gain were modestly lower in the infants, but response variability was also lower, and therefore statistical measures of sensitivity and susceptibility to masking noise showed little difference between infant and adult neurons. Especially, in the 1-week-old animals, a substantial fraction of neurons lacked spontaneous activity. This resulted in ‘hard’ contrast thresholds not seen in adult animals. As a consequence, masking noise often paradoxically enhanced visual responses in these animals by subthreshold summation, a result not seen in the adults.

The maturation of visual responses in macaque LGN consists largely of changes in spatial and temporal scale, accompanied by modest changes in responsiveness and little or no change in sensitivity.
Compression of Visual Space before Saccades
Authors: Burr D C , Morrone M C , Ross J
Published: December 2015

We studied how the human visual system recalibrates visual coordinates to compensate for saccadic eye movements. Observers made 20 horizontal saccades to a target on an otherwise featureless red screen, and reported the apparent position of a vertical green bar that was briefly displayed before, during, or after the saccade. Bars presented 50 ms before the beginning of the saccade, or after its completion, were perceived accurately and veridically. However, bars presented immediately prior to the saccade were systematically mislocated, either in the direction of the saccade or in the opposite direction, depending on the spatial position of the bar. This result has been verified by various techniques including Vernier offset estimation, and a forced-choice annulling task. When four bars (straddling the saccade target) were displayed in the interval −25 to 0 ms, they were seen to be merged into 1 bar (forced choice). None of these effects could be mimicked by causing the scene to move at saccadic speeds and amplitudes. The results suggest that each saccade is accompanied by a non-visual signal that displaces the retinal coordinate system, and a momentary compression of visual space. The perceptual compression may be instrumental in ensuring a smooth transition from fixation to fixation.
Entropy Masking
Authors: Watson A B
Published: December 2015

Current vision theory admits two principal types of visual masking: contrast masking and noise masking. The former is thought to operate via mechanisms such as a nonlinear transducer, contrast gain control, and lateral inhibitory influences. The latter is understood to be due to the randomness of the target + mask combination. Here I propose a new category: entropy masking. Entropy masking is evident when the mask is a complex but deterministic signal. Because the mask is deterministic, and because experiments demonstrate that the masking power of such signals is much greater than predicted by contrast gain control, entropy masking cannot be due to either contrast masking or noise masking. Entropy masking occurs because the observer cannot discount the background, as supposed by ideal-observer theory, and hence must learn the target + mask signal. The ease with which this is done is a function of the entropy of both target and mask. Experiments that illustrate these general principles are described.
Comparative Analysis of Brightness and Lightness Functions
Authors: Biondini A R , de Mattiello M L F , Spinadell V
Published: December 2015

A study is reported of twelve chromatic functions and one achromatic function of brightness, and fourteen chromatic functions and one achromatic function of lightness with the aim of establishing the change in the exponent of the functions, a phenomenon considered as being related to: (a) the brightness or lightness response of a subject; (b) the influence of hue and saturation; (c) the level of the background surrounding the stimulus; (d) the Helmholtz - Kohlrausch effect.

Different authors agree that these differences are produced by specific contributions of the tristimulus value Y and of chromaticity, the complexity of the problem arising from the fact that chromaticity would be expected to play a different role in each hue. They also agree that the Helmholtz - Kohlrausch effect is independent of the appearance of colour.

We show that the variation due to hue and saturation does not prevent the lightness and brightness functions from showing a specific response that permits differentiation between them and other functions, for instance the saturation functions.

Finally, we correlate chromatic and achromatic functions by means of the quadratic fluctuation function which allows us to define a self-correlation function. This mathematical analysis extends to all the samples without taking into account their variation in wavelength and purity. It shows the existence of a solid and stable coding system for luminance, a system that achieves stability through its instability, reflected in the variation of the exponent of the functions.
Anchoring of Surface Lightness by Area and Luminance
Authors: Gilchrist A L
Published: December 2015

Relative luminance is fundamental to lightness perception, but can be used to predict specific lightness values only when coupled with an anchoring rule. Empirical results indicate that, in simple displays, lightness is anchored by the highest luminance (white) rather than by the average luminance (middle gray). This implies that increasing the luminance range of a stimulus causes grayness induction: the lower luminance values become darker gray while the highest luminance remains unchanged, as many so-called brightness induction experiments have shown. Yet sometimes increasing the luminance range of a stimulus causes luminosity induction: the highest luminance becomes increasingly self-luminous while the lowest luminances remain unchanged. Whether grayness induction or luminosity induction results from an increase in stimulus contrast depends on relative area. A few simple, yet hitherto unrecognised, rules that describe how anchoring by highest luminance combines with anchoring by largest area appear to be consistent with the many published reports on area and lightness/brightness. These findings add to the accumulating evidence that many phenomena previously attributed to contrast are much better understood in terms of anchoring.
Measuring Object-Centred Attention Based on a Cueing Paradigm
Authors: Lafosse C , Westerhuis M F , Vandenbussche E
Published: December 2015

Visual attention can be allocated to a location in visual space and/or to a representation of an object in the visual field, independently of their spatial location. In Posner's cueing paradigm, it is assumed that attention is moved to and then engaged at a cued location. If the target appears at an uncued location, attention first has to be disengaged from the cued location before moving to and engaging the target location.

On the basis of this paradigm, we designed an experiment to measure the disengaging of attention from objects, independently of location. For this purpose we used the bistable Necker cube that can be perceived as two different object configurations, depending on the position of the front side of the cube (lower left or upper right). The subject was instructed to react when he perceived the Necker cube as a previously presented model configuration, ie a stable cube. Each condition started with a bistable cube (with equal luminance of the ribs) that gradually evolved into the model configuration, by manipulating the luminance of the ribs of the cube. Prior to this the subject was cued by a cube similar to the model in the valid condition and cued by a dissimilar cube in the invalid condition.

The results showed a significant difference (F1,10=7.35; p<0.05) between the valid and invalid cue condition, indicating a significant cost for the invalid cue condition. This effect is in accord with the well-known set effect found previously with bistable figures. The difference between the valid and the invalid cue conditions will be interpreted as disengaging attention from an object. Thus, object-based components of attention can be examined by paradigms similar to Posner's paradigm for location-based attention.
Identifying Facial Images in the Visual Periphery
Authors: Mäkelä P , Näsänen R , Rovamo J
Published: December 2015

Identification sensitivity for four different faces was measured at the fovea and in the periphery to find out whether foveal and peripheral visual performance in this complex spatial task can be made equivalent simply by changing image magnification. Identification sensitivity was measured as a function of image magnification. The lowest contrast for identification was determined by a 4AFC method. Observers indicated via the keyboard which of the four faces was presented on the CRT screen. The images were shown monocularly at the fovea and at 2.5, 5, and 10 deg eccentricities in the nasal visual field of the right eye (eccentricity measured from the right- hand edge of the image).

If scaling is successful, the foveal and peripheral sensitivity vs size functions collapse together when shifted along the size dimension only (Watson, 1987 Journal of the Optical Society of America A4 1579). Although the foveal and peripheral sensitivity functions could be superimposed, they did not fully superimpose without also shifting them in vertical direction, as foveal sensitivity at largest sizes was slightly superior to that of any eccentric location. Thus, size scaling alone was not adequate for this task, in agreement with the contrast sensitivity results of Valeton and Watson (1990 Perception19 Supplement, 393). In this identification task, performance deteriorated towards the periphery at approximately the same rate as visual acuity, when the size corresponding to half-maximal sensitivity at each eccentricity was used as a measure.
Plasmas: A New Class of Motion-Induced Brightness Illusion
Authors: Tse P
Published: December 2015

When a white figure is set in smooth continuous motion against a dark background, a ‘shadowy’ region forms around and behind it, somewhat similar to the wake that a boat leaves in water. Conversely, when a dark figure moves against a white background, this ‘wake’ appears lighter than the background. When several such figures move, these ‘wakes’ can link up in surprising ways. For example, if four white circles of radius r are positioned on a dark background at the vertices of an imaginary square whose width is, say, 4 r, and this imaginary square rotates about its centre, bands darker than the background form between the four circles, linking them through the centre. This ‘black cross’ is nonrigid, and is accompanied by dark ‘wakes’ on the outside of the circles. I call these illusory brightness regions ‘plasmas’.

The relation of this effect to other motion-induced brightness illusions is considered. In particular, this effect has different properties than either induced gratings (McCourt, 1982 Vision Research22 119 – 134) or phantom gratings (Tynan and Sekuler, 1975 Science188 951 – 952). Plasmas are not due to retinal afterimages or persistence, since they form in regions never traversed by the moving figure. Nor are they an artifact of the computer screen, since the illusion occurs equally well in displays constructed from paper. Plasmas have the opposite phase relative to the inducing figure, disappear when a figure stops moving, are strongest at high background/inducer contrast, and are most visible when the luminance of the inducers is in the photopic range. Experiments measuring the strength of plasmas as a function of angular velocity and visual angle are described. A model is proposed according to which plasmas are due to a phase lag in luminance-edge induced lateral inhibition.
ASIDT: Adaptive Signal Detection Testing
Authors: Treutwein B
Published: December 2015

Two-alternative forced choice (2AFC) methods which are routinely used in psychophysics provide criterion-free sensitivity measures but at a high price: 2AFC procedures generally need 2 – 3 times the number of trials than a simple yes/no design to achieve the same level of threshold variability. Similarly, signal detection methods provide measures for criterion and sensitivity, but they require predetermined signal levels for the presented stimuli and also need a large number of trials.

ASIDT (adaptive signal detection testing) combines rating responses with an adaptive stimulus placement strategy according to the accelerated stochastic approximation method (see Treutwein, 1995 Vision Research35 2503 – 2522). More specifically, during the course of the experiment the rating responses are collapsed into two groups (yes/no). The border between these two groups is set at the current median of the rating categories actually used by the subject. With the collapsed yes/no responses the stimuli are placed according to accelerated stochastic approximation. This placement strategy is based on very simple rules and distributes the stimulus values asymptotically according to a normal distribution around the median. After the experiment, the rating responses can be used to compute a nonparametric and criterion-free sensitivity measure, the area under the empirical ROC, which is equivalent to the percentage correct in a 2AFC experiment. The usage of ASIDT is demonstrated by psychophysical data.
An Illusory Contour Induces Texture Segmentation
Authors: Peromaa T-L , Laurinen P I
Published: December 2015

It is known that neurons in V1 can signal a contour between two out-of-phase gratings [Grosof et al, 1993 Nature (London)365 550 – 552]. We demonstrate that this type of illusory contour can segregate areas of surfaces without any luminance, contrast, or textural difference between the areas.

We have studied the conditions under which the illusory contour induces texture segmentation. The target was a circular contrast-inverted area (diameter 0.22 – 8.3 deg) in the centre of an isotropic narrow-band noise texture (centre spatial frequency 0.4 – 7 cycles deg−1). Generally, segmentation was effortless in low-spatial-frequency textures but gradually disappeared with increasing spatial frequency although the contour remained visible. In a staircase experiment, the highest spatial frequency allowing segmentation was measured for each target size. The task was to tell whether the stimulus contained an object or just a contour. A negative power function relates the target diameter and the highest spatial frequency allowing segmentation. The visibility of the contour was independent of the target size. The illusory contour ‘captures’ the texture inside. However, the process is spatially limited. In a separate experiment, the subject used a cursor to point out how far from the contour the capture spreads. A negative relationship between the spatial spread and the spatial frequency of the texture was found. These findings are consistent with the idea that low-level mechanisms signalling illusory contours are involved in perceptual scene segmentation.
Computation of Binocular Eye Position from Vertical Disparities with the use of Probabilistic Place Coding
Authors: Porrill J , Frisby J P
Published: December 2015

Stereoscopic vision exploits the fact that points in a 3-D scene will in general project to different locations in the images in the left and the right eye. The differences in retinal locations, measured horizontally and vertically, are called horizontal ( H) and vertical ( V) disparities respectively. Their size is affected by the positions of the eyes which determine the viewing geometry parameters, that is distance to the fixation point ( d) and the angle of gaze ( g). H is also affected by the depth of the scene point relative to fixation distance, which is why one can recover 3-D scene structure using binocular vision. Achieving metric reconstruction requires knowledge of d and g to allow for their influence on H. Computational analyses have shown that d and g can in principle be recovered from V because of its relative insensitivity to scene depth variations. As d and g are the only two unknowns in the equation for V, in theory only two measurements of V (at suitable retinal locations) are needed. A practical system, however, dealing with noisy images composed of many points, needs to pool information from measurements of V at numerous retinal locations. A place-coding algorithm of the Hough transform type is well suited to this purpose (S A Peek, J E W Mayhew, J P Frisby, 1984 Image and Vision Computing2 180 – 190), but it has not hitherto been used in a way which deals appropriately with measurement noise. We describe how this can be done and demonstrate with computer simulations greatly improved estimation of d and g as well as improved robustness to noise. The new method also permits the solution of an important aspect of the stereo correspondence problem—that of finding epipolar lines.
The Conditions under which Opposing Motion is Seen in Transparency or as Flicker
Authors: Del Viva M M , Morrone M C
Published: December 2015

We present several examples of moving stimuli comprising several harmonic components that can be perceived either as moving independently, or together as a coherent moving pattern. A simple example is two sinusoidal gratings of equal contrast and spatial frequencies moving in opposite directions that are perceived as a single grating modulated sinusoidally over time (counterphase). However, two square waves drifting in opposite directions, while being a superposition of counterphasing pairs, are perceived as two distinct patterns drifting in transparency one over the other. Intermediate situations such as pairs of counterphase gratings can be perceived alternatively as counterphases or as drifting in two directions.

We show that the relative phase of the components plays a fundamental role in whether they group or become transparent. The tendency of a component to become part of a pattern is maximal when the phase relationship is 0 deg, and gradually decreases to a minimum at 90 deg. This agrees quantitatively with previous measurements on different stimuli (paper presented at Del Viva and Morrone, ARVO96), suggesting a common computational mechanism. The data were well modelled by a nonlinear model of motion analysis based on feature tracking, showing sensitivity to the exact degree of nonlinearity.
Memory and Binocular Visual Direction
Authors: Rozhkova G I , Rodionova E I , Panyutin A K
Published: December 2015

It has been shown by a number of investigators that perceived binocular direction (BD) could be changed by varying the contrast ratio between the left and right images of a target. In this and certain other cases, the former simple idea that BD is the average of the two monocular directions (MD) fails to predict BD correctly. As an alternative, Mansfield and Legge (1996 Vision Research36 27 – 41) suggested that a weighted average of modified MD signals be calculated with weights depending on respective image contrasts. However, in experiments including not only ordinary (monocularly recognisable) but also cyclopean (monocularly imperceptible) targets, we have found that there exist (at least) two principally different mechanisms responsible for calculation of BD and that one of these mechanisms is contrast-dependent while the other one is contrast-independent (Rozhkova et al, 1995 Sensory Systems9 46 – 57). The first one probably corresponds to the model of Mansfield and Legge but the second one (which determines perception of cyclopean targets) has other specific properties. In a new series of experiments based on the same paradigm (to align a test target with a control one under conditions of varying interocular contrast ratios for the test stimuli), we have found that perceived direction of a cyclopean target (coded by Julesz's random-dot stereogram) could be manipulated by memory. It appeared that if, in a given series of measurements, the random-dot patterns for the test targets were chosen at random from six samples, the results were independent of the contrast ratio, but if one and the same random-dot pattern was used many times throughout all the series of measurements, and the subject could memorise the texture of the target, perceived direction became dependent on contrast ratio as in the case of monocularly recognisable targets. One of the possible explanations of this effect is the secondary activation of the first BD mechanism, which usually does not process cyclopean targets.
Separation of Figure from Ground as an Adaptive Image Processing
Authors: Mironov S V , Dudkin K N , Doudkine A K
Published: December 2015

We define the separation of figure from ground as a visual-attribute-dependent and task-dependent representation of sensory information in higher-level visual processes. A computer model for adaptive segmentation of 2-D visual objects (Dudkin et al, 1995 Proceedings of SPIE 122) was developed in these studies. The description and separation of figure from ground are implemented by spatial frequency filters and feature detectors performing as self-organising mechanisms. The simulation of control processes caused by attention (top - down), and lateral, frequency-selective, and cross-orientation inhibition (bottom - up) determines the adaptive image processing.

The first stage is the estimation of input image produced by the analysis of the spatial brightness distribution by algorithms calculating the vector of primary descriptive attributes. These results provide the synthesis of control processes based on several algorithms, each of which transforms descriptive attributes into separate control parameters. The creation of two primary descriptions: ‘sustained’ (contours) and ‘transient’ (fragments with homogeneous intensity), and the selection of feature-detection operators are governed by the complete set of control parameters. The primary descriptions allow formation of the intermediate image description in which similar elements are grouped by identical brightness, colour, spatial position, curvature, and texture according to Gestalt concepts. To divide the image into basic areas and to extract fragments which belong to a putative figure, all these descriptions are combined into the final integrated image representation. The model has been tested on various images.
Top-Down versus Bottom-up Control of Saccades in Texture Perception
Authors: Groner R , von Mühlenen A , Groner M
Published: December 2015

An experiment was conducted to examine the influence of luminance, contrast, and spatial frequency content on saccadic eye movements. 112 pictures of natural textures from Brodatz were low-pass filtered (0.04 – 0.76 cycles deg−1) and high-pass filtered (1.91 – 19.56 cycles deg−1) and varied in luminance (low and high) and contrast (low and high), resulting in eight images per texture. Circular clippings of the central parts of the images (approximately 15% of the whole image) were used as stimuli.

In the condition of bottom - up processing, the eight stimuli derived from one texture were presented for 1500 ms in a circular arrangement around the fixation cross. They were followed by a briefly presented target stimulus in the centre, which in half the trials was identical to one of the eight test stimuli. Participants had to decide whether the target stimulus was identical to any of the preceding stimuli. During a trial, their eye movements were recorded by means of a Dual-Purkinje-Image eye tracker. In the top - down condition, the target stimulus was presented in each trial prior to the display of the test stimulus. It was assumed that the priming with a target produced a top - down processing of the test stimuli. The latency and landing site of the first saccade were computed and compared between the top - down and bottom - up conditions. It is hypothesised that stimulus characteristics (luminance, contrast, and spatial frequency) play a more prominent role in bottom - up processing, while top - down processing is adjusted to the particular characteristics of the prime.
Sequential Stereopsis Demonstrated with High-Bandpass Spatial-Frequency Filtered Textures
Authors: Frisby J P , Catherall C , Porrill J , Buckley D
Published: December 2015

Enright (1995 Vision Research36 307 – 312; 1995 Perception24 Supplement, 32 – 33) described a simple piece of equipment for demonstrating a highly sensitive perceptual mechanism that he called sequential stereopsis. The apparatus prevents ‘conventional’ stereopsis mechanisms from operating as it prevents the comparisons of disparities of targets seen at the same time. Enright suggested that the mechanism of sequential stereopsis relies on a comparison of disparities before and after isovergent saccades between fixations of the two targets. In his apparatus, the observer makes fixations between two textured targets seen behind a pair of viewing ports and the task is to adjust these to appear equidistant. The principle upon which the apparatus depends is the use of textures whose elements cannot be resolved in peripheral vision at the eccentricity determined by the target separation. Enright used a fine sandpaper for this purpose. We describe two similar experiments ( N=3 in both) with high-bandpass filtered textures which eliminate any possibility of the low-spatial-frequency content of sandpaper textures playing a role. Our results corroborate Enright's general conclusions on sequential stereopsis while at the same time showing that high-bandpass textures do not give wholly similar results to sandpaper. Possible reasons for this are discussed.
The Dynamics of Perceptual Learning in Different Visual Search Tasks: Psychophysics and Psychophysiology
Authors: Leonards U , Rettenbach R , Sireteanu R
Published: December 2015

Serial visual search can become parallel with practice (Sireteanu and Rettenbach, 1995 Vision Research35 2037 – 2043). Our purpose was to examine whether psychophysiological indices reflect the changes in reaction time during training.

We used targets and distractors that differed either in orientation (‘tilt’), or in local brightness: closed circles with or without an additional line element (‘added line’), or circles with gaps of different width (‘gap’). The subjects’ task was to indicate the presence or absence of a target on a computer screen by immediately pressing a button and pointing to the location of the target if the trial was positive, or raise the hand if negative. No feedback was given. Response time and error rate were recorded. In addition, electrocardiograms, galvanic skin response, respiration rate and amplitude, horizontal eye movements, and electromyograms were monitored. Two naive and two experienced subjects participated in at least 16 experimental sessions.

Before training, slopes differed for the three tasks, ranging from parallel search for the feature ‘tilt’ to a very steep serial search for the feature ‘gap’. These differences were reflected in the psychophysiological parameters. Reaction time and error rate decreased continuously with learning, leading to parallel search after prolonged practice for all three tasks (see Nase et al, 1995 Perception24 Supplement, 84). Preliminary results indicate that the psychophysiological measures do not follow the perceptual changes during learning. We conclude that, despite the perceptual parallelisation with practice, the attentional load remains high for initially serial tasks.
Are Angles Transformed into Blobs in Early Vision?
Authors: Kehrer L , Meinecke C
Published: December 2015

When textures used in visual segmentation experiments are composed of angle elements, these angles seem to be transformed into ‘blobs’ at early stages of visual processing (‘blob hypothesis’; see Meinecke and Kehrer, 1994 Perception & Psychophysics56 326 – 334). These blobs can be imagined as ellipses in which the angles are embedded. Thus, ellipses/blobs are more or less elongated depending on the width of the underlying angle.

We tested the blob hypothesis in a psychophysical experiment using two experimental conditions with angle textures. In condition A, subjects had to detect a target texture, in which the resulting blobs had a well-defined orientation, within a background texture, in which the blobs had a less-well-defined orientation. Condition B was identical, except that the target elements became the background elements and vice versa.

If angles are really transformed into blobs in early vision, then condition A should yield a better performance than condition B, because it is much easier to detect ellipses among circles than circles among ellipses (Treisman and Gormican, 1988 Psychological Review95 15 – 48). Our results replicated this asymmetry and thus support the blob hypothesis.

In a simulation study, in which Gabor filters were used as the input layer in a spatial filter model, we tried to gain a better understanding of the underlying mechanisms. The simulation indicated that the experimentally observed asymmetry might be due to particular features of the target textures, leading to higher Gabor target responses in condition A than in condition B, whereas background responses were very similar in both conditions.
The Perception of Involuntary Prosaccades in an Antisaccade Task
Authors: Mokler A , Fischer B
Published: December 2015

In an antisaccade task subjects are required to generate a voluntary saccade to the side opposite to a small visual stimulus. With fixation-point offset preceding stimulus onset (gap) subjects produce some involuntary saccades to the stimulus and correct them by a second saccade. We wanted to know whether the subjects recognised their errors and whether a recognised sequence (error followed by correction) is different from an unrecognised sequence. To test the access to the correction mechanism, subjects were asked in subsequent experiments to produce the error-correction sequence voluntarily (voluntary sequence).

We used the gap = 200 ms condition. A valid cue was presented 100 ms before stimulus onset. This manipulation increased the error rate (Fischer and Weber, 1996 Experimental Brain Research109 507 – 512). Subjects indicated errors by key-press. The rate of recognised and unrecognised errors, saccadic size, reaction times (SRT), and correction times (CRT) were determined.

Altogether 93 data sets (400 trials each) from 38 subjects were analysed. The mean error rate was 20%, of which 62% went unrecognised. In sessions with high error rates the fraction of unrecognised errors was high. The SRT of the errors ranged from 80 to 170 ms with a strong mode of express saccades at 100 ms. Both types of errors had the same mean SRT of 117 – 119 ms. The unrecognised errors were 0.4 deg smaller. They were corrected after a mean CRT of 95 ms. The recognised errors were corrected after 127 ms; in the voluntary sequence the correction occurred after 217 ms. The CRT distributions differ from each other with the unrecognised errors having an extra peak around 45 ms, suggesting different modes of correction, to which perception has different access. These results raise the question why the large and long-lasting changes of the retinal image escape the conscious perception so often.
Neuronal Plasticity in the Associative Visual Cortex of the Cat after Callosatomy
Authors: Gabibov I M , Strohm V F , Solnushkin S D
Published: December 2015

Changes in receptive field (RF) properties and connectivity of neurons in cat associative visual cortex were studied after callosatomy. Callosatomy and skull trepanation at area 21 were performed in cats weighing 2.5 – 4 kg (with nembutal 40 mg kg−1). A total of 120 neurons in area 21 were studied. In experiments performed a short time after the operation, RF sizes were found to be smaller on average, and more equal between the two hemispheres compared with intact brains [mean RF sizes previously found in intact brains: 39 deg (left hemisphere) and 28 deg (right hemisphere); after operation: 23 (left hemisphere) and 24 deg (right hemisphere)]. At this stage, all investigated neurons lacked ipsilateral input and were selective for the orientation of the stimulus, whereas we have previously found that 70% of the neurons in the left and 30% of the neurons in the right hemisphere of the intact brain are not selective for orientation. Two months after the operation, RF sizes had increased, associated with the appearance of extensive inhibitory zones including input from the ipsilateral hemifield (in 40% of the neurons). Connectivity was studied by injecting horseradish peroxidase (HRP) in area 21. In intact animals, this has been found to result in a characteristic pattern of staining of neurons in area 17 in groups of 2 – 15. In animals studied two months after callosatomy, the resulting HRP staining in area 17 was more uniform. Further, in area 7 of operated animals large numbers of cells were stained, compared with only single cells in intact brains. More than 20% of the stained area 7 neurons of operated animals were inhibitory interneurons, which might correlate with the large inhibitory zones apparent in area 21 RFs. We conclude that extensive compensatory reorganisations occur in the associative visual cortex after callosatomy, involving changing connections in at least areas 7 and 17.
When Stereopsis Does Not Improve with Increased Contrast
Authors: Wilcox L M , Hess R F
Published: December 2015

It is well established that stereoacuity for conventional (1st-order) stimuli improves with increasing contrast, with slopes ranging from −0.5 to −1.0 on log - log axes (Halpern and Blake, 1988 Perception17 483 – 495; Legge and Gu, 1989 Vision Research29 989 – 1004). In the experiments reported here we used a variety of stimuli (Gabor patches, amplitude-modulated gratings, and 1-D noise patches) and tasks (stereoacuity and Dmax) to determine if 2nd-order stereopsis shows a similar square-root dependence, or if the contrast relationship is relatively flat as observed for monocular localisation tasks (Hess and Holliday, 1992 Vision Research32 1085 – 1097). Where possible, stereopsis was assessed with the use of the same stimuli for both 1st-order and 2nd-order stereopsis to ensure that the resulting data would be comparable. In all cases where stereopsis was based on 2nd-order processing there was little or no influence of contrast on performance; the average slope was approximately −0.15. In comparison, more typical slopes near −0.75 were obtained under 1st-order test conditions.

The indifference of 2nd-order depth perception to changes in stimulus contrast not only supports the proposed distinctiveness of the two types of processing but places constraints on theoretical and computational models of 1st-order and 2nd-order stereopsis. Further, these data support the proposal that the same nonlinear position estimate is used for monocular localisation of nonabutting targets, and for 2nd-order stereopsis.
A New Look at Primate Ganglion Cell Receptive Field Structure
Authors: Lee B B , Kremers J
Published: December 2015

We have reinvestigated receptive field structure of macaque ganglion cells using a novel stimulus—counterphase modulation of a bipartite field—with luminance, chromatic, or cone-selective stimulation. We previously used this stimulus (Kremers et al, 1995 Colour Vision DeficienciesXII 399 – 406) to show that surrounds of middle (M) and long (L) wavelength cone opponent cells of the parvocellular (PC) pathway are cone specific, measuring on a finer scale than in previous experiments [Reid and Shapley, 1992 Nature (London)356 716 – 718]. Modelling of response amplitude and phase now confirms this conclusion. Second, centre sizes measured were consistent with those from the literature, with PC and magnocellular pathway (MC) cell centres having similar size. Modelling on the basis of single cone centres for PC-cells plus optical blur provided a partial description of the data, but some inconsistencies were present. Last, the chromatic nonlinearity of magnocellular pathway ganglion cells appears to result from a subunit structure within the receptive-field surround.
Phenomenal Shrinkage and Expansion of Visual Surfaces: Is Amodal Completion a Factor?
Authors: Purghé F , Olivero A
Published: December 2015

When two figures intersect, two apparently paradoxical size effects take place (Kanizsa, 1979 Organization in Vision chapter 11): (1) the partly occluded figure looks shorter than an identical unoccluded figure, and (2) the modally visible parts of the occluded figure look larger than identical isolated parts. Kanizsa explains such effects as due to amodal completion.

Three experiments were carried out by the method of adjustment to test this explanation: two were devoted to the shrinkage effect, and the third one was devoted to the expansion effect. The first experiment exploited the properties of the so-called Helmholtz - Ratoosh law (T-shaped vs Y-shaped junctions) to test whether a figural shrinkage is also present in a pattern in which amodal completion should not occur. The second experiment was aimed at checking whether stereoscopic displacement of the ‘occluded’ figure ‘behind’ or ‘in front of’ the ‘occluding’ one is capable of affecting the amount of shrinkage. In the third experiment the Helmholtz - Ratoosh law was again applied to the expansion. The results of all three experiments converge in showing that the amount of shrinkage of the whole ‘occluded’ figure as well as the ammount of expansion of the ‘modally visible’ parts are not affected by the presence or the absence of amodal completion. We conclude that both the shrinkage and the expansion effects are due to other factors, probably the same factors as those that affect classical optical-geometrical illusions, such as the Oppel - Kundt or the horizontal - vertical illusions.
A Neuromorphic Recurrent Model for Figure-Ground Segregation of Coherent Motion
Authors: Beaudot W H A
Published: December 2015

A neuromorphic model of the retino-cortical motion processing stream is proposed which incorporates both feedforward and feedback mechanisms. The feedforward stream consists of motion integration from the retina to the MT area. Retinal spatiotemporal filtering provides X-like and Y-like visual inputs with band-pass characteristics to the V1 area (Beaudot, 1996 Perception25 Supplement, 30 – 31). V1 direction-selective cells respond to local motion resulting from nonlinear interactions between retinal inputs. MT direction-selective cells respond to global motion resulting from spatial convergence and temporal integration of V1 signals. This feedforward stream provides a fine representation of local motion in V1 and a coarse representation of global motion in MT. However, it is unable to deal with the aperture problem.

Solving this problem requires the adjunction of local constraints related to both smoothness and discontinuity of coherent motion, as well as some minimisation techniques to obtain the optimal solution. We propose a plausible neural substrate for this computation by incorporating excitatory intracortical feedbacks in V1 and their modulation by reciprocal connections from MT. The underlying enhancement or depression of V1 responses according to the strength of MT responses reflects changes in the spatiotemporal properties of the V1 receptive fields. This mechanism induces a dynamic competition between local and global motion representations in V1. On convergence of these dynamics, responses of V1 direction-selective cells provide a fine representation of ‘true’ motion, thus solving the aperture problem and allowing a figure - ground segregation based on coherent motion. The model is compatible with recent anatomical, physiological, and psychophysical evidence [Bullier et al, 1996 Journal de Physiologie (Paris)90 217 – 220].
Zooming Diamonds: A New Illusion, and a Warning on the Biological Relevance of Parallel Projection
Authors: Cowie R
Published: December 2015

A new illusion is described. Observers watch parallel projections of ‘diamonds’ (rhombi) tumbling in 3-D. The displays are generated by moving a viewpoint round a fixed rhombus on a trajectory with two components—a circle parallel to the plane of the diamond, and a superimposed change in elevation which varies sinusoidally. To a greater or lesser extent, depending on the phase of the sinusoid, elevation change is misinterpreted as ‘zooming’ in and out—though in fact the projection always corresponds to an object at a fixed distance. The illusion was devised to underline the questions surrounding the treatment of parallel projection in biological systems. The standard formulations considered in computational vision preclude the kind of size - distance trade-off that the illusion demonstrates, but they do imply that observers should be able to register the shape of an object from this kind of display. A less familiar formulation, ‘paraperspective projection’, allows size - distance trade-off as in perspective projection, but it suggests the shape of a lamina should be impossible to recover from motion. Stimuli which promote ‘zooming’ do weaken shape discrimination, but the trade-off is incomplete. A possible solution is that human vision picks out size change in a way that is appropriate when either object or motion path is ‘friendly’, but that misleads when awkward combinations occur. Certainly vision research should avoid assuming that the attractively simple consequences associated with standard parallel projection govern the way biological systems operate.
Perceived Speed of Moving Cyclopean Corrugations Increases with Increasing Disparity Amplitude
Authors: Johns A M , Rogers B J , Eagle R A
Published: December 2015

In order to investigate how cyclopean motion is coded by the visual system, the points of subjective equality (PSEs) were measured for (i) speed, (ii) spatial frequency (SF), and (iii) temporal frequency (TF) as a function of peak-to-trough disparity amplitude for cyclopean corrugations. Two panels (3.0 deg × 7.0 deg) of dynamic random-dot stereograms were located 0.5 deg on either side of a central fixation spot. Each panel contained a horizontally oriented sinusoidal cyclopean corrugation whose SF, TF, and disparity amplitude were under experimental control. On each trial, the cyclopean corrugations were displaced vertically in opposite directions. Subjects judged which panel contained the higher SF, TF, or speed depending on condition. The reference stimulus was a sinusoidal corrugation with SF=0.4 cycles deg−1, TF=0.8 Hz, speed of 2.0 deg s−1, and peak-to-trough disparity amplitude of 8 min arc around fixation.

We found that, as the peak-to-trough disparity amplitude of the test stimulus increased from 2 min arc to 32 min arc, the PSE for speed decreased from 2.21 deg s−1 to 1.67 deg s−1, compared to a reference speed of 2.00 deg s−1. However, across the same levels of disparity amplitude, the PSE for SF remained constant and the PSE for TF varied but with no consistent pattern. Thus, perceived speed increases with increased disparity amplitude. As all levels of disparity amplitude were above threshold, cyclopean speed cannot be detected by a purely ‘feature-tracking’ mechanism. These metamers and the poor TF matching performance suggest that cyclopean speed is coded by a sparse number of temporal mechanisms.
Lorazepam: A New Tool to Study the Processing of Discontinuities
Authors: Giersch A , Boucart M
Published: December 2015

The involvement of GABAA in the computation of visual information has been suggested in numerous electrophysiological studies. It might play a role, in particular, in the generation of the responses of hypercomplex cells involved in the processing of discontinuities. Benzodiazepines like lorazepam enhance the fixation of GABA on the GABAA receptor. We tested the hypothesis that lorazepam enhances the detection of discontinuities, with static stimuli composed of horizontal line-segments. There were four types of stimuli, with (1) collinear and discontinuous, (2) collinear and continuous, (3) parallel and discontinuous, and (4) parallel and continuous elements. A stimulus was presented on each trial in the centre of a computer screen for 160 ms. The task was to decide whether the stimulus was continuous or discontinuous. Response was given on two response keys. The results showed that lorazepam-treated subjects were faster at detecting a discontinuity when the stimulus included two aligned line-segments (− −) as compared to two parallel line-segments (=). Placebo-treated subjects displayed equivalent performance in the two conditions. These results support the hypothesis that lorazepam enhances responses to line-ends. They are consistent with previous results, obtained with static drawings, letters, fragmented squares, and dynamic stimuli. Lorazepam might be a useful tool to investigate visuo-perceptual integration processes and to link experimental psychology and electrophysiology.