bluuebunny/crossref_metadata_2025 huggingface dataset:
A Neural Net for Reconstructing 3-D Images of Object Surfaces
Authors: Stanikunas R , Vaitkeviius H
Published: December 2015

There are some algorithms for reconstructing a 3-D surface from two 2-D images. Usually these algorithms are different for shaded objects and for 3-D surfaces from random-dot stereograms. We here propose a neural net for the reconstruction of images of such objects. Two arrays (matrices) of photosensitive elements act as inputs to the neural net. The real 3-D object (shaded sphere and planes) illuminated by a point light source produces two images in the plane of these matrices. The information from photomatrices is processed by many independent channels. Each channel obtains information only from a delimited area of the matrices. This area is called the receptive field (RF) of the channel. RFs of the different channels overlap. If RFs have the same coordinates on the two matrices, they form a pair of corresponding RFs. The information obtained from this pair is integrally processed, determining both the spatial position of the single object surface fragment and its averaged brightness. Two subsystems (coarse and fine) are used to determine the spatial position. First, the orientations of the matrices are chosen so as to bring down to a minimum the integral differences of light intensities between the images in all corresponding RFs. The coarse subsystems determine the disparities of the corresponding RFs, taking into account the orientation (fixation disparity) of the matrices. The fine subsystems calculate the centroids of the images within the RFs and the averaged intensity across the RFs. This information is used to calculate the exact position of the fragment in space and the averaged brightness. All calculations are carried out by neuron detectors sensitive to object location in space. These neurons form the output layer of the neural net.
Awareness and Confidence Ratings in Motion Discrimination
Authors: Sahraie A , Weiskrantz L , Barbur J L
Published: December 2015

We have extended our earlier investigations that demonstrated above chance direction discrimination of a moving single-dot target, presented within the blindfield of a hemianope (Weiskrantz et al, 1995 Proceedings of the National Academy of Sciences of the USA92 6122 – 6126). Previous investigations pointed towards two distinct modes of processing, namely correctly discriminating the stimulus direction of motion when ‘aware’ or ‘unaware’ of its presentation. There have been arguments and counter arguments for the relationship between confidence and discrimination scores in normal observers [Kolb and Braun, 1995 Nature (London)377 336 – 338; Morgan et al, 1997 Nature (London)385 401 – 402].

We have investigated the relationship between correct direction-discrimination scores and both the awareness of a visual event and the confidence with which the discrimination is made as a function of the stimulus speed in a blindsight subject using binary and graded scales. Results show that the introduction of a six-point scale for the expressed level of awareness yielded similar results to those when the subject was given a binary choice to indicate the presence or absence of conscious awareness. Although the subject's confidence ratings in making a discrimination varied monotonically as a function of stimulus speed, the data obtained suggest that confidence and awareness, although related, are not functionally identical. The correct discrimination scores remained significantly above chance for a range of speeds even when the subject reported nonconfident. The correct discrimination scores were poorer but nevertheless well above chance for conditions where the subject was asked to report both confidence and awareness and improved if required to report only one of these parameters.
The Perception of Transient Structures
Authors: Vicario G B , Grosso R Actis
Published: December 2015

The phenomenon studied is the perception of well known structures, the elements of which are moving in the visual field and only briefly join together. Consider a horizontal bar traveling along the equator of the screen, and a capital letter F placed in the middle of the screen, with its foot in contact with the trajectory of the bar. There is a moment when the left end of the bar comes in contact with the foot of the F, producing a capital letter E. Preliminary observations show that an E is perceived only if the bar stops its motion for a considerable amount of time, some 120 – 150 ms. In this case a significant structure (the F) is transformed into another significant structure (the E). Other cases are under observation: a meaningless structure that becomes a significant one (an F completed by a bar that is traveling on its top), and a meaningless structure that remains meaningless (an upside down T constituted by a stationary vertical bar with its foot in contact with the trajectory of a moving horizontal bar). The phenomenon shows strong relations with other findings regarding the perception of a stop in the movement of an object, and the perception of a transition between two levels of brightness of the same object.
Estimation of Lengths and Angles of Kinematically Induced Amodal Rods
Authors: Zdravkovic S
Published: December 2015

In previous experiments computer-generated stimuli consisting of two dots moving on semi-elliptical paths, either on inner or on outer halves of ellipses, induced percepts of amodal rods rotating or oscillating in 3-D space, with the two stimulus dots as the visible endpoints. The task was to estimate the length and the angle of inclination of the perceived rod in a specified position. Results of the experiments were only in partial accord with the frontoparallel principle (Johansson and Jansson, 1968 Perception & Psychophysics4 165 – 170). A geometrical model was proposed to explain the perceived characteristics of the rod. The model was tested in two experiments. In the first experiment the length and the angle of inclination of the perceived rod were varied. Subjects were sensitive to changes of length and angle. Furthermore, when they overestimated the length, they underestimated the angle. In the second experiment the curvature of the semi-elliptical paths was increased. Compared to the first experiment, estimations of lengths increased but estimation of angles were not significantly different.
The Human Contrast Sensitivity Function Reflects Nonlinear Dynamics
Authors: Trifonov M I
Published: December 2015

Many processes of visual perception may be modelled by nonlinear systems. I here present a novel nonlinear analysis of contrast sensitivity data. My point of departure was the similarity in shape of (i) curves giving threshold contrast as a function of spatial modulation frequency, and (ii) the envelope of bifurcational diagrams obtained from the logistic equation. It should be noted that the well-known changes of curvature for spatial frequencies above 20 cycles deg−1 are now being discussed in the literature as a problem of the last or finest channel [Kulikowski, 1991, in Limits of Vision Eds J J Kulikowski, V Walsh, I J Murray, volume 5 of Vision and Visual Dysfunction Ed. J Cronly-Dillon (London: Macmillan) pp 286–329].

Numerical simulations of threshold contrast as a function of spatial frequency were carried out on the basis of the logistic equation appropriately adapted to the problem. Several linear transformations of the equation were used for determining the parameters that would provide the best fit to the experimental data. The model introduces the concept of perceptual cycle that can be used for characterising the dynamics of selectivity in the spatial frequency domain in its dependence on grating contrast.
The Effect of Implied Mass on Representational Momentum: Negative Evidence from a Two-Body Collision Paradigm
Authors: Shyi G C-W , Hsieh F-M
Published: December 2015

The phenomenon of representational momentum (RM) refers to distortions of visual memory for the final position of an object whenever the object is preceded by a series of displays that imply smooth transformations of the object in the picture plane. Past research has demonstrated that the magnitude of memory distortion observed in RM is closedly related to kinematic factors such as implied velocity and acceleration depicted in the inducing displays, suggesting that, as a mental analogue to physical momentum, RM may arise from an internalisation of regularities governing the physical world. It is not clear, however, whether or not RM is equally affected by regularities in the kinetic or dynamic domain (eg mass).

We examined this issue in three experiments, using a two-body collision paradigm. The impression of the implied mass was created by simulating the collision of a moving ball of less mass with a stationary ball of greater mass. Although subjects were able to experience and report momentary sensation of objects with different mass, such impressions were not carried over to affect the magnitude of memory distortions in the subsequent RM task where those objects were used as inducing displays. These results imply that the sensation of mass cannot be reliably sustained over a long period by visual information, and as a consequence RM revealed by distortions of visual memory is not affected by implied mass.
Completion under Isoluminance
Authors: Puts M J H , de Weert C M M
Published: December 2015

It is known that monocular depth cues become much less effective under isoluminance. One of these depth cues, occlusion, gives rise to surface completion. A study is reported in which the loss of completion under isoluminance was tested.

A pair of horizontally aligned bars of different lengths is detected automatically in a display filled with pairs of bars of the same length. The pair is detected serially, when vertical bars are placed over the gaps between the pairs. Because the vertical bars are occluders, and the pairs of horizontal bars are aligned, completion behind the vertical bars takes place and the two parts together behave perceptually as a single bar.

We used this knowledge to measure completion under isoluminance. When occlusion is lost under isoluminance, we expect that an occluding surface, isoluminant with the background, will not lead to object completion and as a consequence, the pair with unequal lengths of the parts will pop out. Using this procedure we have demonstrated that completion is lost under isoluminance.
Classification of Visual Cortex Cells with the Use of a New Neural Network Model
Authors: Chihman V N , Mironov S V , Makarov F N , Dudkin K N
Published: December 2015

To identify the intrinsic connections within different layers of area 17 of the cat visual cortex we studied the initial neurons labelled by horseradish peroxidase retrograde axonal transport in serial sections. A computer model of visual neural networks (Dudkin et al, 1995 Proceedings of SPIE 122) has been specially developed in these studies to classify cortical neurons according to their specific anatomic features. There are two main stages of the recognition process in this model: feature selection by nonlinear neural operators and classification (clustering) connected with algorithms of cluster analysis. In the first stage, the primary image processing and segmentation are performed by interactive algorithms, which allow us to form several primary image descriptions and to extract the basic description elements of the cell. From these elements, a feature vector consisting of 17 normalised measures is extracted. In the second stage of the recognition process several algorithms are used to cluster the cells according to the feature vectors extracted. It was possible to group these vectors into compact clusters and to associate each group of vectors with a certain type of cells (pyramidal, spiny, and smooth stellate cells). These results are part of the task of creating a computer image data base and 3-D reconstruction of the cortico-cortical connections in the visual system.
Real-World Scene Categorisation by a Self-Organising Neural Network
Authors: Oliva A , Hérault J , Guerin-Dugue A
Published: December 2015

There is now a great deal of evidence that the visual system identifies the category of a scene before identifying its component objects. Schyns and Oliva (1994 Psychological Science5 195 – 200) showed that scene recognition could be initiated with only coarse blobs (information from which the identity of objects could not be recognised). Thus, one possible strategy for object recognition would be first to recognise the background scene from very coarse information and then recognise the component objects from fine-scale information. However, this will only be useful to the extent that there is enough coarse-scale information for the background scene to be recognised. We present a scene categorisation model in which low and very low spatial frequencies alone offer sufficient information to produce consistent clusters of five distinct scene categories (beach, city, forest, room, and valley). A new self-organising neural network—curvilinear component analysis (P Demartines, J Herault, 1997 IEEE Transactions on Neural Networks8 149 – 154)—was used to project nonlinearly a filtered version of a real-world image onto a two-dimensional output space. The resulting projection preserved the semantic proximities between the scene categories. These results offer formal evidence that there is enough coarse-scale information for recognition.
Perceptual Integration of Motion and Form in a Visual Search Task
Authors: von Mühlenen A , Müller H J , Groner R
Published: December 2015

Three visual search experiments were designed to investigate the processes involved in the efficient detection of motion - form conjunction targets. In experiment 1 the number of movement directions in the display were varied, and we tried to establish whether or not the target direction was predictable. Search was less efficient when items moved in multiple directions compared to just one direction; whether items moved in two, three, or four directions made relatively little difference. Pre-cuing of the target direction facilitated the search to a small, but non-negligible, extent; the facilitation was not due to better predictability of the display region that contained the target at the start of a trial.

Experiment 2 was designed to estimate the relative contributions of stationary and moving nontargets to the search rate. Search rates were primarily determined by the number of moving nontargets; stationary nontargets sharing the target form also exerted a significant effect, but this was only about half as strong as that of moving nontargets; stationary nontargets not sharing the target form had little influence.

In experiment 3 we examined the effects of movement speed and item size on search performance. Increasing the speed of the moving items (> 1.5 deg s−1) facilitated target detection when the task required segregation of the moving from the stationary items; when no segregation was necessary, increasing the movement speed impaired performance. When the display items were ‘large’, motion speed had little effect on target detection; but when the items were ‘small’, search efficiency declined with item movement faster than 1.5 deg s−1. A ‘parallel continuous processing’ account of motion form conjunction search is proposed.
A Ring Model for Direction-Selective Simple Cells in the Visual Cortex
Authors: Hamada T , Kato K , Yamashima M
Published: December 2015

Experimental studies have shown that (1) direction-selective simple cells in the visual cortex have spatiotemporally inseparable receptive fields, whose spatial profiles at a given time are described by Gabor functions: a sinusoid multiplied by a Gaussian, with a phase parameter; (2) among simple cells, the phases are distributed not merely at 0 and pi/2 as for sine and cosine Gabor functions, but uniformly between 0 and 2pi (DeAngelis et al, 1993 Journal of Neurophysiology69 1091 – 1117); (3) anatomically, these simple cells receive inputs more from other cortical cells than from the lateral geniculate body (LGN) (Ahmed et al, 1994 Journal of Comparative Neurology341 39 – 49). We accordingly propose here a neural model for the simple cells whose receptive fields are assumed to be of the same spatial position and orientation. In the model, several cortical cells are arranged in a ring with mutual excitatory and inhibitory connections, and receive afferent signals from lagged and nonlagged cells in LGN (Saul and Humphrey, 1990 Journal of Neurophysiology64 206 – 224). Computer simulation shows that the cortical cells have spatiotemporally inseparable receptive fields with spatial profiles described by Gabor functions, and are directionally selective to a moving grating. The cells are found to be arranged so that their Gabor phases vary regularly from 0 to 2pi with rotation along the ring. The connection among the cortical cells has a role of amplification as in the canonical microcircuit model (Douglas et al, 1989 Neural Computation1 480 – 488).
Spatial Causality in Bilateral Symmetry Detection
Authors: Hulleman J , Oomes A H J
Published: December 2015

We studied the influence of spatial scale on the detection of vertical and horizontal bilateral symmetry. The causality principle in scale - space theory states that increasing the spatial scale in a representation can only result in a decrease of structure. Consequently, a pattern can be random on the fine scale and symmetric on the coarse scale, never the reverse.

Stimuli were bilaterally symmetric or random patterns, black-and-white on a grey background, with a circular aperture. The minimal scale was systematically varied and stimuli ranged from conventional noise patterns, through Dalmatian texture, to cow-like patterns. Observers had to judge whether a briefly presented pattern was ‘symmetric’ or ‘random’.

Symmetric patterns resulted in a high accuracy (95%) with no influence of scale, and reaction times with a small linear decrease for increasing scale. Random patterns yielded an accuracy increasing from 70% at the smallest scale to 95% at the middle scales. Reaction times showed a similar pattern: largest at the smallest scales and decreasing to values equal to the symmetric condition at the middle scales. Results were similar for vertical and horizontal bilateral symmetry, though the effect for small scales in the random condition was more pronounced in the horizontal case. We conclude that bilateral symmetry is processed at multiple scales with coarse structures available slightly earlier than fine ones. The dramatic decrease of performance for fine-scale patterns is due to the causality effect; random patterns are judged as symmetric when the smallest scale information is not (yet) available.
Peripheral and Central Factors Limiting Spatial Contrast Sensitivity in Infant Macaque Monkeys
Authors: Kiorpes L , Movshon J A
Published: December 2015

To evaluate the contribution of peripheral and central factors to the development of visual sensitivity, we measured contrast thresholds for sinusoidal gratings (0.5 – 4.2 cycles deg−1), presented alone and in the presence of added dynamic visual noise, in infant monkeys. We wished to use the changes in contrast threshold during development, in the presence and absence of masking noise, to establish what mechanisms might be responsible for this developmental process.

We tested Macaca nemestrina monkeys, aged 1 – 18 months, on a spatial contrast detection task, and analysed the masking functions to calculate two parameters: equivalent intrinsic noise ( Neq, the noise contrast at which squared contrast threshold was doubled), and signal-to-noise ratio ( Rsn, the asymptotic ratio of threshold to noise contrast at high noise contrasts). We hypothesise, following Pelli, that Neq measures a peripheral limit while Rsn measures a central one.

Our data suggest that intrinsic noise is high in infants and falls with contrast threshold during development, thus supporting the hypothesis that peripheral factors set an important limit on the development of contrast sensitivity. However, the relationship between Neq and contrast threshold varied somewhat with spatial frequency. At low spatial frequencies, contrast threshold was well specified by knowledge of Neq, whereas at higher spatial frequencies, changes in Rsn had a relatively greater weight. We conclude that peripheral factors set an important limit on the development of sensitivity at all spatial frequencies; central factors are needed to account for some aspects of development at higher frequencies.
Environmental Pitch and Three Types of Pointing
Authors: Stoper A E
Published: December 2015

Many studies have shown that large errors are made when setting a target (T) to visually perceived eye level (VPEL) in a pitched environmental surround. The error in judgement of VPEL is typically about 50% of the environmental pitch angle. An observer can, however, point to the level of the target (T) with much smaller errors (eg Stoper et al, 1992 Bulletin of the Psychonomic Society30 439, found a shift of pointing of only 4% of the environmental pitch). These small pointing errors are found when the observer reaches out with an unseen hand and touches the surface on which T is presented. We call this ‘type I pointing’. If longer distances (183 cm) are used the observer must walk (with closed eyes, as in ‘pin the tail on the donkey’) in order to touch the surface on which T is presented. We call this ‘type II pointing’; it results in much larger errors, approaching in angular magnitude the errors in judgement of VPEL.

In the present experiments the observer indicated the level of T by touching a point on a unseen pole which was just to the right of the observer's eyes, and thus separated from T by the viewing distance [as in the ‘manual task’ used to judge apparent height by Stoper and Bautista (1992 Investigative Ophthalmology and Visual Science, Supplement33 962)]. We call this ‘type III pointing’. This method, for both long and short distances, produced large errors similar in magnitude to those of type II pointing. These results are explained by the assumptions that environmental pitch causes an error in the judgement of the apparent horizontal in the sagittal plane (sagittal apparent horizontal; SAH) and that SAH is used in pointing of types II and III, but not of type I.
Motion Contrast: A New Metric for Direction Discrimination
Authors: Scott-Samuel N E , Georgeson M A
Published: December 2015

The Adelson - Bergen energy model (1985 Journal of the Optical Society of America A2 284 – 299) is a standard framework for understanding 1st-order motion processing. Its output, the opponent energy for a given input, is calculated by subtracting one directional energy measure ( R) from its opposite ( L), and its sign indicates the direction of motion of the input. Our observers viewed a dynamic sequence of gratings (1 cycle deg−1) equivalent to the sum of two gratings moving in opposite directions with different contrasts, and had to report the dominant direction of motion. The ratio of contrasts was varied across trials. We found that opponent energy was a very poor predictor of direction discrimination performance. Heeger (1992 Visual Neuroscience9 181 – 197) has suggested that divisive inhibition amongst striate cells requires a contrast gain control in the energy model. A new metric can be formulated in the spirit of Heeger's model by normalising the opponent energy ( L- R) with flicker energy, the sum of the directional motion energies ( L+ R). This new measure, motion contrast ( L- R)/( L+ R), was found to be a good predictor of direction discrimination performance over a wide range of contrast levels, but opponent energy was not. Discrimination thresholds expressed as motion contrast were around 0.5 for the sampled drifting gratings used in our experiments, corresponding to an energy ratio ( L: R or R: L) of 3:1 at discrimination threshold. Such high values suggest that the outputs of motion energy filters are very noisy (variable over space and time) or that the use of them is inefficient.
Role of Inducer Length in the Formation of the Ehrenstein Illusion
Authors: Salvano-Pardieu V
Published: December 2015

We wanted to study the role of the length of the inducers in the Ehrenstein illusion and, particularly, the possible interdependence of inducer length and gap size in sustaining the illusion.

We used a subthreshold summation paradigm. As described previously (Tassi et al, 1995 Perception24 127), the presence of an illusory contour reduces the threshold for detection of a real line superimposed on it. In agreement with the model of Grossberg and Mingolla (Grossberg and Mingolla, 1985 Psychological Review92 173 – 211; Grossberg, 1994 Perception & Psychophysics55 48 – 120), these authors suggested that the Ehrenstein illusion was generated by a bipole operator joining two line ends. In an Ehrenstein figure induced by four white lines, we varied the size of the central gap from 1.6 to 4 deg. The threshold of a white line joining the ends of two inducers was measured with a spatial alternative (left - right). The Ehrenstein figures involved either long (2.5 deg), or short (0.58 deg) inducers.

The result shows that whatever the length of the inducers, the threshold for detecting the real line increases with increasing gap size. The results obtained with short and long inducers were very similar. Moreover, in this experiment, the spatial limit of integration of the illusion occurs for a gap size of 2.4 deg: the detection threshold for the real line was significantly higher from this value. This result, consistent with previous research (Salvano-Pardieu and Bonnet, 1994 Perception23 104), suggests that the length of the inducer is a factor less important than the gap size in the formation of the illusion.
Effects of Observer Orientation on Perception of Ego- and Exocentric Spatial Locations
Authors: Li J , Cohen M M , DeRoshia C W , Guzy L T
Published: December 2015

Perceived eye position and/or the perceived location of visual targets are altered when the orientation of the surrounding visual environment (Cohen et al, 1995 Perception & Psychophysics571 433) or that of the observer (Cohen and Guzy, 1995 Aviation, Space, and Environmental Medicine66 505) is changed.

Fourteen subjects used biteboards as they lay on a rotary bed that was oriented head-down −15°, −7.5°, supine, head-up +7.5°, and +15°. In the dark, subjects directed their gaze and set a target to the apparent zenith (exocentric location); they also gazed at a subjective ‘straight ahead’ position with respect to their head (egocentric location). Angular deviations of target settings and changes in vertical eye position were recorded with an ISCAN infrared tracking system.

Results indicated that, for exocentric locations, the eyes deviate systematically from the true zenith. The gain for compensating changes in head orientation was 0.69 and 0.73 for gaze direction and target settings, respectively. In contrast, ‘straight ahead’ eye positions were not significantly affected by changes in the subject's orientation. We conclude that subjects make systematic errors when directing their gaze to an exocentric location in near-supine positions. This suggests a systematic bias in the integration of extra-ocular signals with information regarding head orientation. The bias may result from underestimating changes in the orientation of the head in space. In contrast, for egocentric locations, where head orientation information can potentially be discarded, gaze directions were unaffected by head orientation near supine.
Separation Discrimination in the Presence of Flanking Lines
Authors: Bondarko V M , Danilova M V
Published: December 2015

The presence of flanking stimuli decreases sensitivity in many visual tasks. Earlier we showed that size perception depends on the distance between objects (Bondarko and Danilova, 1996 Perception25 Supplement, 94). Now we suggest that flanks not only decrease sensitivity, but also lead to misperception of size. To test this hypothesis we studied the perception of spatial interval for a set of separations from 6 to 33 min−1 in the presence of two vertical flanking lines.

The method of constant stimuli was used in a temporal 2AFC paradigm. Reference stimuli were two vertical lines with separation S, either with or without two symmetrical external flanking lines. The distance to the flanking lines varied from 1/4 S to 3 S in steps of 1/4 S. The test stimuli were just two lines. The first stimulus in the pair could be either the test or the reference. Observers had to report whether the distance between the two central lines in the first stimulus were smaller or larger than the distance in the second stimulus. Probit analysis was used to estimate the threshold and the point of subjective equality (PSE).

We found that the PSE implied overestimation by up to 10% of the spatial intervals for the smallest distances to the flanking lines. Increasing distances to the flanking lines finally led to slight underestimation. The strength of the illusion depended on the order of presentation of the reference and test stimuli. Thresholds increased for the smallest distances to flanking lines. Our results confirmed the hypothesis that flanks can induce distortions in size perception. We suggest that our experimental situation could be considered as a one-dimensional case of geometrical visual illusions such as the Ebbinghaus illusion.
Estimation of Time-to-Contact from Retinal Flow
Authors: Harris M G
Published: December 2015

We investigated four models for estimating time-to-contact (TTC) from retinal flow. Lee's model can deal with sparse flow but fails if the flow contains a rotational component. Koenderink's model, based on div, can deal with rotation but fails if the flow is sparse or if the world does not vary coherently in depth. Two new models were developed by representing retinal flow as the sum of an expansion and a rotation component. The first operates on pairs of points and can deal with sparse flow but fails if the world does not vary coherently in depth. Uniquely, this model provides TTC estimates without prior knowledge of either the focus of expansion (FOE) or focus of rotation (FOR). The second model estimates both the FOE and the FOR and then operates on a point-by-point basis. This model can deal with incoherent depth variations.

We compared human performance with these different model properties by requiring subjects to estimate FOE and TTC from random-dot kinematograms. We used kinematograms depicting smooth planes and random 3-D clouds of points, and systematically varied the density of the flow. Performance was not substantially reduced by sparse flow or by incoherent depth, which argues against Koenderink's and the first of our own models. Performance remained good when rotation was added to the flow, which argues against Lee's model. Overall, the data favour a model that first decomposes flow into expansion and rotation components and then estimates TTC on a point-by-point basis.
A Signal-Detection Model of Visual Short-Term Memory Performance for Random Block Patterns
Authors: Cornelissen F W , Greenlee M W
Published: December 2015

On the basis of signal-detection theory, we have formulated a model that accurately explains performance on a visual short-term memory task involving random block patterns. The model assumes that the internal response of an observer for detecting a change in any given element of the block pattern is noisy and has a Gaussian-shaped distribution. On this basis we can calculate the likelihood that an observer correctly or falsely identifies a change in the pattern after a certain time interval (ISI). Using this likelihood, we can then predict the likelihood that an observer correctly identifies a whole pattern as having changed or not as a function of the number of elements that changed in the pattern. We have previously shown (Cornelissen and Greenlee, 1993 Perception22 Supplement, 46) that memory performance declines when changes occur in pattern elements located on the perimeter of the pattern. Therefore the model also incorporates a circular symmetric ‘memory field’ that shows a Gaussian-shaped decline of memory performance from the point of fixation. The model has three parameters: d' (detectability of a change), lambda (criterion level), and the standard deviation of the Gaussian of the memory field.

In the experiments we performed, block patterns made up of 50 light and 50 dark randomly arranged elements (0.5 deg checks) were briefly (200 ms) shown. In a forced-choice task, subjects judged whether two sequentially presented (with ISIs of 1, 3, or 10 s) block patterns were the same or different. Task difficulty was varied by varying the number of elements in the patterns that changed on ‘different’ trials. The model is able to accurately predict memory performance at the three different ISIs for various levels of pattern differences (changes in 1, 2, 4, 8, 16, 20, and 50 out of 100 elements).
A Pedestal Blocks the Perception of Non-Fourier Motion
Authors: Ukkonen O I , Derrington A M
Published: December 2015

We wanted to know whether the mechanisms that discriminate the motion of first-order patterns (defined by spatial variations of luminance) differ from those that detect the motion of non-Fourier or second-order patterns (defined by spatial variations of contrast). To address this question we tested whether motion discrimination performance of first-order and second-order patterns was affected by a pedestal (Lu and Sperling, 1995 Vision Research35 2697 – 2722). A pedestal is a static replica of a moving pattern. We used pedestals with contrast or modulation depth twice the value at which it becomes possible to discriminate the direction of a moving pattern. A two-interval forced-choice task was used to determine how direction discrimination varies with contrast of sine gratings (1 cycle deg−1) and modulation depth of amplitude-modulated gratings presented either alone or with a pedestal. The amplitude-modulated gratings had a 5 cycles deg−1 carrier modulated at 1 cycle deg−1. Three different temporal frequencies (1, 3, and 12 Hz) were studied.

Performance with sine gratings was unaffected by the pedestal at all temporal frequencies tested. For amplitude-modulated gratings the pedestal raised the modulation depth at which it became possible to discriminate the direction of motion. This elevation in threshold decreased when the mean contrast of the pattern was high. This result shows that immunity to pedestals of texture-contrast patterns (Lu and Sperling, 1996 Journal of the Optical Society of America13 2305 – 2318) does not generalise to other non-Fourier motion stimuli.
The Effect of Attentional Cueing upon Speed Discrimination
Authors: Blakemore M R , Snowden R J
Published: December 2015

Motion processes have often been regarded as being preattentive in nature. Additionally, in a few attention studies attempts have been made to incorporate the methodology common to classical psychophysics. Speed discrimination with multiple grating patches has been investigated with uncertainty/search paradigms (Verghese and Stone, 1995 Vision Research35 2811 – 2823). Another method commonly used in attentional research is the cueing paradigm, which we have used here.

In this study participants had to discriminate, using a 2AFC method, whether one of four grating patterns moved faster or slower than the other patches (of a standard speed 6 deg s−1). The speed of this test stimulus was altered from trial to trial with the use of an adaptive staircase procedure. Whilst the gratings were in motion a brief (32 ms) cue was presented. This could be either valid (58%), invalid (14%), or neutral (28%) in respect to the patch of grating which was moving at a different speed.

The results demonstrate that cueing had a strong effect upon the discrimination judgement. The invalid trials had up to a factor of four increase in discrimination thresholds when compared to the valid trials. Further experiments with post-stimulus cueing suggested that a decision theory model may account for a large proportion of these cueing effects. This is in agreement with similar cueing studies (Kinchla et al, 1995 Perception & Psychophysics57 441 – 450) in which participants discriminated between different array elements.
Dark Adaptation as a Model of the Parkinsonian Visual System
Authors: Wink B , Harris J P
Published: December 2015

It has been suggested that the Parkinsonian visual system is like the normal visual system, but is inappropriately dark-adapted (Beaumont et al, 1987 Clinical Vision Sciences2 123 – 129). Thus it is of interest to ask to what extent dark adaptation of normal subjects produces visual changes like those of Parkinson's disease (PD). One such change is the reduction in apparent contrast of medium and high spatial frequencies in peripheral vision in the illness (Harris et al, 1992 Brain115 1447 – 1457). Normal subjects judged whether the contrast of a peripherally viewed grating was higher or lower than that of a foveally viewed grating, and a staircase technique was used to estimate the point of subjective equality. Judgements were made at four spatial frequencies (0.5 to 4.0 cycles deg−1) and four contrasts (8.0% to 64%). The display, the mean luminance of which was 26 cd m−2, was viewed through a 1.5 lu nd filter in the relatively dark-adapted condition.

The ANOVA showed an interaction between dark adaptation and the spatial frequency of the gratings. Dark adaptation reduces the apparent contrast of high-spatial-frequency gratings, an effect which is greater at lower contrasts. This mimics the effect found with PD sufferers, and suggests that dark adaptation may provide a useful model of the PD visual system.

In a second experiment, the effect of dark adaptation on the relationship between apparent spatial frequency in the fovea and periphery was investigated. The experiment was similar to the first, except that judgements were made about the apparent spatial frequency, rather than the contrast, of the peripheral grating. ANOVA showed no differential effect of dark adaptation on the apparent spatial frequency of the peripheral grating. This suggests that the observed reduction in apparent contrast of the peripheral gratings in dark-adapted normals and Parkinson's sufferers may reflect relative changes in contrast gain, rather than relative changes in the spatial organisation of receptive fields.
The Extrinsic/Intrinsic Classification of Motion Signals is a High-Level Process
Authors: Castet E , Charton V , Dufour A
Published: December 2015

It has been proposed that the 2-D motion signals elicited by the bar endings of a barber-pole stimulus disambiguate 1-D motion signals with a variable strength which depends on depth (Shimojo et al, 1989 Vision Research29 619 – 626): these signals would be ‘abolished’ when they are extrinsic (ie the moving grating is behind the plane of the background containing the aperture), whereas they would be given full strength when they are intrinsic (ie the plane of the grating is in front of the background). These authors have suggested that the intrinsic/extrinsic classification is an early process. However, the very long duration (2300 ms) used in their study suggests other interpretations. Therefore, we tried to test whether the barber-pole illusion could be abolished with a shorter duration when the grating had an uncrossed disparity relative to the aperture plane, as initially described in the above-mentioned study (our 30 observers had to adjust an arrow to indicate the perceived direction of the grating). In accordance with our prediction, we could not replicate their finding with a duration of 400 ms. Surprisingly, increasing the duration up to 2300 ms was not sufficient to obtain a large bias towards 1-D signals. To understand this unexpected result, we tried to isolate the relevant difference between the initial study of Shimojo et al and our. We found that the main determinant of the suppression of the barber-pole illusion was the experimental procedure: when our observers had to assess the perceived direction of the barber-pole by choosing between horizontal and vertical, as in the initial study, the results did show a much larger bias towards 1-D signals. We suggest therefore that the extrinsic/intrinsic classification is a high-level process which can be influenced by the observer's expectations.
Differential Velocity Detection in Motion Parallax Fields
Authors: Masson G S , Mestre D R
Published: December 2015

Processing of motion parallax relies on the ability of the visual system to segregate locally different motion signals and integrate them over space and time in order to reconstruct a 3-D structure. In a series of experiments, we evaluated the thresholds for differential motion detection and their dependences upon the spatial structure of the optic-flow field.

Visual stimuli were large moving random-dot displays (80 deg × 60 deg). Each dot was randomly attributed one of two velocities whose difference ranged between 0% and 80% of their average velocity, which ranged between 2 and 64 deg s−1. Stimulus duration was from 130 to 1040 ms. Subjects were instructed to stare at a central fixation mark and had to decide whether the display specified one or two surfaces. At a duration of 130 ms, subjects needed more than 40% difference between the two velocities to reach a 75%-correct detection criterion. Thresholds reached a minimum value of about 20% at a duration of 500 ms. Thresholds always increased with the average velocity and were not affected when a static form segregation cue was added to the displays. They were always larger than thresholds for velocity discrimination in successive displays (about 10% at 260 ms). We finally investigated the spatial properties of differential velocity detection. The visual field was divided into horizontal stripes of equal widths. Adjacent bars alternately contained random dots moving with one of two velocities. Thresholds for differential velocity detection increased monotonically when the bandwidth decreased, up to an asymptotic value, equal to those observed with a transparent display.
Visual Evoked Potentials to Gratings and Noise
Authors: Shelepin Y E , Harauzov A K , Krasilnikov N N , Pronin S V
Published: December 2015

Visual evoked potentials were recorded in a study of the spatial-frequency characteristics of the human visual system. Stimuli were gratings with and without superposition of white noise. Evoked potentials were recorded in normal subjects from different areas of the occipital cortex, from the temporal and parietal lobes, according to the ‘ten-twenty’ electrode system. A set of black-and-white sine-wave gratings was used with eight different spatial frequencies in the range 0.45 to 14.4 cycles deg−1. The gratings were presented with binary quasi-white noise or with a uniform grey field with mean luminance equal to that of the noise. The amplitudes of the N1, P1, N2, and P2 response components were compared under the two stimulation conditions. Changes in the form of responses as well as changes in spatial-frequency characteristics were found when white noise was superimposed. The results obtained are discussed in terms of the presence and location of matched filtering in the visual system.
Characteristics of Visual and Haptic Functions Relate Similarly to Alpha Rhythm Parameters
Authors: Ryazanova T B , Markina A V
Published: December 2015

Visual short-term memory (STM) for digits and reaction times (RT) has been found to correlate significantly with parameters of the occipital background EEG: the dominating alpha frequency and the alpha spindle duration (Markina and Maltzeva, 1993 Proceedings of 7th EMPS in Trier pp 311 – 314). STM span is more closely related to the latter, RT to the former of these parameters. We studied how certain characteristics of accuracy in haptic performance correlated with the same alpha parameters. We measured the control of line-length scatter, and the quantity of metrically accurate copies, in the standard task of blindly copying zigzag lines. The two measures were statistically independent of each other. We found that these characteristics of haptic performance show a similar pattern of correlation with alpha parameters as do STM and RT. Line-length scatter, as an index of control processes in the motor system, is related to alpha spindle duration. The quantity of metrically accurate copies, as an index of how precisely external metrical parameters are mapped to a subjective representation, is related to the dominating alpha frequency. The results are supported by factor analysis. They imply that (1) there are similar types of structures in visual cognition and haptic processes, and (2) the alpha rhythm parameters considered underly both kinds of processes.
A Method for Rehabilitating Vision after Stroke
Authors: Virsu V , Koskela P
Published: December 2015

We developed a method for rehabilitating eye movements and binocular fusion, and tested the method in one patient. An infarct of the pons caused the paresis of the lateral rectus muscle of the left eye. Beginning from the third week in hospital, the patient was trained in eye movements and binocular fusion. Fusion was made possible by means of prisms that moved the images of targets in central vision to the threshold of fusion in the primary eye position. During practice sessions lasting 0.5 to 2 h daily the patient kept the images fused by making a horizontal head movement when necessary. Several eye-movement sessions were held daily, consisting of voluntary saccades and fixations as far to the left as possible. The strength of the prismatic correction required for fusion decreased, and four months after the onset of stroke the patient could fuse without prisms in the primary position. His binocular vision became practically normal in one year. The plasticity of the visual system can be utilised in rehabilitation by a practice that uses minimal remedial means necessary for correct function at each level of performance.
A Comparison of Electrophysiological and Psychophysical Threshold Detection in Texture Segregation
Authors: Meigen T , Kastner S , Bach M
Published: December 2015

The salience of targets in preattentive tasks depends on local feature gradients between the target and the surrounding background. We have developed an electrophysiological method to isolate texture-segregation-specific components in the human visual evoked potential (tsVEPs). We report here on threshold estimates of orientation-based texture segregation, comparing psychophysical and electrophysiological measures.

Eight visually normal subjects participated in the experiment. In a 4AFC task, subjects indicated the position of a single patch that differed in orientation from a homogeneously oriented background. Orientation gradients ranged from 5° to 90°; orientation threshold was estimated with a best PEST procedure. In further experiments VEPs to the onset of ‘orientation chequerboard’ patterns with orientation gradients ranging from 0° to 90° were recorded from an Oz-Fpz derivation. Orientation threshold was estimated in two steps. First, the VEP to the 0° condition was subtracted from all other VEPs to isolate tsVEPs. Second, tsVEP amplitudes were determined by cross-correlation between the tsVEP to any given orientation and the 90° condition.

We found, first, that the electrophysiological threshold derived from the overall mean tsVEPs was considerably higher than the psychophysical threshold of the orientation gradient, 16.1°±1.0°. Thus the pool of neurons contributing to the VEP signal must be larger or more strongly activated in order to elicit a significant tsVEP than to subserve psychophysical performance. Second, we found that the amplitudes of tsVEPs above threshold increased monotonically with increasing orientation gradient of the chequerboard patterns, suggesting that the tsVEP may have its neural correlate in orientation-contrast-dependent neurons similar to those found in monkey and cat striate cortex.