bluuebunny/crossref_metadata_2025 huggingface dataset:
Preferred Hemifields Learn Less
Authors: Pavlovskaya M , Ring H , Groswasser Z , Hochstein S
Published: December 2015

Subjects were tested on orientation and colour pop-out tasks. Orientation pop-out stimuli were 3 × 3, 5 × 5, or 7 × 7 arrays of vertical light (white) bars (on a black background) with a target element of diagonal (45°) orientation (present on half of the trials). Colour pop-out stimuli consisted of 60° oriented bars with blue distractor and yellow target elements (on a gray background). Inter-element distance was 40 min arc. The arrays were presented eccentrically, in the right or left hemifield, so that the distance of their centres from the fixation cross was 2.5 – 6.5 deg.

First we tested performance in the right and left hemifields, with interleaved trials. Most subjects (7/9) had a right-hemifield preference for the orientation and/or colour task. The remaining 2 had a left-hemifield preference. For the six subjects tested on both tasks, all but one had more laterality for colour than for orientation. Thus, for the 15 conditions tested (9 orientation + 6 colour), most (8/15) showed better performance in the right hemifield; the minority (2/15) were better on the left; and a third were about equal in both hemifields. Subjects were recalled for a second session with the same tasks, to test improvement due to first session training. Almost all showed significantly greater improvement for the non-preferred hemifield. This effect was so strong that often preference was switched. Surprisingly, preferred-hemifield performance actually declined for many subjects. Thus, the effect seems related to competition, and perhaps an automatic attention-directing mechanism.
Measurement of the Shortening Effect on the Perceived Path of Stroboscopic Movement
Authors: Nozawa S
Published: December 2015

When two vertical short lines are alternately flashed at certain SOAs, a shortening of the apparent path of the stroboscopic movement is perceived. In the experiments reported here, factors influencing the shortening effect were studied with lines created on a CRT display.

Experiment 1 was designed to study the effect of SOA. Each stimulus line was always presented for 100 ms, but intervals were varied in the range from 25 to 800 ms. With short and long SOAs almost no shortening illusion was observed, whereas the SOA for optimal stroboscopic motion (200 ms) also produced the largest illusion (ca 16%). This agrees with the classic study by Scholz (1924 Psychologische Forschung5 219 – 272) who found the largest illusion (25%) at the optimal frequency for stroboscopic motion.

Experiment 2 dealt with the effect of inversions (I), mirror reflections (M), and rotations (R) of the line during the stroboscopic movement (see Kolars and Pomerantz, 1971 Journal of Experimental Psychology87 99 – 108). The particular movements were signalled by means of a short horizontal line added to one end of each of the two vertical lines of experiment 1. The configurations were (1), signifying parallel motion in one plane; (2), locomotion with rotation around the vertical axis (M); (3), locomotion with rotation around the horizontal axis (I); and (4), locomotion with rotation in the plane of the display (R). In all these conditions, the shortening illusion was significantly larger than in experiment 1. The differences between the four conditions were not statistically significant, but the illusion under condition (1) seemed smaller than in the other three conditions. With SOAs for optimal stroboscopic motion, ‘rotation’ paths tended to appear three-dimensional.
Estimation of Local and Global Characteristics of Half-Tone Surfaces by the Cat's Lateral Geniculate Body
Authors: Podvigin N , Kiseleva N , Chizh A , Novikov G , Moshonkina T R
Published: December 2015

We investigated neuronal mechanisms in the cat's dorsal LGN that could carry information about the spatial brightness distribution across half-tone visual object surfaces. Stimuli were halftone spheres, cylinders, toroids, etc. We have found that geniculate neuronal structures describe and estimate both local and global characteristics of the brightness distribution on these surfaces. The search for local mechanisms of perception of half-tone surfaces revealed a new property of geniculate neurons: their sensitivity to changes in both the magnitude and vector direction of the spatial brightness gradient in the test stimulus. The quantitative estimation of gradient sensitivity in each case has been calculated by the formula: K=( Nmax- Nmin)/ Nmax, where Nmax is the number of spikes (in a 500 ms interval) in response to the preferred (and Nmin to the nonpreferred) gradient direction. The average value of K was 0.49 (range 0.15 – 0.95) for a mean brightness gradient of 2.7 cd m−2 deg−1. A ninefold decrease of the gradient reduced K by a factor of 6.5. The preferred direction of most (82%) neurons coincided with the radial direction from the centre of the receptive field to area centralis. The basis of this phenomenon probably lies in the influence of the spatial brightness gradient on spatiotemporal relations between excitatory and inhibitory processes in geniculate receptive fields. Global characteristics of the surface brightness distribution were represented in the population response of the neurons. The pattern of neuronal activity elicited in on-neurons and off-neurons in response to surface stimuli correlated with the isophot structure of the surface ( rav=0.86 for 23 cases). Therefore, separate descriptions of local and global brightness distributions originate at subcortical levels of the visual system.
Performance of 5-Year-Old Children on Seven Visual Perceptual Tasks in Relation to Neonatal Ultrasound Abnormalities
Authors: Vandenbussche E , Stiers P , Haers M , van den Hout B M , de Vries L S , van Nieuwenhuizen O
Published: December 2015

We investigated whether neonatal brain damage can give rise to visual perceptual deficits, in addition to the well-documented impairments in visual acuity. To this end, forty-one children (age 5.02 to 5.89 years) were given four visual object identification tasks and three visuo-constructive tasks. These subjects were neonatal at risk owing to prematurity or birth asphyxia. From neonatal ultrasound scans, the occurrence of intracranial hemorrhage (ICH, N = 17), periventricular leukomalacia (PVL, N = 15), and/or white matter damage due to either of these conditions (WMD, N = 9) was determined for each subject. Scans were normal in fourteen of them. The number of subjects performing at or below Pc10 of same-age normal children was significantly above 10% for each task (range 27% – 49%). This was still true when mental instead of chronological age was used for comparison, as shown by the results of nine subjects for which intelligence data were available. This high incidence of impairment is not attributable to loss of visual acuity, since grating acuity was reduced in only four subjects (14 – 19 cycles deg−1). The frequency of scores < Pc10 correlated significantly with WMD in six tasks, with PVL in 4 tasks, but not with ICH. We conclude that neonatal at risk children are more likely to develop impaired visual perceptual skills, independent of mental disability and visual acuity loss. On ultrasound permanent white matter abnormalities are most strongly associated with visual perceptual deficit, whereas intracranial hemorrhage is unrelated.
The Phototactic Sensorimotor Mechanism of the Unicellular Alga Euglena Gracilis. An Application to the Evolutionary Study of Vision
Authors: Insinna E M
Published: December 2015

Our understanding of even basic functions of microtubules (MT) such as intracellular transport and ciliary beating, including their role in sensory cells and nerve cells, is still incomplete. For example, vertebrate photoreceptors contain a cilium whose possible dynamical function is still unknown.

A new model of microtubular dynamics, developed by me together with P Zaborski and J Tuszynski [Insinna et al, 1996 BioSystems39(3) 187 – 226], is capable of accounting for most of the phenomena associated with cell motility. Additionally, it sheds new light not only on the phototactic behaviour of the Protozoan Euglena gracilis but also on some possible evolutionary steps leading from primitive sensory organs to more complex ones such as the photoreceptors of vertebrates. Euglena displays simple perceptive functions (phototaxis) based on a primitive photoreceptor. This unicellular organism offers a unique possibility to study the function of MT in a simple form of vision. The model is based on classical nonlinear physics and suggests a dynamical role for MT in vertebrate photoreceptors. I am convinced that a better understanding of photoreceptor dynamics requires knowledge of the evolutionary steps leading to their development.
A Model of Human Contrast Sensitivity as a Function of Retinal Illuminance
Authors: Krasilnikov N N , Shelepin Y E , Krasilnikova O I
Published: December 2015

We present a mathematical model for signal transformation by the receptor-horizontal-bipolar cell triad, which gives a good prediction of the spatial-contrast modulation transfer of the visual system at different levels of retinal illuminance. We assume that rod-driven and cone-driven bipolar output signals, sr and sc, depend on the mean rates of photoisomerisations in rods and cones, yr and yc, and their derivatives, yr' and yc', as expressed by the equations sr= cr yr'/( dr yr+ dc yc), and sc= cc yc'/( dr yr+ dc yc), where cr, cc, dr, dc are constant coefficients. The total output signal is S= sr+ sc. During scotopic vision sr >> sc and spatial frequency characteristics are determined by the rod system. When illumination increases and rods saturate, contrast sensitivity is determined by the cone system. A formula for calculation of the spatial-contrast modulation transfer function is derived with mean illumination as a parameter. The predictions of the model are in good agreement with experimental data measured at various levels of retinal illumination.
Visual Perception of Optic Flow in the Left and in the Right Visual Hemifield
Authors: Levashov O V , Levashova H
Published: December 2015

Recently we have found right visual hemifield (HF) superiority in detection of local texture optic flow discontinuity (H O Levashova and O V Levashov, 1996 Perception25 Supplement, 128). In the present work we investigated a possible difference between hemifields in the perception of global optic flow.

The stimulus consisted of two large texture fields which moved across the screen to the left and to the right from a central vertical line. It gave the illusion of motion through an infinite corridor with high walls. The left or the right wall could be curvilinear (convex) while the other one was flat. In order to make the task more difficult the central vertical meridian of the screen was occluded by a black mask with a width of 5 deg. The task of the subject was to detect the curvilinear wall and press the ‘left’ or the ‘right’ key in accordance with target position. When the target was absent (in some presentations both the walls were flat) or when the subject could not detect it he/she had to press the third key. The distance from the screen was 57 cm. One experimental session consisted of 100 – 150 trials. All subjects showed a superiority of the right hemifield (the difference was significant, p<0.05).

The observed right-hemifield superiority in the detection of large texture flow could suggest that the left hemisphere has a certain dominance in visual perception of optic flow. This is in agreement with the hypothesis that the left hemisphere is more ‘phasic’ and the right hemisphere is more ‘tonic’ [O V Levashov, 1989 Computational Models of Sensory Systems (Moscow: VINITI) (in Russian)].
On the Validity of Colour Vision Tests in the Field: A Practical Assessment
Authors: Davies I R L , Adams M , Beatie A , Boyles S K
Published: December 2015

We found a surprisingly high incidence of tritan-like (blue - yellow) symptoms in population samples from southern Africa. However, these results could be false positives because the test (The City University Colour Vision Test: CUCVT) was not administered under the prescribed illuminant (CIE Illuminant C). We therefore assessed the robustness of the test in order to estimate the true tritan rates in our data and to assess its usefulness under field conditions. First, we administered the test to normal young people under three illuminances (600, 300, and 150 lux), and three spectral distributions (C, reduced short wavelength, and reduced long wavelength). In addition, two viewing distances were used: the standard (14 inches) and double the standard. At the normal viewing distance, no errors were induced by reducing the illuminance or by changing the spectral distribution. However, at 28 inches, both illuminance and spectral changes induced tritan errors. Second, we assessed a sample of old people (over 65), first, under the prevailing illuminant; second, under Illuminant C; and third, under Illuminant C with increased intensity. In all cases, the old people in the sample made large numbers of tritan responses, but the frequency of tritan errors decreased under Illuminant C and decreased even more under enhanced illuminance. Thus, for normal young people at the prescribed distance the test is robust, but at increased distance the test is vulnerable to deviations from the prescribed illuminant. Further, older people can present as having tritan-like problems unless the illuminance is increased above the standard.
What is the Transition Point between Static and Dynamic Motion Aftereffects?
Authors: Ashida H , Verstraten F A J , Nishida S
Published: December 2015

The motion aftereffect (MAE) measured with a dynamic test pattern (eg a counterphase-flickering grating) is distinguishable by a number of properties from the classical MAE obtained with a static test pattern. For a dynamic MAE, however, it is not sufficient simply to introduce dynamic properties into the test pattern. In two experiments we attempted to determine the transition point in the temporal-frequency domain at which a dynamic MAE becomes distinguishable from the static MAE. First, we examined the interocular transfer (IOT) of the MAE with conventional first-order (luminance) gratings. The amount of IOT increased with temporal frequency, and was almost complete at 1 Hz and above. In addition, the IOT of a dynamic MAE shows a drastic reduction in the peripheral visual field, possibly reflecting difficulties in feature tracking or the loss of involuntary attention. Second, we examined the MAE with second-order motion as the adaptation stimulus (contrast modulation of two-dimensional static noise). Under these conditions, similar results were obtained for first-order and second-order test gratings: MAE was not observed at low temporal frequencies and a substantial MAE was observed only at 1 Hz and above. The results agree with recent findings which showed a gradual loss of spatial-frequency selectivity with increasing temporal frequency of the test pattern (Mareschal et al, 1997 Vision Research37 1755 – 1759). The present results support the idea that two mechanisms underlie the different kinds of MAE: a low-level mechanism responsible for the MAE observed at low temporal frequencies, and a high-level mechanism operating predominantly at high temporal frequencies with a transition point at about 1 Hz.
Masking Effects Induced by Perceptual Colour Components
Authors: Bedat L , Saadane A , Barba D
Published: December 2015

Signals from the three types of cone photoreceptors are generally thought to be combined into two opponent-colour components and an achromatic component. Here we have chosen the cardinal directions Ach (achromatic axis), Cr1 (reddish - greenish axis) and Cr2 (bluish - yellowish axis) defined by Krauskopf to build a colour image coding scheme based on features of the human visual system. In order to design the optimal perceptual quantisers, we studied the perception of colour at different spatial frequencies, measuring the effect of a colour masking signal on the perception threshold for achromatic or chromatic sine-wave gratings. Three cases were considered to describe these interactions.

First, we measured the masking effect of each colour component by itself. For the two chromatic components, we observed a pedestal effect for low masking contrasts and threshold rises for higher masking contrasts. Second, we studied interactions between the two colour axes. With colour masking signals composed of Cr1 and Cr2 components, thresholds for Cr1 stimuli dropped at low contrasts of the Cr2 component of the masking signal. This effect disappeared for high Cr2 masking contrasts. This was true at all contrasts of the Cr1 component of the masking signal. Thresholds for Cr2 stimuli were not modified by Cr1 masking. Finally, interactions between the chromatic and achromatic components were more complex. We observed a strong modification of the achromatic contrast-sensitivity function when a Cr1 masking sine-wave grating was added. When the achromatic frequency was lower than the colour masking frequency, the threshold rose. However, when the achromatic frequency was higher than the colour masking frequency, there was a significant pedestal effect. The global effect is a shift of the achromatic contrast sensitivity function. With a Cr2 masking signal, no significant modification of the achromatic contrast-sensitivity function was observed.
Noise Effects in Perceptual Models
Authors: Wickens T D , Olzak L A
Published: December 2015

In studies of visual perception performance is often measured by statistics that are ratios of a perceptual magnitude to its intrinsic variability, most commonly the signal-detection measure d'=delta sigma. Many models for visual phenomena treat the variability sigma as a constant and describe performance exclusively by delta. However, in models for the combination of stimulus attributes, the combination process affects both terms, and an observed d' reflects both delta and sigma. For example, we have shown that masking and configural effects with sinusoidal plaids can be at least partially interpreted as noise effects.

We have developed methods to analyse these effects. Through a series of concurrent-response experiments using grating stimuli, some reported at earlier ECVP meetings, we have measured the form and magnitude of the noise sources. Our analysis allows us to model the way that primitive Fourier components (spatial frequency by orientation) are integrated to form second-order or third-order combinations (eg spatial frequency pooled over orientation).
Discrimination and Detection of Changes in the Velocity of Visual Motion: Effect of Aperture Size
Authors: Likova L , Dimitrov G , Mateeff S , Hohnsbein J
Published: December 2015

Recently Mateeff and Hohnsbein (1996 Vision Research36 2873 – 2882) showed that the critical frequency for detection of modulated velocity of motion was invariant with respect to the viewing distance. The critical frequency was a function of the ratio between the mean velocity and the size of the aperture through which the motion was observed. In the present study we examined whether the velocity/aperture ratio affects velocity discrimination and detection of single velocity changes. Six subjects observed a random-dot pattern that could move within an invisible square aperture. In the discrimination task, two motions of 250 ms duration with slightly different velocities were presented with a 1 s interval between them. The subject had to report which of the motions was faster. In the change-detection task the same two motions were presented without an interval between them and the subject had to report whether the change was from a low to a high velocity or vice versa. Mean velocities of 8 and 64 deg s−1 and aperture sizes of 10 and 40 deg were employed in both tasks. Weber fractions were determined by the method of constant stimuli. The discrimination accuracy was not affected by aperture size at either mean velocity. The detection task was also unaffected by aperture size at 8 deg s−1. However, at 64 deg s−1 decreasing aperture size impaired the Weber fractions by a factor of about three. We suggest that the decrease of the lifetime of the dots of the pattern at high velocities and small apertures may be the critical factor for the impairment of the change detection. This factor is of less importance for the velocity discrimination task.
The Effects of Chromatic Conditions for Transparency
Authors: Ito Y , Uchikawa K
Published: December 2015

When two rectangles overlap crosswise like a letter X, we perceive them as transparent in certain conditions. Perception of transparency is constrained by the luminance and chromatic relations of the overlapping and non-overlapping areas and the background, but not necessarily by their physical transmission profiles. In this study, we investigated the relation between luminance and chromatic mechanisms for the percept. Using two crossed rectangles (2 deg × 8 deg), we measured the ranges of chromaticity and luminance of the overlapping area (2 deg × 2 deg) that allowed the percept of transparent layers.

We found that the ‘transparency’ luminance range was shifted by the intensity of the background. When the two rectangles were isochromatic and isoluminant (10 cd m−2) on a dark background, the overlapping area had to be around 0.3 log units brighter than the rectangles. When the background was D65 white (60 cd m−2), the luminance range for transparency was lower than the luminance of the rectangles. When the two rectangles differed in colour, however, the ‘transparency’ range of the overlapping area varied around the luminance of the rectangles. Transparency was perceived best when the colour of the overlapping area was on the additive mixture chromaticity line of the two rectangles. We discuss how the background influences the appearance of transparency and how the chromatic mechanism is effective in transparency.
Elementary Colour Perception in Hue Judgment
Authors: Sakata K
Published: December 2015

The ratios of psychological elementary colours perceived in chromatic colour papers with a variety of hues were measured by a paired comparison method. Stimulus sets were composed of two colour papers with hues between two of four elementary colours, but on either side of one elementary colour. Five students observed stimulus sets comprising two colour papers under approximately 1000 lux D65 illumination, and they chose the one which caused the stronger elementary colour percept. The results showed that the sum of the ratios of two elementary colours in a colour whose hue is located between them was less than 100%. This result was the same for every hue condition.

This shows that observers underestimated psychological elementary colours, consistent with previous studies (Sakata, 1996 Perception25 Supplement, 100) which showed that the sum of black and white was less than 100% in the perception of greys. If elementary colours are processed simultaneously from the whole colour percept, their sum would be 100%. The result of these experiments strongly shows that psychological elementary colours are processed independently from the inputs of the three cone systems and the rod system, and they are integrated to form the whole colour percept after each process.
From Simple to Complex or Vice Versa: Sequence Effects on the Detection of Embedded Figures
Authors: Ludwig I
Published: December 2015

In a series of studies concerning part - whole perception we have investigated effects of perceptual learning, and of systematically varied presentation orders on the detection of embedded figures. In the present study the effects of increasing and decreasing complexity orders on detection performance are reported.

Stimuli were 170 pairs of whole and part figures. Half of them were positive items, ie the searched part was contained in the (simultaneously presented) whole. The other half were negative items, ie the searched part was not contained in the whole. The difficulty of all figure pairs was determined from earlier data. On the basis of these difficulty parameters three presentation orders were created: (1) increasing difficulty (from simple to complex), (2) decreasing difficulty (from complex to simple), and (3) randomised order. Sixty subjects performed each of these tasks in three sessions separated by one week. Effects of practice and samples were balanced by permutating the order of the three tasks. The reaction times and error rates for all presentations were registered.

The results showed marked differences between the three tasks: The lowest mean reaction time was obtained in the presentation order with increasing difficulty. Error rate, on the other hand, was lowest in the decreasing-difficulty presentation order. For the random-order presentation error rate and reaction time was in between the other orders. Furthermore, differences in benefit from practice were observed between the three orders of difficulty. Results are discussed with respect to the question of how efficient search strategies may be learned and whether one can learn to bend the rules of Gestalt organisation if required.
Neural Modelling of Disparity Influences in Optic-Flow Processing
Authors: Lappe M , Grigo A
Published: December 2015

Last year, we reported a new disparity effect on optic-flow perception. It occurs when a flow field of radially moving random dots is transparently superimposed by a unidirectional motion pattern. Subjects asked to locate the centre of the expansion pattern perceive it shifted away from its true position (Duffy and Wurtz, 1993 Vision Research33 1481 – 1490). The magnitude of this shift depends on binocular disparity between the two superimposed patterns: presenting the translation in front of the expansion causes a massive reduction of the shift, presenting it behind the expansion yields only a slight reduction.

To determine possible mechanisms for this interaction, we tried to model our results by modifying a previously developed neural model for optic-flow processing (Lappe and Rauschecker, 1995 Vision Research35 1619 – 1631). We compared three possible mechanisms of disparity influence. The first was based on the assumption that the absolute distance of the stimuli from the observer was known. The second was based on a disparity-based spatial integration, as found in ‘tuned’ neurons in primate visual area MT. Neither could account for our data.

Finally we included a disparity-dependent weighting function in the model, assuming that distant flow vectors contribute more strongly than near ones to the processing of optic flow. This reproduced the experimental findings. Surprisingly, the optimal weighting function was similar to the disparity-dependence of the so-called ‘far-neurons’ in area MT. We conclude that the visual system uses disparity in optic-flow processing by putting special emphasis on distant flow vectors, and that far-neurons in MT might serve as a neuronal substrate for this.
Brightness Filling-in of Natural Images
Authors: Elder J H
Published: December 2015

There is both psychophysical and physiological evidence that the perception of brightness variations in an image may be the result of a filling-in process in which the luminance signal is encoded only at image contours and is then neurally diffused to form representations of surface brightness. Despite this evidence, the filling-in hypothesis remains controversial. One problem is that in previous experiments highly simplified synthetic stimuli have been used; it is unclear whether brightness filling-in is feasible for complex natural images containing shading, shadows, and focal blur.

To address this question, we present a computational model for brightness filling-in and results of experiments which test the model on a large and diverse set of natural images. The model is based on a scale-space method for edge detection which computes a contour code consisting of estimates of position, brightness, contrast, and blur at each edge point in an image (Elder and Zucker, 1996, paper presented at ECCV). This representation is then inverted by a diffusion-based filling-in algorithm which reconstructs an estimate of the original image. Psychophysical assessment of results shows that while filling-in of brightness alone leads to significant artifact, parallel filling-in of both brightness and blur produces perceptually accurate reconstructions. The temporal dynamics of blur reconstruction predicted by the model are consistent with psychophysical studies of blur perception (Westheimer, 1991 Journal of the Optical Society of America A8 681 – 685). These results suggest that a scale-adaptive contour representation can in principle capture the information needed for the perceptually accurate filling-in of complex natural images.
Perceived Stability of a Scene Depending on the Gain of Motion Parallax
Authors: Runde D , Ruschin D , Feddersen E
Published: December 2015

An observer moving in a natural environment is usually able to separate the constant changes of his retinal images in such a way that he perceives the environment and the changes of his observation point independently. The necessary and sufficient conditions to perceive a stable environment in spite of the retinal change produced by self-motion are, however, as yet unknown.

We found that under certain conditions a scene that changes during observer motion can appear more stable than a rigid one. In our experiment a scene consisting of a number of LEDs distributed in a dark room was visible through a window. A mechanical device controlled by a head-tracker was used to move the LEDs during head motion to either reduce or enhance motion parallax by a predefined gain factor. The subjects rated the scene with respect to different attributes including apparent deformation and degree of motion perceived. They were also asked to adjust the parallax gain to the value of greatest apparent stability of the scene. Monocular as well as binocular trials were conducted and different fixation points were employed. The result was a general tendency in all conditions to perceive scene motion when the scene was in fact rigid and to perceive the greatest stability when the scene was distorted in such a way as to produce reduced motion parallax.
Performance-Based Measures of Transparency in Locally-Balanced Motions
Authors: Wishart K A , Braddick O J
Published: December 2015

In order to perceive transparent motions the visual system must compute and represent two (or more) motion signals at some level of spatial representation. We have developed performance-based measures of transparency, based on the precision of a joint directional judgment of two superimposed global motions in a random-dot kinematogram (RDK). Qian, Andersen, and Adelson (1994 Journal of Neuroscience14 7357 – 7366) have reported that transparent motion is not perceived in RDK stimuli if each leftward moving dot is paired with a rightward moving dot that it meets at the midpoint of a short trajectory (locally balanced). Using performance-based measures of transparency we investigated the conditions for the occurrence of transparency in locally balanced stimuli. Using stimuli with the same parameters as Qian et al we found that the critical distance that the dots must travel to abolish transparency was 0.2 deg or less. Offsetting one set of dots, orthogonally to its motion direction, by 0.3 deg or more allowed for transparency-based judgments with the same degree of accuracy as for random distribution of the two motion directions. These values differ slightly from those reported by Qian et al (0.4 deg and 0.2 deg respectively), perhaps because Qian et al depended solely on subjective reports of transparency. The data suggest that different processes may be involved in detecting transparency when the trajectories are extended and when they are offset. When the trajectory length was varied, transparency-based judgments were possible when each dot pair had an average separation of approximately 0.1 deg over the course of their lifetimes. For the offset stimuli, transparency-based judgments required the dots to have an average separation of approximately 0.2,deg. However, our data are consistent with transparent motion signals not being represented at the most local levels of motion analysis, as proposed by Qian et al.
A Recurrent Network for Tilt Illusions Based on Retinotopic Coding
Authors: Sakai K , Tanaka S
Published: December 2015

We propose a computational model for the mechanism underlying the perception of tilt illusions. Several studies have suggested that cortical mechanisms based on orientation-dependent lateral connections account for tilt illusions such as the Zöllner and Delboeuf illusions [1970 Nature (London)228 37 – 39; 1989 Biological Cybernetics58 35 – 49]. In these studies, it has been proposed that two intersecting bars evoke strong activation of V1 cells tuned to the orientations of the bars, and that, because of lateral interactions, cells tuned to orientations several degrees off those of the bars are the most strongly activated of all orientation-tuned cells. Thus the peaks in the neural orientation profiles are shifted by several degrees compared with the actual orientations of the bars. We investigate the retinotopic distribution of V1 cell activities in simulations of a large-scale recurrent network based on V1 anatomical connectivity, and show that the orientation profile of cell activities is significantly different for different retinotopic positions. For most positions, a single peak appears in the orientation profile. Taking into account population coding of the cells, we show that the bars will be tilted in a stimulus-configuration-dependent manner. The network shows angle expansion at crossed (X-shaped), branched (Y-shaped), or bent (L-shaped) intersections if the angle between the two bars is acute, and contraction if the angle is obtuse. The maximum expansion appears between 15° and 30°. These results are in good agreement with psychophysical observations on tilt illusions, including the Zöllner, Delboeuf, and Poggendorff illusions.
Non-Linear Interactions in Cortical Responses to Bilateral vs Unilateral Visual Field Stimulation
Authors: Ahlfors S P , Foxe J J , Ilmoniemi R J , Simpson G V
Published: December 2015

Stimuli in different parts of the visual field can be perceived as independent entities and as conjoined wholes. It is of interest to determine whether there are cortical representations of the left and right hemifields which remain as independent entities when both hemifields are stimulated simultaneously and/or whether they interact to form a conjoined representation. We examined whether cortical processing of visual stimuli depends on whether they occur in isolation in one hemifield (unilaterally) or simultaneously in both hemifields (bilaterally).

Visual evoked potentials of six normal subjects were recorded from 128 scalp sites. Wedge-shaped chequerboard stimuli, extending 1 – 4 deg eccentricity, were presented to quadrants of the visual field. Stimulus duration was 250 ms; the stimulus onset asynchrony was random, 500 – 750 ms. The evoked potentials revealed multiple peaks of activity with different surface topography. Prominent deflections occurred around 80, 120 – 180, and 230 ms.

The response to bilateral stimuli was compared with the sum of the responses to unilateral stimuli. On the basis of the multichannel recordings, nonlinear interactions were characterised as either (a) modulations (same generators, but different amplitude) or (b) interactions originating from different generators. Modulation occurred at 230 ms, the response being suppressed for the bilateral case. At 120 – 180 ms, the field patterns suggested that at least some of the sources of the interaction effect are different from the source of the bilateral response. Underlying generators of the evoked responses and the interaction effects were further explored with the use of an equivalent current dipole model.
A Simplified, Low-Level Account of the Bistable Perception Yielded by Objects Drifting toward and Past One Another
Authors: Gorea A , Labarre J
Published: December 2015

It is conjectured that the relative frequency of streaming vs bouncing percepts yielded by two objects drifting toward and past one another reflects the relative ‘strength’ of the underlying ‘continuity’ and ‘discontinuity’ motion vectors in the stimulus. The frequency of the two perceptual states should then be predictable from the relative effective contrast of these components which, in turn, is governed by the relative speed sensitivity and spatiotemporal integration range of the underlying sensors. Three distinct experiments were run to test this hypothesis. Percentage bouncing (%B) was assessed with two equal variance Gaussian blobs of equal contrasts as a function of their trajectory length at a given speed (experiment 1), and as a function of their relative physical contrast at a given speed and trajectory length (experiment 2; this latter manipulation was meant to control the relative input to the two competing motion sensors). Apparent contrast of a single drifting blob was independently measured as a function of its speed and trajectory length (experiment 3). The %B obtained in experiment 1 was inferred from experiments 2 and 3 by means of a transformation relating relative apparent contrast (at a given trajectory length) and %B (for that same trajectory) as a function of the relative physical contrast of the continuity and discontinuity motion components. This transformation translates the effect of trajectory length (and speed) in terms of an objective contrast effect. Measured and predicted %B in experiment 1 (ie as a function of trajectory length at a given speed) are in rather good agreement. The phenomenology of simple (and perhaps more complex) Ternus-type stimuli can thus be derived from the sensitivity/apparent contrast and the spatiotemporal integration characteristics of the underlying motion sensors.
Continuous Assessment of Presence in Stereoscopic Displays
Authors: IJsselsteijn W A , Freeman J , Avons S E , Davidoff J , de Ridder H , Hamberg R
Published: December 2015

Presence, a sense of ‘being there’ evoked by a display, can be regarded as a concept of central importance in the evaluation of broadcasting and entertainment services in general and virtual reality applications in particular. Subjective methods of assessing presence that have either been used or proposed to date do not provide a measure of temporal variation in observers' presence. To overcome this limitation, we have applied the method of continuous assessment (ITU-R, BT 500-7) to the measurement of presence. Thirty observers (twelve at IPO, eighteen at UoE) with normal or corrected-to-normal vision and good stereo-acuity viewed a stereoscopic film. While watching, observers were asked to continuously rate their perceptions of depth, naturalness and presence. The stimulus material varied considerably in the amount and strength of the visual cues presented over time. This enabled us to investigate whether the extent of sensory information presented to an observer was a determinant of presence, as proposed by Sheridan [1992 Presence: Teleoperators and Virtual Environments1(1) 120 – 125].

The results, which were very similar across two independent laboratories, suggested that continuous assessment provides a promising methodology for the subjective assessment of temporal variation in the observer's sense of presence. Further, increasing the extent of sensory information presented to an observer may enhance the sense of presence, provided the depth cues introduced are consistent and within natural bounds. [ Note: First and second author in arbitrary order.]
Luminance and Colour Cues in Perceptual Transparency
Authors: Goda N , Takahashi S , Ejima Y
Published: December 2015

A series of experiments was carried out to determine the dependence of the perception of transparency on colour relationships in a four-region pattern comprising two non-overlapping regions, an overlapping region, and a background. The proportion of trials where the pattern was perceived as transparent, and the relative layering in depth of the two perceived surfaces, were determined as a function of the luminance of colour of the overlapping region while those of the non-overlapping regions were kept constant. It was found that perceptual transparency could arise from displays where the four regions were isoluminant but different in colour. The function obtained for isoluminant colour-varying patterns was quite similar to that obtained for isochromatic luminance-varying patterns. In both types of patterns, the background condition strongly affected perceptual transparency and perceived depth order in a similar way. These results suggest that perceptual transparency for both the isochromatic and the isoluminant pattern was specified in a common manner by contrast relations in a certain region of the image, and was not always constrained by the physics of transparency. The dependence of perceptual transparency on the contrast relation may be accounted for by mechanisms incorporating the filling-in process associated with antagonistic luminance and opponent-colour responses.
Saccadic and Electrodermal Responses to Television Violence
Authors: Kalamas A D , Gruber A L
Published: December 2015

The concern for TV violence has not been followed by any way to discriminate one type of violent show from another. To ascertain which stimulus elements of TV violence elicit emotional responses we attempted to correlate the electrodermal response with the general saccadic responses.

The simultaneous electrodermal and saccadic responses of children watching violent TV shows were analysed. The saccadic movements were derived from the corneal reflection, computer enhanced, and superimposed on the violent video. Allowing for the individualised response latency, the analog recording of the electrodermal response (as well as an audiometric response) was also superimposed. We found that most electrodermal responses (41%) actually occurred to nonviolent but frightening stimuli (eg villain's face; grave). Few responses (8%) occurred to actual violent stimuli (eg stabbing). Auditory stimuli (eg gun shot) elicited intense electrodermal responses, the correlation being positively associated with the audiometric level. Statistical evaluation by Anova confirmed the significance of the above findings at p<0.01.

We conclude that (1) actual violence may provoke less electrodermal responses than implied violence; and (2) sound potentiates the electrodermal response to elements of violence. We hope that this computer superimposition technique will make it easier to discriminate one violent show from another.
Effects of Stereo and Motion Manipulations on Measured Presence in Stereoscopic Displays
Authors: Freeman J , Avons S E , Davidoff J , Pearson D E
Published: December 2015

Methods of assessing presence, a sense of ‘being there’ within a displayed virtual environment, include post-test subjective measures, discrimination tests, and monitoring reflexive responses. Each is limited—either they do not provide a measure of temporal variation, are not feasible with current display technology or are overly content-specific.

A measure of presence derived from the method of continuous evaluation (ITU-R, Recommendation BT.500-7, revised, “Methodology for the subjective assessment of the quality of television pictures”, 1995) has been used to overcome these limitations. The results of two experiments are presented. Those of the first experiment established that the methodology is usable under the optimal viewing conditions for the 20 inch stereoscopic TV display upon which our stimuli were presented. It compares within-subject variation on continuous TV picture quality ratings under two viewing conditions—at six picture heights in the light (standard for quality evaluations) and at two picture heights in the dark (optimal stereo TV viewing). The second experiment investigated the effects of manipulations of the visual parameters of stereo, scene motion, and observer-based motion on participants' presence evaluations within edited sections of a stereoscopic film. The results provide support for theories predicting that the extent of sensory information available to a participant is one of the factors determining presence.
Vernier Image Processing: Model and Experiments
Authors: Chihman A V , Chihman V N , Shelepin Y E
Published: December 2015

Earlier we proposed a model for visual processing of the optical image of Vernier targets (1996 Perception25 Supplement, 115 – 116) based on Fourier analysis of the image. Our model comprises blurring of the thin Vernier bars by the optical point-spread function followed by calculation of the two-dimensional Fourier spectrum. In our model the processing area for Fourier analysis (the receptive field size) is 5 min arc. For a Vernier target, the contrast energy in the low-spatial-frequency range is different in different orientations, and magnification of the Vernier shift changes the orientation of the oblique Fourier components. To test the model, we carried out experiments in which the stimuli were Vernier lines with additional line distractors orthogonal to the orientation of the oblique Fourier components. Thresholds for detecting Vernier displacements were determined by a 2AFC paradigm and compared with model predictions. The results are consistent with our modelling of Vernier performance as a measurement of oblique components of the 2-D Fourier spectrum.
Active Control of Heading and Depth
Authors: Rushton S K , Harris J M
Published: December 2015

The addition of depth information has been reported to improve performance on a locomotor heading judgement task in a cloud of dots. Van den Berg and Brenner [1994 Nature (London)371 700 – 702] used stereoscopic disparity as a depth cue. Coding dots with disparity provides both cues for depth order and for identity between successive frames (each dot may be disambiguated from its 2-D neighbours by its 3-D position). Here, we attempted to determine the relative importance of depth and identity, by dissociating disparity-depth and disparity-identity.

We employed an active steering task that dissociates gaze and heading direction. Participants underwent simulated (18 Hz) locomotion through a cloud (100 dots, lifetime 440 ms) at 2 m s−1 for 8 s from an initial trajectory 15 deg (±20%) to the left or to the right of a target tower. Heading was adjusted with a joystick and simulated gaze was continuously computer adjusted so as to keep the tower centred on the projected display (20 deg horizontal). Mean unsigned heading error (gaze-heading angle) over the time course provided a performance measure.

Four types of stimuli were used: (i) veridical disparity depth; (ii) non-veridical disparity (disparity and associated motion-in-depth was randomly assigned to each point in the display); (iii) the left half image of the previous condition to both eyes; (iv) synoptic images (no disparity). No marked advantage was observed when the display contained veridical disparity, and non-veridical disparity produced only a minor impairment in performance in a subset of conditions. We suggest that depth information per se may not be utilised in the perception and control of heading.