bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral properties of the temporal evolution of brain network structure
Authors: Wang Rong , Zhang Zhen-Zhen , Ma Jun , Yang Yong , Lin Pan , Wu Ying
Published: December 2015

The temporal evolution properties of the brain network are crucial for complex brain processes. In this paper, we investigate the differences in the dynamic brain network during resting and visual stimulation states in a task-positive subnetwork, task-negative subnetwork, and whole-brain network. The dynamic brain network is first constructed from human functional magnetic resonance imaging data based on the sliding window method, and then the eigenvalues corresponding to the network are calculated. We use eigenvalue analysis to analyze the global properties of eigenvalues and the random matrix theory (RMT) method to measure the local properties. For global properties, the shifting of the eigenvalue distribution and the decrease in the largest eigenvalue are linked to visual stimulation in all networks. For local properties, the short-range correlation in eigenvalues as measured by the nearest neighbor spacing distribution is not always sensitive to visual stimulation. However, the long-range correlation in eigenvalues as evaluated by spectral rigidity and number variance not only predicts the universal behavior of the dynamic brain network but also suggests non-consistent changes in different networks. These results demonstrate that the dynamic brain network is more random for the task-positive subnetwork and whole-brain network under visual stimulation but is more regular for the task-negative subnetwork. Our findings provide deeper insight into the importance of spectral properties in the functional brain network, especially the incomparable role of RMT in revealing the intrinsic properties of complex systems.
Carrier leakage into the continuum in diagonal GaAs/Al0.15GaAs terahertz quantum cascade lasers
Authors: Albo Asaf , Hu Qing
Published: December 2015

The maximum operating temperature reported so far for THz-QCLs is ∼200 K. With the well-known degradation mechanism of thermally activated LO-phonon scattering, one straightforward strategy to improve their temperature performances is the use of diagonal structures in which the upper-to-lower state scattering time is lengthened. However, the effectiveness of this method for achieving room temperature operation remains to be demonstrated. Here, we studied the temperature degradation of highly diagonal GaAs/Al0.15GaAs THz-QCLs. By analyzing their output power dependence on temperature, we identified the physical mechanism that limits their performance to be thermally activated leakage into the continuum, as evidenced by the large activation energy of ∼80 meV extracted from the Arrhenius plot. This observation is further supported by a careful analysis of current-voltage characteristics, especially in regions of high biases. In order to significantly improve the temperature performances of diagonal THz-QCLs, this leakage should be eliminated.
Role of low melting point element Ga in pronounced β-relaxation behaviors in LaGa-based metallic glasses
Authors: Xue R. J. , Zhao L. Z. , Zhang B. , Bai H. Y. , Wang W. H. , Pan M. X.
Published: December 2015

In contrast to most metallic glasses (MGs) which exhibit weak β-relaxation peak in their dynamic mechanical spectra, the LaGa-based MGs we report here show a distinct β-relaxation peak with the ratio of β/α-relaxation peaks up to ∼0.32 in the mechanical relaxation measurements. Moreover, the β-relaxation behavior can be tuned by modification of the chemical composition and the concentration of flow units. The effects of gallium and structural origin of the β-relaxation in the MGs have been discussed. The LaGa-based MGs with pronounced slow β-relaxation could provide a model system to investigate some underlying issues of the relaxation and plastic mechanism of MGs.
Optical trap-cavity ringdown spectroscopy as a single-aerosol-particle-scope
Authors: Wang Chuji , Gong Zhiyong , Pan Yong-Le , Videen Gorden
Published: December 2015

We report a single-aerosol-particle-scope using an optical trapping-cavity ringdown spectroscopy technique. The scope can not only view physical parameters such as size, motion, and restoring force constant of a single aerosol particle trapped in air but also display time-, particle-, or wavelength-resolved chemical properties such as single aerosol particle extinction. We demonstrate the scope by trapping and walking single carbon-nanotube particles of ∼50 μm in size and viewing those properties via changes of ringdown time. This single-aerosol-particle-scope offers a powerful tool to study both physical and chemical properties as well as their evolving dynamics.
Modified quadrupole mass analyzer RGA-100 for beam plasma research in forevacuum pressure range
Authors: Zolotukhin D. B. , Tyunkov A. V. , Yushkov Yu. G. , Oks E. M.
Published: December 2015

The industrial quadrupole RGA-100 residual gas analyzer was modified for the research of electron beam-generated plasma at forevacuum pressure range. The standard ionizer of the RGA-100 was replaced by three electrode extracting unit. We made the optimization of operation parameters in order to provide the maximum values of measured currents of any ion species. The modified analyzer was successfully tested with beam plasma of argon, nitrogen, oxygen, and hydrocarbons.
Current fluctuation of electron and hole carriers in multilayer WSe2 field effect transistors
Authors: Ko Seung-Pil , Shin Jong Mok , Kim Yong Jin , Jang Ho-Kyun , Jin Jun Eon , Shin Minju , Kim Young Keun , Kim Gyu-Tae
Published: December 2015

Two-dimensional materials have outstanding scalability due to their structural and electrical properties for the logic devices. Here, we report the current fluctuation in multilayer WSe2 field effect transistors (FETs). In order to demonstrate the impact on carrier types, n-type and p-type WSe2 FETs are fabricated with different work function metals. Each device has similar electrical characteristics except for the threshold voltage. In the low frequency noise analysis, drain current power spectral density (SI) is inversely proportional to frequency, indicating typical 1/f noise behaviors. The curves of the normalized drain current power spectral density (NSI) as a function of drain current at the 10 Hz of frequency indicate that our devices follow the carrier number fluctuation with correlated mobility fluctuation model. This means that current fluctuation depends on the trapping-detrapping motion of the charge carriers near the channel interface. No significant difference is observed in the current fluctuation according to the charge carrier type, electrons and holes that occurred in the junction and channel region.
Shift-Invariance of Pattern Recognition in the Visual Field?
Authors: Jüttner M , Rentschler I , Unzicker A
Published: December 2015

The classification behaviour of human observers with respect to compound Gabor signals was tested at foveal and extrafoveal retinal positions. Classification performance was analysed in terms of a probabilistic classification model recently proposed by Rentschler, Jüttner, and Caelli (1994 Vision Research34 669 – 687). The analysis allows inferences about structure and dimensionality of the individual internal representations underlying the classification task and their temporal evolution during the learning process.

With this technique it was found that the internal representations of direct and eccentric viewing are intrinsically incommensurable in the sense that extrafoveal pattern representations are characterised by a lower perceptual dimension in feature space relative to the corresponding physical input signals, whereas foveal representations are not (Jüttner and Rentschler, 1996 Vision Research in press).

We then addressed the question to what extent observers are capable of generalising class concepts that have been acquired at one particular retinal location to other retinal sites. We found partial generalisation with respect to spatial translation across the visual field. Moreover, there is, in the case of extrafoveal learning, a distinct asymmetry in performance with respect to the visual hemifield in which the signals were originally learned. The latter finding can be related to functional hemispheric specialisation in pattern learning and recognition.
The Cost of Spatial-Attentional Precuing for the Perception of Spatially Quantised Visual Images
Authors: Bachmann T , Kahusk N
Published: December 2015

We have investigated the effects of selective attentional spatial precuing and levels of quantisation on the perception of spatially quantised visual images of faces. The coarseness (number of square-shaped isoluminant pixels per image) of quantisation of eight alternative facial images was systematically varied from 9 to 16 pixels per image (along the horizontal dimension). 16 grey levels of the image were used; both local and global precues that designated the position of the expected stimulus-image were employed. Precue-to-target stimulus onset asynchrony was equal to 120 ms, and target image duration was varied between 28 and 76 ms. Targets were exposed at one of four quasiperipheral positions, either after the precue or without the precue. It was found that precues had a facilitative effect on identification with fine levels of quantisation or with original images, but this changed to a cost for precuing with coarse levels (9 – 11 pixels per image). A dramatic drop in overall identification efficiency with moving from 12 pixels per image to 11 pixels per image condition was also revealed. The results are interpreted as supporting (1) the idea of gradual coarse-to-detailed presetting of spatial filters by the precue, and (2) the existence of some implicit relational metrics of the critical facial identity cues that can be ‘catastrophically’ disintegrated by a small but critical change in quantisation value.
Information Processing by Cortical Subthreshold and Suprathreshold Activity
Authors: Kirschfeld K
Published: December 2015

The point spread in visual cortex (the area of the cortex activated by a minimal visual stimulus) measured by optical recording is very much larger than the point spread measured by spike activity; it develops more slowly and lasts for a longer time (Grinvald et al, 1994 Journal of Neuroscience14 2545 – 2568). It is considered to reflect mainly subthreshold activity of neuronal arborisations. It is shown how this subthreshold activity may contribute to information processing: the sudden onset of a visual cue is known to trigger visual attention, which then enhances visual processing in the zone near the cue. The most likely physiological basis of attention is a subthreshold wave of negativity. This wave develops more slowly than signals leading to perception, and spreads outward from the cortical region associated with the cue, into regions associated with other retinal sites. Such slow potentials are known to influence behavioural responses: negativity reduces neuronal thresholds and hence shortens response latencies (Birbaumer et al, 1990 Physiological Reviews70 1 – 41). As examples, the line motion illusion and the generation of express saccades are discussed on the basis of this concept.
Perceptual Grouping of Dots in Noise
Authors: Tripathy S P , Mussap A J , Barlow H B
Published: December 2015

Observers were asked to report the orientation of alignment (horizontal vs vertical) of a group of target dots embedded in randomly placed noise dots. For this perceptual grouping task, sensitivity (the number of noise dots giving 75% correct performance) was measured as a function of the number of target dots. We report surprising tolerance to positional jitter of target dots, with far less tolerance exhibited to alignment jitter than separation jitter. In this respect, perceptual grouping in our task resembles line detection. In a further series of experiments the target was produced by positional shifts of an appropriate number of noise dots, rather than by addition of target dots to the noise dots (this procedure minimised density cues). The improvement in sensitivity with increasing number of target dots was found to be linear (when plotted on log — log axes) with a slope of 0.5, as opposed to bilinear [cf B Moulden, 1994, in Higher-Order Processing in the Visual System Eds G R Bock, J A Goode (Chichester: John Wiley) pp 170 – 192]. This linearity suggests that, over the range of target dot numbers tested, a single mechanism may be sufficient to explain performance.
The Relation between Target Localisation in Strings of Letters, Contrast, and Refresh Rate
Authors: van Kruysbergen N , Hendriks A
Published: December 2015

Jacobs (1987, Vision Research27 1953 – 1966) obtained psychometric curves to investigate the relationship between target localisability and the latency of primary saccades. These psychometric curves show correct scores as a function of string length, target location, and target-background similarity. They were obtained with white text on a black background. We repeated this experiment, with a few extensions. First, we did not only use white text on a black background, but we used black text on a white background as well. The stimuli were also presented with the use of isoluminant colours. Second, we varied the refresh rate of the screen (from 60 Hz up to 150 Hz). Third, we used two different instructions for the subjects: in one the subject was instructed to indicate the position of the target relative to the beginning of the string; in the other the position of the target relative to the fixation point. When the string was presented to the right of the fixation point, the results of these two instructions were predicted to be similar; in the other case the results might be different. The results of our experiments show a clear effect of the amount of contrast between text and background, and an influence of the screen refresh rate. We also found that the results differ depending on the instruction used. Sadly, we were not able to replicate all of the results of the original experiment by Jacobs.
Smooth Pursuit of a Partially Occluded Object
Authors: Stone L S , Lorenceau J , Beutter B R
Published: December 2015

There has long been qualitative evidence that humans can pursue an object defined only by the motion of its parts (eg Steinbach, 1976 Vision Research16 1371 – 1375). We explored this quantitatively using an occluded diamond stimulus (Lorenceau and Shiffrar, 1992 Vision Research32 263 – 275). Four subjects (one naive) tracked a line-figure diamond moving along an elliptical path (0.9 Hz) either clockwise (CW) or counterclockwise (CCW) behind either an X-shaped aperture (CROSS) or two vertical rectangular apertures (BARS), which obscured the corners. Although the stimulus consisted of only four line segments (108 cd m−2), moving within a visible aperture (0.2 cd m−2) behind a foreground (38 cd m−2), it is largely perceived as a coherently moving diamond. The intersaccadic portions of eye-position traces were fitted with sinusoids. All subjects tracked object motion with considerable temporal accuracy. The mean phase lag was 5°/6° (CROSS/BARS) and the mean relative phase between the horizontal and vertical components was +95°/+92° (CW) and −85°/−75° (CCW), which is close to perfect. Furthermore, a \chi2 analysis showed that 56% of BARS trials were consistent with tracking the correct elliptical shape ( p<0.05), although segment motion was purely vertical. These data disprove the main tenet of most models of pursuit: that it is a system that seeks to minimise retinal image motion through negative feedback. Rather, the main drive must be a visual signal which has already integrated spatiotemporal retinal information into an object-motion signal.
Deficits and Recovery of First-Order and Second-Order Motion Perception in Patients with Unilateral Posterior Parietal Lesions
Authors: Braun D , Fahle M , Schönle P , Zanker J
Published: December 2015

Our aim was to test whether unilateral posterior parietal lesions degrade first-order and second-order motion differentially, and to investigate the time course of any potential recovery. We tested ten patients with circumscribed parietal lesions.

Thresholds were measured for the discrimination of the direction of motion of stimuli presented 5.5° peripherally in the ipsilesional and contralesional visual hemifields. Subjects had to indicate whether a rectangular region (1.6 deg × 3 deg) embedded in dynamic random noise background moved up or down. The region contained moving (signal) and flickering (background) dots and moved for 1 s at 2.36 deg s−1. Signal dots were either (a) coherently moving in the same direction as the region (first-order), (b) stationary (second-order), or (c) coherently moving in the opposite direction (theta). Thresholds were defined as percentage of signal dots within the region yielding 75% correct responses.

All patients had higher thresholds for second-order than for first-order motion. When contralesional and ipsilesional thresholds were compared, three patients showed proportional threshold elevations for all three types of motion stimuli in the contralesional hemifield. Two of these three patients were tested again five months later. Both showed considerable recovery: in one patient, the contralesional deficit was no longer present; in the other, it was reduced by about half. None of our patients had lesions affecting first-order or second-order motion differentially; lesions always affected first-order and second-order motion similarly. Owing to recovery, these deficits might be detectable only for a short time.
Fillers and Spaces in Text: Implications for the Relative Importance of Word Recognition and Physical Features of the Text during Reading
Authors: Epelboim J , Booth J R , Taleghani A , Ashkenazy R , Steinman R M
Published: December 2015

The role of spaces between words in text was studied in many experiments by filling the spaces with irrelevant symbols. This practice is based on the assumption that these fillers occlude spaces without disturbing word recognition appreciably. There is no empirical evidence to support this assumption. We performed a series of experiments to study the role of fillers and spaces in text. We found that texts in all conditions in which words were surrounded by fillers were read more slowly than normal texts, as long as the fillers shared common features with the letters of the text (digits, irrelevant Latin letters, and Greek letters). Reading was as slow when words were surrounded by fillers (1like2 8this6), as when fillers replaced spaces (1like2this6), showing that reading was disrupted by the presence of fillers, not by the absence of spaces. Reading eye movements were recorded with some of the subjects and text conditions. The placement of fillers and the presence or absence of spaces in the text had no effect on percentage regressions, or on where reading saccades landed within words. However, more saccades per line were made in conditions that contained fillers than either with normal or with unspaced text. Observed differences in eye movement patterns and reading speed can be accounted for by a global adjustment to only one eye movement parameter, viz saccade size, made on the basis of the global appearance of the text. No evidence was found that the presence of fillers or the absence of spaces required new strategies for programming individual saccades. This suggests that word recognition and global strategies are more significant in programming reading saccades than the local physical features of the text.
Converging Evidence for Two Separate Processes in Perceptual Grouping
Authors: Gilchrist I D , Findlay J M , Heywood C A
Published: December 2015

Using a visual search paradigm, Gilchrist, Humphreys, Riddoch, and Neumann ( Journal of Experimental Psychology: Human Perception and Performance in press) demonstrated that grouping was mediated by a contrast-polarity-dependent surface-based process, and a contrast-independent edge-based process. A vertical pair of circles ‘popped out’ amongst horizontal pairs of circles as distractor only if the items within each pair had the same contrast polarity. Alternatively, if the circles were replaced with squares that had collinear edges, the vertical pair ‘popped out’ regardless of the contrast polarity of the squares.

Here we report new data in which the role of these two grouping processes in the control of saccadic eye-movements was investigated. If subjects are presented with two items concurrently in an eye-movement task they will often make a saccade to the midpoint between the two items—this has been called the ‘global effect’. This effect is assumed to reflect some underlying integration of the visual information during the programming of saccade landing position. Distractor items that either shared the same polarity or had collinear edges with the target were found to affect the landing position of saccades. Distractors that had neither collinear edges nor common polarity produced a smaller global effect but the landing position was still affected. This suggests that the integration processes that underlie the global effect are sensitive to both common edge and surface properties. However, some integration may occur as a result of the mere presence of activity in the visual field.
Tracing a Metameric Match to Individual Variations of Colour Vision
Authors: Diaz J A , Chiron A , Viénot F
Published: December 2015

A metameric match may be accepted by one observer but rejected by another, indicating that the colour vision of the two observers is different. We analysed the variations of normal colour vision using metameric surfaces. First, we modelled the matches of a theoretical colour-normal observer on the Davidson and Hemmendinger (D&H) colour rule (Kaiser and Hemmendinger, 1980 Color Research and Applications5 65 – 71), using spectroradiometric measurements and a set of fundamentals (Stockman, Macleod, and Johnson, 1993 Journal of the Optical Society of America A10 2491 – 2521). We also derived deviate fundamentals by changing the macular pigment density, the lens density, the photopigment density, and by shifting the long-wave sensitive photopigment. Modelling the deviate normal observer matches yields shifts of no more than 1 sample on the D&H colour rule. The largest shifts are produced by changes in lens density.

Second, we asked six observers to perform a match on the D&H colour rule. We also estimated their macular pigment densities and their lens densities by using heterochromatic flicker photometry matches at 466 nm and 413 nm versus a 558 nm reference, and we recorded their Nagel anomaloscope setting. Then, we computed their personalised fundamentals in order to predict their D&H colour matches. As the most frequent match in a sample of young observers is F-15 and the modelling for the theoretical observer yields G-15, a systematic error occurs in the predictions. After correction for this error, the results show that the metameric match of five out of six observers is better predicted by using the personalised correction of the lens and macular pigment optical density only, than by using the theoretical colour observer data.
Target Detection and Texture Segmentation in Briefly Presented Displays of Curved-line Elements
Authors: Savage C J , Foster D H
Published: December 2015

Similar pre-attentive processes are often thought to underlie rapid texture segmentation and target ‘pop-out’ in multi-element displays (but see Wolfe, 1992 Vision Research32 757 – 763). Performance in target-detection and texture-segmentation tasks was measured here for briefly presented displays of curved-line elements. In both tasks 49 curved-line elements, each subtending 1 deg of visual angle, were presented in a circular display for 100 ms and followed by a mask. The position of each element in the array was jittered to reduce any possible collinearity or luminance cues. In the target-detection task, observers determined whether the display contained a target which differed in curvature from the other, background elements. In the texture-segmentation task, observers determined the orientation, horizontal or vertical, of a foreground region of 4 × 2 elements which differed in curvature from the background elements. Performance, quantified as percent correct, was measured as a function of target (or foreground) and background curvatures. At small background curvatures, performance in the two tasks was very similar: performance was best when target or foreground curvature was large. Performance differed, however, at large background curvatures: for texture segmentation there was a marked peak in performance when foreground curvature was close to zero, but there was no corresponding peak for target detection. It seems that some additional, global cue can be extracted from a group of straight or slightly curved lines that is not available from a single line, thereby facilitating texture segmentation but not target detection.
Texture Segmentation Performance as a Function of Target Size: Psychophysical Data and Simulation Results
Authors: Kehrer L
Published: December 2015

Texture segmentation performance is little affected by the size of the target patch if targets of 0.76 deg × 0.76 deg and 2.16 deg × 2.16 deg are used (Joffe and Scialfa, 1995 Spatial Vision9 325 – 342). In two experiments we reinspected the results of Joffe and Scialfa, but now also included a rather small target patch (target sizes: 0.3 deg × 0.3 deg, 1.4 deg × 1.4 deg, and 2.3 deg × 2.3 deg). In agreement with Joffe and Scialfa, we found only a small difference in segmentation performance between the two large target sizes (1.4 deg × 1.4 deg and 2.3 deg × 2.3 deg) but a marked difference between 0.3 deg × 0.3 deg and 1.4 deg × 1.4 deg. In line with Joffe and Scialfa, we conclude that segmentation performance is affected by the size of the target in a nonlinear manner, leading to an asymptotic performance level at large target sizes.

In a simulation study we asked whether the observed experimental results could be explained within the framework of a standard two-layer ‘back-pocket’ spatial filter model. The results of the computations showed a good fit with the experimental data if spatial filters in the second layer of the model were set to a width of about 1 deg.
Extracting Distributed Orientations in Different Contexts
Authors: Bonnet C , Dufour A
Published: December 2015

It has been shown that discriminating the orientations of a target is influenced by the spatial distribution of the orientations of similar elements in the background. However, this effect appears to be essentially decisional (Dufour and Bonnet, 1995 Spatial Vision9 307 – 324). In the present experiment, we explored the accuracy with which subjects can discriminate relative proportions of orientations distributed over a surface (background). Stimuli were textures made of 100 segments with regular spacing. Each of the segments had one of four possible orientations. For each display, one of the orientations was overrepresented (31%, 37%, 43% and 49%). The task of the subject was to discriminate, in a reaction-time paradigm, which of the four orientations was more frequent within a given display. Three spatial conditions were used. In condition 1, the different orientations were randomly distributed over the surface. In the two other conditions, nine elements of the same orientation formed a group within one region of the texture. In condition 2, these grouped elements had the over-represented orientation. In condition 3, they had one of the three under-represented orientations. Grouping of the oriented elements has a facilitating effect when it contains the over-represented orientation, and a negative effect when it contains one of the under-represented orientations. There are good reasons to think that these interferences are decisional rather than sensorial.
Contrast Dependence of Contextual Effects in Macaque Striate Cortex
Authors: Levitt J B , Lund J S
Published: December 2015

We have studied the effect of varying stimulus contrast on the modulatory effects exerted on V1 receptive fields by surrounding portions of the visual field. We used standard extracellular techniques to record unit activity in striate cortex of paralysed, opiate-anaesthetised macaque monkeys. We measured the orientation and direction tuning of neurons at several contrasts, with and without the presence of a surrounding stimulus that itself evoked minimal responses from the neuron. At both high and low stimulus contrasts, surround stimuli modulated responses to centre stimuli when the orientation and direction of the centre and surround were in the appropriate (though not necessarily matched) relationship. At low contrasts, we observed more profound suppression and facilitation. However, this did not simply reflect release from response saturation. At low contrasts, a greater range of surround orientations could modulate neuronal responses. This sometimes resulted in identical surround stimuli being suppressive when pairs with high-contrast centre stimuli, but facilitative when paired with low-contrast centre stimuli. Further evidence against response saturation at high contrasts was the frequent anisotropy in the suppression, ie suppression was direction-selective at high contrast, but nondirectional at low contrasts. Such contrast-dependent effects were also revealed by measurements of contrast response functions in the presence of the surround. We sometimes observed not only a decrease in the response to high-contrast stimuli and a lower slope of the response-versus-contrast curve but also an increase in responsiveness to low-contrast centre stimuli in the presence of nominally suppressive surround stimuli. Contextual effects in striate cortex thus depend importantly on the relative contrasts of centre and surround stimuli.
Do Difficulties in Stimulus Discrimination Affect Luminance Processing?
Authors: Pins D , Treisman M , Johnston R
Published: December 2015

Simple reaction time is known to decay as a hyperbolic function of luminance (Piéron's function). An identical relationship has also been demonstrated recently (Pins and Bonnet, 1996 Perception & Psychophysics in press) with different choice-reaction-time tasks. Although mean choice reaction time increased with the complexity of the task, the exponents of the functions relating reaction time (RT) to luminance were found to be equal in each experiment. These results suggest that the task specific time required by the different tasks only adds to the time necessary for luminance processing. In these experiments, the different stimuli presented were easily discriminable.

In the present study, we examined the effect of variation in luminance on a more difficult discrimination task involving variation in orientation. Five different luminance levels covering the entire mesopic range were used. In two conditions, tilted lines at nine different angles were used, at a spacing of 2°. In the first condition, the orientations were chosen on both sides of the vertical (the subject responded “left” or “right”); in the second condition, the orientations were on both sides of a line oriented at −40° to the vertical (the subject responded “high” or “low”). The results were compared to those of a second experiment in which only two easily discriminable orientations were used. The results show that RT is greater in the experiments in which nine orientations are used, while the effect of intensity on RT is lower. This effect does not depend on orientation.
Reaction Time to Gratings: A Re-Examination
Authors: Bonnet C , Thomas J P , Fagerholm P
Published: December 2015

We have examined the relationship between the reaction time for detecting a sinusoidal grating stimulus and the stimulus variables of spatial frequency, contrast, window size, and uncertainty with respect to spatial frequency. Detection was measured in a two-alternative spatial-forced-choice procedure. The stimuli were horizontal cosine gratings windowed spatially by two-dimensional Gaussians. Spatial frequency was varied from 0.7 to 6.5 cycles deg−1 and contrast was varied from 0.054 to 0.673. The standard deviation of the Gaussian window was fixed in some conditions and the number of cycles presented in each window covaried with spatial frequency. In other conditions, window size was varied, along the vertical axis only, to hold the number of cycles constant. Contrasts were always randomly intermixed, but frequencies were intermixed in some conditions and blocked in others. We confirm previous findings that reaction time increases as spatial frequency increases and decreases as contrast increases. We also confirm and extend the proposal of Rudd that reaction time closely approximates a single function of the product of contrast and the square of the grating period. We consider the implications of these findings for the nature of the physiological mechanisms which govern reaction time.
Adaptation to Variegated Scenes and Colour Constancy
Authors: Zaidi Q , Spehar B , DeBonnet J S
Published: December 2015

For a visual system to possess colour constancy across varying illumination, chromatic signals from a scene must remain constant at some neural stage. We found that photoreceptor and opponent-colour signals from a large sample of natural and man-made objects under one kind of natural daylight were almost perfectly correlated with the signals from those objects under every other spectrally different phase of daylight. Therefore, in scenes consisting of many objects, the effect of illumination changes on specific colour mechanisms can be simulated by shifting all chromaticities by an additive or multiplicative constant along a theoretical axis. When the effect of the illuminant change was restricted to specific colour axes, thresholds for detecting a change in the perceived colours in a scene were significantly elevated in the presence of spatial variations along the same axis. Probe-flash threshold curves revealed that adaptation to variegated scenes is qualitatively different from independent adaptation to the constituents or to the space-average, but is similar to adaptation to prolonged temporal modulation (Shapiro and Zaidi, 1992 Vision Research32 2065 – 2076), which would be caused by small eye-movements across object boundaries. The data are consistent with a ‘response equalisation’ model, which modifies the response function of each mechanism to match the cumulative frequency distribution of its inputs (Zaidi and Shapiro, 1993 Biological Cybernetics69 415 – 428). In a variegated scene, correlations between spatially local chromatic signals across illuminants, and adaptation caused by eye movements across spatial variations, help the visual system to attenuate the perceptual effects due to changes in illumination.
Generalised Model of Illusory Grouping Accounts for Collinear, Orthogonal, and Vertex Grouping Percepts
Authors: Lehar S
Published: December 2015

A number of illusory phenomena, for example the Kanizsa illusion, exhibit boundary completion by collinearity between visible inducing edges. These phenomena have been addressed by models such as Grossberg's boundary contour system (BCS) (1985 Psychological Review92 173 – 211), which incorporates collinearity cells with receptive fields specialised to detect and enhance collinearity. Other illusory phenomena like the Ehrenstein illusion exhibit boundary completion orthogonal to the oriented inducers. The BCS model explains such orthogonal grouping by disinhibition, due to competition between collinearity cells of orthogonal orientations. There are many illusory grouping phenomena, however, which exhibit boundary completion through sharp corners in a variety of configurations, producing illusory ‘V’ or ‘Y’ vertices. Examples are seen in the diamond percept of the four-line Ehrenstein illusion, the triangular grouping percept of three dots arranged in a triangular configuration, as well as in the hexagonal percept of a grid of dots in a honeycomb pattern. These completions cannot be explained by models based on collinearity. Lehar's orientational harmonic model (1994, PhD thesis, Boston University) offers a single generalised grouping mechanism capable of collinear, orthogonal, and sharp vertex grouping. The proposed mechanism is a harmonic resonance, or pattern of standing waves in the orientational representation which promotes orientational periodicity. Computer simulations show that the model can account for a large number of diverse illusory phenomena using a single simple mechanism, and predicts the specific conditions under which a grouping of one type yields to a different grouping type.
A Neuronal Model Detecting Figure—Ground Direction and Contrast Polarity at Illusory Contours
Authors: Heitger F , Peterhans E
Published: December 2015

A previous model of visual cortical contour processing simulated the perception of illusory contours and the direction of figure and ground at such contours (Heitger and von der Heydt, 1993 Proceedings of International Conference on Computer Vision). This model was based on ‘complex type’ operators. Hence, it lacked representations of contrast polarity and could not account for brightness effects of illusory figures.

Recently, we found that some neurons of area V2 in monkey visual cortex that signalled illusory contours were also sensitive to the figure — ground direction and contrast polarity at such contours. This allowed us to extend the previous scheme by incorporating the signals of even and odd symmetrical ‘simple type’ operators, particularly at the level of the ‘end-stopped’ operators which served the detection of the inducing elements of illusory figures (line ends, corners). To represent illusory contours, the signals of these operators were grouped with the use of fields of wide spatial support. Line ends and corners were handled differently in this process: at corners, signals parallel to the illusory contour were emphasised (para grouping), whereas at line ends signals orthogonal to the contour (ortho grouping) were emphasised. Further, ortho grouping included only end-stopped operators with even-symmetrical fields, para grouping only operators with odd-symmetrical fields. Thus, both processes contributed to the contrast polarity represented at illusory contours.

Results of simulations showed that this model reproduces the shapes of illusory contours, their figure — ground direction, and contrast polarity. Hence, it can explain the selectivity of cortical neurons at such contours and the perceived brightness of illusory figures.
Local and Global Properties of Motion Aftereffects
Authors: Wade N J , Pardieu V , Swanston M T
Published: December 2015

The local motion adaptation at the basis of the motion aftereffect (MAE) can be expressed in a variety of ways, depending upon the structure of the test display (N J Wade, L Spillmann, M T Swanston Vision Research in press). This has been demonstrated with MAEs from induced motion: if adaptation is to two moving (Surround) gratings, an MAE is seen in the central grating if two gratings surround it, but in the flanking gratings when they are themselves surrounded in the test stimulus. We report two experiments in which the characteristics of the test display and of the local adaptation process have been examined. In experiment 1, five vertical gratings were presented during adaptation; the outermost and central gratings remained stationary and those flanking the centre moved laterally. The test display always consisted of three stationary gratings: either the central three or the lower three equivalent to the locations of the adaptation display. MAEs were only recorded in the Centre and not in the Surround, irrespective of whether the Centre or Surround had been exposed to motion during adaptation. MAEs in the Centre were in opposite directions, reflecting the influence of Surround adaptation. The influence of adapting motion in different directions was examined in experiment 2. The upper grating always received the same direction of motion during adaptation, and the lower grating was absent, stationary, or moving in the same or in the opposite direction. The results indicate that an MAE is visible in the upper grating only after differential adaptation between the upper and lower gratings.
The Changing of Perceived Speed as a Function of Stimulus Contrast: An Attempted Replication with a Variety of Stimuli
Authors: Blakemore M R , Snowden R J
Published: December 2015

It has been reported that the perceived speed of a target stimulus varies with its physical contrast (Thompson, 1982 Vision Research22 377 – 380). Yet the effect is somewhat ephemeral with intersubject variations, effects of presentation mode, and test speed. So far this effect has only been reported for grating stimuli and it is unclear whether the effect generalises to other stimuli.

Subjects reported which of two patterns appeared to move more quickly. Using a forced-choice method we compared performance with three different stimuli: simple gratings in linear motion, radial gratings in rotary motion, and Julesz random-dot patterns in linear motion. We further compared simultaneous and successive presentations.

For grating stimuli at slow speeds we obtained an effect of contrast which was greater for simultaneous presentations, thus replicating Stone and Thompson's results (1992 Vision Research32 1535 – 1549). However, at similar temporal frequencies there is no reliable effect of contrast for the radial gratings. With the random-dot patterns we generally found little effect of contrast; however, we did obtain a small effect at the slowest speeds, but only when presenting the two patterns simultaneously. Our results suggest that the contrast effect on perceived speed does not readily generalise to all stimuli. We discuss the implications of these results in terms of attentional tracking mechanisms and models of motion perception.
Speed Judgments of Transparent Stimuli
Authors: Mulligan J B
Published: December 2015

When two moving patterns are combined additively, observers often perceive two transparent surfaces, even when there are no cues supporting this segmentation in a frozen snapshot. The ability of observers to make quantitative judgments about the speed of one of the patterns under these conditions was examined. The component patterns consisted of band-pass-filtered random noise presented in a spatial Gaussian contrast envelope, displayed for 250 ms. On each trial a standard pattern appeared on one side of the fixation point, while a test pattern appeared on the other. The test pattern moved in the same direction as the standard, but with a speed which varied from trial to trial according to a staircase procedure. The subjects' task was to report the side of the fixation point on which faster motion was seen. In some conditions the test stimulus was made to appear transparent by adding a mask pattern. When the mask was stationary, or moved slowly with respect to the test, no significant biases were introduced and discrimination performance was comparable to the no-mask condition (typically 3%). But if the mask moved over the test with similar speed, the task became much harder, regardless of whether the mask moved in a direction opposite or orthogonal to the test. (Some subjects commented on a perceived directional repulsion between tests and orthogonally moving masks.) These results suggest the use of nondirectional temporal channels in the performance of the speed discrimination task.
Velocity Computation from Measures of Spatiotemporal Gradients at Multiple Orientations
Authors: Johnston A , McOwan P W
Published: December 2015

Current models of speed and direction of motion which use measures of spatiotemporal gradients can suffer from ill-conditioning. This problem arises either because local measures of the derivatives of image brightness take zero values or because the motion equations cannot be solved for one-dimensional (1-D) signals in two-dimensional (2-D) images—the aperture problem. One way around this predicament is to select image points or introduce constants to deal with ill-conditioned calculations. Here we describe an analytic method that combines measures of speed in a range of directions to provide a well-conditioned measure of velocity at all points in the moving stimulus. This approach is a natural extension of a one-dimensional model which has been successful in predicting perceived motion in a variety of 1-D spatiotemporal motion patterns (Johnston, McOwan and Buxton 1992 Proceedings of the Royal Society of London, Series B250 297 – 306). Speed is computed with the use of biologically plausible filters that are derivatives of Gaussians in the spatial domain and log Gaussians in the temporal domain. Measures of speed and inverse speed are computed for a range of orientations consistent with the number of direction columns in MT/V5. The pattern of velocities measured over this set of orientations is then used to recover the speed and direction of motion of the stimulus. The model can correctly compute the velocity of moving 1-D patterns, such as gratings, patterns that prove a problem for many current 2-D motion models as they form degenerate cases, as well as the motion of rigid 2-D patterns.
Is the McCollough Effect Coded in Disparity-Sensitive Units?
Authors: McLoughlin N P
Published: December 2015

Previous studies have demonstrated that the McCullough effect (ME) is coded by both binocular and monocular units [see for example Vidyasagar, 1976 Nature (London)261 39]. At ECVP95, McLoughlin and Savoy (1995 Perception24 Supplement, 94) presented a contiguity-learning based model capable of accounting for a large dataset of both binocular and monocular ME variants. One issue not directly addressed by this model concerns the site of binocular ME adaptation. Specifically, are the binocular ME ‘units’ disparity selective?

The methods used were developed from those of McLoughlin and Savoy. Subjects were exposed to alternating coloured gratings at various crossed and uncrossed disparities (relative to fixation points). A colour cancellation paradigm was used to record the strength of the ME for a series of binocularly disparate composite achromatic test gratings both before and after adaptation. These test gratings had a small random amount of colour added to them so that the subjects could not assume that the patterns were achromatic before making adjustments. Subjects waited 5 min in the dark after adaptation before being tested. This removes any possible confounds associated with retinal afterimages. Preliminary results suggest, in agreement with previous studies, that the main component of the ME is monocular. However, the binocular component appears to be sensitive to disparity. A second series of experiments wherein two opponent MEs were induced, one at crossed and the other at uncrossed disparities, supports this finding. As in the Vidyasagar variant, the magnitude of the ME is extremely weak in this case.