bluuebunny/crossref_metadata_2025_split huggingface dataset:
Environmental Pitch and Three Types of Pointing
Authors: Stoper A E 
Published: December 2015

Many studies have shown that large errors are made when setting a target (T) to visually perceived eye level (VPEL) in a pitched environmental surround. The error in judgement of VPEL is typically about 50% of the environmental pitch angle. An observer can, however, point to the level of the target (T) with much smaller errors (eg Stoper et al, 1992 Bulletin of the Psychonomic Society30 439, found a shift of pointing of only 4% of the environmental pitch). These small pointing errors are found when the observer reaches out with an unseen hand and touches the surface on which T is presented. We call this ‘type I pointing’. If longer distances (183 cm) are used the observer must walk (with closed eyes, as in ‘pin the tail on the donkey’) in order to touch the surface on which T is presented. We call this ‘type II pointing’; it results in much larger errors, approaching in angular magnitude the errors in judgement of VPEL. In the present experiments the observer indicated the level of T by touching a point on a unseen pole which was just to the right of the observer's eyes, and thus separated from T by the viewing distance [as in the ‘manual task’ used to judge apparent height by Stoper and Bautista (1992 Investigative Ophthalmology and Visual Science, Supplement33 962)]. We call this ‘type III pointing’. This method, for both long and short distances, produced large errors similar in magnitude to those of type II pointing. These results are explained by the assumptions that environmental pitch causes an error in the judgement of the apparent horizontal in the sagittal plane (sagittal apparent horizontal; SAH) and that SAH is used in pointing of types II and III, but not of type I.
Role of Inducer Length in the Formation of the Ehrenstein Illusion
Authors: Salvano-Pardieu V 
Published: December 2015

We wanted to study the role of the length of the inducers in the Ehrenstein illusion and, particularly, the possible interdependence of inducer length and gap size in sustaining the illusion. We used a subthreshold summation paradigm. As described previously (Tassi et al, 1995 Perception24 127), the presence of an illusory contour reduces the threshold for detection of a real line superimposed on it. In agreement with the model of Grossberg and Mingolla (Grossberg and Mingolla, 1985 Psychological Review92 173 – 211; Grossberg, 1994 Perception & Psychophysics55 48 – 120), these authors suggested that the Ehrenstein illusion was generated by a bipole operator joining two line ends. In an Ehrenstein figure induced by four white lines, we varied the size of the central gap from 1.6 to 4 deg. The threshold of a white line joining the ends of two inducers was measured with a spatial alternative (left - right). The Ehrenstein figures involved either long (2.5 deg), or short (0.58 deg) inducers. The result shows that whatever the length of the inducers, the threshold for detecting the real line increases with increasing gap size. The results obtained with short and long inducers were very similar. Moreover, in this experiment, the spatial limit of integration of the illusion occurs for a gap size of 2.4 deg: the detection threshold for the real line was significantly higher from this value. This result, consistent with previous research (Salvano-Pardieu and Bonnet, 1994 Perception23 104), suggests that the length of the inducer is a factor less important than the gap size in the formation of the illusion.
Effects of Observer Orientation on Perception of Ego- and Exocentric Spatial Locations
Authors: Li J , Cohen M M , DeRoshia C W , Guzy L T 
Published: December 2015

Perceived eye position and/or the perceived location of visual targets are altered when the orientation of the surrounding visual environment (Cohen et al, 1995 Perception & Psychophysics571 433) or that of the observer (Cohen and Guzy, 1995 Aviation, Space, and Environmental Medicine66 505) is changed. Fourteen subjects used biteboards as they lay on a rotary bed that was oriented head-down −15°, −7.5°, supine, head-up +7.5°, and +15°. In the dark, subjects directed their gaze and set a target to the apparent zenith (exocentric location); they also gazed at a subjective ‘straight ahead’ position with respect to their head (egocentric location). Angular deviations of target settings and changes in vertical eye position were recorded with an ISCAN infrared tracking system. Results indicated that, for exocentric locations, the eyes deviate systematically from the true zenith. The gain for compensating changes in head orientation was 0.69 and 0.73 for gaze direction and target settings, respectively. In contrast, ‘straight ahead’ eye positions were not significantly affected by changes in the subject's orientation. We conclude that subjects make systematic errors when directing their gaze to an exocentric location in near-supine positions. This suggests a systematic bias in the integration of extra-ocular signals with information regarding head orientation. The bias may result from underestimating changes in the orientation of the head in space. In contrast, for egocentric locations, where head orientation information can potentially be discarded, gaze directions were unaffected by head orientation near supine.
Separation Discrimination in the Presence of Flanking Lines
Authors: Bondarko V M , Danilova M V 
Published: December 2015

The presence of flanking stimuli decreases sensitivity in many visual tasks. Earlier we showed that size perception depends on the distance between objects (Bondarko and Danilova, 1996 Perception25 Supplement, 94). Now we suggest that flanks not only decrease sensitivity, but also lead to misperception of size. To test this hypothesis we studied the perception of spatial interval for a set of separations from 6 to 33 min−1 in the presence of two vertical flanking lines. The method of constant stimuli was used in a temporal 2AFC paradigm. Reference stimuli were two vertical lines with separation S, either with or without two symmetrical external flanking lines. The distance to the flanking lines varied from 1/4 S to 3 S in steps of 1/4 S. The test stimuli were just two lines. The first stimulus in the pair could be either the test or the reference. Observers had to report whether the distance between the two central lines in the first stimulus were smaller or larger than the distance in the second stimulus. Probit analysis was used to estimate the threshold and the point of subjective equality (PSE). We found that the PSE implied overestimation by up to 10% of the spatial intervals for the smallest distances to the flanking lines. Increasing distances to the flanking lines finally led to slight underestimation. The strength of the illusion depended on the order of presentation of the reference and test stimuli. Thresholds increased for the smallest distances to flanking lines. Our results confirmed the hypothesis that flanks can induce distortions in size perception. We suggest that our experimental situation could be considered as a one-dimensional case of geometrical visual illusions such as the Ebbinghaus illusion.
Estimation of Time-to-Contact from Retinal Flow
Authors: Harris M G 
Published: December 2015

We investigated four models for estimating time-to-contact (TTC) from retinal flow. Lee's model can deal with sparse flow but fails if the flow contains a rotational component. Koenderink's model, based on div, can deal with rotation but fails if the flow is sparse or if the world does not vary coherently in depth. Two new models were developed by representing retinal flow as the sum of an expansion and a rotation component. The first operates on pairs of points and can deal with sparse flow but fails if the world does not vary coherently in depth. Uniquely, this model provides TTC estimates without prior knowledge of either the focus of expansion (FOE) or focus of rotation (FOR). The second model estimates both the FOE and the FOR and then operates on a point-by-point basis. This model can deal with incoherent depth variations. We compared human performance with these different model properties by requiring subjects to estimate FOE and TTC from random-dot kinematograms. We used kinematograms depicting smooth planes and random 3-D clouds of points, and systematically varied the density of the flow. Performance was not substantially reduced by sparse flow or by incoherent depth, which argues against Koenderink's and the first of our own models. Performance remained good when rotation was added to the flow, which argues against Lee's model. Overall, the data favour a model that first decomposes flow into expansion and rotation components and then estimates TTC on a point-by-point basis.
A Signal-Detection Model of Visual Short-Term Memory Performance for Random Block Patterns
Authors: Cornelissen F W , Greenlee M W 
Published: December 2015

On the basis of signal-detection theory, we have formulated a model that accurately explains performance on a visual short-term memory task involving random block patterns. The model assumes that the internal response of an observer for detecting a change in any given element of the block pattern is noisy and has a Gaussian-shaped distribution. On this basis we can calculate the likelihood that an observer correctly or falsely identifies a change in the pattern after a certain time interval (ISI). Using this likelihood, we can then predict the likelihood that an observer correctly identifies a whole pattern as having changed or not as a function of the number of elements that changed in the pattern. We have previously shown (Cornelissen and Greenlee, 1993 Perception22 Supplement, 46) that memory performance declines when changes occur in pattern elements located on the perimeter of the pattern. Therefore the model also incorporates a circular symmetric ‘memory field’ that shows a Gaussian-shaped decline of memory performance from the point of fixation. The model has three parameters: d' (detectability of a change), lambda (criterion level), and the standard deviation of the Gaussian of the memory field. In the experiments we performed, block patterns made up of 50 light and 50 dark randomly arranged elements (0.5 deg checks) were briefly (200 ms) shown. In a forced-choice task, subjects judged whether two sequentially presented (with ISIs of 1, 3, or 10 s) block patterns were the same or different. Task difficulty was varied by varying the number of elements in the patterns that changed on ‘different’ trials. The model is able to accurately predict memory performance at the three different ISIs for various levels of pattern differences (changes in 1, 2, 4, 8, 16, 20, and 50 out of 100 elements).
A Pedestal Blocks the Perception of Non-Fourier Motion
Authors: Ukkonen O I , Derrington A M 
Published: December 2015

We wanted to know whether the mechanisms that discriminate the motion of first-order patterns (defined by spatial variations of luminance) differ from those that detect the motion of non-Fourier or second-order patterns (defined by spatial variations of contrast). To address this question we tested whether motion discrimination performance of first-order and second-order patterns was affected by a pedestal (Lu and Sperling, 1995 Vision Research35 2697 – 2722). A pedestal is a static replica of a moving pattern. We used pedestals with contrast or modulation depth twice the value at which it becomes possible to discriminate the direction of a moving pattern. A two-interval forced-choice task was used to determine how direction discrimination varies with contrast of sine gratings (1 cycle deg−1) and modulation depth of amplitude-modulated gratings presented either alone or with a pedestal. The amplitude-modulated gratings had a 5 cycles deg−1 carrier modulated at 1 cycle deg−1. Three different temporal frequencies (1, 3, and 12 Hz) were studied. Performance with sine gratings was unaffected by the pedestal at all temporal frequencies tested. For amplitude-modulated gratings the pedestal raised the modulation depth at which it became possible to discriminate the direction of motion. This elevation in threshold decreased when the mean contrast of the pattern was high. This result shows that immunity to pedestals of texture-contrast patterns (Lu and Sperling, 1996 Journal of the Optical Society of America13 2305 – 2318) does not generalise to other non-Fourier motion stimuli.
The Effect of Attentional Cueing upon Speed Discrimination
Authors: Blakemore M R , Snowden R J 
Published: December 2015

Motion processes have often been regarded as being preattentive in nature. Additionally, in a few attention studies attempts have been made to incorporate the methodology common to classical psychophysics. Speed discrimination with multiple grating patches has been investigated with uncertainty/search paradigms (Verghese and Stone, 1995 Vision Research35 2811 – 2823). Another method commonly used in attentional research is the cueing paradigm, which we have used here. In this study participants had to discriminate, using a 2AFC method, whether one of four grating patterns moved faster or slower than the other patches (of a standard speed 6 deg s−1). The speed of this test stimulus was altered from trial to trial with the use of an adaptive staircase procedure. Whilst the gratings were in motion a brief (32 ms) cue was presented. This could be either valid (58%), invalid (14%), or neutral (28%) in respect to the patch of grating which was moving at a different speed. The results demonstrate that cueing had a strong effect upon the discrimination judgement. The invalid trials had up to a factor of four increase in discrimination thresholds when compared to the valid trials. Further experiments with post-stimulus cueing suggested that a decision theory model may account for a large proportion of these cueing effects. This is in agreement with similar cueing studies (Kinchla et al, 1995 Perception & Psychophysics57 441 – 450) in which participants discriminated between different array elements.
Dark Adaptation as a Model of the Parkinsonian Visual System
Authors: Wink B , Harris J P 
Published: December 2015

It has been suggested that the Parkinsonian visual system is like the normal visual system, but is inappropriately dark-adapted (Beaumont et al, 1987 Clinical Vision Sciences2 123 – 129). Thus it is of interest to ask to what extent dark adaptation of normal subjects produces visual changes like those of Parkinson's disease (PD). One such change is the reduction in apparent contrast of medium and high spatial frequencies in peripheral vision in the illness (Harris et al, 1992 Brain115 1447 – 1457). Normal subjects judged whether the contrast of a peripherally viewed grating was higher or lower than that of a foveally viewed grating, and a staircase technique was used to estimate the point of subjective equality. Judgements were made at four spatial frequencies (0.5 to 4.0 cycles deg−1) and four contrasts (8.0% to 64%). The display, the mean luminance of which was 26 cd m−2, was viewed through a 1.5 lu nd filter in the relatively dark-adapted condition. The ANOVA showed an interaction between dark adaptation and the spatial frequency of the gratings. Dark adaptation reduces the apparent contrast of high-spatial-frequency gratings, an effect which is greater at lower contrasts. This mimics the effect found with PD sufferers, and suggests that dark adaptation may provide a useful model of the PD visual system. In a second experiment, the effect of dark adaptation on the relationship between apparent spatial frequency in the fovea and periphery was investigated. The experiment was similar to the first, except that judgements were made about the apparent spatial frequency, rather than the contrast, of the peripheral grating. ANOVA showed no differential effect of dark adaptation on the apparent spatial frequency of the peripheral grating. This suggests that the observed reduction in apparent contrast of the peripheral gratings in dark-adapted normals and Parkinson's sufferers may reflect relative changes in contrast gain, rather than relative changes in the spatial organisation of receptive fields.
Differential Velocity Detection in Motion Parallax Fields
Authors: Masson G S , Mestre D R 
Published: December 2015

Processing of motion parallax relies on the ability of the visual system to segregate locally different motion signals and integrate them over space and time in order to reconstruct a 3-D structure. In a series of experiments, we evaluated the thresholds for differential motion detection and their dependences upon the spatial structure of the optic-flow field. Visual stimuli were large moving random-dot displays (80 deg × 60 deg). Each dot was randomly attributed one of two velocities whose difference ranged between 0% and 80% of their average velocity, which ranged between 2 and 64 deg s−1. Stimulus duration was from 130 to 1040 ms. Subjects were instructed to stare at a central fixation mark and had to decide whether the display specified one or two surfaces. At a duration of 130 ms, subjects needed more than 40% difference between the two velocities to reach a 75%-correct detection criterion. Thresholds reached a minimum value of about 20% at a duration of 500 ms. Thresholds always increased with the average velocity and were not affected when a static form segregation cue was added to the displays. They were always larger than thresholds for velocity discrimination in successive displays (about 10% at 260 ms). We finally investigated the spatial properties of differential velocity detection. The visual field was divided into horizontal stripes of equal widths. Adjacent bars alternately contained random dots moving with one of two velocities. Thresholds for differential velocity detection increased monotonically when the bandwidth decreased, up to an asymptotic value, equal to those observed with a transparent display.
A Comparison of Electrophysiological and Psychophysical Threshold Detection in Texture Segregation
Authors: Meigen T , Kastner S , Bach M 
Published: December 2015

The salience of targets in preattentive tasks depends on local feature gradients between the target and the surrounding background. We have developed an electrophysiological method to isolate texture-segregation-specific components in the human visual evoked potential (tsVEPs). We report here on threshold estimates of orientation-based texture segregation, comparing psychophysical and electrophysiological measures. Eight visually normal subjects participated in the experiment. In a 4AFC task, subjects indicated the position of a single patch that differed in orientation from a homogeneously oriented background. Orientation gradients ranged from 5° to 90°; orientation threshold was estimated with a best PEST procedure. In further experiments VEPs to the onset of ‘orientation chequerboard’ patterns with orientation gradients ranging from 0° to 90° were recorded from an Oz-Fpz derivation. Orientation threshold was estimated in two steps. First, the VEP to the 0° condition was subtracted from all other VEPs to isolate tsVEPs. Second, tsVEP amplitudes were determined by cross-correlation between the tsVEP to any given orientation and the 90° condition. We found, first, that the electrophysiological threshold derived from the overall mean tsVEPs was considerably higher than the psychophysical threshold of the orientation gradient, 16.1°±1.0°. Thus the pool of neurons contributing to the VEP signal must be larger or more strongly activated in order to elicit a significant tsVEP than to subserve psychophysical performance. Second, we found that the amplitudes of tsVEPs above threshold increased monotonically with increasing orientation gradient of the chequerboard patterns, suggesting that the tsVEP may have its neural correlate in orientation-contrast-dependent neurons similar to those found in monkey and cat striate cortex.
Effects of Head Orientation on the Variations in the Discrimination of Shading Orientation
Authors: Moshonkina T R 
Published: December 2015

We have previously shown that errors in judging the direction from which a smooth surface is illuminated (ie shading orientation) are not random (Moshonkina et al, 1993 Perception22 Supplement, 100), but vary with this direction in a regular manner. In the experiments described here we investigate whether the same regularities occur when the head is tilted—specifically, whether the gravitational or the head frame of reference is predominantly used in estimation of shading orientation. Stimuli were computer images of spheres (23 deg diameter) presented briefly (for 100 ms, excluding effects of eye movements). The spheres were apparently illuminated from one of 24 directions spaced at equal (15°) intervals around the sphere in the plane of the display. One of these was randomly chosen for each trial. The task was to define the direction of the apparent illumination on a clock scale with half-hour accuracy. The subject's head was either upright or tilted to the left by 30°. Viewing was left monocular. The errors in the estimation of illumination direction showed the same systematic dependence on ‘true’ direction regardless of head orientation. These regularities can be described as follows. (1) Estimates were most accurate when the shading gradient was along the horizontal axis (ie with the illuminated pole of the sphere on the right or the left), with greater errors when the gradient was along the oblique axis or vertical. (2) With oblique shading, the estimates showed a systematic tendency to bring the illumination direction closer to the horizontal axis than in ‘reality’. (3) The largest errors occurred when the illuminated pole was oriented downwards. This suggests that the gravitational frame is important in the discrimination of shading orientation. A significant effect of head orientation was observed when the illuminated pole was oriented downwards. The most probable estimation error was −30° with head upright but −60° with head tilted. This difference is opposite to that expected from application of the head frame of reference.
The Characteristics of VEPs Associated with the Accuracy of Recognition of Facial Emotion
Authors: Mikhailova E S , Davidov D V , Oleichik I V 
Published: December 2015

While investigating the timing of the N180 wave of human VEPs, we found earlier that correct recognition of facial emotions was characterised by the primary activation of the right fronto-central regions followed by the immediate involvement of the left ones. The aim of the present study was to analyse the relations between the parameters of the N400 wave, its regional and hemispheric specificity, and the accuracy of emotion recognition. Twenty healthy right-handed subjects and thirty-two untreated psychiatric patients with mild cognitive dysfunction were instructed to recognise the emotional expression of sad, neutral, or laughing faces presented for 80 ms on a computer screen. The amplitude and peak latency of the N400 response were found to be closely related to the accuracy of recognition. In healthy subjects the main difference between error and correct recognition manifested itself in decreased amplitude and shortened latency of the N400 response in frontal and central zones of the left hemisphere ( p<0.001). Compared to healthy subjects, the patients demonstrated both poor recognition and decreased latency and amplitude of the N400 response in the left frontal regions ( p<0.0001). The results suggest that activation of the left fronto-central regions plays a principal role in the final stages of facial emotion identification.