bluuebunny/crossref_metadata_2025 huggingface dataset:
Accumulator and Random-Walk Models of Psychophysical Discrimination: A Counter-Evaluation
Authors: Vickers Douglas , Smith Philip
Published: January 2007

In a recent assessment of models of psychophysical discrimination, Heath criticises the accumulator model for its reliance on computer simulation and qualitative evidence, and contrasts it unfavourably with a modified random-walk model, which yields exact predictions, is susceptible to critical test, and is provided with simple parameter-estimation techniques. A counter-evaluation is presented, in which the approximations employed in the modified random-walk analysis are demonstrated to be seriously inaccurate, the resulting parameter estimates to be artefactually determined, and the proposed test not critical. It is pointed out that Heath's specific application of the model is not legitimate, his data treatment inappropriate, and his hypothesis concerning confidence inconsistent with experimental results. Evidence from adaptive performance changes is presented which shows that the necessary assumptions for quantitative analysis in terms of the modified random-walk model are not satisfied, and that the model can be reconciled with data at the qualitative level only by making it virtually indistinguishable from an accumulator process. A procedure for deriving exact predictions for an accumulator process is outlined.
Discrimination of Higher-Order Textures
Authors: Nothdurft H C
Published: January 2007

Arrays of figural elements differing in certain features (‘textons’) may be visually segregated to yield the impression of a global figure of different texture. This fact was used to construct texture patterns of a higher level of complexity. In microstructure, these patterns reveal regular arrays of distinguishable figural elements, the segregation of which can be predicted from previous studies of human texture sensitivity. In macrostructure, clusters of such elements form new figural elements which, when repeated over space, themselves give the impression of texture at a perceptually higher level. Discrimination of such macrostructure textures was found to place similar restrictions on the form of figural elements as those of texture discrimination at the microstructure level.
Shape, Height and Angular Movement in Stereokinesis
Authors: Robinson James O , Piggins David J , Wilson John A
Published: January 2007

Cues affecting the apparent height (depth) of a stereokinetic ‘cone’ were investigated by comparing heights for pairs of rotating figures. Apparent height was shown to be independent of the number of bands on the figure and of the geometric perspective by which it was drawn. On the other hand, apparent height was strongly dependent on eccentricity within the figure, with maximum apparent height being achieved at full eccentricity. The reason for the existence of such a definite maximum remains obscure.
Thatcher and the Cheshire Cat: Context and the Processing of Facial Features
Authors: Parks Theodore E , Coss Richard G , Coss Craig S
Published: January 2007

As has been noted before, a face made gruesome by the inversion of its mouth will not be so perceived when the entire construction is inverted. Results are presented which suggest that this is because (a) the mouth and eye features are evaluated individually (although each feature may influence the evaluation of the other) and (b) the mouth, whether normal or inverted, tends to have its uppermost part assigned as ‘top’, providing for either a pleasant smiling-mouth expression or a gruesome ‘biting-intention’ expression. However, the gruesomeness of an inverted mouth is attenuated when eyes are shown below it (producing an inverted smiling face) which suggests that the location of other facial features can also influence the assignments of ‘top’.
Optimizing Stereo Separation in Color Television Anaglyphs
Authors: Mulligan Jeffrey B
Published: January 2007

A method is described for minimizing the ghost images which normally appear when anaglyphs are presented on color television screens. This is done by careful adjustment of the phosphor levels in each of the anaglyph regions.
Tactile Diplopia (Diplesthesia) on the Human Fingers
Authors: Benedetti Fabrizio
Published: January 2007

The third and fourth fingertips of five subjects were pressed against each other so as to produce a skin displacement. A single spherical stimulus was then applied simultaneously to the two fingertips in order to test perceptual experiences with different amounts of skin displacement. The results show that the probability of detecting one stimulus when a single stimulus was applied to the skin in the resting position was 0.90. This probability decreased with increments in skin displacement. At the maximum skin displacement tested the probability of detecting one stimulus when a single stimulus was applied to the two displaced fingertip surfaces was only 0.04: this means that the single stimulus was perceived to be double with a probability of 0.96. The occurrence of this doubling, similar to Aristotle's crossed-finger illusion, shows that a diplopia-like phenomenon is present in the somesthetic system. How this ‘tactile diplopia’ could represent an interesting approach to the study of tactile perception is discussed.
Metaphoric Devices in Drawings of Motion Mean the Same to the Blind and the Sighted
Authors: Kennedy John M , Gabias Paul
Published: January 2007

Five pictures, each with a different way of indicating motion, were given to blind and to sighted subjects—as tactual pictures for the blind, and as ink-print pictures for the sighted. The subjects matched the pictures to kinds of motion. The blind subjects concurred with each other and with the sighted in their matches. Therefore the devices are effective without explicit or formal training in their interpretation. The basis for the devices, it is argued, is not convention. Rather, the devices are metaphoric.
Suprathreshold Stereo-Depth Matches as a Function of Contrast and Spatial Frequency
Authors: Schor Clifton M , Howarth Peter A
Published: January 2007

Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg−1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies.

Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15–9.6 cycles deg−1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10–100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5–72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth.

A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.
Multiple 3-D Interpretations in a Classic Stereokinetic Effect
Authors: Bressan Paola , Vallortigara Giorgio
Published: January 2007

It is known that a flat ellipse rotating in the frontoparallel plane sooner or later appears as a rigid circular disc tilting in 3-D space. An experiment is reported in which prolonged exposure to the same flat pattern produces a second previously unnoticed 3-D percept: an elongated egg slanted in 3-D space, which points towards the observer and the end parts of which describe a circular trajectory in the frontal plane. It is shown that the achievement of this alternative percept is not affected by the particular shape of the ellipse, although the time needed to reach it increases with an ellipse with a 2:3 axis ratio.
Subjective Contours Can Produce Stereokinetic Effects
Authors: Bressan Paola , Vallortigara Giorgio
Published: January 2007

When a pattern of interrupted concentric circles drawn so as to produce an anomalous contour ellipse is slowly rotated in the frontoparallel plane, the subjective figure appears first to deform and then to tilt as a ring in 3-D space over motionless circles. Also, Benussi's floating cone can be obtained by placing an eccentric gray dot upon an anomalous solid ellipse and setting this figure into rotation. These patterns provide strong evidence that subjective contours can produce stereokinetic effects as effectively as real contours can. Implications for current explanations of stereokinetic effects are presented and discussed.
The Impossibly Twisted Pulfrich Pendulum
Authors: Wilson J A , Robinson J O
Published: January 2007

Two Pulfrich pendulums swinging in opposite phase present impossible twisting effects and also size-constancy effects.
Three-Dimensional Structure from Long-Range Apparent Motion
Authors: Prazdny Kvetoslav
Published: January 2007

Experiments are reported which show that three-dimensional structure can be perceived from two-dimensional image motions carried by objects defined solely by the differences in binocular and/or temporal correlation (ie disparity or motion discontinuities). This demonstrates that the kinetic depth effect is independent of motion detection in the luminance domain and that its relevant input comes from detectors based on some form of identity preservation of objects or features over time, ie the long-range processes of apparent motion.
Use of Central and Peripheral Optical Flow in Stance and Locomotion in Young Walkers
Authors: Stoffregen Thomas A , Schmuckler Mark A , Gibson Eleanor J
Published: January 2007

Young walkers (up to 5 years of age) were presented with optical flow in a moving room. Flow was global or was restricted to either the center or the periphery of the visible optic array. On standing trials the response rate was greatest when peripheral flow was available. The availability of central flow had a smaller effect on standing, and the younger children showed greater response rates to frontal flow than did the older ones. There was a strong negative correlation between age and response rate for all conditions. Flow also affected stability during locomotion. Response rate was again related to the location of the available flow. It is concluded that children show the same relative sensitivity for flow in the periphery of the dynamic structure of the optic array as has been observed in adults, but that this differentiation of different areas of optical structure is not yet fully developed when children learn to stand.
Line Correspondence in Binocular Vision
Authors: Clement Richard A
Published: January 2007

The mathematical analysis of binocular vision introduced by Helmholtz is applied to the problem of the use of disparity information to position a stimulus in depth. It is shown that matching the images from the left and right eyes along radial directions is an alternative to matching images along the horizontal direction only.
Flicker Contrast Sensitivity in Normal and Specifically Disabled Readers
Authors: Martin Frances , Lovegrove William
Published: January 2007

Temporal contrast sensitivity for counterphase flicker was determined for specifically disabled and normal readers to investigate whether the two groups differ in the functioning of their transient systems. In experiment 1, temporal contrast sensitivity was measured over a range of temporal frequencies with a spatial frequency of 2 cycles deg−1. Disabled readers were less sensitive than the control subjects at all temporal frequencies. In experiment 2, temporal contrast sensitivity was measured at a temporal frequency of 20 Hz over a range of spatial frequencies. Disabled readers were less sensitive than the controls at all spatial frequencies, with the differences between the groups increasing as spatial frequency increased. Both these findings are interpreted as supporting the hypothesis of a transient-system deficit in the visual systems of disabled readers.
The Gollin Incomplete Figures Test: A Flexible, Computerised Version
Authors: Foreman Nigel , Hemmings Robert
Published: January 2007

The Gollin incomplete figures test has been used as a measure of visual development, as a clinical test for parietal cortex dysfunction, and to examine long-term memory in amnesic patients. It has traditionally been administered by using a series of three or five stimulus cards, successive cards containing progressively more information. A study is reported in which digitised outline drawings of familiar objects were presented via a computer, the percentage of the figure on the screen slowly increasing from 0 to 100. The original findings of Gollin were successfully replicated; children's performance on the task improved markedly over the age range 2–5 years, and performance improved dramatically over three tests in all subjects. Computerisation of the Gollin task provides a precise and versatile alternative to the original card version.
Possible Neural Substrates for Orientation Analysis and Perception
Authors: Wenderoth Peter , Johnstone Syren
Published: January 2007

Recent research into the response properties of extrastriate visual cortical mechanisms has revealed single-cell functional organisation which closely parallels certain global and apparently emergent properties of psychophysical observation. An attempt is made to relate previous data on orientation illusions and aftereffects to these extrastriate mechanisms and new data which cannot be explained adequately by V1 (striate) orientation channels are discussed. Conversely, properties of cells in areas such as V3, V4, MT, and others seem to provide an obvious neural substrate for global interactions. It is suggested that psychological ‘explanations’ couched in terms of ‘hypotheses’ or ‘cognitive problem solving’ lack heuristic value, and that, in contrast, the properties of extrastriate cells can suggest novel experimental psychophysical paradigms which are designed to probe these higher-order global mechanisms more or less selectively.
A Simple Optical Method for Producing Two Identical Stimuli Moving in opposite Directions
Authors: Moulden Bernard
Published: January 2007

A problem in experiments on adaptation to moving stimuli is the tendency of subjects to track the stimuli visually, which can complicate results. A simple optical method is described which eliminates this tendency by presenting two identical stimuli moving in opposite directions.
Sources of Error in the Piagetian Water-Level Task
Authors: Sommerville Douglas , Cox Maureen V
Published: January 2007

Task materials were devised which allowed two different explanations of the Piagetian water-level task to be distinguished. Two experiments are reported with 5–6 and 7–8 year olds in which both a predictive and a perceptual task were used. The results support Ibbotson and Bryant's explanation that the performance of young children is affected by a perpendicular bias. This effect was weaker, particularly among 7–8 year olds, in the perceptual task.
Spatial Asymmetries in Tactile Discrimination of Line Orientation: A Comparison of the Sighted, Visually Impaired, and Blind
Authors: Lechelt Eugene C
Published: January 2007

Thresholds for tactile discrimination of stimulus orientation discrepancy from standard or referent vertical, horizontal, and diagonal orientations were determined for sighted, visually impaired, and blind subject groups. The stimuli were presented to the ventral distal portion of the tip of the subject's left index finger via an Optacon. Although the subject groups did not differ in overall discrimination accuracy, for each group the deviations from vertical and horizontal standard orientations were discriminated reliably more accurately than the deviations from standard diagonals, ie the oblique effect was obtained. The bases for this tactual spatial anisotropic effect appear to reflect both sensory—neurological and experiential factors.
The Effects of Target Borders on Dynamic Visual Acuity: Practical and Theoretical Implications
Authors: Long Gerald M , Garvey Philip M
Published: January 2007

The effects of target borders on the ability of observers to resolve moving targets (Landolt Cs) under a range of conditions were examined. Contrary to reported findings with stationary targets, it was predicted that the presence of borders would improve acuity for slow-moving targets because (i) overall stimulus energy is kept relatively constant as target detail varies, and (ii) a low-spatial-frequency component is held constant as target detail varies. In an experiment in which a two-sided border (above and below the target) was used, the predicted beneficial effect of the border at slow speeds was obtained. The results are discussed in terms of practical implications for the assessment of dynamic visual acuity as well as the potential neural mechanisms underlying performance.
Perceptual Linkage of Multiple Objects Rotating in Depth
Authors: Eby David W , Loomis Jack M , Solomon Eric M
Published: January 2007

When multiple objects rotate in depth, they are frequently perceived to rotate in the same direction even when perspective information signals counterrotation. Three experiments are reported on this tendency to recover the rotation directions of multiple objects in a nonindependent fashion (termed rotational linkage). Rotational linkage was strongly affected by slant in depth of the objects, image perspective, and relative starting phase of the objects. Linkage was found not to vary as a function of the relative rotation speed of the objects or the relative alignment of their rotation axes. Rotational linkage is interpreted as a tendency of the visual system to assign signed depths to objects based on a communality of image point direction.
The Analogy between Stereo Depth and Brightness
Authors: Brookes Allen , Stevens Kent A
Published: January 2007

Apparent depth in stereograms exhibits various simultaneous-contrast and induction effects analogous to those reported in the luminance domain. This behavior suggests that stereo depth, like brightness, is reconstructed, ie recovered from higher-order spatial derivatives or differences of the original signal. The extent to which depth is analogous to brightness is examined. There are similarities in terms of contrast effects but dissimilarities in terms of the lateral inhibition effects traditionally attributed to underlying spatial-differentiation operators.
Structure from Stereo by Associative Learning of the Constraints
Authors: O'Toole Alice J
Published: January 2007

A computational model of structure from stereo that develops smoothness constraints naturally by associative learning of a large number of example mappings from disparity data to surface depth data is proposed. Banks of disparity-selective graded response units at all spatial locations in the visual field were the input data. These cells responded to matches of luminance change at convergent, divergent, or zero offsets in the left and right ‘retina’ samples. Surfaces were created by means of a pseudo-Markov process. From these surfaces, shaded marked and ummarked surfaces were created, along with random-dot versions of the same surfaces. Learning of these example shaded and shaded marked surfaces allowed the system to solve stereo mappings both for the surfaces it had learned and for surfaces it had not learned but which had been created by the same pseudo-Markov process. Further, the model was able to solve some random-dot versions of the surfaces when the surfaces had been learned as shaded marked surfaces.
Inspection Time and High-Speed Ball Games
Authors: Deary Ian J , Mitchell Howard
Published: January 2007

It has been suggested that successful batsmen in cricket are not distinguished by their fast speed of visual information intake. A study is presented in which a season's batting averages for twenty regular cricketers, all members of the same local team, were correlated with the cricketers' visual inspection times. The correlation was −0.63 ( p < 0.005), suggesting that the successful batsmen were faster at picking up information from briefly presented visual displays. When the age factor was eliminated, the partial correlation between inspection time and batting average remained significant at −0.52 ( p < 0.01). This finding is discussed with respect to the sources of information available from a ball in flight.
The Standard Deviation of Luminance as a Metric for Contrast in Random-Dot Images
Authors: Moulden Bernard , Kingdom Fred , Gatley Linda F
Published: January 2007

Michelson's contrast, C, is an excellent metric for contrast in images with periodic luminance profiles, such as gratings, but is not suitable for images consisting of isolated stimulus elements, eg single bars; other metrics have been devised for such stimuli. But what metric should be used for random-dot images such as are commonly used in stereograms and kinematograms?

Previously the standard deviation (SD) of the luminances (equivalent to the root mean square, RMS, of the amplitudes) has been taken as a measure of contrast, but on little more than intuitive grounds. The validity of this speculative usage is tested. Experiments are described in which a wide range of random-dot images of various compositions was used and the adapting power of these images measured. This was taken as an index of their visual effectiveness. The contrast and contrast-reducing effects of the stimuli were expressed in terms of six candidate metrics, including SD, to discover which would give the most lawful description of the experimental data.

The usefulness and generality of the SD measure were confirmed. The effects of mean luminance were also measured and a general expression that would take them into account was derived. Finally, on the basis of computational modelling in which spatial filters with properties approximating those of retinal ganglion cells were used, a possible theoretical account for the success of the SD metric is offered.
Stereo and Motion Cues in Preattentive Vision Processing—Some Experiments with Random-Dot Stereographic Image Sequences
Authors: Pong Ting-Chuen , Kenner Martin A , Otis James
Published: January 2007

Low-level preattentive vision processing is of special interest since it seems the logical starting point of all vision processing. Exploration of the human visual processing system at this level is, however, extremely difficult, but can be facilitated by the use of stroboscopic presentation of sequences of random-dot stereograms, which contain only local spatial and temporal information and therefore limit the processing of these images to the low level. Four experiments are described in which such sequences were used to explore the relationships between various cues (optical flow, stereo disparity, and accretion and deletion of image points) at the low level. To study these relationships in more depth, especially the resolution of conflicting information among the cues, some of the image sequences presented information not usually encountered in ‘natural’ scenes. The results indicate that the processing of these cues is undertaken as a set of cooperative processes.
How the Visual System Detects Changes in the Direction of Moving Targets
Authors: Sekuler Allison B , Sekuler Robert , Sekuler Erica B
Published: January 2007

To determine how the visual system represents information about change in target direction, we studied the detection of such change under conditions of varying stimulus certainty. Target direction was either held constant over trials or was allowed to vary randomly. When target direction was constant the observer could be certain about that stimulus characteristic; randomizing the target direction rendered the observer uncertain. We measured response times (RTs) to changes in target direction following initial trajectories of varying time and distance. In different conditions, the observer was uncertain about either the direction of the initial trajectory, or the direction of change or both. With brief initial trajectories in random directions, uncertainty about initial direction elevated RTs by 50 ms or more. When the initial trajectories were at least 500 ms, this directional uncertainty ceased to affect RTs; then, only uncertainty about the direction of change affected RTs. We discuss the implications of these results for (i) schemes by which the visual system might code directional change; (ii) the visual integration time for directional information; and (iii) adaptational processes in motion perception.
Preattentive Vision and Perceptual Groups
Authors: Bravo Mary , Blake Randolph
Published: January 2007

Recent evidence suggests that preattentive processing may not be limited to the analysis of simple stimulus features as previously suggested. To explore this idea a visual search task was used to test whether the shapes of several perceptual groups can be processed in parallel. Textured displays that give rise to strong perceptual grouping were used to create figures on a background. Search times for a target figure distinguished by a unique shape were found to be independent of the number of distractor figures in the display. This result indicates that perceptual groups may be processed in parallel and suggests an expanded role for preattentive processing in vision.
Low-Frequency Filtering and the Processing of Local—Global Stimuli
Authors: Badcock Johanna C , Whitworth Felicity A , Badcock David R , Lovegrove William J
Published: January 2007

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.
Visual Perception and the Guidance of Locomotion without Vision to Previously Seen Targets
Authors: Rieser John J , Ashmead Daniel H , Talor Charles R , Youngquist Grant A
Published: January 2007

Two experiments were performed to assess the accuracy and precision with which adults perceive absolute egocentric distances to visible targets and coordinate their actions with them when walking without vision. In experiment 1 subjects stood in a large open field and attempted to judge the midpoint of self-to-target distances of between 4 and 24 m. In experiment 2 both highly practiced and unpracticed subjects stood in the same open field, viewed the same targets, and attempted to walk to them without vision or other environmental feedback under three conditions designed to assess the effects on accuracy of time-based memory decay and of walking at an unusually rapid pace. In experiment 1 the visual judgments were quite accurate and showed no systematic constant error. The small variable errors were linearly related to target distance. In experiment 2 the briskly paced walks were accurate, showing no systematic constant error, and the small, variable errors were a linear function of target distance and averaged about 8% of the target distance. Unlike Thomson's (1983) findings, there was not an abrupt increase in variable error at around 9 m, and no significant time-based effects were observed. The results demonstrate the accuracy of people's visual perception of absolute egocentric distances out to 24 m under open field conditions. The accuracy of people's walking without vision to previously seen targets shows that efferent and proprioceptive information about locomotion is closely calibrated to visually perceived distance. Sensitivity to the correlation of optical flow with efferent/proprioceptive information while walking with vision may provide the basis for this calibration when walking without vision.
Estimating Middle-Wavelength and Long-Wavelength Cone Sensitivity with Large, Long-Duration Targets and Small, Brief Targets
Authors: Sharpe Lindsay T , Volbrecht Vicki J
Published: January 2007

Incremental threshold versus field-intensity curves (tvi) and spectral sensitivity functions were measured for 500 nm and 667 nm test flashes, either under the standard two-colour threshold conditions of Stiles (target: 1.0 deg, 200 ms)—which are known to favour detection by the chromatically-opponent pathways—or with a smaller (0.1 deg), shorter-duration (10 ms) target—chosen to favour detection by the non-opponent (achromatic) pathway. The data reveal differences between the two conditions: for the small, brief target, the tvi curves were shallower and less dependent upon wavelength, and the spectal sensitivity functions were narrower than for the standard target.
Training Effects on Dynamic Visual Acuity with Free-Head Viewing
Authors: Long Gerald M , Riggs Cynthia A
Published: January 2007

Training effects on dynamic visual acuity (DVA) were determined over a range of target durations and target velocities under the viewing condition of free-head movement. Dynamic acuity levels were lower than static acuity levels, and training was found to be very effective in improving performance, particularly under the most difficult conditions in which performance was poorest. Improvement was also most evident among those observers who initially exhibited the poorest DVA scores. Preliminary comparisons between DVA scores of a nonselect sample of college students and those of a small group of college athletes indicated that the athletes were not very different from the nonselect group and would also benefit from training on the task.
Colour Changes as a Function of Luminance Contrast
Authors: Valberg Arne , Lange-Malecki Bettina , Seim Thorstein
Published: January 2007

When spectral light increases in luminance, the hues change. Normally, long-wavelength light becomes increasingly yellow, and short-wavelength light turns blue or blue-green. This is known as the Bezold-Brücke hue shift. Less notice has been paid to the change in relative chromatic content (saturation or chromatic strength) that accompanies these shifts in hue. As luminance contrast increases from zero, chromatic strength increases to reach a maximum at a luminance that is wavelength dependent. Short-wavelength blueish light reaches this maximum at low relative luminances, whereas midspectral yellowish stimuli need several log units higher luminance. Red and green are somewhere in between. For luminances above this maximum, the chromatic content usually diminishes, and most light becomes more whitish in appearance. In this study it is demonstrated how the combined chromatic appearance of hue and chromatic strength change with intensity. Both phenomena find a common physiological interpretation in the nonlinear and nonmonotonic responses of colour-opponent P cells in the retina and lateral geniculate nucleus of the primate. A model that combines the outputs of six P-cell types accounts for observers' estimates of hue and chromatic strength.
Recognising Facial Surfaces
Authors: Bruce Vicki , Healey Patrick , Burton Mike , Doyle Tony , Coombes Anne , Linney Alf
Published: January 2007

The extent to which faces depicted as surfaces devoid of pigmentation and with minimal texture cues (‘head models’) could be matched with photographs (when unfamiliar) and identified (when familiar) was examined in three experiments. The head models were obtained by scanning the three-dimensional surface of the face with a laser, and by displaying the surface measured in this way by using standard computer-aided design techniques. Performance in all tasks was above chance but far from ceiling. Experiment 1 showed that matching of unfamiliar head models with photographs was affected by the resolution with which the surface was displayed, suggesting that subjects based their decisions, at least in part, on three-dimensional surface structure. Matching accuracy was also affected by other factors to do with the viewpoints shown in the head models and test photographs, and the type of lighting used to portray the head model. In experiment 2 further evidence for the importance of the nature of the illumination used was obtained, and it was found that the addition of a hairstyle (not that of the target face) did not facilitate matching. In experiment 3 identification of the head models by colleagues of the people shown was compared with identification of photographs where the hair was concealed and eyes were closed. Head models were identified less well than these photographs, suggesting that the difficulties in their recognition are not solely due to the lack of hair. Women's heads were disproportionately difficult to recognise from the head models. The results are discussed in terms of their implications for the use of such three-dimensional head models in forensic and surgical applications.
The Discriminability of Smooth Stereoscopic Surfaces
Authors: Norman J Farley , Lappin Joseph S , Zucker Steven W
Published: January 2007

In this study the sensitivity of human vision to the smoothness of stereoscopic surface structure was investigated. In experiments 1 and 2 random-dot stereograms were used to evaluate the discrimination of smooth versus ‘noisy’ sinusoidal surfaces differing in the percentages of points on a single smooth surface. Fully coherent smooth surfaces were found to be much more discriminable than other less smooth randomly perturbed surfaces. In experiment 3 the discrimination between discontinuous triangle-wave surfaces and similarly shaped smoothly curved surfaces obtained from the addition of the fundamental and the third harmonic of the corresponding triangle-wave surface was evaluated. The triangle-wave surfaces were found to be more accurately discriminated from the smoothly curved surfaces than would be predicted from the detectability of the difference in their Fourier power spectra. This superior discriminability was attributed to differences between the curvature and/or discontinuity of the two surfaces. In experiment 3 the effects of incoherent ‘noise’ points on the discrimination between the two surface types were also evaluated. These randomly positioned noise points had a relatively small effect on the discrimination between the two surfaces. In general, the results of these experiments indicate that smooth surfaces are salient for stereopsis and that isolated local violations of smoothness are highly discriminable.
Distractor Ratio and Grouping Processes in Visual Conjunction Search
Authors: Poisson Marie E , Wilkinson Frances
Published: January 2007

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 cast doubt on this interpretation. In experiment 2 the possible role of ‘pop out’ and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.
Gaze Control in Putting
Authors: Vickers Joan N
Published: January 2007

The gaze of low and higher handicap golfers was assessed while they performed consecutive putts from 3 m, wearing an eye-movement helmet that permitted normal mobility. MANOVA (count and duration), with univariate follow-up, revealed significant differences in gaze between five low (LH, 0–8) and seven higher handicap golfers (HH, 10–16). The LH (ie more highly skilled) golfers were found to use a variable form of gaze control in which longer fixation durations on the ball and target were observed, and there were fewer fixations on the club and surface, with more express saccades and quicker saccades between gaze locations. The HH golfers, in contrast, allocated the same mean durations to each gaze (about 1 s), independent of type of control (fixation, saccade, or tracking) or location (ball, club, target, or surface). In comparing hits to misses, there was an increased probability of hits if the golfers used express saccades to the club during preparation, and a steady fixation on the ball during the backswing/foreswing of the club, as well as a steady fixation on the surface during contact. These results suggest that with the acquisition of the putting skill, there are changes in gaze control, characterized by economy in the number of gaze shifts, the development of priority to specific gaze locations, and economy in the allocation of time between preferred gaze locations. In the discussion two reasons are proposed that may partially explain the results found here, as well as help further our understanding of the role of gaze in targetting skills.
The Perceived Duration of Gratings
Authors: Baro John A , Brzezicki Lynda J , Lehmkuhle Stephen , Hughes Howard C
Published: January 2007

Since visible persistence of grating patterns increases with spatial frequency, it is often inferred that the perceived duration of a grating is also longer at higher spatial frequencies. However, other work has demonstrated that the perceived onset of a grating is also delayed at higher spatial frequencies. Thus it is impossible to infer the subjective duration from the results of visible persistence studies alone. In order to estimate perceived duration in the present study, reaction times (RTs) to grating onsets and offsets were measured for a range of spatial frequencies. The results indicate that although the perceived duration (ie the difference between offset and onset RTs) was consistently longer than the physical duration, the estimates of perceived duration did not vary with changes in spatial frequency. Differences between the present results and earlier findings are interpreted in the context of the different methods used to measure perceived offset.
Vision with Equiluminant Colour Contrast: 2. A Large-Scale Technique and Observations
Authors: Cavanagh Patrick , Adelson Edward H , Heard Priscilla
Published: January 2007

A simple technique is described for producing large-scale, tritanopic displays. The technique reproduces the various phenomena of vision with equiluminous-colour contrast that have previously been reported with red/green stimuli. It is, however, much less demanding technically, robust against artifacts, and can be used on large-scale scenes. One advantage of the technique is that a piece of blue filter can be used individually by each observer to compare quickly tritanopic and luminance conditions.
Use of a Distracting Task to Obtain Defensive Head Movements to Looming Visual Stimuli by Human Adults in a Laboratory Setting
Authors: King Sheila M , Dykeman Caroline , Redgrave Peter , Dean Paul
Published: January 2007

Defensive responses to looming visual stimuli have been obtained in a wide variety of species, including human infants as young as one week. This phenomenon has not, however, been formally demonstrated for adults under laboratory conditions. In this paper it is reported that similar responses, namely avoidance movements of the head, can be obtained in most human adults provided that they are suitably distracted by playing a computer tracking game. Such behaviours were not obtained when subjects were not so distracted. The use of control conditions also ruled out the possibility that simple movement cues from stimuli presented on a noncollision trajectory are sufficient stimulus to obtain defensive responses. It is of interest to note that latencies for avoidance movements were significantly shorter than those for orienting movements in the same situation, but were no different from the latencies for orienting movements when subjects were not distracted. It is argued that these findings are consistent with the proposition that defensive head movements to looming stimuli, like orienting movements to novel peripheral stimuli, represent a basic visual competence that is normally suppressed (or subsumed) by higher competences. The decision to avoid is probably based on the computation of time to contact, and may reflect the operation of a subcortical system for elementary analysis of optic flow.
Stimulus Mislocalization Depends on Spatial Frequency
Authors: Rose David , Halpern D Lynn
Published: January 2007

It was previously reported that briefly presented peripheral stimuli are perceived closer to fixation than continuously presented stimuli at the same eccentricity; this effect has, however, not proved consistently replicable. In this study it was investigated whether the misperception of location might depend upon the spatial frequency content of the stimulus. Spatial-frequency-filtered vertical bars were displayed briefly and their locations were judged relative to continuously visible comparison spots. For monocular stimuli, a significant foveopetal mislocalization of the bar was obtained that increased in size as spatial frequency was lowered. Even larger mislocalizations were obtained for dichoptically presented horizontally disparate pairs of bars, and this effect also increased at low spatial frequencies. Possible underlying mechanisms are discussed, and spatial frequency is suggested to have been the confounding factor in previous studies.
Asymmetric Interactions in the Processing of the Visual Dimensions of Position, Width, and Contrast of Bar Stimuli
Authors: Shechter Sima , Hochstein Shaul
Published: January 2007

The processing of different dimensions of a single stimulus may be either integral or separable. Dimensions are called integral if correlated variation of one improves discrimination on the basis of the other and random variation of one interferes with discrimination on the basis of the other. For separable dimensions on the other hand, subjects can attend to one dimension and disregard variations in the other. These discrimination tests were used to find the interactions between the processing of the visual dimensions of position, width, and contrast of a light bar stimulus. The relations between these dimensions were found to be asymmetric-judgments of position and width are independent of contrast variations, but variations in these dimensions influence contrast discriminations. Furthermore, position variations influence width judgements more than vice versa. The data were analyzed for repetition effects, and it was found that this model is not sufficient to explain all the interaction phenomena. The asymmetries found may be related to the different ways these dimensions are mapped onto cortical neuron responses.
Attention and Interference in Prospective and Retrospective Timing
Authors: Brown Scott W , Stubbs D Alan
Published: January 2007

Subjects listened to a series of musical selections and then judged the duration of each selection. Some subjects were informed beforehand that timing was involved (prospective timing) whereas others were informed afterwards (retrospective timing). Half the groups performed a concurrent proofreading task during stimulus presentation. The results showed a trade-off between temporal and nontemporal task performance: prospective-timing groups were more accurate in judging time and were worse at proofreading, whereas retrospective-timing groups were relatively poor at judging time but better at proofreading. This pattern is consistent with Michon's notion of an essential equivalence between temporal and nontemporal processing, and supports the predictions of an attentional allocation model of timing. The proofreading task interfered both with prospective and with retrospective timing, and both types of time judgments were influenced in the same way by effects of stimulus context. These results imply that similar timing processes operate under prospective and retrospective conditions.
Perceiving Tongue Position
Authors: Grover Cynthia , Craske Brian
Published: January 2007

Two experiments on perceiving the tongue position were conducted in which naive human subjects pointed to gingival targets, or to extensions of their fingertip, with their tongue. The aim in experiment 1 was to inquire about the existence and accuracy of the perception of the horizontal position of the tongue inside and outside the mouth, and whether kinesthetic elements other than the skin might contribute to perceiving tongue position. Vertical positioning and the calibration of the sensory map that can be presumed to underlie position sense in the tongue were examined in experiment 2.

It was found that position sense is equally good in the presence and absence of anesthesia of the mucosa, suggesting that the muscles, tendons, and corollary discharge contribute to position sense in the tongue. Perception of the tongue position both inside and outside the mouth is accurate, with the error in tongue positioning being about 2°. Feedback improved naive subjects' accuracy very little.
Contrast Sensitivity during Horizontal Visual Pursuit: Dynamic Sensitivity Functions
Authors: Long Gerald M , Homolka Joyce L
Published: January 2007

The contrast sensitivity functions of college students for grating targets presented at angular velocities of 0, 30, 60, and 90 deg s−1 were determined for target durations of 200 and 600 ms. The most pronounced effects of target movement were evident at the mid to high spatial frequencies in which sensitivity was markedly reduced as velocity increased. These adverse effects were greatest in the 200 ms condition, in which performance was largely limited to the saccadic eye movement system. In the 600 ms condition, in which both saccadic and smooth pursuit eye movements were possible, contrast sensitivity for the low-frequency target actually improved significantly for the 30 and 60 deg s−1 targets, whereas only adverse effects of target motion were found for targets of mid and high spatial frequencies. The results are discussed in terms of the limitations of traditional visual assessment procedures and the practical and theoretical benefits of conceptualizing the joint effects of target composition and target movement.