bluuebunny/crossref_metadata_2025 huggingface dataset:
A Correlational Analysis of Symmetry between the Arrowhead and Featherhead Müller-Lyer Illusions
Authors: Adam June , Bateman Layna
Published: January 2007

Correlational methods were used to investigate symmetry of effect for the arrowhead and featherhead versions of the Müller-Lyer figure. Two control figures were compared in the determination of baseline levels for measurement of the illusions: a shaft presented without any inducing context, and a shaft with vertical inducing lines attached. In addition, results based on difference-score measures of the illusions were contrasted with results obtained by partial-correlation techniques. Overall, when one considers the results for either one of the arrowhead or featherhead versions, the evidence favours a common underlying mechanism. However, results across the two versions suggest that the mechanisms for the two versions differ fundamentally. In weighing the different kinds of evidence contributing to this conclusion, methodological issues were raised. By obtaining two judgments for each stimulus figure from a large number of subjects, it was possible to demonstrate not only that conventional difference-score measures of illusions are highly unreliable, but also that they can yield biased results.
The Development of Vergence Does Not Account for the Onset of Stereopsis
Authors: Birch Eileen E , Gwiazda Jane , Held Richard
Published: January 2007

The onset of measurable stereopsis in human infants occurs at approximately 4 months of age, directly following a period of rapid oculomotor development. The experiment reported here was designed to investigate whether the onset of stereopsis is determined solely by the onset of accurate oculomotor coordination or whether neural development in the binocular visual system is necessary. These alternatives were examined by means of a testing procedure which ensured that stimuli were presented within 1.4 deg of the horopter. That these stimuli are insensitive to errors of vergence was verified by testing thirty infants aged 6 to 10 months who were known to have stereoacuity of at least 1 min of visual angle. All but one infant retained the ability to make stereo discriminations with simulated vergence errors of up to 30 prism diopters. Results obtained from a group of forty-four infants tested longitudinally between 0 and 6 months showed a mean age of onset of stereopsis of 4.1 months. Thus, ensuring that stimuli are presented near to the horopter does not significantly alter estimates of the age of onset of stereopsis. These results suggest that neural development which is critical for the ability to make stereo discrimination must occur during the first 3 months of life.
Depth from Dichoptic Edges Depends on Vergence Tuning
Authors: Krol Jodi D , van de Grind Wim A
Published: January 2007

Small but reproducible fixation disparities occur in normal subjects when they view certain types of dichoptic stimuli. During dichoptic as well as stereoptic stimulation the motor fusion process determines first the average vergence state of the eyes. The subsequent fine tuning of vergence is shown to depend on the spatial distribution of contrast edges both of the same contrast sign (‘stereoptic edges’) and of opposite contrast sign (‘dichoptic edges’).

Stereoptic edges tend to induce superposition attempts of the vergence control system and dichoptic edges tend to antagonise this process. If a single low-contrast dichoptic edge is presented with zero disparity and within a stereoptic reference frame, a fixation disparity of several minutes of arc results. This influences depth vision since dichoptic edges are perceived (as monocular edges) at the actual rather than at the intended fixation distance. The findings explain previous paradoxical results of eg Kaufman and Pitblado who reported seeing depth in opposite-contrast stereograms. Their results seemed to contradict the well-established ‘same-sign rule’ (SSR) which states that the stereoptic system only detects disparities of edges with the same contrast sign. It is concluded that (i) the SSR holds; (ii) dichoptic (and monocular) edges are seen at the horopter; (iii) the vergence fine tuning prevents superposition of dichoptic edges even if this causes a fixation disparity.
The Effect of Changes in Visible Area on Facial Recognition
Authors: Inui Toshio , Miyamoto Kensaku
Published: January 2007

The proportion of a human face which needs to be visible for correct identification was investigated. Photographs of familiar public faces were presented under two conditions: (i) the facial photographs were original photographs with local information; and (ii) the facial photographs were blurred, with high spatial frequencies removed and with no local information. Subjects could scan the face as they wished through a moveable window generated by a TV system and had to identify the depicted person as quickly as possible. Window size was varied systematically. The main study showed that, in spite of including only its local features, a familiar facial photograph could be perfectly identified when the visual window covered one-fifth of the whole face, and that a global image of the whole face was obtained when nearly half of the whole face was processed simultaneously. The structure of the memory image of the face is discussed in the light of the results obtained.
The Dependence of Monocular Rivalry on Spatial Frequency: Some Interaction Variables
Authors: Mapperson Barry , Lovegrove William
Published: January 2007

The effect of spatial frequency upon the rate of monocular rivalry has been investigated on several occasions, with conflicting results. With increases in spatial frequency the rivalry rate has been variously reported as declining monotonically, increasing slightly and briefly before declining sharply, or showing a sharp increase before a sharp decrease. Experiments are reported which show that all three functions may be obtained depending on whether the number of cycles of the gratings is held constant across spatial frequency, or the field size is held constant (and, if the latter, what field size is used). The point at which any high-frequency decline occurs is also shown to increase with increasing luminance. All functions obtained correspond to visual sensitivity functions obtained under comparable conditions.
Summation of Target and Mask Metacontrast Stimuli
Authors: Burr David C
Published: January 2007

Three experiments are reported; they show the following: (i) Metacontrast occurs at high photopic conditions (250 cd m−2); at this luminance the function peaks at stimulus asynchronies of ∼40 ms (compared with 100 ms at lower luminances). (ii) The strength of metacontrast is contrast-independent, down to contrast very near detection threshold. (iii) Detection thresholds for the combined target and mask presentation are lower than those for the mask alone, implying summation between target and mask. Summation is greatest at the same asynchronies which yield maximum metacontrast masking. The experiments are taken as evidence in favour of a fusion explanation for metacontrast. An argument is made which implicates temporally units of the motion detection system as being instrumental in the fusion process.
Color Logic of Apparent Motion
Authors: Kolers Paul A , Green Marc
Published: January 2007

Two shapes of either the same or different color will seem to be in smooth apparent motion with like-colored mates, at proper conditions of flash timing and spacing. An experiment is reported in which the condition was tested for unlike-colored pairs, for example red–green alternated with green–red. The question of interest was how the visual system would resolve the disparity of color. An ‘intelligent’ solution would rotate the shapes in three dimensions. Like-colored and unlike-colored pairs were found to move and transform similarly, however, the resolution being dependent more upon timing than upon color. The notion of intelligence as it might be applied to vision is discussed in light of these results.
Strategy Choices by Young Braille Readers
Authors: Millar Susanna
Published: January 2007

The hypotheses that in Braille learning (i) coding strategies change with reading level, and (ii) coding differs between normal and retarded readers were tested with oddity judgments by blind children. Experiment 1 showed that strategy choices varied with reading level only in association with mental age. By contrast, shape neglect and preference for phonological strategies were shown by retarded readers rather than by matched normal readers. Experiment 2 showed that under instructions to use given coding strategies the retarded were as accurate as normal readers. Accuracy for all forms of coding increased with reading level, but coding word shape was significantly less accurate than other forms of coding, and even correct coding of shape was no faster than semantic or phonological coding. It is concluded that (i) coding the shape of Braille words is unlikely to be a major factor in producing faster Braille reading, and (ii) retarded Braille readers differ from normal readers in their spontaneous choice of strategy rather than in the ability to use the relevant codes.
On Misoriented Letters
Authors: Bressan Paola , Vicario Giovanni B
Published: January 2007

An experiment is reported in which subjects were presented with misoriented letters and asked to identify the transformations to which normal letters had been subjected to produce the misorientation. When two or more transformations were equally justifiable from the geometrical point of view, they did not have necessarily the same relevance from the perceptual standpoint. For instance, it was easier for the subjects to see an upside-down letter R as the outcome of two reflections (one about the horizontal axis and one about the vertical axis) rather than of a 180° rotation in the picture plane. Contrary to expectation, for the same final result a reflection—a movement in three dimensions—was preferred to a rotation—a movement in two dimensions—and two reflections were preferred to a single rotation. To explain the distribution of the responses, a hypothesis on the ‘mode of appearance’ of equivocally oriented objects is presented. This postulates that the directional axes assigned to the objects tend to remain in correspondence with those of the environment. Moreover, the polarization of the up–down axis tends to be preserved, while that of the left-right axis does not. Implications of this simple model for the perception of misorientations are discussed.
What is Perceived When Two Images are Combined?
Authors: Caelli Terry , Yuzyk Jerome
Published: January 2007

When two images are combined three perceptual outcomes are possible. One, they can be perceived as they are—separate and independent images. Two, they can become perceptually fused into a new image. Three, one image may dominate, or mask, the other. These possibilities are demonstrated with a variety of images and it is proposed that it is their spatial correlation rather than their spatial frequency similarities or differences which is critical in determining the particular outcome.
The Role of Verbal Labels in the Judgment of Orientation and Location
Authors: Maki Ruth H , Braine Lila Ghent
Published: January 2007

In an earlier study it was found that judgments of right—left orientations and locations were more difficult than judgments of up—down only when spatial words were used in the tasks. Experiments are reported in which pictures of many objects were presented to eliminate the possibility that subjects in previous studies had used strategies specific to single-stimulus tasks. In experiment 1, right—left orientations were judged more slowly than up—down orientations both when the spatial words were used and when arbitrary letters replaced the spatial words. In experiment 2, judgments of the right—left locations of pictures took longer than judgments of their up—down locations only when spatial words were used in the task; the right—left difficulty was eliminated when arbitrary letters replaced the words. The differential effect of words and letters in location judgments seems to be due to the different coding strategies adopted by subjects under the two conditions. It is concluded that a right—left difficulty does not depend on the use of spatial terms: word and letter conditions yield different results only when the task permits different judgments to be made under the two conditions.
Apparent Motion of Subjective Surfaces
Authors: Ramachandran Vilayanur S
Published: January 2007

Apparent motion of an illusory surface was produced by presenting two spatially separated illusory squares in an appropriately timed sequence. Control experiments showed that the effect arose from the illusory contours themselves and not from motion of the cut sectors on the discs. When a template of this movie was superimposed on ‘wallpaper’ composed of a regular matrix of spots, the spots appeared to move with the illusory surface even though they were physically stationary. This effect (‘motion capture’) suggests that the motion of certain salient features in the visual field gets spontaneously attributed to even static elements in the vicinity.
Topological Structure in the Perception of Apparent Motion
Authors: Chen Lin
Published: January 2007

Not only translations and rotations, but also intriguing ‘plastic deformations’ are observed in apparent motion. What kinds of invariants does the visual system depend on during these transformations to determine that two figures of different shapes nevertheless represent the same object? Experiments are reported in which seven pairs of stimuli with topological differences were used. The evidence suggests that topological invariants may be used in the perception of apparent motion. In spite of variations in other factors, such as brightness, spatial frequency, terminators, etc, subjects displayed a strong preference for motion from a central figure to a figure with the same topological invariants. The results emphasize the importance of topological structure in figure perception.
Modifying the Gestalt Factor of Proximity: Theories Compared
Authors: McKenna Frank P
Published: January 2007

A theoretical controversy between adaptation-level (AL) theory and response-frequency-equalization (RFE) theory was examined. The stimulus presented for judgment was a matrix of dots which varied in appearance as the relative horizontal and vertical separation between the dots was altered. The matrix appeared to be columns of dots (larger horizontal separation), rows of dots (larger vertical separation), or neither row-like nor column-like (when the two separations were equal). A stimulus with an extreme row organization was presented on alternate trials and resulted in more vertical judgments. Both AL theory and RFE theory are shown to be inadequate on two counts. They both predict that the response latencies to the vertical stimuli should be affected, whereas these were unaffected. They both predict that in terms of the categorical decision, ‘horizontal’ or ‘vertical’, the responses to both horizontal and vertical stimuli should be affected. It is shown that the context effect is almost entirely on the horizontal stimuli. These results are consistent with a spatial-adaptation model.
The Measurement of Differential Thresholds for Lifted Weights by Microcomputer-Controlled Apparatus
Authors: Brodie Eric E
Published: January 2007

The measurement of differential thresholds (DLs) for lifted weights normally involves lengthy test sessions. This is due partly to the statistical techniques for measuring the threshold and partly to the methods adopted to present the stimuli. Attempts to speed up test procedures tend to produce unreliable DL estimates, or to measure the sense of force or effort rather than weight perception. To facilitate the measurement of DLs for lifted weights, an Apple microcomputer was programmed to generate Wetherill tracking procedures and was interfaced with two stimulus bearing turntables. Various lifting methods are easily implemented, and DLs are reliably and quickly measured. The adaptation of the apparatus for general use in investigating the haptic sense modality is discussed.
Haploscopic Colour Mixtures with and without Contours in Subjects with Normal and Disturbed Binocular Vision
Authors: Lange-Malecki Bettina , Creutzfeldt Otto D , Hinse Paul
Published: January 2007

Binocular mixtures of equiluminous components of different wavelengths were matched with additive monoptic mixtures of the same components. After a satisfactory match had been achieved, the luminance of each colour in the monoptic mixture was measured photometrically. After presentation of an orthogonal grating superimposed on the colour shown to one eye, the colour matching was repeated. The grating induced a strong dominance of the colour with which it was combined. Yet the uncontoured colour was not entirely suppressed, but contributed to the binocular colour to various degrees. In three subjects with anisometropic amblyopia in one eye the colour presented to the amblyopic eye contributed little or nothing to the haploscopic colour mixture, depending on the degree of amblyopia. This diminished contribution could not be enhanced by a grid. In three cases with strabismic amblyopia and in one case with strabismus alternans no haploscopic colour mixture effects could be demonstrated. The observations are discussed in the context of neurophysiological findings in the visual system of primates, and it is suggested that colour and contour are not transmitted through independent channels from the retina to the cortex.
Stroop Interference Based on the Multimodal Correlates of Haptic Size and Auditory Pitch
Authors: Walker Peter , Smith Sylvia
Published: January 2007

In a preliminary experiment subjects were asked to explore three wooden knobs of different sizes and to rate each one on a series of 7-point scales. The results confirmed that an object may possess a number of multimodal qualities that are contingent on its haptic size. The pattern of intercorrelations between the qualities was consistent with the pattern that is observed when subjects respond to pure auditory tones varying in pitch. For example, small (high-pitched) sounds, like small objects, are judged to be sharp, thin, light, weak, fast, tense, and bright. The main experiments used a paradigm based on the Stroop interference effect. Subjects were required to press one of two keys as quickly as possible depending on which of four possible words appeared in the centre of the screen. A 50 Hz or a 5500 Hz tone accompanied each test word, and subjects responded on two keys that differed in size. Subjects were found to respond more slowly when either the pitch of the incidental sound or the size of the key on which they responded was incongruent with the multimodal features represented by the test word. The results confirm that people are automatically and immediately sensitive to the multimodal features of a stimulus when direct sensory evidence for the features is absent.
Matching Familiar and Unfamiliar Faces on Internal and External Features
Authors: Young Andrew W , Hay Dennis C , McWeeny Kathryn H , Flude Brenda M , Ellis Andrew W
Published: January 2007

Two experiments are reported in which subjects were asked to match a photograph of a complete face and a simultaneously presented photograph of internal or external features of a face, deciding whether or not the two photographs were pictures of the same person. In experiment 1 ‘same’ pairs were derived from different pictures of the same face, so that subjects had to match the faces and not the particular photographs used. Matches based on internal features were found to be faster for familiar than for unfamiliar faces, whereas there was no difference in reaction time between matches based on the external features of familiar and unfamiliar faces. Faster matching of internal features of familiar faces was found to hold equally for pairs of photographs that differed in orientation of the face or in facial expression. In experiment 2 ‘same’ pairs were derived from the same photograph, which gave subjects the choice of matching on the basis of the features of the depicted faces or matching the photographs. Reaction times were faster than in experiment 1, and there were no differences between familiar and unfamiliar faces. The study confirms reports of differential saliency of the internal features of familiar faces, and shows that this only holds when stimuli are treated as faces. The finding thus reflects properties of structural rather than pictorial codes.
A Spatial Illusion from Motion Rivalry
Authors: Burr David C , Ross John , Morrone M Concetta
Published: January 2007

A new dynamic visual illusion is reported: contrast reversal of a horizontal and vertical plaid pattern (produced by adding two orthogonal sinusoidal gratings) causes the pattern to appear as an array of lustrous diamonds, cut by sharp lines into a diagonal lattice structure. On the basis of computer simulations it is suggested that the illusion results from rivalrous interaction of motion detectors tuned to opposing directions of motion.
High-Resolution Facial Feature Saliency Mapping
Authors: Haig Nigel D
Published: January 2007

For recognition of a target there must be some form of comparison process between the image of that target and a stored representation of that target. In the case of faces there must be a very large number of such stored representations, yet human beings seem able to perform comparisons at phenomenal speed. It is possible that faces are memorised by fitting unusual features or combinations of features onto a bland prototypical face, and such a data-compression technique would help to explain our computational speed. If humans do indeed function in this fashion, it is necessary to ask just what are the features that distinguish one face from another, and also, what are the features that form the basic set of the prototypical face. The distributed apertures technique was further developed in an attempt to answer both questions. Four target faces, stored in an image-processing computer, were each divided up into 162 contiguous squares that could be displayed in their correct positions in any combination of 24 or fewer squares. Each observer was required to judge which of the four target faces was displayed during a 1 s presentation, and the proportion of correct responses for each individual square was computed. The resultant response distributions, displayed as brightness maps, give a vivid impression of the relative saliency of each feature square, both for the individual targets and for all of them combined. The results, while broadly confirming previous work, contain some very interesting and surprising details about the differences between the target faces.
A Test of the Spatial-Frequency Explanation of the Müller-Lyer Illusion
Authors: Carrasco Marisa , Figueroa Jesus G , Willen J Douglas
Published: January 2007

Previous investigations have shown that the response of spatial-frequency-specific channels in the human visual system is differentially affected by adaptation to gratings of distinct spatial frequencies and/or orientations. A study is reported of the effects of adaptation to vertical or horizontal gratings of a high or a low spatial frequency on the extent of the Brentano form of the Müller-Lyer illusion in human observers. It is shown that the illusion decreases after adaptation to vertical gratings of low spatial frequency, but seems unaffected otherwise. These results are consistent with the notion of visual channels that are spatial-frequency and orientation specific, and support the argument that the Müller-Lyer illusion may be due primarily to lower-spatial-frequency components in the Fourier spectra of the image.
The Stiles—Crawford Effect of the First Kind (SCE I): Studies of SCE I in an Aniridic Observer
Authors: Enoch Jay M , Lakshminarayanan Vasudevan , Yamade Shinichi
Published: January 2007

The Stiles—Crawford effect of the first kind (SCE I), discovered in 1933, represented a major breakthrough in our understanding of retinal physiology and the modern beginning of the science of photoreceptor optics. The current status of knowledge in this area is briefly reviewed. A study is presented of a white adult female observer with aniridia, clear media, only traces of nystagmus, rather good visual acuity, and good fixation. It is shown that this individual exhibits approximate alignment of her photoreceptors with the center of the retinal sphere, clear evidence of side lobes on functions, and surprisingly steep SCE I functions. The implications of these findings are considered.
The Stiles—Crawford Effect of the Second Kind (SCII): A Review
Authors: Alpern Mathew
Published: January 2007

The background which led Stiles to the discovery that the color of a monochromatic ray of light varied with its angle of incidence on the retina, and the developments in the subject since 1937 when this discovery was reported are summarized. Stiles's original measurements of the ‘hue shift’ did not successfully quantify SCII in every part of the spectrum, since it provided only two degrees of freedom while the color changes sometimes require three. Nevertheless, they were the paradigm for most subsequent work for the next quarter of a century. Full quantification of the effect was first obtained by trichromatic matching on Stiles's NPL trichromator almost 25 years after the initial discovery. The phenomena were then also fully described quantitatively with an elaboration of Stiles's original theory (‘self-screening’ theory), on the assumption that the ordinary laws of additivity are valid for color matches of three primaries striking the retina at one angle of obliquity to a test incident at a different angle. More recent experiments suggest that this initial assumption may not be generally true and that ‘self-screening’ theory may not generally provide a satisfactory description of the color changes throughout the visible spectrum, even for measurements of the effect obtained under conditions in which the additivity assumption seems to be valid. However, the available data strongly imply that absorptions of photons obliquely incident on foveal cones depend upon spectra clearly different from those upon which absorptions of normally incident photons depend.
Observations on Stroboscopic Induced Motion
Authors: Micella Francesco , Pinna Baingio
Published: January 2007

Subject-relative explanations of motion induction state that induced motion is the result of a misperceived shift of the median plane of the visual field of the subject. This theory does not require relative motion of the spot and frame, in the classical spot-and-frame condition, only asymmetrical stimulation. Three experiments are reported in which stroboscopic induced motion was investigated. The experimental arrangement was unconventional in that the induced object (spot) was presented only during the interstimulus interval between the exposures of the inducing object (frame). This allowed differentiation of the duration of the induced movement and that of the inducing one. In the first experiment it was demonstrated that perception of induced motion depends upon the duration of the interstimulus interval between the presentations of the inducing frame. In the second experiment it was shown that the perceived velocity of the induced movement can be different from that of the inducing one and depends on the duration of exposure of the induced object. In the third experiment a stimulus display was created in which the apparent displacement of an object and its induced motion are incongruous. The results are incompatible with subject-relative displacement as the sole determining factor of motion induction and they present some difficulties for the hypothesis that induced motion is the result of the apportionment of the objective displacement of the frame.
Spatial Frequency and Selective Attention to Local and Global Information
Authors: Shulman Gordon L , Wilson James
Published: January 2007

Probe methods were used to investigate whether the distribution of attention to the local or the global structure of a stimulus affects the detectability of different spatial frequencies. Four experiments are reported in which the detectability of threshold probe gratings of different spatial frequencies was measured while subjects analyzed either the local or the global information from a display. A relative shift in the detectability of low and high frequencies was observed. Low frequencies were facilitated during global processing and/or high frequencies were facilitated during local processing.
Apparent Motion in Depth Resulting from Changing Size and Changing Vergence
Authors: Heuer Herbert
Published: January 2007

Two experiments are reported in which the effect of combining stimuli of changing size and changing vergence on the perception of motion in depth was examined. Changing size and changing vergence corresponded to in-phase or anti-phase sinusoidal motions of an outline circle, with different amplitudes. In-phase stimuli had approximately additive effects on the estimated peak-to-peak amplitude of apparent motion in depth. Anti-phase stimuli did not cancel each other; apparent motion was in-phase with one or the other stimulus. When apparent motion was in-phase with one stimulus, there was only a limited influence of the other stimulus. The results are discussed with regard to a model proposed by Regan and Beverley for the combination of changing size and changing disparity.
Interaction of Simultaneous Visual Events
Authors: Wilson J T Lindsay
Published: January 2007

Five experiments are reported in which a perceptual interaction between simultaneous brief events was investigated. Subjects judged whether a briefly presented stimulus was flickering or not. Performance was much more error prone when a similar unattended stimulus was presented at a relatively remote position in the visual field, thus confirming previous findings. Interference was obtained for stimuli presented both peripherally and centrally in the visual field. The effect was not confined to large stimuli, but the larger the interfering stimulus the greater the effect it had. Events in the auditory modality interfered with judgements made in the visual modality. Possible mechanisms for the interaction are discussed.
Letter Recognition at Low Contrast Levels: Effects of Letter Size
Authors: Blommaert Frans J J , Timmers Han
Published: January 2007

Contrast variation was used to measure recognition thresholds for lowercase letters, with the aim of obtaining a better understanding of the role that early stages of visual processing play in letter recognition. Frequency-of-recognition curves were measured for alphabets of different letter size. Since variation of the adaptational state of the eye changes the characteristics of primary visual processing in a quantifiable way, recognition thresholds were measured both at a high (150 cd m−2) and at a low (0.9 cd m−2) adaptation level. Thresholds decreased as letter size increased, in a way comparable with data on visual acuity. At the lower adaptation level, recognition thresholds became higher, which is also in accordance with visual acuity data. Furthermore, the slopes of the frequency-of-recognition curves for alphabets as a function of log contrast decreased with decreasing letter size. It is argued that this is mainly caused by an increasing dispersion of internal representations of individual letters on the internal psychological scale as letter size decreases.
Subjective Rarefaction in Illusory Figures: The Inadequacy of Apparent Lightness as an Explanation
Authors: Bressan Paola
Published: January 2007

Apparent rarefaction in subjective figures seems to violate the principle of size scaling. It has been claimed that this anomaly is due to a difference in illusory lightness which counteracts the expected effect of illusory depth. Qualitative evidence is presented that neither illusory lightness nor illusory depth have a relevant part in the phenomenon. An alternative account, in which contrast and size are shown to play major roles, is presented and discussed.
Masking by Uniform Field Flicker: Some Practical Problems
Authors: Badcock David R , Sevdalis Elizabeth
Published: January 2007

The technique of uniform field flicker (UFF) masking has frequently been used to address issues concerning the relative performance of sustained and transient neural channels in the human visual system. Unfortunately there has been an artifact in the implementation of this method in most published experiments which has meant that the contrast of the target has been flickered in synchrony with the mean luminance. A study is reported in which the artifact was corrected and the effects of UFF masking on the contrast sensitivity function then examined. With this correction, masking was still restricted to low spatial frequencies but it was much weaker than reported originally. It is argued that the original evidence suggesting that UFF masking can be used to examine the functioning of transient and sustained channels has not been interpreted correctly and that the basis for such a claim is weak.
The Concave Cusp as a Determiner of Figure—Ground
Authors: Stevens Kent A , Brookes Allen
Published: January 2007

The tendency to interpret as figure, relative to background, those regions that are lighter, smaller, and, especially, more convex is well known. Wherever convex opaque objects abut or partially occlude one another in an image, the points of contact between the silhouettes form concave cusps, each indicating the local assignment of figure versus ground across the contour segments. It is proposed that this local geometric feature is a preattentive determiner of figure—ground perception and that it contributes to the previously observed tendency for convexity preference. Evidence is presented that figure—ground assignment can be determined solely on the basis of the concave cusp feature, and that the salience of the cusp derives from local geometry and not from adjacent contour convexity.
Visual Stimulation Affects the Perception of Voluntary Leg Movements during Walking
Authors: Lackner James R , DiZio Paul
Published: January 2007

When a limb is used for locomotion, patterns of afferent and efferent activity related to its own motion are present as well as visual, vestibular, and other proprioceptive information about motion of the whole body. A study is reported in which it was asked whether visual stimulation present during whole-body motion can influence the perception of the leg movements propelling the body. Subjects were tested in conditions in which the stepping movements they made were identical but the amount of body displacement relative to inertial space and to the visual surround varied. These test conditions were created by getting the subjects to walk on a rotatable platform centered inside a large, independently rotatable, optokinetic drum. In each test condition, subjects, without looking at their legs, compared, against a standard condition in which the floor and drum were both stationary, their speed of body motion, their stride length and stepping rate, the direction of their steps, and the perceived force they exerted during stepping. When visual surround motion was incompatible with the motion normally associated with the stepping movements being made, changes in apparent body motion and in the awareness of the frequency, extent, and direction of the voluntary stepping movements resulted.
Interactions in the Discrimination and Absolute Judgement of Orientation and Length
Authors: Dick Miri , Hochstein Shaul
Published: January 2007

An asymmetric model is described for interactions in the perception of two dimensions (length and orientation) of a single visual stimulus. Two methods were used to test these interactions, and models for the interpretation of the possible outcomes of these tests are discussed. A length discrimination task showed facilitation (decreased reaction time) when orientation was covaried with length, and interference (increased reaction time) when random orientation variation was introduced. A smaller effect was seen when length was varied in an orientation discrimination task in a correlated or random fashion. Analysis of sequential effects showed that reaction times are fastest on repetition trials and are slowed by either the need to change the response or the need for additional sensory processing. With the second method, it was found that the amount of information transmitted in the estimation of orientation was not affected by the introduction of the redundant dimension of length, but that there was a significant gain in the amount of information transmitted in the estimation of length by the addition of the redundant dimension of orientation. It is concluded that orientation is probably a perceptual primitive of the visual system whereas length is a computed variable.
Information Influences the Pattern of Eye Fixations during Sentence Comprehension
Authors: Underwood Geoffrey , Bloomfield Richard , Clews Susan
Published: January 2007

How does the pattern of eye fixation vary as an informative part of a word is encountered? If the processing of information lags behind the movement of the eyes, then we should expect no variation in the pattern; but if processing is immediate, then the movements of the reader's eyes should correspond to the distribution of information being inspected. An experiment is reported which examined the ways that the text ahead of the point of current fixation can be used to guide the eyes to future fixations, by monitoring fixations during a sentence comprehension task. The patterns of eye fixations upon words with uneven distributions of information (where, for example, words predictable from the sight of their first few letters but not from their last few letters are defined as containing informative beginnings) were observed, and it was found that more and longer fixations were produced when subjects looked at the informative parts of words, particularly at the informative endings of words. The results support the suggestion that eye movements are under the moment-to-moment control of cognitive mechanisms.
Perceptions of Depth Elicited by Occluded and Shearing Motions of Random Dots
Authors: Royden Constance S , Baker James F , Allman John
Published: January 2007

A computer-controlled display of random dots was used to study perceptions of depth. In this display, a field of stationary random dots surrounded a rectangular area in which random dots moved with uniform velocity in a single direction. The boundaries of this rectangle did not move. When dot motion was perpendicular to the longer boundary of the rectangle (occluded motion), the rectangle seemed to be behind the stationary background surround. Motion parallel to the longer boundary of the rectangle (shearing motion) made it appear in front of the surround. The relative lengths of the sides of the rectangle determined which effect predominated. Thus, for motion perpendicular to the long axis of the rectangle the occlusion predominated and naive subjects reported that the central area seemed farther away than the surround. For shearing motion parallel to the long axis, the subjects reported that the rectangle was closer than the surround and the strength of both effects also depended on the length-to-width ratio of the rectangle. If there was occluded motion along the long axis, as the length-to-width ratio increased so did the likelihood that subjects would report seeing the rectangle behind the surround. Conversely, with shearing motion along the long axis, increasing the length-to-width ratio increased the likelihood that the rectangle would appear unambiguously in front of the surround. Some subjects integrated the two cues with the resulting perception being a rotating cylinder. The occlusion effect was stronger than the shearing effect. In fact, this ‘far’ depth effect was so powerful that it tended to override conflicting depth cues such as height in the visual field or stereopsis. The ‘near’ depth effect produced by shearing motion was definite but these other depth cues could often override it.
Orientation Discrimination and Cortical Function in the Human Newborn
Authors: Slater Alan , Morison Victoria , Somers Marcia
Published: January 2007

There is some controversy concerning whether or not the visual abilities of the newborn are mediated entirely through subcortical pathways or whether the visual cortex is functioning at birth. A critical test of cortical functioning is discrimination of orientation: orientation-selective neurons are found in the visual cortex but not in subcortical parts of the visual system. An experiment is described in which newborn infants were habituated to a square-wave grating oriented 45° from vertical. After habituation, significant preferences for the novel, mirror-image, grating were found, a result which argues for some degree of visual cortical functioning at birth.
Visual Information and Skill Level in Time-To-Collision Estimation
Authors: Cavallo Viola , Laurent Michel
Published: January 2007

Previous studies on the visual origin of time-to-collision ( Tc) information have demonstrated that Tc estimates can be based solely on the processing of target expansion rate (optic variable τ). But in the simulated situations used (film clips), there was little reliable information on speed (owing to reduced peripheral vision) and distance (owing to the absence of binocular distance cues) available. In order to determine whether these kinds of information are also taken into account, it is necessary to take an approach where the subject receives a more complete visual input. Thus, an experiment conducted on a circuit under actual driving conditions is reported. Experienced drivers and beginners, who were passengers in a car, had to indicate the moment they expected a collision with a stationary obstacle to take place. Subjects were blindfolded after a viewing time of 3 s. The conditions for speed evaluation (normal versus restricted visual field) and distance evaluation (binocular versus monocular vision) by subjects were varied. The approach speed (30 and 90 km h−1) and actual Tc (3 and 6 s) were also varied. The results show that accuracy of Tc estimation increased with (i) normal visual field, (ii) binocular vision, (iii) higher speeds, and (iv) driving experience. These findings have been interpreted as indicating that both speed and distance information are taken into account in Tc estimation. They suggest furthermore that these two kinds of information may be used differently depending on the skill level of the subject. The results are discussed in terms of the complementarity of the various potentially usable visual means of obtaining Tc information.
Stereoscopic Illusion Based on the Proximity Principle
Authors: Papathomas Thomas V , Julesz Bela
Published: January 2007

A class of ambiguous random-dot stereograms were created that share the following interesting property: Although the binocular disparity forms a periodic ‘sawtooth’ waveform as a function of row number (the disparity is constant for a given row), these stimuli yield a monotonically increasing depth percept along the rows. The random-dot pattern of each row is periodic along the horizontal direction for the purpose of producing an ambiguous depth percept. It is this ambiguity that makes it possible for the periodic stimulus to give rise to a monotonic percept. This monotonic percept is substantially enhanced when the rows are shown in temporal sequence instead of all being displayed together. Experiments are reported which indicate that this illusion is due to the proximity, or pulling, effect in stereopsis.
Adaptation to Suprathreshold Contrast and Luminance Stimuli
Authors: de Vries Meindert J , Yakimoff Naum , Spekreijse Henk
Published: January 2007

The temporal and spatial properties of the difference in perceived contrast and brightness of two suprathreshold stimuli presented successively in different retinal locations were determined. Stimulus onset asynchrony (SOA) was varied and the perceived contrast or brightness of the first stimulus (S1) was measured as a function of SOA by matching the contrast or luminance of the second stimulus (S2) to that of S1. The two stimuli overlapped in time for 200 ms to allow the comparison to be made. The adjusted values for S2 could well be fitted with an exponential decay function of SOA. For luminance increments and decrements the time constant for this function was 253 ms; for checkerboards with checks of size 16 min square the time constant was 164 ms. The difference in perceived contrast was dependent on initial contrast in a nonlinear fashion. It increased with increasing check size and was independent of the mean luminance and spatial proximity of the two stimuli. The phenomenon was observed with different pattern types and with dichoptic presentation, but could only be seen when direct comparison of the two stimuli was possible.
Motion Aftereffects and Retinal Motion
Authors: Mack Arien , Hill James , Kahn Steven
Published: January 2007

Two experiments are described in which it was investigated whether the adaptation on which motion aftereffects (MAEs) are based is a response to retinal image motion alone or to the motion signal derived from the process which combines the image motion signal with information about eye movement (corollary discharge). In both experiments observers either fixated a stationary point or tracked a vertically moving point while a pattern (in experiment 1, a grating; in experiment 2, a random-dot pattern) drifted horizontally across the field. In the tracking condition the adapting retinal motion was oblique. In the fixation condition it was horizontal. In every case in both conditions the MAE was horizontal, in the direction opposite to that of pattern motion. These results are consistent with the hypothesis that the adaptation is a response to the motion signal derived from the comparison of eye and image motion rather than to retinal motion per se. An alternative explanation is discussed.
Circular Vection as a Function of the Relative Sizes, Distances, and Positions of Two Competing Visual Displays
Authors: Howard Ian P , Heckmann Thomas
Published: January 2007

In studies where it is reported that illusory self-rotation (circular vection) is induced more by peripheral displays than by central displays, eccentricity may have been confounded with perceived relative distance and area. Experiments are reported in which the direction and magnitude of vection induced by a central display in the presence of a surround display were measured. The displays varied in relative distance and area and were presented in isolation, with one moving and one stationary display, or with both moving in opposite directions. A more distant display had more influence over vection than a near display. A central display induced vection if seen in isolation or through a ‘window’ in a stationary surrounding display. Motion of a more distant central display weakened vection induced by a nearer surrounding display moving the other way. When the two displays had the same area their effects almost cancelled. A moving central display nearer than a textured stationary surround produced vection in the same direction as the moving stimulus. This phenomenon is termed ‘contrast-motion vection’ because it is probably due to illusory motion of the surround induced by motion of the centre. Unequivocal statements about the dominance of an eccentric display over a central display cannot be made without considering the relative distances and sizes of the displays and the motion contrast between them.