Psychophysics of vision

To investigate the effect of intraocular straylight (IOS) on threshold measurements made by four types of perimetry: standard automated perimetry (SAP), frequency-doubling technology (FDT) perimetry, flicker-defined form (FDF) perimetry, and the Moorfields Motion Displacement Test (MDT). METHODS. Six healthy young (21-29 years) psychophysically experienced observers were tested five times with each perimeter, each time with one of five different white opacity filters (WOFs), to induce various levels of IOS. Each WOF gave rise to an increase in IOS, measured with a straylight meter, of between 10% and 200% from baseline and ranging in effect from normal ageing to significant cataract. The change in sensitivity from baseline was normalized, allowing comparison of standardized (z) scores (change divided by the standard deviation of normative values) for each instrument. RESULTS. Average SAP, FDT, and FDF thresholds were significantly affected (P Ͻ 0.001) by moderate to large increases in IOS (50%-200%), whereas MDT remained largely unaffected over the same range of straylight levels.

In the theoretical framework of this paper, attention is part of the inference process that solves the visual recognition problem of what is where. The theory proposes a computational role for attention and leads to a model that predicts some of its main properties at the level of psychophysics and physiology. In our approach, the main goal of the visual system is to infer the identity and the position of objects in visual scenes: spatial attention emerges as a strategy to reduce the uncertainty in shape information while feature-based attention reduces the uncertainty in spatial information. Featural and spatial attention represent two distinct modes of a computational process solving the problem of recognizing and localizing objects, especially in difficult recognition tasks such as in cluttered natural scenes.

Gustav Fechner, the German experimental psychologist, coined the term psychophysics in 1860, publishing the first mathematical equation to model human consciousness.  Fechner assumed that any future approaches to consciousness would include mathematical and physical underpinnings. In 1995, the cognitive scientist and philosopher David Chalmers coined the phrase "the hard problem" as being that of connecting consciousness with some physical substrate.  According to Chalmers, "The really hard problem of consciousness is the problem of experience. Most existing theories of consciousness either deny the phenomenon, explain something else, or elevate the problem to an eternal mystery."  As Chalmers points out, unfortunately, the modern scientific community has not yet produced a model of consciousness supported by mathematics and physics, thus no progress in Fecher's "psychophysics." 
However there is growing interest among the scientific community in string theory, the only branch of mathematical geometry that has successfully explained recent discoveries in high energy particle physics. The mathematical proof of string theory requires the existence of a ten dimensional universe.  But in trying to map consciousness, science continues to limit its data search within time and the three spatial dimensions, ignoring six of string theory's ten dimensions  Perhaps because mainstream science tacitly limits itself to four of the ten dimensions, progress has come to a dead end. Accordingly, the most widely accepted theory among neurophysiologists is that consciousness is somehow a "byproduct" of the electrical sparking of neurons, an accidental epiphenomenon of the activities of nerve-filled wet meat.
This paper considers data beyond the four dimensional constraints of the scientific establishment. It is my contention that many recorded accounts of philosophers, mystics, and shamans can and should be considered as data. My thesis is that these accounts express experience and observation within one or more of the additional dimensions predicted by string theory. Any advance in understanding consciousness must take into account not only the language and domains of mathematics, particle physics, and neurobiology, but also the experiential domains of philosophers and mystics. It is in regarding data from these multiple domains of inquiry that a new model of consciousness will emerge. In this paper I hypothesize a psychophysics of consciousness, woven of strands from mathematics, mysticism, and physics, as well as contemplative participatory practice, to address and solve "the hard problem."

The timing of events can influence spatial segmentation. Neighbouring regions flickering asynchronously at a high temporal frequency appear identical, but the visual system signals a 'phantom contour' between them. Using psychophysical procedures, we determined the spatial limitations of this form of temporal segmentation across the visual field. These limitations correspond closely to receptive field diameters of neurons in primary visual cortex (V1), and are also consistent with the spatial limitations of motion detection. Here, we propose a computational model in which the neural signal for phantom contours originates from individual neurons with receptive fields that are space-time separable, like those mapped in cat and macaque V1. Similar units are also employed in the first stages of models for the perception of local motion. The correspondence between theoretical models of temporal segmentation and motion perception in the human visual system, as well as psychophysical evidence that they have similar spatial limitations, support the notion that these processes are mediated by a common neural substrate situated in early in visual cortex.

A quarter of a century ago, the first systematic behavioral experiments were performed to clarify the nature of color constancy—the effect whereby the perceived color of a surface remains constant despite changes in the spectrum of the illumination. At about the same time, new models of color constancy appeared, along with physiological data on cortical mechanisms and photographic colorimetric measurements of natural scenes. Since then, as this review shows, there have been many advances. The theoretical requirements for constancy have been better delineated and the range of experimental techniques has been greatly expanded; novel invariant properties of images and a variety of neural mechanisms have been identified; and increasing recognition has been given to the relevance of natural surfaces and scenes as laboratory stimuli. Even so, there remain many theoretical and experimental challenges, not least to develop an account of color constancy that goes beyond deterministic and relatively simple laboratory stimuli and instead deals with the intrinsically variable nature of surfaces and illuminations present in the natural world.

Here we report the range and reliability of individual differences in stereo acuity in a large, well-characterised population. We tested 1000 participants aged 16-40 on a novel test of stereo acuity and on the TNO test. Participants had their monocular and binocular visual acuity measured and corrected. They were assessed for strabismus by means of a cover test, and by cards for detecting lateral and vertical phorias presented using a telebinocular. Contrast sensitivity was assessed separately for each eye, as was the magnitude of binocular masking. We report the range and distribution of individual variability for each of our measures, and the correlations between them. We also report test–retest reliabilities measured using a random 10% of our sample.

Color constancy is usually measured by achromatic setting, asymmetric matching, or color naming paradigms, whose results are interpreted in terms of indexes and models that arguably do not capture the full complexity of the phenomenon. Here we propose a new paradigm, chromatic setting, which allows a more comprehensive characterization of color constancy through the measurement of multiple points in color space under immersive adaptation. We demonstrated its feasibility by assessing the consistency of subjects' responses over time. The paradigm was applied to twodimensional (2-D) Mondrian stimuli under three different illuminants, and the results were used to fit a set of linear color constancy models. The use of multiple colors improved the precision of more complex linear models compared to the popular diagonal model computed from gray. Our results show that a diagonal plus translation matrix that models mechanisms other than cone gain might be best suited to explain the phenomenon. Additionally, we calculated a number of color constancy indices for several points in color space, and our results suggest that interrelations among colors are not as uniform as previously believed. To account for this variability, we developed a new structural color constancy index that takes into account the magnitude and orientation of the chromatic shift in addition to the interrelations among colors and memory effects.

" A new approach to the treatment of functional eye disorders successfully corrects strabismus, sports vision syndrome, migraine, tired eyes caused by eye strain, learning difficulties and  disturbed clarity of vision associated with emotional or psychological issues, myopia, and presbyopia, without surgery. "

Correctly perceiving the temporal order of events is essential to many tasks. Despite this, the factors constraining our ability to make timing judgments remain largely unspecified. Here we present a new phenomenon demonstrating that perceived timing of visual events may be profoundly impaired by the mere presence of irrelevant events elsewhere in the visual field. Human observers saw two abrupt luminance events presented across a range of onset asynchronies. Temporal order judgment (TOJ) just noticeable differences (JNDs) provided a behavioural index of temporal precision. When target events were presented in isolation or in static distractor environments temporal resolution was very precise (JNDs $20 ms). However, when surrounded by dynamic distractor events, performance deteriorated more than a factor of four. This contextual effect we refer to as Remote Temporal Camouflage (RTC) operates across large spatial and temporal distances and possesses a unique spatial distribution conforming to neither the predictions of attentional capture by transient events, nor by stimulus dependencies associated with other contextual phenomena such as surround suppression, crowding, object-substitution masking or motion-induced blindness. We propose that RTC is a consequence of motion-related masking whereby irrelevant motion signals evoked by dynamic distractors interfere with TOJ-relevant target-related apparent motion. Consistent with this we also show that dynamic visual distractors do not interfere with audio-visual TOJs. Not only is RTC the most spatially extensive contextual effect ever reported, it offers vision science a new technique with which to investigate temporal order performance, free of motionrelated sensory contributions.

Sakkaden, also kurze, schnelle Augenbewegungen dienen dazu, die visuelle Umwelt möglichst genau zu erkunden. Obwohl durch eine Sakkade das gesamte Retinaabbild der Szenerie verschoben wird, bleibt diese Verschiebung aufgrund der sakkadischen Suppression in der Regel unbemerkt. Einem Erklärungsmodell für diesen Effekt zufolge, geht das visuelle System grundlegend von der räumlichen Konstanz des Sakkadenziels aus. Erst wenn diese Annahme erheblich verletzt wird, beispielweise aufgrund einer großen intrasakkadischen Positionsänderung des Ziels oder seinem gänzlichen Fehlen nach Sakkadenende, wird die Diskrepanz zwischen prä- und postsakkadischen Position des Ziels bewusst bemerkt.
Damit Sakkaden möglichst akkurat bleiben wird auf Basis der Diskrepanz zwischen postsakkadischer Blick- und Zielposition die Programmierung des Verlaufs der nachfolgenden Sakkaden angepasst. Dieser Prozess der Sakkadenadaptation kann experimentell mittels intrasakkadischer Sprünge hervorgerufen und auf diese Weise untersucht werden. Dabei lässt sich beobachten, dass eine solche Adaptation auch dann stattfindet, wenn der intrasakkadische Sprung nicht bewusst wahrgenommen wurde.
Diese Dissoziation zwischen Sakkadenadaptation und Wahrnehmung wurde hier in einem Double-step Paradigma experimentell untersucht. Speziell wurde dabei geprüft, ob sich die Annahme der visuellen Stabilität des Sakkadenziels dadurch brechen lässt, dass es sich präsakkadisch erkennbar bewegt. In diesem Fall sollte der intrasakkadische Sprung besser bemerkt werden und anstatt auf eine fehlerhafte Sakkade auf eine reale Positionsveränderung des Ziels attribuiert werden. In der Folge sollte deshalb die Stärke der sakkadischen Adaptation geringer ausfallen. Als Ergebnis des Experiments zeigte sich bei präsakkadischer Zielbewegung keine verbesserte Erkennungsleistung des intrasakkadischen Sprungs verglichen mit einem statischen Ziel. Allerdings wies die Sakkadenadaptation bei beweglichen Zielen eine deutlich geringere Stärke auf, als wenn das Ziel präsakkadisch statisch blieb.
Dieser Befund weist, zusammen mit den weiteren in dieser Richtung durchgeführten Studien, auf eine Dissoziation von transsakkadischer Wahrnehmung und Sakkadenadaptation hin.

For a stable visual world, the colours; of objects should appear the same under different lights. This property of colour constancy has been assumed to be fundamental to vision, and many experimental attempts have been made to quantify it. I contend here, however, that the usual methods of measurement are either too coarse or concentrate not on colour constancy itself, but on other, complementary aspects of scene perception. Whether colour constancy exists other than in nominal terms remains unclear.

It is known that temporal modulation of the source illuminating a scene can affect visual performance. Nevertheless, there is a lack of evidence showing this in a natural setting with a simple, natural task. We devised a simple test of visual function using a pencil and paper visual search technique -identifying low-contrast printed numbers on the sheet of paper. We measured the effect of the temporal modulation of the fluorescent lighting upon visual performance on this task. We constructed two office environments lit with both tubular and compact fluorescent lamps, completely identical except for the lamp ballasting. Office A equipped with electronic ballasts, producing a light emission mainly modulated at 60 kHz having a modulation percentage of 3 percent for 100 Hz component; office B equipped with magnetic ballasts, producing a temporally modulated light which fundamental component-100 Hz frequency-has a modulation percentage of 32 percent).

Ratios of excitations in each cone-photoreceptor class produced by light reflected from pairs of surfaces in a scene are almost invariant under natural illuminant changes. The stability of these spatially defined ratios may explain the remarkable ability of human observers to efficiently discriminate illuminant changes from changes in surface reflectances. Spatial cone-excitation ratios are not, however, exactly invariant. This study is concerned with observers' sensitivity to these invariance violations. Simulations of Mondrian paintings with either 49 or two natural surfaces under Planckian illuminants were presented as images on a computer-controlled display in a two-interval experimental design: in one interval, the surfaces underwent an illuminant change; in the other interval, the surfaces underwent the same change but the images were then corrected so that, for each cone class, ratios of excitations were preserved exactly. Although the intervals with corrected images corresponded individually to highly improbable natural events, observers systematically misidentified them as containing the illuminant changes, the probability of error increasing as the violation of invariance in the other interval increased. For the range of illuminants and surfaces tested, sensitivity to violations of invariance was found to depend on cone class: it was greatest for long-wavelength-sensitive cones and least for short-wavelength-sensitive cones. Spatial cone-excitation ratios, or some closely related quantities, seem to be the cues preferred by observers for making inferences about surface illuminant changes.

Despite the compelling contribution of the study of event related potentials (ERPs) and eye movements to cognitive neuroscience, these two approaches have largely evolved independently. We designed an eye-movement visual search paradigm that allowed us to concurrently record EEG and eye movements while subjects were asked to find a hidden target face in a crowded scene with distractor faces. Fixation event-related potentials (fERPs) to target and distractor stimuli showed the emergence of robust sensory components associated with the perception of stimuli and cognitive components associated with the detection of target faces. We compared those components with the ones obtained in a control task at fixation: qualitative similarities as well as differences in terms of scalp topography and latency emerged between the two. By using single trial analyses, fixations to target and distractors could be decoded from the EEG signals above chance level in 11 out of 12 subjects. Our results show that EEG signatures related to cognitive behavior develop across spatially unconstrained exploration of natural scenes and provide a first step towards understanding the mechanisms of target detection during natural search.

— In a comparison of two asynchronous and spatially-separated luminous disks, the second stimulus has a tendency to be evaluated lighter against a dark background and darker against a lighter background, suggesting a temporal context effect to the perceived contrast rather than on luminosity. The current study confirms this hypothesis by direct evaluation of contrast comparison of Gabor patches, stimuli that have intrinsic contrast.

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries’ coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon.

The bootstrap method provides a powerful, general procedure for estimating the variance of a parameter of a function. The parametric version of the method was used to estimate the standard deviation of a threshold from a psychometric function and the standard deviation of its slope. Bootstrap standard deviations were compared with those obtained by a classical incremental method and by the asymptotic method of probit analysis. Twelve representative experimental conditions were tested in Monte Carlo studies, each of 1,000 data sets. All methods performed equally well with large data sets, but with small data sets the bootstrap was superior in both percentage bias and relative efficiency.

Object substitution masking (OSM) is said to occur when a perceptual object is hypothesised that is mismatched by subsequent sensory evidence, leading to a new hypothesised object being substituted for the first.  An example is when a brief target is accompanied by a longer lasting display of non-overlapping mask elements, and reporting of target features is impaired. Enns & DiLollo (2000) considered it an outstanding question whether OSM masks some or all aspects of a target.  We report three experiments demonstrating that OSM can selectively affect target features.  Participants may be able to detect a target while being unable to report other aspects of it, or to report the colour but not the orientation of a target (or vice-versa).  We discuss these findings in relation to two other visual phenomena.

This thesis examined how limiting the Field of View (FoV) impairs the ability to integrate visual information and make navigational decisions. Participants wore field-restricting goggles with a 40° or 90° horizontal FoV and were asked to locate specific targets within a maze-like environment. Performance was measured by average walking speed. Afterwards, they completed a set of spatial memory tasks that included sketching a map of the search area and distance and relative direction judgments. Participants with the narrower FoV walked significantly slower to the targets, and independent raters showed a significant preference for the maps of the wide FoV group. There was no effect of FoV for the relative direction and distance estimation task indicating a limited impact on object location memory. The results suggest that FoV restriction significantly impacts spatial representation of layout that needs to be considered in the design and use of augmented vision systems.

Body size misperception is common amongst the general public and is a core component of eating disorders and related conditions. While perennial media exposure to the " thin ideal " has been blamed for this misperception, relatively little research has examined visual adaptation as a potential mechanism. We examined the extent to which the bodies of "self" and "other" are processed by common or separate mechanisms in young women. Using a contingent adaptation paradigm, experiment 1 gave participants prolonged exposure to images both of the self and of another female that had been distorted in opposite directions (e.g., expanded other/contracted self), and assessed the aftereffects using test images both of the self and other. The directions of the resulting perceptual biases were contingent on the test stimulus, establishing at least some separation between the mechanisms encoding these body types. Experiment 2 used a cross adaptation paradigm to further investigate the extent to which these mechanisms are independent. Participants were adapted either to expanded or to contracted images of their own body or that of another female. While adaptation effects were largest when adapting and testing with the same body type, confirming the separation of mechanisms reported in experiment 1, substantial misperceptions were also demonstrated for cross adaptation conditions, demonstrating a degree of overlap in the encoding of self and other. In addition, the evidence of misperception of one's own body following exposure to " thin " and to " fat " others demonstrates the viability of visual adaptation as a model of body image disturbance both for those who underestimate and those who overestimate their own size.

A subject’s response to the strength of a stimulus is described by the psychometric function, from which summary measures, such as a threshold or a slope, may be derived. Traditionally, this function is estimated by fitting a parametric model to the experimental data, usually the proportion of successful trials at each stimulus level. Common models include the Gaussian and Weibull cumulative distribution functions. This approach works well if the model is correct, but it can mislead if not. In practice, the correct model is rarely known. Here, a nonparametric approach based on local linear fitting is advocated. No assumption is made about the true model underlying the data, except that the function is smooth. The critical role of the bandwidth is identified, and its optimum value is estimated by a cross-validation procedure. As a demonstration, seven vision and hearing data sets were fitted by the local linear method and by several parametric models. The local linear method frequently performed better and never worse than the parametric ones. Supplemental materials for this article can be downloaded from app.psychonomic-journals.org/content/supplemental.

Montréal. Il a pour mission la promotion et la valorisation de la recherche. Érudit offre des services d'édition numérique de documents scientifiques depuis 1998. Pour communiquer avec les responsables d'Érudit : erudit@umontreal.ca Article « L'émergence d'une psychophysique de la perception en art » Stefania Caliandro Tangence, n° 69, 2002, p. 133-155. Pour citer la version numérique de cet article, utiliser l'adresse suivante : http://id.erudit.org/iderudit/008076ar Note : les règles d'écriture des références bibliographiques peuvent varier selon les différents domaines du savoir. Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y compris la reproduction) est assujettie à sa politique d'utilisation que vous pouvez consulter à l'URI http://www.erudit.org/documentation/eruditPolitiqueUtilisation.pdf Document téléchargé le 22 avril 2009

Previous studies proposed a smart lighting system that can detect object colors and emit spectrally optimized lighting to reduce the light absorbed by surfaces. The spatial resolution of an absorption-minimization light projection system is investigated using images of various visual complexity. Participants with normal color vision and good visual acuity judged the visual clarity of low, medium, and high complexity images by using a mean opinion score (MOS) scale. Results from the visual assessments show that blur acceptability of illuminated images significantly reduces when the circle of confusion (CoC) is increased by 3%. Blur perception also changes with visual complexity.

Howard and Duke . Monocular transparency generates quantitative depth. Vision Research, 43, 2615Research, 43, -2621 recently proposed a new source of binocular information they claim is used to recover depth in stereoscopic displays. They argued that these displays lack conventional disparity and that the metrical depth experienced results from transparency rather than occlusion relations. Using a variety of modified versions of their stimuli, we show here that the conditions for transparency are not required to elicit the depth experienced in their stereograms. We demonstrate that quantitative and precise depth depended not on the presence of transparency but horizontal contours of the same contrast polarity. Depth was attenuated, particularly at larger target offsets, when horizontal contours had opposite contrast polarity for at least a portion of their length. We also show that a demonstration they used to control for the role of horizontal contours can be understood with previously identified mechanisms involved in the computations associated with stereoscopic occlusion. These results imply that the findings reported by Howard and Duke can be understood with mechanisms responsible for the computation of binocular disparity and stereoscopic occlusion.

Discrete and continuous modes of visual pattern discrimination performance are analyzed using a model for the investigation of discrete internal pattern representations described in previous papers (Foster, 1980a, b). A simple quantitative criterion is derived to characterize the two kinds of visual discrimination performance. Values predicted by this criterion are then compared with values obtained from experimental data.

Painters mastered replicating the regularities of the visual patterns that we use to infer different materials and their properties, via meticulous observation of the way light reveals the world’s textures. The convincing depiction of bunches of grapes is particularly interesting. A convincing portrayal of grapes requires a balanced combination of different material properties, such as glossiness, translucency and bloom, as we learn from the 17th-century pictorial recipe by Willem Beurs. These material properties, together with three-dimensionality and convincingness, were rated in experiment 1 on 17th-century paintings, and in experiment 2 on optical mixtures of layers derived from a reconstruction of one of the 17th-century paintings, made following Beurs’s recipe. In experiment 3 only convincingness was rated, using again the 17th-century paintings. With a multiple linear regression, we found glossiness, translucency and bloom not to be good predictors of convincingness of the 17...

Color-motion asynchrony (CMA) refers to an apparent asynchrony of visual features when a repeatedly oscillating moving dots change colour and motion direction simultaneously at the same rate (Moutoussis & Zeki, 1997). It has been demonstrated that a single direction change is enough to induce CMA in cross-feature correspondence tasks (Linares & Lopez-Moliner, 2006). Following a recent study, where Stetson et al (2006) have shown a reversal effect in the subjective order of simultaneous action and sensation after subjects were adapted to a delayed sensory feedback regime, here, we examined whether such visuo-motor temporal recalibration is sensory-feature-specific and transferrable to multi-feature-stimuli in correspondence tasks such as CMA. We first measured the effect of visuo-motor recalibration in an experiment where subjects judged the temporal order of a voluntary key press and a change in the motion direction of an achromatic dot array following a delayed (~100ms) sensory feedback adaptation in response to causal action. For those of subjects, who showed significant adaptation effects, we then conducted the main experiment using a single-direction-change CMA paradigm, where direction change was induced by voluntary key press and subjects were asked to report the predominant motion direction of target colour. Compared to baseline condition, we found that CMA was significantly reduced for the delayed sensory feedback adaptation condition. Control experiments showed that visuo-motor adaptation effects are not observed when the change in direction occurs independent of subject’s key press, which together show a temporal recalibration effect in a visuo-motor CMA paradigm due to voluntary action.

We show that visuo-motor temporal recalibration elicited with respect to a single visual feature (motion direction) in response to a voluntary causal action is transferrable to multi-feature-stimuli (i.e. chromatic dots changing motion direction) and reduce colour-motion asynchrony in a single-direction-change paradigm.

Argomenti:
La luce: cosa è, come si genera.
Diffusione e assorbimento: i colori di cielo, nuvole, tramonto, spazio.
La fisiologia della visione.
La percezione è diversa dalla visione: forme e colori ingannevoli.
Arte figurativa percettiva: le anamorfosi

Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or nondominant eye to assess the relative contributions of 2 cues: changing disparity and interocular velocity difference. Perceived speed for monocular lateral motion and perceived binocular visual direction (BVD) was also assessed. Unlike stereomotion trajectory perception, the BVD of static targets showed an ocular dominance bias, even without adaptation. Adaptation caused equivalent biases in perceived trajectory and monocular motion speed, without significantly affecting perceived BVD. Predictions from monocular motion data closely match trajectory perception data, unlike those from BVD sources. The results suggest that the interocular velocity differences make a significant contribution to stereomotion trajectory perception.

Two experiments are presented assessing the contributions of the rate of change of disparity (CD) and interocular velocity difference (IOVD) cues to stereomotion speed perception. Using a two-interval forced-choice paradigm, the perceived speed of directly approaching and receding stereomotion and of monocular lateral motion in random dot stereogram (RDS) targets was measured. Prior adaptation using dysjunctively moving random dot stimuli induced a velocity aftereffect (VAE). The degree of interocular correlation in the adapting images was manipulated to assess the effectiveness of each cue. While correlated adaptation involved a conventional RDS stimulus, containing both IOVD and CD cues, uncorrelated adaptation featured an independent dot array in each monocular half-image, and hence lacked a coherent disparity signal. Adaptation produced a larger VAE for stereomotion than for monocular lateral motion, implying effects at neural sites beyond that of binocular combination. For motion passing through the horopter, correlated and uncorrelated adaptation stimuli produced equivalent stereomotion VAEs. The possibility that these results were due to the adaptation of a CD mechanism through random matches in the uncorrelated stimulus was discounted in a control experiment. Here both simultaneous and sequential adaptation of left and right eyes produced similar stereomotion VAEs. Motion at uncrossed disparities was also affected by both correlated and uncorrelated adaptation stimuli, but showed a significantly greater VAE in response to the former. These results show that (1) there are two separate, specialised mechanisms for encoding stereomotion: one through IOVD, the other through CD; (2) the IOVD cue dominates the perception of stereomotion speed for stimuli passing through the horopter; and (3) at a disparity pedestal both the IOVD and the CD cues have a significant influence.

A characteristic of early visual processing is a reduction in the effective number of filter mechanisms acting in parallel over the visual field. In the detection of a line target differing in orientation from a background of lines, performance with brief displays appears to be determined by just two classes of orientation-sensitive filter, with preferred orientations close to the vertical and horizontal. An orientation signal represented as a linear combination of responses from such filters is shown to provide a quantitative prediction of the probability density function for identifying the perceived orientation of a target line. This prediction was confirmed in an orientation-matching experiment, which showed that the precision of orientation estimates was worst near the vertical and horizontal and best at about 30-degrees each side of the vertical, a result that contrasts with the classical oblique effect in vision, when scrutiny of the image is allowed. A comparison of predicted and observed frequency distributions showed that the hypothesized orientation signal was formed as an opponent combination and horizontal and vertical filter responses.

Object substitution masking (OSM) is typically studied using a brief search display. The target item may be indicated by a cue/mask surrounding but not overlapping it. Report of the target is reduced when mask offset trails target offset rather than being simultaneous with it. We report 5 experiments investigating whether OSM can be obtained if the search display is on view for a period of up to 830 ms but cueing of the target location is delayed. The question of interest is whether OSM must reflect the initial response of the visual system to target onset or whether it can arise in other ways, possibly during the transition from a pre-attentive representation of the target item to an attentional representation of it. Our results show that OSM decreases in strength as target duration increases. An explanation is suggested in terms of the object individuation hypothesis (Lleras & Moore, 2003).

One of the processes determining object substitution masking (OSM) is thought to be the spatial competition between independent object file representations of the target and mask (e.g., Kahan & Lichtman, 2006). In a series of experiments, we further examined how OSM is influenced by this spatial competition by manipulating the overlap between the surfaces created by the modal completion of the target (an outline square with a gap in one of its sides) and the mask (a four-dot mask). The results of these experiments demonstrate that increasing the spatial overlap between the surfaces of the target and mask increases OSM. Importantly, this effect is not caused by the mask interfering with the processing of the target features it overlaps. Overall, the data indicate, consistent with Kahan and Lichtman, that OSM can arise through competition between independent target and mask representations

We investigate whether there are second-order form and motion mechanisms in human color vision. Second-order stimuli are contrast modulations of a noise carrier. The contrast envelopes are static Gabors of different spatial frequencies (0.125-1 cycles/8) or drifting Gabors of different temporal frequencies (0.25 cycles/8, 0.5-4 Hz). Stimuli are isoluminant red-green or achromatic. Secondorder form processing is measured using a simultaneous 2IFC (two-interval forced-choice) detection and orientation identification task, and direction identification is used for second-order motion processing. We find that for simple detection thresholds, chromatic performance is as good or better than achromatic performance, whereas for both motion and form tasks, chromatic performance is poorer than achromatic. Chromatic second-order form perception is very poor across all spatial and temporal frequencies measured and has a lowpass contrast modulation sensitivity function with a spatial cutoff of 1 cycle/8 and temporal cutoff of 4 Hz. Chromatic second-order motion sensitivity is even poorer than for form and typically is limited to 1-2 Hz. To determine whether this residual motion processing might be based on feature tracking, we used the pedestal paradigm of Lu and Sperling (1995). We find that adding a static pedestal of the same spatial frequency as the drifting Gabor envelope, with its contrast set to 1-2 times its detection threshold, impairs motion direction performance for the chromatic stimuli but not the achromatic. This suggests that the motion of second-order chromatic stimuli is not processed by a second-order system but by a third-order, featuretracking system, although a genuine second-order motion system exists for achromatic stimuli.