Veridical Perception Research Articles

Apodization by the Stiles-Crawford effect moderates the visual impact of retinal image defocus.

Previous optical modeling of the human eye with large pupils has predicted a larger impact of defocus on the human contrast sensitivity function and modulation transfer function than is observed experimentally. Theory predicts that aberrations and the Stiles-Crawford effect (SCE) should both lead to increased depth of focus, resulting in higher contrast sensitivities and veridical (not phase-reversed) perception over a larger range of spatial frequencies in defocused retinal images. Using a wave optics model, we examine these predictions quantitatively and compare them with psychophysical experiments that measure the effect of defocus on contrast sensitivity and perceived phase reversals. We find that SCE apodization has its biggest effect on defocused image quality when defocus and spherical aberration have the same sign. A model including typical amounts of spherical aberration and pupil apodization provides a dramatically improved prediction of the effects of defocus on contrast sensitivity with large pupils. The SCE can significantly improve defocused image quality and defocused vision, particularly for tasks that require veridical phase perception.

The Poggendorff illusion: a bias in the estimation of the orientation of virtual lines by second-stage filters.

The veridical perception of collinearity between two separated lines is distorted by two parallel lines in the space between them (the Poggendorff illusion). This paper tests the conjecture that the perception of collinearity of separated lines is based on a two-stage mechanism. The first stage encodes the orientation of the virtual line between the proximal terminators of the target lines. The second stage compares this virtual orientation with the orientation of the target lines themselves. Errors can and do arise from either process. Two parallel lines, abutting against the target lines, cause the classical Poggendorff misalignment bias. The magnitude of the bias is increased by Gaussian blur, as is a version of the Poggendorff figure containing only acute angles. In the obtuse-angle figure, on the other hand, blur decreases the misalignment bias. We argue that the acute- and obtuse-angle biases depend upon different mechanisms, and that the obtuse-angle effect is more related to the obtuse-angle version of the Muller-Lyer illusion, which is also decreased by blur. If observers attempt to match the orientation of the virtual line between the two line intersections in the Poggendorff figure they make an error in the same direction as the Poggendorff bias. The orientation of the target lines in the figure, however, is veridically matched to a Gabor-patch probe, unless the target lines are very short, in which case the error is in the same direction as the Poggendorff bias. A small bend in the target lines where they abut the parallels increases the Poggendorff bias if it makes the line more orthogonal to the parallel, but has little effect in the opposite direction. The Poggendorff bias is unlikely to depend upon biases in first-stage linear filters because (a) it still exists in figures composed of short, luminance-balanced lines which are defined by contrast only; and (b) it also exists if the parallels are replaced by grating patches with the same mean luminance as the background. The orientation of the grating in the latter case affects the magnitude of the bias, but even an orientation which should reverse the Poggendorff bias by the mechanism of cross-orientation inhibition fails to do so. The Poggendorff bias is a complex effect arising from several sources. Blurring in second-stage filters with large receptive fields can explain many aspects of the phenomenon.

An Airplane Illusion: Apparent Velocity Determined by Apparent Distance

When a small drone plane appears to be a normal-sized airplane, it appears to be very far away and moving too fast. This is the airplane illusion. In the illusory situation, familiar size determines the apparent size and distance of the plane. It sets the depth for the frontal-plane component of the perceived motion and the relative depth difference for the motion-in-depth component. Because these perceived distances are very large, the perceived velocities are very large in the respective directions. Cognition can override familiarity and produce a veridical perception of the drone.

Veridical perception of global motion from disparate component motions.

Although it is in principle possible to determine the direction of motion of an object by combining the motion of its one-dimensional oriented contours (Fennema CL, Thompson WB. Comput. Graph. Image Processing 1979;9:301-315) there is still much debate on whether human observers can do so. The Intersection Of Constraint (IOC) rule proposed by Adelson and Movshon (Adelson EH, Movshon JA. Nature 1982;300:523-525), although compatible with the veridical object's motion, was challenged by recent psychophysical data obtained with type II plaids or lines moving behind apertures: perceived direction of motion is biased toward the vectorial average of the component motions, rather than in the direction predicted by the IOC rule. Since the velocity predicted by the vectorial rule is inconsistent with the physical velocity, its use leads to the puzzling prediction that the perceived position of a moving object becomes inconsistent with its actual position. In the present paper, the perceived path of a figure defined by its one-dimensional contours and moving behind apertures along a circular trajectory is compared with the discrepant predictions of the IOC and of the Vectorial model. The results show that the perceived path is close to veridical with these stimuli, therefore challenging the idea that the visual system uses a vector averaging rule.

Ghosts in the Machine? Pathological Visual Completion Phenomena in the Damaged Brain

Abstract Patlents with hemianopis following unilateral brain damage occasionally report seeing visual shapes such as squares and circles as complete, even though a significant portion of the shape falls within their objectively blind hemffield. In addition, some individuals report that partial shapes appear to be complete (e.g. they see a semicircle as a complete circle) when the half-shapes are presented entirely within the Intact hemffield, such that their amputated edge is aligned with the border of the field defect. These phenomena are referred to collectively as ‘pathological visual completion’. The phenomena are distinct from ‘blindsight’, in the sense that patients claim to ‘see’ parts of visual stimuli that fall within their blind hemffield (i.e. they appear to have conscious, veridical perception within their blind hemffield), or else they claim to see visual information that Is physically absent from the display altogether (i.e. they appear to have conscious, non-veridical or confabulatory perception within their blind hemffield). In this article we review critically all of the published reports of pathological visual completion phenomena, and interpret them in the context of the current debate surrounding visual completion in normals. We show that although there is evidence to suggest that visual completion can arise for particular kinds of stimuli in people without brain damage, the evidence for completion among brain-damaged patients is less clear-cut. We demonstrate that an overwhelming majority of studies claiming to show the existence of pathological completion phenomena can be questioned on methodological or theoretical grounds. Significantly, most patients reported as completing partial figures had parietal lobe damage and concomitant attention disorders such as unilateral neglect and visual extinction. We conclude that in these cases pathological completion phenomena may reflect unawareness of visual loss, rather than active filling-in.

Far Distance Perception

Determining how people perceive distance is a central issue in the study of visual perception. Whilst near-distance perception has been extensively researched, far-distance perception has received little attention. We review the literature and illustrate key points with new data, focusing on analysis of available information/cues, degree of accuracy, measurement procedures, perceptual/cognitive influences, and geometry of perceived space. Analysis of available cues suggests that under natural viewing there is sufficient information for veridical distance perception. However, distance judgements are inaccurate and vary with mode of measurement (eg absolute vs relative judgements). Inaccuracy includes compression of distance. This has been modelled with the use of different geometries—most commonly power functions. Exponents often average 1.0 but this conceals considerable individual variation (in our data individual exponents ranged from 0.5 to over 1.0). Further, even for averaged exponents values vary between 0.8 and 1.25, as a function of viewing conditions, experimental method, and of the relative contribution of cognitive and perceptual factors. Evidence suggests that distance is encoded at an ordinal level, but for many practical tasks the final judgement must be metric (eg range finding) and this transformation is error-prone (in our data numerical estimates could be a factor of 10 out). Further, many natural judgements require perception of the full layout of the scene (including exocentric distances) rather than the more commonly investigated perception of egocentric distance. Evidence suggests that training based on practice with feedback produces some improvement in accuracy, but this is highly context-specific.

In a world of their own: experiencing unconsciousness

Introduction Historical Theories, Assumptions, and Beliefs Paradigms of the Disciplines: Medical Science, Psychology, Parapsychology, and Physics Carol's Experience Total Unconsciousness Perceived Unconsciousness Inner Awareness Distorted Consciousness Extrasensory Experiences Out-of-Body Experiences Near-Death Experiences Near-Death Visits The Grim Reaper Pronounced Dead but Still Alive Conclusions, Current Research, and Clinical Implications Appendix: Further Evidence for Veridical Perception during Near-Death Experiences References Index

A descriptive framework for information processing: an integrative approach

Survival depends on veridical perception of the environment and appropriate response to it. How do high level organisms perceive and compare, in short, evaluate stimuli and how do they select and execute responses? How does information processing proceed? How does the mind work? © 1997 Elsevier Science B.V.

Aliased frequencies enable the discrimination of compound gratings in peripheral vision.

Although gratings beyond the Nyquist limit of the peripheral retina are visible as aliased percepts when presented in isolation, the reported lack of aliasing for targets with complex spectra (edges, lines, letters) suggests that aliased frequency components are invisible in the presence of sub-Nyquist image components. We tested this hypothesis by systematically exploring a range of stimulus parameters in search of conditions which would enable subjects to detect the supra-Nyquist components of a compound grating. A three-alternative forced-choice masking paradigm was used, which required subjects to discriminate a 2.5 deg patch of compound grating (mask + test) from a simple grating (mask only). Using a 2 c/deg grating as the masking component, which is well below the 4 c/deg Nyquist limit to veridical perception at 20 deg in the horizontal nasal field, we varied the spatial frequency of the test grating over a range extending above and below the Nyquist frequency. We found that aliased, supra-Nyquist components are reliably detected in the presence of high contrast, sub-Nyquist gratings, provided they have sufficient contrast. Contrast threshold for detecting the aliased gratings was higher when presented as a component of the compound grating than when presented in isolation. This masking effect of the sub-Nyquist component was not specific to supra-Nyquist test targets, however, since a similar masking effect also occurred for sub-Nyquist targets. These results suggest that the invisibility of aliasing described previously for edges or square wave gratings is due primarily to the combination of the low amplitudes of supra-Nyquist harmonics in such stimuli and a high contrast threshold of the peripheral retina. The additional masking effects of the sub-Nyquist, fundamental component of a square wave on the supra-Nyquist harmonics make the detection of aliasing impossible even for very high contrast square wave gratings presented in the periphery.

Pluralistic Ignorance Across Issues and Over Time: Information Cues and Biases

Pluralistic Ignorance Across Issues and Over Time: Information Cues and Biases

Motion Perception in High-Pass Filtered Random-Dot Patterns: Motion Energy, Element-Matching, or Both?

We have measured perception of the direction of displacement of two-frame random-dot patterns (50% light/dark pixels) that have been spatially high-pass filtered. In the ‘standard’ condition, pairs of high-pass filtered images, identical apart from the displacement, were presented in succession. The displacement could be in either of two opposite directions and the task was to identify the direction. The ‘reverse’ condition was the same except that image contrast was inverted between the two frames. Various element sizes and filter cut-offs were used. Two distinct patterns of results were obtained. For small check sizes, performance alternated cyclically between veridical direction perception and incorrect direction perception (aliasing) as displacement size was increased over a wide range. The period of the cycle was close to one period of the lowest spatial frequency remaining in the image after filtering, ie performance was as would be expected for a grating of that spatial frequency. In the ‘reverse’ condition the cyclical psychometric functions were inverted, ie reversed-phi motion occurred. For large check sizes, and particularly for high filter cut-offs, there was no cyclical alternation of direction perception and reversed phi did not occur in the ‘reverse’ condition. The results suggest that two mechanisms are at play. In most circumstances, detection appears to be based on motion energy since the cyclical alternation is predicted by a consideration of the spatiotemporal energy of the stimulus but not by element-matching theories. But for large elements, particularly when most of the low spatial frequencies have been removed, element-matching takes over. Elements are matched without regard to their contrast polarity. The results are thus inconsistent with the single front-end filter mechanism which Morgan [1992 Nature (London)355 344] has advanced to explain performance in this type of task.

Biological Motion Perception: From Inversion to Upright Display Orientation

How does biological motion perception change with display orientation? As previously shown, display inversion (180°) completely prevents veridical perception of biological motion. However, with upright orientation (0°), observers are able to recover the invariant structure through biological motion despite reverse transformation (showing the film backwards) or changing the presentation rate (Pavlova, 1995 Perception24 Supplement, 112). In the present experiments, observers saw the biological motion pattern at various display deviations, from inverted to upright orientation (180°, 150°, 120°, 90°, 60°, 30°, 0°), in the right or left hemifield, on a circular screen monitor. The display consisted of an array of 11 dots on the main joints of an invisible walker moving as if on a treadmill. While viewing (60 s), observers pressed a key each time their perception changed from one stable percept to another (eg when the direction of apparent rotation of the pattern reversed). The perceived multistability (the number of key-presses) increased as orientation was varied from inverted to 90°, and then decreased between 90° and upright. The recognition of walking figure improved abruptly with changing orientation: at deviations of 60° and 30° most observer reported seeing the walking figure spontaneously, yet the pattern was seen as multistable. The findings imply the relative power of constraints (such as orientation) in perception of biological motion that is discussed in relation to the KSD principle in event perception [Runeson, 1994, in Perceiving Events and Objects Eds Jansson, Epstein, Bergström (Hillsdale, NJ: Erlbaum) pp 383 – 405].

Fiction and Intentionality

A good phenomenological theory must be able to account equally well for our experiences of veridical perception and hallucination, for our thoughts about universities, colors, numbers, mythical figures and more. For all of these are characteristic mental acts, and a theory of intentionality should be a theory of conscious acts in general, not just of consciousness of a specific kind of thing or of a specific kind of consciousness. In so far as phenomenology purports to be a general study of intentional acts of consciousness, it must be able to account for acts with all kinds of objects (existent or non-existent, particular or universal, specific or vague) and with all kinds of thetic character (judgings, perceivings, imaginings...). Any limitation of these features puts the theory in danger of being an explanation of only some kinds of mental act, and of mis-characterizing the nature of intentionality in general. One of the most important features of intentionality is that the object of an intentional act need not exist (as a spatio-temporal, physical particular). Traditional theories of intentionality distort and trivialize the issue of our intentional relations to non-existent objects by considering almost exclusively such marginal and unusual cases as seeing a centaur. Such cases are, of course, important for theories of intentionality, but since they are so unusual, and epistemically so private, we have very few intuitions about how these experiences ought to be analyzed, against which we could evaluate the success of the theory. The selection of such cases is, in my opinion, the reason intentionality theory has not yet dealt adequately with the problems involved in dealing with non-existent objects like fictional characters. The case is very different with fictional objects, for they are objects of frequent and quite ordinary (as opposed to paranormal) daily conscious experiences. Our experience of fictional characters is no doubt one of the most important and common kinds of intentional relation to something outside the realm of ordinary physical existents: we discuss fictional characters with others, write theories about them, interpret them, pass moral and aesthetic judgments on them, admire and emulate them, even dedicate academic disciplines to their study. In short, only the oddest of people has hallucinatory experiences as complex and varied as our normal experiences of fictional objects.

Infant preferential looking: Face-voice synchrony vs. affect

Young infants have been reported to perceive the unity of a center-occluded object when the visible ends of the object are aligned and undergo common motion but not when the edges of the object are misaligned (Johnson & Aslin, 1996). Using a recognition-based paradigm, the authors investigated the possibility that past research failed to provide sufficiently sensitive assessments of infants' perception of the unity of misaligned edges in partial occlusion displays. Positive evidence was obtained in 4-month-olds for veridical perception of the motion and location of a hidden region but not its orientation, whereas 7-month-olds, in contrast to the younger infants, appeared to respond to the orientation of the hidden region. Overall, the results suggest that habituation designs tapping recognition processes may be particularly efficacious in revealing infants' perceptual organization. In addition, the findings provide corroborative evidence for the importance of both motion and orientation in young infants' object segregation and for the difficulty in achieving percepts of the global form of a partly occluded object.

Perception of object unity in young infants: The roles of motion, depth, and orientation

One hundred twenty-eight 4-month-old infants were habituated to one of several displays that depicted two rod pieces above and below a box. The effects of common motion, background texture, and orientation of the rod pieces on infants' perception of unity of the partially occluded rod were examined. Infants who viewed displays in which the rod pieces were aligned and presented in front of a textured background, subsequently looked longer at a broken rod (two rod pieces separated by a gap) than at a complete rod, implying that the infants experienced the rod pieces as connected behind the box in the first display. Infants who viewed displays with no background texture, or displays in which the rod pieces were nonaligned but relatable (i.e., connected if extended behind the occluder), looked equally at the two posthabituation displays. Infants who viewed displays containing nonrelatable rod pieces looked longer at the complete rod, implying that nonrelatable edges specify disjoint objects to 4-month-olds. A threshold model, stipulating that perception of object unity is supported by multiple visual cues, is proposed to account for these results. Veridical perception of motion of display elements, depth ordering, and edge orientation are necessary, but not individually sufficient, to support young infants' perception of object unity.

Perceiving the orientation-in-depth of triangular surfaces: static-monocular, moving-monocular, and static-binocular viewing.

Although data from simulations suggest that motion information and binocular information each elicit veridical depth perception, data from real stimuli, such as trapezoidal surfaces, are equivocal; the discrepancy might be explained by the complexity of nonveridical pictorial information in the latter. In the present study, observers judged the orientations-in-depth of triangular surfaces about a vertical axis: Pictorial information resided only in the visual lengths of surfaces, so it was predicted that motion and binocularity would be fully effective. Viewing conditions were static-monocular (SM), moving-monocular (MM), and static-binocular (SB). SM judgments were largely frontal, reflecting the equidistance tendency that applies in impoverished conditions. SB judgments were broadly veridical, as predicted. However, MM judgments were only partly influenced by motion information; the equidistance tendency and visual length also contributed--the latter in contrast to SM. It is concluded that motion information is only weakly effective, no matter what the complexity of pictorial information. Instead, motion may be valuable in enhancing pictorial information.

Detection thresholds for object motion and self-motion during vestibular and visuo-oculomotor stimulation

We compared the detection threshold for object motion with that of self-motion in space in healthy human subjects. Stimuli consisted of horizontal rotations of subjects' body with a fixation spot kept in fixed alignment with their heads (vestibular stimulus), rotation of the fixation spot relative to the stationary subjects (visuo-oculomotor stimulus), and a combination thereof by applying rotations of subjects' body relative to the stationary object (sinusoidal oscillations, 0.025–0.4 Hz). Two series of experiments were performed. 1) One group of subjects was instructed to attend to, and to indicate the occurrence of, either object or self-motion. 2) A second group was instructed not only to detect the occurrence of a perception, but also to qualify it either as object motion or self-motion, depending on which modality dominated perceptually. With either instruction it was found that all three stimulus conditions could evoke both, either an object motion perception or a self-motion perception. The detection thresholds of both perceptions were essentially similar. Thresholds were highest with the vestibular stimulus, intermediate with the stimulus combination, and lowest with the visuo-oculomotor stimulus. The vestibular threshold depended on stimulus frequency, in that it decreased with increasing frequency. Thereby, it became similar to the visuo-oculomotor one, which was essentially constant across frequency. Probability of occurrence of the perceptions in the first experimental series was considerably higher than in the second series, suggesting an important role of attentional mechanisms. In the second series, percent frequency of occurrence of veridical perception (object motion with visuo-oculomotor stimulus, self-motion with stimulus combination) was at chance level (50%) at low stimulus frequency, but was augmented considerably at high frequency. We assume that the latter effect is brought about by a visual-vestibular conflict measure by which the visual stimulus (light spot) is qualified as representing either a moving object or a spatial reference for self-motion. While at suprathreshold stimulus intensities the conflict can determine perception magnitude, at threshold levels its influence is restricted mainly on the probability of occurrence of object and self-motion perception.

Ghosts in the machine: A Case description of visual and haptic hallucinations after right hemisphere stroke

Abstract Visual and haptic hallucinations are described in a patient with a right hemisphere infarct. The hallucinations, which lasted intermittently for 10 days, occurred in clear consciousness and in the context of left hemiplegia, sensory loss, hemianopia, and visual neglect. These transient visual hallucinations, characteristic of Charles Bonnet Syndrome, featured children, animals, relatives, dripping water, and traffic, and coexisted with otherwise veridical percepts. Unusual features included the patient's claim to have touched some of the hallucinated objects and the persistent delusion, precipitated by the form of certain hallucinations, that the ward on which she was staying was located one floor above its true position. Specific attention is drawn to the problem of assessing the degree to which the patient showed insight into the hallucinatory nature of some of her percepts. We provide a modular account of the causes of the hallucinations and a central account of how they were interpreted by th...

On raising our sights

There are some very strong traditions in contemporary research on spatial vision that have arisen, in large measure, through advances in technology. This is especially true of research on the neurophysiology of vision, e.g. in recording from single cells, and in developments in computing that have promoted the study of machine vision. No one would dispute that these are major advances in the field, but one may ask whether they have unduly constrained the sorts of question that we ask about the visual processes. It is argued that they have, and that it is timely to stand back to see what other problems and questions have suffered neglect. Two items seem to be quite ripe for reconsideration: the question of how to understand the enormous adaptability of visual systems to sensory rearrangement (by means of distorting lenses, for example); and the very broad matter of keeping our perceptual processing in close and coherent touch both with stimulation, and with the properties of the physical/biological environment--what Gibson called the problem of veridical perception. It is argued that the two items are closely allied, and can be approached from a single theoretical stance. The one I favour is outlined, and I claim that it is one that has little to do, in the first instance, with either neurophysiology or computation. In that sense tackling the two questions involves raising our sights above the current traditions of vision research, but that of course does not in any way mean attempting to belittle the immense advances that those traditions have brought about.(ABSTRACT TRUNCATED AT 250 WORDS)

Perception of horizontal head and trunk rotation: modification of neck input following loss of vestibular function.

Chronic loss of vestibular function modifies the role of neck afferents in human perception of self-motion. We characterized this change by comparing the self-motion perception of patients with chronic vestibular loss (Ps) to that of normal subjects (Ns). Stimuli consisted of sinusoidal horizontal rotations (0.025-0.4 Hz) of the trunk relative to the head (neck stimulation) and/or of the head in space (vestibular stimulation). Perception of head rotation relative to the trunk, of trunk rotation in space, or of head rotation in space was assessed in terms of gain and phase (veridical perception, G = 1 and phi = 0 degree) as well as detection threshold using a pointing procedure. (1) Perception of head rotation relative to the trunk (neck proprioception). Ps' detection threshold of head-to-trunk rotation was normal (i.e. similar to that of Ns) across all frequencies tested. Also, with peak angular velocities above 5 degrees/s, the gain of their perception was approximately normal. When peak velocity was decreased below this value, however, either by lowering stimulus frequency with peak displacement kept constant (+/- 8 degrees) or by decreasing peak displacement at constant frequency (0.05 Hz), the gain increased above unity, unlike in Ns. In contrast, the phase remained normal (approximately 0 degree). (2) Perception of trunk rotation in space. Ps perceived their trunks as stationary during neck stimulation and all vestibular-neck combinations at medium to low frequencies. At 0.4 Hz, however, Ps consistently perceived the trunk rotation, conceivably due to somatosensory self-motion cues arising from high body acceleration. In contrast, Ns perceive a trunk-in-space rotation with the neck stimulation and most of the stimulus combinations across the whole frequency range tested. Ns perceived their trunks as stationary only during head rotation on the stationary trunk (presumed to reflect a mutual cancellation of neck and vestibular signals). (3) Perception of head rotation in space. In Ps, unlike Ns, this perception always resembled that of head rotation relative to the trunk. (4) When Ps were presented with a visual or somatosensory space reference (not motion cues), their perception of trunk and head rotation in space became approximately normal. (5) We suggest that there are basically two changes in the neck-induced self-motion perception associated with chronic vestibular loss. First, neck proprioception shows a non-linear gain that overemphasizes low stimulus velocities, for unknown reasons.(ABSTRACT TRUNCATED AT 400 WORDS)