Manifest illusions : a reply to Axel Kohler

The notion of illusion as a discrepancy between physical stimulus and percept (here referred to as illusion d, as long as merely this "error" is meant) is unable to capture the four very different cases in which illusions can arise. The observer may or may not be aware of the discrepancy, and its magnitude may be large or small. I argue that the special case of small error paired with awareness deserves special attention. Only in this case does the observer readily see the illusion, since it becomes manifest (referred to as illusion m). Illusion m is a meaningful category even in cases where illusion d cannot be determined. Illusions m of apparent motion and illusions of intuitive physics are solicited

Beyond illusions : on the limitations of perceiving relational properties

Explaining the perception of our visual world is a hard problem because the visual system has to fill the gap between the information available to the eye and the much richer visual world that is derived from the former. Perceptual illusions continue to fascinate many researchers because they seem to promise a glimpse of how the visual system fills this gap. Illusions are often interpreted as evidence of the error -prone nature of the process. Here I will show that the opposite is true. To do so, I introduce a novel stance on what constitutes an illusion, arguing that the traditional view (illusion as mere discrepancy between stimulus and percept) has to be replaced by illusion as a manifest noticed discrepancy. The two views, unfortunately, are not necessarily related. On the contrary; we experience the most spectacular illusions where our perception is pretty much on target. Once our interpretation of the sensory data is off the mark, we usually no longer experience illusions but live happily without ever noticing the enormous perceptual and conceptual errors we make. The farther we move away from simple pictorial stimuli as the subject of our investigations, the more commonplace a discrepancy between percept and reality does become —and the less likely we are willing to call it illusory. Two case studies of our perception of relational properties will serve to illustrate this idea. The case studies are based on the conviction that perceiving is more than mere sensation, and that some degree of (unconscious) judgment is a necessary ingredient of perception. We understand little about how to balance objects and we make fundamental mistakes when perceiving the slipperiness of surfaces. All the while, we never experience illusions in this context. Thus, when dealing with simple percepts, illusions may be revealing. But when it comes to percepts that involve relational properties, illusions fail to arise, as perception is not concerned with veridicality but appears to be satisfied with the first solution that does not interfere with our daily activities

Functional neuroanatomy of time-to-passage perception

The time until an approaching object passes the observer is referred to as time-to-passage (TTP). Accurate judgment of TTP is critical for visually guided navigation, such as when walking, riding a bicycle, or driving a car. Previous research has shown that observers are able to make TTP judgments in the absence of information about local retinal object expansion. In this paper we combine psychophysics and functional MRI (fMRI) to investigate the neural substrate of TTP processing. In a previous psychophysical study, we demonstrated that when local retinal expansion cues are not available, observers take advantage of multiple sources of information to judge TTP, such as optic flow and object retinal velocities, and integrate these cues through a flexible and economic strategy. To induce strategy changes, we introduced trials with motion but without coherent optic flow (0% coherence of the background), and trials with coherent, but noisy, optic flow (75% coherence of the background). In a functional magnetic resonance imaging (fMRI) study we found that coherent optic flow cues resulted in better behavioral performance as well as higher and broader cortical activations across the visual motion processing pathway. Blood oxygen-level-dependent (BOLD) signal changes showed significant involvement of optic flow processing in the precentral sulcus (PreCS), postcentral sulcus (PostCS) and middle temporal gyrus (MTG) across all conditions. Not only highly activated during motion processing, bilateral hMT areas also showed a complex pattern in TTP judgment processing, which reflected a flexible TTP response strategy.Accepted manuscrip

Time-to-Passage Judgments in Nonconstant Optical Flow Fields

The time until an approaching object will pass an observer (time to passage, or TTP) is optically specified by a global flow field even in the absence of local expansion or size cues. Kaiser and Mowafy have demonstrated that observers are in fact sensitive to this global flow information. The present studies investigate two factors that are usually ignored in work related to TTP: (1) non-constant motion functions and (2) concomitant eye rotation. Non-constant velocities violate an assumption of some TTP derivations, and eye rotations may complicate heading extraction. Such factors have practical significance, for example, in the case of a pilot accelerating an aircraft or executing a roll. In our studies, a flow field of constant-sized stars was presented monocularly on a large screen. TIP judgments had to be made on the basis of one target star. The flow field varied in its acceleration pattern and its roll component. Observers did not appear to utilize acceleration information. In particular, TTP with decelerating motion were consistently underestimated. TTP judgments were fairly robust with respect to roll, even when roll axis and track vector were decoupled. However, substantial decoupling between heading and track vector led to a decrement in performance, in both the presence and the absence of roll

Global Flow Impacts Time-to-passage Judgments Based on Local Motion Cues

We assessed the effect of the coherence of optic flow on time-to-passage judgments in order to investigate the strategies that observers use when local expansion information is reduced or lacking. In the standard display, we presented a cloud of dots whose image expanded consistent with constant observer motion. The dots themselves, however, did not expand and were thus devoid of object expansion cues. Only the separations between the dots expanded. Subjects had to judge which of two colored target dots, presented at different simulated depths and lateral displacements would pass them first. Image velocities of the target dots were chosen so as to correlate with time-to-passage only some of the time. When optic flow was mainly incoherent, subjects’ responses were biased and relied on image velocities rather than on global flow analysis. However, the bias induced by misleading image velocity cues diminished as a function of the coherence of the optic flow. We discuss the results in the context of a global tau mechanism and settle a debate whether local expansion cues or optic flow analysis are the basis for time-to-passage estimation

Bright paint makes interior-space surfaces appear farther away

Previous studies have reported that bright ceilings appear higher than dark ceilings, irrespective of the other colorimetric properties of the ceiling color (hue, saturation) and irrespective of the luminance of the remaining room surfaces (walls, floor). In the present study, we expand these findings to width and depth estimates. We presented stereoscopic full-scale room simulations on a head-mounted display and varied the luminance of the side walls, rear wall, and ceiling independently of each other. Participants judged the width and depth of the simulated rooms. Our results show that the perceived spatial layout of a given room is significantly influenced by the luminance of the direct bounding surfaces (e.g., the side walls when judging perceived width) but less affected by the luminance of the other surfaces. In the discussion, we provide an overall picture of effects of surface luminance on the perceived layout of interior spaces and discuss the conclusions in the context of existing interior-design guidelines

2005 Policy Analysis Report on Migration and Asylum

Der Bericht über Migration und Asyl bietet einen Gesamtüberblick über die wichtigsten politischen Diskussionen und Entwicklungen in den Bereichen Migration und Asyl in der Bundesrepublik Deutschland im Jahr 2005. Dabei werden auch die Implementation von EU-Gesetzgebung in nationales Recht beschrieben sowie Erfahrungen mit der Umsetzung von EU-Gesetzgebung dargestellt

2006 Policy Analysis Report on Migration and Asylum

Der Bericht über Migration und Asyl bietet einen Gesamtüberblick über die wichtigsten politischen Diskussionen und Entwicklungen in den Bereichen Migration und Asyl in der Bundesrepublik Deutschland im Jahr 2006. Dabei werden auch die Implementation von EU-Gesetzgebung in nationales Recht beschrieben sowie Erfahrungen mit der Umsetzung von EU-Gesetzgebung dargestellt.The Policy Report 2006 of the German National Contact Point provides an overview of the most important political discussions, statutory developments and administrative practice in Germany in the areas of immigration and asylum. In addition, the report demonstrates the state of domestic implementation of European Community law. A synthesis report compares the main developments in the Member States, putting them into perspective

Policy Report 2007

Der Bericht über Migration und Asyl bietet einen Gesamtüberblick über die wichtigsten politischen Diskussionen und Entwicklungen in den Bereichen Migration und Asyl in der Bundesrepublik Deutschland im Jahr 2007. Dabei werden auch die Implementation von EU-Gesetzgebung in nationales Recht beschrieben sowie Erfahrungen mit der Umsetzung von EU-Gesetzgebung dargestellt.The Policy Report 2007 of the German National Contact Point provides an overview of the most important political discussions, statutory developments and administrative practice in Germany in the areas of immigration and asylum. In addition, the report demonstrates the state of domestic implementation of European Community law. A synthesis report compares the main developments in the Member States, putting them into perspective