The Ring Rotation Illusion: Properties and Links of a Novel Illusion of Motion.

We report a novel visual illusion we call the Ring Rotation Illusion (RRI). When a ring of stationary points replaces a circular outline, the ring of points appears to rotate to a halt, although no actual motion has been displayed. Three experiments evaluate the clarity of the illusory rotation. Clarity decreased as the diameter of the circle and ring increased and increased as the number of points forming the ring increased. The optimal interstimulus interval (ISI) between the circle and ring was 90 ms when stimulus presentations lasted 100 ms but 0 ms with 500 ms presentations. We compare the RRI to the Motion Bridging Effect (MBE), a similar illusion in which a stationary ring of points replaces an initial ring of points that spins so rapidly it looks like a stationary outline. A rotation of the stationary ring is seen that usually matches the direction of the initial ring's invisible spin. Participants reported a slightly more frequent and clearer motion percept with the MBE than RRI. ISI manipulations had similar effects on the two illusions, but the effects of number of points and ring diameter were largely restricted to the RRI. We suggest that both the RRI and MBE motion percepts are produced by a visual heuristic that holds that the transition from an outline circle to a ring of points is plausibly explained by a rapid spin decelerating to a halt, but in the case of the MBE, an additional direction-sensitive mechanism contributes to this percept.

No effect of very brief exposure to masked food pictures on fear of food in anorexia nervosa.

OBJECTIVE: Very brief exposure to masked images of spider stimuli can facilitate approach behaviour towards spiders in fearful subjects. We hypothesized that a similar effect might occur for fear of food in patients with anorexia nervosa (AN), possibly offering a new treatment approach, with advantages over other methods of food exposure. METHODS: Patients with AN (n = 60) were randomly assigned to one of three experimental conditions and received a single session of exposure to either masked and very briefly presented food images, clearly visible food images, or masked non-food images (i.e. household items). Effects of the three exposure conditions on fear of food and food avoidance were examined. RESULTS: Contrary to our expectations, very brief food cue exposure was not superior to the control conditions regarding fear of food and approach behaviour towards food immediately after the intervention and body mass index four weeks later. CONCLUSION: This finding suggests important differences between fear of food in AN and specific phobias such as fear of spiders. The absence of an effect reveals limitations of the very brief exposure method, which might be better suited for evolutionarily relevant threat stimuli.

Encoding Information From Rotations Too Rapid To Be Consciously Perceived as Rotating: A Replication of the Motion Bridging Effect on a Liquid Crystal Display.

A ring of points that is rotated so rapidly is perceived as a stationary outline circle that can induce an illusory rotation with the same spin direction in a subsequently presented ring of stationary points. This motion bridging effect (MBE) demonstrates that motion information can be conveyed by temporal frequencies generally thought to exceed the processing capabilities of the human visual system. It was first described in displays shown with an analog oscilloscope, but the rapid rotation rates needed to produce the MBE have heretofore prevented it from being investigated with conventional raster scan monitors. Here, we demonstrate the MBE can be reliably generated using the new generation of 240 Hz LCD gaming monitors, and exhibits basic characteristics similar to those reported previously. These monitors therefore provide a readily available resource for research on the MBE and the studies of the visual processing rapid motions in general.

Visual experience forms a multidimensional pattern that is not reducible to a single measure: Evidence from metacontrast masking.

A metacontrast masking paradigm was employed to provide evidence for the richness and diversity of our visual experience. Square- and diamond-shaped targets were followed by square- and diamond-shaped masks at varying stimulus onset asynchronies (SOAs), resulting in shape-congruent and shape-incongruent trials. In Experiment 1, participants reported in each trial how they perceived target and mask. After extended training, seven different aspects of the target could be distinguished as specific percepts in this metacontrast masking paradigm. These percepts encompass aspects including the temporal distance between both stimuli, the perceived contrast of the target, and motion percepts resulting from the interplay between the target and mask. Participants spontaneously reported each of these percepts, and the frequency of reports varied systematically with SOA and the congruency between target and mask. In Experiment 2, we trained a new group of participants to distinguish each of these target percepts. Again, the frequency of reports of the specific percepts varied with SOA and congruency, just as in Experiment 1. In a last session, we measured objective discrimination performance yielding the typical individually different masking functions across SOAs. An examination of the relation between the frequencies of reports of subjective percepts and objective discrimination performance revealed multiple dissociations between these measures. Results suggest a multidimensional pattern of subjective experiences under metacontrast, which is reflected in dissociated subjective and objective measures of visual awareness. As a consequence, awareness cannot be assessed exhaustively by a single measure, thus challenging the use of simple one-dimensional subjective or objective measures in visual masking.

Action priming is linked to visual perception in continuous flash suppression.

Visual prime stimuli can affect the processing of following target stimuli even if their visibility is reduced due to visual masking. Prime visibility depends on the stimulus parameters of the prime and those of the mask. Here we explored the effects of prime stimuli and modulated their visibility by continuous flash suppression (CFS). CFS reduces the visibility of a stimulus presented to one eye by simultaneously presenting a series of high-contrast masking stimuli to the other eye. We manipulated the strength of CFS effects on perception and examined how action priming effects of the masked stimuli varied under the same conditions. Prime visibility was modulated by the contrast of the primes (Experiments 1 and 2), the contrast of the masks (Experiments 2 and 3), and by the stimulus onset asynchrony between prime and target stimuli (all experiments). Surprisingly, action priming effects were modulated by these experimental variables in a parallel way. In addition, individual differences between participants in prime visibility correlated with individual differences in action priming. Our findings suggest that action priming and prime perception depend in similar ways on prime contrast, mask contrast, stimulus onset asynchrony, and individual dispositions in CFS. These findings distinguish CFS from other perceptual suppression techniques, such as backward masking, that allow reducing prime visibility without parallel effects on action priming. Our results corroborate the view that CFS interferes with visual processing at early stages in the cortical hierarchy with similar effects on later processing for perception and action.

Action priming suppression by forward masks.

Visual stimuli may produce strong and reliable effects on subsequent actions irrespective of their visibility. This dissociation between action priming and conscious perception of the stimuli suggests two ways of processing of visual stimuli. One way of processing leads to the emergence of conscious visual perception, and another way leads to action priming effects. Here we examined the influence of forward masks that precede the prime on processing for action. In three experiments, we found that forward masks can suppress and even abolish priming effects. Suppression was larger with strong rather than weak forward masks and with short rather than long prime durations. Similar suppression effects occurred with surrounding paracontrast masks and with overlapping pattern masks. Our findings emphasize that processing for action depends crucially on preceding stimuli suggesting that action priming may depend on the initial part of the response to the prime. Results indicate that the use of forward masks to reduce prime visibility may also reduce action priming and potentially other priming effects.

Stimulus dependencies of an illusory motion: Investigations of the Motion Bridging Effect.

The Motion Bridging Effect (MBE) is an illusion in which a motion that is not consciously visible generates a visible motion aftereffect that is predominantly in the same direction as the adapter motion. In the initial study of the MBE (Mattler & Fendrich, 2010), a ring of 16 points was rotated at angular velocities as high as 2250°/s so that observers saw only an unbroken outline circle and performed at chance when asked to report the ring's rotation direction. However, when the rotating ring was replaced by a veridically stationary ring of 16 points, the stationary ring appeared to visibly spin to a halt, principally in the same direction as the initial ring's rotation. Here we continue to investigate the stimulus dependencies of the MBE. We find the MBE, measured by the correspondence between the direction of the invisible rotation of the spinning ring and perceived rotation of the stationary ring, increases as the number of points used to construct the rings decreases and grows stronger as the diameter of the rings get larger. We consider the potential contributions of temporal frequency, retinal eccentricity, luminance levels, and the separation between the points forming the rings as mediators of these effects. Data is discussed with regard to the detection of real movement and apparent motion. We conclude that the detection of the rapid rotation of the spinning ring is likely to be modulated by temporal frequency of luminance changes along the ring perimeter while the point-distance may modulate an apparent motion produced by the transition from the perceptually unbroken spinning ring to the point-defined stationary ring.

Masking procedures can influence priming effects besides their effects on conscious perception.

Masked priming has been employed to study the role of consciousness for different levels of visual processing. However, masking procedures differ systematically between studies. To examine these procedural differences we contrasted priming effects with metacontrast masking, which is often applied in the context of perceptual priming, and priming effects with sandwich pattern masking, frequently used in studies on semantic priming. Results indicate that the amount of masking neither affects perceptual nor semantic priming effects in a semantic categorization task when a metacontrast masking paradigm was used. However, perceptual and semantic priming effects increased with increasing prime visibility when a sandwich pattern masking paradigm was used. Findings suggest that different types of masking procedures affect the processing of the masked stimuli in substantially different ways even if the masking effect on conscious perception of these stimuli is comparable.

Myocardial T1-mapping at 3T using saturation-recovery: reference values, precision and comparison with MOLLI.

BACKGROUND: Myocardial T1-mapping recently emerged as a promising quantitative method for non-invasive tissue characterization in numerous cardiomyopathies. Commonly performed with an inversion-recovery (IR) magnetization preparation at 1.5T, the application at 3T has gained due to increased quantification precision. Alternatively, saturation-recovery (SR) T1-mapping has recently been introduced at 1.5T for improved accuracy. Thus, the purpose of this study is to investigate the robustness and precision of SR T1-mapping at 3T and to establish accurate reference values for native T1-times and extracellular volume fraction (ECV) of healthy myocardium. METHODS: Balanced Steady-State Free-Precession (bSSFP) Saturation-Pulse Prepared Heart-rate independent Inversion-REcovery (SAPPHIRE) and Saturation-recovery Single-SHot Acquisition (SASHA) T1-mapping were compared with the Modified Look-Locker inversion recovery (MOLLI) sequence at 3T. Accuracy and precision were studied in phantom. Native and post-contrast T1-times and regional ECV were determined in 20 healthy subjects (10 men, 27 ± 5 years). Subjective image quality, susceptibility artifact rating, in-vivo precision and reproducibility were analyzed. RESULTS: SR T1-mapping showed <4 % deviation from the spin-echo reference in phantom in the range of T1 = 100-2300 ms. The average quality and artifact scores of the T1-mapping methods were: MOLLI:3.4/3.6, SAPPHIRE:3.1/3.4, SASHA:2.9/3.2; (1: poor - 4: excellent/1: strong - 4: none). SAPPHIRE and SASHA yielded significantly higher T1-times (SAPPHIRE: 1578 ± 42 ms, SASHA: 1523 ± 46 ms), in-vivo T1-time variation (SAPPHIRE: 60.1 ± 8.7 ms, SASHA: 70.0 ± 9.3 ms) and lower ECV-values (SAPPHIRE: 0.20 ± 0.02, SASHA: 0.21 ± 0.03) compared with MOLLI (T1: 1181 ± 47 ms, ECV: 0.26 ± 0.03, Precision: 53.7 ± 8.1 ms). No significant difference was found in the inter-subject variability of T1-times or ECV-values (T1: p = 0.90, ECV: p = 0.78), the observer agreement (inter: p > 0.19; intra: p > 0.09) or consistency (inter: p > 0.07; intra: p > 0.17) between the three methods. CONCLUSIONS: Saturation-recovery T1-mapping at 3T yields higher accuracy, comparable inter-subject, inter- and intra-observer variability and less than 30 % precision-loss compared to MOLLI.

Individually different weighting of multiple processes underlies effects of metacontrast masking.

Metacontrast masking occurs when a mask follows a target stimulus in close spatial proximity. Target visibility varies with stimulus onset asynchrony (SOA) between target and mask in individually different ways leading to different masking functions with corresponding phenomenological reports. We used individual differences to determine the processes that underlie metacontrast masking. We assessed individual masking functions in a masked target discrimination task using different masking conditions and applied factor-analytical techniques on measures of sensitivity. Results yielded two latent variables that (1) contribute to performance with short and long SOA, respectively, (2) relate to specific stimulus features, and (3) differentially correlate with specific subjective percepts. We propose that each latent variable reflects a specific process. Two additional processes may contribute to performance with short and long SOAs, respectively. Discrimination performance in metacontrast masking results from individually different weightings of two to four processes, each of which contributes to specific subjective percepts.

On the source and scope of priming effects of masked stimuli on endogenous shifts of spatial attention.

Unconscious stimuli can influence participants' motor behavior as well as more complex mental processes. Previous cue-priming experiments demonstrated that masked cues can modulate endogenous shifts of spatial attention as measured by choice reaction time tasks. Here, we applied a signal detection task with masked luminance targets to determine the source and the scope of effects of masked stimuli. Target-detection performance was modulated by prime-cue congruency, indicating that prime-cue congruency modulates signal enhancement at early levels of target processing. These effects, however, were only found when the prime was perceptually similar to the cue indicting that primes influence early target processing in an indirect way by facilitating cue processing. Together with previous research we conclude that masked stimuli can modulate perceptual and post-central levels of processing. Findings mark a new limit of the effects of unconscious stimuli which seem to have a smaller scope than conscious stimuli.

Masked stimuli modulate endogenous shifts of spatial attention.

Unconscious stimuli can influence participants' motor behavior but also more complex mental processes. Recent research has gradually extended the limits of effects of unconscious stimuli. One field of research where such limits have been proposed is spatial cueing, where exogenous automatic shifts of attention have been distinguished from endogenous controlled processes which govern voluntary shifts of attention. Previous evidence suggests unconscious effects on mechanisms of exogenous shifts of attention. Here, we applied a cue-priming paradigm to a spatial cueing task with arbitrary cues by centrally presenting a masked symmetrical prime before every cue stimulus. We found priming effects on response times in target discrimination tasks with the typical dynamic of cue-priming effects (Experiments 1 and 2) indicating that central symmetrical stimuli which have been associated with endogenous orienting can modulate shifts of spatial attention even when they are masked. Prime-Cue Congruency effects of perceptual dissimilar prime and cue stimuli (Experiment 3) suggest that these effects cannot be entirely reduced to perceptual repetition priming of cue processing. In addition, priming effects did not differ between participants with good and poor prime recognition performance consistent with the view that unconscious stimulus features have access to processes of endogenous shifts of attention.

Neural correlates of control operations in inverse priming with relevant and irrelevant masks.

The inverse priming paradigm can be considered one example which demonstrates the operation of control processes in the absence of conscious experience of the inducing stimuli. Inverse priming is generated by a prime that is followed by a mask and a subsequent imperative target stimulus. With "relevant" masks that are composed of the superposition of both prime alternatives, the inverse priming effect is typically larger than with "irrelevant" masks that are free of task-relevant features. We used functional magnetic resonance imaging (fMRI) to examine the neural substrates that are involved in the generation of inverse priming effects with relevant and irrelevant masks. We found a network of brain areas that is accessible to unconscious primes, including supplementary motor area (SMA), anterior insula, middle cingulate cortex, and supramarginal gyrus. Activation of these brain areas were involved in inverse priming when relevant masks were used. With irrelevant masks, however, only SMA activation was involved in inverse priming effects. Activation in SMA correlated with inverse priming effects of individual participants on reaction time, indicating that this brain area reflects the size of inverse priming effects on the behavioral level. Findings are most consistent with the view that a basic inhibitory mechanism contributes to inverse priming with either type of mask and additional processes contribute to the effect with relevant masks. This study provides new evidence showing that cognitive control operations in the human cortex take account of task relevant stimulus information even if this information is not consciously perceived.

Individual differences in subjective experience and objective performance in metacontrast masking.

When participants discriminate stimuli that are masked by a following stimulus via metacontrast masking, stable individually different masking functions have been found despite identical stimulation conditions.In the present study, in one group of observers objective performance increased with increasing target-mask stimulus onset asynchrony (SOA), whereas in another group performance decreased with increasing SOA. In addition, a group of overachievers showed ceiling effects whereas a group of underachievers hardly exceeded chance levels of performance irrespective of SOA. The differences between observers' objective measures of performance correspond to differences in participants' phenomenological reports of subjective experience. This indicates that participants differ in their access to specific perceptual cues that they use spontaneously to solve the task. When we instructed participants to use only one specific cue, the instructed cue determined participants' objective performance considerably in two experiments. Nevertheless, masking functions remained similar with and without the cued instruction, and the effect of cues depended on the initial masking function of individuals. Findings suggest that individuals with different masking functions differ also in terms of phenomenology, used cues, and response strategy. The relation between subjective experience, reported usage of perceptual cues, and objective performance in the metacontrast masking task deserves further investigation.

Inverse cue priming is not limited to masks with relevant features.

Apart from positive priming effects, masked prime stimuli can impair responses to a subsequent target stimulus which shares response-critical features in contrast to a target assigned to the opposite response. This counterintuitive phenomenon is called inverse priming (or negative compatibility effect). Here we examine the generality of this phenomenon beyond priming of motor responses. We used a non-motor cue-priming paradigm to study the underlying mechanism of inverse priming for relevant features masks which include task-relevant stimulus features and for irrelevant masks which omit task-relevant features. We found inverse cue-priming effects with both types of masks. With task-irrelevant masks inverse cue-priming was emphasized in those participants being unable to perceive the prime. The existence of inverse non-motor priming under conditions where simple perceptual interactions between the stimuli are ruled out as the source of inverse priming is at odds with the view that inverse priming reflects motor inhibition. Alternatives are discussed.

Individual differences in metacontrast masking regarding sensitivity and response bias.

In metacontrast masking target visibility is modulated by the time until a masking stimulus appears. The effect of this temporal delay differs across participants in such a way that individual human observers' performance shows distinguishable types of masking functions which remain largely unchanged for months. Here we examined whether individual differences in masking functions depend on different response criteria in addition to differences in discrimination sensitivity. To this end we reanalyzed previously published data and conducted a new experiment for further data analyses. Our analyses demonstrate that a distinction of masking functions based on the type of masking stimulus is superior to a distinction based on the target-mask congruency. Individually different masking functions are based on individual differences in discrimination sensitivities and in response criteria. Results suggest that individual differences in metacontrast masking result from individually different criterion contents.

Time course of free-choice priming effects explained by a simple accumulator model.

Unconscious visual stimuli can be processed by human observers and modulate their behavior. This has been shown for masked prime stimuli that influence motor responses to subsequent target stimuli. Beyond this, masked stimuli can also affect participants' behavior when they are free to choose one of two response alternatives. This finding demonstrates that an apparently free-choice between alternative behaviors can be subject to influences that are outside of awareness. We report three experiments which exhibit that the temporal dynamic of free-choice priming effects corresponds to that of forced-choice priming effects. Forced-choice priming effects were relatively robust against variations of prime stimuli but sensitive to physical features of target stimuli. Free-choice priming effects, in contrast, depended largely on the stimulus-response compatibility of the prime. A simple accumulator model which accounts for forced-choice response priming can also explain free-choice priming effects by the assumption that unconscious stimuli can initiate motor responses when participants are engaged in a speeded choice-reaction time task. According to our analyses free-choice priming results from a response selection mechanism which integrates conscious and unconscious information from external, stimulus driven sources and also from internal sources.

A PRP-study to determine the locus of target priming effects.

Visual stimuli that are made invisible by a following mask can nonetheless affect motor responses. To localize the origin of these target priming effects we used the psychological refractory period paradigm. Participants classified tones as high or low, and responded to the position of a visual target that was preceded by a prime. The stimulus onset asynchrony (SOA) between both tasks varied. In Experiment 1 the tone task was followed by the position task and SOA dependent target priming effects were observed. When the visual position task preceded the tone task in Experiment 2, with short SOA the priming effect propagated entirely to the tone task yielding faster responses to tones on visually congruent trials and delayed responses to tones on visually incongruent trials. Together, results suggest that target priming effects arise from processing before and at the level of the central bottleneck such as sensory analysis and response selection.

PRP-paradigm provides evidence for a perceptual origin of the negative compatibility effect.

Visual stimuli (primes) that are made invisible by masking can affect motor responses to a subsequent target stimulus. When a prime is followed by a mask which is followed by a target stimulus, an inverse priming effect (or negative compatibility effect) has been found: Responses are slow and frequently incorrect when prime and target stimuli are congruent, but fast and accurate when prime and target stimuli are incongruent. To functionally localize the origins of inverse priming effects, we applied the psychological refractory period (PRP-) paradigm which distinguishes a perceptual level, a central bottleneck, and a level of motor execution. Two dual-task experiments were run with the PRP-paradigm to localize the inverse priming effect relative to the central bottleneck. Together, results of the Effect-Absorption and the Effect-Propagation Procedure suggest that inverse priming effects are generated by perceptual mechanisms. We suggest two perceptual mechanisms as the source of inverse priming effects.

Neuro-cognitive mechanisms of conscious and unconscious visual perception: From a plethora of phenomena to general principles.

Psychological and neuroscience approaches have promoted much progress in elucidating the cognitive and neural mechanisms that underlie phenomenal visual awareness during the last decades. In this article, we provide an overview of the latest research investigating important phenomena in conscious and unconscious vision. We identify general principles to characterize conscious and unconscious visual perception, which may serve as important building blocks for a unified model to explain the plethora of findings. We argue that in particular the integration of principles from both conscious and unconscious vision is advantageous and provides critical constraints for developing adequate theoretical models. Based on the principles identified in our review, we outline essential components of a unified model of conscious and unconscious visual perception. We propose that awareness refers to consolidated visual representations, which are accessible to the entire brain and therefore globally available. However, visual awareness not only depends on consolidation within the visual system, but is additionally the result of a post-sensory gating process, which is mediated by higher-level cognitive control mechanisms. We further propose that amplification of visual representations by attentional sensitization is not exclusive to the domain of conscious perception, but also applies to visual stimuli, which remain unconscious. Conscious and unconscious processing modes are highly interdependent with influences in both directions. We therefore argue that exactly this interdependence renders a unified model of conscious and unconscious visual perception valuable. Computational modeling jointly with focused experimental research could lead to a better understanding of the plethora of empirical phenomena in consciousness research.