Invisibility and interpretation

Unconscious Perception of Vernier Offsets

The comparison between conscious and unconscious perception is a cornerstone of consciousness science. However, most studies reporting above-chance discrimination of unseen stimuli do not control for criterion biases when assessing awareness. We tested whether observers can discriminate subjectively invisible offsets of Vernier stimuli when visibility is probed using a bias-free task. To reduce visibility, stimuli were either backward masked or presented for very brief durations (1-3 milliseconds) using a modern-day Tachistoscope. We found some behavioral indicators of perception without awareness, and yet, no conclusive evidence thereof. To seek more decisive proof, we simulated a series of Bayesian observer models, including some that produce visibility judgements alongside type-1 judgements. Our data are best accounted for by observers with slightly suboptimal conscious access to sensory evidence. Overall, the stimuli and visibility manipulations employed here induced mild instances of blindsight-like behavior, making them attractive candidates for future investigation of this phenomenon.

Close - but not distant - conditioned flanker emotion affects crowding

Crowding is affected by conditioned stimulus emotion. This effect is clearly observed for conditioned flankers, but only marginally pronounced for conditioned targets. Studies on the processing of emotional stimuli suggest that the magnitude of the emotional effect depends on the presentation depth in that effects of emotion increase with decreasing distance to the observer in depth. Based on respective findings, we investigate crowding with stimuli of conditioned negative and neutral emotion across real depth; that is, stimuli were either presented closer, at or farther away than the fixation depth. Conditioned emotion of flankers affected crowding when flankers were presented closer than or at fixation depth, which is also the distance the target was presented at. Farther away than the fixation depth, flanker emotion did not alter crowding (Experiment 1a). Conditioned target emotion, however, did only show weak effects on crowding; neither when flankers (Experiment 1b) nor when targets were varied in depth (Experiment 2) there was a clear effect of target emotion, replicating findings in two-dimensional settings. Taken together, the results suggest that flanker's emotional associations can become important for crowding, although, it depends on the special processing characteristics of stimulus emotion in depth. The conditioned emotion of targets scarcely affected crowding.

Unconscious retinotopic motion processing affects non-retinotopic motion perception

Unconscious visual stimuli can affect conscious perception: For example, an invisible prime can affect responses to a subsequent target. The invisible interpretation of an ambiguous figure can have similar effects. Invisibility in these situations is typically explained by stimulus-suppression in early, retinotopic brain areas. We have previously argued that invisibility is closely linked to Gestalt ("object") organization principles. For example, motion is typically perceived in non-retinotopic, object-centered, and not in retinotopic coordinates. Such is the case for a bicycle-reflector that is perceived as circling, although its retinotopic trajectory is cycloidal. Here, we used a modified Ternus-Pikler display in which, just as in everyday vision, the retinotopic motion is invisible and the non-retinotopic motion is perceived. Nevertheless, the invisible retinotopic motion, can strongly degrade the conscious non-retinotopic motion percept. This effect cannot be explained by inhibition at a retinotopic processing stage.

Field-like interactions between motion-based reference frames

A reference frame is required to specify how motion is perceived. For example, the motion of part of an object is usually perceived relative to the motion of the object itself. Johansson (Psychological Research, 38, 379-393, 1976) proposed that the perceptual system carries out a vector decomposition, which rewsults in common and relative motion percepts. Because vector decomposition is an ill-posed problem, several studies have introduced constraints by means of which the number of solutions can be substantially reduced. Here, we have adopted an alternative approach and studied how, rather than why, a subset of solutions is selected by the visual system. We propose that each retinotopic motion vector creates a reference-frame field in the retinotopic space, and that the fields created by different motion vectors interact in order to determine a motion vector that will serve as the reference frame at a given point and time in space. To test this theory, we performed a set of psychophysical experiments. The field-like influence of motion-based reference frames was manifested by increased nonspatiotopic percepts of the backward motion of a target square with decreasing distance from a drifting grating. We then sought to determine whether these field-like effects of motion-based reference frames can also be extended to stationary landmarks. The results suggest that reference-field interactions occur only between motion-generated fields. Finally, we investigated whether and how different reference fields interact with each other, and found that different reference-field interactions are nonlinear and depend on how the motion vectors are grouped. These findings are discussed from the perspective of the reference-frame metric field (RFMF) theory, according to which perceptual grouping operations play a central and essential role in determining the prevailing reference frames.

Putting low-level vision into global context: Why vision cannot be reduced to basic circuits

To cope with the complexity of vision, most models in neuroscience and computer vision are of hierarchical and feedforward nature. Low-level vision, such as edge and motion detection, is explained by basic low-level neural circuits, whose outputs serve as building blocks for more complex circuits computing higher level features such as shape and entire objects. There is an isomorphism between states of the outer world, neural circuits, and perception, inspired by the positivistic philosophy of the mind. Here, we show that although such an approach is conceptually and mathematically appealing, it fails to explain many phenomena including crowding, visual masking, and non-retinotopic processing.

Targets but not flankers are suppressed in crowding as revealed by EEG frequency tagging

Perception of a visual target can strongly deteriorate in the presence of flanking elements (crowding). For example, adding lines next to a vernier makes vernier offset discrimination difficult. Crowding is often considered a bottleneck of low-level vision, determined by the unavoidable limitations of the early visual system. In accordance with this proposal, neural processing of the flankers should be impaired in crowding as much as that of the target. To test this prediction, we used steady-state visually evoked potentials (ssVEPs) to separate target responses from flanker responses. We presented a vernier target either alone or flanked by lines, which had the same color as the vernier or a different color. Crowding by same-color flankers was stronger than by different-color flankers. Mirroring the behavioral results, ssVEP amplitudes corresponding to the target were higher for different-color flankers than for same-color flankers. Flanker related ssVEPs, however, did not depend on crowding strength. It seems that target, but not flanker processing, is susceptible to crowding. In line with previous results, we suggest that crowding is not caused by low-level interferences but is linked to target-flanker grouping instead.

Crowding, grouping, and object recognition: A matter of appearance

In crowding, the perception of a target strongly deteriorates when neighboring elements are presented. Crowding is usually assumed to have the following characteristics. (a) Crowding is determined only by nearby elements within a restricted region around the target (Bouma's law). (b) Increasing the number of flankers can only deteriorate performance. (c) Target-flanker interference is feature-specific. These characteristics are usually explained by pooling models, which are well in the spirit of classic models of object recognition. In this review, we summarize recent findings showing that crowding is not determined by the above characteristics, thus, challenging most models of crowding. We propose that the spatial configuration across the entire visual field determines crowding. Only when one understands how all elements of a visual scene group with each other, can one determine crowding strength. We put forward the hypothesis that appearance (i.e., how stimuli look) is a good predictor for crowding, because both crowding and appearance reflect the output of recurrent processing rather than interactions during the initial phase of visual processing.