Assessing the microstructure of motion correspondences with non-retinotopic feature attribution

The motion correspondence problem, one of the classical examples of perceptual organization, addresses the question of how elements are grouped across space and time. Here, we investigate motion correspondences using a new feature attribution technique. We present, for example, a grating of four lines followed by a spatially shifted grating of three lines. Observers perceive a contracting grating. To study individual line-to-line correspondences, (1) we add, as a "perceptual marker," a small Vernier offset to one line of the first grating and (2) determine to which line of the second grating this offset is attributed. This procedure allows us inferring motion correspondences because this kind of feature attribution follows perceptual grouping in dynamic displays (H. Oğmen, T. U. Otto, & M. H. Herzog, 2006). Our results show that feature attribution between outer lines of the grating is more consistent than between inner lines. We interpret our results according to the principle of the "primacy of bounding contours," which states that bounding contours of an object provide a framework for element correspondences that is more important than the internal structure of that object.

Contrast polarity, chromaticity, and stereoscopic depth modulate contextual interactions in vernier acuity

Vernier alignment thresholds are strongly compromised when the vernier is embedded in an array of equal-length flanking lines. Here, we show that these contextual interactions can be diminished by giving the flanks the opposite contrast polarity, e.g., white flanks surrounding a black vernier. Similar results are obtained for red verniers and equiluminant green flanks and when vernier and flanks have different binocular disparity. Using special flank configurations, we can eliminate location uncertainty as an important factor for this kind of contextual interactions. We interpret these results as evidence that perceptual grouping of the vernier and the flanks plays an important role in the vernier threshold elevation caused by contextual flanks.

Enhanced temporal but not attentional processing in expert tennis players

In tennis, as in many disciplines of sport, fine spatio-temporal resolution is required to reach optimal performance. While many studies on tennis have focused on anticipatory skills or decision making, fewer have investigated the underlying visual perception abilities. In this study, we used a battery of seven visual tests that allowed us to assess which kind of visual information processing is performed better by tennis players than other athletes (triathletes) and non-athletes. We found that certain time-related skills, such as speed discrimination, are superior in tennis players compared to non-athletes and triathletes. Such tasks might be used to improve tennis performance in the future.

Contextual suppression and protection in schizophrenic patients

INTRODUCTION

Contextual processing is often strongly deteriorated in schizophrenic patients as found, for example, in higher cognitive as well as lower visual paradigms. In visual detection tasks, impoverished contextual facilitation was attributed to aberrant excitatory neural circuits. On the other hand, we found contextual suppression, possibly related to neural inhibition, to be fast and intact in a visual backward masking task. Here, we combine a suppressive with a "protective" paradigm to further our understanding of the contextual deficiencies of schizophrenic patients in visual information processing.

METHODS

Twenty three schizophrenic patients and 18 healthy controls were asked to discriminate the offset direction of a vernier target, which was followed by one of a variety of masks for several stimulus onset asynchronies (SOAs).

RESULTS

As in previous studies, patients needed clearly longer SOAs than controls. However, when longer SOAs were taken into account, increases as well as decreases in backward mask strength had comparable effects in patients and controls.

CONCLUSIONS

From these results, we suggest that complex spatial processing is fast and intact in schizophrenic patients.

Time-course and surround modulation of contrast masking in human vision

Neural and perceptual responses to a visual stimulus can be suppressed by the addition of both spatially overlapping and spatially adjacent contextual stimuli. We investigated the temporal characteristics of these suppressive interactions in psychophysical contrast masking experiments using Gabor and grating stimuli with a spatial frequency of 4 cycles per degree. We found that the time course of masking strongly depended on mask orientation. Most interestingly, masking by a spatially overlaid, iso-oriented mask was strongest when the target was presented immediately before or immediately after the mask. This masking was transient, presumably caused by the neural responses to mask onset and offset. Adding a surround to the mask modulated the backward masking effect, but only when the target and the central mask were iso-oriented. Our results provide evidence for a surround suppression mechanism that affected the transient responses to the mask onset, but not the responses to the mask offset. Together, these results demonstrate how the effects of spatial context in visual processing critically depend on stimulus timing.

Modeling spatial and temporal aspects of visual backward masking

Visual backward masking is a versatile tool for understanding principles and limitations of visual information processing in the human brain. However, the mechanisms underlying masking are still poorly understood. In the current contribution, the authors show that a structurally simple mathematical model can explain many spatial and temporal effects in visual masking, such as spatial layout effects on pattern masking and B-type masking. Specifically, the authors show that lateral excitation and inhibition on different length scales, in combination with the typical time scales, are capable of producing a rich, dynamic behavior that explains this multitude of masking phenomena in a single, biophysically motivated model.

Perceptual learning of bisection stimuli under roving: slow and largely specific

In perceptual learning, performance often improves within a short time if only one stimulus variant is presented, such as a line bisection stimulus with one outer-line-distance. However, performance stagnates if two bisection stimuli with two outer-line-distances are presented randomly interleaved. Recently, S. G. Kuai, J. Y. Zhang, S. A. Klein, D. M. Levi, and C. Yu, (2005) proposed that learning under roving conditions is impossible in general. Contrary to this proposition, we show here that perceptual learning with bisection stimuli under roving is possible with extensive training of 18000 trials. Despite this extensive training, the improvement of performance is still largely specific. Furthermore, this improvement of performance cannot be explained by an accommodation to stimulus uncertainty caused by roving.

Consciousness & the small network argument

The last decade has experienced a vivid enthusiasm to unravel the mystery of consciousness believed to be one of the major puzzles of human kind. We share this enthusiasm. Still, we feel that current models are incomplete suffering from a problem that we call the "small network argument".

What static and dynamic properties should slalom skis possess? Judgements by advanced and expert skiers

Flexural and torsional rigidity are important properties of skis. However, the flexural and torsional rigidity that lead to optimal performance remain to be established. In the present study, four pairs of slalom skis that differed in flexural and torsional rigidity were tested by advanced and expert skiers. Using a 10-item questionnaire, different aspects of the skis' performance were rated on a 9-point scale. For each pair of skis, physical measurements were compared with the ratings of the two groups of skiers. Correlations (Spearman) were then determined between (i) different mechanical properties of the skis (static and dynamic), (ii) subjective assessments of the participants, and (iii) properties of the skis and the participants' assessments. The latter showed that expert skiers rate the aspects of the skis more accurately than advanced skiers. Most importantly, expert skiers are particularly sensitive to torsion of the skis. These results suggest that such highly rated elements should be addressed in future ski designs.

Bloch’s law and the dynamics of feature fusion

How the visual brain integrates temporally dispersed information is an open question. Often, it is assumed that the visual system simply sums light over a certain period of time (e.g. Bloch's law). However, in feature fusion, information presented later dominates, suggesting complex temporal dynamics that cannot be described by simple energy summation. For example, if two verniers are presented in rapid succession at the same location, they are not perceived individually but they fuse to one single vernier. The perceived offset of the fused vernier is a combination of the offsets of the two presented verniers, with the later one dominating. Here, we show that indeed, Bloch's law does not hold across verniers in a sequence. However, changes in the luminance of a single vernier can be compensated for by changes in its duration in accordance with Bloch's law. We present a simple model to demonstrate that these findings can be explained by decaying neural activation.

Feature fusion reveals slow and fast visual memories

Although the visual system can achieve a coarse classification of its inputs in a relatively short time, the synthesis of qualia-rich and detailed percepts can take substantially more time. If these prolonged computations were to take place in a retinotopic space, moving objects would generate extensive smear. However, under normal viewing conditions, moving objects appear relatively sharp and clear, suggesting that a substantial part of visual short-term memory takes place at a nonretinotopic locus. By using a retinotopic feature fusion and a nonretinotopic feature attribution paradigm, we provide evidence for a relatively fast retinotopic buffer and a substantially slower nonretinotopic memory. We present a simple model that can account for the dynamics of these complementary memory processes. Taken together, our results indicate that the visual system can accomplish temporal integration of information while avoiding smear by breaking off sensory memory into fast and slow components that are implemented in retinotopic and nonretinotopic loci, respectively.

The effects of the global structure of the mask in visual backward masking

The visibility of a target can be strongly affected by a trailing mask. Research on visual backward masking has typically focused on the temporal characteristics of masking, whereas non-basic spatial aspects have received much less attention. However, recently, it has been demonstrated that the spatial layout is an important determinant of the strength of a mask. Here, we show that not only local but also global aspects of the mask's spatial layout affect target processing. Particularly, it is the regularity of the mask that plays an important role. Our findings are of importance for theoretical research, as well as for applications of visual masking.

Collinear contextual suppression in schizophrenic patients

Schizophrenic patients show aberrancies of contextual processing over a broad range. Of particular importance are low level contextual deficiencies since they might cause higher level processing deficits. It was previously found that schizophrenic patients reveal diminished contextual facilitation in visual detection tasks taken as an indication of a modified neural circuitry. Here, we show that contextual suppression is not affected. Sixteen schizophrenic patients and sixteen healthy controls participated in a backward masking task in which a vernier target was followed by a masking grating. In accordance with a previous publication, schizophrenic patients needed longer SOAs between the vernier and the grating onset to obtain a performance level comparable to healthy controls. To study contextual processing we added single collinear lines to the grating. These lines yielded a strong impairment of performance in patients and controls. This impairment is comparable between the two groups if SOAs were individually adjusted. Hence, whereas contextual facilitation is deficient, contextual suppression seems to be intact in schizophrenic patients.

Grouping of contextual elements that affect vernier thresholds

To reveal the mechanisms of spatial interference in the fovea, we examined the capacity of a variety of lateral flanking configurations to interfere with alignment thresholds of a vertical vernier. A single line on each side of the vernier, at the optimal separation of 2-3 arcmin, raises thresholds threefold or more and masks most effectively when its length equals that of the vernier. For an array of equal lines whose length differs from that of the vernier, masking is reduced but not when vernier and flanks have the same length. The reduction for shorter and longer flanks can be reversed by inserting a gap in the row of flanks. By comparing the masking effect of arrays of mixed line lengths, we show that when a pair of flanks, which by itself masks strongly, becomes a component of a coherent contextual configuration, it loses much of its effectiveness to interact. Observers' ranking of the conspicuity of the vernier test pattern among the flanking elements is negatively correlated with the threshold elevation. We conclude that clustering of contextual patterns influences their capacity to mask. Discrimination of a target deteriorates when the target is grouped within an array of surrounding elements.

What is the strength of a mask in visual metacontrast masking?

After more than a century of research, the mechanisms underlying visual masking are still hotly debated. One key characteristic of masking is that variations in the stimulus onset asynchrony (SOA) between the target and the mask can lead to either monotonic reductions in the effect of the mask on the target (A-type masking) or an increase in masking for intermediate SOAs and then a decrease in masking for longer SOAs (B-type masking). Past experimental and theoretical work suggested that the type of the masking function depends on the strength of the mask relative to the target. Usually, mask strength is related to energy (stimulus intensity x duration). Here, we show that the overall spatial layout of the mask is a much stronger factor than classical energy to explain the type of masking function.

Reverse feedback induces position and orientation specific changes

To investigate the mechanisms of perceptual learning, we recently introduced a paradigm in which incorrect, reverse feedback followed after some but not all vernier presentations. This feedback paradigm exerted a strong effect on performance that seemed to bias decisions rather than to yield perceptual learning. Here, we show that observers can develop independent decision biases for different stimulus orientations as well as for different visual field positions. Our results demonstrate that the effects of incorrect, reverse feedback are surprisingly specific.

Perceptual grouping induces non-retinotopic feature attribution in human vision

The human visual system computes features of moving objects with high precision despite the fact that these features can change or blend into each other in the retinotopic image. Very little is known about how the human brain accomplishes this complex feat. Using a Ternus-Pikler display, introduced by Gestalt psychologists about a century ago, we show that human observers can perceive features of moving objects at locations these features are not present. More importantly, our results indicate that these non-retinotopic feature attributions are not errors caused by the limitations of the perceptual system but follow rules of perceptual grouping. From a computational perspective, our data imply sophisticated real-time transformations of retinotopic relations in the visual cortex. Our results suggest that the human motion and form systems interact with each other to remap the retinotopic projection of the physical space in order to maintain the identity of moving objects in the perceptual space.

The flight path of the phoenix—The visible trace of invisible elements in human vision

How features are attributed to objects is one of the most puzzling issues in the neurosciences. A deeply entrenched view is that features are perceived at the locations where they are presented. Here, we show that features in motion displays can be systematically attributed from one location to another although the elements possessing the features are invisible. Furthermore, features can be integrated across locations. Feature mislocalizations are usually treated as errors and limits of the visual system. On the contrary, we show that the nonretinotopic feature attributions, reported herein, follow rules of grouping precisely suggesting that they reflect a fundamental computational strategy and not errors of visual processing.

Perceptual learning with spatial uncertainties

In perceptual learning, stimuli are usually assumed to be presented to a constant retinal location during training. However, due to tremor, drift, and microsaccades of the eyes, the same stimulus covers different retinal positions on sequential trials. Because of these variations the mathematical decision problem changes from linear to non-linear (). This non-linearity implies three predictions. First, varying the spatial position of a stimulus within a moderate range does not deteriorate perceptual learning. Second, improvement for one stimulus variant can yield negative transfer to other variants. Third, interleaved training with two stimulus variants yields no or strongly diminished learning. Using a bisection task, we found psychophysical evidence for the first and last prediction. However, no negative transfer was found as opposed to the second prediction.

Long lasting effects of unmasking in a feature fusion paradigm

In spite of more than 100 years of research, the mechanisms underlying visual masking are still unknown. In recent publications, we introduced an unmasking paradigm involving the fusion of features that revealed interesting spatial characteristics. Here, we investigate the temporal aspects of this paradigm showing very long lasting effects that impose serious restrictions on models of masking. We used a simple feed-forward neural network model to explain these results.

Attention and feature integration in the feature inheritance effect

Features of neighboring elements are not processed independently. Often, it is assumed that nearby features are integrated by a (pre-attentive) pooling mechanism. Here, we show that in the feature inheritance effect some features are integrated across space whereas others are not. This result may be partly explained by a very focused spatial attention. Our findings challenge models based on a simple pooling mechanism.

Intact and deficient feature fusion in schizophrenia

In patients with schizophrenia, early as well as late stages of information processing can be deficient. Therefore, it is important to determine the earliest occurrences of aberrant processing since deficits on these stages may cause abnormal processing on later, e. g. cognitive, levels. In order to investigate this issue in the visual domain, we studied one of the most basic feature integration mechanisms, namely feature fusion. Our results indicate that in schizophrenic patients this integration mechanism is qualitatively intact but reveals quantitative impairments that may influence later processing stages.

Valences in contextual vision

Elements in the neighborhood of a stimulus can modulate both the subjective perception of and the neural responses to this stimulus. Investigations of this contextual modulation usually focus on low level features such as the orientation difference between the target and its context. Recently, we introduced a paradigm in which contextual modulation cannot be explained by orientation differences between target and context per se. Instead, the overall structure of the context seemed to determine contextual modulation. Here, we show that edges of contextual gratings as well as isolated contextual lines are the main source of contextual suppression in this effect. Such suppressive contextual elements can be blocked by non-suppressive ones. We suggest that contextual elements reveal valences in loose analogy to chemical valences.

Intact figure-ground segmentation in schizophrenia

As revealed by backward masking studies, schizophrenic patients show strong impairments of early visual processing. However, the underlying temporal mechanisms are not yet well understood. To shed light on the exact timing of these deficits, we employed a paradigm in which two masks follow each other. We investigated 16 medicated schizophrenic patients and a matched group of 14 controls with a new backward masking technique, shine-through. In accordance with other masking studies, schizophrenic patients require a dramatically longer processing time to reach a predefined performance level compared with healthy subjects. However, patients are surprisingly sensitive to subtle differences in the timing of the two masks, revealing good temporal resolution. This good temporal resolution indicates intact and fast perceptual grouping and figure-ground segmentation in spite of high susceptibility to masking procedures in schizophrenia.