Frouke Hermens

Long-lasting modulation of feature integration by transcranial magnetic stimulation

The human brain analyzes a visual object first by basic feature detectors. On the objects way to a conscious percept, these features are integrated in subsequent stages of the visual hierarchy. The time course of this feature integration is largely unknown. To shed light on the temporal dynamics of feature integration, we applied transcranial magnetic stimulation (TMS) to a feature fusion paradigm. In feature fusion, two stimuli which differ in one feature are presented in rapid succession such that they are not perceived individually but as one single stimulus only. The fused percept is an integration of the features of both stimuli. Here, we show that TMS can modulate this integration for a surprisingly long period of time, even though the individual stimuli themselves are not consciously perceived. Hence, our results reveal a long-lasting integration process of unconscious feature traces.

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.

Comment on "Competition for consciousness among visual events: the psychophysics of reentrant visual processes" (Di Lollo, Enns, & Rensink, 2000).

V. Di Lolo, J. T. Enns, and R. A. Rensink (2000) reported properties of masking that they claimed were inconsistent with all current models. The current authors show, through computer simulation, that many current models can account for V. Di Lollo et al.s (2000) data. Although V. Di Lollo et al. (2000) argued that their data could be accounted for only with models that incorporate reentrant processing, the current authors show that reentrant processing is not necessary.

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. Blochs 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, Blochs 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 Blochs 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.

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.

Spatial Grouping Determines Temporal Integration

To make sense out of a continuously changing visual world, people need to integrate features across space and time. Despite more than a century of research, the mechanisms of features integration are still a matter of debate. To examine how temporal and spatial integration interact, the authors measured the amount of temporal fusion (a measure of temporal integration) for different spatial layouts. They found that spatial grouping by proximity and similarity can completely block temporal integration. Computer simulations with a simple neural network capture these findings very well, suggesting that the proposed spatial grouping operations may occur already at an early stage of visual information processing.

Eye Movements in Risky Choice

Abstract We asked participants to make simple risky choices while we recorded their eye movements. We built a complete statistical model of the eye movements and found very little systematic variation in eye movements over the time course of a choice or across the different choices. The only exceptions were finding more (of the same) eye movements when choice options were similar, and an emerging gaze bias in which people looked more at the gamble they ultimately chose. These findings are inconsistent with prospect theory, the priority heuristic, or decision field theory. However, the eye movements made during a choice have a large relationship with the final choice, and this is mostly independent from the contribution of the actual attribute values in the choice options. That is, eye movements tell us not just about the processing of attribute values but also are independently associated with choice. The pattern is simple—people choose the gamble they look at more often, independently of the actual numbers they see—and this pattern is simpler than predicted by decision field theory, decision by sampling, and the parallel constraint satisfaction model.

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 masks 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.

The roles of mask luminance and perceptual grouping in visual backward masking

Visual backward masking is a commonly used technique in vision research and psychology. There are two distinct types of masking. Either masking is strongest for a simultaneous presentation of the target and the mask (A-type masking) or masking is strongest when the mask trails the target (B-type masking). To account for the two types of masking, a variety of explanations have been put forward that often rely on low-level features such as the target-mask energy ratio. However, recent studies have demonstrated that the global spatial layout of the mask is an equally important factor. Here, we investigated both factors jointly. Our findings show that both factors strongly interact with each other and that neither one alone can explain the results. This finding indicates that choosing a mask should not be taken lightly when masking is used as a tool to investigate properties of perception or cognition.